Drug sensitivity tests were performed for ethambutol, rifampicin, streptomycin, and isoniazid both alone and in paired combinations, on 16 strains of Mycobacterium avium intracellulare, seven strains of Mycobacterium xenopi, and eight strains of Mycobacterium malmoense. Most strains were resistant to the individual drugs, but all strains of M malmoense, 86% of M xenopi, and 31% of M avium intracellulare were completely suppressed by the lowest concentrations of ethambutol and rifampicin when the two drugs were combined in vitro. Streptomycin combined with ethambutol or with rifampicin in the lowest combined concentrations suppressed 50% and 62% respectively of strains of M malmoense. All strains of M xenopi were suppressed by the lowest combined concentrations of streptomycin with rifampicin. Combinations with isoniazid were less effective. It is postulated that similar effects in vivo might account for the satisfactory clinical response seen in patients with disease caused by these mycobacteria who have received treatment with combinations of standard antituberculosis drugs despite in vitro resistance to the individual agents.
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