Investigation of a multiresistance gene cfr that fails to mediate resistance to phenicols and oxazolidinones in Enterococcus faecalis

Objectives To investigate the basis of susceptibility to phenicols and oxazolidinones of the porcine Enterococcus faecalis CPPF5 despite the presence of the multiresistance gene cfr. Methods Southern blotting, conjugation and transformation analyses were conducted to confirm the plasmid location and transferability of cfr in CPPF5. The genetic environment of cfr was determined by sequence analysis. Transcription and translation of cfr were examined by RT–PCR and western blotting, respectively, and modifications at A2503 within the 23S rRNA sequence were identified by primer extension. Results Electrotransformation and Southern blotting indicated that CPPF5 and its transformant 5B2-3 contained two cfr-carrying plasmids ∼50 and ∼12 kb in size. The complete 12 270 bp sequence of the smaller plasmid, pCPPF5, was determined and shared 99.9% (12 269/12 270 bp) identity with the corresponding region of the cfr-carrying plasmid pEF-01 in E. faecalis of cattle origin. Moreover, the genetic environment of cfr in the ∼50 kb plasmid was the same as that in pCPPF5 according to sequencing results. Although cfr mRNA, Cfr protein and a modification at the A2503 site were detected, the cfr-carrying transformant 5B2-3 did not have elevated MICs of chloramphenicol, florfenicol and linezolid, indicating that cfr fails to mediate resistance to the respective antibiotics in E. faecalis. Conclusions This is the first report of the cfr gene failing to elevate MICs of the corresponding antibiotics. Although the genetic basis for the apparent ‘no resistance’ phenotype remains to be determined, this finding may have implications for surveillance studies that target the cfr gene.