Molecular and phenotypic characterization of Streptococcus uberis isolates from bovine mastitis milk in South Korea

Streptococcus uberis, a major environmental pathogen, is frequently linked to recurrent or persistent intramammary infections, posing a challenge to mastitis control in dairy cattle. In this study, we characterized 112 S. uberis isolates recovered from bovine mastitis milk samples collected across South Korea between 2019 and 2024. Multilocus sequence typing revealed substantial genetic diversity, with all sequence types being newly assigned, and no evidence of a dominant clonal lineage. High levels of resistance to tetracycline (81.3%), erythromycin (75.9%), and pirlimycin (75.0%) were observed. Notably, 74.1% of the isolates were classified as multidrug-resistant. The corresponding resistance genes tetO (82.1%) and ermB (79.5%) were commonly detected and showed strong concordance with resistance phenotypes. Virulence-associated genes, including sua, mtuA, oppF, skc, and pauA, were widely detected (89.3-100%). All isolates were capable of biofilm formation, although the majority demonstrated weak biofilm-formation ability. This study provides important insights into the epidemiology and pathogenic potential of bovine mastitis-associated S. uberis isolates in South Korea and underscores the need for continued surveillance and prudent antimicrobial use to formulate effective mastitis-control strategies.

Streptococcus uberis is a leading environmental cause of bovine mastitis worldwide, yet region-specific molecular epidemiological data remain limited in South Korea. To our knowledge, this study represents the first nationwide molecular characterization of mastitis-associated S. uberis isolates in South Korea, integrating MLST-based population analysis, antimicrobial susceptibility testing, resistance gene detection, and virulence profiling. The identification of exclusively novel sequence types indicates substantial genetic diversity within the regional population. The high prevalence of tetracycline and MLS-type resistance, closely associated with tetO and ermB, highlights the importance of continued monitoring of antimicrobial resistance in the dairy production. Additionally, the widespread detection of core virulence-associated genes and universal biofilm-forming capacity indicates conserved characteristics among mastitis-associated isolates. These findings provide baseline data to support future surveillance and control strategies targeting bovine mastitis-associated S. uberis.