Survival, resistance, and fitness dynamics of Escherichia coli populations after prolonged exposure to copper.

Abstract

Background and objectives: Copper is an essential micronutrient and a widely used antimicrobial, yet its widespread application may accelerate microbial resistance. We investigated how long-term copper (II) sulfate (CuSO₄) exposure drives genetic and phenotypic changes in Escherichia coli, focusing on survival, resistance mechanisms, and antibiotic cross-resistance.

Methodology: Fifty E. coli populations were evolved for 55 days under progressively increasing CuSO₄ concentrations. Whole-genome sequencing (WGS) identified genetic adaptations, while phenotypic changes were assessed using minimum inhibitory concentration (MIC) and fitness assays across CuSO₄ and antibiotic gradients.

Results: CuSO₄ imposed strong selective pressure, with only 16% of populations surviving prolonged exposure. Survivors exhibited up to eight-fold increases in CuSO₄ resistance, though some reverted to ancestral resistance levels when selective pressure was removed. Fitness assays showed that CuSO₄-selected populations maintained significantly higher fitness in high CuSO₄ environments than controls and ancestors (P < .001). WGS revealed diverse mutations in stress-response and metal-tolerance genes (cusA, acrB, corA, fur, and ybhA) without a single resistance signature. Although antibiotic cross-resistance was not observed, some CuSO₄-selected populations displayed elevated MICs for levofloxacin, colistin, trimethoprim, fosfomycin, and meropenem. Similar trends in controls suggest that additional factors, such as adaptation to laboratory media, also contribute to resistance.

Conclusions and implications: CuSO₄ exerts strong and variable selective pressure on E. coli populations, promoting diverse resistance pathways through distinct genetic and physiological mechanisms. While some CuSO₄-selected strains exhibited increased antibiotic resistance, trends in controls highlight the complexity of resistance evolution. These findings emphasize the need to monitor copper-driven antimicrobial resistance.