FIG. 5.

Detection of a rifampin-resistant subpopulation. Cultures containing various proportions of rifampin-resistant (H₃₇Rv [ATCC 35838]) and rifampin-sensitive (H₃₇Ra [ATCC 35836]) bacteria were set up in culture, treated with rifampin at 1 μg/ml, and processed for the MTT reduction assay. Bacterial populations containing a drug-resistant subpopulation of more than 1% could be detected in that their ability to reduce MTT was not inhibited by rifampin at 1 μg/ml. Further, the ability to continue to reduce MTT in the presence of rifampin increased with the proportion of the drug-resistant subpopulation.