Dokyun Kim, M.D., Ph.D., SungYoung Lee, M.S., Jun Sung Hong, Ph.D., Min Hyuk Choi, M.D., Ph.D., Hyun Soo Kim, M.D., Ph.D., Young Ree Kim, M.D., Ph.D., Young Ah Kim, M.D., Ph.D., Young Uh, M.D., Ph.D., Kyeong Seob Shin, M.D., Ph.D., Jeong Hwan Shin, M.D., Ph.D., Jeong Su Park, M.D., Ph.D., Kyoung Un Park, M.D., Ph.D., Soo Hyun Kim, M.D., Ph.D., Jong Hee Shin, M.D., Ph.D., Jungsik Yu, Ph.D., and Seok Hoon Jeong, M.D., Ph.D.. Ann Lab Med -0001;0:. https://doi.org/10.3343/alm.2025.0145

Vancomycin-variable enterococci (VVE), though genetically containing van genes, are phenotypically sensitive to vancomycin. If VVE is undetected or does not grow on the vancomycin-resistant enterococci (VRE) selection medium, or both, it can acquire resistance upon exposure to vancomycin. This characteristic is clinically important for the treatment and prevention of VRE. This study aims to analyze the prevalence and characteristics of VVE in Korea through the Global Antibiotic Resistance Surveillance System (Kor-GLASS) and emphasize the importance of VVE. A total of 3342 enterococcal bloodstream isolates were collected through the Kor-GLASS between 2017 and 2022. Antibiotic susceptibility testing, van gene detection, and multilocus sequence typing were conducted with all the isolates. The trends in the domestic prevalence of VVE were analyzed and compared with global prevalence data. Among the isolates, 197 (5.9%), including 124 Enterococcus faecium and 73 E. faecalis, were identified as VVE. While the VRE incidence has declined in Korea since 2020, the VVE incidence is significantly rising. In Korea, only the vanA gene has been detected in both VRE and VVE, and no other van gene variants have been identified. Most of these isolates belong to CC17 (91.3%), with ST17, ST817, and ST80 as the predominant types. We have shown that continuous surveillance is essential in Korea due to the persistently high prevalence of VRE and the potential evolution of VVE into VRE. Consequently, it is critical to evaluate Enterococcus species isolated from domestic clinical settings for their phenotypic vancomycin resistance and the molecular detection of van genes, irrespective of the strain.

Objectives: Linezolid resistance among Enterococcus species poses a growing clinical and public health concern, especially due to the dissemination of transferable resistance genes, such as optrA. This study aimed to evaluate the prevalence of linezolid resistance and to characterize the molecular epidemiology and genetic contexts of optrA-positive linezolid-resistant Enterococcus (LRE) isolates from clinical and animal sources in South Korea. Methods: A total of 2156 Enterococcus isolates, collected through nationwide surveillance from hospitalized patients and healthy livestock (pigs, cattle, and chickens) between 2017 and 2019, were retrospectively analyzed. Phenotypic susceptibility testing, optrA gene screening, and whole-genome sequencing were performed to investigate genetic environments and phylogenetic relationships. Results: The prevalence of linezolid resistance was 0.2% in clinical isolates, 3.3% in pigs, 4.3% in cattle, and 1.4% in chickens. optrA-positive linezolid-resistant isolates were less frequent, with rates of 0.1%, 1.4%, 0.9%, and 1.0%, respectively. Multilocus sequence typing identified sequence types (STs) 330 and ST476 in E. faecalis from humans, with no shared STs between human and livestock isolates. The optrA gene was located either chromosomally, frequently associated with transposon Tn6674, or on multidrug resistance plasmids. Notably, optrA variants exhibited host-specific distribution patterns. Phylogenetic analysis demonstrated considerable genomic diversity, and Korean ST476 isolates were genetically related to international strains reported from livestock, poultry products, and wild birds, suggesting potential global dissemination. Conclusions: This study provides a comprehensive, nationally representative assessment of linezolid resistance in South Korea. The findings highlight the zoonotic potential and possible international dissemination of optrA-carrying ST476 lineages, underscoring the need for integrated One Health surveillance to monitor and control the spread of transferable resistance genes.

Background/Objectives: The Korean Global Antimicrobial Resistance Surveillance System monitors bloodstream Staphylococcus aureus infections by combining antimicrobial susceptibility testing (AST) with conventional polymerase chain reaction (PCR). Considering the clinical significance of methicillin-resistant S. aureus (MRSA), we performed an in-depth analysis of isolates showing genotypic–phenotypic discrepancies. Methods: Isolates were collected from designated collection centers in the Republic of Korea between 2017 and 2024. The 30 μg cefoxitin disk diffusion method was used to define the phenotypes. PCR targeting mecA and the staphylococcal cassette chromosome mec (SCCmec) was used to identify genotypes through gel electrophoresis. Long-read whole-genome sequencing (WGS) was performed using the Revio system (Pacific Biosciences) for isolates exhibiting discrepancies between phenotypes and genotypes. Results: In total, 5808 isolates were screened, and seven cases of genotypic–phenotypic discrepancies were identified, including one infant and six elderly patients with chromosomal SCCmec type IV. Although WGS confirmed intact PCR primer-binding sites, structural alterations were observed: three isolates had normal-length mecA and mecR1, two had partial deletions in mecA, and two featured either mecA or mecR1 split into two proteins. Notably, although the six isolates with intact mecR1 genes matched the nucleotide length of SCCmec type IV, their sequences exhibited high homology with SCCmec type II. Conclusions: Despite the presence of mecA, the non-standard configuration of regulatory genes within the SCCmec elements suppressed actual resistance expression. Because conventional PCR focusing on partial gene segments could overlook such phenotypic traits, the meticulous observation and implementation of WGS are crucial for the accurate characterization of genotypic–phenotypic discrepancies.

홍성희, 장혜진, 이정은, 강희영, 이성영, 문동찬, 김도균, 권용준, 김수현, 배송미, 신나리. Public Health Weekly Report 2026;19:1465-83. https://doi.org/10.56786/PHWR.2026.19.35.1