Characterization of florfenicol resistance among calf pathogenic Escherichia coli.

Three floR genes were cloned from calf pathogenic Escherichia coli strains, and the efflux-mediated accumulation of florfenicol in the floR gene-JM109 E. coli system was determined by HPLC. The floR genes resulted in a 1356-bp fragment covering the ORF in region 66-1280 coding for 404 amino acids. The common motifs of 12-transmembrane segments efflux pumps family were conserved in the deduced floR amino acid sequences. HPLC results indicated a significant difference in florfenicol accumulation between florfenicol-resistant strains and the susceptible strains, which was almost reversed by the addition of a proton motive force blocker. These results suggest that the florfenicol resistance mediated by the floR gene involves active efflux of florfenicol.