Association with Elevated Somatic Cell Counts and Characterization of Aerococcus viridans Isolates from Bovine Mastitis Milk in South Korea

Aerococcus viridans, an emerging pathogen, is responsible for the recent increase in cases of bovine mastitis. However, its specific effects on mastitis remain largely unexplored. In this study, we examined the correlation between A. viridans-induced mastitis infections and somatic cell counts (SCCs), and characteristics of A. viridans isolates from bovine mastitis milk. Among 1774 mastitis milk samples collected between 2016 and 2021 in South Korea, 69 (3.9%) A. viridans isolates were obtained. Mastitis milk samples containing A. viridans exhibited significantly higher SCCs than did non-mastitis samples. Most isolates (80.5%) were associated with subclinical mastitis (200–1200 × 10³ cells/mL), whereas 19.5% were associated with clinical mastitis (> 1.2 × 10⁶ cells/mL). In pulsed-field gel electrophoresis analysis, A. viridans isolates displayed substantial genetic diversity, with no dominant clones identified. Antimicrobial susceptibility testing revealed high resistance rates to ceftiofur (46.4%) and oxacillin + 2% NaCl (44.9%) among β-lactams, followed by tetracycline (36.2%) and erythromycin (10.1%), with 21.7% isolates being multidrug-resistant. Fifty-four isolates (78.3%) were able to form biofilms, with all recent isolates being biofilm-positive, in contrast to several earlier non-producers. Our findings suggest the necessity for targeted management strategies and continuous monitoring for mitigating A. viridans-induced mastitis in dairy cows.