Characterization of Mutations Conferring Resistance to Rifampin in Mycobacterium tuberculosis Clinical Strains

Abstract

Resistance of <i>Mycobacterium tuberculosis</i> to rifampin (RMP), mediated by mutations in the <i>rpoB</i> gene coding for the beta-subunit of RNA polymerase, poses a serious threat to the efficacy of clinical management and, thus, control programs for tuberculosis (TB). The contribution of many individual <i>rpoB</i> mutations to the development and level of RMP resistance remains elusive. In this study, the incidence of mutations throughout the <i>rpoB</i> gene among 115 <i>Mycobacterium tuberculosis</i> clinical isolates, both resistant and susceptible to RMP, was determined. Of the newly discovered <i>rpoB</i> mutations, the role of three substitutions in the causation of RMP resistance was empirically tested. The results from <i>in vitro</i> mutagenesis experiments were combined with the assessment of the prevalence of <i>rpoB</i> mutations, and their reciprocal co-occurrences, across global <i>M. tuberculosis</i> populations. Twenty-two different types of mutations in the <i>rpoB</i> gene were identified and distributed among 58 (89.2%) RMP-resistant strains. The MICs of RMP were within the range of 40 to 800 mg/liter, with MIC₅₀ and MIC₉₀ values of 400 and 800 mg/liter, respectively. None of the mutations (Gln429His, Met434Ile, and Arg827Cys) inspected for their role in the development of RMP resistance produced an RMP-resistant phenotype in isogenic <i>M. tuberculosis</i> H37Rv strain-derived mutants. These mutations are supposed to compensate for fitness impairment incurred by other mutations directly associated with drug resistance.