RESUMO
Mycobacterium marinum is a photochromogenic and slowly growing acid-fast organism classified in Runyon's group I of atypical mycobacteria. Infection with M. marinum occurs when the organism is inoculated usually in association with exposure to contaminated water and the disease is characteristically limited to the skin but deep soft tissue may be involved. We report a case of M. marinum infection in a 43-year-old man with erythematous verrucous plaque on his right knee, which was caused by a shell wound on a beach 3 months ago. The skin biopsy specimen showed pseudoepitheliomatous hyperplasia and several small epithelioid granulomas in the dermis. Culture of the tissue specimen revealed slowly growing whitish colonies in the dark, which changed yellowish after exposure to light. The microorganism was identified as M. marinum by biochemical tests, and was confirmed by the DNA-DNA hybridization method. The skin lesion disappeared after 3 moaths of treatment with minocycline, 200mg daily.