Streptococcus suis autolysin functions as a molecular bridge for epithelial adhesion via lipoteichoic acid interaction.

As a zoonotic opportunistic pathogen, adhesion to the porcine upper respiratory tract epithelium is a prerequisite for Streptococcus suis serotype 2 (SS2) to breach epithelial barriers and cause systemic infections under certain conditions. In this study, we first demonstrated the adhesive function of the autolysin Atl of SS2 to swine tracheal epithelial cells. We found that Atl functions as a molecular bridge for epithelial adhesion. The Bsp-like domain (AtlBsp) specifically binds bacterial lipoteichoic acid, anchoring to the bacterial surface, while the C-terminal hydrolase domain (AtlCOOH) directly interacts with host epithelial receptor Fibronectin (Fn). With the purified recombinant Atl (rAtl), we demonstrated that both membrane-bound and secreted forms of Atl could mediate adhesion to the epithelial cells. Moreover, the rAtl was able to restore the adhesive capacity of the Atl-deficient SS2 mutant (Δatl) to both porcine and murine upper respiratory tract epithelium. This study reveals a novel function of the autolysin Atl in promoting SS2 adhesion to the upper respiratory tract epithelium, which elucidates the underlying molecular mechanism and identifies the key functional domains involved. These findings offer new insights into autolysin biology and provide a theoretical basis for developing anti-adhesion strategies against streptococcal infections in pigs.