Evolutionary trajectories and transmission dynamics of multidrug-resistant <i>Mycobacterium tuberculosis</i> in Tibet, China

ABSTRACT Objective Tibet has the highest prevalence of both tuberculosis disease and multidrug-resistant tuberculosis (MDR-TB) in China. The circulated Mycobacterium tuberculosis strains from Tibet were sequenced to investigate the underlying drivers for the high burden of MDR-TB. Methods Using whole-genome sequencing data of 576 M. tuberculosis strains isolated from consecutive patients in Tibet, we mapped resistance-conferring mutations onto phylogenetic trees to determine their evolution and spread. The impact of drug resistance on bacterial population growth was assessed with a Bayesian (Skyline Plot) analysis. Multivariable logistic regression was used to identify risk factors for the development of rifampicin resistance. Results Of the 576 isolates, 284 (49.3%), 280 (48.6%), and 236 (41.0%) were, respectively, genetically resistant to isoniazid, rifampicin, or both (MDR-TB). Among the isoniazid- and rifampicin-resistant strains, the proportions in phylogenetically-inferred clusters were 77.8% (221/284) and 62.1% (174/280), respectively. Nearly half (47.2%, 134/284) of the isoniazid-resistant strains were in six major clades, which contained between 8 and 58 strains with katG S315T, katG S315N, or fabG1 promoter −15 C&gt;T resistance mutations. These major clades exponentially expanded after emerging with isoniazid resistance and stabilized before evolving into MDR-TB twenty years later. Isoniazid-resistant isolates showed an increased risk of accumulating rifampicin resistance compared to isoniazid-susceptible strains, with an adjusted odds ratio of 3.81 (95% confidence interval 2.47-5.95). Conclusion Historical expansion of isoniazid-resistant strains and their increased likelihood of acquiring rifampicin resistance both contributed to the high burden of MDR-TB in Tibet, highlighting the need to detect INH-resistant strains promptly and to control their transmission.