This patient has Mycobacterium fortuitum–associated skin infection. Nontuberculous mycobacteria can colonize salon water, and M. fortuitum furunculosis has been well described in patients who obtain pedicures at nail salons that use contaminated whirlpool footbaths. Recent leg shaving with a razor increases the risk for these infections, likely owing to resultant skin nicking. The distal lower-extremity skin lesions, which usually arise 3 to 4 weeks after inoculation of the organism, appear in areas that have been exposed to spa water. Initially papular, these progress to a boil before ulcerating. The fast growth of the organism in culture (<7 days) is consistent with rapidly growing mycobacteria, such as M. fortuitum, Mycobacterium chelonae, or Mycobacterium abscessus.

Mycobacterium gordonae, Mycobacterium kansasii, and Mycobacterium marinum are slowly growing bacteria (that is, growth on solid media requires more than 7 days and usually 2 to 4 weeks). M. gordonae is a ubiquitous organism that rarely causes infection in immunocompetent hosts. Isolation of the organism in patients who are not immunosuppressed usually reflects colonization or contamination.

An environmental pathogen, M. kansasii typically causes a lung infection that mimics tuberculosis. Risk factors include COPD, HIV, alcohol abuse, exogenous immunosuppression, and cancer. Skin infection is uncommon and usually occurs in immunosuppressed patients with disseminated disease.

M. marinum can cause trauma-associated skin infection after exposure to fresh or salt water, including fish tanks and swimming pools. Initially papular before ulcerating, skin lesions are sometimes referred to as “fish tank granuloma.” The rapid growth of the culture from this patient's lesions (<7 days) makes M. marinum unlikely.