Corynebacterium diphtheriae (Diphtheria)

MICROBIOLOGY

• Pleomorphic Gram-positive bacillus that is club-shaped appears like Chinese characters on Gram stain [Fig1].
  
  • Facultative anaerobe.
  • Humans are only known hosts.
• Diphtheria is 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
  • 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 is best performed on tellurite-selective media, e.g., Tinsdale agar.
• The demonstration of an immunoprecipitation band confirms toxin production.
  • CDC provides laboratory support for state/local health departments.
    • Elek immunodiffusion assay for C. diphtheriae and C. ulcerans isolates