Actinomyces naeslundii is a Gram-positive, rod-shaped bacterium found in the mouth of humans.

The species has been implicated in periodontal disease, as well as various tooth cavities.

It is one of the first bacteria to occupy the oral cavity and colonize the tooth's surface.

[3] Many species of the genus Actinomyces, including A. naeslundii, cause a chronic disease called actinomycosis, which is characterized by swelling and formation of an abscess which may exude pus.

[4] Infections of the mouth, face, and neck are the most commonly recognized infections; however, the thoracic region, abdomen, pelvis, and the central nervous system can also be involved.