Copper sulphate is often used as a growth promoter in swine production, according to Dr H.M. Scott of Kansas State Univeristy. Earlier work from Dr Scott's lab (Amachawadi et al., 2011) has illustrated the presence of the transferable copper resistance (tcrB) gene among swine enterococci and its co-location with erm(B) and tet(M) genes, coding for macrolide and tetracycline resistance, respectively, on the same transferable plasmid.

The present study was undertaken to determine the effects of single or multiple feed-grade antimicrobials on selection and co-selection of tcrB-positive enterococci in piglets.

The study consisted of 240 weaned piglets, housed in groups of five animals per pen (n=48 pens). The pens were randomly allocated to six treatments (eight pens per treatment), arranged as an incomplete factorial design, comprising basal diets supplemented with none (control), copper (Cu), chlortetracycline (CTC), tylosin (Tyl), copper and tylosin (CuTyl) or copper and chlortetracycline (CuCTC). No combinations included both CTC and Tyl since this was not an FDA-approved combination.

The treatment phase was for four weeks followed by a wash-out phase for two weeks. Faecal samples were collected on days 0, 7, 14, 21, 28 and 35. All the enterococcal isolates were tested for the presence of tcrB gene by PCR. An equal number of tcrB-positive and matched-negative isolates (by pen, date, and treatment) were also tested for both erm(B) and tet(M) genes.

A total of 372 enterococcal isolates were positive for the tcrB gene with an overall prevalence of 14.4 per cent. The prevalence of tcrB-positive enterococci in each treatment group was: control (47/432; 10.8 per cent), Cu (52/432; 12.0 per cent), CTC (79/432; 18.3 per cent), Tyl (51/432; 11.8 per cent), CuCTC (75/432; 17.4 per cent) and CuTyl (68/432; 15.7 per cent).

The tcrB-positive isolates had a mean copper MIC of 17.8mM, compared to tcrB-negative isolates with an average MIC of 6.6mM. Non-parametric analysis (log-rank test) of median and 90th percentile MIC values revealed a similar conclusion.

All the tcrB-positive and matched-negative isolates also carried both erm(B) and tet(M) genes (conferring phenotypic resistance to erythromycin and tetracycline, respectively).

The supplementation of additional antimicrobials had an additive effect beyond what would have been expected as a simple substitution and resulted in higher prevalence of the tcrB gene when compared to copper supplementation alone.

Further studies are being undertaken to study both the phenotypic and genotypic differences among other enterococcal and E. coli isolates derived from the present study.

Further Reading

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July 2013