Interfaculty Institute of Microbiology and Infection Medicine, University of Tübingen, Elfriede-Aulhorn-Str. 6, 72076 Tübingen, Germany.
Cell Mol Life Sci. 2010 Oct;67(19):3331-44. doi: 10.1007/s00018-010-0384-9. Epub 2010 May 11.
Enterococci are commensal organisms in the alimentary tract. However, they can cause a variety of life-threatening infections, especially in nosocomial settings. We hypothesized that induction of cell death might enable these facultative pathogenic bacteria to evade the innate immune response and to cause infections of their host. We demonstrate that E. faecium when exposed to lysozyme induces cell death in macrophages in vitro and in vivo. Flow cytometric analyses of J774A.1 macrophages infected with E. faecium revealed loss of cell membrane integrity indicated by uptake of propidium iodide and decrease of the inner mitochondrial transmembrane potential DeltaPsi(m). Inhibition of caspases, treatment of macrophages with cytochalasin D, or rifampicin did not prevent cells from dying, suggesting cell death mechanisms that are independent of caspase activation, bacterial uptake, and intracellular bacterial replication. Characteristics of necrotic cell death were demonstrated by both lack of procaspase 3 activation and cell shrinkage, electron microscopy, and release of lactate dehydrogenase. Pretreatment of E. faecium with lysozyme and subsequently with broad spectrum protease considerably reduced cell death, suggesting that a bacterial surface protein is causative for cell death induction. Moreover, in a mouse peritonitis model we demonstrated that E. faecium induces cell death of peritoneal macrophages in vivo. Altogether, our results show that enterococci, under specific conditions such as exposure to lysozyme, induce necrotic cell death in macrophages, which might contribute to disseminated infections by these facultative pathogenic bacteria.
肠球菌是肠道内的共生菌。然而，它们也可能引发多种危及生命的感染，尤其是在医院环境中。我们假设，细胞死亡的诱导可能使这些兼性致病菌逃避先天免疫反应，并导致宿主感染。我们证明，粪肠球菌在接触溶菌酶时，会在体外和体内诱导巨噬细胞发生细胞死亡。对感染粪肠球菌的 J774A.1 巨噬细胞进行流式细胞分析显示，细胞膜完整性丧失，表现为碘化丙啶摄取增加和线粒体跨膜电位 ΔPsi(m) 降低。细胞凋亡抑制剂、巨噬细胞用细胞松弛素 D 或利福平处理均不能阻止细胞死亡，这表明细胞死亡机制不依赖于半胱天冬酶激活、细菌摄取和细胞内细菌复制。缺乏前半胱天冬酶 3 激活和细胞皱缩、电子显微镜以及乳酸脱氢酶释放等特征表明发生了坏死性细胞死亡。溶菌酶预处理粪肠球菌，随后用广谱蛋白酶处理，可显著减少细胞死亡，表明细菌表面蛋白是诱导细胞死亡的原因。此外，在小鼠腹膜炎模型中，我们证明粪肠球菌在体内诱导腹膜巨噬细胞发生细胞死亡。总之，我们的研究结果表明，肠球菌在特定条件下，如暴露于溶菌酶时，会诱导巨噬细胞发生坏死性细胞死亡，这可能有助于这些兼性致病菌的播散性感染。
