Diphtheria toxin

**Source organism:** Corynebacterium diphtheriae is a gram-positive, club-shaped rod with metachromatic (volutin/Babes-Ernst) granules. It appears in "Chinese character" arrangements. Only strains lysogenized by the beta-prophage (corynephage β) carrying the *tox* gene produce toxin. Toxin gene expression is regulated by the iron-dependent repressor DtxR. Low iron derepresses it.

**Structure:** It is an A-B toxin, a single polypeptide cleaved into two fragments linked by a disulfide bond. - B (binding) subunit: binds heparin-binding EGF-like growth factor precursor (HB-EGF) on host cells, especially heart and nerve cells. Receptor-mediated endocytosis follows. - A (active) subunit: enzymatic. After endosomal acidification, A is translocated into the cytosol.

**Mechanism:** The A subunit catalyzes ADP-ribosylation of elongation factor-2 (EF-2) at a modified histidine residue called diphthamide. NAD⁺ is the ADP-ribose donor. EF-2 is inactivated, so translocation during eukaryotic protein synthesis is blocked. Host cell protein synthesis stops and the cell dies. *Pseudomonas aeruginosa* exotoxin A has the same mechanism.

**Mnemonic:** Diphtheria and Pseudomonas exotoxin A both inhibit EF-2. Other toxins that ADP-ribosylate: cholera (Gs, ↑cAMP), pertussis (Gi, ↑cAMP), and LT of ETEC.

**Clinical features:** - Respiratory diphtheria: sore throat, low-grade fever, and a thick gray-white adherent pseudomembrane (necrotic epithelium, fibrin, WBCs, bacteria) on the tonsils, pharynx, or larynx. Attempts to remove it cause bleeding. It can cause airway obstruction. - "Bull neck": marked cervical lymphadenopathy and edema. - Myocarditis: arrhythmias, heart block, and heart failure. It is the main cause of death. - Neuropathy: palatal paralysis, then cranial nerve palsies, and later a demyelinating polyneuropathy (Guillain-Barré-like). - Cutaneous diphtheria: chronic nonhealing ulcers, more common in the tropics.

**Diagnosis:** - Clinical suspicion, because treatment should not be delayed. - Gram stain showing club-shaped rods. - Culture on Löffler medium (enhances metachromatic granules) and on tellurite agar (black colonies). - Elek test: immunodiffusion that confirms toxin production. PCR for the *tox* gene is also used.

**Treatment:** - Diphtheria antitoxin (equine) to neutralize circulating unbound toxin. It does not neutralize toxin already internalized. - Antibiotics (penicillin or erythromycin) to eradicate the organism. - Airway management and cardiac monitoring. - Isolation and contact prophylaxis.

**Prevention:** The toxoid vaccine (formalin-inactivated toxin) is given as DTaP in children and Tdap/Td in adolescents and adults. Boosters are given every 10 years. Immunity is antitoxin-mediated, so the vaccine prevents disease but not colonization.

**High-yield associations:** - Lysogenic conversion (bacteriophage-encoded toxin). Others: Shiga toxin, botulinum toxin, cholera toxin, erythrogenic toxin of *S. pyogenes*, and *Clostridium botulinum* type C/D (the "ABCDEFG" mnemonic: Shiga and Shiga-like toxins, Botulinum, Cholera, Diphtheria, Erythrogenic toxin of *S. pyogenes*, and *Corynebacterium diphtheriae*). - Unvaccinated or under-immunized populations are at risk, including migrants and areas with disrupted vaccination programs.