The emergence of zoonotic Streptococcus suis

Streptococcus suis is an opportunistic porcine pathogen and emerging zoonotic pathogen. In recent decades, emerging zoonotic S. suis lineages have been identified; however, the evolutionary mechanisms underlying the acquisition of zoonotic potential remain poorly elucidated. In this thesis we aimed to gain a better understanding of the evolutionary processes that give rise to emerging zoonotic S. suis lineages and to elucidate the mechanisms underlying the association between specific genetic elements with zoonotic potential. We performed a systematic review, a survey of seven reference laboratories, and a grey literature scan were performed to estimate zoonotic S. suis infections in Europe and characterized the population structure of zoonotic European S. suis using genomics. Thailand is the country with the highest reported incidence of zoonotic S. suis infections and these infections are caused by an unusually diverse group of strains. We used genomics to investigate how novel zoonotic lineages have evolved in Thailand, focusing on antimicrobial resistance and capsule switching. The majority of zoonotic infections are caused by serotype 2 isolates. We used comparative genomics and in vitro infection models to explore the link between the serotype 2 capsule and zoonotic potential, identifying a plausible mechanism via interactions with Siglecs. Additionally, we explored the role of phase-variability in promoting zoonotic potential by characterizing the SsuCC20p type I RM system associated with the Dutch CC20 lineage, examining its spread, phase variability, and role in virulence.