Home is where the host is: evolutionary history of geographic spread, host switching and adaptive genomic signatures in two generalist group B Streptococcus clonal group.

Group B Streptococcus (GBS) is a pathogen of global relevance in neonatal and maternal disease as well as bovine mastitis. Two closely related clonal group, denoted 103 and 314 (CG103/314), have been detected in humans and cattle on multiple continents in recent decades but are poorly characterized compared to other host-generalist clades. We examined their potential origins, host-switching events and the presence of a suite of genetic markers for antimicrobial resistance, virulence and host association using a newly assembled dataset of 248 CG103/314 genomes from humans, cattle and food originating from five continents. We detected multiple host switches between humans and cattle and significant regional differences in antimicrobial resistance gene distribution, possibly reflecting local differences in antimicrobial use across countries and hosts and indicating a capacity for regional adaptation to selective pressures. Across the evolutionary history of CG103/314 from both host species, the prevalence of the Lac.2 operon, a genetic marker associated with bovine host adaptation, was high, whereas the prevalence of the scpB-lmb gene pair, a genetic marker of human host adaptation in other GBS clonal group, was very low. All isolates with scpB-lmb were associated with human disease rather than carriage. Our dataset displayed biases typical of research into multi-host pathogens, when sampling is often focused on a specific host species or setting. Consistent, balanced, contemporaneous and sympatric sampling efforts across host species and sources are needed to fully understand the distribution and emergence of CG103/314 and similar multi-host pathogens that impact food safety and public health.