Actinomyces israelii is the usual cause. A israelii and at least 5 other Actinomyces species are slow-growing, microaerophilic or facultative anaerobic, gram-positive, filamentous branching bacilli that can be part of the normal oral, gastrointestinal, or vaginal flora. Actinomyces species frequently are copathogens in tissues harboring multiple species. Actinobacillus actinomycetemcomitans is a frequent copathogen, and its isolation may predict the presence of actinomycosis.