Plasmid-borne florfenicol and ceftiofur resistance encoded by the floR and blaCMY-2 genes in Escherichia coli isolates from diseased cattle in France.

This study was designed to determine the genetic basis of florfenicol and ceftiofur resistance in Escherichia coli isolates recovered from French cattle. In these isolates, ceftiofur resistance was conferred by bla(CMY-2) located on three distinct conjugative plasmids on a specific DNA fragment, ISEcp1-bla(CMY-2)-blc- sugE. Two of the plasmids also carried the floR gene conferring resistance to florfenicol. The floR gene was shown to be associated with the insertion sequence ISCR2. Mobile elements appear to contribute to the mobilization of floR and bla(CMY-2) genes in E. coli. The presence of bla(CMY-2) and floR on the same plasmid highlights the potential risk for a co-selection of the bla(CMY-2) gene through the use of florfenicol in food animal production.