Actinomyces

Actinomyces
Scanning electron micrograph of Actinomyces israelii
Scientific classification
Domain:	Bacteria
Phylum:	Actinomycetota
Class:	Actinomycetia
Order:	Actinomycetales
Family:	Actinomycetaceae
Genus:	Actinomyces Harz 1877 (Approved Lists 1980)[1]
Species
See text.
Synonyms
"Actinobacterium" Sampietro 1908 "Odontomyces" Howell et al. 1965

Actinomyces is a genus of the Actinomycetia class of bacteria. They all are gram-positive and facultatively anaerobic, growing best under anaerobic conditions.^[2] Actinomyces species may form endospores, and while individual bacteria are rod-shaped, Actinomyces colonies form fungus-like branched networks of hyphae.^[3] The aspect of these colonies initially led to the incorrect assumption that the organism was a fungus and to the name Actinomyces, "ray fungus" (from Greek actis, ray or beam, and mykes, fungus).

Actinomyces species are ubiquitous, occurring in soil and in the microbiota of animals, including the human microbiota. They are known for the important role they play in soil ecology; they produce a number of enzymes that help degrade organic plant material, lignin, and chitin. Thus, their presence is important in the formation of compost. Certain species are commensal in the skin flora, oral flora, gut flora, and vaginal flora^[4] of humans and livestock. They are also known for causing diseases in humans and livestock, usually when they opportunistically gain access to the body's interior through wounds. As with other opportunistic infections, people with immunodeficiency are at higher risk. In all of the preceding traits and in their branching filament formation, they bear similarities to Nocardia.^[5]

Like various other anaerobes, Actinomyces species are fastidious, thus not easy to culture and isolate. Clinical laboratories do culture and isolate them, but a negative result does not rule out infection, because it may be due simply to reluctance to grow in vitro.