Impact of gyrB and eis Mutations in Improving Detection of Second-Line-Drug Resistance among Mycobacterium tuberculosis Isolates from Georgia.

The country of Georgia has a high burden of multi- and extensively drug-resistant tuberculosis (XDR-TB). To evaluate whether mutations in gyrB and eis genes increased the sensitivity of detection of phenotypic resistance to ofloxacin and kanamycin or capreomycin compared to use of the first-generation MTBDRsl assay alone, which tests for mutations in gyrA and rrs genes, a retrospective study of stored Mycobacterium tuberculosis isolates was performed. All isolates underwent DNA sequencing of resistance-determining regions. Among 112 M. tuberculosis isolates with DNA extraction data, targeted sequencing was successfully performed for each gene as follows: for gyrA, 98% sensitivity; for gyrB, 96%; for rrs, 93%; for the eis gene and its promoter, 93%. The specificity and hence the positive predictive value of gyrA and gyrB mutations for detecting ofloxacin resistance were 100%. The addition of gyrB mutations increased the sensitivity of phenotypic ofloxacin resistance detection by 13% (75% to 88%). All rrs resistance-conferring mutations were A1401G, and this mutation had low sensitivity (40% and 18%) and high specificity (95% and 100%) in predicting phenotypic capreomycin and kanamycin resistance, respectively. The eis C-14T mutation increased the sensitivity of phenotypic kanamycin resistance detection by 9% (18% to 27%) and was found solely in kanamycin phenotypic resistance isolates. Our data showed that the inclusion of eis C-14T and gyrB mutations in addition to rrs and gyrA mutations improves the sensitivity of detection of phenotypic ofloxacin and kanamycin resistance, respectively.