Decoding the threat: whole-genome comparison of high- and low-virulence Aerococcus viridans isolates from bovine mastitis

Abstract Aerococcus viridans , an emerging pathogen associated with bovine mastitis, has not been fully characterized in terms of its genetic features. This study systematically evaluated the virulence heterogeneity of A. viridans strains causing bovine mastitis. We used a Galleria mellonella infection model and assessed the cytotoxicity of bovine mammary epithelial cells (bMECs), followed by whole-genome comparisons between high- and low-virulence strains. When categorized in vivo and in vitro, high-virulence strains were found to carry widespread invasion-related genes ( aut ), stress-regulatory genes ( clpC / E / P) , and immune-modulating genes ( galE , gndA , hasB / C , ugd ). In contrast, low-virulence strains exhibited elevated levels of antimicrobial resistance genes, such as fusidic acid ( Saur_fusA_FA ) and elfamycin ( Efac_EFTu_GE2A ) resistance genes, suggesting that this opportunistic pathogen adapts its genome to antibiotic-rich environments while optimizing host-associated fitness. Phylogenetic analysis based on core genome single nucleotide polymorphisms revealed that 1 strain of A. viridans was evolutionarily related to a fish-derived reference strain from France, indicating potential cross-border transmission risks. Based on the Clusters of Orthologous Genes database (COG) results, more core genes were assigned to functional categories related to information storage and processing, as well as to cellular processes and signaling. This study is the first to elucidate the virulence-resistance genomic characteristics of A. viridans isolated from bovine mastitis in China, providing new insights to inform prevention and control strategies.