Mycobacterium marinum: A bacterium found in both fresh and salt water that can cause disease in fish and people. Skin infection with M. marinum is usually acquired from swimming pools, aquariums, or fish-handling. People who have breaks in the skin such as cuts and scrapes are at increased risk. One form of the infection known as "swimming pool granuloma" can occur when there is inadequate chlorination of swimming pools. In the US, most infections with this bacteria have been from contact with fish tanks. M. marinum infection is not spread from person to person.
When M. marinum infects the skin, it causes the formation of localized microscopic nodules called granulomas. They occur at sites of skin trauma where there are scratches and cuts. The granulomas usually appear within 2-3 weeks of exposure. The most frequent sign is a slowly developing nodule (raised bump) at the site the bacteria entered the body. Frequently, the nodule is on the hand or upper arm. Later the nodule can become an enlarging sore (an ulcer). Swelling of nearby lymph nodes occurs. Multiple granulomas may form in a line along the lymphatic vessel that drains the site. These lesions will usually spontaneously heal in several months. This infection can also involve the joints (septic arthritis) and bones (osteomyelitis). For people with compromise of the immune system, M. marinum infection can be especially serious and involve disseminated (widespread) disease.
Anti-tuberculous drugs speed healing. These drugs usually need to be taken for 3-6 months. The preferred combination of drugs is Rifampin + Ethambutol. Depending on the antibiotic sensitivity of the organism, alternative drugs are Rifampin + Clarithromycin, or Minocycline alone.