A genus of slow-growing, nonmotile, nonsporeforming, anaerobic to facultatively anaerobic bacteria (family Actinomycetaceae) containing Gram-positive, irregularly staining filaments; diphtheroid cells are predominant. Has characteristic sulfur granules that exhibit true branching while forming mycelial type colonies. Most of the species produce a filamentous microcolony. The metabolism of these chemoheterotrophs is fermentative; the products of glucose fermentation include acetic, formic, lactic and succinic acids but not propionic acid. These organisms are pathogenic for man and/or other animals and can cause chronic suppurative infection in humans. The type species is Actinomyces bovis.