Incidence of Streptococcus spp. and whole genome sequencing of Streptococcus agalactiae isolated from cow's milk samples in Brazil.

This study assessed the occurrence of Streptococcus spp. in milk samples from cows with visible clinical mastitis signs and from cows with high (≥ 200,000 cells/mL) or low (< 200,000 cells/mL) somatic cell counts (SCC) across five Brazilian regions. Antimicrobial resistance and virulence-associated genes were investigated using phenotypic assays and whole-genome sequencing (WGS). Among 1,447 milk samples, Streptococcus agalactiae was the predominant species, followed by Streptococcus uberis and Streptococcus dysgalactiae, with most isolates associated with high SCC samples. Positive samples presented elevated SCC values, reinforcing the association between Streptococcus spp. and udder inflammation. High resistance rates to streptomycin and tetracycline were observed, and multidrug resistance was detected in one-third of the isolates. Genomic analysis of S. agalactiae identified two sequence types (ST-67 and ST-103), resistance genes associated with aminoglycoside, tetracycline, and macrolide resistance, and a broad repertoire of virulence-related genes. S. agalactiae isolates were detected exclusively in samples from Paraíba, whereas S. uberis was identified across all evaluated regions, suggesting distinct epidemiological patterns among Streptococcus species. Overall, these findings highlight the epidemiological relevance, antimicrobial resistance potential, and virulence-associated characteristics of Streptococcus spp. involved in bovine mastitis in Brazil, reinforcing the importance of continuous surveillance and improved mastitis control strategies in dairy herds.