Actinomyces

Actinomyces is a genus of Gram-positive, non-spore-forming, filamentous bacteria in the phylum Actinobacteria. They are anaerobic to microaerophilic and form branching filaments that often resemble fungi. Actinomyces species are part of the normal microbiota of the human mouth, upper respiratory tract, gastrointestinal tract, and female genital tract.

Actinomyces israelii is the species most commonly associated with human disease. Actinomyces infections can cause actinomycosis, a chronic granulomatous infection characterized by abscess formation, draining sinus tracts, and the presence of sulfur granules—dense colonies of organisms surrounded by eosinophilic material found in discharges.

Infection typically follows disruption of mucosal barriers due to dental disease, trauma, or surgery and is often polymicrobial. The cervicofacial region is the most frequent site, but actinomycosis can involve the thorax, abdomen, pelvis, and, less commonly, other sites. Infections are often indolent and can form mass-like lesions that mimic neoplasia.

Diagnosis relies on clinical suspicion, identification of sulfur granules in exudates, histology showing filamentous, Gram-positive, branching bacteria, and culture under strict anaerobic conditions, though growth is slow. The condition must be differentiated from nocardiosis, which is partially acid-fast.

Treatment consists of prolonged antimicrobial therapy, typically high-dose penicillin G initially, followed by an extended course of oral penicillin or amoxicillin for several months. Alternatives for penicillin allergy include doxycycline, erythromycin, or tetracycline. Surgical drainage or debridement may be necessary for abscesses or sinus tracts. The prognosis is generally favorable with adequate therapy, though relapse can occur if treatment is shortened.