The protective protein Sao (surface antigen one) is not a critical virulence factor for Streptococcus suis serotype 2.

Surface antigen one (Sao) is protein that was shown to protect pigs in vaccination trials against disease caused by Streptococcus suis. Here, we used an isogenic Δsao mutant to investigate the insofar unknown role of Sao in S. suis virulence. The Δsao mutant and its wild-type parent strain (sequence type 1, European strain) did not present significant differences on levels of adhesion/invasion of swine respiratory epithelial cells. Moreover, the Δsao mutant resisted phagocytosis by macrophages similarly to the parent strain and it was still virulent in a mouse model of infection. Our findings suggest a minor role of Sao in S. suis serotype 2 virulence.