A filamentous gram-positive rod recovered from an aspirate of a closed chest abscess grew only under anaerobic conditions and was not acid-fast. What is the most likely identification?

• A. Actinomyces israelii
• B. Bacteroides fragilis
• C. Clostridium septicum
• D. Propionibacterium acnes

Answer: (A)

Actinomyces israelii is a classic match when you see a filamentous, gram-positive rod that grows only under anaerobic conditions and is not acid-fast, in a chest abscess from aspiration. Actinomyces species are part of the normal oral flora and cause chronic granulomatous infections with draining sinus tracts and sulfur granules; thoracic actinomycosis can follow aspiration and present as a closed-chest abscess. The filamentous, branching appearance and strict anaerobic growth fit Actinomyces well, and the lack of acid-fast staining helps differentiate it from other filamentous organisms.

Bacteroides fragilis is anaerobic but gram-negative, not filamentous. Clostridium septicum is a gram-positive rod that is not typically filamentous in the same branching form and has different clinical associations. Propionibacterium acnes is a non-filamentous gram-positive rod, commonly linked to skin conditions. The combination of morphology, growth requirements, and clinical context makes Actinomyces israelii the best fit.