Within-host evolution of drug tolerance in Mycobacterium tuberculosis

Background and objectives: (Mtb) causes tuberculosis (TB) in humans. Poor treatment responses are a threat to global TB control, as such, understanding contributing factors to poor responses is important. We proposed that antibiotic tolerance could contribute to delayed culture conversion (recalcitrant TB), and resistance amplification in patients during TB treatment. We thus ventured to investigate the role of drug tolerance in delayed culture conversion and resistance amplification in TB patients. Methods: We collected serial Mtb isolates from patients with (i) drug-susceptible TB who remained culture positive for up to 6 years (i.e. recalcitrant TB), and (ii) multidrug-resistant TB (MDR-TB) where resistance amplified during treatment. We measured tolerance to rifampicin in drug-susceptible TB strains and tolerance to moxifloxacin in MDR-TB strains using a real-time time-kill assay. Results and discussion: Rifampicin tolerance evolved within-host, increasing up to and ∼1.5-fold, however, there was no apparent contribution of rifampicin tolerance to delayed culture conversion. Tolerance to moxifloxacin in MDR-TB patients appeared negatively associated with resistance amplification and consistently decreased over time in patients. Conclusion: Our findings confirm that antibiotic tolerance evolves in Mtb within patients over time during treatment. However, there was no evidence that this tolerance influences treatment responses, calling for further investigation of contributors to adverse treatment responses and their mitigation.