An aerobic, filamentous, branching gram-positive organism that stains weakly acid-fast in a brain abscess from an immunosuppressed patient is most likely which genus?

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

Answer: (B)

Recognition of Nocardia features: an aerobic, filamentous, branching gram-positive bacterium that stains weakly acid-fast. Nocardia species are soil bacteria that grow aerobically and form branching, filamentous structures. Their cell walls contain mycolic acids, which makes them partially acid-fast; they are best seen with a modified acid-fast stain that highlights the weak acid-fastness. This combination—aerobic growth, branching filamentous morphology, and weak acid-fast staining—points to Nocardia. Clinically, Nocardia commonly causes brain abscesses in immunocompromised patients, often after a pulmonary infection.

Actinomyces israelii is anaerobic and not acid-fast, and it typically presents with draining sinus tracts and sulfur granules rather than brain abscesses in this immunosuppressed context. Mycobacterium tuberculosis is acid-fast but appears as non-branching rods, not true filamentous branching forms. Propionibacterium acnes is an anaerobic gram-positive rod associated with acne and lacks acid-fast staining.