Corynebacterium diphtheriae becomes pathogenic and causes diphtheria in humans solely through infection by a specific bacteriophage carrying the tox gene. The correct answer to the question is (A) phage β. This lysogenic conversion mechanism highlights a classic example of phage-mediated virulence in microbiology.

Correct Answer

Option (A) phage β enables Corynebacterium diphtheriae to produce diphtheria toxin, transforming non-toxigenic strains into toxigenic ones responsible for the disease. The β-phage (also called corynebacteriophage β) integrates as a prophage, encoding the tox structural gene regulated by iron levels via the DtxR repressor. Extensive studies, including genetic mapping, confirm its role in toxinogenesis during lysogeny.​

Option Explanations

Phage β (Correct)

Phage β is a temperate corynebacteriophage that lysogenizes C. diphtheriae, inserting the tox+ gene between host range (h) and immunity (imm) loci on its genome. Low iron triggers toxin expression by inactivating the iron-bound aporepressor, leading to ADP-ribosylation of EF-2 and host cell death. This phage-specific conversion is why only lysogenized strains cause clinical diphtheria.​

Epsilon Phage (Incorrect)

Epsilon phage is not associated with C. diphtheriae or diphtheria toxin production; no literature links it to toxigenicity in this bacterium. It may refer to unrelated phages in other systems, but it fails to confer virulence here, making it irrelevant to human diphtheria pathogenesis.​

T4 Phage (Incorrect)

T4 phage infects Escherichia coli, not Corynebacterium species, and lacks any tox gene for diphtheria. As a lytic virulent phage, it causes host lysis without lysogenic conversion, so it cannot induce toxin production in C. diphtheriae.​

Lambda Phage (Incorrect)

Lambda phage targets E. coli, serving as a model for lysogeny but unrelated to corynebacterial hosts or diphtheria toxin. Despite similarities in integration mechanisms, it does not infect C. diphtheriae or carry the tox gene.​