What is the genetic basis for diphtheria toxin production in Corynebacterium diphtheriae?

Explanation

## Genetic Basis of Diphtheria Toxin Production **Key Point:** Diphtheria toxin is produced only by *Corynebacterium diphtheriae* strains that are lysogenized by a temperate β-phage (corynephage) carrying the *tox* gene. This is a classic example of **lysogenic conversion** in bacteriology. ### Lysogenic Conversion Mechanism ```mermaid flowchart TD A["Wild-type C. diphtheriae<br/>(non-toxigenic)"]:::outcome --> B{"β-phage infection"}:::decision B -->|"Integration of phage DNA<br/>with tox gene"|C["Lysogenized C. diphtheriae<br/>(toxigenic)"]:::outcome C --> D["Prophage in chromosome"]:::action D --> E["Constitutive or inducible<br/>toxin expression"]:::action E --> F["Diphtheria toxin production"]:::outcome ``` ### Key Features of Lysogeny 1. **Phage integration**: The β-phage DNA becomes integrated into the *C. diphtheriae* chromosome as a **prophage** 2. **tox gene location**: The *tox* gene is located on the phage genome, not on the bacterial chromosome 3. **Expression regulation**: Toxin expression is regulated by: - **Iron availability**: Low iron (via DtxR repressor) induces toxin expression - **Phage induction**: Stress conditions may induce prophage replication **High-Yield:** Only **lysogenized strains** produce toxin; non-lysogenized (non-toxigenic) strains of *C. diphtheriae* do NOT produce diphtheria toxin despite being the same species. ### Clinical Significance | Feature | Toxigenic Strain | Non-Toxigenic Strain | |---|---|---| | Phage status | Lysogenized with β-phage | No phage | | tox gene | Present (on prophage) | Absent | | Toxin production | Yes (regulated by iron) | No | | Disease potential | Causes systemic diphtheria | Causes only local infection | | Schick test | Positive (toxin-mediated) | Negative | **Clinical Pearl:** The Elek test (immunodiffusion) is used to detect toxin production in culture and confirm toxigenicity. Only toxigenic strains are reportable as *C. diphtheriae* in most jurisdictions. **Mnemonic: PHAGE-TOX** — **P**rophage integration, **H**orizontal gene transfer (phage-mediated), **A**ctivation by iron depletion, **G**enetic basis (lysogeny), **E**xpression regulated, **T**oxin on phage, **O**nly lysogenized strains, **X**enogeneic origin.