Corynebacterium diphtheriae (Diphtheria)

MICROBIOLOGY

• Pleomorphic Gram-positive bacillus that is club-shaped, appears like Chinese characters on Gram stain [Fig1].
  
  • Facultative anaerobe
  • Humans are the only known C. diphtheriae 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, unlike C. diphtheriae, are zoônotic, associated with farm/dairy contacts.
  • Virulent C. diphtheria strains carry a bacteriophage with the diphtheria toxin gene. Without this bacteriophage, the microbe cannot cause serious disease.
• Nontoxigenic C. diphtheriae can still cause significant invasive disease (e.g., wound infection, bacteremia, endocarditis)
• 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