Evolutionary trajectories of resistant mutants during sub-MIC antibiotic exposure

The emergence of antibiotic resistance is one of the most important examples of contemporary evolution. Selection for resistance can occur over a wide concentration range, both above and below the minimum inhibitory concentration (MIC) of the antibiotic. In a majority of cases, resistance mutations confer fitness costs and several studies have shown the importance of these costs for the emergence, ascendance and maintenance of resistance in a population. Importantly, these costs can often be ameliorated by compensatory mutations and rate and efficiency of compensation is a key parameter in determining the evolutionary success of a costly resistance mutation. Despite this knowledge, we still have a limited understanding of how resistance evolution (to increase resistance) and compensatory evolution (to reduce fitness costs) interact during growth in presence of low, sub-MIC, antibiotic concentrations. To examine the impact of these two processes, we carried out evolution experiments at sub-MIC levels of streptomycin using two E. coli mutants (with loss of function mutations in the selB and ubiH genes, respectively) that show low-level streptomycin resistance, and have different fitness costs. For both mutants, evolution at sub-MIC levels enriched for mutations that increased resistance, but selection for compensatory mutations was also common over the course of the experiment. Our study highlights that costly low-level resistant mutants adapt to sub-MIC antibiotic exposure by either increasing resistance, reducing cost or both and that this evolution can result in the maintenance of these mutants in the population.