Antimicrobial resistance selection in avian pathogenic E. coli during treatment.

An experiment was performed to compare the microbiological efficacy of four treatments (oxytetracycline, trimethoprim-sulphonamide, amoxicillin (AMX) or enrofloxacin (ENR)) to control experimental colibacillosis induced by an avian pathogenic Escherichia coli (APEC) with reduced susceptibility to fluoroquinolones. The protocol was also developed in order to study resistance gene transfer. Broilers were first orally inoculated with multiresistant E. coli bearing plasmid genes conferring resistance to fluoroquinolones (qnr), cephalosporins (blaCTX-M or blaFOX), tetracycline or trimethoprim-sulphonamide. They were then inoculated in their air sacs with the APEC and treated as soon as symptoms appeared. Internal organs from dead or sacrificed birds were cultivated on non-supplemented or supplemented media. The inoculated O78 APEC was recovered significantly less frequently in ENR treated group (26%) compared to untreated group (47%). This was not true for other treated groups. Isolates obtained on non-supplemented media had the same susceptibility profile as the inoculated APEC. However, one isolate from the AMX-treated group obtained on AMX-supplemented media was resistant to AMX only, and one isolate from the same group obtained on ENR-supplemented media, showed a resistance profile suggesting acquisition of one of the multiresistance plasmids present in the intestinal microbiota. Molecular analysis performed on this multiresistant isolate confirmed the presence of a conjugative plasmid with qnr and blaCTX-M resistance genes. Thus, the experiment illustrated the emergence of resistant isolates in internal organs, probably via acquisition of a plasmid from the intestinal microbiota.