Color Plate 29 depicts a filamentous gram-positive rod recovered from a closed chest abscess that grows only under anaerobic conditions and is not acid-fast. What is the most likely presumptive identification?

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

Answer: (A)

Actinomyces israelii fits this picture because it is a filamentous, gram-positive rod that thrives only under anaerobic conditions and is not acid-fast. In thoracic infections, it often causes chronic abscesses with draining sinus tracts and sulfur granules—a hallmark finding that a chest abscess image might depict. The not-acid-fast characteristic helps distinguish Actinomyces from Nocardia, which is also filamentous but is partially acid-fast and typically aerobically cultured. The other organisms listed are less consistent: a gram-negative anaerobe would not be filamentous and would not produce the sulfur granule–forming chronic abscess pattern commonly seen with Actinomyces; a spore-forming anaerobe like Clostridium septicum is more associated with gas gangrene and different clinical contexts; Propionibacterium acnes is a skin commensal that doesn’t usually present as a thoracic, sulfur-granule–producing chest abscess.