Objectives: Mutations in the β-subunit of RNA polymerase (RNAP), encoded by rpoB, are responsible for rifampicin resistance (Rif(R)). Although many mutations in rpoB can reduce susceptibility, only a few are frequent amongst Rif(R) clinical Mycobacterium tuberculosis (MTB) isolates. It has been suggested that there is a negative correlation between the fitness costs of Rif(R) mutations and their respective clinical frequency, but so far comparable fitness cost measurements have only been conducted for a very limited number of Rif(R) mutations. We tested this hypothesis using Salmonella and Mycobacterium smegmatis as model organisms.
Methods: We constructed 122 different Rif(R) mutations in Salmonella. MICs and relative fitness costs in the presence and absence of rifampicin were determined for each mutant, including for a smaller number of Rif(R) M. smegmatis strains. Results were compared with available mutation frequency data from clinical MTB isolates.
Results: (i) Rif(R) mutations frequently found in MTB isolates have a fitness cost in Salmonella Typhimurium and M. smegmatis. (ii) Clinically frequent Rif(R) mutations have a high rifampicin MIC. (iii) There is a strong correlation between the magnitude of the fitness cost of a Rif(R) mutation in Salmonella Typhimurium or M. smegmatis and the frequency with which that mutation is associated with secondary (putative compensatory) mutations in RNAP of clinical MTB isolates.
Conclusions: This suggests that the success of Rif(R) mutations in clinical MTB isolates may be dependent not only on a low initial fitness cost, but rather the results of three factors: (i) a high rifampicin MIC; (ii) a relatively low initial fitness cost; and (iii) the ability to additionally acquire compensatory mutations selected to further reduce fitness cost.
Keywords: M. tuberculosis; RifR; resistance genetics.
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