Characterization of Cytotoxicity and Antimicrobial Susceptibility of Streptococcus agalactiae Isolated from Bovine Mastitis in Northeastern Thailand

Streptococcus agalactiae is a major contagious pathogen that causes bovine mastitis and substantial economic losses in the dairy industry. This study investigated the capsular genotypes, antimicrobial susceptibility, and virulence-associated phenotypes of S. agalactiae isolated from bovine mastitis cases in northeastern Thailand. A total of 177 S. agalactiae isolates were identified, and characterized. Antimicrobial susceptibility was determined by broth microdilution. Biofilm formation was evaluated using Congo red agar and crystal violet staining, whereas adhesion, invasion, and cytotoxicity were assessed using A549 and HeLa epithelial cell lines using representative isolates. Capsular genotype Ia predominated (95.48%), followed by genotype IX (3.39%), and the remaining were non-typeable (1.13%). All isolates were susceptible to penicillin, vancomycin, erythromycin, and clindamycin, whereas resistance to ceftriaxone (1.13%) and levofloxacin (0.56%) was detected. Among the three representative isolates, the non-typeable isolate exhibited the strongest biofilm-forming ability, followed by the genotype IX and genotype Ia isolates. The genotype IX isolate showed the highest mean adhesion, invasion, and cytotoxicity in both A549 and HeLa cell lines, although not all pairwise differences were statistically significant. This isolate also exhibited higher cyl and bac mRNA levels than the other two isolates. These findings indicate that S. agalactiae isolated in bovine mastitis in this area remained susceptible to most antimicrobials tested and that virulence-associated phenotypes varied among the representative bovine isolates. Epidemiological surveillance, responsible antimicrobial use, farmer education, and improved farm remain important priorities for the prevention and control of bovine mastitis and antimicrobial resistance.