A 5-year-old unvaccinated child presents with a greyish-white pseudomembrane in the throat. Corynebacterium diphtheriae toxin is produced only by lysogenic strains carrying which bacteriophage?

• A T4 phage
• B Lambda (λ) phage
• C M13 filamentous phage
• D Beta (β) phage (also called corynephage β) ✓

Explanation

Diphtheria toxin production requires lysogeny with corynephage β (tox+ phage). The tox gene is carried on the phage genome and is expressed only under conditions of iron depletion via a transcriptional repressor (DtxR — diphtheria toxin repressor). Non-lysogenic C. diphtheriae strains are non-toxigenic and non-pathogenic in terms of systemic disease, though they can still cause local throat infection. Elek's plate test and PCR for tox gene confirm toxigenicity.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.