Streptococcus suis exports WapA polymorphic toxins to compete with tonsil microbiota for an optimal colonization

Streptococcus suis is a zoonotic pathogen, and its colonization of the host tonsil is believed to be a vital source causing infection, while its mechanism competing for a stable tonsil niche is unknown. Rearrangement hotspot (Rhs) proteins are characterized to facilitate interbacterial competition by their polymorphic C-terminal toxins (CTs) in diverse bacteria, while their distant homologues emerged in S. suis, referred to as wall-associated protein A (WapA), has not been identified.

Bioinformatics, western blot and interbacterial competition analyses were performed to identify Rhs/WapA toxins and their roles during S. suis infection.

The 350 kDa WapA-CT1, linked with a SecF-like protein and a SrtB sortase, was verified to manipulate the tonsil microbiota for S. suis optimal colonization. The unfolded WapA-CT1 was translocated across the cell membrane via the canonical Sec pathway. Afterward, autocleavage generated four fragments: the N-terminal NCWB fragment, two middle Rhs domains (Rhs1&2) that may fold as a β-barrel structure, and a C-terminal PreT-CT toxin domain. SrtB interacts with the NCWB region, and plays vital roles for the interbacterial antagonism mediated by the toxic CT1.

This discovery underscores the diversity of mechanisms by which pathogens delivering Rhs/WapA polymorphic toxins, and their roles in competing with the host microbiota.