Male with non-healing cutaneous granulomatous lesions after aquarium exposure; most likely etiologic agent?

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

Answer: (A)

Aquarium exposure with a chronic, non-healing cutaneous granulomatous lesion is a classic clue for infection with Mycobacterium marinum, a non-tuberculous mycobacterium that thrives in aquatic environments. This organism commonly causes “fish-tank granuloma,” presenting as nodules or ulcers on the hands or arms that may slowly spread or track along lymphatics. It grows best at cooler temperatures (around 30°C), which explains why the infection is cutaneous rather than deep-tisted. Diagnosis is supported by observing acid-fast bacilli in tissue and by culturing the organism from the lesion on media incubated at about 30°C. Treatment usually requires a multi-drug regimen for several months (commonly rifampin plus ethambutol, sometimes with a macrolide), as monotherapy is insufficient.

Nocardia asteroides can produce cutaneous granulomatous lesions but is more commonly linked to soil and decaying matter, and it is not the classic aquarium-associated pathogen. Pseudomonas aeruginosa can cause water-associated infections like hot tub folliculitis or burn-wound infections, which don’t fit the chronic granulomatous hand/arm lesions after aquarium exposure. Vibrio vulnificus is tied to seawater or raw seafood exposure and tends to cause rapidly progressive cellulitis or septicemia rather than a chronic localized granulomatous skin infection from an aquarium.