Definition: A genus of nonmotile, non-spore-forming, anaerobic to aerotolerant bacteria (family Propionibacteriaceae) containing gram-positive rods that are usually pleomorphic, diphtheroid, or club shaped, with one end rounded, the other tapered or pointed. Some cells may be coccoid, elongate, bifid, or even branched. The cells usually occur singly, in pairs, in V and Y configurations, short chains, or clumps in “Chinese character” arrangement. The metabolism of these organisms is fermentative, and the products of fermentation include combinations of propionic and acetic acids. These organisms occur in dairy products, on human skin, and in the intestinal tracts of humans and other animals. They may be pathogenic. The type species is Propionibacterium freudenreichii.