A college student who repaired a marina outboard motor sustained several lacerations on his forearm. A biopsy of a lesion revealed a cutaneous granulomatous condition. Which microorganism would most likely be the etiologic agent?

• A. Mycobacterium marinum
• B. Pseudomonas aeruginosa
• C. Vibrio vulnificus
• D. Nocardia asteroides

Answer: (A)

Mycobacterium marinum is the organism most consistent with this scenario. It is a non-tuberculous mycobacterium classically linked to aquatic exposures, especially in and around marine environments and fish tanks. After a skin wound sustained in water, it often causes a chronic cutaneous granulomatous infection on the extremities, described histologically as granulomatous inflammation with acid-fast bacilli. A key clue is the marine/fitness context (repairing a marina motor) with forearm lacerations, pointing to an infection acquired from seawater or tank water. Mycobacterium marinum grows best at cooler temperatures (around 25–33°C), which confines the infection to the skin and superficial tissues rather than deep structures.

Pseudomonas aeruginosa can cause skin infections in water-related settings, but those are typically purulent, rapidly progressive, and not granulomatous. Vibrio vulnificus causes aggressive wound infections with rapid tissue destruction and systemic illness after seawater exposure, not a chronic granulomatous lesion. Nocardia asteroides can produce cutaneous granulomas after soil exposure, but it is more commonly associated with soil or decaying vegetation and does not fit the aquatic, “fish-tank” exposure pattern as neatly as M. marinum.