The truncated major pilin subunit Sbp2 of the srtBCD pilus cluster still contributes to Streptococcus suis pathogenesis in the absence of pilus shaft.

Streptococcus suis serotype 2 (SS2) is an emerging zoonotic agent responsible for a number of infections in pigs and humans. Pili have been proposed as virulence factors in Gram-positive bacteria. However, due to the abolition of pili production, the function of the srtBCD pilus cluster, especially the truncated major pilin subunit Sbp2 (Sbp2', Sbp2″), has not been explored. In this study, isogenic mutants (Δsbp2', Δsbp2″) were constructed by homologous replacement in SS2 strain P1/7. Deletion of sbp2' attenuated the virulence in a zebrafish model as shown by more than an eightfold increase in the LD50 of Δsbp2', compared with that of the parent strain. In addition, the adhesion of Δsbp2' to HEp-2 cell monolayers decreased significantly. Compared with the parent strain, no obvious differences in virulence and adherence efficiency were observed for Δsbp2″. Our data suggest that Sbp2' could be involved in SS2 pathogenesis despite absence of its pilus shaft.