SCHIZOMYCETES, a term proposed by Nägeli in 1857 to include all those minute organisms known as Bacteria, Microphytes, Microbes, &c., and allied forms. These terms have been used at various times by different authors with widely different meanings in detail, but it is now agreed that the Schizomycetes 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 other aggregates of such cells. True spores occur in several, but no trace whatever of sexual organs exists. From their mode of growth, division, and spore-formation (in part), as well as their habit of forming deliquescent, swollen cell-walls, and other peculiarities, there can be no doubt of the close alliance between the Schizomycetes and certain lower Algae; whence both groups have been conjoined under the name Schizophyta. No one character except the want of chlorophyll—which of course entails physiological differences—separates the Schizomycetes from other Schizophyta; morphologically and phylogenetically the two groups are united. From this point of view wo relegate all the so-called bacteria which contain chlorophyll (e.g., Engelmann’s Bacterium chlorinum, Van Tieghem’s B. viride and Bacillus virens, Cohn’s Micrococcus chlorinus, &c.) to the Algae.
Schizomycetes, then, are saprophytic or parasitic Schizophyta devoid of chlorophyll, though they may secrete other colouring matters. In size their cells are commonly about 0·001 mm. (called 1 micro-millimetre = lμ) in diameter, or from two to five times that length; but smaller ones and a few larger are known. The various shapes assumed by the cells are shown in fig. 1; the filamentous and other aggregates will be described below.
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 foodstuff (meat, preserves, vegetables, &c.), allowed to stand exposed to the air soon swarms with bacteria, if moisture is present and the temperature not abnormal. Though they occur all the world over in the 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 men 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. Leeuwenhoek figured Bacteria as far back as the 17th century, and O. F. Müller 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 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 violent 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 Billroth came forward in 1874 with the startling view that the various “form-species” and “formgenera” are only different states of one and the same organism. From that time to the present the discussion as to the limits of “species” among the Schizomycetes has been maintained; much extravagance has resulted, as well as valuable additions to our knowledge of the forms. Klebs (1875) and Nägeli (1877) upheld similar views to those suggested by Lankester; and the researches of Cienkowski, Zopf, Kurth, and De Bary have rendered it clear that forms employed by Cohn to define genera and species (it should be borne in mind that Cohn recognized their provisional nature) occur as phases in one and the same life-history. Zopf showed (1882) that minute spherical “cocci,” short rodlets (“bacteria”), longer rodlets (“bacilli”), and filamentous (“leptothrix”) forms as well as curved and spiral threads (“vibrio,” “spirillum”), &c., occur as vegetative stages in one and the same Schizomycete (cf. fig. 16). In the meantime, while various observers were building up our knowledge of the morphology of the Schizomycetes, others were laying the foundations of what is known of the relations of these organisms to fermenta- tion 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 fluids. 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 Schizomycetes 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 Schizomycetes. 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 importance.
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 exist 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 reproducing itself had been entertained. Such vague notions began to take more definite shape as the ferment theory of Cagniard-Latour (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. 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. 10 and 11). In 1870 Pasteur had proved that a disease of silkworms was due to a ferment-organism of the nature of a Schizomycete; 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 a few other diseases are now brought definitely within the range of biology, and several other contagious and infectious diseases are known to be also due to Schizomycetes.
Other questions of the highest importance have arisen from the foregoing. A few years ago Pasteur 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 Chaveau confirmed, and others have extended the observations. More remarkable still, animals inoculated with such “attenuated” bacilli proved to be curiously resistent 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 now burning,—Metschnikoff’s recent observations (1884), showing that the white corpuscles eliminate the bacilli from the blood, being one of the most startling contributions to the answers.
Another burning question has already been in part touched upon. Experiments have shown that Schizomycetes are pleomorphic; they are also very sensitive, so to speak, to the influences of the environment. The investigations of Cohn, Pasteur, Koch, Nägeli, Kurth, De Bary, and others leave no doubt that many Schizomycetes are sensibly affected by the media in which they are cultivated: not only are the forms modified, but also the physiological activity varies in degree, and even in kind. These and similar facts seem to be largely responsible for recent ideas as to the possibility of being able to cultivate or “educate” certain Schizomycetes. One case only need be referred to. Bacillus anthracis and B. subtilis are only distinguishable with great difficulty morphologically (cf. figs. 10-12); the former is parasitic in its vegetative stages, the latter is always a saprophyte. Now B. anthracis, as said, can become harmless by cultivation, and so it has been thought that the two forms were convertible. Buchner even went so far as to declare that he had transformed B. anthracis into B. subtilis, i.e., that the differences which botanists detect are only due to the influence of the environment at the time. These assertions cannot be regarded as proved; but the question whether harmless forms can become educated, as it were, to a parasitic mode of life within periods which we can control is of course of the highest importance. Such are a few of the questions now under discussion, together with others as to the mode of action of patho- genic Schizomycetes, as to the nature of immunity, and as to the limitation of “species” among such simple forms. 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 1 μ (·001 mm.) in diameter, others have cells measuring 4μ, or 5 μ or even 7 μ or 8 μ 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. As to the structure of the cell, little more can be said than that it consists of a mass of homogeneous or very slightly granular protoplasm, with a pearl-like lustre, and without vacuoles; this is enveloped by a membranous envelope, which is so delicate as to be scarcely perceptible. In the actively vegetating or mobile conditions this cell wall appears very thin and sharp, and is extremely flexible and elastic, but at other times it is swollen and diffluent, furnishing the intercellular gelatinous matrix of the zoogloea condition (fig. 3). It is doubtful whether the thin envelope closely applied to the protoplasm is not always simply the innermost layer of a very diffluent covering, which is continuously thickening and throwing off its outermost swollen and disorganized lamellae. The facts to hand seem to show that, while in some cases this envelope consists mainly of cellulose, in others (zoogloea of Bacteria, e.g.) it contains relatively large proportions of nitrogenous compounds. In some cases the cell-walls form a lamellated sheath. No cuticularization occurs, nor are deposits of lime or silex known in the cell walls. Colouring pigments, however (red, yellow, and even green and blue), are sometimes met with, and a rusty or brown tinge is in some cases produced by the precipitation of iron oxides in the walls. In the typical Schizomycetes the protoplasmic contents (which are said to consist largely of a peculiar substance named mycoprotein) are colourless, or more rarely tinged with colouring matters—bright red, yellow, &c.—which cannot be mistaken for chlorophyll. The few forms described as containing a green pigment, allied to or identical with chlorophyll, will not be considered here, but relegated to the Algse. The occurrence of starch or a granulose-like substance in some Bacteria is undoubted; it yields a deep blue colour with iodine solutions, is diffused in bands or patches, and arises in cases where the Schizomycete is nourished by a matrix which does not contain starch. Trécul noticed this formation of amyloid substance in Clostridium, Van Tieghem in a Spirillum, and several other cases are known; Ward detected starch in a Bacillus found in decaying coffee seeds, and in other media devoid of starch. In the filamentous Schizomycetes (Beggiatoa, e.g.) are found extremely minute dark granules; Cramer and Cohn have shown that these consist of sulphur in fine crystals (fig. 14). Oily or fatty substances and minute granules of undetermined nature occur in the protoplasm, but no nucleus has as yet been discovered in any Schizomycete.
Vegetative States.— While many forms are fixed to a substratum, others are free; and in certain conditions single cells or groups may be motile. In some cases the movements are mere oscillations, in others there are rapid movements of translation, sometimes ascribed to the action of flagella or cilia; these movements are of course not to be confounded with the dancing “Brownian motion” observed in the case of all such minute bodies suspended in fluids. Cilia have now been described in some of the smallest Bacteria by several good observers (Dallinger and Drysdale, Cohn, Koch, Zopf), though, on account of their extreme fineness, and the difficulty of fixing them, much discussion has taken place as to their nature, functions, origin, numbers, and even existence; that they occur is proved by the photographs, but whether they are not sometimes mere filaments drawn out from the cell-walls is very doubtful
(figs. 2 and 12).
While some Schizomycetes appear to have no active stage, and many are only motile under certain conditions when swarming, others are described as possessing two or even three distinct active forms.
When vigorously growing and dividing, the Schizomycetes as a rule present certain definite forms, which are at any rate so constant under constant conditions that they can be figured and described with such accuracy and certainty that good observers have regarded them as fixed species, or at least as “form-species” or “form-genera.” We now know, however, that many Schizomycetes pass through several such phases, and we may therefore regard them in these cases as “vegetative forms,” which pass into one another too gradually to admit of their being employed as sharply distinctive of genera.
As the chief of these forms may be mentioned the following (see fig. 1) :— 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 biscuit-shaped ones are Bacteria in the early stages of division. Clostridia, &c., are spindle-shaped.
Filaments (Leptothrix forms) 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. Various observera have, moreover, described particular cases where the cells or cell-filaments exhibit irregularities of form; such “involution forms,” “torula forms,” &c., appear to bo fairly constant in some cases.
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 (as in the zooglcea conditions, &c., see fig. 3).
Growth and Division.— Whatever the shape and size of the individual cell, cell-filament, or cell-colony, the immediate visible re sults of active nutrition are elongation of the cell and its division into two equal halves, across the long axis, by the formation of a septum, which either splits at once or re mains intact for a shorter or longer time. This process is then repeated, and so on. In the first case the separated cells assume the characters of the parent-cell whose division gave rise to them; in the second case they form filaments, or, if the further elongation and divisions of the cells proceed in different directions, plates or spheroidal or othershaped 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 repeating the growth, division, and fragmentation as before (cf. figs. 15
and 16). Finally, such filaments may break up into their individual cells, forming “bacilli,” “bacteria,” or “cocci” as the case may be. By these means 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 lifehistory of the Schizomycetes 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 irre gular clumps or branched masses
(sometimes several inches across) sub merged in it, and consists of more or less gelatinous ma trix enclosing in numerable “cocci,”
“bacteria,” or other elements of the
Schizomycete con cerned. Formerly regarded as a distinct genus—the natural fate of all the various forms—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 “my-coprotein,” the substance said to be met with in the protoplasm; the matrix in some cases is horny and resistent, 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, fruticose, lamellar,
&c.,—but these vary considerably as the mass increases or comes in contact with others. Older 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 (fig. 4). If the zoogloea is formed on a solid substratum it may become firm and horny; immersion in water softens it as described above.
Spores.— Spores or resting-cells are now known in many Schizomycetes (fig. 5). They may be formed in two ways.
In Leuconostoc, Bacterium zopfii, Crenothrix, Beggiatoa, and Cladothrix the spore is simply one of the smallest segments (“cocci”) into which the filament at length breaks up. De Bary terms such forms “arthrosporous” (cf. figs. 8, 13, 14, and 16). In others the formation of the spore is “endosporous” (De Bary). It 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 dense envelope, and is a ripe ovoid spore, smaller than the mother-cell, and lying loosely in it (cf . figs. 9, 11, and 12). In the case of the simplest and most minute Schizomycetes (Micrococcus, &c.) no definite spores have been discovered; any one of the vegetative micrococci may commence a new series of cells 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 cell-wall 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. 5, D). The so-called “Köpfchenbacterien” of older writers are simply bacterioid segments with a spore at one end, the mother cell-wall having adapted itself to the outline of the spore (fig. 5, F). The ripe spores of Schizomycetes are spherical, ovoid, or long-ovoid in shape, and extremely minute (e.g., those of Bacillus subtilis measure 0·0012 mm. long by 0·0006 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. The spores are capable of germination at once, or they may be kept for months and even years, and are very resistent 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 11); 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. Pleomorphism.— As already stated, some Schizomycetes have been shown to present as vegetative forms, or phases in one and the same life-history, “cocci,” “bacteria,”
“leptothrix-filaments,” and even spiral and curved forms known as “spirillum,” “vibrio,” «fcc. On the other hand, several Schizomycetes which have been long and diligently investigated by the best observers show no such pleomorphism. As examples of the latter we may select Bacillus megaterium (fig. 9) and numerous Micrococci which produce similar cells generation after generation. A remarkable example of a pleomorphic form is Clado-thrix dichotoma (fig. 16). According to Zopf this species passes successively through the stages known as “coccus,” “bacterioid,” “bacillar,” and “leptothrix,” by mere elongation and division by transverse septa; the observer named declares that these simple filaments have formerly received generic and specific names {Leptothrix parasitica and L. ochracea, Kiitz.). Certain of the threads then partially break up, and the portions become slightly displaced from the linear series; these portions go on growing in a direction at an angle with the previous one, but still in contact, and thus produce the “false-branching” to which Cladothrix owes its name. Finally the filaments break up into segments corresponding with the septa which have been formed across them. This fragmentation is peculiar in that the filaments separate first into shorter filaments, then into rodlets, and finally into “cocci.” Portions of the filaments or branches may become separated and travel with a gliding movement, or even become more active and swarm by means of cilia. Such portions may break up into shorter filaments or rods which also swarm. But, in addition to these straight and more or less rigid forms (which, it will be noticed, simulate Ehrenberg and Cohn’s “genera” Micrococcus, Bacterium, Bacillus, and Leptothrix so closely that any of them observed alone would undoubtedly have been formerly placed apart in one of those “genera ”), it is interesting to find that some of the filaments become spirally twisted and simulate Spirillum, Spirochaete, and Vibrio, the dis tinctions depending on the relative length and thickness of the filament, and the closeness or steepness of the coils. Moreover these twisted filaments also break up into shorter gliding or ciliated portions, which at length fall into rodlets and “cocci” as before.
A branched zoogloea form also occurs, and this contains cocci, bacterium-like or bacillar rods, or filaments resembling Leptothrix or Vibrio according to circumstances. In Lankester’s Bacterium rubescens we have another species which is variable in a high degree. Many other Schizomycetes have now been shown to be more or less pleomorphic, and the researches of Lankester, Nägeli, Zopf, Miller, Kurth, De Bary, and others have laid the foundation for a knowledge of the circumstances which induce the changes in form referred to; it is at least certain that alterations in the nutritive medium, in the quantity of oxygen at the disposal of the organism, and in the temperature, &c., play their part in the matter.
It by no means follows, however, that because some species are pleomorphic all must be so, and still less that no species of Schizomycetes—or only one—exist at all; those who deny the existence of species among the Schizomycetes on the evidence to hand must, to be logically consistent, deny the existence of species altogether. But even if that be allowed, some name of similar intention must be employed to denote any group of organisms which within our experience exhibit periodical repetitions of a process of development, i.e., all the individuals of successive generations go through the same phases periodically. It matters not that variations—ill-defined deviations from an average or “type”—occur on the part of individuals or generations; the periodically repeated life-history or development marks what we term a species.
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 observed; or, at any rate, the strictest rules should be followed in accepting the evidence adduced to render the union of any forms probable. Classification. —The limits of this article prevent our examining in detail the system of classification proposed by Cohn, or the modifications of it followed by other authorities. Zopf, in the third edition of his work (1885), proposes a scheme based on the modern views as to the pleomorphism: we must refer to the original for the details, simply remarking that, apart from the extreme views accepted by the author, his system is impracticable to a degree and recognized by him as provisional only. Indeed any such classification must be provisional, for we are at the threshold only of a knowledge of the Schizomycetes.
The best starting-point for a modern classification of these organisms is that suggested by De Bary—the two modes of formation of the spores,—and as a provisional scheme, and simply to facilitate comparison of the groups, we might perhaps employ De Bary’s two groups, and a third one to include those simple forms which show no trace of spore-formation. Many gaps exist, and many changes will probably have to be made. Meanwhile it might be advisable to classify the Schizomycetes provisionally as follows :—
Group A. Asporeae.
There are no spores distinct from the vegetative cells.
I. Coccaceae (figs. 6 and 7).
Genera: 1, Micrococcus (and Streptococcus) ; 2, Sarcina (and Zopf’s Merismopedia) ; 3, Ascococcus. Group B. Arthrosporeae (De Bary).
The vegetative cells differ in shape, size, growth, or other characters from the spores: the latter are produced by segmentation.
II. Arthrobacteriaceae.
Genera: 4, Bacterium (fig. 8); 5, Leuconostoc; 6, Spirochaete (†).
III. Leptotricheae.
Genera: 7, Crenothrix (fig. 13); 8, Beggiatoa (figs. 14 and 15); 9, Phragmidothrix (?); 10, Leptothrix.
IV. Cladotricheae.
Genus: 11, Cladothrix (fig. 16).
Group C. Endosporeae (De Bary).
Genera: 12 (figs. 9-12), Bacillus (and Clostridium) ; 13, Vibrio (?); 14, Spirillum (at least in part). Physiology. —As in the case of other plants, we are here concerned with the functions of the Schizomycetes and their relations to the environment; for convenience, the subject may be treated under various headings. Limitation of space prevents our doing more than touch lightly upon such matters as the action of the Schizomycetes as ferments, and their relations to disease, though both subjects belong strictly to the physiology of their nutrition and actions on the environment.
Nutrition.— Having no chlorophyll, the Schizomycetes of course depend on other organisms for their carbonaceous food, and are either saprophytes— i.e., live on the remains of dead organisms— or parasites— i.e., obtain their food direct from living organisms. Pasteur, Nägeli, and others have shown that these organisms can derive their carbon from very numerous and widely different organic substances, e.g., sugars of all kinds, mannite, glycerine, tartaric and other vegetable acids, &c., and even from ethylalcohol, benzoic, salicylic, and carbolic acids to some extent. Carbonic, formic, and oxalic acids, cyanogen, urea, and oxamide are, however, useless for this purpose. The nitrogen and carbon together may be obtained from leucin, asparagin, methylamine, &c., or the nitrogen alone from these or urea, and compounds of ammonia with vegetable acids or phosphorus. The best nutritive substances are proteids (peptones) and sugars (glucoses); others must be passed over here. The nature of the particular Schizomycete has to be studied as well as the solution, and external agents affect the matter also. Certain minerals are of course necessary,—sulphur, phosphorus, potassium (or rubidium or caesium), and calcium (or magnesium, barium, or strontium) being indispensable. As one of many suitable nutritive solutions we may select the following :—
Di-potassium phosphate 0·20 gramm.
Magnesium sulphate 0·04 ,,
Calcium chloride 0·02 „
Peptone 1·00 ,,
Water 100·00 ,,
For other solutions, particulars as to changes of concentration, &c., and the peculiarities of different Schizomycetes in this connexion, special works must be consulted.
The chief sources of error in cultures of these very minute forms are the introduction of spores, &c., from without into the vessels, and on the instruments, &c., and the difficulty of continuously observing a developing individual with the necessary high powers. Numerous errors have arisen from inferences being employed to fill up gaps in life-histories which have only been partly observed. The first object of the cultivator, then, is to guarantee the purity of his materials, instruments, &c., and then to keep one form (or even a single specimen) under observation for a sufficiently long period and under suitable conditions. The practical difficulties are enormous, of course, and are very rarely entirely overcome for periods at all long. Here again we must refer to the special works for details as to the beautiful and refined methods now devised or employed by De Bary, Cohn, Koch, Brefeld, Lister, Nägeli, and others, calling special attention to the gelatine method devised by Vittadini and Brefeld and so successfully used and improved by Koch. Thoroughly conducted cultivations should decide in what medium the Schizomycete flourishes best, and how it behaves in others,—what vegetative forms it presents normally, and how changes in the environment affect these. They should also decide the characters of the aggregates or colonics; at what temperatures germination, growth, division, spore-formation, &c., take place or cease, and so on; the necessity or otherwise of free oxygen; the effects of the organism on its substratum or medium—whether it cause fermentation, or putrefaction, or excrete soluble ferments, and so on. Moreover, the products of these actions should be examined in detail. Where the particular Schizomycete is a parasite (wholly or partially) the methods of culture are even more refined. Here the fluids or tissues of the host must be regarded as a soil in which (by means of “infection,” “inoculation,” &c.) the observer sows the spores or vegetative cells of the parasitic organism. It is impossible to go more into details in the limits of this article, however, and we must dismiss the subject with the remark that, having regard to the complexity of the medium (e.g., blood) and the organization of the host, the difficulties of manipulation become greater than ever.
Temperature.— As with other plants, so with the Schizomycetes, their various functions, e.g., germination, growth, division, formation of spores, &c., can only be carried on at certain temperatures: the best average temperature is about 35° C., but the optimum may differ for each species and for each function. The same is generally true for the minimum and maximum temperatures, which have to bo determined separately also. Remarkable phenomena are connected with the death-points of certain Bacilli, &c. The spores of some of these forms have been frozen for days or weeks without injury, and some are said to have resisted temperatures as low as —100° C., or even lower: it appears to be all but impossible to kill such spores by cold. High temperatures are more fatal; but the spores of Bacilli have germinated after the fluid containing them was boiled for an hour, and even a temperature of 110° C. and higher has been withstood. The vegetative states are less resistent; nevertheless the bacilli of anthrax were not killed by heating the fluid to 75-80° for an hour or more. Speaking generally, ripe spores are most resistent and germinating ones least so; dry cells or spores resist extreme temperatures better than normally saturated ones. Of course time is an important factor; and other conditions also affect the matter, e.g., slightly acid media are more fatal than neutral or feebly alkaline ones, denser less so than thin ones (caeteris paribus), and so on.
To illustrate the importance of these facts we may note Tyndall’s method of “discontinuous heating”: by boiling the solutions containing the spores for 5-10 minutes daily all the life was destroyed in two or three days, though an exposure of an hour or more to a temperature of 100° C. did not kill the spores if not repeated. The explanation is that the spores which resist the first or second short boiling have time to begin germinating in the interval, and they then succumb at once when the liquid is again boiled. Light, Electricity, Gravitation, &c.—The relations between these and the functions of Schizomycetes have been partly investigated, but the results must be passed over here. A few of the higher genera show polarity—or at any rate difference between base and apex. Effects of Chemical Agents. —Oxygen. — Pasteur showed that, while some Schizomycetes require free oxygen like other plants, there are some which need none, or at most very little—the extreme case is perhaps still doubtful; but “anaerobiotic” forms like Bacillus butyricus stand in sharp contrast to such exquisitely “aerobiotic” ones as Bacterium aceti, Bacillus subtilis, &c. A few are known to flourish best—or at any rate they are more active—when supplied with oxygen in proportion less than that in the atmosphere. Engelmann showed that, while some species congregated close to a bubble of air, others collected at a certain distance from it, and came nearer when the bubble contained less oxygen. The same is true for the same species when brought near an Alga which is evolving oxygen—the aerobiotic forms collect where the oxygen is being evolved (in the yellow-red, &c., of the spectrum). Some Schizomycetes are powerful deoxidizing and reducing agents: it has already been stated that Beggiatoa deposits pure sulphur in its filaments. Bacterium aceti and others, on the contrary, transfer oxygen in large quantities to the medium in which they live, and the carbon in that may be entirely consumed. Fermentation once started may go on without free oxygen or not (according to the particular Schizomycete, &c. ), but it is necessary at the commencement. Oxygen is of course necessary for the respiration of the growing Schizomycete.
Water is absolutely necessary for the life and growth of the Schizomycetes, but the spores (and to a less extent the vegetative cells) of some can resist desiccation for long periods; others (e.g ., Bacterium zopfii) soon die. Those of Bacillus subtilis have been kept air-dry for years; and those of B. anthracis were not killed after several weeks in absolute alcohol. A year in water failed to kill the spores of B. subtilis. Zoogloea and vegetative cells of some resist drying for some time—how long is uncertain. In the dry state spores and cells are disseminated by currents of air: how far spores may be buried and still retain life (carried down by rain, &c.) is uncertain. The importance of these facts, however, is obvious. Acids, Poisons, &c.—The reader must be referred to the literature for details as to the quantities of acids and other products of their own decomposition which can be endured by given Schizomycetes (see especially the literature on fermentation and cultivation, and also respecting the action of poisons, antiseptics, &c.). Attraction towards Proteid Food-Substances.— Bacteria have long been known to swarm around pieces of organic food-materials, but although Ehrenberg and Cohn noticed the fact it was not investigated in detail until quite recently. Pfeffer finds that Bacteria and Spirilla are attracted in a definite manner towards minute tubes containing extract of meat or solution of asparagin, just as he finds antherozoids and zoospores of various kinds attracted by definite substances into tubes designed to imitate archegonia. For Pfeifer’s proofs that the substances mentioned exert a specific attraction on the organism the reader is referred to his treatise, “Locomotorische Richtungsbewegungen durch chemische Reize,” in Unters. aus dem bot. Inst. zu Tübingen, i. Hft. 3, 1884.
Fermentation and Butrefaction.— The growth and development of a Schizomycete in any organic medium results in a breaking down of the complex food-materials into simpler bodies, which may then become oxidized and still further decomposed. Such processes are known as fermentation in the wider sense. The particular kind of fermentation depends on the medium and on the species of Schizomycete, and may be affected by other circumstances; as the process goes on volatile substances may escape and others remain behind. Where proteid substances are being decomposed by Schizomycetes and evil-smelling gases escape, the fermentation is spoken of as putrefaction; in certain cases, where intense oxidation follows and still further consumes the products of decomposition, the process has been termed eremocausis. In a few instances a process of reduction sets in, as when sulphur salts are decomposed by Beggiatoa. The theory of Fermentation (q.v .) cannot be treated in detail here, but it is important to note that side by side with the actions referred to another kind of action may go on. Many Schizomycetes excrete what are called “soluble ferments,” which are capable of changing proteids into peptones, sugar into glucose, and so on. These processes of inversion, &c., result simply in an alteration of the proteid, &c., from the non-diffusible and non-assimilable condition to the diffusible and assimilable one, and are in no way destructive as are the fermentations described above. Nevertheless it is the custom to speak of both as cases of fermentation; the one series of changes renders the medium less and less capable of supporting life at every stage, the other series does not do so, yet the same name is frequently given to both kinds of action. It is a curious fact that the same Schizomycete may produce a different fermentation in each of two different media. The various fermentations are distinguished and valued according to the products which result; these bye-products are usually injurious to the ferment organism as they accumulate, and often complicate the investigation.
Of important fermentations due to Schizomycetes may be mentioned those concerned in the making of vinegar and cheese, in the preparation of flax, hemp, &c., in the souring and diseases of beer, wines, &c., the destruction of sugars, preserved food, &c. Others are of importance in the soil, and in the destruction of organic matter in ponds, rivers, drains, &c. In fact, much of the raison d'être of sanitary science may be referred here; and it may turn out to be still more true than we now know that Schizomycetes are important in agriculture.
In pathology the changes due to these organisms are at length being duly recognized. Apart from the comparatively harmless actions of those forms normally existing in the alimentary canal— Leptothrix aids in the decay of teeth, &c.—it is now certain that some invasions are dangerous. The injurious effects of some Schizomycetes when introduced into open wounds, &c., against which the brilliant labours of Lister have been so successfully directed, are acknowledged everywhere; but it is important to recognize that on the whole the diseases due to organisms in the blood depend fundamentally upon changes of the same category as those referred to. Of course the fluids of a living body present complicated conditions, and the action of a pathogenous Schizomycete cannot be treated and studied simply as a typical fermentation; but, although the conditions presented are involved and special, it cannot be doubted that common principles lie at the base of all the phenomena, and that the fluids of the diseased organism must be treated, so to speak, as fermentable media.
Numerous other fermentations of scientific interest are due to Schizomycetes: e.g., those in which colours are formed, certain cases of phosphorescence, the ammoniacal fermentation of urine, &c. Schizomycetes and Disease. —The presence of Schizomycetes in the blood, tissues, or organs of animals and man suffering from certain specific diseases is admitted, and has naturally suggested the question—Are they accompaniments only or have they any causal relations to the diseased conditions? Their constancy in given cases excluded the former view. Next arose the discussion as to how the causal connexion comes about and in what it consists, a discussion which is still going on as to the details. The chief points now established may be expressed generally somewhat as follows.
In a given specific disease, due to the action of a definite Schizomycete, the latter may be conceived to be injurious in several ways. If it robs the blood or tissues of oxygen or of any other valuable constituent, or if its activity results in the excretion of poisonous substances or in their formation as products of degradation of the matrix, or if it simply acts more or less as a mechanical obstruction or irritant,—in any of these cases harm may result to the delicately adjusted organism of the host. It being known that Schizomycetes act thus in nutrient pabula outside the body, their rapid growth and multiplication inside can of course only be explained as due to their success in the pabula there met with, and are indications that they produce changes there which must result in abnormality so far as the host is concerned. This does not end the matter, however. The living tissues of a healthy animal exert actions which are antagonistic to those of the parasitic invader; and it is now generally admitted that the mere admission of a Schizomycete into an animal does not necessarily cause disease. Were it otherwise it is difficult to see how the higher organisms could escape at all. Schizomycetes abound all over, about and around us; many, of course, are unable to live in the fluids of the body, but many are able to do so. Something must therefore be placed to the action of the tissues of the host, which when healthy can “resist” the attempts of a Schizomycete to settle, grow, and multiply with fatal effect. Much can undoubtedly be explained by this struggle for existence between the cells of the parasite and those of the healthy tissues invaded. But the higher organisms, again, present obstacles of other kinds to the lodgment of Schizomycetes: ciliary actions, active excretions, isolating processes of tissue-formation, &c., may be mentioned. Thus not every Schizomycete met with in the body can do harm.
But even when a Schizomycete has gained access to the bloodvessels, lymph-passages, &c., and has succeeded in establishing itself and multiplying, there are other facts to be taken into account before we dismiss the question as to its relations to disease. The rapidity of its growth may vary according to many circumstances,—temperature, oxidation, &c.,—as well as the still partially obstructive action of the invaded organism; whether the parasite excretes a poison, or simply robs the host, or distributes injurious agents of any kind, it is clear that everything which favours it aids in intensifying its action. And this may be local or general also according to complex circumstances. Of course sores, open wounds, &c., may render the access of a given Schizomycete very easy, and pave the way for its success in the tissues, &c., different strata of which may be exerting less and less resistance to its attacks. The study of this subject has led to the methods of modern surgery devised by Lister. It may be mentioned that Schizomycetes which produce bad effects on injured or dead tissues of wounds are not necessarily able to live in the healthy organism, however deadly the poisonous products of their action may be when they succeed in establishing themselves.
All these and many other facts, then, point to the conclusion that the mere presence of a Schizomycete in an organ or tissue is not sufficient proof of its causal relation to disease, and lead us to the following requirements to be satisfied before any such relation can be admitted (Koch) :—(1) given a specific disease in which a definite Schizomycete is constantly detected, and with a constant disposition with respect to the tissues, organs, &c.,—this organism should be absent from animals free from the disease; (2) the Schizomycete should be cultivated in nutrient media outside the body, kept pure for several “generations,” and obtained in some quantity by these means; (3) inoculation of a small amount of this pure cultivation should reproduce the specific disease in a healthy animal; (4) the same foreign elements as before should be clearly detected in the tissues of the now diseased subject, and in the same relations as before.
The satisfying of all these requirements is difficult, and the necessity of overcoming the difficulties has led to what may almost be termed a special branch of medical art. At the same time the majority of the principles which are here becoming recognized have long been known to biologists, and especially to botanists, and there are still numerous indications of a want of botanical training on the part of writers on these subjects. It is impossible here to even mention all the methods devised for staining, preparing, and examining tissues, &c, and the Schizomycetes they contain, or for cultivating these minute organisms under constant conditions on sterilized potatoes, bread-paste, jelly, blood-serum, &c., or in animal infusions or fluids, &c. Some of the more important points in cultivation have already been referred to; the literature must be consulted for further details.
