General Characteristics
- Gram Stain: Gram-positive rods (often appear club-shaped or with a beaded appearance).
- Arrangement: “Chinese letter” or V/Y formations due to snapping division.
- Oxygen Requirement: Aerobic or facultative anaerobes.
- Motility: Non-motile.
- Spores/Capsule: Non-spore forming, no capsule.
- Special Features: Contains metachromatic granules (Ernst-Babes bodies) – stain with Neisser stain.
- Selective Media: Loeffler’s serum slope and Tellurite medium (potassium tellurite inhibits other flora).
- Toxin-producing strains: Only strains lysogenized by bacteriophage (β-phage) produce the diphtheria toxin.
Epidemiology
- Reservoir: Humans (asymptomatic carriers in oropharynx and skin).
- Transmission: Respiratory droplets or direct contact with cutaneous lesions.
- Geography: More prevalent in underdeveloped and tropical countries.
- High-risk groups: Unvaccinated children, immunocompromised individuals.
Pathogenesis
Corynebacterium diphtheriae causes disease primarily through toxin-mediated effects.
Steps:
- 1. Colonization of oropharyngeal epithelium or skin.
- 2. Release of diphtheria exotoxin → systemic effects.
Virulence Factor: Diphtheria Toxin
- The gene for toxin is carried by prophage (β-phage).
- Affects heart, nerves, and kidneys.
Clinical Forms of Diphtheria
1. Respiratory Diphtheria
- Symptoms: Sore throat, low-grade fever, malaise.
- Pseudomembrane formation in pharynx – grey-white, adherent, bleeds if scraped.
- Severe cases: Myocarditis, polyneuritis (due to circulating toxin).
2. Cutaneous Diphtheria
- Symptoms: Chronic, non-healing ulcer with grayish membrane.
3. Non-toxigenic Strains
- Can cause endocarditis, septic arthritis, and catheter-related infections in immunocompromised.