"A genus of Gram-positive, facultatively anaerobic bacteria with rod-shaped and branching filamentous forms, which form part of the normal mucosal flora in mammals but may cause opportunistic infections. Also (in form actinomyces): a bacterium of this genus (also called ray fungus)", Late 19th cent. From scientific Latin Actinomyces, genus name from classical Latin actīno- + scientific Latin myces.[Actinomyces etymology, Actinomyces origin, 英语词源]