Actinomyces naeslundii are Gram positive rod shaped bacteria that occupy the oral cavity. They have been implicated in periodontal disease and root caries. These bacteria are also associated with good oral health. These bacteria are some of the first to occupy the oral cavity and colonize the tooth's surface.
Propionibacterium propionicus, and related species of bacteria, also can cause actinomycosislike disease. Other bacteria that frequently are isolated from clinical specimens concomitantly with Actinomyces in human infection include Aggregatibacter actinomycetemcomitans, Eikenella corrodens, and species of Fusobacterium, Bacteroides, Capnocytophaga, Staphylococcus, Streptococcus, and Enterococcus.
In general, Actinomyces species, being members of the normal flora, are agents of low pathogenicity and require disruption of the mucosal barrier to cause disease. Oral and cervicofacial diseases commonly are associated with dental procedures, trauma, oral surgery, or dental sepsis. Pulmonary infections usually arise after aspiration of oropharyngeal or GI secretions. GI infection frequently follows loss of mucosal integrity, such as with surgery, appendicitis, diverticulitis, trauma, or foreign bodies. Numerous reports have linked the use of intrauterine contraceptive devices to the development of actinomycosis of the female genital tract. The presence of a foreign body in this setting appears to trigger infection.
Other bacterial species that often are copathogens to Actinomyces species may aid spread of infection by inhibiting host defenses and reducing local oxygen tension. Once the organism is established locally, it spreads to surrounding tissues in a progressive manner, leading to a chronic, indurated, suppurative infection often with draining sinuses and fibrosis. In tissues, Actinomyces grow in microscopic or macroscopic clusters of tangled filaments surrounded by neutrophils. When visible, these clusters are pale yellow and exude through sinus tracts; they are called sulfur granules. This is not an exclusive finding of actinomycosis, and its absence does not rule out the diagnosis. Other conditions, such as eumycetoma and nocardiosis, have been linked to the production of sulfur granules.