Mycobacterium Marinum Infection of the Skin

Updated: May 24, 2017
  • Author: Kirstin Altman, MD; Chief Editor: Dirk M Elston, MD  more...
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Mycobacterium marinum is an atypical Mycobacterium found in salt water and fresh water. M marinum is the most common atypical Mycobacterium to cause infection in humans. Infection occurs following inoculation of a skin abrasion or puncture and manifests as a localized granuloma or sporotrichotic lymphangitis (see the image below).

Photograph of Mycobacterium marinum infection lesi Photograph of Mycobacterium marinum infection lesions.

Diagnosis and treatment are often delayed because of a lack of suspicion for mycobacterial involvement, ie, versus more common bacterial pathogens. Due to the increased use of immunosuppressants for transplant recipients and tumor necrosis factor (TNF) inhibitors for a variety of conditions, infections with mycobacteria other than tuberculosis (MOTT) are increasing.



M marinum is a slow-growing species that resides in both freshwater and saltwater environments, with optimal growth at 30-32°C. It is carried by many fish species and can result in human infection via inoculation of the skin by a fish bite, exposure of an open wound to contaminated water, contact with an aquarium, or contact with marine animals such as fish or turtles. [1] Exposure to M marinum via swimming pools is rare because most pools are chlorinated. [2]

The pathogen is classified as a photochromagen in Runyon group 1, which means that it produces yellow pigment when cultured and exposed to light. Culture growth occurs over 7-21 days and is optimal at 25-32°C (77-89.6°F) given the organism is adapted to infect ectotherms, such as fish. When endotherms, such as humans, are infected the infection favors the cooler extremities more than central sites. Systemic infection, usually of an immunocompromised host, has been reported. This indicates that the organism is capable of adapting to grow in conditions closer to 37°C. [3]

After inoculation into the host tissues via an abrasion or other wound, the mycobacteria are phagocytosed by macrophages. Inside the macrophage, they are able to interrupt the formation of the phagolysosome, which would normally kill the organisms. The mycobacteria, however, are able escape the lysosome and can move intracellularly and extracellularly via actin-based motility. This may contribute to cell-to-cell spread.

Tumor necrosis factor (TNF) is important for the immune response against mycobacteria. Studies have demonstrated that in the absence of TNF, macrophages engulf but do not destroy the mycobacteria. Instead, the mycobacteria survive and grow, finally killing the macrophage. [4] The importance of TNF is also supported by a number of reports of infection occurring in patients treated with TNF inhibitors, and these medications should be stopped during the course of antibiotic therapy. If not, the lesions may rapidly extend. [5, 6, 7]

Studies have revealed two pathophysiologically and genetically (ie, via amplified restriction-based polymorphism analysis) distinct populations of M marinum. One group can infect humans and causes acutely lethal disease in fish, while a second group cannot infect humans and causes chronic progressive disease in fish.

Special concerns

Utility of M marinum as an immunotherapy agent to elicit an antituberculosis response is currently being explored. [8] There is specific scientific interest in M marinum because of its genetic relatedness to Mycobacterium tuberculosis and because experimental infection of M marinum in fish mimics tuberculosis pathogenesis. [7]




United States

Infections caused by M marinum are uncommon but well described in the literature. The estimated annual incidence is 0.05-0.27 case per 100,000 adult patients. [9, 7] Of the more than 160 cases described, most are case reports of cutaneous infection; some report concomitant osteomyelitis, tenosynovitis, arthritis, and/or disseminated infection. Nosocomial infection has never been described.


Infection occurs worldwide, most commonly in individuals with occupational and recreational exposure to nonchlorinated fresh water or salt water. [10]


No racial predilection is apparent for M marinum skin infection.


No sexual predilection has been noted for M marinum skin infection.


M marinum infection has been reported in persons of every age group; however, it appears to be rare in the pediatric population. [11, 12]



Once identified and appropriately treated, M marinum infection can typically be successfully eradicated, usually with no major sequelae. M marinum skin infection typically remains localized and does not cause significant morbidity in patients who are immunocompetent. Cases reported in patients who are severely immunocompromised document disseminated infection via lymphatics and can involve the bone marrow and viscera, with rare reports of death secondary to the infection. [1]