Actinomyces is a genus of bacteria. The bacteria that grouped in this genus share several characteristics. The bacteria are rod-like in shape. Under the light microscope, Actinomyces appear fungus-like. They are thin and joined together to form branching networks. Bacteria of this genus retain the primary stain in the Gram stain reaction, and so are classified as being Gram positive. Actinomycetes are not able to form the dormant form known as a spore. Finally, the bacteria are able to grow in the absence of oxygen.

Members of the genus Actinomyces are normal residents of the mouth, throat, and intestinal tract. But they are capable of causing infections both in humans and in cattle if they are able to enter other regions. This can occur as the result of an accident such as a cut or abrasion.

An infection known as Actinomycosis is characterized by the formation of an abscess--a process "walling off" the site of infection as the body responds to the infection--and by swelling. Pus can also be present. The pus, which is composed of dead bacteria, is granular, because of the presence of granules of sulfur that are made by the bacteria.

The diagnosis of an Actinomyces infection can be challenging, as the symptoms and appearance of the infection is reminiscent of a tumor or of a tuberculosis lesion. A well-established infection can produce a great deal of tissue damage. Additionally, the slow growth of the bacteria can make the treatment of infection with antibiotics very difficult, because antibiotics rely on bacterial growth in order to exert their lethal effect.

The culturing of Actinomyces in the laboratory is also challenging. The bacteria do not grow on nonselective media, but instead require the use of specialized and nutritionally complex selective media. Furthermore, incubation needs to be in the absence of oxygen. The growth of the bacteria is quite slow. Solid growth medium may need to be incubated for up to 14 days to achieve visible growth. In contrast, a bacterium like Escherichia coli yields visible colonies after overnight growth on a variety of nonselective media. The colonies of Actinomyces are often described as looking like bread crumbs.

Currently, identification methods such as polymerase chain reaction (PCR), chromatography to detect unique cell wall constituents, and antibody-based assays do always perform effectively with Actinomyces.