Second-line anti-tuberculosis drug resistance and its genetic determinants in multidrug-resistant Mycobacterium tuberculosis clinical isolates.

BACKGROUND: Mutations in several genetic loci have been implicated in the development of resistance to second-line anti-tuberculosis (TB) drugs (SLDs). The purpose of this study was to investigate the prevalence of resistance to SLDs and its association with specific mutations in multidrug-resistant (MDR) Mycobacterium tuberculosis clinical isolates.

MATERIALS AND METHODS: The study included 46 MDR-TB isolates. Mutation profiling was performed by amplifying and sequencing the following six genes: gyrA/gyrB, rrs, tlyA, and ethA/ethR, in which mutations are implicated in resistance of tubercle bacilli to ofloxacin (OFX), amikacin (AMK), capreomycin, and ethionamide (ETH), respectively.

RESULTS: Of the strains analyzed, 14 (30.4%) showed resistance to at least one of the four SLDs tested. Mutations in the gyrA gene occurred in 34 (73.9%) strains, with the most common amino acid change being Ser95Thr. The Asp94Asn and Ala90Val substitutions in the gyrA were present exclusively in OFX-resistant strains, yet represented only 40% of all OFX-resistant strains. The only mutation in the gyrB gene was substitution Ser447Phe, detected in one OFX-resistant isolate. None of the AMK-resistant strains carried a mutation in the rrs gene. Mutations in the ethA/ethR loci were found in one ETH-resistant and 11 ETH-susceptible strains.

CONCLUSIONS: The results of this study challenge the usefulness of sequence analyses of tested genes (except gyrA) for the prediction of SLD resistance patterns and highlight the need for searching other genetic loci for detection of mutations conferring resistance to SLDs in M. tuberculosis.