A LXG toxin stabilized by DUF4176 contributes to Streptococcus suis competition and pathogenicity

Streptococcus suis poses a significant threat to both humans and pigs, causing severe disease such as meningitis, septicaemia, and sudden death. The type VII secretion system (T7SS) plays a crucial role in exporting effectors that mediate bacterial competition and colonization in polymicrobial environments. Although Leu-x-Gly (LXG) polymorphic toxins are among the main effectors secreted by the T7SS, the mechanism underlying their secretion remains to be fully elucidated.

Here, we identified a complex repertoire of LXG effectors in S. suis, comprising at least five distinct LXG-domain groups and 14 C-terminal toxin types. We focused on one LXG effector, LXG1-CT3, from the virulent strain WUSS351, demonstrating its pore-forming activity and essential role in S. suis competition and virulence. Additionally, we discovered that a small protein family, DUF4176, partners with each cognate C-terminal toxin, stabilizing the corresponding LXG effector prior to secretion, thereby enhancing bacterial competitiveness.

These findings deepen our understanding of S. suis competition and pathogenesis, while offering new insights into the T7SS-mediated secretion of LXG effectors.