Corynebacterium diphtheriae bacteria, coloured scanning electron micrograph (SEM). Corynebacterium diphtheriae is a Gram-positive, rod-shaped (bacillus) bacterium that causes diphtheria, an acute infection, normally of the throat. The bacillus affects an area of mucous membrane, causing the tissue to swell. The danger to the patient is two-fold: the swelling may form an obstruction to breathing, and the bacillus secretes a toxin which passes into the blood. The toxin may cause failure of the kidneys, heart muscles or local paralysis. Treatment is with an antitoxin; prevention is by immunisation. Magnification: x15,000 when printed 15cm wide.