A family of nonsporeforming, nonmotile, ordinarily facultatively anaerobic (some species are aerobic and others are anaerobic) bacteria (order Actinomycetales) containing Gram-positive, non-acid-fast, predominantly diphtheroid cells which tend to form branched filaments in tissue or in some stages of cultural development; the filaments readily fragment, producing diphtheroid or coccoid forms. The metabolism of these chemoheterotrophic bacteria is fermentative. This family contains the genera Actinomyces (type genus), Arachnia, Bacterionema, Bifidobacterium, and Rothia.