Ultrathin section of Corynebacterium diptheriae pw8 (pdi) cultivated under high Fe condition. Toxin production is limited. Corynebacteria are rod-shaped, gram-positive, non motile, non spore-forming bacilli. Though many of the species are pathogenic in domestic animals, birds, and plants, the most important is the species C. diptheriae with causes diphtheria in humans. The disease is characterized by the formation of a false membrane on any mucous surface and occasionally on the skin. In nasal diphtheria, fever is a much more evident symptom. Adenitis is often severe, with serious discharge from nostrils, which may be blood tinged. Myocarditids and late neuritis are commonly present.