USE OF 3-(4,5-DIMETHYLTHIAZOL-2-YL)-2,5-DIPHENYL TETRAZOLIUM BROMIDE FOR RAPID DETECTION OF RIFAMPIN-RESISTANT MYCOBACTERIUM-TUBERCULOSIS

We describe a test which uses the ability of viable cells to reduce 3- (4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) to d etect resistance to a bactericidal drug, rifampin, in in vitro-culture d Mycobacterium tuberculosis. The assay shows a linear relationship be tween the number of viable bacteria and the ability to reduce MTT. Dea d mycobacteria were unable to reduce MTT. Rifampin-sensitive M. bovis (BCG) and M. tuberculosis exposed to rifampin showed a rifampin concen tration-dependent inhibition of the ability to reduce MTT, while the r esistant strains were unaffected. The inhibition of MTT reduction afte r treatment with rifampin paralleled the reduction in the number of CF U. By using mixing experiments in which the population percentages of rifampin-sensitive and -resistant strains were varied, the assay could detect the presence of rifampin resistance in the mixture when at lea st 1% of the bacterial population was composed of drug-resistant strai ns. The assay is cheap, can be usually read, and requires less than 3 days to obtain susceptibility results. The total time required to obta in results, from the time sputum is received in the laboratory, is, in most cases, less than 4 to 5 weeks, which is the time required for pr imary culture of the bacteria. The MTT assay could, in combination wit h a test to detect resistance to isoniazid, be a cheap and rapid scree ning method for multidrug-resistant M. tuberculosis that is affordable even by low-income countries where tuberculosis is a major public hea lth problem.