Occurrence of the Transferable Copper Resistance Gene tcrB among Fecal Enterococci of U.S. Feedlot Cattle Fed Copper-Supplemented Diets

Copper, an essential micronutrient, is supplemented in the diet at elevated levels to reduce morbidity and mortality and to promote growth in feedlot cattle. Gut bacteria exposed to copper can acquire resistance, which among enterococci is conferred by a transferable copper resistance gene (tcrB ) borne on a plasmid. The present study was undertaken to investigate whether the feeding of copper at levels sufficient to promote growth increases the prevalence of thetcrB gene among the fecal enterococci of feedlot cattle. The study was performed with 261 crossbred yearling heifers housed in 24 pens, with pens assigned randomly to a 2�2 factorial arrangement of treatments consisting of dietary copper and a commercial linseed meal-based energy protein supplement. A total of 22 isolates, each identified asEnterococcus faecium , were positive fortcrB with an overall prevalence of 3.8% (22/576). The prevalence was higher among the cattle fed diets supplemented with copper (6.9%) compared to normal copper levels (0.7%). ThetcrB -positive isolates always contained botherm (B) andtet (M) genes. Median copper MICs fortcrB -positive andtcrB -negative enterococci were 22 and 4 mM, respectively. The transferability of thetcrB gene was demonstrated via a filter-mating assay. Multilocus variable number tandem repeat analysis revealed a genetically diverse population of enterococci. The finding of a strong association between the copper resistance gene and other antibiotic (tetracycline and tylosin) resistance determinants is significant because enterococci remain potential pathogens and have the propensity to transfer resistance genes to other bacteria in the gut.