Infection of bone by Mycobacterium fortuitum masquerading as Nocardia asteroides

A case of traumatic osteomyelitis of the leg yielded on culture a branching partially acid-fast organism that failed to respond to therapy directed at Nocardia asteroides. Subsequent laboratory investigation revealed the organism to be Mycobacterium fortuitum. N. asteroides and M. fortuitum can demonstrate similar staining and morphologic patterns microscopically, as well as common colonial and cultural characteristics. Separation can be aided by careful examination of the branching pattern, and can be established by thin-layer chromatography of lipid extracts of the organism. Correct identification of these species in the laboratory is important because of some overlap in their clinical syndromes and because of differences in their susceptibilities to antibiotics.