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Bacteria, the name applied to certain organisms of microscopic size, which constitute the lowest division of those forms of vegetable life called fungi. The divisions of the group of fungi have undergone many changes of nomenclature of late years; it is now customary to apply the term "bacteria" as a synonym for the division known to botanists as the Schizomycetes or fission fungi. The fact that bacteria multiply by repeated division justifies the application of this term, derived as it is from two Greek words, for "to split, and "a fungus." The word bacterium means a little rod, and was at one time reserved for certain members of the group of Schizomycetes, but as already stated the whole group is now commonly spoken of as bacteria. The bacteria are single cells. They may assume various shapes. There are spherical forms known as "micrococci;" two of these may adhere together forming a dumb-bell shaped double coccus or "diplococcus;" rod-shaped forms are called "bacilli" (bacillus, a little staff); intermediate forms between cocci and bacilli, i.e. short rods, used to be called, and are still spoken of, as "bacteria"; and thus, as already incidentally observed, this word is unfortunately used in a double sense. Again several rods may adhere together forming filaments known as "Leptothrix" forms, while chains of micrococci are spoken of as "streptococci."

Curved rods also occur, as, for instance, in the organism known as Koch's cholera bacillus, and if several such curved bacilli are united, end to and, the resulting spiral form is known as spirillum, while a long and closely wound spiral is called a spirochaeta.

Some bacteria are provided with a whip-like "flagellum," which gives them the power of active movement, others are non-motile. Very near relations of the bacteria are met with in certain humble members of the great family of algae or seaweeds. These lowliest algae are, like the bacteria, unicellular, devoid of sexual organs, and present many other points of similarity, but one great difference, namely, that they contain the peculiar green colouring matter known as chlorophyll. The absence of chlorophyll in bacteria prevents their obtaining carbon from carbonic acid gas, and they must therefore live upon ready-formed carbon compounds, such as exist in animals or plants. In other words, the bacteria are parasitic, feeding upon organic matter, and in some cases actually attacking living organisms. It is this last peculiarity which attaches such vast importance to the study of bacteria, and the researches of Pasteur and others, which have shown how the life history of fission fungi is bound up with certain fermentations, with putrefaction, and finally with disease, gave a powerful impetus to the scientific study of these minute plants, which are now recognised to be fraught with the most wonderful power for working good or ill to higher forms of life.

The importance of the study of bacteria, then, was first recognised in investigating the role played by them in fermentation processes. Pasteur showed that milk turns sour because of the growth within it of a bacterium, which converts the sugar of milk into lactic acid; again, in the manufacture of vinegar a bacterium is at work, and is the cause of the conversion of alcohol into acetic acid. After the establishment of these facts the question arose whether the phenomena of putrefaction might not also be due to bacterial growth, and this led to a great controversy. It was maintained, on the one hand, that bacteria could never develop in nutrient material unless similar bacteria already existed there, or were introduced from without; on the other hand, the doctrine of spontaneous generation was upheld, and it was urged that it was impossible to prevent putrefactive processes from occurring in organic infusions, however carefully they were preserved from bacterial intrusion. The difficulty was not easily set aside, so small were the living units in question and so universal is their distribution; their minute spores are readily borne from place to place by currents of air, and every drop of water teems with bacterial life. It was found, however, in course of time that prolonged boiling was uniformly effectual in destroying all germs, and that nutrient material which had been exposed to this treatment in flasks plugged with cotton wool could be kept for an indefinite period without undergoing putrefactive changes. The cotton-wool plug served the purpose of a filter, permitting interchange of gases between the inside of the flask and the outer world, but preventing any organisms reaching the interior of the flask from outside. Nutrient media which have thus been prevented from putrefying are said to be "sterilised;" that their remaining unchanged is due to the absence of bacterial life within them is easily shown by noting the effect of introducing germs into them from without. Such sterile media are now largely employed in studying the growth of bacteria", and when due precautions are taken it is not difficult to ensure securing what is called a "pure cultivation" of a given organism; that is to say, one and only one kind of organism being introduced into the medium, there is a development within it of organisms of that kind and of that kind only. In this way the fallacy of spontaneous generation has been completely demonstrated; putrefactive processes are now clearly shown to be due to the growth of bacteria, and by studying the differing ways in which different organisms affect nutrient material an invaluable method of classifying bacteria and of studying their life history has been placed at the disposal of science.

Meanwhile, however, further and yet more important truths were being elicited with regard to the functions of bacteria. The part played by them in fermentation and in putrefaction was demonstrated, and then came the great discovery of their importance in disease.

It had been noticed that the blood of animals dying of a disease known as splenic fever or anthrax contained bacilli; a minute drop of such blood was found to be capable of conveying anthrax to other animals, and the question arose whether the bacilli were not the cause of the disease. Davaine upheld this view, and the subsequent researches of Koch have placed the matter beyond all doubt. The bacillus anthracis, the bacillus in question, has now been carefully studied in pure cultivations; it has been found to grow into long threads, to produce spores, and to grow and affect the nutrient material in a manner peculiar to itself, and infinitesimal portions of the growth taken from cultures many times removed from the original source produce the disease known as anthrax in suitable animals. Anthrax is but rarely met with in the human subject; it occasionally, however, presents itself among those whose wrork brings them in contact with the hides of diseased cattle, and for that reason anthrax in man is known as "woolsorter's disease."

The great discovery of the cause of "splenic fever" established on a firm footing the germ theory of disease, and led to a vast display of activity in this field of work. It was soon found, however, that the difficulties of the subject were considerable, and many rash generalisations have been made. None the less, however, a number of facts have been demonstrated sufficient to revolutionise some of the conceptions of twenty years ago. Consumption has been shown by Koch to be caused by a bacillus, the tubercle bacillus; the bacilli by which the diseases glanders and leprosy are produced have been demonstrated, and there are good reasons for supposing that the germs of tetanus, diphtheria, and perhaps of cholera, typhoid, erysipelas, and other diseases occurring in man are now known; while several more disorders affecting animals have been undoubtedly placed in the category of germ diseases.

Great advances have been made, too, in technique, so that further additions to the knowledge of germs should be speedily forthcoming. The use of aniline dyes in staining bacteria, the employment of gelatine and agaragar in culture media, and the method of plate cultivation, introduced by Koch, may be alluded to in passing.

The " gelatine tube" is a sterilised mixture of gelatine and broth, which is transparent, and can be liquefied by exposure to a temperature of about 25° C. This degree of heat does not destroy the germs; and admits of agitation of the resulting liquid, and thus of the uniform diffusion throughout its substance of any bacteria it may contain. The liquefied gelatine can then be poured out and allowed to set, and wherever a germ happens to be fixed, there a colony produced by the multiplication of that germ will in time appear. By inoculating sterile gelatine with a minute droplet (diluted if necessary) of material, the bacteria therein contained can thus be separated from one another.

Agaragar, or Japanese isinglass, is used where it is desirable to grow bacteria at a relatively high temperature; gelatine would, of course, be liquefied if exposed to the body temperature, whereas the melting-point of agaragar is considerably higher than this.

The possibility of separating germs from one another by plate cultivation depends upon the varying characteristics which the colonies of different organisms present. In some cases colours are produced by bacteria, as for example the brilliant red of the micrococcus prodigiosus, a fungus of wide distribution which so often presents itself on mouldy bread; the yellow colour of staphylococcus aureus, the bluish green of bacillus pyocyaneus, and so on; by these colour phenomena and by other characteristics it is possible in many cases to pronounce upon the nature of a colony without examining its constituent bacteria microscopically.

To turn now to the various means which have been suggested for combating the ravages of bacteria when they attack the bodies of men and animals. Germs are destroyed by certain chemical substances which are known as antiseptics (q.v.); and the antiseptic treatment of wounds advocated by Lister was one of the first practical applications of the facts of bacteriology to therapeutics. But the question was how to kill germs flourishing inside the body, maybe in the blood itself, and to this problem Pasteur addressed himself.

The great Frenchman found that by various means bacilli could be deprived of their virulence, "attenuated" as it is called; so that cultures of an organism, which would ordinarily prove fatal to an animal, could be rendered inert, or else modified so that they only produce the disease in a mild form. Moreover, Pasteur knew that many disorders only occur once in an individual's lifetime ; for example, one attack of scarlet fever protects the patient against a subsequent attack, and thus arose the idea of protective vaccination with attenuated cultures; the theory being to produce the disease in a mild form and so render the vaccinated person "immune," incapable of subsequent infection. Pasteur has applied his method in anthrax, hydrophobia and other diseases. Another theory of protective vaccination is that the chemical substances produced by germs in the course of their growth are inimical to their development, and when inoculated into a patient hinder or prevent the development of the disease in question. This method has been applied by Koch to the treatment of consumption.

The doctrine of Phagocytosis (q.v.) may here be alluded to. It has been supposed by Metschnikoff that disease is in many cases a struggle for existence between invading bacteria and certain cells of the body possessed of amoeboid movement; either the bacteria destroy the cells, or the cells, hence called phagocytes or devouring cells, eat up the bacteria. In the first case the patient dies; in the second, germs succumb and the patient recovers. It is questionable, however, fascinating as the theory is at first sight, whether the cells are the actual destroyers of the germs; at all events, animal fluids, apart from cells, have very definite germicidal powers.

The study of the chemical substances produced by germs in the course of their development promises to be fertile in results as regards the treatment of diseases. Certain it appears to be that most powerful poisons result from bacterial growth, belonging either to the class known as alkaloids or to the albumose group. The hope may be entertained that as the nature of these poisons becomes more accurately known methods of dealing with them msy be devised, and that thus the labours of bacteriologists may not be without result upon the medicine of the near future.