Determination of in vitro susceptibility of mycobacteria to ansamycin.

The in vitro susceptibility of different mycobacterial species to ansamycin (LM427) in concentrations of 2.0, 1.0, 0.5, and 0.2 micrograms/ml was determined by the agar dilution method. For those strains of M. tuberculosis and M. avium complex tested, susceptibility to ansamycin was compared with susceptibility to rifampin. All M. tuberculosis strains susceptible to rifampin were susceptible to ansamycin; the strains that were highly resistant to rifampin also were resistant to ansamycin. The majority of M. avium complex strains were "naturally" resistant to rifampin (1.0 micrograms/ml and higher), but only approximately 13% of them were resistant to ansamycin in a concentration of 1.0 microgram/ml. The cross-resistance in M. avium strains was made apparent only by comparing patterns of resistance to low concentrations of ansamycin (0.5 microgram/ml) with patterns of resistance to higher concentrations of rifampin (5.0 and 10.0 micrograms/ml).