Gram-positive, club-shaped, non-motile rods.
Classically arranged at angles forming “Chinese letter” shapes (V, L, W).
Grow best at 37 °C in oxygen.
Only phage-infected strains produce exotoxin.
Exotoxin has two subunits:
- A component: Blocks protein synthesis (RNA translation inhibition via ADP-ribosylation of EF-2).
- B component: Targets oropharynx, heart, and nerves.
Exotoxin is a 58 kDa A–B polypeptide linked by disulphide bond.
Effects: local necrosis, systemic toxaemia, myocarditis, and demyelinating neuritis.

⚡ Virulence

Corynephage β-infected strains release exotoxin.
A subunit: ADP-ribosyl transferase → inhibits elongation factor-2.
B subunit: Guides toxin to target tissues.

📤 Transmission

Direct person-to-person contact.
Inhalation of aerosolised droplets from infected individuals.

🧑‍⚕️ Clinical Presentation (Incubation: 2–4 days)

⚠️ Suspect toxin-induced myocarditis if tachycardia is out of proportion to fever - perform frequent ECGs.

Pharyngitis: Grey “dirty” pseudomembrane (fibrin + dead cells) in throat.
Bull neck: Cervical lymphadenopathy + swelling.
Affected sites: nose, pharynx, larynx.
Airway obstruction risk with stridor.
Symptoms: sore throat, fever, malaise.
Complications:
- Myocarditis → arrhythmias, heart block, heart failure.
- Cranial nerve palsies (diplopia, dysphagia, dysarthria).
- Laryngeal nerve palsy → hoarseness.

🔬 Investigations

Swab culture below pseudomembrane + PCR.
Culture: Hoyle’s medium (blood tellurite) → black colonies; Tinsdale’s medium is an alternative.
Strain types: gravis (severe), intermedius, mitis (milder) - based on colony/haemolysis pattern.
Biochemical tests: Catalase, urease, nitrate reduction, cystinase, pyrazinamidase.
Toxin detection: Elek test (precipitin line with antitoxin).
Extra: Serotyping, phage typing, cardiac monitoring (ECG, echo).

💉 Mass vaccination in the mid-20th century led to near-eradication of diphtheria in developed countries.

🩺 Management

Supportive: ABCs, isolation, cardiac monitoring, bed rest.
Diphtheria Antitoxin: 10,000–30,000U IM (any age; higher if severe). Neutralises unbound toxin. ⚠️ Risk of anaphylaxis (horse serum origin).
Antibiotics: IV/PO Erythromycin. Close contacts: 7 days of erythromycin (dosing by age).
Airway: Laryngoscopy/bronchoscopy for pseudomembrane removal if obstruction risk.
Contacts: Vaccinate or give booster; routine booster every 10 years for ongoing immunity.

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Diphtheria (Corynebacterium diphtheriae)

📖 About

🦠 Microbiology

⚡ Virulence

📤 Transmission

🧑‍⚕️ Clinical Presentation (Incubation: 2–4 days)

🔬 Investigations

🩺 Management

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