Contribution of a LysM domain-containing protein regulated by VicRK to <i>streptococcus sanguinis</i> virulence

<i>Streptococcus sanguinis</i> is a commensal member of the oral microbiome involved in opportunistic cardiovascular infections. In the present study, we investigated the contribution of <i>ssa_0094</i>, a gene strongly regulated by the two-component system VicRK, to functions associated with biofilm formation, immune evasion and cardiovascular virulence. <i>In silico</i> analysis showed that <i>ssa_0094</i> encodes a protein with a LysM domain, which is highly conserved among <i>S. sanguinis</i>. Although not an ubiquitous gene, several commensal streptococcal species of the oronasopharynx and zoonotic strains of <i>Streptococcus suis</i> harbour <i>ssa_0094</i> homologues. A <i>ssa_0094</i> isogenic mutant (SK0094) showed defects in initiating biofilms on saliva-coated surfaces, reduced hydrophobicity and lower production of amyloid-like components when compared to the parent strain (SK36) or to the complemented mutant (SK0094+), although it showed mild changes in DNA release and production of H₂O₂. Deletion of <i>ssa_0094</i> also impaired <i>S. sanguinis</i> binding to multiple human glycoproteins of plasma and/or extracellular matrix (ECM) (plasminogen, fibronectin, fibrinogen, fibrin, type I collagen and elastin), and promoted clear increases in C3b deposition, and in induction of NEtosis by neutrophils of peripheral blood. Moreover, SK0094 showed impaired invasiveness into HCAEC cells and reduced <i>ex vivo</i> persistence in human blood, but no clear change in virulence in a <i>Galleria mellonella</i> infection model. These findings indicate that <i>ssa_0094</i> is highly conserved within <i>S. sanguinis</i> strains required for biofilm initiation as well as for multiple functions of immune evasion and cardiovascular virulence in <i>S. sanguinis</i> in a host-specific fashion.

血链球菌（*Streptococcus sanguinis*）是口腔微生物组的共生成员，可引发机会性心血管感染。本研究探讨了受双组分系统VicRK严格调控的基因*ssa_0094*在生物膜形成、免疫逃逸及心血管毒力相关功能中的作用。计算机模拟（In silico）分析显示，*ssa_0094*编码一种带有LysM结构域（LysM domain）的蛋白质，该结构域在血链球菌中高度保守。尽管该基因并非普遍存在，但口咽腔的多种共生链球菌属物种以及猪链球菌（*Streptococcus suis*）的人畜共患菌株均携带*ssa_0094*的同源基因。与亲本菌株SK36及互补突变株SK0094+相比，*ssa_0094*同基因缺失突变株（SK0094）在唾液包被表面启动生物膜形成的能力存在缺陷，疏水性降低，类淀粉样成分的生成量减少，不过其DNA释放与过氧化氢（H₂O₂）生成仅出现轻微变化。删除*ssa_0094*还会削弱血链球菌与血浆及/或细胞外基质（ECM）的多种人类糖蛋白——包括纤溶酶原、纤连蛋白、纤维蛋白原、纤维蛋白、I型胶原蛋白及弹性蛋白——的结合能力，并显著促进C3b沉积以及外周血中性粒细胞发生中性粒细胞胞外诱捕（NEtosis）。此外，SK0094侵袭人冠状动脉内皮细胞（HCAEC）的能力受损，离体（ex vivo）状态下在人血液中的存留能力下降，但在大蜡螟（*Galleria mellonella*）感染模型中的毒力未出现明显变化。本研究结果显示，*ssa_0094*在血链球菌菌株中高度保守，且对于血链球菌的生物膜起始形成、以及以宿主特异性方式发挥的多项免疫逃逸与心血管毒力功能均具有重要作用。