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" both 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 , ray, beam and , fungus).