The Contribution of Active Efflux in Reduced Susceptibilities to Copper Sulfate and Zinc Chloride in Escherichia coli Isolates from Swine

Abstract

A hundred-eighty Escherichia coli isolates from swine were tested for minimal inhibitory concentrations (MICs) of 9 antimicrobials and two heavy metals copper sulfate and zinc chloride. All the E. coli isolates were resistant to at least one antibiotic and 175 (97.2%) isolates were multi-drug resistant. Forty antibiotic resistance patterns were identified, of which the most common pattern was AMP-CHP-CIP-ERY-GEN-STR-SUL-TET-TRI (13.3%). All the isolates formed one large population of susceptibility to copper sulfate and zinc chloride at the concentrations of 6.4-12.8 mM and 3.2-6.4 mM, respectively. Exposure to copper sulfate and zinc chloride at sublethal concentration did not change the susceptibility. Addition of carbonyl cyanide m-chlorophenylhydrazone (CCCP) and reserpine caused at least 4-fold reduction of MIC value of copper sulfate and zinc chloride in 66 (36.7%) isolates and
142 (78.9%) isolates, respectively. The results highlighted the contribution of active efflux systems driven by proton motive force and/or ATP in reduced susceptibilities to copper sulfate and zinc chloride in E. coli from swine.