Effect of residual doxycycline concentrations on resistance selection and transfer in porcine commensal escherichia coli
2017
Peeters, Laura E.J. | Croubels, Siska | Rasschaert, Geertrui | Imberechts, Hein | Daeseleire, Els | Dewulf, Jeroen | Heyndrickx, Marc | Butaye, Patrick | Haesebrouck, Freddy | Smet, Annemieke
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.This study aimed to determine the effect of residual DOX concentrations (1 and 4 mg/L) in vitro on selection of DOX resistant porcine commensal E. coli and transfer of their resistance plasmids.Three different DOX resistant porcine commensal E. coli strains and their plasmids were characterized. These strains were each brought in competition with a susceptible strain in a medium containing 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 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 to the blank control, but no significant difference between 1 and 4 mg/L was seen. 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.
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