A genus of gram-positive bacteria comprising at least two species, E. rhusiopathiae and E. tonsillarum. Erysipelothrix rhusiopathiae is the more pathogenic and causes disease in a variety of animal species, including humans. It is a rod-shaped organism, 0.3 × 1–2 micrometers, that may form filaments in old cultures and in chronically infected tissues. It occurs in the tonsil of healthy swine and on the surfaces of fish and other aquatic species; it is shed in the urine, oral secretions, and feces of infected animals. Organisms deposited in the soil may survive for years and may be a source of infection for susceptible domestic and wild animals and birds.