Corynebacterium diphtheriae

Corynebacterium diphtheriae is a gram-positive, club-shaped (coryneform) bacillus and the causative agent of diphtheria. On microscopy the organisms often arrange in V or Y shapes, and staining with aniline dyes reveals metachromatic (volutin) granules. It is cultured on cystine-tellurite agar, where colonies appear black, or on Loeffler medium.

Its virulence comes from diphtheria toxin, an AB exotoxin encoded by a gene acquired through lysogenic conversion by a bacteriophage — only phage-infected strains are toxigenic. The toxin's B subunit binds host cells and the A subunit ADP-ribosylates elongation factor 2 (EF-2), shutting down protein synthesis. Locally, this kills pharyngeal epithelium, producing the classic gray-white pseudomembrane over the tonsils and pharynx that bleeds when scraped and can obstruct the airway. Systemically, absorbed toxin damages the heart and nerves, causing myocarditis and demyelinating neuropathy; cervical lymphadenopathy can create a "bull neck" appearance.

Diphtheria is now rare where vaccination is routine: the DTaP and Tdap vaccines contain diphtheria toxoid, an inactivated toxin that elicits neutralizing antitoxin antibodies. Treatment of active disease centers on diphtheria antitoxin plus antibiotics such as penicillin or erythromycin.

USMLE Step 1 tests C. diphtheriae as a classic microbiology-biochemistry crossover: expect questions linking the EF-2 ADP-ribosylation mechanism (shared with Pseudomonas exotoxin A), lysogenic conversion, the tellurite/Loeffler culture media, and the pseudomembrane-plus-myocarditis clinical picture.