Mycobacterium marinum is an atypical Mycobacterium species found in cold or warm, fresh or salted water. M marinum infection occurs following skin and soft-tissue injuries that are exposed to an aquatic environment or marine animals. The infection usually presents as a localized granuloma but can evolve into an ascending lymphangitis that resembles sporotrichosis or can spread to deeper tissues. M marinum is a pathogen classified in Runyon group 1 and is a photochromogen, meaning it produces pigment when cultured and exposed to light. Culture growth occurs over 7-14 days and is optimal at 32°C. See the image below.
For additional information on cutaneous M marinum infection, see the Medscape Reference article Dermatologic Manifestations of Mycobacterium Marinum Infection of the Skin.
M marinum is water-borne atypical Mycobacterium species that commonly infects fish and amphibians. It was first recognized to cause human disease in 1951. M marinum infection commonly develops as a complication of skin and soft-tissue injuries exposed to aquatic equipment such as fish lines and fishhooks, among others. Domestic exposures involved in infection commonly involve fish tank manipulations. M marinum infection was once called swimming pool granuloma, but that term is now rather obsolete because of the widespread use of chlorination in swimming pools. Chlorinated swimming pools are not considered an exposure risk. 
M marinum grows best at 32°C; therefore, cooler extremities, particularly hands, are affected more often than central areas. This feature is also important for optimal growth in the microbiology laboratory. M marinum can disseminate in severely immunosuppressed individuals (eg, transplant recipients).
M marinum infections are rare but well described in the literature. The estimated annual incidence is 0.27 cases per 100,000 adult patients. The infection is typically limited to the skin, mostly involving limbs, but spread to deeper structures has been reported. This can result in clinical entities such as tenosynovitis, septic arthritis, and osteomyelitis. Dissemination is extremely rare and has been reported mainly in severely immunocompromised individuals.
M marinum is ubiquitous and is found in both salt and fresh waters. At least 150 fish and frog species, aquatic mammals (eg, dolphins), eels, oysters, African toads, and royal pythons are known to acquire natural M marinum infection. In Africa, M marinum has been isolated from healthy human skin and soil. Individuals who fish or work with aquariums are at an increased risk of exposure. To the authors’ knowledge, nosocomial M marinum infection has never been reported.
The international incidence and prevalence of M marinum infection are unknown owing to a lack of surveillance. One French study found the incidence of M marinum infection to be 0.04 per 100,000 inhabitants per year.
Feng et al described an outbreak of a cutaneous M marinum infection in China in 2008. 
M marinum infection responds slowly to appropriate antibiotic therapy. Infected patients may require treatment for 2 weeks or up to 18 months.
M marinum infection may result in persistent ulceration, draining sinuses, or septic arthritis. Aggressive M marinum infection may cause extensive osteomyelitis, resulting in amputation of the involved digit.
Disseminated M marinum infection and more invasive skin infections have been reported in significantly immunosuppressed individuals. Reports describe dissemination to the bone marrow and visceral involvement; however, the reports do not include deaths directly related to M marinum infection.
Delayed diagnosis can result from the indolent nature of early lesions and a lack of clinical suspicion. The average interval from clinical presentation to correct diagnosis varies from 1-27 months, with a mean interval of 7 months. 
M marinum infection has no known racial predilection.
M marinum infection has no known sexual predilection. Infection in men is typically linked to occupational exposures.
M marinum infection has no known age predilection.
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