Microscopic Shapes

Bacteria vary in size from 0.4 to 2 !lm. They occur in three basic shapes (Figure 1-6):

Cocci (spherical)

Bacilli (rod-shaped)’

Spirochetes (spiral or helical)

Individual bacteria may form characteristic groupings. Cocci (plural of coccus) may occur singly, in pairs (diplococci), in chains (streptococci), or in clusters (staphylococci). Bacilli (plural of bacillus) may vary greatly in size and length from very short coccobacilli to long filamentous rods. The ends may be square or rounded. Bacilli with tapered, pointed ends are termed fusiform. Some bacilli are curved. When a species varies in size and shape within a pure culture, the bacterium is pleomorphic. Bacilli may occur as single rods or in chains or may align themselves side by side (palisading). Spirochetes vary in length and in the number of helical turns (not all helical bacteria are called spirochetes).


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