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Corynebacterium diphtheriae

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Corynebacterium diphtheriae 

  • Characteristics
    • Gram + rods
      • club-shaped 
    • Metachromatic (blue-red) granules
      • intracellular polyphosphate granules visualized with methylene blue stain on Loeffler medium
    • Elek test positive
      • Tests for the diphtheria toxin
    • Black colonies on cystine-tellurite agar
      • Occur in angular arrangements
  • Diphtheria toxin
    • Exotoxin encoded by beta-prophage (Tox gene)
      • Derived from Corynephage beta bacteriophage
      • Therefore, not all strains of C. diphtheriae are pathogenic
    • Also known as AB toxin, composed of 2 subunits
      • B (binding) subunit allows penetration of A (active) subunit into the cell
    •  Inactivates EF-2 (elongation factor) via ADP-ribosylation
      • Inhibits protein synthesis, leading to cell death and necrosis
      • Similar to Pseudomonas exotoxin A
  •  Symptoms
    • Pseudomembranous pharyngitis
      • grayish-white pseudomembrane
      • Bleeds easily if irritated 
    • Myocarditis
      • Can progress to a dilated cardiomyopathy (most common cause of death!) or precipitate arrhythmias
    • Polyneuropathy (peripheral neuropathy)
    • Lymphadenopathy (bull neck) 
  • Treatment
    • Diphtheria antitoxin
      • Passive immunization (antibodies against toxin)
    • Erythromycin
    • Penicillin G