Effects of carry-over of fluoroquinolones on the susceptibility of commensal Escherichia coli in the intestinal microbiota of poultry

Due to the frequent use of antibacterials in veterinary medicine as well as in human medicine the occurrence of antibacterial resistance rises worldwide. But independent of the usage of antimicrobials the microbiota from animals as well as from humans already harbour a diversity of resistance genes. As a consequence of manufacturing animal production the treatment of livestock in case of illness is carried out via feed or drinking water. This automatically implies several risks. It has been demonstrated that an antibiotic treatment of livestock via feed or drinking water cause an accumulation of antibiotics and their metabolites in the direct environment of animals. This can lead to a carry-over or rather a resumption of the antimicrobials and their metabolites. Thus, the aim of this study was to determine the influence of carry-over of enrofloxacin as a representative of the fluoroquinolones on the development of bacterial resistance of commensal E. coli in the intestinal microbiota of poultry. Therefore four different treatment groups were provided and the minimal inhibitory concentrations (MICs) of commensal E. coli were measured: One group acted as untreated control, another one was therapeutically treated with the recommended dosage. The third and fourth group were exposed to different"carry-over dosages" for three weeks, 3% of the recommended dosage were applied to the third and 10% to the fourth group. To determine the influence of a therapeutic treatment on a prestressed microbiota, both groups were treated with the recommended dosage for five days. The present study demonstrates that every kind of exposure of the commensal microbiota of poultry with enrofloxacin leads to an amplification and selection of resistant E coli, which persist in the commensal microbiota. A long term exposure of gut microbiota, which already harbour non-wildtype E. coli, with high levels of carry-over of fluoroquinolones may lead to a development of high-level clinically resistant E. coli in the commensal microbiota. It has to be investigated to which extent antimicrobial leftovers occur in animal production.