轉糖鏈球菌對於抗生素預防感染性心內膜炎的抵抗性

Abstract

轉糖鏈球菌(Streptococcus mutans)屬於兼性厭氧革蘭氏陽性菌，存在於人體的口腔之中，當它們有機會進入血液中，形成暫時性菌血症時，便可能經由血液循環進入心臟，進而黏附於先天有缺陷的心臟瓣膜或是人工心瓣膜上，形成贅疣(vegetation)，往感染性心內膜炎(infective endocarditis)發展。以往在預防感染性心內膜炎時，美國心臟協會(AHA)將 β-lactam類藥物作為首選藥物，但在本實驗室先前的研究發現，在大鼠感染性心內膜炎模式中，使用盤尼西林預防轉糖鏈球菌引起的感染性心內膜炎並不會降低菌血症的發生。另外，血小板和轉糖鏈球菌會形成凝集構造，此凝集構造在本實驗室之前研究顯示可以有助於轉糖鏈球菌抵抗全血中嗜中性球的殺菌。因此，本實驗主要探討存在血液中的血小板和嗜中性球是否和盤尼西林有交互作用，導致轉糖鏈球菌對於抗生素的術前預防有抵抗性。本實驗結果發現轉糖鏈球菌在富含血小板的血漿(PRP)和血小板貧乏的血漿(PPP)中可以提升轉糖鏈球菌對盤尼西林的最低抑菌濃度(MIC)以及最低殺菌濃度(MBC)，並且，在盤尼西林殺菌能力試驗中，當低濃度的盤尼西林存在時，轉糖鏈球菌在富含血小板的血漿中的存活率比在血小板貧乏的血漿中來得高，但是當高濃度的盤尼西林存在時，轉糖鏈球菌在富含血小板的血漿中的存活率比在血小板貧乏的血漿中來得高的現象變得不明顯或是消失，而且，利用高效液相層析儀(HPLC)測定富含血小板的血漿和血小板貧乏的血漿中的游離態盤尼西林發現沒有差異，另外，盤尼西林的存在並不會影響血小板和轉糖鏈球菌形成凝集構造，在嗜中性球殺菌試驗中亦不會影響嗜中性球的殺菌能力。因此，血小板和嗜中性球對於幫助轉糖鏈球菌抵抗抗生素可能不是很主要的因子。其他可能導致轉糖鏈球菌對於抗生素術前預防有抵抗性的因素仍需進一步探討。

Streptococcus mutans is one of the principal dental caries causative agents and an opportunistic pathogen of infective endocarditis (IE). S. mutans can enter the bloodstream through the dental procedure to induce transient bacteremia which is the early indicator of the IE. Antibiotics prophylaxis is routinely recommended at the time of dental procedures in patients deemed to be at risk of infective endocarditis. However, no clear benefit of the traditional approach for antibiotic prophylaxis has been reported. In our previous study indicated that penicillin prophylaxis did not reduce the bacteremia caused by S. mutans in the IE rat model. Therefore, the resistant mechanism of S. mutans against antibiotic killing effect was evaluated in in vitro killing assay and in vivo bacteremia rat model. In whole blood, platelets and neutrophils have the bactericidal effect. The minimum inhibitory concentration and minimum bactericidal concentration of penicillin for S. mutans in platelet-rich plasma (PRP) and platelet-poor plasma (PPP) were higher than in medium control. In addition, S. mutans in PRP showed higher survival rates than PPP in low-dose penicillin killing assay suggested that platelets might help S. mutans escape the killing of antibiotics. But, S. mutans in PRP did not clearly showed higher survival rates than PPP in high-dose penicillin killing assay. The concentration of free-form penicillin after incubated with PRP or PPP was similar demonstrated that permeability of penicillin into platelet was not involved in this antibiotics resistant mechanism. The morphology of bacteria-platelet aggregate varied in penicillin treatment was observed in immuno-florescence staining. On the other hand, penicillin could not change the bactericidal ability of neutrophils. In conclusion, neither the platelets nor the neutrophils were involved in S. mutans resistant to penicillin prophylaxis.