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- Pleomorphic Gram-positive bacillus that is club-shaped, appears like Chinese characters on Gram stain [ Fig].
- Facultative anaerobe.
- Humans are only known hosts.
- Diphtheria caused only by exotoxin-producing strains of C. diphtheriae.
- Three Corynebacterium strains may produce diphtheria toxin:
- C. diphtheriae (epidemic diphtheria w/ person-person spread)
- C. ulcerans
- C. pseudotuberculosis
- Both C. ulcerans and C. pseudotuberculosis are less common and associated with farm/dairy contacts.
- Virulent C. diphtheria strains carry a bacteriophage with the diphtheria toxin gene. Without this bacteriophage, the microbe is unable to cause serious disease.
- Culture best performed on tellurite-selective media, e.g., Tinsdale agar.
- Confirmation of toxin production by demonstration of an immunoprecipitin band done at CDC.