Etiology

• Pathogen: Corynebacterium diphtheriae
  • A gram-positive, nonsporulating, club-shaped bacillus
  • Contains metachromatic granules (volutin granules; stain red with a blue dye)
• Route of infection
  • Droplet transmission

Pathophysiology

• C. diphtheriae has both toxigenic and nontoxigenic strains; toxigenic strains contain a beta-prophage gene (tox), which encodes for the exotoxin diphtheria toxin
  • Conversion from nontoxigenic to toxigenic C diphtheriae occurs due to infection with a lysogenic bacteriophage called Corynephage beta.
  • This phage inserts the tox gene into the C diphtheriae genome, which results in the bacterial expression of the diphtheria AB toxin.
  • Diphtheria toxin irreversibly halts protein synthesis due to ADP-ribosylation of elongation factor-2 and causes severe local (eg, pseudomembranous pharyngitis) and systemic (eg, myocarditis, neuritis) effects.

Clinical features

Local features

• Tonsillar and pharyngeal diphtheria
  • Grayish-white pseudomembrane over the posterior pharyngeal wall, and/or tonsils. Also see Gray-white exudates of throat.
  • Any attempt to scrape off the pseudomembrane exposes the underlying capillaries and results in heavy bleeding.
  • Bull neck due to cervical lymphadenopathy and swelling of the soft tissue of the neck → airway obstruction

Systemic features (due to dissemination of toxin)

• Cardiac (can be lethal)
  • Myocarditis
  • Arrhythmias
• Reversible polyneuropathy

Diagnostics

• Cultures
  • Microscopic examination: multiple Gram-positive club-shaped bacilli clustered in angular arrangements
  • Culture media of choice
    • Cystine-tellurite agar: C. diphtheriae appears as black colonies.
    • Loffler medium: shows metachromatic granules
• Tests to identify toxigenic strains (if the culture reveals C. diphtheriae)
  • Elek test
    • Positive if the strain is toxicogenic

Treatment

• Diphtheria antitoxin (most important)
• Antibiotics