Mycobacteria exclusively affecting skin:

Ref: Ananthanarayan, Paniker CJ. Textbook of Microbiology. 8th Edition. London: John Wiley & Sons: 2009. Pg. 361Explanation:M. uicerans and M. marinum are exclusively skin pathogens, causing chronic ulcers and granulomatous lesions on the skin.Systemic invasion does not occur and the regional lymph glands are not involved.Cutaneous localisation is because they multiply optimally at skin temperature.M. marinum:It is a natural pathogen of cold blooded animals (fish and amphibians)Human infection originates from contaminated swimming pools or fish tank.The lesion usually follows abrasions and therefore occurs on the prominences--elbows, knees, ankles, nose, fingers or toes. It begins as a papule and breaking down to form an indolent ulcerThe lesion caused by M. marinum is also known as 'swimming pool granuloma"It has been reported from temperate countries such as North America and European countriesThis bacilli grows in about two weeks at 30degC (range 25-35 degC.Infection with M. marinum may cause a low- grade tuberculin reaction.M. ulcerans:Ulcers caused by M. ulcerans are usually seen on the legs or arms and are believed to follow infection through minor injuries.After an incubation period of a few weeks, indurated nodules appear which break down, forming indolent ulcers which slowly extend under the skin.The bacillus grows on Lowenstein-Jensen medium slowly, in 4-8 weeks; growth occurs only between 30 degC and 33 degCA toxin is produced by M. ulcerans that causes inflammation and necrosis when injected into the skin of guinea pigs. This is the only mycobacterium species known to produce a toxin