Effect of residual doxycycline concentrations on resistance selection and transfer in porcine commensal Escherichia coli.

Pig feed may contain various levels of antimicrobial residues due to cross-contamination. A previous study showed that a 3% carry-over level of doxycycline (DOX) in the feed results in porcine faecal concentrations of approximately 4 mg/L. The aim of this study was to determine the effect of residual DOX concentrations (1 and 4 mg/L) in vitro on selection of DOX-resistant porcine commensal Escherichia coli and transfer of their resistance plasmids. Three different DOX-resistant porcine commensal E. coli strains and their plasmids were characterised. These strains were each brought in competition with a susceptible strain in a medium containing 0, 1 and 4 mg/L DOX. Resistant bacteria, susceptible bacteria and transconjugants were enumerated after 24 h and 48 h. The tet(A)-carrying plasmids showed genetic backbones that are also present among human E. coli isolates. Ratios of resistant to susceptible bacteria were significantly higher at 1 and 4 mg/L DOX compared with the blank control, but there was no significant difference between 1 and 4 mg/L. Plasmid transfer frequencies were affected by 1 or 4 mg/L DOX in the medium for only one of the resistance plasmids. In conclusion, DOX concentrations of 1 and 4 mg/L can select for resistant E. coli in vitro. Further research is needed to determine the effect of these concentrations in the complex environment of the porcine intestinal microbiota.