IX. 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;
Pathogenic, or disease-producing bacteria;

or according to their food requirements into—

Prototrophic, requiring no organic food (e. g., nitrifying bacteria);
Metatrophic, requiring organic food (e. g., saprophytes and facultative parasites);
Paratrophic, requiring living food (obligate parasites);

or according to their metabolic products into—

Chromogenic, or pigment-producing bacteria;
Photogenic, or light-producing bacteria;
Aerogenic, or gas-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. Cocci. (Fig. 83).—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.

Sarcinæ, where division takes place in three planes successively, and the individuals remain attached in cubical packets of eight and its multiples.

Micrococci or 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.

2. Bacilli (Fig. 84, 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.

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 leptothriciæ (vide infra).

3. Spirilla (Fig. 84, 4 to 6).—Curved and twisted filaments. Classified, according to shape, into—

Spirillum.
Vibrio (comma).
Spirochæta.

Many Spirochætes appear to belong to the animal kingdom and are grouped under protozoa; other organisms to which this name has been given are undoubtedly bacteria.

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. } Free swimming forms, which
2. Thiothrix. } contain sulphur granules.

3. Crenothrix. }
4. Cladothrix. } These forms do not contain
5. Leptothrix. } sulphur granules.

6. Streptothrix. A group which exhibits true but
not dichotomous branching, and contains some pathogenic
species.

The morphology of the same bacterium may vary greatly under different conditions.

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. This variation in morphology is known as "pleomorphism."

Some of the factors influencing pleomorphism are:

1. The composition, reaction, etc., of the nutrient medium in which the organism is growing.

2. The atmosphere in which it is cultivated.

3. The temperature at which it is incubated.

4. Exposure to or protection from light.

The various points in the anatomy morphology and physiology of bacteria upon which stress is laid in the following pages should be studied as closely as is possible in preparations of the micro-organisms named in connection with each.