Drug Susceptibility Testing of Recurrent TB

All MTB strains isolated from recurrent TB cases were previously stored in 7H9 medium containing 25% glycerin at–80°C refrigerator, and then were thawed and re-cultured on L-J medium for further study. MTB isolates in the logarithmic phase were subjected to drug susceptibility testing against rifampicin, isoniazid, ethambutol, streptomycin, ofloxacin, moxifloxacin, kanamycin, and amikacin using MYCOTB plate (Thermo Fisher Scientific, United States). Previous studies have demonstrated the good accuracy and reproducibility of the MYCOTB plate, which can be used as an alternative method for DST (Xia et al., 2017 (link); Wu et al., 2019 (link)). All procedures were performed by trained staff at the national TB reference laboratory of China, as described elsewhere (He et al., 2022 (link)). H37Rv (ATCC 27294) was used as pan-susceptible control in each batch of DST. The concentration ranges and cut-off values for determining resistance or sensitivity for each drug used in this study were depicted previously (He et al., 2022 (link)). All DSTs were conducted twice to ensure the accuracy of DST results.