Characterization of mutations conferring resistance to rifampicin in Mycobacterium tuberculosis clinical strains [PublishAheadOfPrint]

Resistance of Mycobacterium tuberculosis to rifampicin, mediated by mutations in the rpoB gene, coding for the beta-subunit of RNA polymerase, poses a serious threat to the efficacy of clinical management and thus control programs of TB. The contribution of many individual rpoB mutations to the development and level of RMP resistance remains elusive. In this study, the incidence of mutations throughout the rpoB gene among 115 Mycobacterium tuberculosis clinical isolates, both resistant and susceptible to RMP was determined. Of the newly-discovered rpoB mutations, the role of three substitutions in the causation of RMP resistance was empirically tested. The results from in vitro mutagenesis experiments were combined with the assessment of the prevalence of rpoB mutations, and their reciprocal co-occurrences, across global M. tuberculosis populations.

Twenty-two different types of mutations in the rpoB gene were identified and distributed among 59 (90.8%) RMP-resistant strains. The minimum inhibitory concentrations (MICs) of RMP were within the range of 40-800 mg/L, with MIC50 and MIC90 values of 400 and 800 mg/L, respectively. None of the mutations (Gln429His, Met434Ile, Arg827Cys) inspected for their role in the development of RMP resistance produced a RMP-resistant phenotype in isogenic M. tuberculosis H37Rb strain-derived mutants. These mutations are supposed to compensate for fitness impairment incurred by other mutations directly associated with drug resistance.

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