In Vitro Activity of Trimethoprim and Sulfamethoxazole Against the Nontuberculous Mycobacteria

Trimethoprim (TMP) and sulfamethoxazole (SMZ) were studied alone and in combination (TMP-SMZ) against 141 nontuberculous mycobacteria; an agar dilution method was used. All strains of Mycobacterium kansasii (8), Mycobacterium marinum (16), and Mycobacterium scrofulaceum (3); 97% (63 of 65) of strains of Mycobacterium fortuitum; and 27% (3 of 11) of strains of Mycobacterium avium-intracellulare were inhibited by less than or equal to 32 micrograms of SMZ/ml. In contrast, all 38 isolates of Mycobacterium chelonei were resistant to 32 micrograms of SMZ/ml. All species were highly resistant to TMP, with minimal inhibitory concentrations (MICs) of greater than or equal to 64 micrograms/ml. The MICs of TMP-SMZ for these organisms were similar to the MICs of SMZ alone. These results support the use of sulfonamides for treatment of infections due to M. fortuitum and M. marinum and suggest the need for further clinical and laboratory studies of the activity of these drugs against several additional mycobacterial species. TMP-SMZ appears to offer no advantage (in vitro) over SMZ alone against any of these organisms.