A. naeslundii, A. oris and A. israelii

A. naeslundii appears Gram-positive and pleomorphic by microscopy. It does not form spores, and is considered anaerobic or microaerophilic. In 2009, the species A. naeslundi was split. Bacteria formerly known as "Actinomyces naeslundi genospecies 2" were reclassified under the new species name Actinomyces oris. Actinomyces naeslundii and Actinomyces oris are part of the commensal oral microbiota, may play a role in pathogenesis of caries and have been isolated from extra-oral infections. Actinomyces israelii is a species of Gram-positive, rod-shaped bacteria within theActinomyces. Known to live commensally on and within humans, A. israelii is an opportunistic pathogen and a cause of actinomycosis.