Phenotypic and genotypic characterization of drug-resistant Mycobacterium tuberculosis strains

Of 142 pulmonary tuberculosis patients, 76 were considered high risk for the development of resistance, and 24 were confirmed as resistant strain carriers. Resistant isoniazid strains presented a high frequency of katG and ahpC mutations (90%) correlated with an MIC >4 microg/mL (94%). inhA mutations were not seen. rpoB mutations were identified in 78.6% of rifampicin-resistant strains, usually in codon 531 (72.7%), and 75% had an MIC >16 microg/mL. katG and rpoB mutations recognized 88.2% of multidrug-resistant strains and proved more efficient than the katG and rpoB mutations alone. Seventy percent of resistant pyrazinamide strains had pncA mutations between genes 136 and 188, 62.5% of them with an MIC >900 microg/mL. Pyrazinamidase inactivity was not an efficient resistance marker because 60% of pncA-mutated strains maintained enzymatic activity despite displaying good correlation with high resistance levels. Resistant ethambutol strains had embB mutations in codon 306, with MIC >16 microg/mL.