Selection and transfer of an IncI1-tet(A) plasmid of Escherichia coli in an exￂ vivo model of the porcine caecum at doxycycline concentrations caused by crosscontaminated feed

Aims The aim of this study was to investigate the effect of subtherapeutic intestinal doxycycline (DOX) concentrations (4 and 1 mg l-1), caused by cross-contamination of feed, on the enrichment of a DOX-resistant commensal Escherichia coli and its resistance plasmid in an ex vivo model of the porcine caecum. Methods and Results A DOX-resistant, tet(A)-carrying, porcine commensal E. coli strain (EC 682) was cultivated for 6 days in the porcine caecum model under different conditions (0, 1 and 4 mg l-1 DOX). EC 682, other coliforms and anaerobic bacteria were enumerated daily. A selection of isolated DOX-resistant coliforms (n = 454) was characterized by rep-PCR clustering, PCR assays (Inc1 and tet(A)) and micro broth dilution susceptibility tests (Sensititre). Both 1 and 4 mg l-1 DOX-enriched medium had a significantly higher selective effect on EC 682 and other resistant coliforms than medium without DOX. Transconjugants of EC 682 were isolated more frequently in the presence of 1 and 4 mg l-1 DOX compared to medium without DOX. Conclusions Subtherapeutic intestinal DOX concentrations have the potential to select for DOX-resistant E. coli, and promote the selection of transconjugants in a porcine caecum model. Significance and Impact of the Study Cross-contamination of feed with antimicrobials such as DOX likely promotes the spread of antimicrobial resistance. Therefore, it is important to develop or fine-tune guidelines for the safe use of antimicrobials in animal feed and its storage.