The Actinomyces species are straight to slightly curved gram-positive bacilli that can range from short rods to long, branching rods. This anaerobic bacterium is found normally in the oral cavity, upper respiratory tract and, to a lesser extent, the gastrointestinal tract. They are not a particularly virulent species of bacteria and usually only cause infection when there is some disruption in the surface of the oral cavity or upper respiratory tract (opportunistic infection).
Also, the causative organisms, Actinomyces species (most commonly A. israelii), are often present commensally on the gums, tonsils, and teeth and may be present in the intestines and vagina.
Actinomycosis most often occurs in adult males and takes several forms:
Cervicofacial (lumpy jaw): The most common portal of entry is periodontal disease.
Thoracic: Pulmonary disease results from aspiration of oral secretions.
Abdominal: Disease presumably results from a break in the mucosa of a diverticulum or the appendix or from trauma.
Uterine: This localized pelvic form is a complication of certain types of intrauterine device (IUD).
Generalized: Rarely, the infection spreads from primary sites, presumably by hematogenous seeding.