Sizes, Forms, Structure, etc. - The Schizomycetes Form and Structure. 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 4 µ 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 Cell-wall. 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 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, 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. 3. - Types of Zoogloea. (After Zopf.)

A. Mixed zoogloea found as a pellicle on the surface of vegetable infusions, etc.; it consists of various forms, and contains cocci (a) and rodlets, in series (b and c), etc.

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.

D, E, H. Colonies of Myconostoc enveloped in diffluent matrix.

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

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

The substance of the bacterial cell when suitably prepared Cell-contents. 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 arc usually one or more deeply-staining granules. The protoplasm itself may be tinged with colouring matter, bright red, yellow, etc., 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 Nucleus. 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, 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 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 finely-divided particles suspended in a fluid. Independent movement is effected by special motile organs, the cilia or flagella. These 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 Löffler. By these methods the cilia are seen to be fine protoplasmic outgrowths of the cell (fig. 1) of the same nature as those of the zoospores and antherozoids of algae, mosses, etc. Cilia. 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 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, etc., 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. 1, C, D), or one at each pole; others again have a tuft of several cilia at one pole (Lophotrichous), e.g. B. syncyaneus (fig. 1, E), or at each pole (Amphitrichous) (fig. 1, J, K, L); and, finally, many actively motile forms have the cilia springing all round (Peritrichous), e.g. B. vulgaris (fig. 1, 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 Vegetative State. movements are to be observed, reminding us of the movements of Oscillatoria among the Cyanophyceae. In these cases no cilia have been observed, and there is a firm cell-wall, so the movement remains quite unexplained.

Fig. 4. - Types of Spore-formation in Schizomycetes. (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 chain 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, a) Schizomycetes.

F. Long rod-like form containing a spore (these are the so-called "Köpfchenbacterien" 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.

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.)

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, etc., 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, etc. 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 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.