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]