Contributions to the diagnosis of pirazinamide resistance in Mycobacterium tuberculosis

the direct detection of pyrazinamide resistance in mycobacterium tuberculosis is not routinely performed in many laboratories in the world because the drug is active only at acid ph, which also affects the growth of m. tuberculosis. the pyrazinamidase enzyme, encoded by pnca gene, is necessary to convert prodrug pyrazinamide to its active form. taking into account that nicotinamide, a structural analogue of pyrazinamide, converted in its active forms by the pyrazinamidase enzyme at a physiological ph does not affect bacterial growth, the aim of this research was to evaluate two colorimetric methods: nitrate reductase and malachite green microtube assays, using nicotinamide to perform susceptibility testing in 102 m. tuberculosis strains. the results were compared with those obtained by the classic wayne assay. mutations in the pnca gene were identified by sequencing the pnca gene from all isolates in which pyrazinamide resistance was detected by any of the three methods. both the nitrate reductase and malachite green microtube assays showed sensitivities of 93.75% and specificities of 97.67%. mutations in the pnca gene were found in 14 of 16 strains (87.5%) that were pyrazinamide resistant.