Effect of low 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 concentrations in pig intestines of approximately 4 mg/L. Resistant bacteria, enriched in the pig microbiota due to these low DOX concentrations, could cycle between different reservoirs and transfer their genes to pathogenic bacteria, imposing a threat to human health.This study aimed to determine the effect of low DOX concentrations (1 and 4 mg/L) on the selection of DOX resistant porcine commensal E. coli and transfer of their plasmids.Three DOX resistant porcine commensal E. coli strains and their plasmids were characterized. These strains were each brought in competition with a susceptible strain at 0, 1 and 4 mg/L of DOX. Resistant bacteria, susceptible bacteria and transconjugants were enumerated after 24h and 48h. The tet(A) carrying plasmids showed to have backbones that also circulate among human E. coli isolates. Ratios of resistant to susceptible bacteria were significantly higher at 1 and 4 mg/L DOX compared to the blank control, but no significant difference between 1 and 4 mg/L was seen. Plasmid transfer frequencies were not significantly affected by 1 or 4 mg/L DOX in the medium.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.