Corynebacterium diphtheriae

1. Diphtheria toxin mechanism — ADP-ribosylation of EF-2 — is a classic MCQ favourite. Distinguish from cholera toxin (ADP-ribosylates Gs) and pertussis toxin (ADP-ribosylates Gi). Pseudomonas exotoxin A shares the same EF-2 target but is a different organism ^[1].

2. Elek test is the definitive in vitro test for toxin production (immunoprecipitation in agar); PCR for the tox gene is faster but does not confirm actual toxin expression. Exams may test the difference ^[1].

3. Toxin is produced ONLY by strains lysogenised by corynephage β — this is a frequently tested virulence concept. Non-lysogenised strains are non-toxigenic ^[1].

4. Diphtheria antitoxin (DAT) must be given as early as possible because it neutralises circulating (unbound) toxin only. Once toxin is cell-bound, DAT is ineffective → this is why delayed treatment carries higher mortality ^[1].

5. Myocarditis and neuropathy are the two major toxin-mediated complications. Myocarditis occurs in the first 1–2 weeks; neuropathy is delayed (2–8 weeks). Palatal palsy (nasal speech, regurgitation) is the earliest and most characteristic neurological finding ^[1].

6. Vaccine is a toxoid (not live, not killed whole organism) — important for MCQs asking about vaccine type classification. Safe in immunocompromised patients ^[1][2].

• Pseudomembrane bleeding on removal = classic diphtheria clue; EBV tonsillar exudate wipes off more easily.
• "Bull neck" in a child with sore throat + grey membrane → think diphtheria before Ludwig's angina (which originates from dental/floor-of-mouth infection, not pharyngeal membrane) ^[3].
• Cutaneous diphtheria is often missed in MCQs; it presents as a chronic non-healing ulcer with a grey base, more common in tropical/resource-limited settings.
• C. diphtheriae vs C. jeikeium (JK): C. jeikeium is a nosocomial multi-drug-resistant skin organism causing line infections in neutropenic patients — do NOT confuse with C. diphtheriae.
• If asked "which toxin inhibits protein synthesis by the same mechanism as diphtheria toxin?" → answer is Pseudomonas exotoxin A (both ADP-ribosylate EF-2).

• 2024 Fourth Summative SAQ Q3 ^[3]: "A 65-year-old gentleman went to the A&E for a 4-day history of sore throat and painful neck swelling. Physical examination showed fever of 39°C, swollen floor of mouth and right painful neck swelling." — (a) Most likely diagnosis; (b) Most common infective origin; (d) Crepitus of the right neck → most likely diagnosis now.
  • Answer: (a) Ludwig's angina; (d) Necrotising fasciitis / descending necrotising mediastinitis. This is a discriminator question — Ludwig's angina (floor of mouth origin, dental source) vs pharyngeal diphtheria (pseudomembrane, toxin complications). The question tests the student's ability to distinguish deep neck space infections from diphtheria when "sore throat + neck swelling" is the stem. Diphtheria would feature a pharyngeal membrane and systemic toxin signs rather than floor-of-mouth swelling and crepitus.

No other past paper question in the indexed set directly tests C. diphtheriae by name. Students should be prepared for MCQ stems featuring: pseudomembrane on the pharynx in an unvaccinated/under-vaccinated child or traveller, toxin mechanism identification, and vaccine type classification.