Anatomy

1. Capsule (Fig. 85, b).—A gelatinous envelope (probably akin to mucin in composition) surrounding each individual organism, and preventing absolute contact between any two. In some species the capsule (e. g., B. pneumoniæ) is well marked, but it cannot be demonstrated in all. In very well marked cases of gelatinisation of the cell wall, the individual cells are cemented together in a coherent mass, to which the term "zooglœa" is applied (e. g., Streptococcus mesenteroides). In some species colouring matter or ferric oxide is stored in the capsule.

2. Cell Wall (Fig. 85, c).—A protective differentiation of the outer layer of the cell protoplasm; difficult to demonstrate, but treatment with iodine or salt solution sometimes causes shrinkage of the cell contents—"plasmolysis"—and so renders the cell wall apparent (e. g., B. megatherium) in the manner shown in figure 85. Stained bacilli, when examined with the polarising microscope, often show a doubly refractile cell wall (e. g., B. tuberculosis and B. anthracis).

In some of the higher bacteria the cell wall exhibits this differentiation to a marked degree and forms a hard sheath within which the cell protoplasm is freely movable; and during the process of reproduction the cell protoplasm may be extruded, leaving the empty tube unaltered in shape.

3. Cell Contents.—Protoplasm (mycoprotein) contains a high percentage of nitrogen, but is said to differ from proteid in that it is not precipitated by C2H6O. It is usually homogeneous in appearance—sometimes granular—and may contain oil globules or sap vacuoles (Fig. 85, d), chromatin granules, and even sulphur granules. Sap vacuoles must be distinguished from spores, on the one hand, and the vacuolated appearance due to plasmolysis, on the other.

The cell contents may sometimes be differentiated into a parietal layer, and a central body (e. g., beggiotoa) when stained by hæmatoxylin.

4. Nucleus.—This structure has not been conclusively proved to exist, but in some bacteria chromatin particles have been observed near the centre of the bacterial cell and denser masses of protoplasm situated at the poles which exhibit a more marked affinity than the rest of the cell protoplasm for aniline dyes. These latter are termed polar granules or Polkoerner (Fig. 85, e). Occasionally these aggregations of protoplasm alter the colour of the dye they take up. They are then known as metachromatic bodies or Ernstschen Koerner (e. g., B. diphtheriæ).

5. Flagella (Organs of Locomotion, Fig. 85, a).—These are gelatinous elongations of the cell protoplasm (or more probably of the capsule), occurring either at one pole, at both poles, or scattered around the entire periphery. Flagella are not pseudopodia. The possession of flagella was at one time suggested as a basis for a system of classification, when the following types of ciliation were differentiated (Fig. 87):

1. Polar: (a) Monotrichous (a single flagellum situated at one pole; e. g., B. pyocyaneus).

(b) Amphitrichous (a single flagellum at each pole; e. g., Spirillum volutans).

2. Diffuse: Peritrichous (flagella scattered around the entire periphery e. g., B. typhosus).