SBP1 is an adhesion-associated factor without the involvement of virulence in Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen that infects swine and humans with high mortality and morbidity. Although a number of virulence-associated factors have been reported, the understanding of the molecular mechanism underlying SS2 pathogenicity remains limited. Our previous studies revealed that srtBCD-associated protein 2' (SBP2') contributed to the pathogenesis of SS2, but the function of another member in the srtBCD cluster, srtBCD-associated protein 1 (SBP1) was still unknown. Here, we found that sbp1 was widely distributed among high virulent SS2 strains, suggesting that sbp1 may be involved in the pathogenesis of SS2. To investigate the function of SBP1, we firstly conducted Western blotting analyses to confirm that SBP1 was expressed in the high virulent SS2 strain ZY05719 both in vivo and in vitro, then constructed the deletion mutant of sbp1 by homologous recombination. Bacterial adhesion assay, indirect immunofluorescence assay and protein binding assay all demonstrated that SBP1 was associated with adhesion of SS2 to HEp-2 cells. However, SBP1 did not influence the invasion, phagocytosis or intracellular survival of SS2. Furthermore, infection assays in vivo showed that inactivation of sbp1 failed to impair the ability of SS2 to cause zebrafish and mouse mortality. Overall, these results indicate that SBP1 is an adhesion-associated factor without the involvement of virulence in Streptococcus suis serotype 2.