The species of Mycobacterium that is most commonly associated with contamination of hot water systems in large institutions such as hospitals is which?

• A. M. haemophilum
• B. M. marinum
• C. M. ulcerans
• D. M. xenopi

Answer: (D)

Hot-water plumbing in large institutions can harbor environmental mycobacteria that persist in biofilms and survive the warmth of taps and heaters. Among these, Mycobacterium xenopi is especially linked to hot-water environments and is frequently recovered from hospital hot-water systems, taps, and boilers. This association with warm water makes it the most common contaminant in such settings. Other options fit different niches: M. haemophilum grows best at cooler temperatures and in iron-rich environments, often causing skin infections in immunocompromised patients; M. marinum is typically found in aquatic environments like swimming pools and aquariums and causes skin infections after exposure; M. ulcerans is associated with tropical settings and Buruli ulcers, not hospital hot-water systems.