The Mycobacterium that is the etiologic agent of 'swimming pool granuloma' is?

• A. M. fortuitum
• B. M. kansasii
• C. M. marinum
• D. M. xenopi

Answer: (C)

Exposure to aquatic environments causing a skin infection points to Mycobacterium marinum. This organism thrives at cooler temperatures (around 30°C) and classically produces a localized skin granuloma after contact with fish tanks, swimming pools, or seawater—often on the hands or arms. That pattern—swimming pool granuloma or fish-tank granuloma—is a hallmark of M. marinum. Other non-tuberculous mycobacteria have different clinical pictures: some cause rapid skin infections after trauma, while others primarily cause pulmonary disease. Diagnostically, cultures grown at lower temperatures yield colonies, and the organism is acid-fast and may pigment with light exposure. Treatment usually involves a prolonged course of antibiotics chosen based on susceptibility.