Clostridium

Corynebacteria (from the Greek koryne, club) are small and pleomorphic. The genus
Corynebacterium includes many species of aerobic and facultative Gram-positive rods.
The cells tend to have clubbed ends, and often remain attached after division, forming
“Chinese letter” or palisade arrangements. Spores are not formed. Growth is generally
best under aerobic conditions on media enriched with blood or other animal products, but
many strains will grow anaerobically. Colonies on blood agar are typically small (1 to
2 mm), and most are nonhemolytic. Catalase is produced, and many strains form acid
(usually lactic acid) through carbohydrate fermentation.

C. diphtheriae produces a powerful exotoxin that is responsible for diphtheria. Other
corynebacteria are nonpathogenic commensal inhabitants of the pharynx, nasopharynx,
distal urethra, and skin; they are collectively referred to as “diphtheroids.” The species
that have disease associations are included in Table 18–2.