FIG 4.

20-day serial passage mutant selection. For HUMC1, the starting MIC for both drugs was 3.13 mg/L in MHII. In RPMI, however, the starting MICs for rifampin and rifabutin were 3.13 mg/L and <0.05 mg/L, respectively. For ABNIH1, the starting MIC for both drugs was 3.13 mg/L in both MHII and RPMI. Despite HUMC1 initially being more susceptible to rifabutin in RPMI, it was still possible to select high-level resistance mutants. Similarly, no difference in rifabutin resistance emergence was observed between the hypersusceptible strain HUMC1 and the susceptible strain ABNIH1 after serial passage.