In vitro activity of rifabutin and rifampin against clinical isolates of Mycobacterium tuberculosis in Taiwan

Background and purpose: To determine the in vitro activity of rifabutin against Mycobacterium tuberculosis (MTB) and the cross-resistance rate between rifampin and rifabutin. Methods:A total of 56 clinical isolates of MTB, including 23 multidrug-resistant (MDR) isolates and 33 susceptible isolates, were tested for susceptibility to rifampin and rifabutin using the absolute concentration method. The concentrations of drugs tested were 2.5 and 5 mg/mL for rifampin and 0.1, 0.5, 1, 2,5, 5, and 10 mg/mL for rifabutin. Results: All 33 MTB isolates that were susceptible to rifampin were also susceptible to rifabutin. None of the 23 MDR-MTB isolates were inhibited by rifabutin at a concentration of 0.1 mg/mL. Among these 23 MDR isolates, three were susceptible to rifabutin at concentrations ≥ 0.5 mg/mL, six were susceptible to rifabutin at concentrations ≥ 5 mg/mL, 18 were susceptible to rifabutin at concentrations ≥ 10 mg/mL and five were not inhibited at any of the concentrations tested. The cross-resistance rate between rifampin and rifabutin was 87%. Conclusions: Our results indicate that the in vitro activity of rifabutin against drug- susceptible MTB isolates is greater than that of rifampin. For MDR-MTB isolates, the cross-resistance is high between rifampin and rifabutin.