FIG. 4.

Different MTT reduction patterns of rifampin-resistant (A) and -sensitive (B) M. tuberculosis isolates obtained after treatment with rifampin for 48 h. Bacilli (5 × 10⁷) were set up in cultures containing rifampin and processed for MTT reduction. Drug-resistant isolates were not inhibited in the ability to reduce MTT, while drug-sensitive isolates showed a drug dose-dependent reduction in the ability to reduce MTT after treatment with rifampin. The sensitivity of the clinical isolates had previously been determined by using the BACTEC system. Strains 35838 and 35836 are rifampin-resistant and rifampin-sensitive reference strains, respectively.