Corynebacterium diphtheriae

Corynebacterium diphtheriae is an aerobic, gram-positive, pleomorphic rod that appears club-shaped and arranges in V or L formations ('Chinese letter' pattern) on Gram stain. It is grown on Loeffler's or tellurite (Tinsdale) agar, producing black colonies. Toxin production requires lysogenic conversion by a corynephage carrying the tox gene, which encodes diphtheria toxin—an AB exotoxin. The B subunit binds host cell receptors (heparin-binding EGF-like growth factor) allowing entry; the A subunit ADP-ribosylates elongation factor-2 (EF-2), inhibiting protein synthesis, similar mechanistically to Pseudomonas aeruginosa exotoxin A.

Clinically, diphtheria presents with sore throat, low-grade fever, and formation of a thick, grayish-white pseudomembrane on the tonsils/pharynx that bleeds when scraped (distinguishing it from strep throat). Systemic absorption of the toxin can cause: myocarditis (arrhythmias, heart block—major cause of death), demyelinating peripheral neuropathy (palatal/pharyngeal paralysis, later distal extremity weakness), and rarely renal or hepatic damage.

Diagnosis involves culture on selective media plus the Elek test to detect toxin production (toxigenicity testing). Treatment includes diphtheria antitoxin (equine-derived) to neutralize circulating toxin plus antibiotics (penicillin or erythromycin) to eradicate the organism and halt further toxin production. Prevention is achieved through the diphtheria toxoid component of DTaP/Tdap vaccines, which is highly effective, making diphtheria rare in vaccinated populations but still a threat in unvaccinated groups or where vaccination coverage lapses (e.g., post-Soviet outbreaks).

High-yield board points: gram-positive club-shaped rod, exotoxin via corynephage, ADP-ribosylation of EF-2, pseudomembranous pharyngitis, myocarditis and neuropathy as key complications, treated with antitoxin + antibiotics, prevented by toxoid vaccine.