Abstract
OBJECTIVE: Increasing reports of resistance to newer antituberculosis drugs have prompted the search for other alternative drugs. Streptomycin (STR) could be used for the treatment of drug-resistant tuberculosis if susceptibility of Mycobacterium tuberculosis isolate to STR could be accurately detected. We performed phenotypic and genotypic drug susceptibility testing (DST) of 118 M. tuberculosis isolates for STR. MATERIALS AND METHODS: Fifty pansusceptible and 68 multidrug-resistant M. tuberculosis (MDR-TB) isolates were used. Phenotypic DST for STR, rifampicin, isoniazid, and ethambutol was performed by mycobacteria growth indicator tube 960 System. Genotypic DST was done by GenoTypeMTBDRplus assay for rifampicin and isoniazid and by PCR-sequencing of rpsL, rrs, and gidB genes for STR. MDR-TB isolates were genotyped by spoligotyping. RESULTS: Phenotypic DST identified 50 isolates susceptible to all four drugs (pansusceptible). Sixty-one of 68 MDR-TB isolates were resistant to STR. Genotypic testing for rifampicin and isoniazid yielded expected results. Fifty pansusceptible and 7 STR-susceptible MDR-TB isolates contained no mutation in rpsL or rrs, while 47, 2, and 1 STR-resistant isolate contained rpsL, rrs, and rpsL + rrs mutations, respectively. Of the remaining 11 STR-resistant MDR-TB, 9 isolates contained deletion frame-shift/nonsynonymous mutations in gidB. Surprisingly, 13 pansusceptible isolates also contained deletion frame-shift/nonsense/nonsynonymous mutations in gidB. Also, 30 of 68 MDR-TB but only 2 of 50 pansusceptible isolates belonged to the Beijing genotype. CONCLUSIONS: Our data show that, like rifampicin, ethambutol, and pyrazinamide, STR also exhibits discordant phenotypic and genotypic DST results for some M. tuberculosis isolates. Hence, STR should be included in therapy regimens only if both phenotypic and genotypic resistance testing indicate susceptibility to avoid amplification of resistance and drug toxicity.