Stk and Stp1 participate in Streptococcus suis serotype 2 pathogenesis by regulating capsule thickness and translocation of certain virulence factors

Eukaryotic-like serine/threonine protein kinase (eSTK) and phosphatase (eSTP) play multiple roles in pathogenesis of many Gram-positive bacteria. eSTK (Stk) and eSTP (Stp1) of Streptococcus suis serotype 2 (S. suis 2) have also been reported to be virulence-associated, but their roles and underlying mechanisms in S. suis 2 pathogenesis require further investigation. We constructed mutants of stk or stp1 deletion using the virulent S. suis 2 isolate 05ZYH33 as the parental strain. Both Δstk and Δstp1 mutants showed abnormal cell division shown as increased chain length. This might be due to regulation by Stk and Stp1 of the phosphorylation status of the bacterial division protein DivIVA. Both mutants showed increased adhesion but reduced invasion to epithelial and endothelial cells. The two mutants were more readily phagocytosed by murine RAW264.7 macrophages. Western blotting revealed that GAPDH (glyceraldehyde-3-phosphate dehydrogenase), an important adhesin of S. suis, was significantly increased in the surface-associated and secreted fractions of the two mutant strains. Because increased adhesion of the mutant strains Δstk and Δstp1 to endothelial cells could be significantly inhibited by anti-GAPDH serum, we suppose that aberrant translocation of GAPDH due to deletion of the stk or stp1 gene contributed to increased interaction with host cells. The Δstk mutant showed reduced survival in macrophages, while the Δstp1 mutant showed increased survival probably as a result of increased capsule thickness. Enhanced hemolytic activity of the Δstk mutant could be due to increased secretion of suilysin. Both mutants exhibited reduced survival in pig whole blood and attenuated virulence to mice. Taken together, these results suggest that Stk and Stp1 can modulate S. suis cell division by post-translational modification of DivIVA, and regulate translocation of certain virulence factors, thereby benefiting its pathogenicity by compromising its interactions with the host.