Distribution of the pco Gene Cluster and Associated Genetic Determinants among Swine Escherichia coli from a Controlled Feeding Trial

Abstract

Copper is used as an alternative to antibiotics for growth promotion and disease prevention. However, bacteria developed tolerance mechanisms for elevated copper concentrations, including those encoded by the <i>pco</i> operon in Gram-negative bacteria. Using cohorts of weaned piglets, this study showed that the supplementation of feed with copper concentrations as used in the field did not result in a significant short-term increase in the proportion of <i>pco</i>-positive fecal <i>Escherichia coli</i>. The <i>pco</i> and <i>sil</i> (silver resistance) operons were found concurrently in all screened isolates, and whole-genome sequencing showed that they were distributed among a diversity of unrelated <i>E. coli</i> strains. The presence of <i>pco</i>/<i>sil</i> in <i>E. coli</i> was not associated with elevated copper minimal inhibitory concentrations (MICs) under a variety of conditions. As found in previous studies, the <i>pco</i>/<i>sil</i> operons were part of a Tn<i>7</i>-like structure found both on the chromosome or on plasmids in the <i>E. coli</i> strains investigated. Transfer of a <i>pco</i>/<i>sil</i> IncHI2 plasmid from <i>E. coli</i> to <i>Salmonella</i> <i>enterica</i> resulted in elevated copper MICs in the latter. <i>Escherichia coli</i> may represent a reservoir of <i>pco</i>/<i>sil</i> genes transferable to other organisms such as <i>S. enterica</i>, for which it may represent an advantage in the presence of copper. This, in turn, has the potential for co-selection of resistance to antibiotics.