chlortetracycline and sulfadiazine--trimethoprim-drug-combination

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.

Topics: Animal Feed; Animals; Anti-Bacterial Agents; Antiporters; Bacterial Proteins; Chlortetracycline; Doxycycline; Drug Combinations; Drug Resistance, Bacterial; Escherichia coli; Feces; Gene Transfer, Horizontal; Intestines; Plasmids; Sulfadiazine; Swine; Trimethoprim

Cross-contamination of feed with low concentrations of antimicrobials can occur at production, transport and/or farm level. Concerns are rising about possible effects of this contaminated feed on resistance selection in the intestinal microbiota. Therefore, an experiment with pigs was set up, in which intestinal and fecal concentrations of chlortetracycline (CTC), doxycycline (DOX) and sulfadiazine-trimethoprim (SDZ-TRIM) were determined after administration of feed containing a 3 % carry-over level of these antimicrobials.. The poor oral bioavailability of tetracyclines resulted in rather high concentrations in cecal and colonic content and feces at steady-state conditions. A mean concentration of 10 mg/kg CTC and 4 mg/kg DOX in the feces was reached, which is higher than concentrations that were shown to cause resistance selection. On the other hand, lower mean levels of SDZ (0.7 mg/kg) and TRIM (< limit of detection of 0.016 mg/kg) were found in the feces, corresponding with the high oral bioavailability of SDZ and TRIM in pigs.. The relation between the oral bioavailability and intestinal concentrations of the tested antimicrobials, may be of help in assessing the risks of cross-contaminated feed. However, future research is needed to confirm our results and to evaluate the effects of these detected concentrations on resistance selection in the intestinal microbiota of pigs.

Topics: Animal Feed; Animals; Anti-Bacterial Agents; Chlortetracycline; Doxycycline; Drug Combinations; Drug Residues; Feces; Food Contamination; Gastrointestinal Contents; Sulfadiazine; Swine; Trimethoprim