The present work addresses the effect of excess levels of ZnCl2 and CuSO4 in the growth medium on the conjugative transfer of plasmids carrying the antibiotic resistance gene blaCMY-2 from extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli. Norwegian poultry are not treated prophylactically with antibiotics, but still, ESBL-producing E. coli are found in the chicken populations. Chickens receive higher amounts of Zn and Cu than their biological need, and several metals have been shown to act as drivers of antimicrobial resistance. In the present study, ESBL-producing E. coli strains collected from retail chicken meat were mated in broth containing various concentrations of ZnCl2 and CuSO4. Manual counting of transconjugants showed that ZnCl2 and CuSO4 reduced the conjugation frequency between E. coli strains in a concentration-dependent manner. Quantitative real-time PCR analyses showed that the presence of ZnCl2 and CuSO4 in the growth media reduced expression of the conjugation genes traB and nikB. By propagating monocultures over several generations, it was found that the blaCMY-2 plasmids remained stable in the recipient strains. Together the results show that exposure of ESBL-producing E. coli to Zn and Cu reduce horizontal transfer of the blaCMY-2 resistance plasmid by reducing expression of genes involved in conjugation in the plasmid donor strain.
Keywords: CMY-2; antimicrobial resistance; conjugation; copper; poultry; zinc.