Intestinal commensal bacteria are considered good indicators for monitoring antimicrobial resistance. We investigated the antimicrobial resistance profiles and resistance trends of Enterococcus faecium and Enterococcus faecalis isolated from food animals in Korea between 2010 and 2019. E. faecium and E. faecalis, isolated from chickens and pigs, respectively, presented a relatively high resistance rate to most of the tested antimicrobials. We observed high ciprofloxacin (67.9%), tetracycline (61.7%), erythromycin (59.5%), and tylosin (53.0%) resistance in E. faecium isolated from chickens. Similarly, more than half of the E. faecalis isolates from pigs and chickens were resistant to erythromycin, tetracycline and tylosin. Notably, we observed ampicillin, daptomycin, tigecycline and linezolid resistance in a relatively small proportion of enterococcal isolates. Additionally, the enterococcal strains exhibited an increasing but fluctuating resistance trend (p < 0.05) to some of the tested antimicrobials including daptomycin and/or linezolid. E. faecalis showed higher Multidrug resistance (MDR) rates than E. faecium in cattle (19.7% vs. 8.6%, respectively) and pigs (63.6% vs. 15.6%, respectively), whereas a comparable MDR rate (≈60.0%) was noted in E. faecium and E. faecalis isolated from chickens. Collectively, the presence of antimicrobial-resistant Enterococcus in food animals poses a potential risk to public health.

Antimicrobial-resistant bacteria in food animals pose a major public health threat worldwide. In this study, we aimed to assess the antimicrobial resistance profiles and resistance trends of commensal Escherichia coli isolated from the feces of healthy cattle, pigs, and chickens in South Korea during 2010 and 2020. A total of 7237 E. coli isolates (2733 cattle, 2542 pig, and 1962 chicken isolates) were tested for susceptibility towards 12 antimicrobials. About 48%, 90%, and 97% of cattle, pig, and chicken isolates, respectively, were resistant to one or more antimicrobial agents. Cattle isolates presented low resistance (45%) resistance rate to ampicillin, chloramphenicol, streptomycin, and tetracycline. We observed high ciprofloxacin and nalidixic acid resistance rates in chicken (76.1% and 88.6%, respectively), isolates in pig (12.7% and 26.7%, respectively) and cattle (2.7% and 8.2%, respectively) isolates. Notably, a very small proportion of isolates (<5%) from cattle, chickens, and pigs demonstrated resistance to amoxicillin/clavulanic acid, cefoxitin, and colistin. We identified ceftiofur resistance in a small proportion of chicken (8.8%), pig (3.7%), and cattle (0.7%) isolates. We noted an increasing but fluctuating trend of ampicillin, amoxicillin/clavulanic acid, ceftiofur, cefoxitin, chloramphenicol, ciprofloxacin, and streptomycin resistance in pig isolates. Similarly, the ampicillin, ceftiofur, and chloramphenicol resistance rates were increased but fluctuated through time in chicken isolates. Overall, 56% of the isolates showed multidrug-resistant (MDR). The proportion of MDR isolates was low in cattle (17.1%); however, this proportion was high in chickens (87.1%) and pigs (73.7%). Most of the resistance patterns included streptomycin and tetracycline in pigs and cattle, and ciprofloxacin and nalidixic acid in chickens. In conclusion, this study showed high resistance of commensal E. coli isolated from major food animals in Korea to commonly used antimicrobials including critically important antimicrobials. These bacteria could not only be a resistance reservoir but also could have potential to spread this resistance through gene transfer to pathogenic bacteria. Thus, the high prevalence of antimicrobial resistance in food animals highlights the urgent need for measures to restrict and ensure the prudent use of antimicrobials in Korea.

We aimed to investigate the presence of the phenicol–oxazolidinone resistance gene poxtA in linezolid-resistant enterococci from food-producing animals and analyze its molecular characteristics. We collected 3941 Enterococcus faecium and 5088 E. faecalis isolates from all provinces of South Korea from 2008 to 2018. We found linezolid resistance in 0.79% (94/3941) of E. faecium and 1.22% (62/5088) of E. faecalis isolates. Overall, 23.1% (36/156) of the linezolid-resistant isolates had the poxtA gene, including 31 E. faecium and five E. faecalis isolates. The poxtA-positive enterococci were mainly isolated from chicken (86.1%; 26/36). Fifteen poxtA-harboring isolates co-carried another linezolid-resistance gene, optrA. Eight E. faecium isolates had an N130K mutation in the ribosomal protein L4, while no mutations were observed in E. faecalis isolates. The poxtA gene was transferred into 10 enterococci by conjugation. Multi-locus sequence typing (MLST) and pulsed-field gel electrophoresis (PFGE) analysis indicated that poxtA-carrying isolates were heterogeneous. Three E. faecium isolates belonged to CC17 (ST32, ST121, and ST491). To our knowledge, this is the first report on the poxtA gene in Korea. Prudent use of antimicrobials and active surveillance on antimicrobial resistance are urgently needed to reduce the risk of dissemination of the linezolid-resistant isolates in humans and animals.