A 45 year old male presents with jaw swelling and pain around a recently extracted tooth. There is drainage around the incision and a minor fever. An aspirate from the abscess was cultured. The culture shows white-gray molar-tooth-like colonies on blood agar by day 3; Brucella agar also shows small similar colonies; thioglycolate broth appears turbid; Gram stain shows Gram-positive branching filaments with sulfur granules; Penicillin is susceptible. Which organism is suspected?

• A. Actinomyces species
• B. Nocardia asteroides
• C. Propionibacterium acnes
• D. Streptococcus anginosus

Answer: (A)

Cervicofacial actinomycosis from Actinomyces israelii fits this presentation. These filamentous, Gram-positive bacteria are part of the normal oral flora and cause chronic jaw infections after dental work when the mucosa is breached. The hallmark is sulfur granules—yellowish grains of organism and debris—in draining sinus tracts. On culture, Actinomyces forms slow-growing, filamentous colonies that can resemble tiny “molar teeth” or have molar-tooth–like morphology on blood agar after a few days. The infection is classically anaerobic, and penicillin is the treatment of choice, often for an extended period, because tissues becomeFibrotic and infections are chronic.

This pattern helps distinguish it from other organisms. Nocardia can form branching filaments too but is typically weakly acid-fast and tends to cause pulmonary or disseminated infections rather than a cervicofacial jaw abscess with sulfur granules. Propionibacterium acnes (Cutibacterium) is associated more with skin conditions and does not produce sulfur granules or that distinctive colony morphology. Streptococcus anginosus can cause abscesses but appears as Gram-positive cocci, not branching filaments, and lacks sulfur granules.

So, the combination of jaw infection after dental work, draining sinus with sulfur granules, Gram-positive branching filaments, and molar-tooth–like colonies that grow under anaerobic conditions, all point to Actinomyces israelii.