Comparison of two rapid colorimetric methods for determining resistance ofMycobacterium tuberculosis to rifampin, isoniazid, and streptomycin in liquid medium

The usefulness of two colorimetric methods for the determination of the sus ceptibility or resistance of Mycobacterium tuberculosis to rifampin, strept omycin, and isoniazid in liquid medium based on the reduction of 2,3-bis(2- methoxy-4-nitro-5-sulfo-phenyl)-2H-tetrazolium-5-carboxanilide (XTT) and 3- (4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was invest igated. The agar proportion method was used as the reference method. Result s obtained indicate that the sensitivity of the XTT reduction assay for the detection of rifampin resistance was comparable to that observed, and prev iously described, for the MTT assay. However, the reduction of XTT yields a water-soluble formazan that can be easily quantified without performing ad ditional steps such as addition of lysing buffer and solubilization. Furthe rmore, the colorimetric assays, based on the reduction of XTT and MTT for t he detection of isoniazid and streptomycin resistance in Mycobacterium tube rculosis, were standardized. The inhibition of MTT and XTT reduction after treatment with rifampin, streptomycin, or isoniazid was directly proportion al to the reduction in the number of viable bacteria, and a strain of Mycob acterium tuberculosis could be reported as susceptible or resistant to rifa mpin, streptomycin, or isoniazid after 3, 6, or 8 days, respectively. The X TT and MTT reduction assays are rapid, reliable, and affordable and do not require the use of radioisotopes. Moreover, they can be performed with comm on laboratory equipment.