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actinomycete, moldlike bacterium of the order Actinonsycetales, a group of micro-organisms noted for a filansentous and branching growth pattern, which results, in most forms, in an extensive colony, or mycelium. Many species occur in soil and are harmless to animals and higher plants; others are important pathogens.
Most authorities recognize four families. The family Mycobacteriaceae includes several dozen species, some of which cause diseases of animals. Among the most important are Mycobacterium tuberculosis, the cause of tuberculosis (q.v.) in man, and M. Ieprae, the agent of leprosy (q.v.; also called Hansen’s disease). Others of this genus cause tuberculotic diseases in nonhuman animals. The rod- shaped cells in the family occasionally form filaments but usually lack the branching characteristic of the other three.
Within the family Actinomycetaceae are many bacteria that aid in the decay of plant and animal material in soil. A number of Nocardia species are believed to cause severe (sometimes fatal) lesions of the lungs, spleen, and other organs of mammals, including man. The family Streptomycetaceae includes many Streptotnyces species that are associated with serious (but poorly known) diseases in animals. Most species, however, are soil bacteria, many of which form antibiotics (e.g., streptomycin).
The fourth family, Actinoplanaceae, is a little-known group of aquatic bacteria that forms mycelia on plant and animal surfaces.
actinomycosis, a noncontagious fungal infection of man and cattle caused by two anaerobic species of the genus Actinomyces and characterized by multiple painful, hard swellings filled with pus, most often seen on the face, neck, chest, and abdomen. Actinomyces bovis is responsible for the disease in cattle and Actinomyces israeli for that in man.
In man, the organism lives chiefly in the mouth and bowel, growing best in the absence of oxygen, and disease is produced by direct invasion of devitalized tissues. Lesions of the neck and face (cervicofacial actinomycosis), notably the jaw, which account for about one- half of all cases, may appear following a wound in the mouth or a tooth extraction, and lesions of the abdomen may follow appendicitis or perforation of the stomach or large intestine. Infection of the lungs and surrounding structures (thoracic actinomycosis) may result from inhalation of the organism into the air passages and is usually associated with weight loss, night sweats, coughing, and high fever. In rare cases, the disorder may involve the brain, heart valves, or extremities; itmay also be disseminated, via the bl stream, in which case lesions appear in parts of the body. Treatment is with anti ics; surgical drainage or excision of acces lesions are valuable adjuncts.
