Gram-⊕, aerobic rod.- Transmitted by respiratory droplets.

- Causes diphtheria via exotoxin encoded by β-prophage. Potent exotoxin inhibits protein synthesis via ADP-ribosylation of EF-2.

Symptoms:- Pseudomembranous pharyngitis (grayish-white membrane) over posterior wall/tonsils- Bull neck, which may result in airway obstruction.- Lymphadenopathy- Myocarditis → Arrhythmias

Lab diagnosis:- Gram-⊕ rods with metachromatic (blue and red) granules.- ⊕ Elek test for toxin.- Black colonies on cysteine-tellurite agar/Löffler medium.

- Toxoid vaccine prevents diphtheria- Treatment: IM injections of penicillin G for 14 days