GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

5. STRUCTURE OF THE MICROBIAL CELL

5.4. ENDOSPORES AND OTHER RESTING FORMS IN BACTERIA

5.4.1. Characteristics of spore-forming bacteria

Spore-forming Bacteria, with a single exception, belong to rod-shaped Gram-positive bacteria. The Gram-negative spore-forming bacteria are represented by members of the genus Desulfotomaculum. Over 10 genera of bacteria capable of forming endospores have been described, for example, Bacillus, Clostridium, Desulfotomaculum, Sporolactobacillus, Oscillospora, and others. Most of these bacteria are motile due to peritrichous flagella. Bacteria belonging to the genus Bacillus are strict aerobes or facultative anaerobes. The genera Clostridium and Desulfotomaculum group together anaerobic spore-forming bacteria. Clostridia obtain energy via Fermentation, whereas Desulfotomaculum species do so through Anaerobic Respiration. Sporolactobacillus belongs to lactic acid bacteria. Sporosarcina possesses spherical Cells, but based on its physiological characteristics, it belongs to bacilli.

A characteristic feature of spore-forming bacteria is the low GC (guanine + cytosine) content in their cellular DNA. For instance, the DNA of clostridia contains from 22 to 27% GC.

Spores of different bacterial species vary in shape, size, and position within The Cell (Fig. 5.11). In shape, they can be round or oval. The spore surface may be smooth, wavy, spine-like, etc. Spores can occupy the following positions within the cell:

1) bacillary, when the spore is localized centrally, eccentrically, or terminally, and the cell does not change its shape. Such cells are called bacilli (Bacillus);

2) clostridial, when during spore formation the cell changes its shape, acquiring the appearance of a shuttle or spindle. Such cells are called clostridia (Clostridium, from Greek closter — spindle);

Class="center">3) plectridial, when the spore is localized terminally, causing the cell to expand at the site of its Location and take on the shape of a racket.

Fig. 5.11. Spore placement within the cell (a) and spore surface (b): 1 — Bacillus megaterium; 2 — B. thuringiensis; 3 — Clostridium polymyxa; 4 — C. butyricum; 5 — C. tetani; 6 — spine-like; 7 — with polygonal protrusions; 8 — smooth; 9 and 10 — wavy; 11 — spine-like in actinomycete conidiospores

The clostridial and plectridial types of spore placement are characteristic mainly of species of the genus Clostridium and are frequently found simultaneously within a culture of a single species.

Detection of endospores. Under microscopic examination, spores are visible due to their high light-refractive index, which is similar to that of dehydrated protein. This indicates that within the spores, a large amount of protein-rich material is concentrated in a small volume. The spore contains almost all the dry matter of the mother cell, yet occupies 10 times less volume. In doubtful cases, true endospores can be detected in cells using special staining. If a heat-fixed bacterial smear is boiled with carbol fuchsin solution, the spores tightly bind the dye and do not become decolorized even upon Treatment with ethanol or 1M acetic acid (under conditions where the rest of the cell content becomes colorless).