Putative compensatory mutations in the rpoC gene of rifampin-resistant Mycobacterium tuberculosis are associated with ongoing transmission.

Rifampin resistance in clinical isolates of Mycobacterium tuberculosis arises primarily through the selection of bacterial variants harboring mutations in the 81-bp rifampin resistance-determining region of the rpoB gene. While these mutations were shown to infer a fitness cost in the absence of antibiotic pressure, compensatory mutations in rpoA and rpoC were identified which restore the fitness of rifampin-resistant bacteria carrying mutations in rpoB. To investigate the epidemiological relevance of these compensatory mutations, we analyzed 286 drug-resistant and 54 drug-susceptible clinical M. tuberculosis isolates from the Western Cape, South Africa, a high-incidence setting of multidrug-resistant tuberculosis. Sequencing of a portion of the RpoA-RpoC interaction region of the rpoC gene revealed that 23.5% of all rifampin-resistant isolates tested carried a nonsynonymous mutation in this region. These putative compensatory mutations in rpoC were associated with transmission, as 30.8% of all rifampin-resistant isolates with an IS6110 restriction fragment length polymorphism (RFLP) pattern belonging to a recognized RFLP cluster harbored putative rpoC mutations. Such mutations were present in only 9.4% of rifampin-resistant isolates with unique RFLP patterns (P < 0.01). Moreover, these putative compensatory mutations were associated with specific strain genotypes and the rpoB S531L rifampin resistance mutation. Among isolates harboring this rpoB mutation, 44.1% also harbored rpoC mutations, while only 4.1% of the isolates with other rpoB mutations exhibited mutations in rpoC (P < 0.001). Our study supports a role for rpoC mutations in the transmission of multidrug-resistant tuberculosis and illustrates how epistatic interactions between drug resistance-conferring mutations, compensatory mutations, and different strain genetic backgrounds might influence compensatory evolution in drug-resistant M. tuberculosis.