Comparative Genomic Analysis of Bovine and Publicly Available Human Streptococcus agalactiae Genomes

Background/Objectives: Streptococcus agalactiae is one of the most significant pathogens causing infections in humans and mastitis in dairy cattle. This work focused on a comprehensive comparative pan-genomic analysis of bovine and human Group B Streptococcus to elucidate the genetic mechanisms underlying host adaptation and dissemination. Methods: Isolates of S. agalactiae from quarter milk samples from dairy herds in Lombardy (Italy), along with human strains, were considered. Whole genome sequencing was used to compare core and accessory genomes, assign sequence types, and find virulence and resistance factors. Results: 30 sequence types were detected, of which two (ST12 and ST23) in common between bovine and human. The allele frequencies for resistance determinants revealed elevated rates for tetM (59.8% overall, 66.5% in human clinical isolates), ermB (17.3% overall, 20.4% in human clinical isolates), and ant(6)la (8.9% overall, 12.8% in human clinical isolates). Bovine strains had accessory gene clusters linked to lactose metabolism and immunological evasion, whereas human isolates were concentrated in regions related to adhesion and antibiotic resistance. Conclusions: Comparative pan-genomics show that there is a small genetic overlap between bovine and human Group B Streptococcus populations.