|Scanning electron micrograph of Actinomyces israelii.|
Actinomyces (from Greek "actis" ray, beam and "mykes" mucus, fungus) is a genus of the actinobacteria class of bacteria. They are all Gram-positive. Actinomyces are facultatively anaerobic (except A. meyeri, a strict anaerobe). All species grow best under anaerobic conditions. Actinomyces species do not form endospores, and, while individual bacteria are rod-shaped, Actinomyces colonies form fungus-like branched networks of hyphae. The aspect of these colonies initially led to the incorrect assumption that the organism was a fungus and to the name "Actinomyces" or ray fungus.
Actinomyces are known for causing disease in humans, and for the important role they play in soil ecology. They produce a number of enzymes that help degrade organic plant material, lignin, and chitin. As such, their presence is important in the formation of compost.
Phylogenetic trees based on 16S rRNA sequences have shown that the genus Actinomyces is quite diverse, exhibiting polyphyletic branching into several clusters. The genera Actinomyces and Mobiluncus form a monophyletic clade in a phylogenetic tree constructed using RpoB, RpoC and DNA Gyrase B protein sequences. This clade is also strongly supported by a conserved signature indel consisting of a 3 amino acid insertion in isoleucine tRNA synthetase found only in the species of the genera Actinomyces and Mobiluncus.
Actinomyces species are normally present in the gums and are the most common cause of infection in dental procedures and oral abscesses. Many Actinomyces species are opportunistic pathogens of humans and other mammals, particularly in the oral cavity. In rare cases, these bacteria can cause actinomycosis, a disease characterized by the formation of abscesses in the mouth, lungs, or the gastrointestinal tract. Actinomycosis is most frequently caused by Actinomyces israelii. A. israelii may also cause endocarditis, though the resulting symptoms may be similar to those resulting from infections by other bacterial species. Aggregatibacter actinomycetemcomitans has been identified as being of note in periodontal disease.
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