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The minute organisms which are commonly called " bacteria " 1 are also known popularly under other designations, e.g. " microbes," " micro-organisms," " microphytes," " bacilli," " micrococci." All these terms, including the usual one of bacteria, are unsatisfactory; for " bacterium," " bacillus " and " micrococcus " have narrow technical meanings, and the other terms are too vague to be scientific. The most satisfactory designation is that proposed by Nageli in 1857, namely " schizomycetes," and it is by this term that they are usually known among botanists; the less exact term, however, is also used and is retained in this article since the science is commonly known as " bacteriology." The first part of this article deals with the general scientific aspects of the subject, while a second part is concerned with the medical aspects.

I. THE Study Of Bacteria 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 19th 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. They were then known under the name of " animalculae," and were confounded with all kinds of other small organisms. At that time nothing was known of their life-history, and no one dreamed of their being of importance to man and other living beings, or of their capacity to produce the profound chemical changes with which we are now so familiar. At the present day, however, not only have hundreds of forms or species been described, but our knowledge of their biology has so extended that we have entire laboratories equipped for their study, and large libraries devoted solely to this subject. Furthermore, this branch of science has become so complex that the bacteriological departments of medicine, of agriculture, of sewage, &c., have become more or less separate studies.

The schizomycetes or bacteria are minute vegetable organisms devoid of chlorophyll and multiplying by repeated bipartitions. They consist of single cells, which may be spherical, oblong or cylindrical in shape, or of filamentous or Definition. other aggregates of cells. They are characterized by the absence of ordinary sexual reproduction and by the absence of an ordinary nucleus. In the two last-mentioned characters and in their manner of division the bacteria resemble Schizophyceae (Cyanophyceae or blue-green algae), and the two groups of Schizophyceae and Schizomycetes are usually united in the class Schizophyta, to indicate the generally received view that most of the typical bacteria have been derived from the Cyanophyceae. Some forms, however, such as " Sarcina," have their algal analogues in Palmellaceae among the green algae, while Thaxter's group of Myxobacteriaceae suggests a relationship with the Myxornycetes. The existence of ciliated micrococci together with the formation of endospores - structures not known in the Cyanophyceae - reminds us of the flagellate Protozoa, e.g. Monas, Chromulina. Resemblances also exist between the endospores and the spore-formations in the Saccharomycetes, and if Bacillus inflatus, B. ventriculus, &c., really form more than one spore in the cell, these analogies are strengthened. Schizomycetes such as Clostridium, Plectridium, &c., where the sporiferous cells enlarge, bear out the same argument, and we must not forget that there are extremely minute " yeasts," easily mistaken for Micrococci, and that yeasts occasionally form only one spore in the cell.

Nor must we overlook the possibility that the endosporeformation in non-motile bacteria more than merely resembles the development of azygospores in the Conjugatae, and some Ulothricaceae, if reduced in size, would resemble them. Meyer regards them as chlamydospores, and Klebs as " carpospores " or possibly chlamydospores similar to the endospores of yeast.

1 Gr. gai -i A cov, Lat. bacillus, little rod or stick.

The former also looks on the ordinary disjointing bacterial cell as an oidium, and it must be admitted that since Brefeld's discovery of the frequency of minute oidia and chlamydospores among the fungi, the probability that some so-called bacteria - and this applies especially to the branching forms accepted by some bacteriologists - are merely reduced fungi is increased. Even the curious one-sided growth of certain species which form sheaths and stalks - e.g. Bacterium vermiforme, B. pediculatum - can be matched by Algae such as Oocardium, Hydrurus, and some Diatoms. It is clear then that the bacteria are very possibly a heterogeneous group, and in the present state of our knowledge their phylogeny must be considered as very doubtful.

Nearly all bacteria, owing to the absence of chlorophyll, are saprophytic or parasitic forms. Most of them are colourless, but FIG. 1. - Preparations showing various types of cilia A. Bacillus subtilis, Cohn, and Spirillum undula, Ehrenb.

B. Planococcus citreus (Menge), Migula. [[[sard]]), Migula.

C. Pseudomonas pyocyanea (Ges D. P. macroselmis, Migula.

E. P. syncyanea (Ehrenb.), Migula.

a few secrete colouring matters other than chlorophyll. In size their cells are commonly about o

ooi mm. (1 micromillimetre or 1 µ ) in diameter, and from two to five times that length, but smaller ones and a few larger ones are known. Some of the shapes assumed by the cells are shown in fig. 1.

That bacteria have existed from very early periods is clear from their presence in fossils; and although we cannot accept all the conclusions drawn from the imperfect records of the rocks, and may dismiss as absurd the statements that geologically immured forms have been found still living, the researches of Renault and van Tieghem have shown pretty clearly that large numbers of bacteria existed in Carboniferous and Devonian times, and probably earlier.

Schizomycetes are ubiquitous as saprophytes in still ponds and ditches, in running streams and rivers, and in the sea, and especially in drains, bogs, refuse heaps, and in the soil, and wherever organic infusions are allowed to stand for a short time. Any liquid (blood, urine, milk, beer, &c.) containing organic matter, or any solid food-stuff (meat preserves, vegetables, &c.), allowed to stand exposed to the air soon swarms with bacteria, if moisture is present and the temperature not ab- Distribu- normal. Though they occur all the world over in the g Y air and on the surface of exposed bodies, it is not to be supposed that they are by any means equally distributed, and it is questionable whether the bacteria suspended in the air ever exist in such enormous quantities as was once believed. The evidence to hand shows that on heights and in open country, especially in the north, there may be few or even no Schizomycetes detected in the air, and even in towns their distribution varies greatly; sometimes they appear to exist in minute clouds, as it were, with interspaces devoid of any, but in laboratories and closed spaces where their cultivation has been promoted Lhe air may be considerably laden with them Of course the distribution of bodies so light and small is easily influenced by movements, rain, wind, changes of temperature, &c. As parasites, certain Schizomycetes inhabit and prey upon the organs of man and animals in varying degrees, and the conditions for their growth and distribution are then very complex. Plants appear to be less subject to their attacks - possibly, as has been suggested, because the acid fluids of the higher vegetable organisms are less suited for the development of Schizomycetes; nevertheless some are known to be parasitic on plants. Schizomycetes exist in every part of the alimentary canal of animals, except, perhaps, where acid secretions prevail; these are by no means necessarily harmful, though, by destroying the teeth for instance, certain forms may incidentally be the forerunners of damage which they do not directly cause.

Little was known about these extremely minute organisms before 1860. A. van Leeuwenhoek figured bacteria as far back as the 17th century, and O. F. Muller knew several important forms in 1773, while Ehrenberg in 1830 had advanced to the commencement of a scientific separation and grouping of them, and in 1838 had proposed at least sixteen species, distributing them into four genera. Our modern more accurate though still fragmentary knowledge of the forms of Schizomycetes, 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 the bacteria, published in 1872 and extended in 1875, has in fact dominated the study of these organisms almost ever since. He proceeded in the main on the assumption that the forms of bacteria as met with and described by him are practically constant, at any rate within limits which are not wide: observing that a minute spherical micrococcus or a rod-like bacillus regularly produced similar micrococci and bacilli respectively, 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 1857, though it was expressed dubiously in 1872; these spores had no doubt been observed previously. In 1876, however, Cohn had seen the spores germinate, and Koch, Brefeld, Pratzmowski, van Tieghem, de Bary and others confirmed the discovery in various species.

The supposed constancy of forms in Cohn's species and genera received a shock when Lankester in 1873 pointed out that his Bacterium rubescens (since named Beggiatoa roseo-persicina, Zopf) passes through conditions 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, Hicrococcus, Bacillus, Leptothrix, &c., occur as phases in one life-history. Lister put forth similar ideas about the same time; and Billroth came forward in 1874 with the extravagant 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 (mutability of shape) of the bacteria has been hotly discussed; but it is now generally agreed that, while a various forms of bacteria and the and their arrangement.

F. Bacillus typhi, Gaffky.

G. B. vulgaris (Hauser), Migula.

H. Microspira Comma (Koch), Schroeter.

J, K. Spirillum rubrum, Esmarsch.

L, M. S. undula (Mtill er), Ehrenb. ( All after Migula.) certain number of forms may show different types of cell during the various phases of the life-history,' yet the majority of forms are uniform, showing one type of cell throughout their lifehistory. 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 classificatory position its whole lifehistory must be studied, so that all the phases may be known. In the meantime, while various observers were building up our knowledge of the morphology of bacteria, others were laying the foundation of what is known of the relations of these organisms to fermentation and disease - that ancient will-o'-the-wisp " spontaneous generation " being revived by the way. When Pasteur in 1857 showed that the lactic fermentation depends on the presence of an organism, it was already known from the researches of Schwann (1837) and 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 reasonable doubt that the ammoniacal fermentation of urea is due to the action of a minute Schizomycete; 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 Schizomycetes. Meanwhile two branches of inquiry had arisen, so to speak, from the above. In the first place, the ancient 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; Spallanzani (1777) had replied that more careful heating and other precautions prevent the appearance of organisms in the fluid. Various experiments by Schwann, Helmholtz, Schultz, Schroeder, 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, &c., had failed, - the boiled infusions, &c., 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 Brefeld, BurdonSanderson, de Bary, Tyndall, Roberts, Lister and others, the various links in the chain of evidence 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 investigation; 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 Schizomycetes, and led to more exact ideas of antiseptic treatments. Methods were also improved, and the application of some of them to surgery at the hands of Lister, Koch and others has yielded results of the highest value.

Long before any clear ideas as to the relations of Schizomycetes to fermentation and disease were possible, various thinkers at different times had suggested that resemblances existed between the phenomena of certain diseases and those of fermentation, and the idea that a virus or contagium might be something of the nature of a minute organism capable of spreading and 1 Cladothrix dichotoma, for example, which is ordinarily a branched, filamentous, sheathed form, at certain seasons breaks up into a number of separate cells which develop a tuft of cilia and escape from the sheath. Such a behaviour is very similar to the production of zoospores which is so common in many filamentous algae.

reproducing itself had been entertained. Such vague notions began to take more definite shape as the ferment theory of Cagniard de la Tour (1828), Schwann (1837) and Pasteur made way, especially in the hands of the last-named savant. From about 1870 onwards the " germ theory of disease " has passed into acceptance. P. F. O. Rayer in 1850 and Davaine had observed the bacilli in the blood of animals dead of anthrax (splenic fever), and Pollender discovered them anew in 1855

In 1863, imbued with ideas derived from Pasteur's researches on fermentation, Davaine reinvestigated the matter, and put forth the opinion that the anthrax bacilli caused the splenic fever; this was proved to result from inoculation. Koch in 1876 published his observations on Davaine's bacilli, placed beyond doubt their causal relation to splenic fever, discovered the spores and the saprophytic phase in the life-history of the organism, and cleared up important points in the whole question (figs. 7 and 9). In 1870 Pasteur had proved that a disease of silkworms was due to an organism of the nature of a bacterium; and in 1871 Oertel showed that a Micrococcus already known to exist in diphtheria is intimately concerned in producing that disease. In 1872, therefore, Cohn was already justified in grouping together a number of " pathogenous " Schizomycetes. Thus arose the foundations of the modern " germ theory of disease;" and, in the midst of the wildest conjectures and the worst of logic, a nucleus of facts was won, which has since grown, and is growing daily. Septicaemia, tuberculosis, glanders, fowl-cholera, relapsing fever, and other diseases are now brought definitely within the range of biology, and it is clear that all contagious and infectious diseases are due to the action of bacteria or, in a few cases, to fungi, or to protozoa or other animals.

Other questions of the highest importance have arisen from the foregoing. About 1880 Pasteur first showed that Bacillus anthracis cultivated in chicken broth, with plenty of oxygen and at a temperature of 42-43° C., lost its virulence after a few " generations," and ceased to kill even the mouse; Toussaint and Chauveau confirmed, and others have extended the observations. More remarkable still, animals inoculated with such " attenuated " bacilli proved to be curiously resistant to the deadly effects of subsequent inoculations of the non-attenuated form. In other words, animals vaccinated with the cultivated bacillus showed immunity from disease when reinoculated with the deadly wild form. The questions 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, &c., are of the greatest interest and importance. These matters, however, and others such as phagocytosis (first described by Metchnikoff in 1884), and the epoch-making discovery of the opsonins of the blood by Wright, do not here concern us (see II. below).

Morphology. - Sizes, Forms, Structure, &c. - The Schizomycetes consist of single cells, or of filamentous or other groups of cells, according as the divisions are completed at once or not. While some unicellular forms are less than ( . 001 mm.) in diameter, others have cells measur ing 4µ or 5µ or even 7 1 2 or 8µ in thickness, while the length may vary from that of the diameter to many times that measurement. In the filamentous forms the individual cells are often difficult to observe until reagents are applied (e.g. fig. 14), and the length of the rows of cylindrical cells may be many hundred times greater than the breadth. Similarly, the diameters of flat or spheroidal colonies may vary from a few times to many hundred times that of the individual cells, the divisions of which have produced the colony. The shape of the individual cell (fig. 1) varies from that of a minute sphere to that of a straight, curved, or twisted filament or cylinder, which is not necessarily of the same diameter throughout, and may have flattened, rounded, or even pointed ends. The rule is that the cells divide in one direction only - i.e. transverse to the long axis - and therefore produce aggregates of long cylindrical shape; but in rarer cases iso-diametric cells divide in two or three directions, producing flat, or spheroidal, or irregular colonies, the size of which is practically unlimited. The bacterial cell is always clothed by a definite cell-membrane, as was shown by the plasmolysing experiments of Fischer and others. Unlike Cell=wall. the cell-wall of the higher plants, it gives usually no react i ons of cellulose, nor is chitin present as in the fungi, but it consists of a proteid substance and is apparently a modification of the general protoplasm. In some cases, however, as in B. tuberculosis, analysis of the cell shows a large amount of cellulose. The cell-walls in some forms swell up into a gelatinous mass so that the cell appears to be surrounded in the unstained condition by a clear, transparent space. When the swollen wall is dense and regular in appearance the term " capsule " is applied to the sheath as in Leuconostoc. Secreted pigments (red, yellow, green and blue) are sometimes deposited in the wall, and some of the iron-bacteria have deposits of oxide of iron in the membranes.

FIG. 2. - The various phases of germination of spores of Bacillus ramosus (Fraenkel), as actually observed in hanging drops under very high powers.

A. The spore sown at I I A.M., as shown at a, had swollen ( b ) perceptibly by noon, and had germinated by 3.30 P.M., as shown at c: in d at 6 P.M., and e at 8.30 P.M.; the resulting filament is segmenting into bacilli as it elongates, and at midnight ( f ) consisted of twelve such segments.

B, C. Similar series of phases in the order of the small letters in each case, and with the times of observation attached. At f and g occurs the breaking up of the filament into rodlets.

D. Germinating spores in various stages, more highly magnified, and showing the different ways of escape of the filament from the spore-membrane. (H. M. W.) The substance of the bacterial cell when suitably prepared and stained shows in the larger forms a mass of homogeneous protoplasm containing irregular spaces, the vacuoles, which enclose a watery fluid. Scattered in the protoplasm are usually one or more deeply-staining granules. The protoplasm itself may be tinged with colouring matter, bright red, yellow, &c., and may occasionally contain substances other than the deeply-staining granules. The occurrence of a starch-like substance which stains deep blue with iodine has been clearly shown in some forms even where the bacterium is growing on a medium containing no starch, as shown by Ward and others. In other forms a substance (probably glycogen or amylo-dextrin) which turns brown with iodine has been observed. Oil and fat drops have also been shown to occur, and in the sulphur-bacteria numerous fine granules of sulphur.

The question of the existence of a nucleus in the bacteria is one that has led to much discussion and is a problem of some difficulty. In the majority of forms it has not hitherto been possible to demonstrate a nucleus of the type which is so characteristic of the higher plants. Attention has accordingly been directed to the deeply-staining granules mentioned above, and the term chromatin-granules has been applied to them, and they have been considered to represent a rudimentary nucleus. That these granules consist of a material similar to the chromatin of the nucleus of higher forms is very doubtful, and the comparison with the nucleus of more highly organized cells rests on a very slender basis. The most recent works (Vejdovsky, Mend), however, appear to show that nuclei of a structure and mode of division almost typical are to be found in some of the largest bacteria. It is possible that a similar structure has been overlooked or is invisible in other forms owing to their small size, and that there may be another type of nucleus - the diffuse nucleus - such as Schaudinn believed to be the case in B. butschlii. Many bacteria when suspended in a fluid exhibit a power of independent movement which is, of course, quite distinct from the Brownian movement - a non-vital phenomenon common to all finelydivided particles suspended in a fluid. Independent movement is effected by special motile or-

gans, the cilia or flagella. These H structures are invisible, with ordinary illumination in living cells or unstained preparations, and can only be made clearly visible by special methods of preparation and staining first used by Loffler. By these methods the cilia are seen to be fine protoplasmic outgrowths of the cell (fig. i) of the same nature as those of the zoospores and antherozoids of algae, mosses, &c.

These cilia appear to be attached to the cell-wall, being unaffected by plasmolysis, but Fischer states that they really are derived from the central protoplasm and pass through minute pores in cfl,8 the wall. The cilia may be present during a short period only in the life of a Schizomycete, and their number may vary according to the medium on which the organism is growing. Nevertheless, there is more or less constancy in the type of distribution, &c., of the cilia for each species when growing at its best. The chief results may be summed up as follows: some species, e.g. B. anthracis, have no cilia; others have only one flagellum at one pole ( Monotrichous), e.g. Bacillus pyocyaneus (fig. r, C, D), or one at each pole; others again have a tuft of several cilia Celli contents. Nucleus. FIG. 3. - Types of Zoogloea. (After Zopf.) A. Mixed zoogloea found as a pellicle on the surface of vegetable infusions, &c.; it consists of various forms, and contains cocci (a) and rodlets, in series ( b and c ), &c. (x 540).

B. Egg-shaped mass of zoogloea of Beggiatoa roseo-persicina (Bacterium rubescens of Lankester); the gelatinous swollen walls of the large crowded cocci are fused into a common gelatinous envelope.

C. Reticulate zoogloea of the same (x 250). D, E, H. Colonies of Myconostoc enveloped in diffluent matrix (x 540).

F. Branched fruticose zoogloea of Cladothrix (slightly magnified).

G. Zoogloea of Bacterium merismopedioides, Zopf, containing cocci arranged in tablets.

at one pole (Lophotrichous), e.g. B. syncyaneus (fig. I, E), or at each pole ( Amphitrichous ) (fig. r, J, K, L); and, finally, many actively motile forms have the cilia springing all round (Peritrichous), e.g. B. vulgaris (fig. I, G). It is found, however, that strict reliance cannot be placed on the distinction between the Monotrichous, Lophotrichous and Amphitrichous conditions, since one and the same species may have one, two or more cilia at one or both poles; nevertheless some stress may usually be laid on the existence of one or two as opposed to several - e.g. five or six or more - at one or each pole.

In Beggiatoa, a filamentous form, peculiar, slow, oscillatory movements are to be observed, reminding us of the movements of Oscillatoria among the Cyanophyceae. In these Vegetative cases no cilia have been observed, and there is a state. firm cell-wall, so the mol r ement remains quite unexplained.

FIG. q.. - Types of Sporeformation in Schizomyc cotes. (After Zopf.) A. Various stages in the development of the endogenous spores in a Clostridium - the small letters indicate the order.

B. Endogenous spores of the hay bacillus.

C. A chair of cocci of Leuconostoc mesenterioides, with two " resting spores," i.e. arthrospores. (After van Tieghem.) D. A motile rodlet with one cilium and with a spore formed inside. 0 000% v E.' Spore - formation in O Vibrio- like ( c) and o' I O Spirillum-like (a, b, d) o Schizomycetes.

0 F. Long rod-like form con QI a a taining a spore (these b II ,. are the so - called " Kopfchenbacterien " of German authors).

G. Vibrio form with spore. (After Prazmowski.) H. Clostridium - one cell contains two spores. (After Prazmowski.) I. Spirillum containing many spores (a), which are liberated at b by the breaking up of the parent cells.

K. Germination of the spore of the hay bacillus (B. subtilis) - the axis of growth of the germinal rodlet is at right angles, to the long axis of the spore.

L. Germination of spore of Clostridium butyricum - the axis of growth coincides with the long axis of the spore.

While many forms are fixed to the substratum, others are free, being in this condition either motile or immotile. The chief of these forms are described below.

Cocci: spherical or spheroidal cells, which, according to their relative (not very well defined) sizes are spoken of as Micrococci, Macrococci, and perhaps Monas forms.

Rods or rodlets: slightly or more considerably elongated cells which are cylindrical, biscuit-shaped or somewhat fusiform. The cylindrical forms are short, i.e. only three or four times as long as broad ( Bacterium ), or longer (Bacillus); the biscuitshaped ones are Bacteria in the early stages of division. Clostridia, &c., are spindle-shaped.

Filaments really consist of elongated cylindrical cells which remain united end to end after division, and they may break up later into elements such as those described above. Such filaments are not always of the same diameter throughout, and their segmentation varies considerably. They may be free or attached at one (the " basal ") end. A distinction is made between simple filaments (e.g. Leptothrix ) and such as exhibit a false branching (e.g. Cladothrix). Curved and spiral forms. Any of the elongated forms described above may be curved or sinuous or twisted into a corkscrewlike spiral instead of straight. If the sinuosity is slight we have the Vibrio form; if pronounced, and the spiral winding well marked, the forms are known as Spirillum, Spirochaete, &c. These and similar terms have been applied partly to individual cells, but more often to filaments consisting of several cells; and much confusion has arisen form the difficulty of defining the terms themselves.

In addition to the above, however, certain Schizomycetes present aggregates in the form of plates, or solid or hollow and irregular branched colonies. This may be due to the successive divisions occurring in two or three planes instead of only across the long axis (Sarcina ), or to displacements of the cells after division.

Growth and Division. - Whatever the shape and size of the individual cell, cell-filament or cell-colony, the immediate visible results of active nutrition are elongation of the cell and its division into two equal halves, t;o?. across the long axis, by the formation of a septum, which either splits at once or remains intact for a shorter or longer time. This process is then repeated and so on. In the first case the separated A cells assume the character of the parent cell whose division a $ °° °

gave rise to them; in the second case they

o se form filaments, or, if the further elongation and divisions of the cells proceed in different directions, plates or spheroidal or other shaped colonies. It not unfrequently happens, however, that groups of cells break away from their former connexion as longer or shorter straight or curved filaments, or as solid masses. In some filamentous forms this " fragmentation " into multicellular pieces of equal length or nearly so is a normal phenomenon, each partial filament repeat s ing the growth, division and q? b n f? x ?d fragmentation as before (cf. figs. 2 and 6). By rapid division hundreds of thousands of cells may be produced in a few hours,' and, according to the species and the conditions (the medium, temperature, &c.), enormous collections of isolated cells may cloud the fluid in which they are cultivated, or form deposits below or films on its surface; valuable characters are sometimes obtained from these appearances. When these dense " swarms " of vegetative cells become fixed in a matrix of their own swollen contiguous cell-walls, they pass over into a sort of resting state as a so-called zoogloea (fig. 3).

One of the most remarkable phenomena in the life-history of the Schizomy cetes is the formation of this zoogloea stage, which corresponds to the " palmella " condition of the lower Algae. This occurs as a membrane on the surface of the medium, or as irregular clumps or branched masses (sometimes several inches across) submerged in it, and consists of more or less gelatinous matrix enclosing innumerable " cocci," " bacteria," or other elements of the Schizomycete concerned. Formerly regarded as a distinct genus - the natural fate of all the various ' Brefeld has observed that a bacterium may divide once every half-hour, and its progeny repeat the process in the same time. One bacterium might thus produce in twenty-four hours a number of segments amounting to many millions of millions.

D 0K2, a b c m Q c 6 L -...


FIG. 5. - Characteristic groups of Micrococci. (After Cohn.) A. Micrococcus prodigiosus. B. M. vaccinae. C. Zoogloea stage of a Micrococcus, forming a close membrane on infusion - Pasteur's Mycoderma. (Very highly magnified.) a 6. Bacillus megaterium. (After de Bary.) a, a chain of motile rodlets still growing and dividing ( bacilli). b, a pair of bacilli actively growing and dividing.

p, a rodlet in this condition (but divided into four segments) after treatment with alcoholic iodine solution.

c, d, e, f, successive stages in the development of the spores.

r, a rodlet segmented in four, each segment containing one ripe spore.

g ' , g 2, g3, early stages in the germination of the spores (after being dried several days); h2, i, k, 1 and m, successive stages in the germination of the spore. (aX250; all the rest X 600).


1 form

2 Myxobacteriaceae

3 Authorities

4 r. Active Immunity


the zoogloea is now known to be a sort of resting condition of the Schizomycetes, the various elements being glued together, as it were, by their enormously swollen and diffluent cell-walls becoming contiguous. The zoogloea is formed by active division of single or of several mother-cells, and the progeny appear to go on secreting the cell-wall substance, which then absorbs many times its volume of water, and remains as a consistent matrix, in which the cells come to rest. The matrix - i.e. the swollen cell-walls - in some cases consists mainly of cellulose, in others chiefly of a proteid substance; the matrix in some cases is horny and resistant, in others more like a thick solution of gum. It is intelligible from the mode of formation that foreign bodies may become entangled in the gelatinous matrix, and compound zoogloeae may arise by the apposition of several distinct forms, a common event in macerating troughs (fig. 3, A). Characteristic forms may be assumed by the young zoogloea of different species, - spherical, ovoid, reticular, filamentous, fruiticose, lamellar, &c., - but these vary considerably as the mass increases or comes in contact with others. Older A FIG. 7. - Bacillus e nthracis. (After Koch.) A. Bacilli mingled with blood-corpuscles from the blood of a guinea-pig; some of the bacilli dividing.

humour. They grow out into long leptothrix-like filaments, which B. The rodlets after three hours' culture in a drop of aqueous become septate later, and spores are developed in the segments. (X 650) .

zoogloeae may precipitate oxide of iron in the matrix, if that metal exists in small quantities in the medium. Under favourable conditions the elements in the zoogloea again become active, and move out of the matrix, distribute themselves in the surrounding medium, to grow and multiply as before. If the zoogloea is formed on a solid substratum it may become firm and horny; immersion in water softens it as described above.

The growth of an ordinary bacterium consists in uniform elongation of the rodlet until its length is doubled, followed by division by a median septum, then by the simul- Measure- taneous doubling in length of each daughter cell, again ment of followed by the median division, and soon (figs. 13, 14). growth. If the cells remain connected the resulting filament repeats these processes of elongation and subsequent division uniformly so long as the conditions are maintained, and very accurate measurements have been obtained on such a form, e.g. B. ramosus. If a rodlet in a hanging drop of. nutri' nt gelatine is fixed under the microscope and kept at constant temperature, a curve of growth can be obtained recording the behaviour during many hours or days. The measured lengths are marked off on ordinates erected on an abscissa, along which the times are noted. The curve obtained on joining the former points then brings out a number of facts, foremost among which are (1) that as long as the conditions remain constant the doubling periods - i.e. the times taken by any portion of the filament to double its length - are constant, because each cell is equally active along the whole length; (2) there are optimum, minimum and maximum temperatures, other conditions remaining constant, at which growth begins, runs at its best and is soon exhausted, respectively; (3) that the most rapid cell-division and maximum growth do not necessarily accord with the best conditions for the life of the organism; and (4) that any sudden alteration of temperature brings about a check, though a slow rise may accelerate growth (fig. 8). It was also shown that exposure to light, dilution or exhaustion of the food-media, the presence of traces of poisons or metabolic products check growth or even bring it to a standstill; and the death or injury of any single cell in the filamentous series shows its effect on the curve by lengthening the doubling period, because its potential progeny have been put out of play. Hardy has shown that such a destruction of part of the filament may be effected by the attacks of another organism.










80 85 1 J FIG. 8. - Curve of growth of a filament of Bacillus ramosus (Fraenkel), constructed from data such as in fig. 4. The abscissae represent intervals of time, the ordinates the measured lengths of the growing filament. Thus, at 2.33 the length of the filament was 6; at 5.45, at 8 P.M., 70 and so on. Such curves show differences of steepness according to the temperature (see temp. curve), and to alterations of light (lamp) and darkness. (H. M. W.) A very characteristic method of reproduction is that of sporeformation, and these minute reproductive bodies, which represent a resting stage of the organism, are now known in many spores. forms. Formerly two kinds of spores were described, arthrospores and endospores. An arthrospore, however, is not a true spore but merely an ordinary vegetative cell which separates and passes into a condition of rest, and such may occur in forms which form endospores, e.g. B. subtilis, as well as in species not known to form endospores. The true spore or endospore begins with the appearance of a minute granule in the protoplasm of a vegetative cell; this granule enlarges and in a few hours has taken to itself all the protoplasm, secreted a thin but very resistive envelope, and is a ripe ovoid spore, smaller than the mother-cell and lying loosely in it (cf. figs. 6, 9, io, and II). In the case of the simplest and most minute Schizomycetes III. 6 280 260 240 220 200 180 160 140 120 100 80 60 40 20 2pm, 2.33 S.3S 3.5 3.30 S.9S 620 7.7 ( Micrococcus, &c.) no definite spores have been discovered; any one of the vegetative micrococci may commence a new series of cell by growth and division. We may call these forms " asporous," at any rate provisionally.

The spore may be formed in short or long segments, the cellwall of which may undergo change of form to accommodate itself to the contents. As a rule only one spore is formed in a cell, and the process usually takes place in a bacillar segment. In some cases the spore-forming protoplasm gives a blue reaction with iodine solutions. The spores may be developed in cells which are actively swarming, the movements not being interfered with by the process (fig. 4, D). The so-called " KOpfchenbacterien " of older writers are simply bacterioid segments with a spore at one end, the mother cell-wall having j adapted itself to the outline of the spore 2 (fig. 4, F). The ripe spores of Schizomy y 0 cetes are spherical, ovoid or long-ovoid in ab shape and extremely minute (e.g. those of Bacillus subtilis measure o. 0012 mm. long by o.0006 mm. broad according to Zopf), highly refractive and colourless (or very dark, probably owing to the high index of refraction and minute size). The membrane may be relatively thick, and even exhibit shells or strata.

The germination of the spores has now been observed in several forms with care.

A The spores are capable of germination at once, or they may be kept for months and even years, and are very resistant against desiccation, heat and cold, &c. In a suitable medium and at a proper temperature the germination is completed in a few hours. The spore swells and elongates and the contents grow forth to a cell like that which produced it, in some cases clearly breaking through the membrane, the remains of which may be seen attached to the young germinal rodlet (figs. 5, 9 and II); in other cases the surrounding membrane of the spore swells and dissolves. The germinal cell then grows forth into the forms typical for the particular Schizomycete concerned.

The conditions for spore-formation differ. Anaerobic species usually require little oxygen, but aerobic species a free supply. Each species has an optimum temperature and many are known to require very special food-media. The systematic interference with these conditions has enabled bacteriologists to induce the development of socalled asporogenous races, in which the formation of spores is indefinitely postponed, changes in vigour, virulence and other properties being also involved, in some cases at any rate. The addition of minute traces of acids, poisons, &c., leads to this change in some forms; high temperature has also been used successfully.

The difficult subject of the classification of bacteria dates ' The difficulties presented by such minute and simple organisms as the Schizomycetes are due partly to the few " characters " which they possess and partly to the dangers of error in manipulating them; it is anything but an easy matter either to trace the whole development of a single form or to recognize with certainty any one stage in the development unless the others are known. This being the case, and having regard to the minuteness and ubiquity of these organisms, we should be very careful in accepting evidence as to the continuity or otherwise of any two forms which falls short of direct and uninterrupted observation. The outcome of all these considerations is that, while recognizing that the " genera " and " species " as defined by Cohn must be recast, we are not warranted in uniting any forms the continuity of which has not been directly from the year 1872, when Cohn published his system, which was extended in 1875; this scheme has in fact dominated the study of bacteria ever since. Zopf in 1885 proposed a scheme based on the acceptance of extreme views of pleomorphism; his system, however, was extraordinarily A FIG. I o. - Bacillus subtilis. (After Strasburger.) A. Zoogloea pellicle (X 500). B. Motile rodlets (X 1000). C. Development of spores (X 800).


impracticable and was recognized by him as provisional only. Systems have also been brought forward based on the formation of arthrospores and endospores, but as explained above this is eminently unsatisfactory, as arthrospores are not true spores and both kinds of reproductive bodies are found in one and the same form. Numerous attempts have been made to construct schemes of classification based on the power of growing colonies _ ? J ?

P.M. - -.__-- ,:?

.., ?,,?

?;?. ????? ?4 ?. -- FIG. I I. - Stages in the development of spores of Bacillus ramosus (Fraenkel), in the order and at the times given, in a hanging drop culture, under a very high power. The process begins with the formation of brilliant granules (A, B); these increase, and the brilliant substance gradually balls together (C) and forms the spores (D), one in each segment, which soon acquire a membrane and ripen (E). (H. M. W.) to liquefy gelatine, to secrete coloured pigments, to ferment certain media with evolution of carbon dioxide or other gases, or to induce pathological conditions in animals. None of these systems, which are chiefly due to the medical bacteriologists, has maintained its position, owing to the difficulty of applying the characters and to the fact that such properties are physiological and liable to great fluctuations in culture, because a given organism may vary greatly in such respects according to its degree of vitality at the time, its age, the mode of nutrition observed; or, at any rate, the strictest rules should be followed in accepting the evidence adduced to render the union of any forms probable.

1 l ?,?I?, ? li ?

i ,,? 1 ;ti?'?I„ i? I?, ,41lh'?ll!

?? ??I`` ?I?1?i????? ?i ?Irililli??ili;??I ? ?1'" ? l ? ,?, i III I ???? ?? i ,?,???i?fl II '111?

,;)?i?; i?? ? ?? ??'d,`??, 1 '1;?? AV 0 ?

FIG. 9.

A, Bacillus anthracis. (After de Bary.) Two of the long filaments (B, fig. io), in which spores are being developed. The specimen was cultivated in broth, and spores are drawn a little too small - they should be of the same diameter transversely as the segments. (X 600.) B. Bacillus subtilis. (After de Bary.) "1, fragments of filaments with ripe spores; 2-5, successive stages in the germination of the spores, the remains of the spore attached to the germinal rodlets. (X 600.) P.M. and the influence of external factors on its growth. Even when used in conjunction with purely morphological characters, these physiological properties are too variable to aid us in the discrimination of species and genera, and are apt to break down at critical periods. Among the more characteristic of these schemes adopted at various times may be mentioned those of Miguel (1891), Eisenberg (1891), and Lehmann and Neumann (1897). Although much progress has been made in determining the value and constancy of morphological characters, we are still in need of a sufficiently comprehensive and easily applied scheme of classification, partly owing to the existence in the literature of imperfectly described forms, the life-history of which is not yet known, or the microscopic characters of which have not been examined with sufficient accuracy and thoroughness. The principal attempts at morphological classifications recently brought forward are those of de Toni and Trevisan (1889), Fischer (1897) and Migula (1897). Of these systems, which alone are available in any practical scheme of classification, the two most important and most modern are those of Fischer and Migula. The extended investigations of the former on the number and distribution of cilia (see fig. I) led him to propose a scheme of classification based on these and other morphological characters, and differing essentially from any preceding one. This scheme may be tabulated as follows: I. O [[Rder - I]]-laplobacterinae. Vegetative body unicellular; spheroidal, cylindrical or spirally twisted; isolated or connected in filamentous or other growth series.

I. Family - CoecACEAE. Vegetative cells spheroidal.

(a) Sub-family - ALLOcoccAcEAE. Division in all or any planes, colonies indefinite in shape and size, of cells in short chains, irregular clumps, pairs or isolated :- Micrococcus (Cohn), cells non-motile; Planococcus (Migula), cells motile.

(b) Sub-family - HoMococcACEAE. Division planes regular and definite : - Sarcina (Goods.), cells non-motile; growth and division in three successive planes at right angles, resulting in packet-like groups; Planosarcina (Migula), as before, but motile; Pediococcus (Lindner), division planes at right angles in two successive planes, and cells in tablets of four or more; Streptococcus (Billr.), divisions in one plane only, resulting in chains of cells.

2. Family - Bacillaceae. Vegetative cells cylindric (rodlets), ellipsoid or ovoid, and straight. Division planes always perpendicular to the long axis.

(a) Sub-family - Bacilleae. Sporogenous rodlets cylindric, not altered in shape : - Bacillus (Cohn), non-motile; Bactrinium (Fischer), motile, with one polar flagellum (monotrichous); Bactrillum (Fischer), motile, with a terminal tuft of cilia (lophotrichous); Bactridium (Fischer), motile, with cilia all over the surface (peritrichous) .

Sub-family - Clostridieae. Sporogenous rodlets, spindle-shaped : - Clostridium (Prazm.), motile (peritrichous).

(c) Sub-family - Plectridieae. Sporogenous rodlets, drumstick-shaped : - Plectridium (Fischer), motile (peritrichous).

3. Family - SPIRILLAcEAE. Vegetative cells, cylindric but curved more or less spirally. Divisions perpendicular to the long axis: - Vibrio (Muller-Ldffler), commashaped, motile, monotrichous; Spirillum (Ehrenb.), more strongly curved in open spirals, motile, lopho trichous; Spirochaete (Ehrenb.), spirally coiled in numerous close turns, motile, but apparently owing to flexile movements, as no cilia are found.

II. O Rder - Trichobacterinae. Vegetative body of branched or unbranched cell-filaments, the segments of which separate as swarm-cells ( Gonidia). I. Family - Trichobacteriaceae. Characters those of the Order.

(a) Filaments rigid, non-motile, sheathed: - Crenothrix (Cohn), filaments unbranched and devoid of sulphur particles; Thiothrix (Winogr.), as before, but with sulphur particles; Cladothrix (Cohn), filaments branched in a pseudo-dichatomous manner.

(b ) Filaments showing slow pendulous and creeping movements, and with no distinct sheath: - Beggiatoa (Tre y .), with sulphur particles.

The principal objections to this system are the following: - (1) The extraordinary difficulty in obtaining satisfactory preparations showing the cilia, and the discovery that these motile organs are not formed on all substrata, or are only developed during short periods of activity while the organism is young and vigorous, render this character almost nugatory. For instance, B. megatherium and B. subtilis pass in a few hours after commencement of growth from a motile stage with peritrichous cilia, into one of filamentous growth preceded by casting of the cilia. (2) By far the majority of the described species (over woo) fall into the three genera - Micro- coccus (about 400), Bacillus (about 200) and Bactridium (about 150), so that only a quarter or so of the forms are selected out by the other genera. (3) The monotrichous and lophotrichous conditions are by no means constant even in the motile stage; thus Pseudomonas rosea (Mig.) may have I, 2 or 3 cilia at either end, and would be distributed by Fischer's classification between Bactrinium and Bactrillum, according to which state was observed. In Migula's scheme the attempt is made to avoid some of these difficulties, but others are introduced by his otherwise clever devices for dealing with these puzzling little organisms.

The question, What is an individual ? has given rise to much difficulty, and around it many of the speculations regarding pleomorphism have centred without useful result. If a tree fall apart into its constituent cells periodically we should have the same difficulty on a larger and more complex scale. The fact that every bacterial cell in a species in most cases appears equally capable of performing all the physiological functions of the species has led most authorities, however, to regard it as the individual - a view which cannot be consistent in those cases where a simple or branched filamentous series exhibits differences between free apex and fixed base and so forth. It may be doubted whether the discussion is profitable, though it appears necessary in some cases - e.g. concerning pleomorphy - to adopt some definition of individual.


To the two divisions of bacteria, Haplobacterinae and Trichobacterinae, must now be added a third division, Myxobacterinae. One of the first members of this group, Chondromyces crocatus, was described as long ago as 1857 by Berkeley, but its nature was not understood and it was ascribed to the Hyphomycetes. In 1892, however, Thaxter rediscovered it and showed its bacterial nature, founding for it and some allied forms the group Myxobacteriaceae. Another form, which he described as Myxobacter, was shown later to be the same as Polyangium vitellinum described by Link in 1795, the exact nature of which had hitherto been in doubt. Thaxter's observations and conclusions were called in question by some botanists, but his later observations and those of Baur have established firmly the position of the group. The peculiarity of the group lies in the fact that the bacteria form plasmodiumlike aggregations ' and build themselves up into sporogenous structures of definite form superficially similar to the cysts of the Mycetozoa (fig. 12). Most of the forms in question are found growing on the dung of herbivorous animals, but the bacteria occur not only in the alimentary canal of the animal but also free in the air. The Myxobacteria are most easily obtained by keeping at a temperature of 30-35° C. in the dark dung which has lain exposed to the air for at least eight days. The high temperature is favourable to the growth of the bacteria but FIG. 12.

A. Myxococcus digelatus,bright red fructification occurring on dung (X 120).

B. Polyangium primigenum, red fructification on dog's dung (X40)

C. Chondromyces apiculatus, orange fructification on antelope's dung.

D. Young fructification (X45)

E. Single cyst germinating (X200).

(A, B. after Quehl; C-E,after Thaxter.) From Strasburger's Lehrbuch der Bolanik, by permission of Gustav Fischer.

inimical to that of the fungi which are so common on this substratum.

The discoveries that some species of nitrifying bacteria and perhaps pigmented forms are capable of carbon-assimilation, that others can fix free nitrogen and that a number of decompositions hitherto unsuspected are accom fished by Schizomycetes have ut thequestions of P Y Y, P d nutrition and fermentation in quite new lights. Apart from numerous fermentation processes such as rotting, the soaking of skins for tanning, the preparation of indigo and of tobacco, hay, ensilage, &c., in all of which bacterial fermentations are concerned, attention may be especially directed to the following evidence of the supreme importance of Schizomycetes in agriculture and daily life. Indeed, nothing marks the attitude of modern bacteriology more clearly than the increasing attention which is being paid to useful fermentations. The vast majority of these organisms are not pathogenic, most are harmless and 4.,52 5.20 5.20 FIG. 13, - A series of phases of germination of the spore of B. ramosus sown at 8.30 (to the extreme left), showing how the growth can be measured. If we place the base of the filament in each case on a base line in the order of the successive times of observation recorded, and at distances apart proportional to the intervals of time (8.30, 10.0, 10.30, 11.40, and so on) and erect the straightened-out filaments, the proportional length of each of which is here given for each period, a line joining the tips of the filaments gives the curve of growth. (H. M. W.) many are indispensable aids in natural operations important to man.

Fischer has proposed that the old division into saprophytes and parasites should be replaced by one which takes into account other peculiarities in the mode of nutrition of bacteria. The nitrifying, nitrogen-fixing, sulphurand iron-bacteria he regards as monotrophic, i.e. as able to carry on one particular series of fermentations or decompositions only, and since they require no organic food materials, or at least are able to work up nitrogen or carbon from inorganic sources, he regards them as primitive forms in this respect and terms them Prototrophic. They may be looked upon as the nearest existing representatives of the primary forms of life which first obtained the power of working up non-living into living materials, and as playing a correspondingly important role in the evolution of life on our globe. The vast majority of bacteria, on the other hand, which are ordinarily termed saprophytes, are saprogenic, i.e. bring organic material to the putrefactive state - or saprophilous, i.e. live best in such putrefying materials - or become zymogenic, i.e. their metabolic products may induce blood-poisoning or other toxic effects (facultative parasites) though they are not true parasites. These forms are termed by Fischer Metatrophic, because they require various kinds of organic materials obtained from the dead remains of other organisms or from the surfaces of their bodies, and can utilize and decompose them in various ways ( Polytrophic) or, if monotrophic, are at least unable to work them up. The true parasites - obligate parasites of de Bary - are placed by Fischer in a third biological group, Paratrophic bacteria, to mark the importance of their mode of life in the interior of living organisms where they live and multiply in the blood, juices or tissues.

When we reflect that some hundreds of

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Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Bacteriology'. 1911 Encyclopedia Britanica. 1910.

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