The evolution of no-cost resistance at sub-MIC concentrations of streptomycin in Streptomyces coelicolor

At the high concentrations used in medicine, antibiotics exert strong selection on bacterial populations for the evolution of resistance. However, these lethal concentrations may not be representative of the concentrations bacteria face in soil, a recognition that has lead to questions of the role of antibiotics in soil environments as well as the dynamics of resistance evolution during sub-lethal challenge. Here we examine the evolution of resistance to sub-MIC concentrations of streptomycin in the filamentous soil bacterium Streptomyces coelicolor. First, we show that spontaneous resistance to streptomycin causes an average fitness deficit of ~21% in the absence of drugs; however, these costs are eliminated at concentrations as low as 1/10 the MIC of susceptible strains. Using experimental evolution, we next show that resistance readily evolves at these non-lethal doses. More important, S. coelicolor resistance that evolves at sub-MIC streptomycin is cost-free. Whole-genome analyses reveal that sub-MIC evolved clones fix a distinct set of mutations to those isolated at high drug concentrations. Our results broaden the conditions under which resistance can evolve in nature and suggest that the long-term persistence of these strains is facilitated by the absence of pleiotropic fitness costs. Finally, our data cast doubt on arguments that low-concentration antibiotics in nature are signals, instead supporting models that resistance evolves in response to antibiotics used as weapons.