轉糖鏈球菌與血小板形成團聚構造提升在血液循環中的存活率

Abstract

轉糖鏈球菌(Streptococcus mutans)為草綠色鏈球菌屬(viridans streptococci)，是人類口腔中常見的菌叢，會造成牙菌斑及齲齒，平時並不會對人體造成傷害，當口腔因外科手術或咀嚼造成傷口後，細菌就有機會進入血液中，形成短暫性菌血症，若再加上缺陷的心臟瓣膜或是人工心瓣膜，細菌就有機會黏附住瓣膜形成贅生物，有機會發展為感染性心內膜炎(infective endocarditis)。因此細菌在血液循環中抵抗先天性免疫反應並存活，是成為感染性心內膜炎最重要的致病機轉之一。先前的研究已知轉糖鏈球菌可以藉由其表面的一些分子直接或間接活化血小板或活化凝血機轉，並有抗體及纖維蛋白（fibrin）的參與。故本實驗即假設轉糖鏈球菌在進入血液循環後，會團聚血小板，並藉此團聚作用抵抗先天免疫反應，增加其在血液中的存活率。從實驗結果證實在血液循環中，轉糖鏈球菌確能與血小板形成團聚，而且在動物體內模式及體外人血模式下都發現類似的團聚現象。從全血殺菌能力測試(whole blood killing assay)中也發現，若添加血小板抑制劑阿斯匹靈（aspirin），轉糖鏈球菌在全血內的存活率明顯下降；若先將轉糖鏈球菌與血小板濃厚血漿(platelet rich plasma) 培養，可發現轉糖鏈球菌在全血殺菌能力測試中的存活率明顯上升，相同的現象也在白血球殺菌試驗中被發現，顯示血小板在細菌抵抗先天免疫反應的作用上扮演了一部份的角色。同樣的給與大鼠注射阿斯匹靈以後，會增進轉糖鏈球菌在血液循環中被清除的數量。此外血小板萃取物會刺激細菌，提升轉糖鏈球菌在血液中的存活率，並可能被細菌代謝利用，以促進細菌生長。但在以往研究也發現血小板能夠促進嗜中性球的殺菌能力，而在我的系統下，血小板與嗜中性球是有很高的機會可以進行交互作用。這樣的結果也顯示在血液循環中的血小板對於細菌性感染可扮演促進清除細菌的角色，也能扮演幫助轉糖鏈球菌抵抗免疫系統的角色。

Streptococcus mutans, a Gram-positive bacterium, is one of the pathogens of dental caries and also an opportunistic pathogen of infective endocarditis (IE). Dissemination of Streptococcus mutans into the bloodstream is known to be induced bacteremia caused during dental surgery or oral trauma. Circulating bacteria may escape host immune surveillance and subsequently adhere to the damaged valve to form vegetation (fibrin-platelet bacterial biofilm). Our laboratory demonstrated previously that Streptococcus mutans could directly bind to platelets and induce platelet aggregation. In this study, I demonstrated found that upon entering circulation in rats, Streptococcus mutans interact with platelets and form aggregation with platelets. Similar results were also observed in human blood in vitro and the bacteria survival rates were reduced by platelet activation inhibitor, aspirin. Preincubating with platelet rich plasma, Streptococcus mutans could form aggregation with platelets and also increase survival rates in whole blood or neutrophil killing; but reduce survival rate by aspirin. After the injection of aspirin in vivo, Streptococcus mutans clearance in bloodstream was also enhanced. These results suggested that bacteria aggregation with platelets is important in the bacteria resistance to host immune surveillance. But platelets sonication supernatant induced growth enhancement of Streptococcus mutans occurs in GS5 and clinical-isolate strain. These results suggest that in blood circulation platelets could play dual roles in bacterial infection: Streptococcus mutans subvert platelet function to help bacteria survival; platelets enhance bacteria clearance through neutrophils