Reduction of the fitness cost of antibiotic resistance caused by chromosomal mutations under poor nutrient conditions.

The prevalence of antibiotic resistance in drinking water system is pressing public health risk. Antibiotic resistance conferred by chromosomal mutations often produces fitness cost, which may affect its spread and persistence. In this study, the rifampin-resistant strains were competed with their wild-type counterparts at different nutrient levels. It was observed that the ratio of the absolute number between resistant and wild-type cells quickly decreased under rich nutrient conditions, but it slowly reduced or remained stable in the poor nutrient medium. This finding suggested that poor nutrient conditions resulted in the reduction of fitness cost of antibiotic resistance, i.e. the resistant bacteria became more competitive. Implying mechanisms analysis found that the differences of metabolic activity between wild-type and rifampin-resistant strains was significant smaller (P < 0.05) at low nutrient levels. Additionally, distinguishable large colony size rifampin-resistant strains were observed during competition assay. DNA sequencing of RNA polymerase subunit genes further revealed that these colonies could be adaptive mutants from wild-type strain in rpoB gene. To our knowledge, this is the first study to reveal that the oligotrophic conditions facilitate the persistence of antibiotic resistance in drinking water by reducing the fitness cost of the resistant strains.