Detection of Linezolid-Resistant Enterococcus faecalis and Enterococcus faecium Isolates from the Layer Operation System in Korea

Linezolid (LNZ) is one of the most important antimicrobial agents against infections caused by gram-positive bacteria, including enterococci. In a layer operation system, antimicrobial resistance can be transferred to commercial layers via the fecal-oral route. This study investigated the presence and distribution of LNZ-resistant Enterococcus faecalis and Enterococcus faecium in a layer operation system. Among 117 E. faecalis and 154 E. faecium, 10 (8.5%) E. faecalis and 5 (3.2%) E. faecium isolates showed resistance to LNZ and chloramphenicol, and they exhibited multidrug resistance against 5 or more classes of antimicrobial agents. Among the resistant isolates, 9 (90.0%) and 2 (20.0%) E. faecalis harbored optrA and cfr genes, respectively. The optrA and fexA genes were not detected in five LNZ-resistant E. faecium. None of the 15 LNZ-resistant isolates harbored the fexA gene, and no mutations were observed in the genes encoding domain V of 23S ribosomal RNA (rRNA) and ribosomal proteins L3 (rplC) and L4 (rplD). Transferability was identified in three of the nine optrA-positive LNZ-resistant isolates. The tetM, tetL, and ermB genes were cotransferred with the optrA gene in all optrA-positive transconjugants. The results indicate that optrA is well-distributed in E. faecalis, implying a greater level of transferability. Thus, enhanced surveillance efforts are needed to monitor the emergence and spread of optrA in enterococci in layer operation system.