Group B Streptococcus is a major human neonatal and maternal pathogen, an emerging pathogen of non-pregnant humans, and a multi-host pathogen affecting many animal species. For all our talk about One Health, many people do not realize this as in the veterinary world, GBS mostly goes by the name Streptococcus agalactiae, referring to its original detection by Rebecca Lancefield as causative agent of mastitis in dairy cattle.
In 2015, the first major outbreak of foodborne GBS disease was detected in Singapore, attributed to consumption of fish with sequence type 283, first described in Australia by Professor Lyn Gilbert. This strain is now recognized as a major pathogen of fish species, and a significant cause of sepsis in adults without comorbidities in Southeast Asia, possibly with higher incidence than neonatal GBS sepsis in some regions. Meanwhile, in northern Europe, GBS has re-emerged in dairy industries where it had almost been eliminated, quite possibly due to human-to-animal spillover of GBS with subsequent host-adaptation and onward transmission, and in East Africa, the intensification of camel milk production is leading to an expansion of GBS infection in dromedary camels.
Through analysis of over 1250 GBS genomes collected from 1953 to 2021 across 6 continents and 9 host categories, we sought to understand the host spectrum and genomic mechanisms of host-adaptation in GBS. In doing so, we identified host-generalist and host-specialist lineages, including lineages with host-preference and those with host-restriction. Using Genome Wide Association Studies, we then identified molecular markers of host adaptation and showed that, remarkably, three gene clusters appear to be both required and sufficient to explain host adaptation to the three major host categories of GBS: humans, cattle and fishes. The three gene clusters are located on mobile genetic elements that are shared with other streptococcal species occupying the same host niche, demonstrating the importance of intersecting biological species and "ecological species", and alerting us to the possibility and risk of spill-over and spill-back of GBS between human and animal hosts and its consequences for public health and food security.