General Information: This genus comprises a number of Gram-positive, acid-fast, rod-shaped aerobic bacteria and is the only member of the family Mycobacteriaceae within the order Actinomycetales. Like other closely related Actinomycetales, such as Nocardia and Corynebacterium, mycobacteria have unusually high genomic DNA GC content and are capable of producing mycolic acids as major components of their cell wall. Mycobacterium marinum is commonly found in fresh and salt water in North America. This bacterium, a close relative of M. tuberculosis, causes a tuberculosis-like disease in frogs, fish and other cold-blooded animals, and a peripheral granulomatous disease in humans. Human infections can be persistent and may lead to systemic infection and arthritis. M. marinum is photochromogenic, i.e. it produces bright yellow pigmentation (mainly beta-carotene) upon exposure to visible light. Infection is followed by a 2-8 week incubation period resulting in suppuration (pus) and organized ulcerating granulomas (tumorous lesions), predominantly localized in lymphatic and tendon sheaths, and cooler body tissues in the extremities.