Actinomycetes are traditionally considered to be transitional forms between bacteria and fungi. Because, like fungi they form a mycelial network of branching filaments but like bacteria, they are thin, possess cell walls containing muramic acid, have prokaryotic nuclei and are susceptible to antibacterial antibiotics. They are therefore true bacteria, bearing a superficial resemblance to fungi.
Actinomycetes are related to mycobacteria and corynebacteria. They are Gram positive, nonmotile, non sporing, non capsulated filaments that break up into bacillary and coccoid elements. Most of them are free living, particularly in soil.
Actinomyces is one of the, medical interest, genera included in Actinomycetes. The major pathogenic genus Actinomyces is anaerobic or microaerphilic and non acid fast, while Nocardia species are aerobic and may be acid fast.Bollinger (1877) found a mound like organism in the lesion of ‘lumpy jaw’ (actinomycosis) in cattle. Wolff and Israel (1891) isolated an anaerobic bacillus from human lesions and produced experimental infection in rabbits and guinea pigs. This was named Actinomyces israelii. It causes human actinomycosis. Actinomycosis in cattle is produced by A. bovis.