USMLE

Corynebacterium diphtheriae

1,316 views
Bacteria - Gram Positive
  1. Staph aureus: Overview
  2. Staph aureus: Presentation
  3. Methicillin-Resistant Staph aureus (MRSA)
  4. Staph saprophyticus
  5. Strep pneumoniae: Overview
  6. Strep pneumoniae: Presentation
  7. Strep viridans
  8. Strep pyogenes: Overview
  9. Strep pyogenes: Presentation
  10. Strep agalactiae
  11. Strep bovis
  12. Enterococcus
  13. Bacillus anthracis
  14. Bacillus cereus
  15. Clostridium tetani
  16. Clostridium perfringens
  17. Clostridium botulinum
  18. Clostridium difficile
  19. Corynebacterium diphtheriae
  20. Listeria monocytogenes
  21. Nocardia
  22. Actinomyces

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