Recombination, mobile genetic elements, and genetic transfer contribute to the adaptation of Streptococcus uberis...

Streptococcus uberis is a major cause of bovine mastitis. However, the genomic mechanisms that facilitate adaptation of the pathogen within different host-associated environment or selection pressures remain poorly understood. This study analyzed whole-genome sequence data from three Thai dairy herds to investigate the contributions of recombination and mobile genetic elements (MGEs) to S. uberis evolution and adaptation. Among the 138 S. uberis genomes, 42 core genome sequence types (cgSTs) were identified, along with frequent detection of MGEs such as plasmid-associated genes (81.1% of isolates), prophages (67.4% of isolates), and insertion sequences (26.1% of isolates). The isolates from farm A exhibited the longest recombined fragment size, but with extremely low recombination frequency and recombination-to-mutation ratio. By contrast, the isolates from farm B, which had the highest prevalence of antimicrobial resistance (AMR) gene, showed a high recombination-to-mutation ratio (R/θ = 4.42) and more frequently contained MGEs associated with AMR genes. Finally, isolates from farm C shared a single core genome and AMR profile but harbored diverse prophages. Several prophages shared high sequence similarity (>99%) with phages infecting other bacterial genera, suggesting that ecological overlap between bacterial species may facilitate cross-genus genetic exchange, highlighting the influence of microbial ecology on the evolution of S. uberis. Collectively, our results illustrate the variety of mechanisms and genetic elements that contribute to the adaptive evolution of S. uberis in dairy farming environments.