ABSTRACT
Bacterial species that are Gram-positive, aerobic, branching and partially acid-fast are members of the order Actinomycetales and are commonly found as soil saphrophytes. Included among over 40 genera of partially acid-fast bacteria are Nocardia, Rhodococcus, Gordona, and Tsukamurella. These genera are enjoined because they contain the glycolipid mycolic acid which accounts for their partial acid-fast staining characteristic. While mycobacteria, e.g. Mycobacterium tuberculosis, are also acid-fast, they resist decolorization with 3% hydrochloric acid in alcohol (acid-alcohol-fast), whereas the aerobic actinomycetes do not. Aerobic actinomycetes, however, retain some of the acid-fast stain when decolorized with 1% sulfuric acid. Corynebacterium species also contain mycolic acid in their cell wall but do not exhibit either true or partial acid-fast staining because of differences in their mycolic acid chemical structure. Aerobic actinomycetes exhibit delicate branching filamentous forms, which may extend upward (aerial hyphae) from a growing colony. Fragmentation of the filaments into small rods and cocci is common. Human infections in susceptible hosts usually follow inhalation of fragmented filaments or direct subcutaneous inoculation through trauma.
