Actinomyces /ak′tinōmī″sēz/ [Gk, aktis, ray, mykes, fungus] , a genus of anaerobic or facultative anaerobic, gram-positive bacteria. Species that may cause disease in humans, such as Actinomyces israelii (which causes actinomycosis), are normally present in the mouth and throat. Disease activity is normally limited to periodontal disease.