Molecular Screening Versus Phenotypic Susceptibility Testing of Multidrug-Resistant Mycobacterium tuberculosis Isolates for Streptomycin and Ethambutol.

Proper management of multidrug-resistant tuberculosis (MDR-TB) requires accurate drug susceptibility testing (DST) of Mycobacterium tuberculosis isolates to other (ethambutol [EMB], pyrazinamide, and streptomycin [SM]) first-line drugs. This study compared the performance of Mycobacterium Growth Indicator Tube (MGIT) 960 system for DST of MDR-TB isolates with polymerase chain reaction (PCR) sequencing of embB, rpsL, and rrs genes for detecting resistance to EMB and SM. MDR-TB strains (n = 60) and 25 pansusceptible M. tuberculosis isolates collected during 2011-2016 were tested. Phenotypic DST was performed by MGIT 960 system by using SIRE drug kit. EMB and SM resistance-conferring mutations in embB and rpsL+rrs genes, respectively, were detected by PCR sequencing. No mutations were detected in pansusceptible isolates. Among 60 MDR-TB strains, 35 of 40 SM-resistant and none of 20 SM-susceptible isolates contained rpsL and/or rrs mutations (κ = 0.82, very good agreement). However, all 18 EMB-resistant MDR-TB strains and 33 of 42 EMB-susceptible MDR-TB strains contained an embB mutation (κ = 0.14, poor agreement). Thus, 40 of 60 (67%) and 35 of 60 (58%) isolates were resistant to SM (p = 0.451), while 18 of 60 (30%) and 51 of 60 (85%) isolates were resistant to EMB (p = 0.000) by MGIT 960 system and PCR sequencing, respectively. MGIT 960 system showed acceptable performance for DST for SM; however, it performed poorly for EMB as many MDR-TB strains with embB mutations, which confer low-level resistance to EMB, were detected as EMB susceptible. Molecular screening for resistance-conferring mutations in embB gene is thus superior to MGIT 960 system when accurate EMB susceptibility results are needed for proper management of MDR-TB patients.