Selection of Fecal Enterococci Exhibiting tcrB -Mediated Copper Resistance in Pigs Fed Diets Supplemented with Copper

Abstract

ABSTRACT Copper, as copper sulfate, is increasingly used as an alternative to in-feed antibiotics for growth promotion in weaned piglets. Acquired copper resistance, conferred by a plasmid-borne, transferable copper resistance ( tcrB ) gene, has been reported in Enterococcus faecium and E. faecalis . A longitudinal field study was undertaken to determine the relationship between copper supplementation and the prevalence of tcrB- positive enterococci in piglets. The study was done with weaned piglets, housed in 10 pens with 6 piglets per pen, fed diets supplemented with a normal (16.5 ppm; control) or an elevated (125 ppm) level of copper. Fecal samples were randomly collected from three piglets per pen on days 0, 14, 28, and 42 and plated on M- Enterococcus agar, and three enterococcal isolates were obtained from each sample. The overall prevalence of tcrB- positive enterococci was 21.1% (38/180) in piglets fed elevated copper and 2.8% (5/180) in the control. Among the 43 tcrB- positive isolates, 35 were E. faecium and 8 were E. faecalis . The mean MICs of copper for tcrB- negative and tcrB- positive enterococci were 6.2 and 22.2 mM, respectively. The restriction digestion of the genomic DNA of E. faecium or E. faecalis with S1 nuclease yielded a band of ∼194-kbp size to which both tcrB and the erm (B) gene probes hybridized. A conjugation assay demonstrated cotransfer of tcrB and erm (B) genes between E. faecium and E. faecalis strains. The higher prevalence of tcrB- positive enterococci in piglets fed elevated copper compared to that in piglets fed normal copper suggests that supplementation of copper in swine diets selected for resistance.