Plasmid-borne florfenicol resistance in Pasteurella multocida

Objectives: A florfenicol-resistant Pasteurella multocida isolate from a calf was investigated for the genetic basis of florfenicol resistance and the location of the resistance gene. Methods: The P. multocida isolate 381 was investigated for its in vitro susceptibility to antimicrobial agents and its plasmid content. A 10.8 kb florfenicol-chloramphenicol resistance plasmid, designated pCCK381, was identified by transformation into Escherichia coli. The plasmid was mapped with restriction endonucleases, cloned and sequenced completely. Results: Of the antimicrobials tested, plasmid pCCK381 conferred resistance only to chloramphenicol and florfenicol. It showed extended similarity to the 5.1 kb plasmid pDN1 from Dichelobacter nodosus in the part carrying the mobilization and replication genes. An adjacent 3.2 kb segment was highly homologous to the florfenicol resistance gene region of plasmid pMBSF1 from E. coli. In pCCK381, combined resistance to chloramphenicol and florfenicol was based on the presence of a floR gene that showed 97.2-99.7% identity to so far known floR genes. Conclusions: The results of this study showed that a plasmid-borne floR gene was responsible for chloramphenicol and florfenicol resistance in the bovine respiratory tract pathogen P. multocida. This is, to the best of our knowledge, the first report of a florfenicol resistance gene in a target bacterium.