Pretomanid MIC distribution and resistance profile in Mycobacterium tuberculosis using middlebrook 7H9 broth

Objective To assess the prevalence and genetic basis of pretomanid (Pa) resistance, further to generate additional MIC data to support ECOFF/breakpoint setting in the microdilution system. Methods Mycobacterium tuberculosis (MTB) isolates were collected from national drug resistance surveillance regions, with no prior pretomanid exposure. All strains were sub-cultured on Löwenstein-Jensen medium and tested against 12 anti-tuberculosis drugs with the Sensititre ® MYCOTB plate. Pretomanid was serially diluted in 7H9 broth supplemented with 10% OADC, with a concentration range spanning from 0.015 mg/L to 16 mg/L. Results A total of 335 MTB isolates were analyzed, with lineage 2 (77.9%, 261/335) predominantly. The 99th percentile MIC for pretomanid was equal to 0.25 mg/L. 15 (4.5%) isolates were classified as probably resistant, with MICs ranging from 0.5 to ≥ 16 mg/L. Only weak correlations were observed between Paand the other drugs ( r < 0.3). Genome analysis revealed that among the 15 probably resistant isolates, only three carried mutations ( fbiA , fbiC , fgd1 ), while none had mutations in fbiB , ddn or fbiD . Conclusions We determined the 99th percentile MIC of pretomanid in 7H9 broth to be 0.25 mg/L for L2 and L4 isolates in China . Moreover, our findings reveal a high baseline resistance rate (4.5%) and suggest the existence of yet-uncharacterized genetic mechanisms underlying pretomanid resistance.