A throat swab from a child with membranous pharyngitis grows grey-black colonies on tellurite medium. Toxigenicity is confirmed using the Elek gel precipitation test. The diphtheria toxin gene (tox) is carried by:

• A A conjugative plasmid acquired by horizontal gene transfer
• B The lysogenic bacteriophage beta (corynephage beta), integrated into the bacterial chromosome ✓
• C A pathogenicity island on the C. diphtheriae chromosome regulated by the DtxR iron-regulatory protein
• D A transposon (Tn916) that inserts into the chromosome when iron concentrations are low

Explanation

The diphtheria toxin structural gene (tox) is encoded by the genome of a lysogenic bacteriophage, corynephage beta (also called corynebacteriophage beta or phage C). This phage integrates into the C. diphtheriae chromosome; only phage-infected (lysogenic) strains are toxigenic. Expression of the tox gene is regulated by the host's DtxR (diphtheria toxin repressor) protein: when iron is abundant, DtxR binds iron, dimerizes, and represses tox transcription; in iron-poor conditions (as in inflamed mucosa), iron-free DtxR cannot repress tox, leading to toxin production. The gene is not plasmid-borne or on a transposon.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.