Mycobacteria marinum

M. marinum is a photochromogen, producing yellow-orange pigment only after light exposure, and it grows optimally at around 30 degrees Celsius, which explains its restriction to the cooler peripheral skin rather than deep tissue or viscera. Infection follows inoculation of abraded skin during cleaning of a fish tank, hence 'fish tank granuloma' or 'swimming pool granuloma', and appears after two to four weeks as a violaceous nodule or plaque that may ulcerate, often on a finger or the dorsum of the hand. Spread proximally in a chain of nodules along lymphatics is called sporotrichoid, a pattern shared with Sporothrix schenckii from rose thorns and gardening, Nocardia, and Leishmania. Diagnosis requires biopsy for culture at low incubation temperature, and the laboratory must be told the suspicion so it does not incubate at 37 degrees. Treatment is prolonged with clarithromycin plus ethambutol or rifampin, or doxycycline; it is notably resistant to isoniazid and pyrazinamide.