A novel phenicol exporter gene, fexB, found in enterococci of animal origin

Objectives: To investigate two porcine Enterococcus isolates for the genetic basis of phenicol resistance and to determine the location and the genetic environment of the novel resistance gene. Methods: A total of 391 isolates with reduced florfenicol susceptibility (MIC ≥16 mg/L), obtained from 557 nasal swabs of individual pigs, were screened by PCR for the known florfenicol resistance genes. Isolates that were negative in these PCRs were analysed for their species assignment and antimicrobial susceptibility. Plasmids were extracted and subjected to transformation and conjugation assays. Restriction fragments of the phenicol resistance plasmids were cloned and sequenced. The sequences obtained were analysed and compared with sequences deposited in the databases. Results: The two isolates, Enterococcus faecium EFM-1 and Enterococcus hirae EH-1, exhibited MICs of chloramphenicol and florfenicol of 64 mg/L and carried a new phenicol resistance gene, designated fexB. This gene codes for a phenicol exporter of 469 amino acids organized in 14 transmembrane domains. The fexB gene was located on the 35 kb pEFM-1 from E. faecium and on the 25.3 kb pEH-1 from E. hirae, respectively. Both plasmids were non-conjugative. The fexB gene was found to be embedded in virtually the same genetic environment of 14.8 kb in both plasmids. Conclusion: To the best of our knowledge, this is the first report of the new florfenicol exporter gene fexB. Based on its plasmid location, horizontal transfer from the enterococci to other bacteria is possible.