A. naeslundii and A. oris

Actinomyces naeslundii is a Gram-positive, rod-shaped bacterium found in human mouths, which has been implicated in periodontal disease, as well as various tooth cavities. In other cases, A. naeslundii is associated with good oral health, one of the first bacteria to occupy the oral cavity and colonize the tooth's surface. It has also been isolated from women with bacterial vaginosis. A. naeslundii appears Gram-positive and pleomorphic by microscopy, which does not form spores, and is considered anaerobic or microaerophilic. Many species of the genus Actinomyces, including A. naeslundii cause a chronic disease called actinomycosis, characterized by swelling and formation of an abscess which may exude pus. This can be accompanied by tissue fibrosis. Not only infections of the mouth, face, and neck are the most commonly recognized infections, but also the thoracic region, abdomen, pelvis, and the central nervous system can also get involved.