Actinomycetes

            Like fungi they form a mycelial network of branching filaments, like bacteria they are thin, possess cell walls containing muramic acid, prokaryotic nuclei. So actinomycetes are considered as transitional forms between bacteria and fungi. They are Gram positive, nonmotile, non sporing, non capsulated filaments that break up into bacillary and coccoid elements. Most are free living, particularly in soil. Human pathogenic actinomycetes belong to four genera : Actinomyces, Nocardia, Streptomyces & Actinomadura. Actinomyces is non acid fast and anaerobic or microaerophilic,  Nocardiais aerobe and acid fast,  Streptomyces& Actinomadura are aerobes and non acid fast. Some species of Streptomyces may cause disease rarely, but their importance is as the major source of antibiotics.