False-colour scanning electron micrograph (SEM) of Corynebacterium diphtheria, the Gram-positive, non-motile bacteria responsible for diphtheria. Individual bacterium are club-shaped at one end (Corynebacterium = clubbed rod). Diphtheria is caused by a potent exotoxin released from the bacterium; infection occurs through direct contact or contaminated milk. Symptoms include sore throat & fever; a grey membrane forms across throat causing breathing difficulties. The exotoxin damages heart & nervous tissues; death may result from heart failure. Treatment involves giving an antitoxin, penicillin, & complete rest. Magnification: X 3,000 at 35mm size.