VIII. SCHIZOMYCETES. Classification and Morphology.-Bacteria are often classified, in general terms, according to their life functions, into Saprogenic, or putrefactive bacteria; Zymogenic, or fermentative bacteria; or according to their metabolic products into— Chromogenic, or pigment-producing bacteria; and so on. Such broad groupings as these have, however, but little practical value when applied to the systematic study of the fission fungi. On the other hand, no really scientific classification of the schizomycetes has yet been drawn up, and the varying morphological appearances of the members. of the family are still utilised as a basis for classification, as under 1. Micrococci, or Cocci (Fig. 65).—Rounded or oval cells, subdivided according to the arrangement of the individuals after fission, into Diplococci and Streptococci, where division takes place in one plane only, and the individuals remain attached (a) in pairs or (b) in chains. Tetrads, Merismopedia, or Pediococci, where division takes place alternately in two planes at right angles to each other, and the individuals remain attached in flat tablets of four, or its multiples. Sarcina, where division takes place in three planes successively, and the individuals remain attached in cubical packets of eight and its multiples. Staphylococci, where division takes place in three planes, but with no definite sequence; consequently the individuals remain attached in pairs, short chains, plates of four, cubical packets of eight, and irregular masses containing numerous cocci. Fig. 65.-Types of bacteria-cocci: 1, Diagram of sphere indicating planes of fission; 2, diplococci; 3, streptococci; 4, tetrads; 5, sarcina; 6, staphylococci. 2. Bacilli (Fig. 66, 1 to 3).-Rod-shaped cells. A bacillus, however short, can usually be distinguished from a coccus in that two sides are parallel. Some bacilli after fission retain a characteristic arrangement and may be spoken of as diplobacilli or streptobacilli. Fig. 66.-Types of bacteria-bacilli, etc. : 1, Bacilli; 2, diplobacilli; 3, streptobacilli; 4, spirilla; 5, vibrios; 6, spirochætæ. (Leptothrix is a term that in the past has been loosely used to signify a long thread, but is now restricted to such forms as belong to the leptothricia (vide page 113). 3. Spirilla (Fig. 66, 4 to 6).-Curved and twisted filaments. Classified, according to shape, into Spirillum. Vibrio (comma). Spirochæta. Higher forms of bacteria are also met with, which possess the following characteristics: They are attached, unbranched, filamentous forms, showing (a) Differentiation between base and apex; (b) Growth apparently apical; (c) Exaggerated pleomorphism; (d) "Pseudo-branching" from apposition of cells; and are classified into 1. Beggiotoa. 2. Thiothrix. 3. Crenothrix. 4. Cladothrix. 5. Leptothrix. 6. Streptothrix. Free swimming forms, which contain sulphur granules. These forms do not contain sulphur granules. A group which exhibits true branching, and contains some pathogenic species. The morphology of the same bacterium may vary greatly under different conditions of growth as to— 1. The composition, reaction, etc., of the nutrient medium. 2. The atmosphere in which it is cultivated. 3. The temperature at which it is incubated. 4. Exposure to or protection from light. For example, under one set of conditions the examination of a pure cultivation of a bacillus may show a short oval rod as the predominant form, whilst another culture of the same bacillus, but grown under different conditions, may consist almost entirely of long filaments or threads, a condition known as " pleomorphism." ANATOMY. 1. Capsule (Fig. 67, b).—A gelatinous envelope (probably akin to mucin in composition) surrounding each individual organism, and preventing absolute con tact between any two. In some species the capsule is well marked, but it cannot be demonstrated in all. In very well marked cases of gelatinisation of the cell wall, Fig. 67.-Structure of bacteria. the individual cells are cemented together in a coherent mass, to which the term "zooglœa" is applied. In some species colouring matter or ferric oxide is stored in the capsule. 2. Cell Wall (Fig. 67,c). A protective differentiation of the outer layer of the cell protoplasm; difficult to demonstrate, but treatment with iodine or salt salt sometimes causes shrinkage of the cell contents solution "plas molysis"—and so renders the cell. wall apparent (e. g., B. megatherium) in the manner shown in figure 68. Stained bacilli, when examined with polarising microscope, often show a doubly refractile cell wall (e. g., B. tuberculosis and B. anthracis). Fig. 68.-Plasmolysis. 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 C2HO. It is usually homogeneous in appearance-sometimes granular—and may contain oil globules or sap vacuoles (Fig. 67, 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 denser masses of protoplasm situated at the poles and exhibiting a more marked affinity than the rest of the cell protoplasm for aniline dyes have been observed. These are termed polar granules or Polkörner (Fig. 67, e). Occasionally these aggregations of protoplasm alter the colour of the dye they take up. They are then known as metachromatic bodies or Ernst'schen Körner. 5. Flagella (Organs of Locomotion, Fig. 67, 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. 69.-Types of ciliation. 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). (c) Lophotrichous (a tuft or bunch of flagella situated at each pole; e. g., B. cyanogenus). 2. Diffuse: Peritrichous (flagella scattered around the entire periphery; e. g., B. typhosus). |