In vitro susceptibility of atypical mycobacteria to rifampin

Atypical mycobacteria (209 strains) were examined for susceptibility to rifampin by the proportion method by using Middlebrook 7H-10 agar. All strains of Mycobacterium kansasii and tap-water scotochromogens were inhibited by 0.25 to 1 mug of the drug per ml. Seventy-six per cent of M. scrofulaceum and 61% of M. intracellulare strains were susceptible to 4 mug/ml or less; 5% of the former and 8% of the latter were resistant to 16 mug/ml. All strains of M. gastri and M. triviale and most strains of M. terrae were sensitive to 1 to 4 mug/ml. Two strains of M. borstelense were both inhibited by 8 mug/ml. Nearly all strains of M. fortuitum were resistant to the drug. The results of this study suggest that rifampin may be a valuable agent for the treatment of many atypical mycobacterial infections.