Actinomyces is a genus of the Actinobacteria class of bacteria. They all are Gram-positive. Actinomyces species are facultatively anaerobic (except A. meyeri and A. israelii are obligate anaerobe), and they grow best under anaerobic conditions. Actinomyces species may form endospores, and while individual bacteria are rod-shaped, Actinomyces colonies form fungus-like branched networks of hyphae. The aspect of these colonies initially led to the incorrect assumption that the organism was a fungus and to the name Actinomyces, "ray fungus" (from Greek actis, ray or beam, and mykes, fungus).