Diphtheria bacteria. Coloured transmission electron micrograph (TEM) of Corynebacterium diphtheriae bacteria, the cause of diphtheria. The bacteria are Gram-positive, non-motile rods (bacilli). Surrounding each cell is a mucus cell wall (red). Diphtheria is caused by a potent exotoxin released from the bacterium. Symptoms include sore throat, fever and breathing difficulties. Exotoxin damage to the heart and nerves can occur. Treatment is with antitoxins, antibiotics and complete rest.