A branching, gram-positive rod that is weakly acid-fast and can cause brain abscesses in immunocompromised hosts belongs to which genus?

• A. Actinomyces
• B. Nocardia
• C. Mycobacterium
• D. Propionibacterium

Answer: (B)

Nocardia are branching, Gram-positive rods that stain weakly acid-fast on a modified acid-fast stain. They are aerobic and commonly found in soil; infection often starts in the lungs and can disseminate to the brain, causing abscesses, a pattern seen especially in immunocompromised hosts. This combination of branching morphology, partial acid-fastness, and the potential to form brain abscesses is characteristic of Nocardia, making it the best match. Actinomyces can branch but is not acid-fast; Mycobacterium is acid-fast but typically not branching filaments and presents differently clinically; Propionibacterium is not acid-fast and is usually associated with acne or device-related infections.