Antimicrobial resistance patterns of Streptococcus uberis isolates recovered from bovine mastitis

Streptococcus uberis, which is a major cause of bovine mastitis in the dairy industry, is commonly treated with antimicrobials. The objective of this study was to investigate antimicrobial resistance (AMR) patterns of S. uberis isolates recovered from bovine mastitis. Totally 120 milk samples were collected in 2025 from cows with mastitis on a farm. S. uberis isolates were identified using standard bacteriological analyses and confirmed by sequence analysis of the 16S rRNA gene. Six different antimicrobials (gentamicin, streptomycin, penicillin, vancomycin, erythromycin, and tetracycline) were used for AMR test by the Kirby-Bauer disc diffusion method. Eighteen S. uberis isolates (15.0%, 18/120) were recovered, none of which were susceptible to all the tested antimicrobials. All isolates (100.0%, 18/18) were resistant to aminoglycosides, gentamycin, and streptomycin. In contrast, all isolates (100.0%, 18/18) were susceptible to penicillin and vancomycin. This finding supports the use of beta lactams as first-line antimicrobials to treat S. uberis infections. Additionally, 27.8% (5/18) and 61.1% (11/18) of the isolates were resistant to erythromycin and tetracycline, respectively. The high resistance rate for tetracycline indicates that it is not an effective treatment option for S. uberis infections. Four different AMR patterns were determined. Isolates with the CN-S-E-TE pattern were defined as multidrug-resistant (MDR) (16.7%, 3/18). The identification of MDR isolates emphasizes the necessity of conducting AMR testing prior to initiating antimicrobial treatment for bovine mastitis. In conclusion, these findings align with existing literature and demonstrate the significance of regular surveillance to guide evidence-based and sustainable approaches in bovine mastitis management.