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, contracted from contact with a carrier by air-bourne infection. 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: x10,000, when printed 15cm wide.