Detection of novel oxazolidinone and phenicol resistance gene optrA in enterococcal isolates from food animals and animal carcasses.

Altogether 7720 Enterococcus faecalis and 3939 E. faecium isolated from food animals and animal carcasses during 2003-2014 in Korea were investigated to determine if linezolid-resistant (LR) enterococci (≥8μg/ml) are present. Overall, 12 E. faecalis and 27 E. faecium recovered from chickens (n=32), pigs (n=6), and cattle (n=1) were resistant to linezolid and were further characterized using molecular methods Most LR isolates were also resistant to chloramphenicol (97.44%) and florfenicol (92.31%). Molecular analysis showed no mutations in the 23S ribosomal RNA and in the ribosomal protein L3. The optrA gene was found in 89.74% of the LR enterococci, including 12 E. faecalis and 23 E. faecium isolates. Among them, 30 optrA-positive isolates co-carried phenicol exporter gene fexA. Seven LR E. faecium isolates had Asn130Lys mutations in the ribosomal protein L4, of which six also carried optrA gene. None of the isolates carried the mutliresistance gene cfr. Transfer of optrA gene was observed in 16 of the 35 optrA-positive isolates by conjugation. Pulsed-field gel electrophoresis demonstrated that the vast majority of Enterococcus strains carrying optrA gene were genetically heterogeneous. Multi-locus sequence typing revealed eight novel Sequence types among E. faecalis and E. faecium strains. To our knowledge, this is the first report of optrA gene in isolates from cattle and animal carcasses. This is also the first report of optrA gene in Korea. Active surveillance of optrA in enterococci is urgently warranted.