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160
BACTERIOLOGY

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 severale.g. five or six or more—at one or each pole.

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

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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.

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

Growth and Division.—Whatever the shape and size of the individual cell, cell-filament or cell-colony, the immediate visible results of active nutrition are elongation of the cell and its division into two equal Reproduc-
tion.
halves, 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 character 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 other shaped colonies. It not unfrequently happens, however, that groups of cells break away from their former connexion as longer or shorter straight or curved filaments, or as solid masses. In some filamentous forms this “fragmentation” into multicellular pieces of equal length or nearly so is a normal phenomenon, each partial filament repeating 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,[1] 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).

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Fig. 6.—Bacillus megaterium. (After de Bary.)
a, a chain of motile rodlets still growing and dividing (bacilli).
b, a pair of bacilli actively growing and dividing.
p, a rodlet in this condition (but divided into four segments)
after treatment with alcoholic iodine solution.
c, d, e, f, successive stages in the development
of the spores.
r, a rodlet segmented in four, each segment containing
one ripe spore.
g1, g2, g3, early stages in the germination of
the spores (after being dried several days);
h1, h2, k, l and m, successive stages in the germination
of the spore.
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Fig. 7.—Bacillus anthracis. (After Koch.)
A. Bacilli mingled with blood-corpuscles from the blood of a guinea-pig; some of the
bacilli dividing.
B. The rodlets after three hours' culture in a drop of aqueous humour. They grow out
into long leptothrix-like filaments, which become septate later, and spores are
developed in the segments.

One of the most remarkable phenomena in the life-history 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 Zoogloeae.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

  1. 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.