Rifabutine: minimal inhibitory and bactericidal concentrations for Mycobacterium tuberculosis

The minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) of rifabutine (ansamycin LM427, spiropiperidyl rifamycin) against rifampin-susceptible and rifampin-resistant strains of Mycobacterium tuberculosis were determined. The MICs for 17 rifampin-susceptible strains were low, falling in a narrow range of 0.06 microgram/ml or less when determined by either the agar-dilution method (in 7H10 or 7H11 agar) or in 7H12 broth (radiometrically or by sampling and colony-forming units [CFU/ml] determination). The MICs of rifabutine for 21 rifampin-resistant strains were much higher, ranging from 0.25 to 16.0 micrograms/ml. The MBCs of rifabutine were 0.125 or 0.25 microgram/ml for rifampin-susceptible strains, and were 4.0 and 32.0 micrograms/ml for two tested rifampin-resistant strains. The MIC/MBC ratio was 1:4 for rifampin-susceptible strains.