Mutation frequency of Escherichia coli isolated from river water: potential role in the development of antimicrobial resistance

Bacteria acquire genetic variations that help them to adapt to stressful environmental conditions, and these changes may be associated with the development of antimicrobial resistance. In this study, we investigated the mutation frequencies of 270 isolates of Escherichia coli from river water, which represents a relatively unstressful environment. As we predicted, mutation frequencies of the E. coli isolates ranged from <1 x 10(-11) to 6.3 x 10(-8) (median, 1.7 x 10(-9)), and a strong mutator (>= 4 x 10(-7)) was not detected. To better understand the role of mutation frequency in the development of antimicrobial resistance, we assessed antimicrobial sensitivity after exposure of the E. coli isolates to subinhibitory concentrations of ciprofloxacin, as a surrogate for stress. We found that antimicrobial resistance increased in bacteria with a low mutation frequency after exposure, and the relative increase in antimicrobial resistance generally increased, depending on the mutation frequency. Thus, mutation frequency may contribute to the development of antimicrobial resistance of bacteria in natural environments.