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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/4962.txt b/4962.txt new file mode 100644 index 0000000..98a321a --- /dev/null +++ b/4962.txt @@ -0,0 +1,5490 @@ +The Project Gutenberg EBook of The Story Of Germ Life, by H. W. Conn + +Copyright laws are changing all over the world. Be sure to check the +copyright laws for your country before downloading or redistributing +this or any other Project Gutenberg eBook. + +This header should be the first thing seen when viewing this Project +Gutenberg file. Please do not remove it. Do not change or edit the +header without written permission. + +Please read the "legal small print," and other information about the +eBook and Project Gutenberg at the bottom of this file. Included is +important information about your specific rights and restrictions in +how the file may be used. You can also find out about how to make a +donation to Project Gutenberg, and how to get involved. + + +**Welcome To The World of Free Plain Vanilla Electronic Texts** + +**eBooks Readable By Both Humans and By Computers, Since 1971** + +*****These eBooks Were Prepared By Thousands of Volunteers!***** + + +Title: The Story Of Germ Life + +Author: H. W. Conn + +Release Date: January, 2004 [EBook #4962] +[Yes, we are more than one year ahead of schedule] +[This file was first posted on April 5, 2002] + +Edition: 10 + +Language: English + +Character set encoding: ASCII + +*** START OF THE PROJECT GUTENBERG EBOOK THE STORY OF GERM LIFE *** + + + + +Produced by Robert Rowe, Charles Franks +and the Online Distributed Proofreading Team. + + + + +THE STORY OF GERM LIFE + +BY H. W. CONN + +PROFESSOR OF BIOLOGY AT WESLEYAN UNIVERSITY, + +AUTHOR OF EVOLUTION OF TO-DAY, +THE LIVING WORLD, ETC. + + + + + +PREFACE. + + +Since the first edition of this book was published the popular +idea of bacteria to which attention was drawn in the original +preface has undergone considerable modification. Experimental +medicine has added constantly to the list of diseases caused by +bacterial organisms, and the general public has been educated to +an adequate conception of the importance of the germ as the chief +agency in the transmission of disease, with corresponding +advantage to the efficiency of personal and public hygiene. At the +same time knowledge of the benign bacteria and the enormous role +they play in the industries and the arts has become much more +widely diffused. Bacteriology is being studied in colleges as one +of the cultural sciences; it is being widely adopted as a subject +of instruction in high schools; and schools of agriculture and +household science turn out each year thousands of graduates +familiar with the functions of bacteria in daily life. Through +these agencies the popular misconception of the nature of micro- +organisms and their relations to man is being gradually displaced +by a general appreciation of their manifold services. It is not +unreasonable to hope that the many thousands of copies of this +little manual which have been circulated and read have contributed +materially to that end. If its popularity is a safe criterion, the +book has amply fulfilled its purpose of placing before the general +reader in a simple and direct style the main facts of +bacteriology. Beginning with a discussion of the nature of +bacteria, it shows their position in the scale of plant and animal +life. The middle chapters describe the functions of bacteria in +the arts, in the dairy, and in agriculture. The final chapters +discuss the relation of bacteria to disease and the methods by +which the new and growing science of preventive medicine combats +and counteracts their dangerous powers. + +JULY, 1915. + + + + + +CONTENTS. + + +I.--BACTERIA AS PLANTS + +Historical.--Form of bacteria.--Multiplication of bacteria.--Spore +formation.--Motion.--Internal structure.--Animals or plants?-- +Classification.--Variation.--Where bacteria are found. + +II.--MISCELLANEOUS USES OF BACTERIA IN THE ARTS. + +Maceration industries.--Linen.--Jute.--Hemp.--Sponges.--Leather. +--Fermentative industries.--Vinegar--Lactic acid.--Butyric acid.-- +Bacteria in tobacco curing.--Troublesome fermentations. + +III.--BACTERIA IN THE DAIRY. + +Sources of bacteria in milk.--Effect of bacteria on milk.-- +Bacteria used in butter making.--Bacteria in cheese making. + +IV.--BACTERIA IN NATURAL PROCESSES. + +Bacteria as scavengers.--Bacteria as agents in Nature's food +cycle.--Relation of bacteria to agriculture.--Sprouting of seeds. +--The silo.--The fertility of the soil.--Bacteria as sources of +trouble to the farmer.--Coal formation. + +V.--PARASITIC BACTERIA AND THEIR RELATION TO DISEASE + +Method of producing disease.--Pathogenic germs not strictly +parasitic.--Pathogenic germs that are true parasites.--What +diseases are due to bacteria.--Variability of pathogenic powers.-- +Susceptibility of the individual.--Recovery from bacteriological +diseases.--Diseases caused by organisms other than bacteria. + +VI.--METHODS OF COMBATING PARASITIC BACTERIA + +Preventive medicine.--Bacteria in surgery.--Prevention by +inoculation.--Limits of preventive medicine.--Curative medicine. +--Drugs--Vis medicatrix naturae.--Antitoxines and their use.-- +Conclusion. + + + + + +THE STORY OF GERM LIFE. + + +CHAPTER I. + +BACTERIA AS PLANTS. + + +During the last fifteen years the subject of bacteriology +[Footnote: The term microbe is simply a word which has been coined +to include all of the microscopic plants commonly included under +the terms bacteria and yeasts.] has developed with a marvellous +rapidity. At the beginning of the ninth decade of the century +bacteria were scarcely heard of outside of scientific circles, and +very little was known about them even among scientists. Today they +are almost household words, and everyone who reads is beginning to +recognise that they have important relations to his everyday life. +The organisms called bacteria comprise simply a small class of low +plants, but this small group has proved to be of such vast +importance in its relation to the world in general that its study +has little by little crystallized into a science by itself. It is +a somewhat anomalous fact that a special branch of science, +interesting such a large number of people, should be developed +around a small group of low plants. The importance of bacteriology +is not due to any importance bacteria have as plants or as members +of the vegetable kingdom, but solely to their powers of producing +profound changes in Nature. There is no one family of plants that +begins to compare with them in importance. It is the object of +this work to point out briefly how much both of good and ill we +owe to the life and growth of these microscopic organisms. As we +have learned more and more of them during the last fifty years, it +has become more and more evident that this one little class of +microscopic plants fills a place in Nature's processes which in +some respects balances that filled by the whole of the green +plants. Minute as they are, their importance can hardly be +overrated, for upon their activities is founded the continued life +of the animal and vegetable kingdom. For good and for ill they are +agents of neverceasing and almost unlimited powers. + +HISTORICAL. + +The study of bacteria practically began with the use of the +microscope. It was toward the close of the seventeenth century +that the Dutch microscopist, Leeuwenhoek, working with his simple +lenses, first saw the organisms which we now know under this name, +with sufficient clearness to describe them. Beyond mentioning +their existence, however, his observations told little or nothing. +Nor can much more be said of the studies which followed during the +next one hundred and fifty years. During this long period many a +microscope was turned to the observation of these minute +organisms, but the majority of observers were contented with +simply seeing them, marvelling at their minuteness, and uttering +many exclamations of astonishment at the wonders of Nature. A few +men of more strictly scientific natures paid some attention to +these little organisms. Among them we should perhaps mention Von +Gleichen, Muller, Spallanzani, and Needham. Each of these, as well +as others, made some contributions to our knowledge of +microscopical life, and among other organisms studied those which +we now call bacteria. Speculations were even made at these early +dates of the possible causal connection of these organisms with +diseases, and for a little the medical profession was interested +in the suggestion. It was impossible then, however, to obtain any +evidence for the truth of this speculation, and it was abandoned +as unfounded, and even forgotten completely, until revived again +about the middle of the 19th century. During this century of +wonder a sufficiency of exactness was, however, introduced into +the study of microscopic organisms to call for the use of names, +and we find Muller using the names of Monas, Proteus, Vibrio, +Bacillus, and Spirillum, names which still continue in use, +although commonly with a different significance from that given +them by Muller. Muller did indeed make a study sufficient to +recognise the several distinct types, and attempted to classsify +these bodies. They were not regarded as of much importance, but +simply as the most minute organisms known. + +Nothing of importance came from this work, however, partly because +of the inadequacy of the microscopes of the day, and partly +because of a failure to understand the real problems at issue. +When we remember the minuteness of the bacteria, the impossibility +of studying any one of them for more than a few moments at a time +--only so long, in fact, as it can be followed under a microscope; +when we remember, too, the imperfection of the compound +microscopes which made high powers practical impossibilities; and, +above all, when we appreciate the looseness of the ideas which +pervaded all scientists as to the necessity of accurate +observation in distinction from inference, it is not strange that +the last century gave us no knowledge of bacteria beyond the mere +fact of the existence of some extremely minute organisms in +different decaying materials. Nor did the 19th century add much to +this until toward its middle. It is true that the microscope was +vastly improved early in the century, and since this improvement +served as a decided stimulus to the study of microscopic life, +among other organisms studied, bacteria received some attention. +Ehrenberg, Dujardin, Fuchs, Perty, and others left the impress of +their work upon bacteriology even before the middle of the +century. It is true that Schwann shrewdly drew conclusions as to +the relation of microscopic organisms to various processes of +fermentation and decay--conclusions which, although not accepted +at the time, have subsequently proved to be correct. It is true +that Fuchs made a careful study of the infection of "blue milk," +reaching the correct conclusion that the infection was caused by a +microscopic organism which he discovered and carefully studied. It +is true that Henle made a general theory as to the relation of +such organisms to diseases, and pointed out the logically +necessary steps in a demonstration of the causal connection +between any organism and a disease. It is true also that a general +theory of the production of ail kinds of fermentation by living +organisms had been advanced. But all these suggestions made little +impression. On the one hand, bacteria were not recognised as a +class of organisms by themselves--were not, indeed, distinguished +from yeasts or other minute animalcuise. Their variety was not +mistrusted and their significance not conceived. As microscopic +organisms, there were no reasons for considering them of any more +importance than any other small animals or plants, and their +extreme minuteness and simplicity made them of little interest to +the microscopist. On the other hand, their causal connection with +fermentative and putrefactive processes was entirely obscured by +the overshadowing weight of the chemist Liebig, who believed that +fermentations and putrefactions were simply chemical processes. +Liebig insisted that all albuminoid bodies were in a state of +chemically unstable equilibrium, and if left to themselves would +fall to pieces without any need of the action of microscopic +organisms. The force of Liebig's authority and the brilliancy of +his expositions led to the wide acceptance of his views and the +temporary obscurity of the relation of microscopic organisms to +fermentative and putrefactive processes. The objections to +Liebig's views were hardly noticed, and the force of the +experiments of Schwann was silently ignored. Until the sixth +decade of the century, therefore, these organisms, which have +since become the basis of a new branch of science, had hardly +emerged from obscurity. A few microscopists recognised their +existence, just as they did any other group of small animals or +plants, but even yet they failed to look upon them as forming a +distinct group. A growing number of observations was accumulating, +pointing toward a probable causal connection between fermentative +and putrefactive processes and the growth of microscopic +organisms; but these observations were known only to a few, and +were ignored by the majority of scientists. + +It was Louis Pasteur who brought bacteria to the front, and it was +by his labours that these organisms were rescued from the +obscurity of scientific publications and made objects of general +and crowning interest. It was Pasteur who first successfully +combated the chemical theory of fermentation by showing that +albuminous matter had no inherent tendency to decomposition. It +was Pasteur who first clearly demonstrated that these little +bodies, like all larger animals and plants, come into existence +only by ordinary methods of reproduction, and not by any +spontaneous generation, as had been earlier claimed. It was +Pasteur who first proved that such a common phenomenon as. the +souring of milk was produced by microscopic organisms growing in +the milk. It was Pasteur who first succeeded in demonstrating that +certain species of microscopic organisms are the cause of certain +diseases, and in suggesting successful methods of avoiding them. +All these discoveries were made in rapid succession. Within ten +years of the time that his name began to be heard in this +connection by scientists, the subject had advanced so rapidly that +it had become evident that here was a new subject of importance to +the scientific world, if not to the public at large. The other +important discoveries which Pasteur made it is not our purpose to +mention here. His claim to be considered the founder of +bacteriology will be recognised from what has already been +mentioned. It was not that he first discovered the organisms, or +first studied them; it was not that he first suggested their +causal connection with fermentation and disease, but it was +because he for the first time placed the subject upon a firm +foundation by proving with rigid experiment some of the +suggestions made by others, and in this way turned the attention +of science to the study of micro-organisms. + +After the importance of the subject had been demonstrated by +Pasteur, others turned their attention in the same direction, +either for the purpose of verification or refutation of Pasteur's +views. The advance was not very rapid, however, since +bacteriological experimentation proved to be a subject of +extraordinary difficulty. Bacteria were not even yet recognised as +a group of organisms distinct enough to be grouped by themselves, +but were even by Pasteur at first confounded with yeasts. As a +distinct group of organisms they were first distinguished by +Hoffman in 1869, since which date the term bacteria, as applying +to this special group of organisms, has been coming more and more +into use. So difficult were the investigations, that for years +there were hardly any investigators besides Pasteur who could +successfully handle the subject and reach conclusions which could +stand the test of time. For the next thirty years, although +investigators and investigations continued to increase, we can +find little besides dispute and confusion along this line. The +difficulty of obtaining for experiment any one kind of bacteria by +itself, unmixed with others (pure cultures), rendered advance +almost impossible. So conflicting were the results that the whole +subject soon came into almost hopeless confusion, and very few +steps were taken upon any sure basis. So difficult were the +methods, so contradictory and confusing the results, because of +impure cultures, that a student of to-day who wishes to look up +the previous discoveries in almost any line of bacteriology need +hardly go back of 1880, since he can almost rest assured that +anything done earlier than that was more likely to be erroneous +than correct. + +The last fifteen years have, however, seen a wonderful change. The +difficulties had been mostly those of methods of work, and with +the ninth decade of the century these methods were simplified by +Robert Koch. This simplification of method for the first time +placed this line of investigation within the reach of scientists +who did not have the genius of Pasteur. It was now possible to get +pure cultures easily, and to obtain with such pure cultures +results which were uniform and simple. It was now possible to take +steps which had the stamp of accuracy upon them, and which further +experiment did not disprove. From the time when these methods were +thus made manageable the study of bacteria increased with a +rapidity which has been fairly startling, and the information +which has accumulated is almost formidable. The very rapidity with +which the investigations have progressed has brought considerable +confusion, from the fact that the new discoveries have not had +time to be properly assimilated into knowledge. Today many facts +are known whose significance is still uncertain, and a clear +logical discussion of the facts of modern bacteriology is not +possible. But sufficient knowledge has been accumulated and +digested to show us at least the direction along which +bacteriological advance is tending, and it is to the pointing out +of these directions that the following pages will be devoted. + +WHAT ARE BACTERIA? + +The most interesting facts connected with the subject of +bacteriology concern the powers and influence in Nature possessed +by the bacteria. The morphological side of the subject is +interesting enough to the scientist, but to him alone. Still, it +is impossible to attempt to study the powers of bacteria without +knowing something of the organisms themselves. To understand how +they come to play an important part in Nature's processes, we must +know first how they look and where they are found. A short +consideration of certain morphological facts will therefore be +necessary at the start. + +FORM OF BACTERIA. + +In shape bacteria are the simplest conceivable structures. +Although there are hundreds of different species, they have only +three general forms, which have been aptly compared to billiard +balls, lead pencils, and corkscrews. Spheres, rods, and spirals +represent all shapes. The spheres may be large or small, and may +group themselves in various ways; the rods may be long or short, +thick or slender; the spirals may be loosely or tightly coiled, +and may have only one or two or may have many coils, and they may +be flexible or stiff; but still rods, spheres, and spirals +comprise all types. + +In size there is some variation, though not very great. All are +extremely minute, and never visible to the naked eye. The spheres +vary from 0.25 u to 1.5 u (0.000012 to 0.00006 inches). The rods +may be no more than 0.3 u in diameter, or may be as wide as 1.5 u +to 2.5 u, and in length vary all the way from a length scarcely +longer than their diameter to long threads. About the same may be +said of the spiral forms. They are decidedly the smallest living +organisms which our microscopes have revealed. + +In their method of growth we find one of the most characteristic +features. They universally have the power of multiplication by +simple division or fission. Each individual elongates and then +divides in the middle into two similar halves, each of which then +repeats the process. This method of multiplication by simple +division is the distinguishing mark which separates the bacteria +from the yeasts, the latter plants multiplying by a process known +as budding. Fig. 2 shows these two methods of multiplication. + +While all bacteria thus multiply by division, certain differences +in the details produce rather striking differences in the results. +Considering first the spherical forms, we find that some species +divide, as described, into two, which separate at once, and each +of which in turn divides in the opposite direction, called +Micrococcus, (Fig. 3). Other species divide only in one direction. +Frequently they do not separate after dividing, but remain +attached. Each, however, again elongates and divides again, but +all still remain attached. There are thus formed long chains of +spheres like strings of beads, called Streptococci (Fig. 4). Other +species divide first in one direction, then at right angles to the +first division, and a third division follows at right angles to +the plane of the first two, thus producing solid groups of fours, +eights, or sixteens (Fig 5), called Sarcina. Each different +species of bacteria is uniform in its method of division, and +these differences are therefore indications of differences in +species, or, according to our present method of classification, +the different methods of division represent different genera. All +bacteria producing Streptococcus chains form a single genus +Streptococcus, and all which divide in three division planes form +another genus, Sarcina, etc. + +The rod-shaped bacteria also differ somewhat, but to a less +extent. They almost always divide in a plane at right angles to +their longest dimension. But here again we find some species +separating immediately after division, and thus always appearing +as short rods (Fig. 6), while others remain attached after +division and form long chains. Sometimes they appear to continue +to increase in length without showing any signs of division, and +in this way long threads are formed (Fig. 7). These threads are, +however, potentially at least, long chains of short rods, and +under proper conditions they will break up into such short rods, +as shown in Fig. 7a. Occasionally a rod species may divide +lengthwise, but this is rare. Exactly the same may be said of the +spiral forms. Here, too, we find short rods and long chains, or +long spiral filaments in which can be seen no division into +shorter elements, but which, under certain conditions, break up +into short sections. + +RAPIDITY OF MULTIPLICATION. + +It is this power of multiplication by division that makes bacteria +agents of such significance. Their minute size would make them +harmless enough if it were not for an extraordinary power of +multiplication. This power of growth and division is almost +incredible. Some of the species which have been carefully watched +under the microscope have been found under favourable conditions +to grow so rapidly as to divide every half hour, or even less. The +number of offspring that would result in the course of twenty-four +hours at this rate is of course easily computed. In one day each +bacterium would produce over 16,500,000 descendants, and in two +days about 281,500,000,000. It has been further calculated that +these 281,500,000,000 would form about a solid pint of bacteria +and weigh about a pound. At the end of the third day the total +descendants would amount to 47,000,000,000,000, and would weigh +about 16,000,000 pounds. Of course these numbers have no +significance, for they are never actual or even possible numbers. +Long before the offspring reach even into the millions their rate +of multiplication is checked either by lack of food or by the +accumulation of their own excreted products, which are injurious +to them. But the figures do have interest since they show faintly +what an unlimited power of multiplication these organisms have, +and thus show us that in dealing with bacteria we are dealing with +forces of almost infinite extent. + +This wonderful power of growth is chiefly due to the fact that +bacteria feed upon food which is highly organized and already in +condition for absorption. Most plants must manufacture their own +foods out of simpler substances, like carbonic dioxide (Co2) and +water, but bacteria, as a rule, feed upon complex organic material +already prepared by the previous life of plants or animals. For +this reason they can grow faster than other plants. Not being +obliged to make their own foods like most plants, nor to search +for it like animals, but living in its midst, their rapidity of +growth and multiplication is limited only by their power to seize +and assimilate this food. As they grow in such masses of food, +they cause certain chemical changes to take place in it, changes +doubtless directly connected with their use of the material as +food. Recognising that they do cause chemical changes in food +material, and remembering this marvellous power of growth, we are +prepared to believe them capable of producing changes wherever +they get a foothold and begin to grow. Their power of feeding upon +complex organic food and producing chemical changes therein, +together with their marvellous power of assimilating this material +as food, make them agents in Nature of extreme importance. + +DIFFERENCES BETWEEN DIFFERENT SPECIES OF BACTERIA. + +While bacteria are thus very simple in form, there are a few other +slight variations in detail which assist in distinguishing them. +The rods are sometimes very blunt at the ends, almost as if cut +square across, while in other species they are more rounded and +occasionally slightly tapering. Sometimes they are +surrounded by a thin layer of some gelatinous substance, which +forms what is called a capsule (Fig. 10). This capsule may connect +them and serve as a cement, to prevent the separate elements of a +chain from falling apart. + +Sometimes such a gelatinous secretion will unite great masses of +bacteria into clusters, which may float on the surface of the +liquid in which they grow or may sink to the bottom. Such masses +are called zoogloea, and their general appearance serves as one of +the characters for distinguishing different species of bacteria +(Fig. 10, a and b). When growing in solid media, such as a +nutritious liquid made stiff with gelatine, the different species +have different methods of spreading from their central point of +origin. A single bacterium in the midst of such a stiffened mass +will feed upon it and produce descendants rapidly; but these +descendants, not being able to move through the gelatine, will +remain clustered together in a mass, which the bacteriologist +calls a colony. But their method of clustering, due to different +methods of growth, is by no means always alike, and these colonies +show great differences in general appearance. The differences +appear to be constant, however, for the same species of bacteria, +and hence the shape and appearance of the colony enable +bacteriologists to discern different species (Fig. II). All these +points of difference are of practical use to the bacteriologist in +distinguishing species. + +SPORE FORMATION. + +In addition to their power of reproduction by simple division, +many species of bacteria have a second method by means of spores. +Spores are special rounded or oval bits of bacteria protoplasm +capable of resisting adverse conditions which would destroy the +ordinary bacteria. They arise among bacteria in two different +methods. + +Endogenous spores.--These spores arise inside of the rods or the +spiral forms (Fig. 12). They first appear as slight granular +masses, or as dark points which become gradually distinct from the +rest of the rod. Eventually there is thus formed inside the rod a +clear, highly refractive, spherical or oval spore, which may even +be of a greater diameter than the rod producing it, thus causing +it to swell out and become spindle formed [Fig. 12 c]. These +spores may form in the middle or at the ends of the rods (Fig. +12). They may use up all the protoplasm of the rod in their +formation, or they may use only a small part of it, the rod which +forms them continuing its activities in spite of the formation of +the spores within it. They are always clear and highly refractive +from containing little water, and they do not so readily absorb +staining material as the ordinary rods. They appear to be covered +with a layer of some substance which resists the stain, and which +also enables them to resist various external agencies. This +protective covering, together with their small amount of water, +enables them to resist almost any amount of drying, a high degree +of heat, and many other adverse conditions. Commonly the spores +break out of the rod, and the rod producing them dies, although +sometimes the rod may continue its activity even after the spores +have been produced. + +Arthrogenous spores (?).--Certain species of bacteria do not +produce spores as just described, but may give rise to bodies that +are sometimes called arthrospores. These bodies are formed as +short segments of rods. A long rod may sometimes break up into +several short rounded elements, which are clear and appear to have +a somewhat increased power of resisting adverse conditions. The +same may happen among the spherical forms, which only in rare +instances form endogenous spores. Among the spheres which form a +chain of streptococci some may occasionally be slightly different +from the rest. They are a little larger, and have been thought to +have an increased resisting power like that of true spores (Fig. +13 b). It is quite doubtful, however, whether it is proper to +regard these bodies as spores. There is no good evidence that they +have any special resisting power to heat like endogenous spores, +and bacteriologists in general are inclined to regard them simply +as resting cells. The term arthrospores has been given to them to +indicate that they are formed as joints or segments, and this term +may be a convenient one to retain although the bodies in question +are not true spores. + +Still a different method of spore formation occurs in a few +peculiar bacteria. In this case (Fig. 14) the protoplasm in the +large thread breaks into many minute spherical bodies, which +finally find exit. The spores thus formed may not be all alike, +differences in size being noticed. This method of spore formation +occurs only in a few special forms of bacteria. + +The matter of spore formation serves as one of the points for +distinguishing species. Some species do not form spores, at least +under any of the conditions in which they have been studied. +Others form them readily in almost any condition, and others again +only under special conditions which are adverse to their life. The +method of spore formation is always uniform for any single +species. Whatever be the method of the formation of the spore, its +purpose in the life of the bacterium is always the same. It serves +as a means of keeping the species alive under conditions of +adversity. Its power of resisting heat or drying enables it to +live where the ordinary active forms would be speedily killed. +Some of these spores are capable of resisting a heat of 180 +degrees C. (360 degrees F.) for a short time, and boiling water +they can resist for a long time. Such spores when subsequently +placed under favourable conditions will germinate and start +bacterial activity anew. + +MOTION. + +Some species of bacteria have the power of active motion, and may +be seen darting rapidly to and fro in the liquid in which they are +growing. This motion is produced by flagella which protrude from +the body. These flagella (Fig. 15) arise from a membrane +surrounding the bacterium, but have an intimate connection with +the protoplasmic content. Their distribution is different in +different species of bacteria. Some species have a single +flagellum at one end (Fig. 15 a). Others have one at each end +(Fig. 15 b). Others, again, have, at least just before dividing, a +bunch at one or both ends (Fig. 15 c and d), while others, again, +have many flagella distributed all over the body in dense +profusion (Fig. 15 e). These flagella keep up a lashing to and fro +in the liquid, and the lashing serves to propel the bacteria +through the liquid. + +INTERNAL STRUCTURE. + +It is hardly possible to say much about the structure of the +bacteria beyond the description of their external forms. With all +the variations in detail mentioned, they are extraordinarily +simple, and about all that can be seen is their external shape. Of +course, they have some internal structure, but we know very little +in regard to it. Some microscopists have described certain +appearances which they think indicate internal structure. Fig. 16 +shows some of these appearances. The matter is as yet very +obscure, however. The bacteria appear to have a membranous +covering which sometimes is of a cellulose nature. Within it is +protoplasm which shows various uncertain appearances. Some +microscopists have thought they could find a nucleus, and have +regarded bacteria as cells with inclosed nucleii (Figs. 10 a and +15 f). Others have regarded the whole bacterium as a nucleus +without any protoplasm, while others, again, have concluded that +the discerned internal structure is nothing except an appearance +presented by the physical arrangement of the protoplasm. While we +may believe that they have some internal structure, we must +recognise that as yet microscopists have not been able to make it +out. In short, the bacteria after two centuries of study appear to +us about as they did at first. They must still be described as +minute spheres, rods, or spirals, with no further discernible +structure, sometimes motile and sometimes stationary, sometimes +producing spores and sometimes not, and multiplying universally by +binary fission. With all the development of the modern microscope +we can hardly say more than this. Our advance in knowledge of +bacteria is connected almost wholly with their methods of growth +and the effects they produce in Nature. + +ANIMALS OR PLANTS? + +There has been in the past not a little question as to whether +bacteria should be rightly classed with plants or with animals. +They certainly have characters which ally them with both. Their +very common power of active independent motion and their common +habit of living upon complex bodies for foods are animal +characters, and have lent force to the suggestion that they are +true animals. But their general form, their method of growth and +formation of threads, and their method of spore formation are +quite plantlike. Their general form is very similar to a group of +low green plants known as Oscillaria. Fig. 17 shows a group of +these Oscillariae, and the similarity of this to some of the +thread-like bacteria is decided. The Oscillariae are, however, +true plants, and are of a green colour. Bacteria are therefore to- +day looked upon as a low type of plant which has no chlorophyll, +[Footnote: Chlorophyll is the green colouring matter of plants.] +but is related to Oscillariae. The absence of the chlorophyll has +forced them to adopt new relations to food, and compels them to +feed upon complex foods instead of the simple ones, which form the +food of green plants. We may have no hesitation, then, in calling +them plants. It is interesting to notice that with this idea their +place in the organic world is reduced to a small one +systematically. They do not form a class by themselves, but are +simply a subclass, or even a family, and a family closely related +to several other common plants. But the absence of chlorophyll and +the resulting peculiar life has brought about a curious anomaly. +Whereas their closest allies are known only to botanists, and are +of no interest outside of their systematic relations, the bacteria +are familiar to every one, and are demanding the life attention of +hundreds of investigators. It is their absence of chlorophyll and +their consequent dependence upon complex foods which has produced +this anomaly. + +CLASSIFICATION OF BACTERIA. + +While it has generally been recognised that bacteria are plants, +any further classification has proved a matter of great +difficulty, and bacteriologists find it extremely difficult to +devise means of distinguishing species. Their extreme simplicity +makes it no easy matter to find points by which any species can be +recognised. But in spite of their similarity, there is no doubt +that many different species exist. Bacteria which appear to be +almost identical, under the microscope prove to have entirely +different properties, and must therefore be regarded as distinct +species. But how to distinguish them has been a puzzle. +Microscopists have come to look upon the differences in shape, +multiplication, and formation of spores as furnishing data +sufficient to enable them to divide the bacteria into genera. The +genus Bacillus, for instance, is the name given to all rod-shaped +bacteria which form endogenous spores, etc. But to distinguish +smaller subdivisions it has been found necessary to fall back upon +other characters, such as the shape of the colony produced in +solid gelatine, the power to produce disease, or to oxidize +nitrites, etc. Thus at present the different species are +distinguished rather by their physiological than their +morphological characters. This is an unsatisfactory basis of +classification, and has produced much confusion in the attempts to +classify bacteria. The problem of determining the species of +bacteria is to-day a very difficult one, and with our best methods +is still unsatisfactorily solved. A few species of marked +character are well known, and their powers of action so well +understood that they can be readily recognised; but of the great +host of bacteria studied, the large majority have been so slightly +experimented upon that their characters are not known, and it is +impossible, therefore, to distinguish many of them apart. We find +that each bacteriologist working in any special line commonly +keeps a list of the bacteria which he finds, with such data in +regard to them as he has collected. Such a list is of value to +him, but commonly of little value to other bacteriologists from +the insufficiency of the data. Thus it happens that a large part +of the different species of bacteria described in literature to- +day have been found and studied by one investigator alone. By him +they have been described and perhaps named. Quite likely the same +species may have been found by two or three other bacteriologists, +but owing to the difficulty of comparing results and the +incompleteness of the descriptions the identity of the species is +not discovered, and they are probably described again under +different names. The same process may be repeated over and over +again, until the same species of bacterium will come to be known +by several different names, as it has been studied by different +observers. + +VARIATION OF BACTERIA. + +This matter is made even more confusing by the fact that any +species of bacterium may show more or less variation. At one time +in the history of bacteriology, a period lasting for many years, +it was the prevalent opinion that there was no constancy among +bacteria, but that the same species might assume almost any of the +various forms and shapes, and possess various properties. Bacteria +were regarded by some as stages in the life history of higher +plants. This question as to whether bacteria remain constant in +character for any considerable length of time has ever been a +prominent one with bacteriologists, and even to-day we hardly know +what the final answer will be. It has been demonstrated beyond +peradventure that some species may change their physiological +characters. Disease bacteria, for instance, under certain +conditions lose their powers of developing disease. Species which +sour milk, or others which turn gelatine green, may lose their +characters. Now, since it is upon just such physiological +characters as these that we must depend in order to separate +different species of bacteria from each other, it will be seen +that great confusion and uncertainty will result in our attempts +to define species. Further, it has been proved that there is +sometimes more or less of a metamorphosis in the life history of +certain species of bacteria. The same species may form a short +rod, or a long thread, or break up into spherical spores, and thus +either a short rod, or a thread, or a spherical form may belong to +the same species. Other species may be motile at one time and +stationary at another, while at a third period it is a simple mass +of spherical spores. A spherical form, when it lengthens before +dividing, appears as a short rod, and a short rod form after +dividing may be so short as to appear like a spherical organism. + +With all these reasons for confusion, it is not to be wondered at +that no satisfactory classification of bacteria has been reached, +or that different bacteriologists do not agree as to what +constitutes a species, or whether two forms are identical or not. +But with all the confusion there is slowly being obtained +something like system. In spite of the fact that species may vary +and show different properties under different conditions, the +fundamental constancy of species is everywhere recognised to-day +as a fact. The members of the same species may show different +properties under different conditions, but it is believed that +under identical conditions the properties will be constant. It is +no more possible to convert one species into another than it is +among the higher orders of plants. It is believed that bacteria do +form a group of plants by themselves, and are not to be regarded +as stages in the history of higher plants. It is believed that, +together with a considerable amount of variability and an +occasional somewhat long life history with successive stages, +there is also an essential constancy. A systematic classification +has been made which is becoming more or less satisfactory. We are +constantly learning more and more of the characters, so that they +can be recognised in different places by different observers. It +is the conviction of all who work with bacteria that, in spite of +the difficulties, it is only a matter of time when we shall have a +classification and description of bacteria so complete as to +characterize the different species accurately. + +Even with our present incomplete knowledge of what characterizes a +species, it is necessary to use some names. Bacteria are commonly +given a generic name based upon their microscopic appearance. +There are only a few of these names. Micrococcus, Streptococcus, +Staphylococcus, Sarcina, Bacterium, Bacillus, Spirillum, are all +the names in common use applying to the ordinary bacteria. There +are a few others less commonly used. To this generic name a +specific name is commonly added, based upon some physiological +character. For example, Bacillus typhosus is the name given to the +bacillus which causes typhoid fever. Such names are of great use +when the species is a common and well-known one, but of doubtful +value for less-known species It frequently happens that a +bacteriologist makes a study of the bacteria found in a certain +locality, and obtains thus a long list of species hitherto +unknown. In these cases it is common simply to number these +species rather than name them. This method is frequently +advisable, since the bacteriologist can seldom hunt up all +bacteriological literature with sufficient accuracy to determine +whether some other bacteriologist may not have found the same +species in an entirely different locality. One bacteriologist, for +example, finds some seventy different species of bacteria in +different cheeses. He studies them enough for his own purposes, +but not sufficiently to determine whether some other person may +not have found the same species perhaps in milk or water. He +therefore simply numbers them--a method which conveys no suggestion +as to whether they may be new species or not. This method avoids +the giving of separate names to the same species found by +different observers, and it is hoped that gradually accumulating +knowledge will in time group together the forms which are really +identical, but which have been described by different observers. + +WHERE BACTERIA ARE FOUND. + +There are no other plants or animals so universally found in +Nature as the bacteria. It is this universal presence, together +with their great powers of multiplication, which renders them of +so much importance in Nature. They exist almost everywhere on the +surface of the earth. They are in the soil, especially at its +surface. They do not extend to very great depths of soil, however, +few existing below four feet of soil. At the surface they are very +abundant, especially if the soil is moist and full of organic +material. The number may range from a few hundred to one hundred +millions per gramme. [Footnote: One gramme is fifteen grains.] The +soil bacteria vary also in species, some two-score different +species having been described as common in soil. They are in all +bodies of water, both at the surface and below it. They are found +at considerable depths in the ocean. All bodies of fresh water +contain them, and all sediments in such bodies of water are filled +with bacteria. They are in streams of running water in even +greater quantity than in standing water. This is simply because +running streams are being constantly supplied with water which has +been washing the surface of the country and thus carrying off all +surface accumulations. Lakes or reservoirs, however, by standing +quiet allow the bacteria to settle to the bottom, and the water +thus gets somewhat purified. They are in the air, especially in +regions of habitation. Their numbers are greatest near the surface +of the ground, and decrease in the upper strata of air. Anything +which tends to raise dust increases the number of bacteria in the +air greatly, and the dust and emanations from the clothes of +people crowded in a close room fill the air with bacteria in very +great numbers. They are found in excessive abundance in every bit +of decaying matter wherever it may be. Manure heaps, dead bodies +of animals, decaying trees, filth and slime and muck everywhere +are filled with them, for it is in such places that they find +their best nourishment. The bodies of animals contain them in the +mouth, stomach, and intestine in great numbers, and this is, of +course, equally true of man. On the surface of the body they cling +in great quantity; attached to the clothes, under the finger +nails, among the hairs, in every possible crevice or hiding place +in the skin, and in all secretions. They do not, however, occur in +the tissues of a healthy individual, either in the blood, muscle, +gland, or any other organ. Secretions, such as milk, urine, etc., +always contain them, however, since the bacteria do exist in the +ducts of the glands which conduct the secretions to the exterior, +and thus, while the bacteria are never in the healthy gland +itself, they always succeed in contaminating the secretion as it +passes to the exterior. Not only higher animals, but the lower +animals also have their bodies more or less covered with bacteria. +Flies have them on their feet, bees among their hairs, etc. + +In short, wherever on the face of Nature there is a lodging place +for dust there will be found bacteria. In most of these localities +they are dormant, or at least growing only a little. The bacteria +clinging to the dry hair can grow but little, if at all, and those +in pure water multiply very little. When dried as dust they are +entirely dormant. But each individual bacterium or spore has the +potential power of multiplication already noticed, and as soon as +it by accident falls upon a place where there is food and moisture +it will begin to multiply. Everywhere in Nature, then, exists this +group of organisms with its almost inconceivable power of +multiplication, but a power held in check by lack of food. Furnish +them with food and their potential powers become actual. Such food +is provided by the dead bodies of animals or plants, or by animal +secretions, or from various other sources. The bacteria which are +fortunate enough to get furnished with such food material continue +to feed upon it until the food supply is exhausted or their growth +is checked in some other way. They may be regarded, therefore, as +a constant and universal power usually held in check. With their +universal presence and their powers of producing chemical changes +in food material, they are ever ready to produce changes in the +face of Nature, and to these changes we will now turn. + + + + + +CHAPTER II. + +MISCELLANEOUS USE OF BACTERIA IN THE ARTS. + + +The foods upon which bacteria live are in endless variety, almost +every product of animal or vegetable life serving to supply their +needs. Some species appear to require somewhat definite kinds of +food, and have therefore rather narrow conditions of life, but the +majority may live upon a great variety of organic compounds. As +they consume the material which serves them as food they produce +chemical changes therein. These changes are largely of a nature +that the chemist knows as decomposition changes. By this is meant +that the bacteria, seizing hold of ingredients which constitute +their food, break them to pieces chemically. The molecule of the +original food matter is split into simpler molecules, and the food +is thus changed in its chemical nature. As a result, the compounds +which appear in the decomposing solution are commonly simpler than +the original food molecules. Such products are in general called +decomposition products, or sometimes cleavage products. Sometimes, +however, the bacteria have, in addition to their power of pulling +their food to pieces, a further power of building other compounds +out of the fragments, thus building up as well as pulling down. +But, however they do it, bacteria when growing in any food +material have the power of giving rise to numerous products which +did not exist in the food mass before. Because of their +extraordinary powers of reproduction they are capable of producing +these changes very rapidly and can give rise in a short time to +large amounts of the peculiar products of their growth. + +It is to these powers of producing chemical changes in their food +that bacteria owe all their importance in the world. Their power +of chemically destroying the food products is in itself of no +little importance, but the products which arise as the result of +this series of chemical changes are of an importance in the world +which we are only just beginning to appreciate. In our attempt to +outline the agency which bacteria play in our industries and in +natural processes as well, we shall notice that they are sometimes +of value simply for their power of producing decomposition; but +their greatest value lies in the fact that they are important +agents because of the products of their life. + +We may notice, in the first place, that in the arts there are +several industries which may properly be classed together as +maceration industries, all of which are based upon the +decomposition powers of bacteria. Hardly any animal or vegetable +substance is able to resist their softening influence, and the +artisan relies upon this power in several different directions. + +BENEFITS DERIVED FROM POWERS OF DECOMPOSITION. + +Linen.--Linen consists of certain woody fibres of the stem of the +flax. The flax stem is not made up entirely of the valuable +fibres, but largely of more brittle wood fibres, which are of no +use. The valuable fibres are, however, closely united with the +wood and with each other in such an intimate fashion that it is +impossible to separate them by any mechanical means. The whole +cellular substance of the stem is bound together by some cementing +materials which hold it in a compact mass, probably a salt of +calcium and pectinic acid. The art of preparing flax is a process +of getting rid of the worthless wood fibres and preserving the +valuable, longer, tougher, and more valuable fibres, which are +then made into linen. But to separate them it is necessary first +to soften the whole tissue. This is always done through the aid of +bacteria. The flax stems, after proper preparation, are exposed to +the action of moisture and heat, which soon develops a rapid +bacterial growth. Sometimes this is done by simply exposing the +flax to the dew and rain and allowing it to lie thus exposed for +some time. By another process the stems are completely immersed in +water and allowed to remain for ten to fourteen days. By a third +process the water in which the flax is immersed is heated from 75 +degrees to 90 degrees F., with the addition of certain chemicals, +for some fifty to sixty hours. In all cases the effect is the +same. The moisture and the heat cause a growth of bacteria which +proceeds with more or less rapidity according to the temperature +and other conditions. A putrefactive fermentation is thus set up +which softens the gummy substance holding the fibres together. The +process is known as "retting," and after it is completed the +fibres are easily isolated from each other. A purely mechanical +process now easily separates the valuable fibres from the wood +fibres. The whole process is a typical fermentation. A +disagreeable odour arises from the fermenting flax, and the liquid +after the fermentation is filled with products which make valuable +manure. The process has not been scientifically studied until very +recently. The bacillus which produces the "retting" is known now, +however, and it has been shown that the "retting" is a process of +decomposition of the pectin cement. No method of separating the +linen fibres in the flax from the wood fibres has yet been devised +which dispenses with the aid of bacteria. + +Jute and Hemp.--Almost exactly the same use is made of bacterial +action in the manufacture of jute und hemp. The commercial aspect +of the jute industry has grown to be a large one, involving many +millions of dollars. Like linen, jute is a fibre of the inner bark +of a plant, and is mixed in the bark with a mass of other useless +fibrous material. As in the case of linen, a fermentation by +bacteria is depended upon as a means of softening the material so +that the fibres can be disassociated. The process is called +"retting," as in the linen manufacture. The details of the process +are somewhat different. The jute is commonly fermented in tanks of +stagnant water, although sometimes it is allowed to soak in river +water for a sufficient length of time to produce the softening. +After the fermentation is thus started the jute fibre is separated +from the wood, and is of a sufficient flexibility and toughness to +be woven into sacking, carpets, curtains, table covers, and other +coarse cloth. + +Practically the same method is used in separating the tough fibres +of the hemp. The hemp plant contains some long flexible fibres +with others of no value, and bacterial fermentation is relied upon +to soften the tissues so that they may be separated. + +Cocoanut fibre, a somewhat similar material is obtained from the +husk of the cocoanut by the same means. The unripened husk is +allowed to steep and ferment in water for a long time, six months +or a year being required. By this time the husk has become so +softened that it can be beaten until the fibres separate and can +be removed. They are subsequently made into a number of coarse +articles, especially valuable for their toughness. Door mats, +brushes, ships' fenders, etc., are illustrations of the sort of +articles made from them. + +In each of these processes the fermentation must have a tendency +to soften the desired fibres as well as the connecting substance. +Putrefaction attacks all kinds of vegetable tissue, and if this +"retting" continues too long the desired fibre is decidedly +injured by the softening effect of the fermentation. It is quite +probable that, even as commonly carried on, the fermentation has +some slight injurious effect upon the fibre, and that if some +purely mechanical means could be devised for separating the fibre +from the wood it would produce a better material. But such +mechanical means has not been devised, and at present a +putrefactive fermentation appears to be the only practical method +of separating the fibres. + +Sponges.--A somewhat similar use is made of bacteria in the +commercial preparation of sponges. The sponge of commerce is +simply the fibrous skeleton of a marine animal. When it is alive +this skeleton is completely filled with the softer parts of the +animal, and to fit the sponge for use this softer organic material +must be got rid of. It is easily accomplished by rotting. The +fresh sponges are allowed to stand in the warm sun and very +rapidly decay. Bacteria make their way into the sponge and +thoroughly decompose the soft tissues. After a short putrefaction +of this sort the softened organic matter can be easily washed out +of the skeleton and leave the clean fibre ready for market. + +Leather preparation.--The tanning of leather is a purely chemical +process, and in some processes the whole operation of preparing +the leather is a chemical one. In others, however, especially in +America, bacteria are brought into action at one stage. The dried +hide which comes to the tannery must first have the hair removed +together with the outer skin. The hide for this purpose must be +moistened and softened. In some tanneries this is done by steeping +it in chemicals. In others, however, it is put into water and +slightly heated until fermentation arises. The fermentation +softens it so that the outer skin can be easily removed with a +knife, and the removal of hair is accomplished at the same time. +Bacterial putrefaction in the tannery is thus an assistance in +preparing the skin for the tanning proper. Even in the subsequent +tanning a bacterial fermentation appears to play a part, but +little is yet known in regard to it. + +Maceration of skeletons.--The making of skeletons for museums and +anatomical instruction in general is no very great industry, and +yet it is one of importance. In the making of skeletons the +process of maceration is commonly used as an aid. The maceration +consists simply in allowing the skeleton to soak in water for a +day or two after cleaning away the bulk of the muscles. The +putrefaction that arises softens the connective tissues so much +that the bones may be readily cleaned of flesh. + +Citric acid.--Bacterial fermentation is employed also in the +ordinary preparation of citric acid. The acid is made chiefly from +the juice of the lemon. The juice is pressed from the fruit and +then allowed to ferment. The fermentation aids in separating a +mucilaginous mass and making it thus possible to obtain the citric +acid in a purer condition. The action is probably similar to the +maceration processes described above, although it has not as yet +been studied by bacteriologists. + +BENEFITS DERIVED FROM THE PRODUCTS OF BACTERIAL LIFE. + +While bacteria thus play a part in our industries simply from +their power of producing decomposition, it is primarily because of +the products of their action that they are of value. Wherever +bacteria seize hold of organic matter and feed upon it, there are +certain to be developed new chemical compounds, resulting largely +from decomposition, but partly also from constructive processes. +These new compounds are of great variety. Different species of +bacteria do not by any means produce the same compounds even when +growing in and decomposing the same food material. Moreover, the +same species of bacteria may give rise to different products when +growing in different food materials. Some of the compounds +produced by such processes are poisonous, others are harmless. +Some are gaseous, others are liquids. Some have peculiar odours, +as may be recognised from the smell arising from a bit of decaying +meat. Others have peculiar tastes, as may be realized in the gamy +taste of meat which is in the incipient stages of putrefaction. By +purely empirical means mankind has learned methods of encouraging +the development of some of these products, and is to-day making +practical use of this power, possessed by bacteria, of furnishing +desired chemical compounds. Industries involving the investment of +hundreds of millions of dollars are founded upon the products of +bacterial life, and they have a far more important relation to our +everyday life than is commonly imagined. In many cases the artisan +who is dependent upon this action of microscopic life is unaware +of the fact. His processes are those which experience has taught +produce desired results, but, nevertheless, his dependence upon +bacteria is none the less fundamental. + +BACTERIA IN THE FERMENTATIVE INDUSTRIES. + +We may notice, first, several miscellaneous instances of the +application of bacteria to various fermentative industries where +their aid is of more or less value to man. In some of the examples +to be mentioned the influence of bacteria is profound and +fundamental, while in others it is only incidental. The +fermentative industries of civilization are gigantic in extent, +and have come to be an important factor in modern civilized life. +The large part of the fermentation is based upon the growth of a +class of microscopic plants which we call yeasts. Bacteria and +yeasts are both microscopic plants, and perhaps somewhat closely +related to each other. The botanist finds a difference between +them, based upon their method of multiplication, and therefore +places them in different classes (Fig. 2, page 19). In their +general power of producing chemical changes in their food +products, yeasts agree closely with bacteria, though the kinds of +chemical changes are different. The whole of the great +fermentative industries, in which are invested hundreds of +millions of dollars, is based upon chemical decompositions +produced by microscopic plants. In the great part of commercial +fermentations alcohol is the product desired, and alcohol, though +it is sometimes produced by bacteria, is in commercial quantities +produced only by yeasts. Hence it is that, although the +fermentations produced by bacteria are more common in Nature than +those produced by yeasts and give rise to a much larger number of +decomposition products, still their commercial aspect is decidedly +less important than that of yeasts. Nevertheless, bacteria are not +without their importance in the ordinary fermentative processes. +Although they are of no importance as aids in the common +fermentative processes, they are not infrequently the cause of +much trouble. In the fermentation of malt to produce beer, or +grape juice to produce wine, it is the desire of the brewer and +vintner to have this fermentation produced by pure yeasts, unmixed +with bacteria. If the yeast is pure the fermentation is uniform +and successful. But the brewer and vintner have long known that +the fermentation is frequently interfered with by irregularities. +The troubles which arise have long been known, but the +bacteriologist has finally discovered their cause, and in general +their remedy. The cause of the chief troubles which arise in the +fermentation is the presence of contaminating bacteria among the +yeasts. These bacteria have been more or less carefully studied by +bacteriologists, and their effect upon the beer or wine +determined. Some of them produce acid and render the products +sour; others make them bitter; others, again, produce a slimy +material which makes the wine or beer "ropy." Something like a +score of bacteria species have been found liable to occur in the +fermenting material and destroy the value of the product of both +the wine maker and the beer brewer. The species of bacteria which +infect and injure wine are different from those which infect and +injure beer. They are ever present as possibilities in the great +alcoholic fermentations. They are dangers which must be guarded +against. In former years the troubles from these sources were much +greater than they are at present. Since it has been demonstrated +that the different imperfections in the fermentative process are +due to bacterial impurities, commonly in the yeasts which are used +to produce the fermentation, methods of avoiding them are readily +devised. To-day the vintner has ready command of processes for +avoiding the troubles which arise from bacteria, and the brewer is +always provided with a microscope to show him the presence or +absence of the contaminating bacteria. While, then, the alcoholic +fermentations are not dependent upon bacteria, the proper +management of these fermentations requires a knowledge of their +habits and characters. + +There are certain other fermentative processes of more or less +importance in their commercial aspects, which are directly +dependent upon bacterial action, Some of them we should +unhesitatingly look upon as fermentations, while others would +hardly be thought of as belonging to the fermentation industries. + +VINEGAR. + +The commercial importance of the manufacture of vinegar, though +large, does not, of course, compare in extent with that of the +alcoholic fermentations. Vinegar is a weak solution of acetic +acid, together with various other ingredients which have come from +the materials furnishing the acid. In the manufacture of vinegar, +alcohol is always used as the source of the acetic acid. The +production of acetic acid from alcohol is a simple oxidation. The +equation C2H6O + O2 = C2H4O2 + H2O shows the chemical change that +occurs. This oxidation can be brought about by purely chemical +means. While alcohol will not readily unite with oxygen under +common conditions, if the alcohol is allowed to pass over a bit of +platinum sponge the union readily occurs and acetic acid results. +This method of acetic-acid production is possible experimentally, +but is impracticable on any large scale. In the ordinary +manufacture of vinegar the oxidation is a true fermentation, and +brought about by the growth of bacteria. + +In the commercial manufacture of vinegar several different weak +alcoholic solutions are used. The most common of these are +fermented malt, weak wine, cider, and sometimes a weak solution of +spirit to which is added sugar and malt. If these solutions are +allowed to stand for a time in contact with air, they slowly turn +sour by the gradual conversion of the alcohol into acetic acid. At +the close of the process practically all of the alcohol has +disappeared. Ordinarily, however, not all of it has been converted +into acetic acid, for the oxidation does not all stop at this +step. As the oxidation goes on, some of the acid is oxidized into +carbonic dioxide, which is, of course, dissipated at once into the +air, and if the process is allowed to continue unchecked for a +long enough period much of the acetic acid will be lost in this +way. + +The oxidation of the alcohol in all commercial production of +vinegar is brought about by the growth of bacteria in the liquid. +When the vinegar production is going on properly, there is formed +on the top of the liquid a dense felted mass known as the "mother +of vinegar." This mass proves to be made of bacteria which have +the power of absorbing oxygen from the air, or, at all events, of +causing the alcohol to unite with oxygen. It was at first thought +that a single species of bacterium was thus the cause of the +oxidation of alcohol, and this was named Mycoderma aceti. But +further study has shown that several have the power, and that even +in the commercial manufacture of vinegar several species play a +part (Fig. 18), although the different species are not yet very +thoroughly studied. Each appears to act best under different +conditions. Some of them act slowly, and others rapidly, the slow- +growing species appearing to produce the larger amount of acid in +the end. After the amount of acetic acid reaches a certain +percentage, the bacteria are unable to produce more, even though +there be alcohol still left unoxidized. A percentage as high as +fourteen per cent, commonly destroys all their power of growth. +The production of the acid is wholly dependent upon the growth of +the bacteria, and the secret of the successful vinegar manufacture +is the skilful manipulation of these bacteria so as to keep them +in the purest condition and to give them the best opportunity for +growth. + +One method of vinegar manufacture which is quite rapid is carried +on in a slightly different manner. A tall cylindrical chamber is +filled with wood shavings, and a weak solution of alcohol is +allowed to trickle slowly through it. The liquid after passing +over the shavings comes out after a number of hours well charged +with acetic acid. This process at first sight appears to be a +purely chemical one, and reminds us of the oxidation which occurs +when alcohol is allowed to pass over a platinum sponge. It has +been claimed, indeed, that this is a chemical oxidation in which +bacteria play no part. But this appears to be an error. It is +always found necessary in this method to start the process by +pouring upon the shavings some warm vinegar. Unless in this way +the shavings become charged with the vinegar-holding bacteria the +alcohol will not undergo oxidation during its passage over them, +and after the bacteria thus introduced have grown enough to coat +the shavings thoroughly the acetic-acid production is much more +rapid than at first. If vinegar is allowed to trickle slowly down +a suspended string, so that its bacteria may distribute themselves +through the string, and then alcohol be allowed to trickle over it +in the same way, the oxidation takes place and acetic acid is +formed. From the accumulation of such facts it has come to be +recognised that all processes for the commercial manufacture of +vinegar depend upon the action of bacteria. While the oxidation of +alcohol into acetic acid may take place by purely chemical means, +these processes are not practical on a large scale, and vinegar +manufacturers everywhere depend upon bacteria as their agents in +producing the oxidation. These bacteria, several species in all, +feed upon the nitrogenous matter in the fermenting mass and +produce the desired change in the alcohol. + +This vinegar fermentation is subject to certain irregularities, +and the vinegar manufacturers can not always depend upon its +occurring in a satisfactory manner. Just as in brewing, so here, +contaminating bacteria sometimes find their way into the +fermenting mass and interfere with its normal course. In +particular, the flavour of the vinegar is liable to suffer from +such causes. As yet our vinegar manufacturers have not applied to +acetic fermentation the same principle which has been so +successful in brewing--namely, the use, as a starter of the +fermentation, of a pure culture of the proper species of bacteria. +This has been done experimentally and proves to be feasible. In +practice, however, vinegar makers find that simpler methods of +obtaining a starter--by means of which they procure a culture +nearly though not absolutely pure--are perfectly satisfactory. It +is uncertain whether really pure cultures will ever be used in +this industry. + +LACTIC ACID. + +The manufacture of lactic acid is an industry of less extent than +that of acetic acid, and yet it is one which has some considerable +commercial importance. Lactic acid is used in no large quantity, +although it is of some value as a medicine and in the arts. For +its production we are wholly dependent upon bacteria. It is this +acid which, as we shall see, is produced in the ordinary souring +of milk, and a large number of species of bacteria are capable of +producing the acid from milk sugar. Any sample of sour milk may +therefore always be depended upon to contain plenty of lactic +organisms. In its manufacture for commercial purposes milk is +sometimes used as a source, but more commonly other substances. +Sometimes a mixture of cane sugar and tartaric acid is used. To +start the fermentation the mixture is inoculated with a mass of +sour milk or decaying cheese, or both, such a mixture always +containing lactic organisms. To be sure, it also contains many +other bacteria which have different effects, but the acid +producers are always so abundant and grow so vigorously that the +lactic fermentation occurs in spite of all other bacteria. Here +also there is a possibility of an improvement in the process by +the use of pure cultures of lactic organisms. Up to the present, +however, there has been no application of such methods. The +commercial aspects of the industry are not upon a sufficiently +large scale to call for much in this direction. + +At the present time the only method we have for the manufacture of +lactic acid is dependent upon bacteria. Chemical processes for its +manufacture are known, but not employed commercially. There are +several different kinds of lactic acid. They differ from each +other in the relations of the atoms within their molecule, and in +their relation to polarized light, some forms rotating the plane +of polarized light to the right, others to the left, while others +are inactive in this respect. All the types are produced by +fermentation processes, different species of bacteria having +powers of producing the different types. + +BUTYRIC ACID. + +Butyric acid is another acid for which we are chiefly dependent +upon bacteria. This acid is of no very great importance, and its +manufacture can hardly be called an industry; still it is to a +certain extent made, and is an article of commerce. It is an acid +that can be manufactured by chemical means, but, as in the case of +the last two acids, its commercial manufacture is based upon +bacterial action. Quite a number of species of bacteria can +produce butyric acid, and they produce it from a variety of +different sources. Butyric acid is a common ingredient in old milk +and in butter, and its formation by bacteria was historically one +of the first bacterial fermentations to be clearly understood. It +can be produced also in various sugar and starchy solutions. +Glycerine may also undergo a butyric fermentation. The presence of +this acid is occasionally troublesome, since it is one of the +factors in the rancidity of butter and other similar materials. + +INDIGO PREPARATION. + +The preparation of indigo from the indigo plant is a fermentative +process brought about by a specific bacterium. The leaves of the +plant are immersed in water in a large vat, and a rapid +fermentation arises. As a result of the fermentation the part of +the plant which is the basis of the indigo is separated from the +leaves and dissolved in the water; and as a second feature of the +fermentation the soluble material is changed in its chemical +nature into indigo proper. As this change occurs the +characteristic blue colour is developed, and the material is +rendered insoluble in water. It therefore makes its appearance as +a blue mass separated from the water, and is then removed as +indigo. + +Of the nature of the process we as yet know very little. That it +is a fermentation is certain, and it has been proved that it is +produced by a definite species of bacterium which occurs on the +indigo leaves. If the sterilized leaves are placed in sterile +water no fermentation occurs and no indigo is formed. If, however, +some of the specific bacteria are added to the mass the +fermentation soon begins and the blue colour of the indigo makes +its appearance. It is plain, therefore, that indigo is a product +of bacterial fermentation, and commonly due to a single definite +species of bacterium. Of the details of the formation, however, we +as yet know little, and no practical application of the facts have +yet been made. + +BACTERIA IN TOBACCO CURING. + +A fermentative process of quite a different nature, but of immense +commercial value, is found in the preparation of tobacco. The +process by which tobacco is prepared is a long and somewhat +complicated one, consisting of a number of different stages. The +tobacco, after being first dried in a careful manner, is +subsequently allowed to absorb moisture from the atmosphere, and +is then placed in large heaps to undergo a further change. This +process appears to be a fermentation, for the temperature of the +mass rises rapidly, and every indication of a fermentative action +is seen. The tobacco in these heaps is changed occasionally, the +heap being thrown down and built up again in such a way that the +portion which was first at the bottom comes to the top, and in +this way all parts of the heap may become equally affected by the +process. After this process the tobacco is sent to the different +manufacturers, who finish the process of curing. The further +treatment it receives varies widely according to the desired +product, whether for smoking or for snuff, etc. In all cases, +however, fermentations play a prominent part. Sometimes the leaves +are directly inoculated with fermenting material. In the +preparation of snuff the details of the process are more +complicated than in the preparation of smoking tobacco. The +tobacco, after being ground and mixed with certain ingredients, is +allowed to undergo a fermentation which lasts for weeks, and +indeed for months. In the different methods of preparing snuff the +fermentations take place in different ways, and sometimes the +tobacco is subjected to two or three different fermentative +actions. The result of the whole is the slow preparation of the +commercial product. It is during the final fermentative processes +that the peculiar colour and flavour of the snuff are developed, +and it is during the fermentation of the leaves of the smoking +tobacco--either the original fermentation or the subsequent ones-- +that the special flavours and aromas of tobacco are produced. + +It can not be claimed for a moment that these changes by which the +tobacco is cured and finally brought to a marketable condition are +due wholly to bacteria. There is no question that chemical and +physical phenomena play an important part in them. Nevertheless, +from the moment when the tobacco is cut in the fields until the +time it is ready for market the curing is very intimately +associated with bacteria and fermentative organisms in general. +Some of these processes are wholly brought about by bacterial +life; in others the micro-organisms aid the process, though they +perhaps can not be regarded as the sole agents. + +At the outset the tobacco producer has to contend with a number of +micro-organisms which may produce diseases in his tobacco. During +the drying process, if the temperature or the amount of moisture +or the access of air is not kept in a proper condition, various +troubles arise and various diseases make their appearance, which +either injure or ruin the value of the product. These appear to be +produced by micro-organisms of different sorts. During the +fermentation which follows the drying the producer has to contend +with micro-organisms that are troublesome to him; for unless the +phenomena are properly regulated the fermentation that occurs +produces effects upon the tobacco which ruin its character. From +the time the tobacco is cut until the final stage in the curing +the persons engaged in preparing it for market must be on a +constant watch to prevent the growth within it of undesirable +organisms. The preparation of tobacco is for this reason a +delicate operation, and one that will be very likely to fail +unless the greatest care is taken. In the several fermentative +processes which occur in the preparation there is no question that +micro-organisms aid the tobacco producer and manufacturer. +Bacteria produce the first fermentation that follows the drying, +and it is these organisms too, in large measure, that give rise to +all the subsequent fermentations, although seemingly in some cases +purely chemical processes materially aid. Now the special quality +of the tobacco is in part dependent upon the peculiar type of +fermentation which occurs in one or another of these fermenting +actions. It is the fermentation that gives rise to the peculiar +flavour and to the aroma of the different grades of tobacco. +Inasmuch as the various flavours which characterize tobacco of +different grades are developed, at least to a large extent, during +the fermentation processes, it is a natural supposition that the +different qualities of the tobacco, so far as concerns flavour, +are due to the different types of fermentation. The number of +species of bacteria which are found upon the tobacco leaves in the +various stages of its preparation is quite large, and from what we +have already learned it is inevitable that the different kinds of +bacteria will produce different results in the fermenting process. +It would seem natural, therefore, to assume that the different +flavours of different grades may not unlikely be due to the fact +that the tobacco in the different cases has been fermented under +the influence of different kinds of bacteria. + +Nor is this simply a matter of inference. To a certain extent +experimental evidence has borne out the conclusion, and has given +at least a slight indication of practical results in the future. +Acting upon the suggestion that the difference between the high +grades of tobacco and the poorer grades is due to the character of +the bacteria that produce the fermentation, certain +bacteriologists have attempted to obtain from a high quality of +tobacco the species of bacteria which are infesting it. These +bacteria have then been cultivated by bacteriological methods and +used in experiments for the fermentation of tobacco. If it is true +that the flavour of high grade tobacco is in large measure, or +even in part, due to the action of the peculiar microbes from the +soil where it grows, it ought to be possible to produce similar +flavours in the leaves of tobacco grown in other localities, if +the fermentation of the leaves is carried on by means of the pure +cultures of bacteria obtained from the high grade tobacco. Not +very much has been done or is known in this connection as yet. Two +bacteriologists have experimented independently in fermenting +tobacco leaves by the action of pure cultures of bacteria obtained +from such sources. Each of them reports successful experiments. +Each claims that they have been able to improve the quality of +tobacco by inoculating the leaves with a pure culture of bacteria +obtained from tobacco having high quality in flavour. In addition +to this, several other bacteriologists have carried on experiments +sufficient to indicate that the flavours of the tobacco and the +character of the ripening may be decidedly changed by the use of +different species of micro-organisms in the fermentations that go +on during the curing processes. + +In regard to the whole matter, however, we must recognise that as +yet we have very little knowledge. The subject has been under +investigation for only a short time; and, while considerable +information has been derived, this information is not thoroughly +understood, and our knowledge in regard to the matter is as yet in +rather a chaotic condition. It seems certain, however, that the +quality of tobacco is in large measure dependent upon the +character of the fermentations that occur at different stages of +the curing. It seems certain also that these fermentations are +wholly or chiefly produced by microorganisms, and that the +character of the fermentation is in large measure dependent upon +the species of micro-organisms that produce it. If these are +facts, it would seem not improbable that a further study may +produce practical results for this great industry. The study of +yeasts and the methods of keeping yeast from contaminations has +revolutionised the brewing industry. Perhaps in this other +fermentative industry, which is of such great commercial extent, +the use of pure cultures of bacteria may in the future produce as +great revolutions in methods as it has in the industry of the +alcoholic fermentation. + +It must not, however, be inferred that the differences in grades +of tobacco grown in different parts of the world are due solely to +variations in the curing processes and to the types of +fermentation. There are differences in the texture of the leaves, +differences in the chemical composition of the tobaccoes, which +are due undoubtedly to the soils and the climatic conditions in +which they grow, and these, of course, will never be affected by +changing the character of the ferment active processes. It is, +however, probable that in so far as the flavours that distinguish +the high and low grades of tobacco are due to the character of the +fermentative processes, they may be in the future, at least to a +large extent, controlled by the use of pure cultures in curing +processes. Seemingly, then, there is as great a future in the +development of this fermentative industry as there has been in the +past in the development of the fermentative industry associated +with brewing and vinting. + +OPIUM. + +Opium for smoking purposes is commonly allowed to undergo a curing +process which lasts several months. This appears to be somewhat +similar to the curing of tobacco. Apparently it is a fermentation +due to the growth of microorganisms. The organisms in question are +not, however, bacteria in this case, but a species of allied +fungus. The plant is a mould, and it is claimed that inoculation +of the opium with cultures of this mould hastens the curing. + +TROUBLESOME FERMENTATIONS. + +Before leaving this branch of the subject it is necessary to +notice some of the troublesome fermentations which are ever +interfering with our industries, requiring special methods, or, +indeed, sometimes developing special industries to meet them. As +agents of decomposition, bacteria will of course be a trouble +whenever they get into material which it is desired to preserve. +Since they are abundant everywhere, it is necessary to count upon +their attacking with certainty any fermentable substance which is +exposed to air and water. Hence they are frequently the cause of +much trouble. In the fermentative industries they occasionally +cause an improper sort of fermentation to occur unless care is +taken to prevent undesired species of bacteria from being present. +In vinegar making, improper species of bacteria obtaining access +to the solution give rise to undesirable flavours, greatly +injuring the product. In tobacco curing it is very common for the +wrong species of bacteria to gain access to the tobacco at some +stage of the curing and by their growth give rise to various +troubles. It is the ubiquitous presence of bacteria which makes it +impossible to preserve fruits, meats, or vegetables for any length +of time without special methods. This fact in itself has caused +the development of one of our most important industries. Canning +meats or fruits consists in nothing more than bringing them into a +condition in which they will be preserved from attack of these +micro-organisms. The method is extremely simple in theory. It is +nothing more than heating the material to be preserved to a high +temperature and then sealing it hermetically while it is still +hot. The heat kills all the bacteria which may chance to be lodged +in it, and the hermetical sealing prevents other bacteria from +obtaining access. Inasmuch as all organic decomposition is +produced by bacterial growth, such sterilized and sealed material +will be preserved indefinitely when the operation is performed +carefully enough. The methods of accomplishing this with +sufficient care are somewhat varied in different industries, but +they are all fundamentally the same. It is an interesting fact +that this method of preserving meats was devised in the last +century, before the relation of micro-organisms to fermentation +and putrefaction was really suspected. For a long time it had been +in practical use while scientists were still disputing whether +putrefaction could be avoided by preventing the access of +bacteria. The industry has, however, developed wonderfully within +the last few years, since the principles underlying it have been +understood. This understanding has led to better methods of +destroying bacterial life and to proper sealing, and these have of +course led to greater success in the preservation, until to-day +the canning industries are among those which involve capital +reckoned in the millions. + +Occasionally bacteria are of some value in food products. The gamy +flavour of meats is nothing more than incipient decomposition. +Sauer Kraut is a food mass intentionally allowed to ferment and +sour. The value of bacteria in producing butter and cheese +flavours is noticed elsewhere. But commonly our aim must be to +prevent the growth of bacteria in foods. Foods must be dried or +cooked or kept on ice, or some other means adopted for preventing +bacterial growth in them. It is their presence that forces us to +keep our ice box, thus founding the ice business, as well as that +of the manufacture of refrigerators. It is their presence, again, +that forces us to smoke hams, to salt mackerel, to dry fish or +other meats, to keep pork in brine, and to introduce numerous +other details in the methods of food preparation and preservation. + + + + + +CHAPTER III. + +RELATION OF BACTERIA TO THE DAIRY INDUSTRY. + + +Dairying is one of the most primitive of our industries. From the +very earliest period, ever since man began to keep domestic +cattle, he has been familiar with dairying. During these many +centuries certain methods of procedure have been developed which +produce desired results. These methods, however, have been devised +simply from the accumulation of experience, with very little +knowledge as to the reasons underlying them. The methods of past +centuries are, however, ceasing to be satisfactory. The advance of +our civilization during the last half century has seen a marked +expansion in the extent of the dairy industry. With this expansion +has appeared the necessity for new methods, and dairymen have for +years been looking for them. The last few years have been teaching +us that the new methods are to be found along the line of the +application of the discoveries of modern bacteriology. We have +been learning that the dairyman is more closely related to +bacteria and their activities than almost any other class of +persons. Modern dairying, apart from the matter of keeping the +cow, consists largely in trying to prevent bacteria from growing +in milk or in stimulating their growth in cream, butter, and +cheese. These chief products of the dairy will be considered +separately. + +SOURCES OF BACTERIA IN MILK. + +The first fact that claims our attention is, that milk at the time +it is secreted from the udder of the healthy cow contains no +bacteria. Although bacteria are almost ubiquitous, they are not +found in the circulating fluids of healthy animals, and are not +secreted by their glands. Milk when first secreted by the milk +gland is therefore free from bacteria. It has taken a long time to +demonstrate this fact, but it has been finally satisfactorily +proved. Secondly, it has been demonstrated that practically all of +the normal changes which occur in milk after its secretion are +caused by the growth of bacteria. This, too, was long denied, and +for quite a number of years after putrefactions and fermentations +were generally acknowledged to be caused by the growth of micro- +organisms, the changes which occurred in milk were excepted from +the rule. The uniformity with which milk will sour, and the +difficulty, or seeming impossibility, of preventing this change, +led to the belief that the souring of milk was a normal change +characteristic of milk, just as clotting is characteristic of +blood. This was, however, eventually disproved, and it was finally +demonstrated that, beyond a few physical changes connected with +evaporation and a slight oxidation of the fat, milk, if kept free +from bacteria, will undergo no change. If bacteria are not +present, it will remain sweet indefinitely. + +But it is impossible to draw milk from the cow in such a manner +that it will be free from bacteria except by the use of +precautions absolutely impracticable in ordinary dairying. As milk +is commonly drawn, it is sure to be contaminated by bacteria, and +by the time it has entered the milk pail it contains frequently as +many as half a million, or even a million, bacteria in every cubic +inch of the milk. This seems almost incredible, but it has been +demonstrated in many cases and is beyond question. Since these +bacteria are not in the secreted milk, they must come from some +external sources, and these sources are the following: + +The first in importance is the cow herself; for while her milk +when secreted is sterile, and while there are no bacteria in her +blood, nevertheless the cow is the most prolific source of +bacterial contamination. In the first place, the milk ducts are +full of them. After each milking a little milk is always left in +the duct, and this furnishes an ideal place for bacteria to grow. +Some bacteria from the air or elsewhere are sure to get into these +ducts after the milking, and they begin at once to multiply +rapidly. By the next milking they become very abundant in the +ducts, and the first milk drawn washes most of them at once into +the milk pail, where they can continue their growth in the milk. +Again, the exterior of the cow's body contains them in abundance. +Every hair, every particle of dirt, every bit of dried manure, is +a lurking place for millions of bacteria. The hind quarters of a +cow are commonly in a condition of much filth, for the farmer +rarely grooms his cow, and during the milking, by her movements, +by the switching of her tail, and by the rubbing she gets from the +milker, no inconsiderable amount of this dirt and filth is brushed +off and falls into the milk pail The farmer understands this +source of dirt and usually feels it necessary to strain the milk +after the milking. But the straining it receives through a coarse +cloth, while it will remove the coarser particles of dirt, has no +effect upon the bacteria, for these pass through any strainer +unimpeded. Again, the milk vessels themselves contain bacteria, for +they are never washed absolutely clean. After the most thorough +washing which the milk pail receives from the kitchen, there will +always be left many bacteria clinging in the cracks of the tin or +in the wood, ready to begin to grow as soon as the milk once more +fills the pail The milker himself contributes to the supply, for +he goes to the milking with unclean hands, unclean clothes, and +not a few bacteria get from him to his milk pail. Lastly, we find +the air of the milking stall furnishing its quota of milk bacteria. +This source of bacteria is, how ever, not so great as was formerly +believed. That the air may contain many bacteria in its dust is +certain, and doubtless these fall in some quantity into the milk, +especially if the cattle are allowed to feed upon dusty hay before +and during the milking. But unless the air is thus full of dust +this source of bacteria is not very great, and compared with the +bacteria from the other sources the air bacteria are unimportant. + +The milk thus gets filled with bacteria, and since it furnishes an +excellent food these bacteria begin at once to grow. The milk when +drawn is warm and at a temperature which especially stimulates +bacterial growth. They multiply with great rapidity, and in the +course of a few hours increase perhaps a thousandfold. The numbers +which may be found after twenty-four hours are sometimes +inconceivable; market milk may contain as many as five hundred +millions per cubic inch; and while this is a decidedly extreme +number, milk that is a day old will almost always contain many +millions in each cubic inch, the number depending upon the age of +the milk and its temperature. During this growth the bacteria +have, of course, not been without their effect. Recognising as we +do that bacteria are agents for chemical change, we are prepared +to see the milk undergoing some modifications during this rapid +multiplication of bacteria. The changes which these bacteria +produce in the milk and its products are numerous, and decidedly +affect its value. They are both advantageous and disadvantageous +to the dairyman. They are nuisances so far as concerns the milk +producer, but allies of the butter and cheese maker. + +THE EFFECT OF BACTERIA ON MILK. + +The first and most universal change effected in milk is its +SOURING. So universal is this phenomenon that it is generally +regarded as an inevitable change which can not be avoided, and, as +already pointed out, has in the past been regarded as a normal +property of milk. To-day, however, the phenomenon is well +understood. It is due to the action of certain of the milk +bacteria upon the milk sugar which converts it into lactic acid, +and this acid gives the sour taste and curdles the milk. After +this acid is produced in small quantity its presence proves +deleterious to the growth of the bacteria, and further bacterial +growth is checked. After souring, therefore, the milk for some +time does not ordinarily undergo any further changes. + +Milk souring has been commonly regarded as a single phenomenon, +alike in all cases. When it was first studied by bacteriologists +it was thought to be due in all cases to a single species of +micro-organism which was discovered to be commonly present and +named Bacillus acidi lactici (Fig. 19). This bacterium has +certainly the power of souring milk rapidly, and is found to be +very common in dairies in Europe. As soon as bacteriologists +turned their attention more closely to the subject it was found +that the spontaneous souring of milk was not always caused by the +same species of bacterium. Instead of finding this Bacillus acidi +lactici always present, they found that quite a number of +different species of bacteria have the power of souring milk, and +are found in different specimens of soured milk. The number of +species of bacteria which have been found to sour milk has +increased until something over a hundred are known to have this +power. These different species do not affect the milk in the same +way. All produce some acid, but they differ in the kind and the +amount of acid, and especially in the other changes which are +effected at the same time that the milk is soured, so that the +resulting soured milk is quite variable. In spite of this variety, +however, the most recent work tends to show that the majority of +cases of spontaneous souring of milk are produced by bacteria +which, though somewhat variable, probably constitute a single +species, and are identical with the Bacillus acidi lactici (Fig. +19). This species, found common in the dairies of Europe, +according to recent investigations occurs in this country as well. +We may say, then, that while there are many species of bacteria +infesting the dairy which can sour the milk, there is one which is +more common and more universally found than others, and this is +the ordinary cause of milk souring. + +When we study more carefully the effect upon the milk of the +different species of bacteria found in the dairy, we find that +there is a great variety of changes which they produce when they +are allowed to grow in milk. The dairyman experiences many +troubles with his milk. It sometimes curdles without becoming +acid. Sometimes it becomes bitter, or acquires an unpleasant +"tainted" taste, or, again, a "soapy" taste. Occasionally a +dairyman finds his milk becoming slimy, instead of souring and +curdling in the normal fashion. At such times, after a number of +hours, the milk becomes so slimy that it can be drawn into long +threads. Such an infection proves very troublesome, for many a +time it persists in spite of all attempts made to remedy it. +Again, in other cases the milk will turn blue, acquiring about the +time it becomes sour a beautiful sky-blue colour. Or it may become +red, or occasionally yellow. All of these troubles the dairyman +owes to the presence in his milk of unusual species of bacteria +which grow there abundantly. + +Bacteriologists have been able to make out satisfactorily the +connection of all these infections with different species of the +bacteria. A large number of species have been found to curdle milk +without rendering it acid, several render it bitter, and a number +produce a "tainted" and one a "soapy" taste. A score or more have +been found which have the power of rendering the milk slimy. Two +different species at least have the power of turning the milk to +sky-blue colour; two or three produce red pigments (Fig. 20), and +one or two have been found which produce a yellow colour. In +short, it has been determined beyond question that all these +infections, which are more or less troublesome to dairymen, are +due to the growth of unusual bacteria in the milk. + +These various infections are all troublesome, and indeed it may be +said that, so far as concerns the milk producer and the milk +consumer, bacteria are from beginning to end a source of trouble. +It is the desire of the milk producer to avoid them as far as +possible--a desire which is shared also by everyone who has +anything to do with milk as milk. Having recognised that the +various troubles, which occasionally occur even in the better +class of dairies, are due to bacteria, the dairyman is, at least +in a measure, prepared to avoid them. The avoiding of these +troubles is moderately easy as soon as dairymen recognise the +source from which the infectious organisms come, and also the fact +that low temperatures will in all cases remedy the evil to a large +extent. With this knowledge in hand the avoidance of all these +troubles is only a question of care in handling the dairy. It must +be recognised that most of these troublesome bacteria come from +some unusual sources of infection. By unusual sources are meant +those which the exercise of care will avoid. It is true that the +souring bacteria appear to be so universally distributed that they +can not be avoided by any ordinary means. But all other +troublesome bacteria appear to be within control. The milkman must +remember that the sources of the troubles which are liable to +arise in his milk are in some form of filth: either filth on the +cow, or dust in the hay which is scattered through the barn, or +dirt on cows' udders, or some other unusual and avoidable source. +These sources, from what we have already noticed, will always +furnish the milk with bacteria; but under common conditions, and +when the cow is kept in conditions of ordinary cleanliness, and +frequently even when not cleanly, will only furnish bacteria that +produce the universal souring. Recognising this, the dairyman at +once learns that his remedies for the troublesome infections are +cleanliness and low temperatures. If he is careful to keep his +milk vessels scrupulously clean; if he will keep his cow as +cleanly as he does his horse; and if he will use care in and +around the barn and dairy, and then apply low temperatures to the +milk, he need never be disturbed by slimy or tainted milk, or any +of these other troubles; or he can remove such infections speedily +should they once appear. Pure sweet milk is only a question of +sufficient care. But care means labour and expense. As long as we +demand cheap milk, so long will we be supplied with milk procured +under conditions of filth. But when we learn that cheap milk is +poor milk, and when we are willing to pay a little more for it, +then only may we expect the use of greater care in the handling of +the milk, resulting in a purer product. + +Bacteriology has therefore taught us that the whole question of +the milk supply in our communities is one of avoiding the too +rapid growth of bacteria. These organisms are uniformly a nuisance +to the milkman. To avoid their evil influence have been designed +all the methods of caring for the dairy and the barn, all the +methods of distributing milk in ice cars. Moreover, all the +special devices connected with the great industry of milk supply +have for their foundation the attempt to avoid, in the first +place, the presence of too great a number of bacteria, and. in the +second place, the growth of these bacteria. + +BACTERIA IN BUTTER MAKING. + +CREAM RIPENING.--Passing from milk to butter, we find a somewhat +different story, inasmuch as here bacteria are direct allies to +the dairyman rather than his enemies. Without being aware of it, +butter makers have for years been making use of bacteria in their +butter making and have been profiting by the products which the +bacteria have furnished them. Cream, as it is obtained from milk, +will always contain bacteria in large quantity, and these bacteria +will grow as readily in the cream as they will in the milk. The +butter maker seldom churns his cream when it is freshly obtained +from the milk. There are, it is true, some places where sweet +cream butter is made and is in demand, but in the majority of +butter-consuming countries a different quality of butter is +desired, and the cream is subjected to a process known as +"ripening" or "souring" before it is churned. In ripening, the +cream is simply allowed to stand in a vat for a period varying +from twelve hours to two or three days, according to +circumstances. During this period certain changes take place +therein. The bacteria which were in the cream originally, get an +opportunity to grow, and by the time the ripening is complete they +become extremely numerous. As a result, the character of the cream +changes just as the milk is changed under similar circumstances. +It becomes somewhat soured; it becomes slightly curdled, and +acquires a peculiarly pleasant taste and an aroma which was not +present in the original fresh cream. After this ripening the cream +is churned. It is during the ripening that the bacteria produce +their effect, for after the churning they are of less importance. +Part of them collect in the butter, part of them are washed off +from the butter in the buttermilk and the subsequent processes. +Most of the bacteria that are left in the butter soon die, not +finding there a favourable condition for growth; some of them, +however, live and grow for some time and are prominent agents in +the changes by which butter becomes rancid. The butter maker is +concerned with the ripening rather than with later processes. + +The object of the ripening of cream is to render it in a better +condition for butter making. The butter maker has learned by long- +experience that ripened cream churns more rapidly than sweet +cream, and that he obtains a larger yield of butter therefrom. The +great object of the ripening, however, is to develop in the butter +the peculiar flavour and aroma which is characteristic of the +highest product. Sweet cream butter lacks flavour and aroma, +having indeed a taste almost identically the same as cream. +Butter, however, that is made from ripened cream has a peculiar +delicate flavour and aroma which is well known to lovers of +butter, and which is developed during the ripening process. + +Bacteriologists have been able to explain with a considerable +degree of accuracy the object of this ripening. The process is +really a fermentation comparable to the fermentation that takes +place in a brewer's malt. The growth of bacteria during the +ripening produces chemical changes of a somewhat complicated +character, and concerns each of the ingredients of the milk. The +lactic-acid organisms affect the milk sugar and produce lactic +acid; others act upon the fat, producing slight changes therein; +while others act upon the casein and the albumens of the milk. As +a result, various biproducts of decomposition arise, and it is +these biproducts of decomposition that make the difference between +the ripened and the unripened cream. They render it sour and +curdle it, and they also produce the flavours and aromas that +characterize it. Products of decomposition are generally looked +upon as undesirable for food, and this is equally true of these +products that arise in cream if the decomposition is allowed to +continue long enough. If the ripening, instead of being stopped at +the end of a day or two, is allowed to continue several days, the +cream becomes decayed and the butter made therefrom is decidedly +offensive. But under the conditions of ordinary ripening, when the +process is stopped at the right moment, the decomposition products +are pleasant rather than unpleasant, and the flavours and aromas +which they impart to the cream and to the subsequent butter are +those that are desired. It is these decomposition products that +give the peculiar character to a high quality of butter, and this +peculiar quality is a matter that determines the price which the +butter maker can obtain for his product. + +But, unfortunately, the butter maker is not always able to depend +upon the ripening. While commonly it progresses in a satisfactory +manner, sometimes, for no reason that he can assign, the ripening +does not progress normally. Instead of developing the pleasant +aroma and flavour of the properly ripened cream, the cream develops +unpleasant tastes. It may be bitter or somewhat tainted, and just +as sure as these flavours develop in the cream, so sure does the +quality of the butter suffer. Moreover, it has been learned by +experience that some creameries are incapable of obtaining an +equally good ripening of their cream. While some of them will +obtain favourable results, others, with equal care, will obtain a +far less favourable flavour and aroma in their butter. The reason +for all this has been explained by modern bacteriology. In the +milk, and consequently in the cream, there are always found many +bacteria, but these are not always of the same kinds. There are +scores, and probably hundreds, of species of bacteria common in and +around our barns and dairies, and the bacteria that are abundant +and that grow in different lots of cream will not be always the +same. It makes a decided difference in the character of the +ripening, and in the consequent flavours and aromas, whether one or +another species of bacteria has been growing in the cream. Some +species are found to produce good results with desired flavours, +while others, under identical conditions, produce decidedly poor +results with undesired flavours. If the butter maker obtains cream +which is filled with a large number of bacteria capable of +producing good flavours, then the ripening of his cream will be +satisfactory and his butter will be of high quality. If, however, +it chances that his cream contains only the species which produce +unpleasant flavours, then the character of the ripening will be +decidedly inferior and the butter will be of a poorer grade. +Fortunately the majority of the kinds of bacteria liable to get +into the cream from ordinary sources are such as produce either +good effects upon the cream or do not materially influence the +flavour or aroma. Hence it is that the ripening of cream will +commonly produce good results. Bacteriologists have learned that +there are some species of bacteria more or less common around our +barns which produce undesirable effects upon flavour, and should +these become especially abundant in the cream, then the character +of the ripening and the quality of the subsequent butter will +suffer. These malign species of bacteria, however, are not very +common in properly kept barns and dairies. Hence the process that +is so widely used, of simply allowing cream to ripen under the +influence of any bacteria that happen to be in it, ordinarily +produces good results. But our butter makers sometimes find, at the +times when the cattle change from winter to summer or from summer +to winter feed, that the ripening is abnormal. The reason appears +to be that the cream has become infested with an abundance of +malign species. The ripening that they produce is therefore an +undesirable one, and the quality of the butter is sure to suffer. + +So long as butter was made only in private dairies it was a matter +of comparatively little importance if there was an occasional +falling off in quality of this sort. When it was made a few pounds +at a time, and only once or twice a week, it was not a very +serious matter if a few churnings of butter did suffer in quality. +But to-day the butter-making industries are becoming more and more +concentrated into large creameries, and it is a matter of a good +deal more importance to discover some means by which a uniformly +high quality can be insured. If a creamery which makes five +hundred pounds of butter per day suffers from such an injurious +ripening, the quality of its butter will fall off to such an +extent as to command a lower price, and the creamery suffers +materially. Perhaps the continuation of such a trouble for two or +three weeks would make a difference between financial success and +failure in the creamery. With our concentration of the butter- +making industries it is becoming thus desirable to discover some +means of regulating this process more accurately. + +The remedy of these occasional ill effects in cream ripening has +not been within the reach of the butter maker. The butter maker +must make butter with the cream that is furnished him, and if that +cream is already impregnated with malign species of bacteria he is +helpless. It is true that much can be done to remedy these +difficulties by the exercise of especial care in the barns of the +patrons of the creamery. If the barns, the cows, the dairies, the +milk vessels, etc., are all kept in condition of strict +cleanliness, if especial care is taken particularly at the seasons +of the year when trouble is likely to arise, and if some attention +is paid to the kind of food which the cattle eat, as a rule the +cream will not become infected with injurious bacteria. It may be +taken as a demonstrated fact that these malign bacteria come from +sources of filth, and the careful avoidance of all such sources of +filth will in a very large measure prevent their occurrence in the +cream. Such measures as these have been found to be practicable in +many creameries. Creameries which make the highest priced and the +most uniform quality of butter are those in which the greatest +care is taken in the barns and dairies to insure cleanliness and +in the handling of the milk and cream. With such attention a large +portion of the trouble which arises in the creameries from malign +bacteria may be avoided. + +But these methods furnish no sure remedy against evils of improper +species of bacteria in cream ripening, and do not furnish any sure +means of obtaining uniform flavour in butter. Even under the very +best conditions the flavour of the butter will vary with the +season of the year. Butter made in the winter is inferior to that +made in the summer months; and while this is doubtless due in part +to the different food which the cattle have and to the character +of the cream resulting therefrom, these differences in the flavour +of the butter are also in part dependent upon the different +species of bacteria which are present in the ripening of cream at +different seasons. The species of bacteria in June cream are +different from those that are commonly present in January cream, +and this is certainly a factor in determining the difference +between winter and summer butter. + +USE OF ARTIFICIAL BACTERIA CULTURES FOR CREAM RIPENING. + +Bacteriologists have been for some time endeavouring to aid butter +makers in this direction by furnishing them with the bacteria +needful for the best results in cream ripening. The method of +doing this is extremely simple in principle, but proves to be +somewhat difficult in practice. It is only necessary to obtain the +species of bacteria that produce the highest results, and then to +furnish these in pure culture and in large quantity to the butter +makers, to enable them to inoculate their cream with the species +of bacteria which will produce the results that they desire. For +this purpose bacteriologists have been for several years searching +for the proper species of bacteria to produce the best results, +and there have been put upon the market for sale several distinct +"pure cultures" for this purpose. These have been obtained by +different bacteriologists and dairymen in the northern European +countries and also in the United States. These pure cultures are +furnished to the dairymen in various forms, but they always +consist of great quantities of certain kinds of bacteria which +experience has found to be advantageous for the purpose of cream +ripening. + +There have hitherto appeared a number of difficulties in the way +of reaching complete success in these directions. The most +prominent arises in devising a method of using pure cultures in +the creamery. The cream which the butter makers desire to ripen +is, as we have seen, already impregnated with bacteria, and would +ripen in a fashion of its own even if no pure culture of bacteria +were added thereto. Pure cultures can not therefore be used as +simply as can yeast in bread dough. It is plain that the simple +addition of a pure culture to a mass of cream would not produce +the desired effects, because the cream would be ripened then, not +by the pure culture alone, but by the pure culture plus all of the +bacteria that were originally present. It would, of course, be +something of a question as to whether under these conditions the +results would be favourable, and it would seem that this method +would not furnish any means of getting rid of bad tastes and +flavours which have come from the presence of malign species of +bacteria. It is plainly desirable to get rid of the cream bacteria +before the pure culture is added. This can be readily done by +heating it to a temperature of 69 degrees C. (155 degrees F.) for +a short time, this temperature being sufficient to destroy most of +the bacteria. The subsequent addition of the pure culture of +cream-ripening bacteria will cause the cream to ripen under the +influence of the added culture alone. This method proves to be +successful, and in the butter making countries in Europe it is +becoming rapidly adopted. + +In this country, however, this process has not as yet become very +popular, inasmuch as the heating of the cream is a matter of +considerable expense and trouble, and our butter makers have not +been very ready to adopt it. For this reason, and also for the +purpose of familiarizing butter makers with the use of pure +cultures, it has been attempted to produce somewhat similar though +less uniform results by the use of pure cultures in cream without +previous healing. In the use of pure cultures in this way, the +butter maker is directed to add to his cream a large amount of a +prepared culture of certain species of bacteria, upon the +principle that the addition of such a large number of bacteria to +the cream, even though the cream is already inoculated with +certain bacteria, will produce a ripening of the cream chiefly +influenced by the artificially added culture. The culture thus +added, being present in very much greater quantity than the other +"wild" species, will have a much greater effect than any of them. +This method, of course, cannot insure uniformity. While it may +work satisfactorily in many cases, it is very evident that in +others, when the cream is already filled with a large number of +malign species of bacteria, such an artificial culture would not +produce the desired results. This appears to be not only the +theoretical but the actual experience. The addition of such pure +cultures in many cases produces favourable results, but it does +not always do so, and the result is not uniform. While the use of +pure cultures in this way is an advantage over the method of +simply allowing the cream to ripen normally without such +additions, it is a method that is decidedly inferior to that which +first pasteurizes the cream and subsequently adds a starter. + +There is still another method of adding bacteria to cream to +insure a more advantageous ripening, which is frequently used, +and, being simpler, is in many cases a decided advantage. This +method is by the use of what is called a natural starter. A +natural starter consists simply of a lot of cream which has been +taken from the most favourable source possible--that is, from the +cleanest and best dairy, or from the herd producing the best +quality of cream--and allowing this cream to stand in a warm place +for a couple of days until it becomes sour. The cream will by that +time be filled with large numbers of bacteria, and this is then +put as a starter into the vat of cream to be ripened. Of course, +in the use of this method the butter maker has no control over the +kinds of bacteria that will grow in the starter, but it is found, +practically, that if the cream is taken from a good source the +results are extremely favourable, and there is produced in this +way almost always an improvement in the butter. + +The use of pure cultures is still quite new, particularly in this +country. In the European butter-making countries they have been +used for a longer period and have become very much better known. +What the future may develop along this line it is difficult to +say; but it seems at least probable that as the difficulties in +the details are mastered the time will come when starters will be +used by our butter makers for their cream ripening, just as yeast +is used by housewives for raising bread, or by brewers for +fermenting malt. These starters will probably in time be furnished +by bacteriologists. Bacteriology, in other words, is offering in +the near future to our butter makers a method of controlling the +ripening of the cream in such a way as to insure the obtaining of +a high and uniform quality of butter, so far, at least, as +concerns flavour and aroma. + +BACTERIA IN CHEESE. + +Cheese ripening.--The third great product of the dairy industry is +cheese, and in connection with this product the dairyman is even +more dependent upon bacteria than he is in the production of +butter. In the manufacture of cheese the casein of the milk is +separated from the other products by the use of rennet, and is +collected in large masses and pressed, forming the fresh cheese. +This cheese is then set aside for several weeks, and sometimes for +months, to undergo a process that is known as ripening. During the +ripening there are developed in the cheese the peculiar flavours +which are characteristic of the completed product. The taste of +freshly made cheese is extremely unlike that of the ripened +product. While butter made from unripened cream has a pleasant +flavour, and one which is in many places particularly enjoyed, +there is nowhere a demand for unripened cheese, for the freshly +made cheese has a taste that scarce any one regards as pleasant. +Indeed, the whole value of the cheese is dependent upon the +flavour of the product, and this flavour is developed during the +ripening. + +The cheese maker finds in the ripening of his cheese the most +difficult part of his manufacture. It is indeed a process over +which he has very little control. Even when all conditions seem to +be correct, when cheese is made in the most careful manner, it not +infrequently occurs that the ripening takes place in a manner that +is entirely abnormal, and the resulting cheese becomes worthless. +The cheese maker has been at an entire loss to understand these +irregularities, nor has he possessed any means of removing them. +The abnormal ripening that occurs takes on various types. +Sometimes the cheese will become extraordinarily porous, filled +with large holes which cause the cheese to swell out of proper +shape and become worthless. At other times various spots of red or +blue appear in the manufactured cheese; while again unpleasant +tastes and flavours develop which render the product of no value. +Sometimes a considerable portion of the product of the cheese +factory undergoes such irregular ripening, and the product for a +long time will thus be worthless. If some means could be +discovered of removing these irregularities it would be a great +boon to the cheese manufacturer; and very many attempts have been +made in one way or another to furnish the cheese maker with some +details in the manufacture which will enable him in a measure to +control the ripening. + +The ripening of the cheese has been subjected to a large amount of +study on the part of bacteriologists who have been interested in +dairy products. That the ripening of cheese is the result of +bacterial growth therein appears to be probable from a priori +grounds. Like the ripening of cream, it is a process that occurs +somewhat slowly. It is a chemical change which is accompanied by +the destruction of proteid matter; it takes place best at certain +temperatures, and temperatures which we know are favourable to the +growth of micro-organisms, all of which phenomena suggest to us +the action of bacteria. Moreover, the flavours and the tastes that +arise have a decided resemblance in many cases to the +decomposition products of bacteria, strikingly so in Limburger +cheese. When we come to study the matter of cheese ripening +carefully we learn beyond question that this a priori conclusion +is correct. The ripening of any cheese is dependent upon several +different factors. The method of preparation, the amount of water +left in the curd, the temperature of ripening, and other +miscellaneous factors connected with the mechanical process of +cheese manufacture, affect its character. But, in addition to all +these factors, there is undoubtedly another one, and that is the +number and the character of the bacteria that chance to be in the +curd when the cheese is made. While it is found that cheeses which +are treated by different processes will ripen in a different +manner, it is also found that two cheeses which have been made +under similar conditions and treated in identically the same way +may also ripen in a different manner, so that the resulting +flavour will vary. The variations between cheeses thus made may be +slight or they may be considerable, but variations certainly do +occur. Every one knows the great difference in flavours of +different cheeses, and these flavours are due in considerable +measure to factors other than the simple mechanical process of +making the cheese. The general similarity of the whole process to +a bacterial fermentation leads us to believe at the outset that +some of the differences in character are due to different kinds of +bacteria that multiply in the cheese and produce decomposition +therein. + +When the matter comes to be studied by bacteriology, the +demonstration of this position becomes easy. That the ripening of +cheese is due to growth of bacteria is very easily proved by +manufacturing cheeses from milk which is deprived of bacteria. For +instance, cheeses have been made from milk that has been either +sterilized or pasteurized--which processes destroy most of the +bacteria therein--and, treated otherwise in a normal manner, are +set aside to ripen. These cheeses do NOT ripen, but remain for +months with practically the same taste that they had originally. +In other experiments the cheese has been treated with a small +amount of disinfective, which is sufficient to prevent bacteria +from growing, and again ripening is found to be absolutely +prevented. Furthermore, if the cheese under ordinary conditions is +studied during the ripening process, it is found that bacteria are +growing during the whole time. These facts all taken together +plainly prove that the ripening of cheese is a fermentation due to +bacteria. It will be noticed, however, that the conditions in the +cheese are not favourable for very rapid bacterial growth. It is +true that there is plenty of food in the cheese for bacterial +life, but the cheese is not very moist; it is extremely dense, +being subjected in all cases to more or less pressure. The +penetration of oxygen into the centre of the mass must be +extremely slight. The density, the lack of a great amount of +moisture, and the lack of oxygen furnish conditions in which +bacteria will not grow very rapidly. The conditions are far less +favourable than those of ripening cream, and the bacteria do not +grow with anything like the rapidity that they grow in cream. +Indeed, the growth of these organisms during the ripening is +extremely slow compared to the possibilities of bacterial growth +that we have already noticed. Nevertheless, the bacteria do +multiply in the cheese, and as the ripening goes on they become +more and more abundant, although the number fluctuates, rising and +falling under different conditions. + +When the attempt is made to determine the relation of the +different kinds of ripening to different kinds of bacteria, it has +thus far met with extremely little success. That different +flavours are due to the ripening produced by different kinds of +bacteria would appear to be almost certain when we remember, as we +have already noticed, the different kinds of decomposition +produced by different species of bacteria. It would seem, +moreover, that it ought not to be very difficult to separate from +the ripened cheese the bacteria which are present, and thus obtain +the kind of bacteria necessary to produce the desired ripening. +But for some reason this does not prove to be so easy in practice +as it seems to be in theory. Many different species of bacteria +have been separated from cheeses. One bacteriologist, studying +several cheeses, separated about eighty different species +therefrom, and others have found perhaps as many more from +different sources. Moreover, experiments have been made with a +considerable number of these different kinds of bacteria to +determine whether they are capable of producing normal ripening. +These experiments consist of making cheese out of milk that has +been deprived of its bacteria, and which has been inoculated with +large quantities of the species in question. Hitherto these +experiments have not been very satisfactory. In some cases the +cheese appears to ripen scarcely at all; in other cases the +ripening occurs, but the resulting cheese is of a peculiar +character, entirely unlike the cheese that it is desired to +imitate. There have been one or two experiments in recent times +that give a little more promise of success than the earlier ones, +for a few species of bacteria have been used in ripening with what +the authors have thought to be promising success. The cheese made +from the milk artificially inoculated with these species ripens in +a satisfactory manner and gives some of the character desired, +though up to the present time in no case has the typical normal +ripening been produced in any of these experiments. + +But these experiments have demonstrated beyond question that the +abnormal ripening which is common in cheese factories is due to +the presence of undesirable species of bacteria in the milk. Many +of the experiments in making cheeses by means of artificial +cultures of bacteria have resulted in decidedly abnormal cheeses. +Many of the cheeses thus manufactured have shown imperfections in +ripening which are identical with those actually occurring in the +cheese factory. Several different species of bacteria have been +found which, when artificially used thus for ripening cheese, will +give rise to the porosity and the abnormal swelling of the cheese +already referred to (Fig. 24). Others produced bad tastes and +flavours, and enough has been done in this line to demonstrate +beyond peradventure that the abnormal ripening of cheese is due +primarily to the growth of improper species therein. Quite a long +list of species of bacteria which produce abnormal ripening have +been isolated from cheeses, and have been studied and experimented +with by bacteriologists. As a result of this study of abnormal +ripening, there has been suggested a method of partially +controlling these--remedying them. The method consists simply in +testing the fermenting qualities of the milk used. A small sample +of milk from different dairies is allowed to stand in the cheese +factory by itself until it undergoes its normal souring. If the +fermentation or souring that thus occurs is of a normal character, +the milk is regarded as proper for cheese making. But if the +fermentation that occurs in any particular sample of milk is +unusual; if an extraordinary amount of gas bubbles are produced, +or if unpleasant smells and tastes arise, the sample is regarded +as unfavourable for cheese making, and as likely to produce +abnormal ripening in the cheeses. Milk from this source would +therefore be excluded from the milk that is to be used in cheese +making. This, of course, is a tentative and an unsatisfactory +method of controlling the ripening, and yet it is one of some +practical value to cheese makers. It is the only method that has +yet been suggested of controlling the ripening. + +Our bacteriologists, of course, are quite confident that in the +future more practical results will be obtained along this line +than in the past. If it is true that cheeses are ripened by +bacteria; if it is true that different qualities in the cheese are +due to the growth of different species of bacteria during the +ripening, it would seem to be possible to obtain the proper kind +of bacteria and to furnish them to the cheese maker for +artificially inoculating his cheese, just as it has been possible +to furnish artificially cultivated yeasts to the brewer, and as it +has become possible to furnish artificially cultivated bacteria to +the butter maker. We must, however, recognise this to be a matter +for the future. Up to the present time no practical results along +the lines of bacteria have been obtained which our cheese +manufacturers can make use of in the way of controlling with any +accuracy this process of cheese ripening. + +Thus it will be seen that in this last dairy product bacteria play +even a more important part than in any of the others. The food +value of cheese is dependent upon the casein which is present. The +market price, however, is controlled entirely by the flavour, and +this flavour is a product of bacterial growth. Upon the action of +bacteria, then, the cheese maker is absolutely dependent; and when +our bacteriologists are able in the future to investigate this +matter further, it seems to be at least possible that they may +obtain some means of enabling the cheese maker to control the +ripening accurately. Not only so, but recognising the great +variety in the flavours of cheese, and recognising that different +kinds of bacteria undoubtedly produce different kinds of +decomposition products, it seems to be at least possible that a +time will come when the cheese maker will be able to produce at-- +will any particularly desired flavour in his cheese by the +addition to it of particular species of bacteria, or particular +mixtures of species of bacteria which have been discovered to +produce the desired effects. + + + + + +CHAPTER IV. + +BACTERIA IN NATURAL PROCESSES.--AGRICULTURE. + + +Thus far, in considering the relations of bacteria to mankind, we +have taken into account only the arts and manufactures, and have +found bacteria playing no unimportant part in many of the +industries of our modern civilized life. So important are they +that there is no one who is not directly affected by them. There +is hardly a moment in our life when we are not using some of the +direct or indirect products of bacterial action. We turn now, +however, to the consideration of a matter of even more fundamental +importance; for when we come to study bacteria in Nature, we find +that there are certain natural processes connected with the life +of animals and plants that are fundamentally based upon their +powers. Living Nature appears limitless, for life processes have +been going on in the world through countless centuries with +seemingly unimpaired vigour. At the very bottom we find this +never-ending exhibition of vital power dependent upon certain +activities of micro-organisms. So thoroughly is this true that, as +we shall find after a short consideration, the continuance of life +upon the surface of the world would be impossible if bacterial +action were checked for any considerable length of time. The life +of the globe is, in short, dependent upon these micro-organisms. + +BACTERIA AS SCAVENGERS. + +In the first place, we may notice the value of these organisms +simply as scavengers, keeping the surface of the earth in the +proper condition for the growth of animals and plants. A large +tree in the forest dies and falls to the ground. For a while the +tree trunk lies there a massive structure, but in the course of +months a slow change takes place in it. The bark becomes softened +and falls from the wood. The wood also becomes more or less +softened; it is preyed upon then by insect life; its density +decreases more and more, until finally it crumbles into a soft, +brownish, powdery mass, and eventually the whole sinks into the +soil, is overgrown by mosses and other vegetation, and the tree +trunk has disappeared from view. In the same way the body of the +dead animal undergoes the process of the softening of its tissues +by decay. The softer parts of the body rapidly dissipate, and even +the bones themselves eventually are covered with the soil and +disintegrated, until in time they, too, disappear from any visible +existence. This whole process is one of decay, and the result is +that the solid mass of the body of the tree or of the animal has +been decomposed. What has become of it? The answer holds the +secret of Nature's eternal freshness. Part of it has dissipated +into the air in the form of gases and water vapour; part of it has +changed its composition and has become incorporated into the soil, +the final result being that the body of the plant or animal +disappears as such, and its substance is converted into gaseous +form, which is dissipated in the air or into simple compounds +which sink into the earth. + +This whole process of decay of organic life is one in which +bacteria play the most important part. In the case of the +decomposition of the woody matter of the tree trunk, the process +is begun by the agency of moulds, for this group of organisms +alone appears to be capable of attacking such hard woody +structure. The later part of the decay, however, is largely carried +on by bacterial life. In the decomposition of the animal tissues, +bacteria alone are the agents. Thus the process by which organic +matter is dissipated into the air or incorporated into the soil is +one which is primarily presided over by bacterial life. + +Viewing this matter in a purely mechanical light, the importance +of bacteria in thus acting as scavengers can hardly be +overestimated. If we think for a moment of the condition of the +world were there no such decomposing agents to rid the earth's +surface of the dead bodies of animals and plants, we shall see +that long since the earth would have been uninhabitable. If the +dead bodies of plants and animals of past ages simply accumulated +on the surface of the ground without any forces to reduce them +into simple compounds for dissipation, by their very bulk they +would have long since completely covered the surface of the earth +so as to afford no possible room for further growth of plants and +animals. In a purely mechanical way, then, bacteria as +decomposition agents are necessary to keep the surface of the +earth fresh and unencumbered so that life can continue. + +BACTERIA AS AGENTS IN NATURE'S FOOD CYCLE. + +But the matter by no means ends here. When we come to think of it, +it is a matter of considerable surprise that the surface of the +earth has been able to continue producing animals and plants for +the many millions of years during which life has been in +existence. Plants and animals both require food, animals depending +wholly upon plants therefor. Plants, however, equally with +animals, require food, and although they obtain a considerable +portion of their food from the air, yet no inconsiderable part of +it is obtained from the soil. The question is forced upon us, +therefore, as to why the soil has not long since become exhausted +of food. How could the soil continue to support plants year after +year for millions of years, and yet remain as fertile as ever? + +The explanation of this phenomenon is in the simple fact that the +processes of Nature are such that the same food is used over and +over again, first by the plant, then by the animal, and then again +by the plant, and there is no necessity for any end of the process +so long as the sun furnishes energy to keep the circulation +continuous. One phase of this transference of food from animal to +plant and from plant to animal is familiar to nearly every one. It +is a well-known fact that animals in their respiration consume +oxygen, but exhale it again in combination with carbon as carbonic +dioxide. On the other hand, plants in their life consume the +carbonic dioxide and exhale the oxygen again as free oxygen. Thus +each of these kingdoms makes use of the excreted product of the +other, and this process can go on indefinitely, the animals +furnishing our atmosphere with plenty of carbonic acid for plant +life, and the plants excreting into the atmosphere at the same +time an abundant sufficiency of oxygen for animal life. The oxygen +thus passes in an endless round from animal to plant and from +plant to animal. + +A similar cycle is true of all the other foods of animal and plant +life, though in regard to the others the operation is more complex +and more members are required to complete the chain. The +transference of matter through a series of changes by which it is +brought from a condition in which it is proper food for plants +back again into a condition when it is once more a proper food for +plants, is one of the interesting discoveries of modern science, +and one in which, as we shall see, bacteria play a most important +part. This food cycle is illustrated roughly by the accompanying +diagram; but in order to understand it, an explanation of the +various steps in this cycle is necessary. + +It will be noticed that at the bottom of the circle represented in +Fig. 25, at A, are given various ingredients which are found in +the soil and which form plant foods. Plant foods, as may be seen +there, are obtained partly from the air as carbonic dioxide and +water; but another portion comes from the soil. Among the soil +ingredients the most prominent are nitrates, which are the forms +of nitrogen compounds most easily made use of by plants as a +source of this important element. It should be stated also that +there are other compounds in the soil which furnish plants with +part of their food--compounds containing potassium, phosphorus, +and some other elements. For simplicity's sake, however, these +will be left out of consideration. Beginning at the bottom of the +cycle (Fig. 25 A), plant life seizes the gases from the air and +these foods from the soil, and by means of the energy furnished it +by the sun's rays builds these simple chemical compounds into more +complex ones. This gives us the second step, as shown in Fig. 25 +B, the products of plant life. These products of plant life +consist of such materials as sugar, starches, fats, and proteids, +all of which have been manufactured by the plant from the +ingredients furnished it from the soil and air, and through the +agency of the sun's rays. These products of plant life now form +foods for the animal kingdom. Starches, fats, and proteids are +animal foods, and upon such complex bodies alone can the animal +kingdom be fed. Animal life, standing high up in the circle, is +not capable of extracting its nutriment from the soil, but must +take the more complex foods which have been manufactured by plant +life. These complex foods enter now into the animal and take their +place in the animal body. By the animal activities, some of the +foods are at once decomposed into carbonic acid and water, which, +being dissipated into the air, are brought back at once into the +condition in which they can serve again as plant food. This part +of the food is thus brought back again to the bottom of the circle +(Fig. 25, dotted lines). But while it is true that animals do thus +reduce some of their foods to the simple condition of carbonic +acid and water, this is not true of most of the foods which +contain nitrogen. The nitrogenous foods are as necessary for the +life as the carbon foods, and animals do not reduce their +nitrogenous foods to the condition in which plants can prey upon +them. While plants furnish them with nitrogenous food, they can +not give it back to the plants. Part of the nitrogenous foods +animals build into new albumins (Fig. 25 C); but a part of them +they reduce at once into a somewhat simpler condition known as +urea. Urea is the form in which the nitrogen is commonly excreted +from the animal body. But urea is not a plant food; for ordinary +plants are entirely unable to make use of it. Part of the nitrogen +eaten by the animal is stored up in its body, and thus the body of +the animal, after it has died, contains these nitrogen compounds +of high complexity. But plants are not able to use these +compounds. A plant can not be fed upon muscle tissue, nor upon +fats, nor bones, for these are compounds so complex that the +simple plant is unable to use them at all. So far, then, in the +food cycle the compounds taken from the soil have been built up +into compounds of greater and greater complexity; they have +reached the top of this circle, and no part of them, except part +of the carbon and oxygen, has become reduced again to plant food. +In order that this material should again become capable of +entering into the life of plants so as to go over the circle +again, it is necessary for it to be once more reduced from its +highly complex condition into a simpler one. + +Now come into play these decomposition agencies which we have been +studying under the head of scavengers. It will be noticed that the +next step in the food cycle is taken by the decomposition +bacteria. These organisms, existing, as we have already seen, in +the air, in the soil, in the water, and always ready to seize hold +of any organic substance that may furnish them with food, feed +upon the products of animal life, whether they are such products +as muscle tissue, or fat, or sugar, or whether they are the +excreted products of animal life, such as urea, and produce +therein the chemical decomposition changes already noticed. As a +result of this chemical decomposition, the complex bodies are +broken into simpler and simpler compounds, and the final result is +a very thorough destruction of the animal body or the material +excreted by animal life, and its reduction into forms simple +enough for plants to use again as foods. Thus the bacteria come in +as a necessary link to connect the animal body, or the excretion +from the animal body, with the soil again, and therefore with that +part of the circle in which the material can once more serve as +plant food. + +But in the decomposition that thus occurs through the agency of +the putrefactive bacteria it very commonly happens that some of +the food material is broken down into compounds too simple for use +as plant food. As will be seen by a glance at the diagram (Fig. 25 +D), a portion of the cleavage products resulting from the +destruction of these animal foods takes the form of carbonic-acid +gas and water. These ingredients are at once in condition for +plant life, as shown by the dotted lines. They pass off into the +air, and the green leaves of vegetation everywhere again seize +them, assimilate them, and use them as food. Thus it is that the +carbon and the oxygen have completed the cycle, and have come back +again to the position in the circle where they started. In regard +to the nitrogen portion of the food, however, it very commonly +happens that the products which arise as the result of the +decomposition processes are not yet in proper condition for plant +food. They are reduced into a condition actually too simple for +the use of plants. As a result of these putrefactive changes, the +nitrogen products of animal life are broken frequently into +compounds as simple as ammonia (NH3), or into compounds which the +chemists speak of as nitrites (Fig. 25 at D). Now these compounds +are not ordinarily within the reach of plant life. The luxuriant +vegetation of the globe extracts its nitrogen from the soil in a +form more complex than either of the compounds here mentioned; +for, as we have seen, it is nitrates chiefly that furnish plants +with their nitrogen food factor. But nitrates contain considerable +oxygen. Ammonia, which is one of the products of putrefactive de- +composition, contains no oxygen, and nitrites, another factor, +contains less oxygen than nitrates. These bodies are thus too +simple for plants to make use of as a source of nitrogen. The +chemical destruction of the food material which results from the +action of the putrefactive bacteria is too thorough, and the +nitrogen foods are not yet in condition to be used by plants. + +Now comes in the agency of still another class of micro-organisms, +the existence of which has been demonstrated to us during the last +few years. In the soil everywhere, especially in fertile soil, is +a class of bacteria which has received the name of nitrifying +bacteria (Fig. 26). These organisms grow in the soil and feed upon +the soil ingredients. In the course of their life they have +somewhat the same action upon the simple nitrogen cleavage +products just mentioned as we have already noticed the vinegar- +producing species have upon alcohol, viz., the bringing about a +union with oxygen. There are apparently several different kinds of +nitrifying bacteria with different powers. Some of them cause an +oxidation of the nitrogen products by means of which the ammonia +is united with oxygen and built up into a series of products +finally resulting in nitrates (Fig. 26). By the action of other +species still higher nitrogen compounds, including the nitrites, +are further oxidized and built up into the form of nitrates. Thus +these nitrifying organisms form the last link in the chain that +binds the animal kingdom to the vegetable kingdom (Fig. 25 at 4). +For after the nitrifying organisms have oxidized nitrogen cleavage +products, the results of the oxidation in the form of nitrates or +nitric acid are left in the soil, and may now be seized upon by +the roots of plants, and begin once more their journey around the +food cycle. In this way it will be seen that while plants, by +building up compounds, form the connecting link between the soil +and animal life, bacteria in the other half of the cycle, by +reducing them again, give us the connecting link between animal +life and the soil. The food cycle would be as incomplete without +the agency of bacterial life as it would be without the agency of +plant life. + +But even yet the food cycle is not complete. Some of the processes +of decomposition appear to cause a portion of the nitrogen to fly +out of the circle at a tangent. In the process of decomposition +which is going on through the agency of micro-organisms, a +considerable part of the nitrogen is dissipated into the air in +the form of free nitrogen. When a bit of meat decays, part of the +meat is, indeed, converted into ammonia or other nitrogen +compounds, but if the putrefaction is allowed to go on, in the end +a considerable portion of it will be broken into still simpler +forms, and the nitrogen will finally be dissipated into the air in +the form of free nitrogen. This dissipation of free nitrogen into +the air is going on in the world wherever putrefaction takes +place. Wherever decomposition of nitrogen products occurs some +free nitrogen is eliminated. Now, this part of the nitrogen has +passed beyond the reach of plants, for plants can not extract free +nitrogen from the air. In the diagram this is represented as a +portion of the material which, through the agency of the +decomposition bacteria, has been thrown out of the cycle at a +tangent (Fig. 25 E). It will, of course, be plain from this that +the store of nitrogen food must be constantly diminishing. The +soil may have been originally supplied with a given quantity of +nitrogen compound, but if the decomposition products are causing +considerable quantities of this nitrogen to be dissipated in the +air, it plainly follows that the total amount of nitrogen food +upon which the animal and vegetable kingdoms can depend is +becoming constantly reduced by such dissipation. + +There are still other methods by which nitrogen is being lost from +the food cycle. First, we may notice that the ordinary processes +of vegetation result in a gradual draining of the soil and a +throwing of its nitrogen into the ocean. The body of any animal or +any plant that chances to fall into a brook or river is eventually +carried to the sea, and the products of its decomposition pass +into the ocean and are, of course, lost to the soil. Now, while +this gradual extraction of nitrogen from the soil by drainage is a +slow one, it is nevertheless a sure one. It is far more rapid in +these years of civilized life than in former times, since the +products of the soil are given to the city, and then are thrown +into its sewage Our cities, then, with our present system of +disposing of sewage, are draining from the soil the nitrogen +compounds and throwing them away. + +In yet another direction must it be noticed that our nitrogen +compounds are being lost to plant life--viz., by the use of various +nitrogen compounds to form explosives. Gunpowder, nitro-glycerine, +dynamite, in fact, nearly all the explosives that are used the +world over for all sorts of purposes, are nitrogen compounds. When +they are exploded the nitrogen of the compound is dissipated into +the air in the form of gas, much of it in the form of free +nitrogen. The basis from which explosive compounds are made +contains nitrogen in the form in which it can be used by plants. +Saltpetre, for example, is equally good as a fertilizer and as a +basis for gunpowder. The products of the explosion are gases no +longer capable of use by plants, and thus every explosion of +nitrogen compounds aids in this gradual dissipation of nitrogen +products, taking them from the store of plant foods and throwing +them away. + +All of these agencies contribute to reduce the amount of material +circulating in the food cycle of Nature, and thus seem to tend +inevitably in the end toward a termination of the processes of +life; for as soon as the soil becomes exhausted of its nitrogen +compounds, so soon will plant life cease from lack of nutrition, +and the disappearance of animal life will follow rapidly. It is +this loss of nitrogen in large measure that is forcing our +agriculturists to purchase fertilizers. The last fifteen years +have shown us, however, that here again we may look upon our +friends, the bacteria, as agents for counteracting this +dissipating tendency in the general processes of Nature. Bacterial +life in at least two different ways appears to have the function +of reclaiming from the atmosphere more or less of this dissipated +free nitrogen. + +In the first place, it has been found in the last few years that +soil entirely free from all common plants, but containing certain +kinds of bacteria, if allowed to stand in contact with the air, +will slowly but surely gain in the amount of nitrogen compounds +that it contains. These nitrogen compounds are plainly +manufactured by the bacteria in the soil; for unless the bacteria +are present they do not accumulate, and they do accumulate +inevitably if the bacteria are present in the proper quantity and +the proper species. It appears that, as a rule, this fixation of +nitrogen is not performed by any one species of microorganisms, +but by two or three of them acting together. Certain combinations +of bacteria have been found which, when inoculated in the soil, +will bring about this fixation of nitrogen, but no one of the +species is capable of producing this result alone. We do not know +to what extent these organisms are distributed in the soil, nor +how widely this nitrogen fixation through bacterial life is going +on. It is only within a short time that it has been demonstrated +to exist, but we must look upon bacteria in the soil as one of the +factors in reclaiming from the atmosphere the dissipated free +nitrogen. + +The second method by which bacteria aid in the reclaiming of this +lost nitrogen is by a combined action of certain species of +bacteria and some of the higher plants. Ordinary green plants, as +already noted, are unable to make use of the free nitrogen of the +atmosphere It was found, however, some fifteen years ago that some +species of plants, chiefly the great family of legumes, which +contains the pea plant, the bean, the clover, etc, are able, when +growing in soil that is poor in nitrogen, to obtain nitrogen from +some source other than the soil in which they grow. A pea plant in +soil that contains no nitrogen products and watered with water +that contains no nitrogen, will, after sprouting and growing for a +length of time, be found to have accumulated a considerable +quantity of fixed nitrogen in its tissues The only source of this +nitrogen has been evidently from the air which bathes the leaves +of the plant or permeates the soil and bathes its roots This fact +was at first disputed, but subsequently demonstrated to be true, +and was found later to be associated with the combined action of +these legumes and certain soil bacteria. When a legume thus gains +nitrogen from the air, it develops upon its roots little bunches +known as root nodules or root tubercles. The nodules are sometimes +the size of the head of a pm, and sometimes much larger than this, +occasionally reaching the size of a large pea, or even larger. +Upon microscopic examination they are found to be little nests of +bacteria In some way the soil organisms (Fig 27) make their way +into the roots of the sprouting plant, and finding there congenial +environment, develop in considerable quantities and produce root +tubercles in the root. Now, by some entirely unknown process, the +legume and the bacteria growing together succeed in extracting +the nitrogen from the atmosphere which permeates the soil, and +fixing this nitrogen in the tubercles and the roots in the form of +nitrogen compounds. The result is that, after a proper period of +growth, the amount of fixed nitrogen in the plant is found to have +very decidedly increased (Fig 25 E). + +This, of course, furnishes a starting point for the reclaiming of +the lost atmospheric nitrogen. The legume continues to live its +usual life, perhaps increasing the store of nitrogen in its roots +and stems and leaves during the whole of its normal growth. +Subsequently, after having finished its ordinary life, the plant +will die, and then the roots and stems and leaves, falling upon +the ground and becoming buried, will be seized upon by the +decomposition bacteria already mentioned. The nitrogen which has +thus become fixed in their tissues will undergo the destructive +changes already described. This will result eventually in the +production of nitrates. Thus some of the lost nitrogen is restored +again to the soil in the form of nitrates, and may now start on +its route once more around the cycle of food. + +It will be seen, then, that the food cycle is a complete one. +Beginning with the mineral ingredients in the soil, the food +matter may start on its circulation from the soil to the plant, +from the plant to the animal, from the animal to the bacterium and +from the bacterium through a series of other bacteria back again +to the soil in the condition in which it started. If, perchance, +in this progress around the circle some of the nitrogen is thrown +off at a tangent, this, too, is brought back again to the circle +through the agency of bacterial life. And so the food material of +animals and plants continues in this never-ceasing circulation. It +is the sunlight that furnishes the energy for the motion. It is +the sunlight that forces the food around the circle and keeps up +the endless change; and so long as, the sun continues to shine +upon the earth there seems to be no reason why the process should +ever cease. It is this repeated circulation that has made the +continuation of life possible for the millions and millions of +years of the earth's history. It is this continued circulation +that makes life possible still, and it is only this fact that the +food is thus capable of ever circulating from animal to plant and +from plant to animal that makes it possible for the living world +to continue its existence. But, ah we have seen, one half of this +great circle of food change is dependent upon bacterial life. +Without the bacterial life the animal body and the animal +excretion could never be brought back again within the reach of +the plant; and thus, were it not for the action of these micro- +organisms the food cycle would be incomplete and life could not +continue indefinitely upon the surface of the earth. At the very +foundation, the continuation of the present condition of Nature +and the existence of life during the past history of the world has +been fundamentally based upon the ubiquitous presence of bacteria +and upon their continual action in connection with both +destructive and constructive processes. + +RELATION OF BACTERIA TO AGRICULTURE. + +We have already noticed that bacteria play an important part in +some of the agricultural industries, particularly in the dairy. +From the consideration of the matters just discussed, it is +manifest that these organisms must have an even more intimate +relation to the farmer's occupation. At the foundation, farming +consists in the cultivation of plants and animals, and we have +already seen how essential are the bacteria in the continuance of +animal and plant life. But aside from these theoretical +considerations, a little study shows that in a very practical +manner the farmer is ever making use of bacteria, as a rule, quite +unconsciously, but none the less positively. + +SPROUTING OF SEEDS. + +Even in the sprouting of seeds after they are sown in the soil +bacterial life has its influence. When seeds are placed m moist +soil they germinate under the influence of heat. The rich +albuminous material in the seeds furnishes excellent food, and +inasmuch as bacteria abound in the soil, it is inevitable that +they should grow in and feed upon the seed. If the moisture is +excessive and the heat considerable, they very frequently grow so +rapidly in the seed as to destroy its life as a seedling. The seed +rots in the ground as a result. This does not commonly occur, +however, in ordinary soil. But even here bacteria do grow in the +seed, though not so abundantly as to produce any injury. Indeed, +it has been claimed that their presence in the seed in small +quantities is a necessity for the proper sprouting of the seed. It +has been claimed that their growth tends to soften the food +material in the seed, so that the young seedling can more readily +absorb it for its own food, and that without such a softening the +seed remains too hard for the plant to use. This may well be +doubted, however, for seeds can apparently sprout well enough +without the aid of bacteria. But, nevertheless, bacteria do grow +in the seed during its germination, and thus do aid the plant in +the softening of the food material. We can not regard them as +essential to seed germination. It may well be claimed that they +ordinarily play at least an incidental part in this fundamental +life process, although it is uncertain whether the growth of +seedlings is to any considerable extent aided thereby. + +THE SILO. + +In the management of a silo the farmer has undoubtedly another +great bacteriological problem. In the attempt to preserve his +summer-grown food for the winter use of his animals, he is +hindered by the activity of common bacteria. If the food is kept +moist, it is sure to undergo decomposition and be ruined in a +short time as animal food. The farmer finds it necessary, +therefore, to dry some kinds of foods, like hay. While he can thus +preserve some foods, others can not be so treated. Much of the +rank growth of the farm, like cornstalks, is good food while it is +fresh, but is of little value when dried. The farmer has from +experience and observation discovered a method of managing +bacterial growth which enables him to avoid their ordinary evil +effects. This is by the use of the silo. The silo is a large, +heavily built box, which is open only at the top. In the silo the +green food is packed tightly, and when full all access of air is +excluded, except at its surface. Under these conditions the food +remains moist, but nevertheless does not undergo its ordinary +fermentations and putrefactions, and may be preserved for months +without being ruined. The food in such a silo may be taken out +months after it is packed, and will still be found to be in good +condition for food. It is true that it has changed its character +somewhat, but it is not decayed, and is eagerly eaten by cattle. + +We are yet very ignorant of the nature of the changes which occur +m the food while in the silo. The food is not preserved from +fermentation. When the siloxis packed slowly, a very decided +fermentation occurs by which the mass is raised to a high +temperature (140 degrees F. to 160 degrees F.). This heating is +produced by certain species of bacteria which grow readily even at +this high temperature. The fermentation uses up the air in the +silo to a certain extent and produces a settling of the material +which still further excludes air. The first fermentation soon +ceases, and afterward only slow changes occur. Certain acid- +producing bacteria after a little begin to grow slowly, and in +time the silage is rendered somewhat sour by the production of +acetic acid. But the exclusion of air, the close packing, and the +small amount of moisture appear to prevent the growth of the +common putrefactive bacteria, and the silage remains good for a +long time. In other methods of filling the silo, the food is very +quickly packed and densely crowded together so as to exclude as +much air as possible from the beginning. Under these conditions +the lack of moisture and air prevents fermentative action very +largely. Only certain acid-producing organisms grow, and these +very slowly. The essential result in either case is that the +common putrefactive bacteria are prevented from growing, probably +by lack of sufficient oxygen and moisture, and thus the decay is +prevented. The closely packed food offers just the same +unfavourable condition for the growth of common putrefactive +bacteria that we have already seen offered by the hard-pressed +cheese, and the bacteria growth is in the same way held in check. +Our knowledge of the matter is as yet very slight, but we do know +enough to understand that the successful management of a silo is +dependent upon the manipulation of bacteria. + +THE FERTILITY OF THE SOIL. + +The farmer's sole duty is to extract food from the soil. This he +does either directly by raising crops, or indirectly by raising +animals which feed upon the products of the soil. In either case +the fertility of the soil is the fundamental factor in his +success. This fertility is a gift to him from the bacteria. + +Even in the first formation of soil he is in a measure dependent +upon bacteria. Soil, as is well known, is produced in large part +by the crumbling of the rocks into powder. This crumbling we +generally call weathering, and regard it as due to the effect of +moisture and cold upon the rocks, together with the oxidizing +action of the air. Doubtless this is true, and the weathering +action is largely a physical and chemical one. Nevertheless, in +this fundamental process of rock disintegration bacterial action +plays a part, though perhaps a small one. Some species of +bacteria, as we have seen, can live upon very simple foods, +finding in free nitrogen and carbonates sufficiently highly +complex material for their life. These organisms appear to grow on +the bare surface of rocks, assimilating nitrogen from the air, and +carbon from some widely diffused carbonates or from the CO2 in the +air. Their secreted products of an acid nature help to soften the +rocks, and thus aid in performing the first step in weathering. + +The soil is not, however, all made up of disintegrated rocks. It +contains, besides, various ingredients which combine to make it +fertile. Among these are various sulphates which form important +parts of plant foods. These sulphates appear to be formed, in +part, at least, by bacterial agency. The decomposition of proteids +gives rise, among other things, to hydrogen sulphide (H2S). This +gas, which is of common occurrence in the atmosphere, is oxidized +by bacterial growth into sulphuric acid, and this is the basis of +part of the soil sulphates. The deposition of iron phosphates and +iron silicates is probably also in a measure aided by bacterial +action. All of these processes are factors in the formation of +soil. Beyond much question the rock disintegration which occurs +everywhere in Nature is chiefly the result of physical and +chemical changes, but there is reason for believing that the +physical and chemical processes are, to a slight extent at least, +assisted by bacterial life. + +A more important factor of soil fertility is its nitrogen content, +without which it is completely barren. The origin of these +nitrogen ingredients has been more or less of a puzzle. Fertile +soil everywhere contains nitrates and other nitrogen compounds, +and in certain parts of the world there are large accumulations of +these compounds, like the nitrate beds of Chili. That they have +come ultimately from the free atmospheric nitrogen seems certain, +and various attempts have been made to explain a method of this +nitrogen fixation. It has been suggested that electrical +discharges in the air may form nitric acid, which would readily +then unite with soil ingredients to form nitrates. There is little +reason, however, for believing this to be a very important factor +But in the soil bacteria we find undoubtedly an efficient agency m +this nitrogen fixation. As already seen, the bacteria are able to +seize the free atmospheric nitrogen, converting it into nitrite +and nitrates. We have also learned that they can act in connection +with legumes and some other plants, enabling them to fix +atmospheric nitrogen and store it m their roots. By these two +means the nitrogen ingredient in the soil is prevented from +becoming exhausted by the processes of dissipation constantly +going on. Further, by some such agency must we imagine the +original nitrogen soil ingredient to have been derived. Such an +organic agency is the only one yet discerned which appears to have +been efficient in furnishing virgin soil with its nitrates, and we +must therefore look upon bacteria as essential to the original +fertility of the soil. But in another direction still does the +farmer depend directly upon bacteria The most important factor in +the fertility of the soil is the part of it called humus. This +humus is very complex, and never alike in different soils It +contains nitrogen compounds in abundance, together with sulphates, +phosphates, sugar, and many other substances. It is this which +makes the garden soil different from sand, or the rich soil +different from the sterile soil. If the soil is cultivated year +after year, its food ingredients are slowly but surely exhausted. +Something is taken from the humus each year, and unless this be +replaced the soil ceases to be able to support life. To keep up a +constant yield from the soil the farmer understands that he must +apply fertilizers more or less constantly. + +This application of fertilizers is simply feeding the crops. Some +of these fertilizers the farmer purchases, and knows little or +nothing as to their origin. The most common method of feeding the +crops is, however, by the use of ordinary barnyard manure. The +reason why this material contains plant food we can understand, +since it is made of the undigested part of food, together with all +the urea and other excretions of animals, and contains, therefore, +besides various minerals, all of the nitrogenous waste of animal +life. These secretions are not at first fit for plant food. The +farmer has learned by experience that such excretions, before they +are of any use on his fields, must undergo a process of slow +change, which is sometimes called ripening. Fresh manure is +sometimes used on the fields, but it is only made use of by the +plants after the ripening process has occurred. Fresh animal +excretions are of little or no value as a fertilizer. The farmer, +therefore, commonly allows it to remain in heaps for some time, +and it undergoes a slow change, which gradually converts it into a +condition in which it can be used by plants. This ripening is +readily explained by the facts already considered The fresh animal +secretions consist of various highly complex compounds of +nitrogen, and the ripening is a process of their decomposition. +The proteids are broken to pieces, and their nitrogen elements +reduced to the form of nitrates, leucin, etc, or even to ammonia +or free nitrogen. Further, a second process occurs, the process of +oxidation of these nitrogen compounds already noticed, and the +ammonia and nitrites resulting from the decomposition are built +into nitrates. In short, in this ripening manure the processes +noticed in the first part of this chapter are taking place, by +which the complex nitrogenous bodies are first reduced and then +oxidized to form plant food. The ripening of manure is both an +analytical and a synthetical process. By the analysis, proteids +and other bodies are broken into very simple compounds, some of +them, indeed, being dissipated into the air, but other portions +are retained and then oxidized, and these latter become the real +fertilizing materials. Through the agency of bacteria the compost +heap thus becomes the great source of plant food to the farmer. +Into this compost heap he throws garbage, straw, vegetable and +animal substances in general, or any organic refuse which may be +at hand. The various bacteria seize it all, and cause the +decomposition which converts it into plant food again. The rotting +of the compost heap is thus a gigantic cultivation of bacteria. + +This knowledge of the ripening process is further teaching the +farmer how to prevent waste. In the ordinary decomposition of the +compost heap not an inconsiderable portion of the nitrogen is lost +in the air by dissipation as ammonia or free nitrogen. Even his +nitrates may be thus lost by bacterial action. This portion is +lost to the farmer completely, and he can only hope to replace it +either by purchasing nitrates in the form of commercial +fertilizers, or by reclaiming it from the air by the use of the +bacterial agencies already noticed. With the knowledge now at his +command he is learning to prevent this waste. In the decomposition +one large factor of loss is the ammonia, which, being a gas, is +readily dissipated into the air. Knowing this common result of +bacterial action, the scientist has told the farmer that, by +adding certain common chemicals to his decomposing manure heap, +chemicals which will readily unite with ammonia, he may retain +most of the nitrogen in this heap in the form of ammonia salts, +which, once formed, no longer show a tendency to dissipate into +the air. Ordinary gypsum, or superphosphates, or plaster will +readily unite with ammonia, and these added to the manure heap +largely counteract the tendency of the nitrogen to waste, thus +enabling the farmer to put back into his soil most of the nitrogen +which was extracted from it by his crops and then used by his +stock. His vegetable crops raise the nitrates into proteids. His +animals feed upon the proteids, and perform his work or furnish +him with milk. Then his bacteria stock take the excreted or refuse +nitrogen, and in his manure heap turn it back again into nitrates +ready to begin the circle once more. This might go on almost +indefinitely were it not for two facts, the farmer sends +nitrogenous material off his farm in the milk or grains or other +nitrogenous products, which he sells, and the decomposition +processes, as we have seen, dissipate some of the nitrogen into +the air as free nitrogen. + +To meet this emergency and loss the farmer has another method of +enriching the soil, again depending upon bacteria. This is the so- +called green manuring. Here certain plants which seize nitrogen +from the air are cultivated upon the field to be fertilized, and, +instead of harvesting a crop, it is ploughed into the soil. Or +perhaps the tops may be harvested, the rest being ploughed into +the soil. The vegetable material thus ploughed in lies over a +season and enriches the soil. Here the bacteria of the soil come +into play in several directions. First, if the crop sowed be a +legume, the soil bacteria assist it to seize the nitrogen from the +air. The only plants which are of use in this green manuring are +those which can, through the agency of bacteria, obtain nitrogen +from the air and store it in their roots. Second, after the crop +is ploughed into the soil various decomposing bacteria seize upon +it, pulling the compounds to pieces. The carbon is largely +dissipated into the air as carbonic dioxide, where the next +generation of plants can get hold of it. The minerals and the +nitrogen remain in the soil. The nitrogenous portions go through +the same series of decomposition and synthetical changes already +described, and thus eventually the nitrogen seized from the air by +the combined action of the legumes and the bacteria is converted +into nitrates, and will serve for food for the next set of plants +grown on the same soil. Here is thus a practical method of using +the nitrogen assimilation powers of bacteria, and reclaiming +nitrogen from the air to replace that which has been lost. Thus it +is that the farmer's nitrogen problem of the fertile soil appears +to resolve itself into a proper handling of bacteria. These +organisms have stocked his soil in the first place. They convert +all of his compost heap wastes into simple bodies, some of which +are changed into plant foods, while others are at the same time +lost. Lastly, they may be made to reclaim this lost nitrogen, and +the fanner, so soon as he has requisite knowledge of these facts, +will be able to keep within his control the supply of this +important element. The continued fertility of the soil is thus a +gift from the bacteria. + +BACTERIA AS SOURCES OF TROUBLE TO THE FARMER. + +While the topics already considered comprise the most important +factors in agricultural bacteriology, the farmer's relations to +bacteria do not end here. These organisms come incidentally into +his life in many ways. They are not always his aids as they are in +most of the instances thus far cited. They produce disease in his +cattle, as will be noticed in the next chapter. Bacteria are +agents of decomposition, and they are just as likely to decompose +material which the farmer wishes to preserve as they are to +decompose material which the farmer desires to undergo the process +of decay. They are as ready to attack his fruits and vegetables as +to ripen his cream. The skin of fruits and vegetables is a +moderately good protection of the interior from the attack of +bacteria; but if the skin be broken in any place, bacteria get in +and cause decay, and to prevent it the farmer uses a cold cellar. +The bacteria prevent the farmer from preserving meats for any +length of time unless he checks their growth in some way. They get +into the eggs of his fowls and ruin them. Their troublesome nature +in the dairy in preventing the keeping of milk has already been +noticed. If he plants his seeds in very moist, damp weather, the +soil bacteria cause too rapid a decomposition of the seeds and +they rot in the ground instead of sprouting. They produce +disagreeable odours, and are the cause of most of the peculiar +smells, good and bad, around the barn. They attack the organic +matter which gets into his well or brook or pond, decomposing it, +filling the water with disagreeable and perhaps poisonous products +which render it unfit to drink. They not only aid in the decay of +the fallen tree in his forests; but in the same way attack the +timber which he wishes to preserve, especially if it is kept in a +moist condition. Thus they contribute largely to the gradual +destruction of wooden structures. It is therefore the presence of +these organisms which forces him to dry his hay, to smoke his +hams, to corn his beef, to keep his fruits and vegetables cool and +prevent skin bruises, to ice his dairy, to protect his timber from +rain, to use stone instead of wooden foundations for buildings, +etc. In general, when the farmer desires to get rid of any organic +refuse, he depends upon bacteria, for they are his sole agents +(aside from fire) for the final destruction of organic matter. +When he wishes to convert waste organic refuse into fertilizing +material, he uses the bacteria of his compost heap. On the other +hand, whenever he desires to preserve organic material, the +bacteria are the enemies against which he must carefully guard. + +Thus the farmer's life from year's end to year's end is in most +intimate association with bacteria. Upon them he depends to insure +the continued fertility of his soil and the constant continued +production of good crops. Upon them he depends to turn into plant +food all the organic refuse from his house or from his barn. Upon +them he depends to replenish his stock of nitrogen. It is these +organisms which furnish his dairy with its butter flavours and +with the taste of its cheese. But, on the other hand, against them +he must be constantly alert. All his food products must be +protected from their ravages. A successful farmer's life, then, +largely resolves itself into a skilful management of bacterial +activity. To aid them in destroying or decomposing everything +which he does not desire to preserve, and to prevent their +destroying the organic material which he wishes to keep for future +use, is the object of a considerable portion of farm labour; and +the most successful farmer to-day, and we believe the most +successful farmer of the future, is the one who most intelligently +and skilfully manipulates these gigantic forces furnished him by +the growth of his microscopical allies. + +RELATION OF BACTERIA TO COAL. Another one of Nature's processes in +which bacteria have played an important part is in the formation +of coal. It is unnecessary to emphasize the importance of coal in +modern civilization. Aside from its use as fuel, upon which +civilization is dependent, coal is a source of an endless variety +of valuable products. It is the source of our illuminating gas, +and ammonia is one of the products of the gas manufacture. From +the coal also comes coal tar, the material from which such a long +series of valuable materials, as aniline colours, carbolic acid, +etc, is derived. The list of products which we owe to coal is very +long, and the value of this material is hardly to be overrated. In +the preparation of these ingredients from coal bacteria do not +play any part. Most of them are derived by means of distillation. +But when asked for the agents which have given us the coal of the +coal beds, we shall find that here, too, we owe a great debt to +bacteria. + +Coal, as is well known, has come from the accumulation of the +luxuriant vegetable growth of the past geological ages. It has +therefore been directly furnished us by the vegetation of the +green plants of the past, and, in general, it represents so much +carbonic dioxide which these plants have extracted from the +atmosphere. But while the green plants have been the active agents +in producing this assimilation, bacteria have played an important +part in coal manufacture in two different directions. The first +appears to be in furnishing these plants with nitrogen. Without a +store of fixed nitrogen in the soil these carboniferous plants +could not have grown. This matter has already been considered. We +have no very absolute knowledge as to the agency of bacteria in +furnishing nitrogen for this vegetation in past ages, but there is +every reason to believe that in the past, as in the present, the +chief source of organic nitrogen has been from the atmosphere and +derived from the atmosphere through the agency of bacteria. In the +absence of any other known factor we may be pretty safe in the +assumption that bacteria played an important part in this nitrogen +fixation, and that bacteria must therefore be regarded as the +agents which have furnished us the nitrogen stored in the coal. + +But in a later stage of coal formation bacteria have contributed +more directly to the formation of coal. Coal is not simply +accumulated vegetation. The coal of our coal beds is very +different in its chemical composition from the wood of the trees. +It contains a much higher percentage of carbon and a lower +percentage of hydrogen and oxygen than ordinary vegetable +substances. The conversion of the vegetation of the carboniferous +ages into coal was accompanied by a gradual loss of hydrogen and a +consequent increase in the percentage of carbon. It is this change +that has added to the density of the substance and makes the +greater value of coal as fuel. There is little doubt now as to the +method by which this woody material of the past has been converted +into coal. The same process appears to be going on in a similar +manner to-day in the peat beds of various northern countries. The +fallen vegetation, trees, trunks, branches, and leaves, accumulate +in masses, and, when the conditions of moisture and temperature +are right, begin to undergo a fermentation. Ordinarily this action +of bacteria, as already noticed, produces an almost complete +though slow oxidation of the carbon, and results in the total +decay of the vegetable matter. But if the vegetable mass be +covered by water and mud under proper conditions of moisture and +temperature, a different kind of fermentation arises which does +not produce such complete decay. The covering of water prevents +the access of oxygen to the fermenting mass, an oxidation of the +carbon is largely prevented, and the vegetable matter slowly +changes its character. Under the influence of this slow +fermentation, aided, probably by pressure, the mass becomes more +and more solid and condensed, its woody character becomes less and +less distinct, and there is a gradual loss of the hydrogen and the +oxygen. Doubtless there is a loss of carbon also, for there is an +evolution of marsh gas which contains carbon. But, in this slow +fermentation taking place under the water in peat bogs and marshes +the carbon loss is relatively small; the woody material does not +become completely oxidized, as it does in free operations of +decay. The loss of hydrogen and oxygen from the mass is greater +than that of carbon, and the percentage of carbon therefore +increases. This is not the ordinary kind of fermentation that goes +on in vegetable accumulations. It requires special conditions and +possibly special kinds of fermenting organisms. Peat is not formed +in all climates. In warm regions, or where the woody matter is +freely exposed to the air, the fermentation of vegetable matter is +more complete, and it is entirely destroyed by oxidation. It is +only in colder regions and when covered with water that the +destruction of the organic matter stops short of decay. But such +incomplete fermentation is still going on in many parts of the +world, and by its means vegetable accumulations are being +converted into peat. + +This formation of peat appears to be a first step in the formation +of denser coal. By a continuation of the same processes the mass +becomes still more dense and solid. As we pass from the top to the +bottom of such an accumulation of peat, we find it becoming denser +and denser, and at the bottom it is commonly of a hard consistence, +brownish in colour, and with only slight traces of the +original woody structure. Such material is called lignite. It +contains a higher percentage of carbon than peat, but a lower +percentage than coal, and is plainly a step in coal formation. But +the process goes on, the hydrogen and oxygen loss continuing until +there is finally produced true coal. + +If this is the correct understanding of the formation of coal, we +see that we have plainly a process in which bacterial life has had +a large and important share. We are, of course, densely ignorant +of the exact processes going on. We know nothing positively as to +the kind of microorganisms which produce this slow, peculiar +fermentation. As yet, the fermentation going on in the formation +of the peat has not been studied by the bacteriologists, and we do +not know from direct experiment that it is a matter of bacterial +action. It has been commonly regarded as simply a slow chemical +change, but its general similarity to other fermentative processes +is so great that we can have little hesitation in attributing it +to micro-organisms, and doubtless to some forms of plants allied +to bacteria. There is no reason for doubting that bacteria existed +in the geological ages with essentially the same powers as they +now possess, and to some forms of bacteria which grow in the +absence of oxygen can we probably attribute the slow change which +has produced coal. Here, then, is another great source of wealth +in Nature for which we are dependent upon bacteria. While, of +course, water and pressure were very essential factors in the +deposition of coal, it was a peculiar kind of fermentation +occurring in the vegetation that brought about the chemical +changes in it which resulted in its transformation into coal. The +vegetation of the carboniferous age was dependent upon the +nitrogen fixed by the bacteria, and to these organisms also do we +owe the fact that this vegetation was stored for us in the rocks. + + + + + +CHAPTER V. + +PARASITIC BACTERIA AND THEIR RELATION TO DISEASE. + + +Perhaps the most universally known fact in regard to bacteria is +that they are the cause of disease. It is this fact that has made +them objects of such wide interest. This is the side of the +subject that first attracted attention, has been most studied, and +in regard to which there has been the greatest accumulation of +evidence. So persistently has the relation of bacteria to disease +been discussed and emphasized that the majority of readers are +hardly able to disassociate the two. To most people the very word +bacteria is almost equivalent to disease, and the thought of +swallowing microbes in drinking water or milk is decidedly +repugnant and alarming. In the public mind it is only necessary to +demonstrate that an article holds bacteria to throw it under +condemnation. + +We have already seen that bacteria are to be regarded as agents +for good, and that from their fundamental relation to plant life +they must be looked upon as our friends rather than as our +enemies. It is true that there is another side to the story which +relates to the parasitic species. These parasitic forms may do us +direct or indirect injury. But the species of bacteria which are +capable of doing us any injury, the pathogenic bacteria, are +really very few compared to the great host of species which are +harmless. A small number of species, perhaps a score or two, are +pathogenic, while a much larger number, amounting to hundreds and +perhaps thousands of species, are perfectly harmless. This latter +class do no injury even though swallowed by man in thousands. They +are not parasitic, and are unable to grow in the body of man. +Their presence is entirely consistent with the most perfect +health, and, indeed, there are some reasons for believing that +they are sometimes directly beneficial to health. It is entirely +unjust to condemn all bacteria because a few chance to produce +mischief. Bacteria in general are agents for good rather than ill. + +There are, however, some species which cause mankind much trouble +by interfering in one way or another with the normal processes of +life. These pathogenic bacteria, or disease germs, do not all act +alike, but bring about injury to man in a number of different +ways. We may recognise two different classes among them, which, +however, we shall see are connected by intermediate types. These +two classes are, first, the pathogenic bacteria, which are not +strictly parasitic but live free in Nature; and, second, those +which live as true parasites in the bodies of man or other +animals. To understand the real relation of these two classes, we +must first notice the method by which bacteria in general produce +disease. + +METHOD BY WHICH BACTERIA PRODUCE DISEASE. + +Since it was first clearly recognised that certain species of +bacteria have the power of producing disease, the question as to +how they do so has ever been a prominent one Even if they do grow +in the body, why should their presence give rise to the symptoms +characterizing disease? Various answers to this question have been +given in the past It has been suggested that in their growth they +consume the food of the body and thus exhaust it, that they +produce an oxidation of the body tissues, or that they produce a +reduction of these tissues, or that they mechanically interfere +with the circulation None of these suggestions have proved of much +value Another view was early advanced, and has stood the test of +time. This claim is that the bacteria while growing in the body +produce poisons, and these poisons then have a direct action on +the body We have already noticed that bacteria during their growth +in any medium produce a large number of biproducts of +decomposition. We noticed also that among these biproducts there +are some which have a poisonous nature; so poisonous are they that +when inoculated into the body of an animal they may produce +poisoning and death. We have only to suppose that the pathogenic +bacteria, when growing as parasites in man, produce such poisons, +and we have at once an explanation of the method by which they +give rise to disease. + +This explanation of germ disease is more than simple theory. It +has been in many cases clearly demonstrated. It has been found +that the bacteria which cause diphtheria, tetanus, typhoid, +tuberculosis, and many other diseases, produce, even when growing +in common culture media, poisons which are of a very violent +nature. These poisons when inoculated into the bodies of animals +give rise to much the same symptoms as the bacteria do themselves +when growing as parasites in the animals. The chief difference in +the results from inoculating an animal with the poison and with +the living bacteria is in the rapidity of the action. When the +poison is injected the poisoning symptoms are almost immediately +seen, but when the living bacteria are inoculated the effect is +only seen after several days or longer, not, in short, until the +inoculated bacteria have had time enough to grow in the body and +produce the poison in quantity. It has not by any means been shown +that all pathogenic germs produce their effect in this way, but it +has been proved to be the real method in quite a number of cases, +and is extremely probable in others. While some bacteria perhaps +produce results by a different method, we must recognise the +production of poisons as at all events the common direct cause of +the symptoms of disease. This explanation will enable us more +clearly to understand the relation of different bacteria to +disease. + +PATHOGENIC GERMS WHICH ARE NOT STRICTLY PARASITIC + +Recognising that bacteria may produce poisons, we readily see that +it is not always necessary that they should be parasites in order +to produce trouble. In their ordinary growth in Nature such +bacteria will produce no trouble The poisons will be produced in +decaying material but will seldom be taken into the human body. +These poisons, produced in the first stages of putrefaction, are +oxidized by further stages of decomposition into harmless +products. But should it happen that some of these bacteria +obtained a chance to grow vigorously for a while in organic +products that are subsequently swallowed as man's food, it is +plain that evil results might follow. If such food is swallowed by +man after the bacteria have produced their poisonous bodies, it +will tend to produce an immediate poisoning of his system. The +effect may be sudden and severe if considerable quantity of the +poisonous material is swallowed, or slight but protracted if small +quantities are repeatedly consumed in food. Such instances are not +uncommon. Well-known examples are cases of ice-cream poisoning, +poisoning from eating cheese or from drinking milk, or in not a +few instances from eating fish or meats within which bacteria have +had opportunity for growth. In all these cases the poison is +swallowed in quantity sufficient to give rise quickly to severe +symptoms, sometimes resulting fatally, and at other times passing +off as soon as the body succeeds in throwing off the poisons. In +other cases still, however, the amount of poison swallowed may be +very slight, too slight to produce much effect unless the same be +consumed repeatedly. All such trouble may be attributed to +fermented or partly decayed food. It is difficult to distinguish +such instances from others produced in a slightly different way, +as follows: + +It may happen that the bacteria which grow in food products +continue to grow in the food even after it is swallowed and has +passed into the stomach or intestines. This appears particularly +true of milk bacteria. Under these conditions the bacteria are not +in any proper sense parasitic, since they are simply living in and +feeding upon the same food which they consume outside the body, +and are not feeding upon the tissues of man. The poisons which +they produce will continue to be developed as long as the bacteria +continue to grow, whether in a milk pail or a human stomach. If +now the poisons are absorbed by the body, they may produce a mild +or severe disease which will be more or less lasting, continuing +perhaps as long as the same food and the same bacteria are +supplied to the individual. The most important disease of this +class appears to be the dreaded cholera infantum, so common among +infants who feed upon cow's milk in warm weather. It is easy to +understand the nature of this disease when we remember the great +number of bacteria in milk, especially in hot weather, and when we +remember that the delicate organism of the infant will be thrown +at once into disorder by slight amounts of poison which would have +no appreciable effect upon the stronger adult. We can easily +understand, further, how the disease readily yields to treatment +if care is taken to sterilize the milk given to the patient. + +We do not know to-day the extent of the troubles which are +produced by bacteria of this sort. They will, of course, be +chiefly connected with our food products, and commonly, though not +always, will affect the digestive functions. It is probable that +many of the cases of summer diarrhoea are produced by some such +cause, and if they could be traced to their source would be found +to be produced by bacterial poisons swallowed with food or drink, +or by similar poisons produced by bacteria growing in such food +after it is swallowed by the individual. In hot weather, when +bacteria are so abundant everywhere and growing so rapidly, it is +impossible to avoid such dangers completely without exercising +over all food a guard which would be decidedly oppressive. It is +well to bear in mind, however, that the most common and most +dangerous source of such poisons is milk or its products, and for +this reason one should hesitate to drink milk in hot weather +unless it is either quite fresh or has been boiled to destroy its +bacteria. + +PATHOGENIC BACTERIA WHICH ARE TRUE PARASITES. + +This class of pathogenic bacteria includes those which actually +invade the body and feed upon its tissues instead of living simply +upon swallowed food. It is difficult, however, to draw any sharp +line separating the two classes. The bacteria which cause +diphtheria (Fig. 28), for instance, do not really invade the body. +They grow in the throat, attached to its walls, and are confined +to this external location or to the superficial tissues. This +bacillus is, in short, only found in the mouth and throat, and is +practically confined to the so-called false membranes. It never +enters any of the tissues of the body, although attached to the +mucous membrane. It grows vigorously in this membrane, and there +secretes or in some way produces extremely violent poisons. These +poisons are then absorbed by the body and give rise to the general +symptoms of the disease. Much the same is true of the bacillus +which causes tetanus or lockjaw (Fig. 29). This bacillus is +commonly inoculated into the flesh of the victim by a wound made +with some object which has been lying upon the earth where the +bacillus lives. The bacillus grows readily after being inoculated, +but it is localized at the point of the wound, without invading +the tissue to any extent. It produces, however, during its growth +several poisons which have been separated and studied. Among them +are some of the most violent poisons of which we have any +knowledge. While the bacillus grows in the tissues around the +wound it secretes these poisons, which are then absorbed by the +body generally. Their poisoning effects produce the violent +symptoms of the disease. Of much the same nature is Asiatic +cholera. This is caused by a bacillus which is able to grow +rapidly in the intestines, feeding perhaps in part on the food in +the intestines and perhaps in part upon the body secretions. To a +slight extent also it appears to be able to invade the tissues of +the body, for the bacilli are found in the walls of the +intestines. But it is not a proper parasite, and the fatal disease +it produces is the result of the absorption of the poisons +secreted in the intestines. + +It is but a step from this to the true parasites. Typhoid fever, +for example, is a disease produced by bacteria which grow in the +intestines, but which also invade the tissues more extensively +than the cholera germs (Fig. 30). They do not invade the body +generally, however, but become somewhat localized in special +glands like the liver, the spleen, etc. Even here they do not +appear to find a very favourable condition, for they do not grow +extensively in these places. They are likely to be found in the +spleen in small groups or centres, but not generally distributed +through it. Wherever they grow they produce poison, which has been +called typhotoxine, and it is this poison chiefly which gives rise +to the fever. + +Quite a considerable number of the pathogenic germs are, like the +typhoid bacillus, more or less confined to special places. Instead +of distributing themselves through the body after they find +entrance, they are restricted to special organs. The most common +example of a parasite of this sort is the tuberculosis bacillus, +the cause of consumption, scrofula, white swelling, lupus, etc. +(Fig. 31). Although this bacillus is very common and is able to +attack almost any organ in the body, it is usually very restricted +in growth. It may become localized in a small gland, a single +joint, a small spot in the lungs, or in the glands of the +mesentery, the other parts of the body remaining free from +infection. Not infrequently the whole trouble is thus confined to +such a small locality that nothing serious results. But in other +instances the bacilli may after a time slowly or rapidly +distribute themselves from these centres, attacking more and more +of the body until perhaps fatal results follow in the end. This +disease is therefore commonly of very slow progress. + +Again, we have still other parasites which are not thus confined, +but which, as soon as they enter the body, produce a general +infection, attacking the blood and perhaps nearly all tissues +simultaneously. The most typical example of this sort is anthrax +or malignant pustule, a disease fortunately rare in man (Fig. 32). +Here the bacilli multiply in the blood, and very soon a general +and fatal infection of the whole body arises, resulting from the +abundance of the bacilli everywhere. Some of the obscure diseases +known as blood poisoning appear to be of the same general nature, +these diseases resulting from a very general invasion of the whole +body by certain pathogenic bacteria. + +In general, then, we see that the so-called germ diseases result +from the action upon the body of poisons produced by bacterial +growth. Differences in the nature of these poisons produce +differences in the character of the disease, and differences in +the parasitic powers of the different species of bacteria produce +wide differences in the course of the diseases and their relation +to external phenomena. + +WHAT DISEASES ARE DUE TO BACTERIA? + +It is, of course, an extremely important matter to determine to +what extent human diseases are caused by bacteria. It is not easy, +nor indeed possible, to do this to-day with accuracy. It is no +easy matter to prove that any particular disease is caused by +bacteria. To do this it is necessary to find some particular +bacterium present in all cases of the disease; to find some method +of getting it to grow outside the body in culture media; to +demonstrate its absence in healthy animals, or healthy human +individuals if it be a human disease; and, finally, to reproduce +the disease in healthy animals by inoculating them with the +bacterium. All of these steps of proof present difficulties, but +especially the last one. In the study of animals it is +comparatively easy to reproduce a disease by inoculation. But +experiments upon man are commonly impossible, and in the case of +human diseases it is frequently very difficult or impossible to +obtain the final test of the matter. After finding a specific +bacterium associated with a disease, it is usually possible to +experiment with it further upon animals only. But some human +diseases do not attack animals, and in the case of diseases that +may be given to animals it is frequently uncertain whether the +disease produced in the animal by such inoculation is identical +with the human disease in question, owing to the difference of +symptoms in the different animals. As a consequence, the proof of +the germ nature of different diseases varies all the way from +absolute demonstration to mere suspicion. To give a complete and +correct list of the diseases caused by bacteria, or to give a list +of the bacteria species pathogenic to man, is therefore at present +impossible. + +The difficulty of giving such a list is rendered greater from the +fact that we have in recent years learned that the same species of +pathogenic bacterium may produce different results under different +conditions. When the subject of germ disease was first studied and +the connection between bacteria and disease was first +demonstrated, it was thought that each particular species of +pathogenic bacteria produced a single definite disease; and +conversely, each germ disease was supposed to have its own +definite species of bacterium as its cause. Recent study has +shown, however, that this is not wholly true. It is true that some +diseases do have such a definite relation to definite bacteria. +The anthrax germ, for example, will always produce anthrax, no +matter where or how it is inoculated into the body. So, also, in +quite a number of other cases distinct specific bacteria are +associated with distinct diseases. But, on the other hand, there +are some pathogenic bacteria which are not so definite in their +action, and produce different results in accordance with +circumstances, the effect varying both with the organ attacked and +with the condition of the individual. For instance, a considerable +number of different types of blood poisoning, septicaemia, +pyaemia, gangrene, inflammation of wounds, or formation of pus +from slight skin wounds--indeed, a host of miscellaneous troubles, +ranging all the way from a slight pus formation to a violent and +severe blood poisoning--all appear to be caused by bacteria, and +it is impossible to make out any definite species associated with +the different types of these troubles. There are three common +forms of so-called pus cocci, and these are found almost +indiscriminately with various types of inflammatory troubles. +Moreover, these species of bacteria are found with almost absolute +constancy in and around the body, even in health. They are on the +clothing, on the skin, in the mouth and alimentary canal. Here +they exist, commonly doing no harm. They have, however, the power +of doing injury if by chance they get into wounds. But their power +of doing injury varies both with the condition of the individual +and with variations in the bacteria themselves. If the individual +is in a good condition of health these bacteria have little power +of injuring him even when they do get into such wounds, while at +times of feeble vitality they may do much more injury, and take +the occasion of any little cut or bruise to enter under the skin +and give rise to inflammation and pus. Some people will develop +slight abscesses or slight inflammations whenever the skin is +bruised, while with others such bruises or cuts heal at once +without trouble. Both are doubtless subject to the same chance of +infection, but the one resists, while the other does not. In +common parlance, we say that such a tendency to abscesses +indicates a bad condition of the blood--a phrase which means +nothing. Further, we find that the same species of bacterium may +have varying powers of producing disease at different times. Some +species are universal inhabitants of the alimentary canal and are +ordinarily harmless, while under other conditions of unknown +character they invade the tissues and give rise to a serious and +perhaps fatal disease. We may thus recognise some bacteria which +may be compared to foreign invaders, while others are domestic +enemies. The former, like the typhoid bacillus, always produce +trouble when they succeed in entering the body and finding a +foothold. The latter, like the normal intestinal bacilli, are +always present but commonly harmless, only under special +conditions becoming troublesome. All this shows that there are +other factors in determining the course of a disease, or even the +existence of a disease, than the simple presence of a peculiar +species of pathogenic bacterium. + +From the facts just stated it will be evident that any list of +germ diseases will be rather uncertain. Still, the studies of the +last twenty years or more have disclosed some definite relations +of bacteria and disease, and a list of the diseases more or less +definitely associated with distinct species of bacteria is of +interest. Such a list, including only well-known diseases, is as +follows: + + Name of disease. Name of bacterium producing the disease. + Anthrax (Malignant pustule). Bacillus anthracis. + Cholera. Spirillum cholera: asiaticae + Croupous pneumonia. Micrococcus pneumonia crouposa. + Diphtheria. Bacillus diphtheria. + Glanders. Bacillus mallei. + Gonorrhoea. Micrococcus gonorrhaeae + Influenza. Bacillus of influenza. + Leprosy. Bacillus leprae. + Relapsing fever. Spirillum Obermeieri. + Tetanus (lockjaw). Bacillus tetani. + Tuberculosis (including + consumption, scrofula, etc.) Bacillus tuberculosis. + Typhoid fever. Bacillus typhi abdominalis. + +Various wound infections, including septicaemia, pyaemia, acute +abscesses, ulcers, erysipelas, etc., are produced by a few forms +of micrococci, resembling each other in many points but differing +slightly. They are found almost indiscriminately in any of these +wound infections, and none of them appears to have any definite +relation to any special form of disease unless it be the +micrococcus of erysipelas. The common pus micrococci are grouped +under three species, Staphylococcus pyogenes aureus, +Staphylococcus pyogenes, and Streptococcus pyogenes. These three +are the most common, but others are occasionally found. + +In addition to these, which may be regarded as demonstrated, the +following diseases are with more or less certainty regarded as +caused by distinct specific bacteria: Bronchitis, endocarditis, +measles, whooping-cough, peritonitis, pneumonia, syphilis. + +Still another list might be given of diseases whose general nature +indicates that they are caused by bacteria, but in connection with +which no distinct bacterium has yet been found. As might be +expected also, a larger list of animal diseases has been +demonstrated to be caused by these organisms. In addition, quite a +number of species of bacteria have been found in such material as +faeces, putrefying blood, etc., which have been shown by +experiment to be capable of producing diseases in animals, but in +regard to which we have no evidence that they ever do produce +actual disease under any normal conditions. These may contribute, +perhaps, to the troubles arising from poisonous foods, but can not +be regarded as disease germs proper. + +VARIABILITY OF PATHOGENIC POWERS. + +As has already been stated, our ideas of the relation of bacteria +to disease have undergone quite a change since they were first +formulated, and we recognise other factors influencing disease +besides the actual presence of the bacterium. These we may briefly +consider under two heads, viz., variation in the bacterium, and +variation in the susceptibility of the individual. The first will +require only a brief consideration. + +That the same species of pathogenic bacteria at different times +varies in its powers to produce disease has long been known. +Various conditions are known to affect thus the virulence of +bacteria. The bacillus which is supposed to give rise to pneumonia +loses its power to produce the disease after having been +cultivated for a short time in ordinary culture media in the +laboratory. This is easily understood upon the suggestion that it +is a parasitic bacillus and does not thrive except under parasitic +conditions. Its pathogenic powers can sometimes be restored by +passing it again through some susceptible animal. One of the most +violent pathogenic bacteria is that which produces anthrax, but +this loses its pathogenic powers if it is cultivated for a +considerable period at a high temperature. The micrococcus which +causes fowl cholera loses its power if it be cultivated in common +culture media, care being taken to allow several days to elapse +between the successive inoculations into new culture flasks. Most +pathogenic bacteria can in some way be so treated as to suffer a +diminution or complete loss of their powers of producing a fatal +disease. On the other hand, other conditions will cause an +increase in the virulence of a pathogenic germ. The virus which +produces hydrophobia is increased in violence if it is inoculated +into a rabbit and subsequently taken from the rabbit for further +inoculation. The fowl cholera micrococcus, which has been weakened +as just mentioned, may be restored to its original violence by +inoculating it into a small bird, like a sparrow, and inoculating +a second bird from this. A few such inoculations will make it as +active as ever. These variations doubtless exist among the species +in Nature as well as in artificial cultures. The bacteria which +produce the various wound infections and abscesses, etc., appear +to vary under normal conditions from a type capable of producing +violent and fatal blood poisoning to a type producing only a +simple abscess, or even to a type that is entirely innocuous. It +is this factor, doubtless, which in a large measure determines the +severity of any epidemic of a bacterial contagious disease. + +SUSCEPTIBILITY OF THE INDIVIDUAL. + +The very great modification of our early views has affected our +ideas as to the power which individuals have of resisting the +invasion of pathogenic bacteria. It has from the first been +understood that some individuals are more susceptible to disease +than others, and in attempting to determine the significance of +this fact many valuable and interesting discoveries have been +made. After the exposure to the disease there follows a period of +some length in which there are no discernible effects. This is +followed by the onset of the disease and its development to a +crisis, and, if this be passed, by a recovery. The general course +of a germ disease is divided into three stages: the stage of +incubation, the development of the disease, and the recovery. The +susceptibility of the body to a disease may be best considered +under the three heads of Invasion, Resistance, Recovery. + +Means of Invasion.--In order that a germ disease should arise in +an individual, it is first necessary that the special bacterium +which causes the disease should get into the body. There are +several channels through which bacteria can thus find entrance; +these are through the mouth, through the nose, through the skin, +and occasionally through excretory ducts. Those which come through +the mouth come with the food or drink which we swallow; those +which enter through the nose must be traced to the air; and those +which enter through the skin come in most cases through contact +with some infected object, such as direct contact with the body of +an infected person or his clothing or some objects he has handled, +etc. Occasionally, perhaps, the bacteria may get into the skin +from the air, but this is certainly uncommon and confined to a few +diseases. There are here two facts of the utmost importance for +every one to understand: first, that the chance of disease +bacteria being carried to us through the air is very slight and +confined to a few diseases, such as smallpox, tuberculosis, +scarlet fever; etc., and, secondly, that the uninjured skin and +the uninjured mucous membrane also is almost a sure protection +against the invasion of the bacteria. If the skin is whole, +without bruises or cuts, bacteria can seldom, if ever, find +passage through it. These two facts are of the utmost importance, +since of all sources of infection we have the least power to guard +against infection through the air, and since of all means 'of +entrance we can guard the skin with the greatest difficulty. We +can easily render food free from pathogenic bacteria by heating +it. The material we drink can similarly be rendered harmless, but +we can not by any known means avoid breathing air, nor is there +any known method of disinfecting the air, and it is impossible for +those who have anything to do with sick persons to avoid entirely +having contact either with the patient or with infected clothing +or utensils. + +From the facts here given it will be seen that the individual's +susceptibility to disease produced by parasitic bacteria will +depend upon his habits of cleanliness, his care in handling +infectious material, or care in cleansing the hands after such +handling, upon his habit of eating food cooked or raw, and upon +the condition of his skin and mucous membranes, since any kind of +bruises will increase susceptibility. Slight ailments, such as +colds, which inflame the mucous membrane, will decrease its +resisting power and render the individual more susceptible to the +entrance of any pathogenic germs should they happen to be present. +Sores in the mouth or decayed teeth may in the same way be +prominent factors in the individual's susceptibility. Thus quite a +number of purely physical factors may contribute to an +individual's susceptibility. + +Resisting Power of the Body.--Even after the bacteria get into the +body it is by no means certain that they will give rise to +disease, for they have now a battle to fight before they can be +sure of holding their own. It is now, indeed, that the actual +conflict between the powers of the body and these microscopic +invaders begins. After they have found entrance into the body the +bacteria have arrayed against them strong resisting forces of the +human organism, endeavouring to destroy and expel them. Many of +them are rapidly killed, and sometimes they are all destroyed +without being able to gain a foothold. In such cases, of course, +no trouble results. In other cases the body fails to overcome the +powers of the invaders and they eventually multiply rapidly. In +this struggle the success of the invaders is not necessarily a +matter of numbers. They are simply struggling to gain a position +in the body, where they can feed and grow. A few individuals may +be entirely sufficient to seize such a foothold, and then these by +multiplying may soon become indefinitely numerous. To protect +itself, therefore, the human body must destroy every individual +bacterium, or at least render them all incapable of growth. Their +marvellous reproductive powers give the bacteria an advantage in +the battle. On the other hand, it takes time even for these +rapidly multiplying beings to become sufficiently numerous to do +injury. There is thus an interval after their penetration into the +body when these invaders are weak in numbers. During this +interval--the period of incubation--the body may organize a +resistance sufficient to expel them. + +We do not as yet thoroughly understand the forces which the human +organism is able to array against these invading foes. Some of its +methods of defence are, however, already intelligible to us, and +we know enough, at all events, to give us an idea of the intensity +of the conflict that is going on, and of the vigorous and powerful +forces which the human organism is able to bring against its +invading enemies. + +In the first place, we notice that a majority of bacteria are +utterly unable to grow in the human body even if they do find +entrance. There are known to bacteriologists to-day many hundreds, +even thousands of species, but the vast majority of these find in +the human tissues conditions so hostile to their life that they +are utterly unable to grow therein. Human flesh or human blood +will furnish excellent food for them if the individual be dead, +but living human flesh and blood in some way exerts a repressing +influence upon them which is fatal to the growth of a vast +majority of species. Some few species, however, are not thus +destroyed by the hostile agencies of the tissues of the animal, +but are capable of growing and multiplying in the living body. +These alone are what constitute the pathogenic bacteria, since, of +course, these are the only bacteria which can produce disease by +growing in the tissues of an animal. The fact that the vast +majority of bacteria can not grow in the living organism shows +clearly enough that there are some conditions existing in the +living tissue hostile to bacterial life. There can be little +doubt, moreover, that it is these same hostile conditions, which +enable the body to resist the attack of the pathogenic species in +cases where resistance is successfully made. + +What are the forces arrayed against these invaders? The essential +nature of the battle appears to be a production of poisons and +counter poisons. It appears to be an undoubted fact that the first +step in repelling these bacteria is to flood them with certain +poisons which check their growth. In the blood and lymph of man +and other animals there are present certain products which have a +direct deleterious influence upon the growth of micro-organisms. +The existence of these poisons is undoubted, many an experiment +having directly attested to their presence in the blood of +animals. Of their nature we know very little, but of their +repressing influence upon bacterial growth we are sure. They have +been named alexines, and they are produced in the living tissue, +although as to the method of their production we are in ignorance. +By the aid of these poisons the body is able to prevent the growth +of the vast majority of bacteria which get into its tissues. +Ordinary micro-organisms are killed at once, for these alexines +act as antiseptics, and common bacteria can no more grow in the +living body than they could in a solution containing other poisons +Thus the body has a perfect protection against the majority of +bacteria. The great host of species which are found in water, +milk, air, in our mouths or clinging to our skin, and which are +almost omnipresent in Nature, are capable of growing well enough +in ordinary lifeless organic foods, but just as soon as they +succeed in finding entrance into living human tissue their growth +is checked at once by these antiseptic agents which are poured +upon them. Such bacteria are therefore not pathogenic germs, and +not sources of trouble to human health. + +There are, on the other hand, a few species of bacteria which may +be able to retain their lodgment in the body m spite of this +attempt of the individual to get rid of them. These, of course, +constitute the pathogenic species, or so called "disease germs". +Only such species as can overcome this first resistance can be +disease germs, for they alone can retain their foothold in the +body. + +But how do these species overcome the poisons, which kill the +other harmless bacteria? They, as well as the harmless forms, find +these alexines injurious to their growth, but in some way they are +able to counteract the poisons. In this general discussion of +poisons we are dealing with a subject which is somewhat obscure, +but apparently the pathogenic bacteria are able to overcome the +alexines of the body by producing in their turn certain other +products which neutralize the alexines, thus annulling their +action. These pathogenic bacteria, when they get into the body, +give rise at once to a group of bodies which have been named +lysines. These lysines are as mysterious to us as the alexines, +but they neutralize the effect of the alexines and thus overcome +the resistance the body offers to bacterial growth The invaders +can now multiply rapidly enough to get a lasting foothold in the +body and then soon produce the abnormal symptoms which we call +disease Pathogenic bacteria thus differ from the non-pathogenic +bacteria primarily in this power of secreting products which can +neutralize the ordinary effects of the alexines, and so overcome +the body's normal resistance to their parasitic life. + +Even if the bacteria do thus overcome the alexines the battle is +not yet over, for the individual has another method of defence +which is now brought into activity to check the growth of the +invading organisms. This second method of resistance is by means +of a series of active cells found in the blood, known as white +blood-corpuscles (Fig. 33 a, b). They are minute bits of +protoplasm present in the blood and lymph in large quantities. +They are active cells, capable of locomotion and able to crawl out +of the blood-vessels Not infrequently they are found to take into +their bodies small objects with which they come in contact. One of +their duties is thus to engulf minute irritating bodies which may +be in the tissues, and to carry them away for excretion. They thus +act as scavengers These corpuscles certainly have some agency in +warding off the attacks of pathogenic bacteria Very commonly they +collect in great numbers in the region of the body where invading +bacteria are found. Such invading bacteria exist upon them a +strong attraction, and the corpuscles leave the blood-vessels and +sometimes form a solid phalanx completely surrounding the invading +germs. Their collection at these points may make itself seen +externally by the phenomenon we call inflammation. + +There is no question that the corpuscles engage in conflict with +the bacteria when they thus surround them. There has been not a +little dispute, however, as to the method by which they carry on +the conflict. It has been held by some that the corpuscles +actually take the bacteria into their bodies, swallow them, as it +were, and subsequently digest them (Fig. 33 c, d, e). This idea +gave rise to the theory of phagocytosis, and the corpuscles were +consequently named phagocytes. The study of several years has, +however, made it probable that this is not the ordinary method by +which the corpuscles destroy the bacteria. According to our +present knowledge the method is a chemical one. These cells, when +they thus collect in quantities around the invaders, appear to +secrete from their own bodies certain injurious products which act +upon the bacteria much as do the alexines already mentioned. These +new bodies have a decidedly injurious effect upon the multiplying +bacteria; they rapidly check their growth, and, acting in union +with the alexines, may perhaps entirely destroy them. + +After the bacteria are thus killed, the white blood-corpuscles may +load themselves with their dead bodies and carry them away (Fig. +33 d, e). Sometimes they pass back into the blood stream and carry +the bacteria to various parts of the body for elimination. Not +infrequently the white corpuscles die in the contest, and then may +accumulate in the form of pus and make their way through the skin +to be discharged directly. The battle between these phagocytes and +the bacteria goes on vigorously. If in the end the phagocytes +prove too strong for the invaders, the bacteria are gradually all +destroyed, and the attack is repelled. Under these circumstances +the individual commonly knows nothing--of the matter. This +conflict has taken place entirely without any consciousness on his +part, and he may not even know that he has been exposed to the +attack of the bacteria. In other cases the bacteria prove too +strong for the phagocytes. They multiply too rapidly, and +sometimes they produce secretions which actually drive the +phagocytes away. Commonly, as already noticed, the corpuscles are +attracted to the point of invasion, but in some cases, when a +particularly deadly and vigorous species of bacteria invades the +body, the secretions produced by them are so powerful as actually +to drive the corpuscles away. Under these circumstances the +invading hosts have a chance to multiply unimpeded, to distribute +themselves over the body, and the disease rapidly follows as the +result of their poisoning action on the body tissues. + +It is plain, then, that the human body is not helpless in the +presence of the bacteria of disease, but that it is supplied with +powerful resistant forces. It must not be supposed, however, that +the outline of the action of these forces just given is anything +like a complete account of the matter; nor must it be inferred +that the resistance is in all respects exactly as outlined. The +subject has only recently been an object of investigation, and we +are as yet in the dark in regard to many of the facts. The future +may require us to modify to some extent even the brief outline +which has been given. But while we recognise this uncertainty in +the details, we may be assured of the general facts. The living +body has some very efficacious resistant forces which prevent most +bacteria from growing within its tissues, and which in large +measure may be relied upon to drive out the true pathogenic +bacteria. These resistant forces are in part associated with the +productions of body poisons, and are in part associated with the +active powers of special cells which have been called phagocytes. +The origin of the poisons and the exact method of action of the +phagocytes we may well leave to the future to explain. + +These resisting powers of the body will vary with conditions. It +is evident that they are natural powers, and they will doubtless +vary with the general condition of vigour of the individual. +Robust health, a body whose powers are strong, well nourished, and +vigorous, will plainly furnish the conditions for the greatest +resistance to bacterial diseases. One whose bodily activities are +weakened by poor nutrition can offer less resistance. The question +whether one shall suffer from a germ disease is not simply the +question whether he shall be exposed, or even the question whether +the bacteria shall find entrance into his body. It is equally +dependent upon whether he has the bodily vigour to produce +alexines in proper quantity, or to summon the phagocytes in +sufficient abundance and vigour to ward off the attack. We may do +much to prevent disease by sanitation, which aids in protecting +the individual from attack; but we must not forget that the other +half of the battle is of equal importance, and hence we must do +all we can to strengthen the resisting forces of the organism. + +RECOVERY FROM GERM DISEASES. + +These resisting forces are not always sufficient to drive off the +invaders. The organisms may retain their hold in the body for a +time and eventually break down the resistance. After this they may +multiply unimpeded and take entire possession of the body. As they +become more numerous their poisonous products increase and begin +to produce direct poisoning effects on the body. The incubation +period is over and the disease comes on. The disease now runs its +course. It becomes commonly more and more severe until a crisis is +reached. Then, unless the poisoning is so severe that death +occurs, the effects pass away and recovery takes place. + +But why should not a germ disease be always fatal? If the bacteria +thus take possession of the body and can grow there, why do they +not always continue to multiply until they produce sufficient +poison to destroy the life of the individual? Such fatal results +do, of course, occur, but in by far the larger proportion of cases +recovery finally takes place. + +Plainly, the body must have another set of resisting forces which +is concerned in the final recovery. Although weakened by the +poisoning and suffering from the disease, it does not yield the +battle, but somewhat slowly organizes a new attack upon the +invaders. For a time the multiplying bacteria have an unimpeded +course and grow rapidly; but finally their further increase is +checked, their vigour impaired, and after this they diminish in +numbers and are finally expelled from the body entirely. Of the +nature of this new resistance but little is yet known. We notice, +in the first place, that commonly after such a recovery the +individual has decidedly increased resistance to the disease. This +increased resistance may be very lasting, and may be so +considerable as to give almost complete immunity from the disease +for many years, or for life. One attack of scarlet fever gives the +individual great immunity for the future. On the other hand, the +resistance thus derived may be very temporary, as in the case of +diphtheria. But a certain amount of resistance appears to be +always acquired. This power of resisting the activities of the +parasites seems to be increased during the progress of the +disease, and, if it becomes sufficient, it finally drives off the +bacteria before they have produced death. After this, recovery +takes place. To what this newly acquired resisting power is due is +by no means clear to bacteriologists, although certain factors +are already known. It appears beyond question that in the case of +certain diseases the cells of the body after a time produce +substances which serve as antidotes to the poisons produced by the +bacteria during their growth in the body-antitoxines. In the case +of diphtheria, for instance, the germs growing in the throat +produce poisons which are absorbed by the body and give rise to +the symptoms of the disease; but after a time the body cells +react, and themselves produce a counter toxic body which +neutralizes the poisonous effect of the diphtheria poison. This +substance has been isolated from the blood of animals that have +recovered from an attack of diphtheria, and has been called +diphtheria antitoxine. But even with this knowledge the recovery +is not fully explained. This antitoxine neutralizes the effects of +the diphtheria toxine, and then the body develops strength to +drive off the bacteria which have obtained lodgment in the throat. +How they accomplish this latter achievement we do not know as yet. +The antitoxme developed simply neutralizes the effects of the +toxine. Some other force must be at work to get rid of the +bacteria, a force which can only exert itself after the poisoning +effect of the poison is neutralized. In these cases, then, the +recovery is due, first, to the development in the body of the +natural antidotes to the toxic poisons, and, second, to some other +unknown force which drives off the parasites. + +These facts are certainly surprising. If one had been asked to +suggest the least likely theory to explain recovery from disease, +he could hardly have found one more unlikely than that the body +cells developed during the disease an antidote to the poison which +the disease bacteria were producing. Nevertheless, it is beyond +question that such antidotes are formed during the course of the +germ diseases. It has not yet been shown in all diseases, and it +would be entirely too much to claim that this is the method of +recovery in all cases. We may say, however, in regard to bacterial +diseases in general, that after the bacteria enter the body at +some weak point they have first a battle to fight with the +resisting powers of the body, which appear to be partly biological +and partly chemical. These resisting powers are in many cases +entirely sufficient to prevent the bacteria from obtaining a +foothold. If the invading host overcome the resisting powers, then +they begin to multiply rapidly, and take possession of the body or +some part of it. They continue to grow until either the individual +dies or something occurs to check their growth. After the +individual develops the renewed powers of checking their growth, +recovery takes place, and the individual is then, because of these +renewed powers of resistance, immune from a second attack of the +disease for a variable length of time. + +This, in the merest outline, represents the relation of bacterial +parasites to the human body But while this is a fair general +expression of the matter, it must be recognised that different +diseases differ much in their relations, and no general outline +will apply to all They differ in their method of attack and in the +point of attack. Not only do they produce different kinds of +poisons giving rise to different symptoms of poisoning; not only +do they produce different results in different animals; not only +do the different pathogenic species differ much in their power +to develop serious disease, but the different species are very +particular as to what species of animal they attack. Some of them +can live as parasites in man alone; some can live as parasites +upon man and the mouse and a few other animals; some can live in +various animals but not in man; some appear to be able to live in +the field mouse, but not in the common mouse; some live in the +horse; some in birds, but not in warm-blooded mammals; while +others, again, can live almost equally well in the tissues of a +long list of animals. Those which can live as parasites upon man +are, of course, especially related to human disease, and are of +particular interest to the physician, while those which live in +animals are in a similar way of interest to veterinarians. + +Thus we see that parasitic bacteria show the widest variations. +They differ in point of attack, in method of attack, and in the +part of the body which they seize upon as a nucleus for growth. +They differ in violence and in the character of the poisons they +produce, as well as in their power of overcoming the resisting +powers of the body. They differ at different times in their powers +of producing disease. In short, they show such a large number of +different methods of action that no general statements can be made +which will apply universally, and no one method of guarding +against them or in driving them off can be hoped to apply to any +extended list of diseases. + +DISEASES CAUSED BY OTHER ORGANISMS THAN BACTERIA. + +Although the purpose of this work is to deal primarily with the +bacterial world, it would hardly be fitting to leave the subject +without some reference to diseases caused by organisms which do +not belong to the group of bacteria. While most of the so-called +germ diseases are caused by the bacteria which we have been +studying in the previous chapters, there are some whose inciting +cause is to be found among organisms belonging to other groups. +Some of these are plants of a higher organization than bacteria, +but others are undoubtedly microscopic animals. Their life habits +are somewhat different from those of bacteria, and hence the +course of the diseases is commonly different. Of the diseases thus +produced by microscopic animals or by higher plants, one or two +are of importance enough to deserve special mention here. + +Malaria.--The most important of these diseases is malaria in its +various forms, and known under various names--chills and fever, +autumnal fever, etc. This disease, so common almost everywhere, +has been studied by physicians and scientists for a long time, and +many have been the causes assigned to it. At one time it was +thought to be the result of the growth of a bacterium, and a +distinct bacillus was described as producing it. It has finally +been shown, however, to be caused by a microscopic organism +belonging to the group of unicellular animals, and somewhat +closely related to the well-known amoeba. This organism is shown +in Fig. 34. The whole history of the malarial organism is not yet +known. The following statements comprise the most important facts +known in regard to it, and its relation to the disease in man. + +Undoubtedly the malarial germ has some home outside the human +body, but it is not yet very definitely known what this external +home is; nor do we know from what source the human parasite is +derived. It appears probable that water serves in some cases as +its means of transference to man, and air in other cases. From +some external source it gains access to man and finds its way into +the blood. Here it attacks the red blood-corpuscles, each malarial +organism making its way into a single one (Fig. 340). Here it now +grows, increasing in size at the expense of the substance of the +corpuscle. As it becomes larger it becomes granular, and soon +shows a tendency to separate into a number of irregular masses. +Finally it breaks up into many minute bodies called spores. These +bodies break out of the corpuscle and for a time live a free life +in the blood. After a time they make their way into other red +blood-corpuscles, develop into new malarial amoeboid parasites, +and repeat the growth and sporulation. This process can apparently +be repeated many times without check. + +These organisms are thus to be regarded as parasites of the red +corpuscles. It is, of course, easy to believe that an extensive +parasitism and destruction of the corpuscles would be disastrous +to the health of the individual, and the severity of the disease +will depend upon the extent of the parasitism. Corresponding to +this life history of the organism, the disease malaria is commonly +characterized by a decided intermittency, periods of chill and +fever alternating with periods of intermission in which these +symptoms are abated. The paroxysms of the disease, characterized +by the chill, occur at the time that the spores are escaping from +the blood-corpuscles and floating in the blood. After they have +again found their way into a blood-corpuscle the fever diminishes, +and during their growth in the corpuscle until the next +sporulation the individual has a rest from the more severe +symptoms. + +There appears to be more than one variety of the malarial +organism, the different types differing in the length of time it +takes for their growth and sporulation. There is one variety, the +most common one, which requires two days for its growth, thus +giving rise to the paroxysm of the disease about once in forty- +eight hours; another variety appears to require three days for its +growth; while still another variety appears to be decidedly +irregular in its period of growth and sporulation. These facts +readily explain some of the variations in the disease. Certain +other irregularities appear to be due to a different cause. More +than one brood of parasites may be in the blood of the individual +at the same time, one producing sporulation at one time and +another at a different time. Such a simultaneous growth of two +independent broods may plainly produce almost any kind of +modification in the regularity of the disease. + +The malarial organism appears to be very sensitive to quinine, a +very small quantity being sufficient to kill it. Upon this point +depends the value of quinine as a medicine. If the drug be present +in the blood at the time when the spores are set free from the +blood-corpuscle, they are rapidly killed by it before they have a +chance to enter another corpuscle. During their growth in the +corpuscle they are far less sensitive to quinine than when they +exist in the free condition as spores, and at this time the drug +has little effect. + +The malarial organism is an animal, and can not be cultivated in +the laboratory by any artificial method yet devised. Its whole +history is therefore not known. It doubtless has some home outside +the blood of animals, and very likely it may pass through other +stages of a metamorphosis in the bodies of other animals. Most +parasitic animals have two or more hosts upon which they live, +alternating from one to the other, and that such is the case with +the malarial parasite is at least probable. But as yet +bacteriologists have been unable to discover anything very +definite in regard to the matter. Until we can learn something in +regard to its life outside the blood of man we can do little in +the way of devising methods to avoid it. + +Malaria differs from most germ diseases in the fact that the +organisms which produce it are not eliminated from the body in any +way. In most germ diseases the germs are discharged from the +patient by secretions or excretions of some kind, and from these +excretions may readily find their way into other individuals. The +malarial organism is not discharged from the body in any way, and +hence is not contagious. If the parasite does pass part of its +history in some other animal than man, there must be some means by +which it passes from man to its other host. It has been suggested +that some of the insects which feed upon human blood may serve as +the second host and become inoculated when feeding upon such +blood. This has been demonstrated with startling success in regard +to the mosquito (Anopheles), some investigators going so far as to +say that this is the only way in which the disease can be +communicated. + +Several other microscopic animals occur as parasites upon man, and +some of them are so definitely associated with certain diseases as +to lead to the belief that they are the cause of these diseases. +The only one of very common occurrence is a species known as +Amaeba coli, which is found in cases of dysentery. In a certain +type of dysentery this organism is so universally found that there +is little doubt that it is in some very intimate way associated +with the cause of the disease. Definite proof of the matter is, +however, as yet wanting. + +On the side of plants, we find that several plants of a higher +organization than bacteria may become parasitic upon the body of +man and produce various types of disease. These plants belong +mostly to the same group as the moulds, and they are especially +apt to attack the skin. They grow in the skin, particularly under +the hair, and may send their threadlike branches into some of the +subdermal tissues. This produces irritation and inflammation of +the skin, resulting in trouble, and making sores difficult to +heal. So long as the plant continues to grow, the sores, of +course, can not be healed, and when the organisms get into the +skin under the hair it is frequently difficult to destroy them. +Among the diseases thus caused are ringworm, thrush, alopecia, +etc. + + + + + +CHAPTER VI. + +METHODS OF COMBATING PARASITIC BACTERIA. + + +The chief advantage of knowing the cause of disease is that it +gives us a vantage ground from which we may hope to find means of +avoiding its evils. The study of medicine in the past history of +the world has been almost purely empirical, with a very little of +scientific basis. Great hopes are now entertained that these new +facts will place this matter upon a more strictly scientific +foundation. Certainly in the past twenty-five years, since +bacteriology has been studied, more has been done to solve +problems connected with disease than ever before. This new +knowledge has been particularly directed toward means of avoiding +disease. Bacteriology has thus far borne fruit largely in the line +of preventive medicine, although to a certain extent also along +the line of curative medicine. This chapter will be devoted to +considering how the study of bacteriology has contributed directly +and indirectly to our power of combating disease. + +PREVENTIVE MEDICINE. + +In the study of medicine in the past centuries the only aim has +been to discover methods of curing disease; at the present time a +large and increasing amount of study is devoted to the methods of +preventing disease. Preventive medicine is a development of the +last few years, and is based almost wholly upon our knowledge of +bacteria. This subject is yearly becoming of more importance. +Forewarned is forearmed, and it has been found that to know the +cause of a disease is a long step toward avoiding it. As some of +our contagious and epidemic diseases have been studied in the +light of bacteriological knowledge, it has been found possible to +determine not only their cause, but also how infection is brought +about, and consequently how contagion may be avoided. Some of the +results which have grown up so slowly as to be hardly appreciated +are really great triumphs. For instance, the study of bacteriology +first led us to suspect, and then demonstrated, that tuberculosis +is a contagious disease, and from the time that this was thus +proved there has been a slow, but, it is hoped, a sure decline in +this disease. Bacteriological study has shown that the source of +cholera infection in cases of raging epidemics is, in large part +at least, our drinking water; and since this has been known, +although cholera has twice invaded Europe, and has been widely +distributed, it has not obtained any strong foothold or given rise +to any serious epidemic except in a few cases where its ravages +can be traced to recognised carelessness. It is very significant +to compare the history of the cholera epidemics of the past few +years with those of earlier dates. In the epidemics of earlier +years the cholera swept ruthlessly through communities without +check. In the last few years, although it has repeatedly knocked +at the doors of many European cities, it has been commonly +confined to isolated cases, except in a few instances where these +facts concerning the relation to drinking water were ignored. + +The study of preventive medicine is yet in its infancy, but it has +already accomplished much. It has developed modern systems of +sanitation, has guided us in the building of hospitals, given +rules for the management of the sick-room which largely prevent +contagion from patient to nurse; it has told us what diseases are +contagious, and in what way; it has told us what sources of +contagion should be suspected and guarded against, and has thus +done very much to prevent the spread of disease. Its value is seen +in the fact that there has been a constant decrease in the death +rate since modern ideas of sanitation began to have any influence, +and in the fact that our general epidemics are less severe than in +former years, as well as in the fact that more people escape the +diseases which were in former times almost universal. + +The study of preventive medicine takes into view several factors, +all connected with the method and means of contagion. They are the +following: + +The Source of Infectious Material.--t has been learned that for +most diseases the infectious material comes from individuals +suffering with the disease, and that except in a few cases, like +malaria, we must always look to individuals suffering from disease +for all sources of contagion. It is found that pathogenic bacteria +are in all these cases eliminated from the patient in some way, +either from the alimentary canal or from skin secretions or +otherwise, and that any nurse with common sense can have no +difficulty in determining in what way the infectious material is +eliminated from her patients. When this fact is known and taken +into consideration it is a comparatively easy matter to devise +valuable precautions against distribution of such material. It is +thus of no small importance to remember that the simple presence +of bacteria in food or drink is of no significance unless these +bacteria have come from some source of disease infection. + +The Method of Distribution.--The bacteria must next get from the +original source of the disease to the new susceptible individual. +Bacteria have no independent powers of distribution unless they be +immersed in liquids, and therefore their passage from individual +to individual must be a passive one. They are readily +transferred, however, by a number of different means, and the +study of these means is aiding much in checking contagion Study +along this line has shown that the means by which bacteria are +carried are several. First we may notice food as a distributor. +Food may become contaminated by infectious material in many ways; +for example, by contact with sewage, or with polluted water, or +even with eating utensils which have been used by patients. Water +is also likely to be contaminated with infectious material, and is +a fertile source for distributing typhoid and cholera. Milk may +become contaminated in a variety of ways, and be a source of +distributing the bacteria which produce typhoid fever, +tuberculosis, diphtheria, scarlet fever, and a few other less +common diseases. Again, infected clothing, bedding, or eating +utensils may be taken from a patient and be used by another +individual without proper cleansing. Direct contact, or contact +with infected animals, furnishes another method. Insects sometimes +carry the bacteria from person to person, and in some diseases +(tuberculosis, and perhaps scarlet fever and smallpox) we must +look to the air as a distributor of the infectious material. +Knowledge of these facts is helping to account for multitudes of +mysterious cases of infection, especially when we combine them +with the known sources of contagious matter. + +Means of Invasion.--Bacteriology has shown us that different +species of parasitic bacteria have different means of entering the +body, and that each must enter the proper place in order to get a +foothold. After we learn that typhoid infectious material must +enter the mouth in order to produce the disease; that tuberculosis +may find entrance through the nose in breathing, while types of +blood poisoning enter only through wounds or broken skin, we learn +at once fundamental facts as to the proper methods of meeting +these dangers. We learn that with some diseases care exercised to +prevent the swallowing of infectious material is sufficient to +prevent contagion, while with others this is entirely +insufficient. When all these facts are understood it is almost +always perfectly possible to avoid contagion; and as these facts +become more and more widely known direct contagion is sure to +become less frequent. + +Above all, it is telling us what becomes of the pathogenic +bacteria after being eliminated from the body of the patient; how +they may exist for a long time still active; how they may lurk in +filth or water dormant but alive, or how they may even multiply +there. Preventive medicine is telling us how to destroy those thus +lying in wait for a chance of infection, by discovering +disinfectants and telling us especially where and when to use +them. It has already taught us how to crush out certain forms of +epidemics by the proper means of destroying bacteria, and is +lessening the dangers from contagious diseases. In short, the +study of bacteriology has brought us into a condition where we are +no longer helpless in the presence of a raging epidemic. We no +longer sit in helpless dismay, as did our ancestors, when an +epidemic enters a community, but, knowing their causes and +sources, set about at once to remove them. As a result, severe +epidemics are becoming comparatively short-lived. + +BACTERIA IN SURGERY. + +In no line of preventive medicine has bacteriology been of so much +value and so striking in its results as in surgery. Ever since +surgery has been practised surgeons have had two difficulties to +contend with. The first has been the shock resulting from the +operation. This is dependent upon the extent of the operation, and +must always be a part of a surgical operation. The second has been +secondary effects following the operation. After the operation, +even though it was successful, there were almost sure to arise +secondary complications known as surgical fever, inflammation, +blood poisoning, gangrene, etc., which frequently resulted +fatally. These secondary complications were commonly much more +serious than the shock of the operation, and it used to be the +common occurrence for the patient to recover entirely from the +shock, but yield to the fevers which followed. They appeared to be +entirely unavoidable, and were indeed regarded as necessary parts +of the healing of the wound. Too frequently it appeared that the +greater the care taken with the patient the more likely he was to +suffer from some of these troubles. The soldier who was treated on +the battlefield and nursed in an improvised field hospital would +frequently recover, while the soldier who had the fortune to be +taken into the regular hospital, where greater care was possible, +succumbed to hospital gangrene. All these facts were clearly +recognised, but the surgeon, through ignorance of their cause, was +helpless in the presence of these inflammatory troubles, and felt +it always necessary to take them into consideration. + +The demonstration that putrefaction and decay were caused by +bacteria, and the early proof that the silkworm disease was +produced by a micro-organism, led to the suggestion that the +inflammatory diseases accompanying wounds were similarly caused. +There are many striking similarities between these troubles and +putrefaction, and the suggestion was an obvious one. At first, +however, and for quite a number of years, it was impossible to +demonstrate the theory by finding the distinct species of micro- +organisms which produced the troubles. We have already seen that +there are several different species of bacteria which are +associated with this general class of diseases, but that no +specific one has any particular relation to a definite type of +inflammation. This fact made discoveries in this connection a slow +matter from the microscopical standpoint. But long before this +demonstration was finally reached the theory had received +practical application in the form of what has developed into +antiseptic or aseptic surgery. + +Antiseptic surgery is based simply upon the attempt to prevent the +entrance of bacteria into the surgical wound. It is assumed that +if these organisms are kept from the wound the healing will take +place without the secondary fevers and inflammations which occur +if they do get a chance to grow in the wound. The theory met with +decided opposition at first, but accumulating facts demonstrated +its value, and to-day its methods have been adopted everywhere in +the civilized world. As the evidence has been accumulating, +surgeons have learned many important facts, foremost among which +is a knowledge of the common sources from which the infection of +wounds occurs. At first it was thought that the air was the great +source of infection, but the air bacteria have been found to be +usually harmless. It has appeared that the more common sources are +the surgeon's instruments, or his hands, or the clothing or +sponges which are allowed to come in contact with the wounds. It +has also appeared that the bacteria which produce this class of +troubles are common species, existing everywhere and universally +present around the body, clinging to the clothing or skin, and +always on hand to enter the wound if occasion offers. They are +always present, but commonly harmless. They are not foreign +invaders like the more violent pathogenic species, such as those +of Asiatic cholera, but may be compared to domestic enemies at +hand. It is these ever-present bacteria which the surgeon must +guard against. The methods by which he does this need not detain +us here. They consist essentially in bacteriological cleanliness. +The operation is performed with sterilized instruments under most +exacting conditions of cleanliness. + +The result has been a complete revolution in surgery. As the +methods have become better understood and more thoroughly adopted, +the instances of secondary troubles following surgical wounds have +become less and less frequent until they have practically +disappeared in all simple cases. To-day the surgeon recognises +that when inflammatory troubles of this sort follow simple +surgical wounds it is a testimony to his carelessness. The skilful +surgeon has learned that with the precautions which he is able to +take to-day he has to fear only the direct effect of the shock of +the wound and its subsequent direct influence; but secondary +surgical fevers, blood poisoning, and surgical gangrene need not +be taken into consideration at all. Indeed, the modern surgeon +hardly knows what surgical gangrene is, and bacteriologists have +had practically no chance to study it. Secondary infections have +largely disappeared, and the surgeon is concerned simply with the +effect of the wound itself, and the power of the body to withstand +the shock and subsequently heal the wound. + +With these secondary troubles no longer to disturb him, the +surgeon has become more and more bold. Operations formerly not +dreamed of are now performed without hesitation. In former years +an operation which opened the abdominal cavity was not thought +possible, or at least it was so nearly certain to result fatally +that it was resorted to only on the last extremity; while to-day +such operations are hardly regarded as serious. Even brain surgery +is becoming more and more common. Possibly our surgeons are +passing too far to the other extreme, and, feeling their power of +performing so many operations without inconvenience or danger, +they are using the knife in cases where it would be better to +leave Nature to herself for her own healing. But, be this as it +may, it is impossible to estimate the amount of suffering +prevented and the number of lives saved by the mastery of the +secondary inflammatory troubles which used to follow surgical +wounds. + +Preventive medicine, then, has for its object the prevention +rather than the cure of disease. By showing the causes of disease +and telling us where and how they are contracted, it is telling us +how they may to a large extent be avoided. Unlike practical +medicine, this subject is one which has a direct relation to the +general public. While it may be best that the knowledge of +curative methods be confined largely to the medical profession, it +is eminently desirable that a knowledge of all the facts bearing +upon preventive medicine should be distributed as widely as +possible. One person can not satisfactorily apply his knowledge of +preventive medicine, if his neighbour is ignorant of or careless +of the facts. We can not hope to achieve the possibilities lying +along this line until there is a very wide distribution of +knowledge. Every epidemic that sweeps through our communities is a +testimony to the crying need of education in regard to such simple +facts as the source of infectious material, the methods of its +distribution, and the means of rendering it harmless. + +PREVENTION IN INOCULATION. + +It has long been recognised that in most cases recovery from one +attack of a contagious disease renders an individual more or less +immune against a second attack. It is unusual for an individual to +have the same contagious disease twice. This belief is certainly +based upon fact, although the immunity thus acquired is subject to +wide variations. There are some diseases in which there is little +reason for thinking that any immunity is acquired, as in the case +of tuberculosis, while there are others in which the immunity is +very great and very lasting, as in the case of scarlet fever. +Moreover, the immunity differs with individuals. While some +persons appear to acquire a lasting immunity by recovery from a +single attack, others will yield to a second attack very readily. +But in spite of this the fact of such acquired immunity is beyond +question. Apparently all infectious diseases from which a real +recovery takes place are followed by a certain amount of +protection from a second attack; but with some diseases the +immunity is very fleeting, while with others it is more lasting. +Diseases which produce a general infection of the whole system +are, as a rule, more likely to give rise to a lasting immunity +than those which affect only small parts. Tuberculosis, which, as +already noticed, is commonly quite localized in the body, has +little power of conveying immunity, while a disease like scarlet +fever, which affects the whole system, conveys a more lasting +protection. + +Such immunity has long been known, and in the earlier years was +sometimes voluntarily acquired; even to-day we find some +individuals making use of the principle. It appears that a mild +attack of such diseases produces immunity equally well with a +severe attack, and acting upon this fact mothers have not +infrequently intentionally exposed their children to certain +diseases at seasons when they are mild, in order to have the +disease "over with" and their children protected in the future. +Even the more severe diseases have at times been thus voluntarily +acquired. In China it has sometimes been the custom thus to +acquire smallpox. Such methods are decidedly heroic, and of course +to be heartily condemned. But the principle that a mild type of +the disease conveys protection has been made use of in a more +logical and defensible way. + +The first instance of this principle was in vaccination against +smallpox, now practised for more than a century. Cowpox is +doubtless closely related to smallpox, and an attack of the former +conveys a certain amount of protection against the latter. It was +easy, therefore, to inoculate man with some of the infectious +material from cowpox, and thus give him some protection against +the more serious smallpox. This was a purely empirical discovery, +and vaccination was practised long before the principle underlying +it was understood, and long before the germ nature of disease was +recognised. The principle was revived again, however, by Pasteur, +and this time with a logical thought as to its value. While +working upon anthrax among animals, he learned that here, as in +other diseases, recovery, when it occurred, conveyed immunity. +This led him to ask if it were not possible to devise a method of +giving to animals a mild form of the disease and thus protect them +from the more severe type. The problem of giving a mild type of +this extraordinarily severe disease was not an easy one. It could +not be done, of course, by inoculating the animals with a small +number of the bacteria, for their power of multiplication would +soon make them indefinitely numerous. It was necessary in some way +to diminish their violence. Pasteur succeeded in doing this by +causing them to grow in culture fluids for a time at a high +temperature. This treatment diminished their violence so much that +they could be inoculated into cattle, where they produced only the +mildest type of indisposition, from which the animals speedily +recovered. But even this mild type of the disease was triumphantly +demonstrated to protect the animals from the most severe form of +anthrax. The discovery was naturally hailed as a most remarkable +one, and one which promised great things in the future. If it was +thus possible, by direct laboratory methods, to find a means of +inoculating against a serious disease like anthrax, why could not +the same principle be applied to human diseases? The enthusiasts +began at once to look forward to a time when all diseases should +be thus conquered. + +But the principle has not borne the fruit at first expected. There +is little doubt that it might be applied to quite a number of +human diseases if a serious attempt should be made. But several +objections arise against its wide application. In the first place, +the inoculation thus necessary is really a serious matter. Even +vaccination, as is well known, sometimes, through faulty methods, +results fatally, and it is a very serious thing to experiment upon +human beings with anything so powerful for ill as pathogenic +bacteria. The seriousness of the disease smallpox, its +extraordinary contagiousness, and the comparatively mild results +of vaccination, have made us willing to undergo vaccination at +times of epidemics to avoid the somewhat great probability of +taking the disease. But mankind is unwilling to undergo such an +operation, even though mild, for the purpose of avoiding other +less severe diseases, or diseases which are less likely to be +taken. We are unwilling to be inoculated against mild diseases, or +against the more severe ones which are uncommon. For instance, a +method has been devised for rendering animals immune against +lockjaw, which would probably apply equally well to man. But +mankind in general will never adopt it, since the danger from +lockjaw is so small. Inoculation must then be reserved for +diseases which are so severe and so common, or which occur in +periodical epidemics of so great severity, as to make people in +general willing to submit to inoculation as a protection. A +further objection arises from the fact that the immunity acquired +is not necessarily lasting. The cattle inoculated against anthrax +retain their protective powers for only a few months. How long +similar immunity might be retained in other cases we can not say, +but plainly this fact would effectually prevent this method of +protecting mankind from being used except in special cases. It is +out of the question to think of constant and repeated inoculations +against various diseases. + +As a result, the principle of inoculation as an aid in preventive +medicine has not proved of very much value. The only other human +disease in which it has been attempted seriously is Asiatic +cholera. This disease in times of epidemics is so severe and the +chance of infection is so great as to justify such inoculation. +Several bacteriologists have in the last few years been trying to +discover a harmless method of inoculating against this disease. +Apparently they have succeeded, for experiments in India, the home +of the cholera, have been as successful as could be anticipated. +Bacteriological science has now in its possession a means of +inoculation against cholera which is perhaps as efficacious as +vaccination is against smallpox. Whether it will ever be used to +any extent is doubtful, since, as already pointed out, we are in a +position to avoid cholera epidemics by other means. If we can +protect our communities by guarding the water supply, it is not +likely that the method of inoculation will ever be widely used. + +Another instance of the application of preventive inoculation has +been made, but one based upon a different principle. Hydrophobia +is certainly one of the most horrible of diseases, although +comparatively rare. Its rarity would effectually prevent mankind +from submitting to a general inoculation against it, but its +severity would make one who had been exposed to it by the bite of +a rabid animal ready to submit to almost any treatment that +promised to ward off the disease. In the attempt to discover a +means of inoculating against this disease it was necessary, +therefore, to find a method that could be applied after the time +of exposure--i.e., after the individual had been bitten by the +rabid animal. Fortunately, the disease has a long period of +incubation, and one that has proved long enough for the purpose. A +method of inoculation against this disease has been devised by +Pasteur, which can be applied after the individual has been bitten +by the rabid animal. Apparently, however, this preventive +inoculation is dependent upon a different principle from +vaccination or inoculation against anthrax. It does not appear to +give rise to a mild form of the disease, thus protecting the +individual, but rather to an acquired tolerance of the chemical +poisons produced by the disease. It is a well-known physiological +fact that the body can become accustomed to tolerate poisons if +inured to them by successively larger and larger doses. It is by +this power, apparently, that the inoculation against hydrophobia +produces its effect. Material containing the hydrophobia poison +(taken from the spinal cord of a rabbit dead with the disease) is +injected into the individual after he has been bitten by a rabid +animal. The poisonous material in the first injection is very +weak, but is followed later by a more powerful inoculation. The +result is that after a short time the individual has acquired the +power of resisting the hydrophobia poisons. Before the incubation +period of the original infectious matter from the bite of the +rabid animal has passed, the inoculated individual has so +thoroughly acquired a tolerance of the poison that he successfully +resists the attack of the infection. This method of inoculation +thus neutralizes the effects of the disease by anticipating them. + +The method of treatment of hydrophobia met with extraordinarily +violent opposition. For several years it was regarded as a +mistake. But the constantly accumulating statistics from the +Pasteur Institute have been so overwhelmingly on one side as to +quiet opposition and bring about a general conviction that the +method is a success. + +The method of preventive inoculation has not been extensively +applied to human diseases in addition to those mentioned. In a few +cases a similar method has been used to guard against diphtheria. +Among animals, experiment has shown that such methods can quite +easily be obtained, and doubtless the same would be true of +mankind if it was thought practical or feasible to apply them. +But, for reasons mentioned, this feature of preventive medicine +will always remain rather unimportant, and will be confined to a +few of the more violent diseases. + +It may be well to raise the question as to why a single attack +with recovery conveys immunity. This question is really a part of +the one already discussed as to the method by which the body cures +disease. We have seen that this is in part due to the development +of chemical substances which either neutralize the poisons or act +as germicide upon the bacteria, or both, and perhaps due in part +to an active destruction of bacteria by cellular activity +(phagocytosis). There is little reason to doubt that it is the +same set of activities which renders the animal immune. The forces +which drive off the invading bacteria in one case are still +present to prevent a second attack of the same species of +bacterium. The length of time during which these forces are active +and sufficient to cope with any new invaders determines the length +of time during which the immunity lasts. Until, therefore, we can +answer with more exactness just how cure is brought about in case +of disease, we shall be unable to explain the method of immunity. + +LIMITS OF PREVENTIVE MEDICINE. + +With all the advance in preventive medicine we can not hope to +avoid disease entirely. We are discovering that the sources of +disease are on all sides of us, and so omnipresent that to avoid +them completely is impossible. If we were to apply to our lives +all the safeguards which bacteriology has taught us should be +applied in order to avoid the different diseases, we would +surround ourselves with conditions which would make life +intolerable. It would be oppressive enough for us to eat no food +except when it is hot, to drink no water except when boiled, and +to drink no milk except after sterilization; but these would not +satisfy the necessary conditions for avoiding disease. To meet all +dangers, we should handle nothing which has not been sterilized, +or should follow the handling by immediately sterilizing the +hands; we should wear only disinfected clothes, we should never +put our fingers in our mouths or touch our food with them; we +should cease to ride in public conveyances, and, indeed, should +cease to breathe common air. Absolute prevention of the chance of +infection is impossible. The most that preventive medicine can +hope for is to point out the most common and prolific sources of +infection, and thus enable civilized man to avoid some of his most +common troubles. It becomes a question, therefore, where we will +best draw the line in the employment of safeguards. Shall we drink +none except sterilized milk, and no water unless boiled? or shall +we put these occasional sources of danger in the same category +with bicycle and railroad accidents, dangers which can be avoided +by not using the bicycle or riding on the rail, but in regard to +which the remedy is too oppressive for application? + +Indeed, when viewed in a broad philosophical light it may not be +the best course for mankind to shun all dangers. Strength in the +organism comes from the use rather than the disuse of our powers. +It is certain that the general health and vigour of mankind is to +be developed by meeting rather than by shunning dangers. +Resistance to disease means bodily vigour, and this is to be +developed in mankind by the application of the principle of +natural selection. In accordance with this principle, disease will +gradually remove the individuals of weak resisting powers, leaving +those of greater vigour. Parasitic bacteria are thus a means of +preventing the continued life of the weaker members of the +community, and so tend to strengthen mankind. By preventive +medicine many a weak individual who would otherwise succumb +earlier in the struggle is enabled to live a few years longer. +Whatever be our humanitarian feeling for the individual, we can +not fail to admit that this survival of the weak is of no benefit +to the race so far as the development of physical nature is +concerned. Indeed, if we were to take into consideration simply +the physical nature of man we should be obliged to recommend a +system such as the ancient Spartans developed, of exposing to +death all weakly individuals, that only the strong might live to +become the fathers of future generations. In this light, of +course, parasitic diseases would be an assistance rather than a +detriment to the human race. Of course such principles will never +again be dominant among men, and our conscience tells us to do all +we can to help the weak. We shall doubtless do all possible to +develop preventive medicine in order to guard the weak against +parasitic organisms. But it is at all events well for us to +remember that we can never hope to develop the strength of the +human race by shunning evil, but rather by combating it, and the +power of the human race to resist the invasions of these organisms +will never be developed by the line of action which guards us from +attack. Here, as in other directions, the principles of modern +humanity have, together with their undoubted favourable influence +upon mankind, certain tendencies toward weakness. While we shall +still do our utmost to develop preventive medicine in a proper +way, it may be well for us to remember these facts when we come to +the practical question of determining where to draw the limits of +the application of methods for preventing infectious diseases. + +CURATIVE MEDICINE. + +Bacteriology has hitherto contributed less to curative than to +preventive medicine. Nevertheless, its contributions to curative +medicine have not been unimportant, and there is promise of much +more in the future. It is, of course, unsafe to make predictions +for the future, but the accomplishments of the last few years give +much hope as to further results. + +DRUGS. + +It was at first thought that a knowledge of the specific bacteria +which cause a disease would give a ready means of finding specific +drugs for the cure of such disease. If a definite species of +bacterium causes a disease and we can cultivate the organism in +the laboratory, it is easy to find some drugs which will be fatal +to its growth, and these same drugs, it would seem, should be +valuable as medicines in these diseases. This hope has, however, +proved largely illusive. It is very easy to find some drug which +proves fatal to the specific germs while growing in the culture +media of the laboratory, but commonly these are of little or no +use when applied as medicines. In the first place, such substances +are usually very deadly poisons. Corrosive sublimate is a +substance which destroys all pathogenic germs with great rapidity, +but it is a deadly poison, and can not be used as a drug in +sufficient quantity to destroy the parasitic bacteria in the body +without at the same time producing poisonous effects on the body +itself. It is evident that for any drug to be of value in thus +destroying bacteria it must have some specially strong action upon +the bacteria. Its germicide action on the bacteria should be so +strong that a dose which would be fatal or very injurious to them +would be too small to have a deleterious influence on the body of +the individual. It has not proved an easy task to discover drugs +which will have any value as germicides when used in quantities so +small as to produce no injurious effect on the body. + +A second difficulty is in getting the drug to produce its effect +at the right point. A few diseases, as we have noticed, are +produced by bacteria which distribute themselves almost +indiscriminately over the body; but the majority are somewhat +definitely localized in special points. Tuberculosis may attack a +single gland or a single lobe of the lung. Typhoid germ is +localized in the intestines, liver, spleen, etc. Even if it were +possible to find some drug which would have a very specific effect +upon the tuberculosis bacillus, it is plain that it would be a +very questionable method of procedure to introduce this into the +whole system simply that it might have an effect upon a very small +isolated gland. Sometimes such a bacterial affection may be +localized in places where it can be specially treated, as in the +case of an attack on a dermal gland, and in these cases some of +the germicides have proved to be of much value. Indeed, the use of +various disinfectants connected with abscesses and superficial +infections has proved of much value. To this extent, in +disinfecting wounds and as a local application, the development of +our knowledge of disinfectants has given no little aid to curative +medicine. + +Very little success, however, has resulted in the attempt to find +specific drugs for specific diseases, and it is at least doubtful +whether many such will ever be found. The nearest approach to it +is quinine as a specific poison for malarial troubles. Malarious +diseases are not, however, produced by bacteria but by a +microscopic organism of a very different nature, thought to be an +animal rather than a plant. Besides this there has been little or +no success in discovering specifics in the form of drugs which can +be given as medicines or inoculated with the hope of destroying +special kinds of pathogenic bacteria without injury to the body. +While it is unwise to make predictions as to future discoveries, +there seems at present little hope for a development of curative +medicine along these lines. + +VIS MEDICATRIX NATURAE. + +The study of bacterial diseases as they progress in the body has +emphasized above all things the fact that diseases are eventually +cured by a natural rather than by an artificial process. If a +pathogenic bacterium succeeds in passing the outer safeguards and +entering the body, and if it then succeeds in overcoming the +forces of resistance which we have already noticed, it will begin +to multiply and produce mischief. This multiplication now goes on +for a time unchecked, and there is little reason to expect that we +can ever do much toward checking it by means of drugs. But after a +little, conditions arise which are hostile to the further growth +of the parasite. These hostile conditions are produced perhaps in +part by the secretions from the bacteria, for bacteria are unable +to flourish in a medium containing much of their own secretions. +The secretions which they produce are poisons to them as well as +to the individual in which they grow, and after these have become +quite abundant the further growth of the bacterium is checked and +finally stopped. Partly, also, must we conclude that these hostile +conditions are produced by active vital powers in the body of the +individual attacked. The individual, as we have seen, in some +cases develops a quantity of some substance which neutralizes the +bacterial poisons and thus prevents their having their maximum +effect. Thus relieved from the direct effects of the poisons, the +resisting powers are recuperated and once more begin to produce a +direct destruction of the bacteria. Possibly the bacteria, being +now weakened by the presence of their own products of growth, more +readily yield to the resisting forces of the cell life of the +body. Possibly the resisting forces are decidedly increased by the +reactive effect of the bacteria and their poisons. But, at all +events, in cases where recovery from parasitic diseases occurs, +the revived powers of resistance finally overcome the bacteria, +destroy them or drive them off, and the body recovers. + +All this is, of course, a natural process. The recovery from a +disease produced by the invasion of parasitic bacteria depends +upon whether the body can resist the bacterial poisons long enough +for the recuperation of its resisting powers. If these poisons are +very violent and produced rapidly, death will probably occur +before the resisting powers are strong enough to drive off the +bacteria. In the case of some diseases the poisons are so violent +that this practically always occurs, recovery being very +exceptional. The poison produced by the tetanus bacillus is of +this nature, and recovery from lockjaw is of the rarest +occurrence. But in many other diseases the body is able to +withstand the poison, and later to recover its resisting powers +sufficiently to drive off the invaders. In all cases, however, the +process is a natural one and dependent upon the vital activity of +the body. It is based at the foundation, doubtless, upon the +powers of the body cells, either the phagocytes or other active +cells. The body has, in short, its own forces for repelling +invasions, and upon these forces must we depend for the power to +produce recovery. + +It is evident that all these facts give us very little +encouragement that we shall ever be able to cure diseases directly +by means of drugs to destroy bacteria, but, on the contrary, that +we must ever depend upon the resisting powers of the body. They +teach us, moreover, along what line we must look for the future +development of curative medicine. It is evident that scientific +medicine must turn its attention toward the strengthening and +stimulating of the resisting and curative forces of the body. It +must be the physician's aim to enable the body to resist the +poisons as well as possible and to stimulate it to re-enforce its +resistant forces. Drugs have a place in medicine, of course, but +this place is chiefly to stimulate the body to react against its +invading hosts. They are, as a rule, not specific against definite +diseases. We can not hope for much in the way of discovering +special medicines adapted to special diseases. We must simply look +upon them as means which the physician has in hand for stimulating +the natural forces of the body, and these may doubtless vary with +different individual natures. Recognising this, we can see also +the logic of the small dose as compared to the large dose. A small +dose of a drug may serve as a stimulant for the lagging forces, +while a larger dose would directly repress them or produce +injurious secondary effects. As soon as we recognise that the aim +of medicine is not to destroy the disease but rather to stimulate +the resisting forces of the body, the whole logic of therapeutics +assumes a new aspect. + +Physicians have understood this, and, especially in recent years, +have guided their practice by it. If a moderate dose of quinine +will check malaria in a few days, it does not follow that twice +the dose will do it in half the time or with twice the certainty. +The larger doses of the past, intended to drive out the disease, +have been everywhere replaced by smaller doses designed to +stimulate the lagging body powers. The modern physician makes no +attempt to cure typhoid fever, having long since learned his +inability to do this, at least if the fever once gets a foothold; +but he turns his attention to every conceivable means of +increasing the body's strength to resist the typhoid poison, +confident that if he can thus enable the patient to resist the +poisoning effects of the typhotoxine his patient will in the end +react against the disease and drive off the invading bacteria. The +physician's duty is to watch and guard, but he must depend upon +the vital powers of his patient to carry on alone the actual +battle with the bacterial invaders. + +ANTITOXINES. + +In very recent times, however, our bacteriologists have been +pointing out to the world certain entirely new means of assisting +the body to fight its battles with bacterial diseases. As already +noticed, one of the primal forces in the recovery, from some +diseases, at least, is the development in the body of a substance +which acts as an antidote to the bacterial poison. So long as this +antitoxine is not present the poisons produced by the disease will +have their full effect to weaken the body and prevent the revival +of its resisting powers to drive off the bacteria. Plainly, if it +is possible to obtain this antitoxine in quantity and then +inoculate it into the body when the toxic poisons are present, we +have a means for decidedly assisting the body in its efforts to +drive off the parasites. Such an antidote to the bacterial poison +would not, indeed, produce a cure, but it would perhaps have the +effect of annulling the action of the poisons, and would thus give +the body a much greater chance to master the bacteria. It is upon +this principle that is based the use of antitoxines in diphtheria +and tetanus + +It will be clear that to obtain the antitoxine we must depend upon +some natural method for its production. We do not know enough of +the chemical nature of the antitoxines to manufacture them +artificially. Of course we can not deny the possibility of their +artificial production, and certain very recent experiments +indicate that perhaps they may be made by the agency of +electricity. At present, however, we must use natural methods, and +the one commonly adopted is simple. Some animal is selected whose +blood is harmless to man and that is subject to the disease to be +treated. For diphtheria a horse is chosen. This animal is +inoculated with small quantities of the diphtheria poison without +the diphtheria bacillus. This poison is easily obtained by causing +the diphtheria bacillus to grow in common media in the laboratory +for a while, and the toxines develop in quantity; then, by proper +filtration, the bacteria themselves can be removed, leaving a pure +solution of the toxic poison. Small quantities of this poison are +inoculated into the horse at successive intervals. The effect on +the horse is the same as if the animal had the disease. Its cells +react and produce a considerable quantity of the antitoxine which +remains in solution in the blood of the animal. This is not +theory, but demonstrated fact. The blood of a horse so treated is +found to have the effect of neutralizing the diphtheria poison, +although the blood of the horse before such treatment has no such +effect. Thus there is developed in the horse's blood a quantity of +the antitoxine, and now it may be used by physicians where needed. +If some of this horse's blood, properly treated, be inoculated +into the body of a person who is suffering from diphtheria, its +effect, provided the theory of antitoxines is true, will be to +counteract in part, at least, the poisons which are being produced +in the patient by the diphtheria bacillus. This does not cure the +disease nor in itself drive off the bacilli, but it does protect +the body from the poisons to such an extent as to enable it more +readily to assert its own resisting powers. + +This method of using antitoxines as a help in curing disease is +very recent, and we can not even guess what may come of it. It has +apparently been successfully applied in diphtheria. It has also +been used in tetanus with slight success. The same principle has +been used in obtaining an antidote for the poison of snake bites, +since it has appeared that in this kind of poisoning the body will +develop an antidote to the poison if it gets a chance. Horses have +been treated in the same way as with the diphtheria poison, and in +the same way they develop a substance which neutralizes the snake +poison. Other diseases are being studied to-day with the hope of +similar results. How much further the principle will go we can not +say, nor can we be very confident that the same principle will +apply very widely. The parasitic diseases are so different in +nature that we can hardly expect that a method which is +satisfactory in meeting one of the diseases will be very likely to +be adapted to another. Vaccination has proved of value in +smallpox, but is not of use in other human diseases. Inoculation +with weakened germs has proved of value in anthrax and fowl +cholera, but will not apply to all diseases. Each of these +parasites must be fought by special methods, and we must not +expect that a method that is of value in one case must necessarily +be of use elsewhere. Above all, we must remember that the +antitoxines do not cure in themselves; they only guard the body +from the weakening effects of the poisons until it can cure +itself, and, unless the body has resisting powers, the antitoxine +will fail to produce the desired results. + +One further point in the action of the antitoxines must be +noticed. As we have seen, a recovery from an attack of most germ +diseases renders the individual for a time immune against a second +attack. This applies less, however, to a recovery after the +artificial inoculation with antitoxine than when the individual +recovers without such aid. If the individual recovers quite +independently of the artificial antitoxine, he does so in part +because he has developed the antitoxines for counteracting the +poison by his own powers. His cellular activities have, in other +words, been for a moment at least turned in the direction of +production of antitoxines. It is to be expected, therefore, that +after the recovery they will still have this power, and so long as +they possess it the individual will have protection from a second +attack. When, however, the recovery results from the artificial +inoculation of antitoxine the body cells have not actively +produced antitoxine. The neutralization of the poisons has been a +passive one, and after recovery the body cells are no more engaged +in producing antitoxine than before. The antitoxine which was +inoculated is soon eliminated by secretion, and the body is left +with practically the same liability to attack as before. Its +immunity is decidedly fleeting, since it was dependent not upon +any activity on the part of the body, but upon an artificial +inoculation of a material which is rapidly eliminated by +secretion. + +CONCLUSION. + +It is hoped that the outline which has been given of the bacterial +life of Nature may serve to give some adequate idea of these +organisms and correct the erroneous impressions in regard to them +which are widely prevalent. It will be seen that, as our friends, +bacteria play a vastly more important part in Nature than they do +as our enemies. These plants are minute and extraordinarily +simple, but, nevertheless, there exists a large number of +different species. The number of described forms already runs far +into the hundreds, and we do not yet appear to be approaching the +end of them. They are everywhere in Nature, and their numbers are +vast beyond conception. Their powers of multiplication are +inconceivable, and their ability to produce profound chemical +changes is therefore unlimited. This vast host of living beings +thus constitutes a force or series of forces of tremendous +significance. Most of the vast multitude we must regard as our +friends. Upon them the farmer is dependent for the fertility of +his soil and the possibility of continued life in his crops. Upon +them the dairyman is dependent for his flavours. Upon them +important fermentative industries are dependent, and their +universal powers come into action upon a commercial scale in many +a place where we have little thought of them in past years. We +must look upon them as agents ever at work, by means of which the +surface of Nature is enabled to remain fresh and green. Their +power is fundamental, and their activities are necessary for the +continuance of life. A small number of the vast host, a score or +two of species, unfortunately for us, find their most favourable +living place in the human body, and thus become human parasites. +By their growth they develop poisons and produce disease. This +small class of parasites are then decidedly our enemies. But, +taken all together, we must regard the bacteria as friends and +allies. Without them we should not have our epidemics, but without +them we should not exist. Without them it might be that some +individuals would live a little longer, if indeed we could live at +all. It is true that bacteria, by producing disease, once in a +while cause the premature death of an individual; once in a while, +indeed, they may sweep off a hundred or a thousand individuals; +but it is equally true that without them plant and animal life +would be impossible on the face of the earth. + + + + + +End of the Project Gutenberg EBook of The Story Of Germ Life, by H. W. 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