Corynephage

Corynephage refers to bacteriophages that infect Corynebacterium species, most notably Corynebacterium diphtheriae, the bacterium that causes diphtheria. Corynephages are typically temperate phages capable of establishing lysogeny, whereby their genomes integrate into the host chromosome and exist as prophages alongside the bacterial DNA.

A key feature of many corynephages is their ability to carry the tox gene, which encodes diphtheria toxin. When a C. diphtheriae cell is lysogenized by a tox-encoding corynephage, the prophage-encoded tox gene can be expressed, converting a non-toxigenic strain into a toxigenic one. The toxin produced is a potent A-B exotoxin that inhibits protein synthesis and underpins the clinical manifestations of diphtheria. Toxin production typically depends on the presence of the tox-encoding prophage and related phage regulatory elements.

Corynephages have historical and clinical significance because phage-mediated lysogenic conversion explains how diphtheria toxin production can arise in bacterial populations. Many corynephages can be induced to enter the lytic cycle under stress or DNA damage, leading to production of new phage particles and potential horizontal transfer of the tox gene among Corynebacterium strains. In laboratory and diagnostic settings, phages are studied through plaque assays, prophage induction experiments, and molecular detection of tox genes by PCR or sequencing.

Diversity within the group includes several tox-encoding phages, sometimes referred to in the literature as corynephage beta and corynephage gamma, among others. Understanding corynephages illuminates the dynamics of virulence gene transfer and the epidemiology of diphtheria.