Prevalence of gyrA and B gene mutations in fluoroquinolone-resistant and -sensitive clinical isolates of Mycobacterium tuberculosis and their relationship with MIC of ofloxacin

The study was done to know the prevalent mutations of gyrA and gyrB genes, and their significance with drug resistance in clinical isolates of Mycobacterium tuberculosis. A total of 100 ofloxacin- (OFX) resistant and 100 OFX-sensitive isolates of M. tuberculosis were consecutively selected from routine Tuberculosis laboratory. Drug resistance pattern of these isolates was recorded. MIC of OFX was tested in all these isolates by absolute concentration method. Quinolone resistance determining region (QRDR) of gyrA and gyrB genes of 320 and 428 bp, respectively, were amplified and sequenced. Sequencing data were analyzed by BLAST on NCBI with reference strain H37Rv. Of 100 OFX-sensitive isolates, 30 were pansusceptible, 28 were monoresistant, 10 were polyresistant and 32 were multidrug resistant (MDR). Among 100 OFX-resistant isolates, 19 were OFX monoresistant, 16 were polyresistant and 65 were MDR. Mutations in gyrA and gyrB genes were observed in 79% and 5% of OFX-resistant isolates, respectively. Most prevalent mutation was found at codon 94 in QRDR of gyrA gene. Double mutations found in gyrA gene and in both gyrA and gyrB genes signifies higher levels of OFX resistance. In one isolate, a substitution at codon 592 (Pro592Ser) was found as a novel mutation outside the QRDR of gyrB gene. Our findings support previous studies that the OFX resistance to M. tuberculosis is associated with mutations in the QRDR of gyrA gene; however, the level of OFX resistance may not be predicted based on the mutation patterns in the gyrA gene.