In an immunocompromised patient with a pulmonary abscess and brain abscess, aspirated material grew an aerobic, filamentous, branching gram-positive organism that stains weakly acid-fast. The most likely etiologic agent would be which?

• A. Actinomyces israelii
• B. Nocardia asteroides
• C. Mycobacterium tuberculosis
• D. Propionibacterium acnes

Answer: (B)

Nocardia species fit this scenario because they are aerobic, filamentous, branching Gram-positive bacteria that stain weakly acid-fast. In immunocompromised patients, inhalation can lead to pulmonary nocardiosis, and the infection can disseminate through the blood to the brain, causing brain abscesses. This combination of lung and brain involvement with a filamentous, branching, weakly acid-fast organism is characteristic of Nocardia. Actinomyces israelii is anaerobic and not acid-fast, so it doesn’t match the staining or the typical clinical pattern. Mycobacterium tuberculosis is strongly acid-fast and, while it can affect the lungs and brain, it usually doesn’t present as a branching filamentous organism in this context. Propionibacterium acnes isn’t acid-fast and isn’t described as branching filaments.