Determination of MICs for Mycobacterium avium-M. intracellulare complex in liquid medium by a colorimetric method.

We investigated the potential of a rapid colorimetric microassay based on the reduction of dimethylthiazol-diphenyltetrazolium bromide (MTT) for determining the growth of Mycobacterium avium-M. intracellulare complex (MAC) and MICs of clofazimine, resorcinomycin A, and the quinolone PD 127391 against MAC. The reduction of MTT was directly proportional to the number of viable bacteria. A comparison of the MTT reduction test with the [3H]glycerol uptake assay showed the former to possess higher analytical sensitivity for detecting MAC growth in microtiter plates. The MIT reduction test avoids the use of radioisotopes and costly material and equipment; it is reliable, reproducible, and convenient for rapid routine susceptibility testing of MAC.