Corynebacterium diphtheriae


  • Pleomorphic Gram-positive bacillus that is club-shaped, appears like Chinese characters on Gram stain [Fig1].
    • 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 the demonstration of an immunoprecipitation band done at CDC.

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Last updated: June 6, 2020