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BACTERIOLOGY

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Originally appearing in Volume V03, Page 160 of the 1911 Encyclopedia Britannica.
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BACTERIOLOGY. 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 ex-tended 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 Myxomycetes. 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 Micro-cocci, 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. ,BaKrrtpiov, 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 (-A'\ H ) A. Bacillus subtilis, Cohn, and F. Bacillus typhi, Gaffky. Spirillum undula, Ehrenb. G. B. vulgaris (Hauser), Migula. B. Planococcus citreus (Menge), H. Microspira Comma (Koch), Migula. [sard), Migula. Schroeter. C. Pseudomonas pyocyanea (Ges- J, K. Spirillum rubrum, Es- D. P. macroselmis, Migula. marsch. E. P. syncyanea (Ehrenb.), L, M. S. undula(Mtiller) , Ehrenb. Migula. (All after Migula.) a few secrete colouring matters other than chlorophyll. In size their cells are commonly about moos 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 Distrlbu- rocks, and may dismiss as absurd the statements that timeln 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 especiallyin drains, bogs, refuse heaps, and in the soil, and wherever organic infusions are. allowed to stand for a short time. Anyliquid (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- Ulstr3L!s- normal. Though they occur all the world over in the non la space. 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 the 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 186o. A. van Leeuwenhoek figured bacteria as far back as the ,7th century, and O. F. Miller knew several History. 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 appeals 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, Micrococcus, Bacillus, Leptothrix, &c., occur as phases in one life-history. Lister put forth similar ideas about the same time; and Biliroth 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: hat it is now generally agreed that, while a 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 life-history. 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 life-history 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 pf 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 ton 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 ,rreae. 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, Burdon-Sanderson, 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 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 187o onwards the " germ theory of disease " has passed into acceptance. P. F. O. Rayer in 185o 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 187o 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 188o 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). MoRPFioL0GY.-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 Form and sfructure. 1 u (•oor mm.) in diameter, others have cells measur- ing 4µ or 5µ or even 7µ 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 Ce»-xati. 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 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, Mencl), 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 c,... a ..{',IIIIIv:••: II:I : ,7,1,;1% ' I Ii I I+ , I, .!,I 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 reactions 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, how-ever, 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. A. The spore sown at s s 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 8.3o 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, cell- which enclose a watery fluid. Scattered in the proto- contenta. plasm arc 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 Nucleus. 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 overlooked or is in-visible 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 dis- C t i n c t from the Brownian movement—a non-vital phenomenon common to all finely- divided particles G suspended in a fluid. Independent movement is effected by special motile organs, the cilia or flagella. These structures are in-visible, with ordinary illumination in living cells or un- • stained 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. r) of the same nature as those of the zoospores and anthcrozoids 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 Cma 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. I, C, D), or one. at each pole; others again have a tuft of several cilia 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. B. Egg-shaped mass of zoogloea of Beggiatoa roseo-persicina (Bacterium .rubescens of Lankester) ; the gelatinous swollen walk; of the large crowded cocci are fused into a common gelatinous envelope. C. Reticulate zoogloea of the same. D, E, H. Colonies of Myconostoc enveloped in diffluent matrix. F. Branched fruticose zoogloea of Cladothrix (slightly magnified). G. Zoogloea of Bacterium merismopedioides, ,Zo2 f, containing cocci arranged in tablets. 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. E. Spore; formation in Vibrio- like (c) and Spirillum-like (a. b, d) Schizomycetes. F. Long rod-likeform containing a spore (these 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 Micro-cocci, 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 biscuit-shaped 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 corkscrew-like 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 from the difficulty of defining the terms themselves. In addition to the above, however, certain Schizomycetes presentaggregates 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, tio Renp ap 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 cells assume the char- •sA• °$ g its acter of the parent- ti ti • cell whose division gave rise to them; in • `; •• the second case they °• s '• 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 hap-pens, 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- ing the growth, division and 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 de-posits 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 forma- tion 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. at one pole (Lophotrichous), e.g. B. syncyaneus (fig. r, 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 0scillatoria among the Cyanophyceae. In these vegetative cases no cilia have been observed, and there is a state. firm cell-wall, so the movement remains quite unexplained. rc e5 e ~ d~ A B O c 6 0 a 0 0 o. 3 a H L z G
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