The aerobic Actinomycetales represent a group of gram-positive organisms that range from aerobic gram-positive bacilli (diphtheroids) to branching, filamentous gram-positive bacteria. Some genera are capable of forming well-developed aerial and vegetative branched filaments. Genera include: Nocardia, Streptomyces, Mycobacterium, Actinomadura, Nocardiopsis, Rhodococcus, Micromonospora, Oerskovia, Saccharomonospora, Saccharopolyspora, Tsukamurella, Thermoactinomyces and Dermatophilus. The genera Saccharomonospora, Saccharopolyspora, and Thermoactinomyces are considered thermophilic actinomycetes because of their ability to grow at 50oC (122oF). The aerobic Actinomycetales have been isolated from the skin, pharynx and gastrointestinal tract of humans and animals and from soil, beach sand, house dust, tap water, swimming pools, plants, compost, hay, decaying vegetation, manure, air-conditioning ducts and humidifier water. They cause human infections following inhalation of these bacteria or by contamination of an area of trauma.