BACTERIOLOGY. The common idea of bacteria in the minds of most people is that of a hidden and sinister scourge lying in wait for mankind. This popular conception is born of the fact that attention was first focused upon bacteria through the dis covery, some 7o years ago, of the relationship of bacteria to dis ease in man, and that in its infancy the study of bacteriology was a branch of medical science. Relatively few people assign to bacteria the important position in the world of living things that they rightly occupy, for it is only a few of the bacteria known to day that have developed in such a way that they can live in the human body, and for every one of this kind there are scores of others which are perfectly harmless and, far from being regarded as the enemies of mankind, must be numbered among his best friends. It is, in fact, no exaggeration to say that upon the ac tivities of bacteria the very existence of man depends; nay more, that without bacteria there could be no other living thing in the world; for every animal and plant owes its existence to the fer tility of the soil and this in turn depends upon the activity of the micro-organisms which inhabit the soil in almost inconceivable numbers. It is one of the main objects of this article to show how true is this statement ; there will be found in it only passing refer ence to the organisms which produce disease in man and animals; for information on these see PATHOLOGY and IMMUNITY.
Bacteria are all unicellular and consist of masses of protoplasm (see CYTOLOGY), which at one time were believed to be naked masses as are some of the Protozoa. They were on this account first placed in the animal kingdom ; but by colouring the organisms with aniline dyes it can be shown that most of the bacteria are surrounded by a cell-wall and are more correctly to be regarded as plants. They differ from the higher plants in being devoid of green colouring matter, in which respect they appear to be closely related to the Fungi. As to the contents of the proto plasm very little is known on account of the minute size of the cell. It is certain, however, that in the make-up of their protoplasm much variability exists as between one kind and another. This is indicated by their capacity to produce mark edly different chemical changes which go far to distinguish one species from another. The denser part of the protoplasm, the nucleus of the cell, believed to play such an important role in controlling the heredity of higher plants and animals, is not known definitely to exist in the bacterial cell. The generally held view is that in the bacteria it is represented by a rudimentary form in which particles of nuclear matter, nucleoproteins, are distributed uniformly throughout the protoplasm in the form of granules or so-called chromidia. Recently, however, by the aid of micropho tography and using the ultra-violet end of the spectrum, J. Barnard has obtained pictures of bacteria showing much more complicated structures than can be seen either by white light or in a stained preparation, and showing also granules which suggest strongly the presence of definite nuclear bodies. Nuclei have been observed in stained specimens of Bacillus gammari, B. gigas and B. Biitschlii. These organisms are so comparatively large as to make it at least a little doubtful whether they are correctly included among the bacteria.
Bacteria multiply as do other organisms, but they differ from most others in the absence of sexual reproduction ; at any rate none has as yet been observed. It is well known that the fusion of sex nuclei tends to maintain constancy of a species. Absence of this feature may possibly account for the ease with which bacterial species tend to break up into races or strains. The characteristics of a species remain fixed only under rather narrowly prescribed conditions of cultivation. Departures from these conditions often produce profound changes in the shape, size and chemical be haviour of an organism—so much so that adaptation to environ ment may give rise to a strain of an organism which bears but little resemblance to the original form from which it developed. All of which tends to show that the nuclei, if present, do not function quite as do the sex nuclei of higher plants.
The shape of bacteria is simple, being modelled on three main types, the spherical or coccus form, the rod or bacillus type and the spirally twisted spirillum; many bacteria are provided also with thin whip-like appendages, projections of the protoplasm called flagella, which by lashing in the surrounding fluid propel the organisms with considerable rapidity. Some of the shapes assumed by the cells are shown in fig. i.
The general advances which have been made of late years in the study of bacteria are clearly brought to mind when we reflect that in the middle of the i 9th century these organisms were only known to a few experts and in a few forms as curiosities of the microscope, chiefly interesting for their minuteness and motility. The beginnings of bacteriology ran parallel with the development of the microscope. The first compound microscope was introduced by H. Jansen in 1590. This instrument gave only small magnifica tion and it is doubtful whether A. Kircher, who wrote in 1659 of "minute living worms in putrid meat, milk, vinegar, etc.," had seen anything smaller than Protozoa or possibly the larvae of in sects. The first to see micro-organisms was probably the Dutch naturalist A. van Leeuwenhoek who in 1683 sent a paper to the Royal Society in London in which he described some "Animalculae," as they were then called, in water, saliva and den tal tartar. These had been seen with a simple lens magnifying some 1oo-15o di ameters. The organisms (fig. 2) seem to correspond with some of the very large forms of bacteria such as Bacillus buccalis maximus and Spirillum sputigenum. In 1762 M. A. Plenciz propounded a theory of infectious disease, namely, that a special organism is associated with each disease and that the organisms are capable of reproduction outside the body and can be carried from place to place by the air. In this enlightened view, Plenciz was well in advance of his time, for little was known of these mi nute creatures before 186o. Great assistance came with the intro duction of the oil-immersion lens by Dollond in 1844 but, though by the use of this instrument the magnification of 1,000 diameters became possible, the definition at this magnification was very im perfect till the light was focused on the object by means of the substage condenser. E. Abbe introduced his condenser in 1870 and C. Zeiss completed the microscopic equipment with his apochro matic lenses in 1880. It is clear that bacteria were recognized be fore they were distinctly seen ; 0. F. Muller knew several impor tant forms in 1773, while F. Ehrenberg in 183o had advanced to the commencement of a scientific separation and grouping of them, and in 1838 had proposed at least 16 species, distributing them into four groups or genera. Our modern more accurate knowledge of the forms of bacteria, however, dates from F. J. Cohn's brilliant researches, the chief results of which were published at various periods between 1853 and 1872. Cohn's classification of bacteria, published in 1872 and extended in 1875, has in fact dominated the study of these organisms almost ever since. He based his classification on what may be considered the constancy of forms which he called species and genera. As to the constancy of form, however, Cohn maintained certain reservations which have been ignored by some of his followers. The fact that Schizomycetes produce spores appears to have been discovered by Cohn in though it was expressed dubiously in 1872. In 1876, however, Cohn had seen the spores germinate, and Robert Koch, 0. Bre feld, P. van Tieghem, A. de Bary and others confirmed the dis covery.
The supposed constancy of forms in Cohn's species and genera received a shock when E. R. Lankester in 1873 pointed out that his Bacterium rubescens (since named Beggiatoa roseo-persicina, Zopf) passes through phases which would have been described by most observers, influenced by the current doctrine, as so many separate species or even genera—that in fact forms known as Bacterium, Micrococcus, Bacillus, Leptotlirix, etc., occur as stages in one life-history. J. Lister put forth similar ideas about the same time, and T. Billroth came forward in 1874 with the ex travagant view that the various bacteria are only different states of one and the same organism which he called Cocco-bacteria septica. From that time the question of the pleomorphism (muta bility of shape) of the bacteria has been hotly discussed: and quite recently pleomorphism exhibited in cultures of Azotobacter chroococcum has led F. Lohnis to the belief that all soil organisms are modifications of this one species. (F. Lohnis and N. R. Smith 1923.) It is now generally agreed that, while a certain number of forms may show different types of cell during the various phases of their life-history, yet the majority of forms are uniform, show ing only one type of cell throughout their life. The question of species in the bacteria is essentially the same as in other groups of plants. Before a form can be placed in a satisfactory classifi catory position its whole life-history must be studied, so that all the phases may be known. In the meantime, while various ob servers were building up our knowledge of the morphology of bacteria, others were laying the foundation of what is known of the relations of the organisms to fermentation and disease— that ancient will-o'-the-wisp, spontaneous generation, being re vived by the way. When L. Pasteur in 1857 showed that lactic fermentation depends on the presence of an organism, it was already known from the researches of T. Schwann (183 7) and H. L. F. v. Helmholtz (1843) that fermentation and putrefaction are intimately connected with the presence of organisms derived from the air, and that the preservation of putrescible substances depends on this principle. In 1862 Pasteur placed it beyond rea sonable doubt that the ammoniacal fermentation of urea is due to the action of a minute bacterium. In 1864 this was confirmed by van Tieghem, and in 1874 by Cohn, who named the organism Micrococcus ureae. Pasteur and Cohn also pointed out that putrefaction is but a special case of fermentation, and before 1872 the doctrines of Pasteur were established with respect to bacteria. Meanwhile two branches of enquiry had arisen from the above. In the first place, the disputed question of spontaneous generation received fresh impetus from the difficulty of keeping such minute organisms as bacteria from reaching and developing in organic infusions; and, secondly, the long-suspected analogies between the phenomena of fermentation and those of certain diseases again made themselves felt as both became better understood. Needham in 1745 had declared that heated infusions of organic matter were not deprived of living beings; Abbe Spallanzani (17 7 7) had re plied that more careful heating and other precautions prevent the appearance of organisms in the fluid. Various experiments by Schwann, Helmholtz, M. Schultze, K. Schroeder, Th. v. Dusch and others led to the refutation, step by step, of the belief that the more minute organisms, and particularly bacteria, arose de novo in the special cases quoted. Nevertheless, instances were adduced where the most careful heating of yolk of egg, milk, hay infusions, etc., had failed—the boiled infusions, etc., turning putrid and swarming with bacteria after a few hours.
In 1862 Pasteur repeated and extended such experiments, and paved the way for a complete explanation of the anomalies; Cohn in 1872 published confirmatory results; and it became clear that no putrefaction can take place without bacteria or some other living organism. In the hands of 0. Brefeld, A. de Bary, J. Tyn dall, J. Lister and others, the various links in the chain of evi dence grew stronger and stronger, and every case adduced as one of spontaneous generation fell to the ground when examined. No case of so-called spontaneous generation has withstood rigid in vestigation ; but the discussion contributed to more exact ideas as to the ubiquity, minuteness and high powers of resistance to physical agents of the spores of bacteria, and led to more exact ideas of antiseptic treatment. Methods were also improved, and the application of some of them to surgery by J. Lister, Robert Koch and others has yielded results of the highest value. The methods of bacteriological technique which are employed to-day had their beginnings as recently as 187o-1885 with the introduction of the use of stains by C. Weigert in 1871 and the discovery by Robert Koch (1881) of the method of separating mixtures of organisms on plates of nutrient media solidified with gelatine and agar. Following closely on the introduction of this technique came the separation of pure cultures of many bacteria. In 1882 F. Loeffler and F. Schulze discovered the cause of glan ders; in 1883 Koch isolated the organism of Asiatic cholera and the same year E. Klebs found that of diphtheria. In 1885, A. Nicolaier observed the Tetanus bacillus in pus produced by in oculating mice and rabbits with soil. It was left, however, to the famous Japanese Kitasato to discover the way in which these or ganisms could be cultivated. In 188g he showed that previous failures in this connection had been due to a lack of appreciation of the necessary conditions, namely, that complete absence of oxygen was essential. A very important discovery came in 188o when Pasteur first showed that Bacillus anthracis cultivated in chicken broth at a temperature of 42°-43° C., lost its virulence af ter a few generations and ceased to kill even the mouse ; this re markable finding which was destined to lay the fcundation of serum-therapy (treatment of disease by serum inoculation), like so many other great discoveries, resulted from an accident ; the temperature regulation of the incubator being faulty the cultures were submitted to 42° C., instead of 37° C. More remarkable still, animals inoculated with such "attenuated" bacilli proved to be curiously resistant to the deadly effects of subsequent inocula tions with the non-attenuated form. In other words, animals vaccinated with the cultivated bacillus showed immunity from disease when re-inoculated with the deadly wild form. The ques tions as to the causes and nature of the changes in the bacillus and in the host, as to the extent of immunity enjoyed by the latter, etc., are of the greatest interest and importance.