Zinc and Copper Reduce Conjugative Transfer of Resistance Plasmids from Extended-Spectrum Beta-Lactamase-Producing <i>Escherichia coli</i>
May Linn Buberg, Ingun Lund Witsø, Trine M. L’Abée-Lund, Yngvild Wasteson
Abstract
The present work addresses the effect of excess levels of ZnCl 2 and CuSO 4 in the growth medium on the conjugative transfer of plasmids carrying the antibiotic resistance gene bla CMY-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 ZnCl 2 and CuSO 4 . Manual counting of transconjugants showed that ZnCl 2 and CuSO 4 reduced the conjugation frequency between E. coli strains in a concentration-dependent manner. Quantitative real-time PCR analyses showed that the presence of ZnCl 2 and CuSO 4 in the growth media reduced expression of the conjugation genes traB and nikB . By propagating monocultures over several generations, it was found that the bla CMY-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 bla CMY-2 resistance plasmid by reducing expression of genes involved in conjugation in the plasmid donor strain.