Streptococcus serine protease Ssp and CPS

The cell envelope of gram-positive organisms serves as the primary defense barrier against noxious substances and environmental interactions. Despite the pivotal role of subtilisin-like serine proteases in<i> Streptococcus suis </i>(<i>S. suis</i>)<i> </i>pathogenicity, their involvement in maintaining the cell envelope and subsequent effects on stress defense and immune evasion remains inadequately underexplored. In this study, growth characterization showed that the Δ<i>sspA-</i>2 strain exhibited a slower growth rate than the parent strain SC-19, as assessed by absorbance and bacterial counts. Conversely, the Δ<i>sspA-1 </i>and<i> </i>Δ<i>sspA-1/2 </i>strains displayed accelerated growth rates based on bacterial counts. Scanning electron microscopy (SEM) morphology analysis revealed that Δ<i>sspA-1 </i>and<i> </i>Δ<i>sspA-1/2</i> tended to form short chains and exhibited an abnormal “rugby” shape, potentially resulting in increased bacterial counts under equivalent absorbance conditions. Δ<i>sspA-2, </i>on the other hand,<i> </i>tended to form oversized cells with longer and wider chains. Transmission electron microscopy (TEM) further elucidated that Δ<i>sspA-1 </i>and<i> </i>Δ<i>sspA-1/2 </i>exhibited incomplete capsules and thicker cell walls, while cell wall synthesis in Δ<i>sspA-2</i> was impaired.<i> </i>Moreover, cell infection assays demonstrated that the Δ<i>sspA-1 </i>and<i> </i>Δ<i>sspA-1/2</i> strains increased adhesion and invasion abilities while decreasing the anti-phagocytosis ability of <i>S. suis</i>, facilitating<i> </i>easier clearance. Stress assays revealed that both SspA-1 and SspA-2 are essential for <i>S. suis</i> survival in hypertonic, oxidation, low pH, and high-temperature environments, with a more pronounced role observed for SspA-2. Collectively, our findings highlight the pivotal roles of SspA-1 and SspA-2 in non-complement-mediated immune evasion and environmental adaption of <i>S. suis </i>through their involvement in maintaining the cell envelope.