School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA; Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
Centre for Bacterial Cell Biology, Institute for Cell and Molecular Biosciences, The Medical School, Newcastle University, Newcastle upon Tyne NE2 4HH, United Kingdom.
Biochim Biophys Acta Biomembr. 2018 Dec;1860(12):2566-2575. doi: 10.1016/j.bbamem.2018.09.020. Epub 2018 Sep 29.
The cell envelope in Gram-negative bacteria comprises two distinct membranes with a cell wall between them. There has been a growing interest in understanding the mechanical adaptation of this cell envelope to the osmotic pressure (or turgor pressure), which is generated by the difference in the concentration of solutes between the cytoplasm and the external environment. However, it remains unexplored how the cell wall, the inner membrane (IM), and the outer membrane (OM) effectively protect the cell from this pressure by bearing the resulting surface tension, thus preventing the formation of inner membrane bulges, abnormal cell morphology, spheroplasts and cell lysis. In this study, we have used molecular dynamics (MD) simulations combined with experiments to resolve how and to what extent models of the IM, OM, and cell wall respond to changes in surface tension. We calculated the area compressibility modulus of all three components in simulations from tension-area isotherms. Experiments on monolayers mimicking individual leaflets of the IM and OM were also used to characterize their compressibility. While the membranes become softer as they expand, the cell wall exhibits significant strain stiffening at moderate to high tensions. We integrate these results into a model of the cell envelope in which the OM and cell wall share the tension at low turgor pressure (0.3 atm) but the tension in the cell wall dominates at high values (>1 atm).
革兰氏阴性菌的细胞包膜由两层截然不同的膜组成，它们之间是细胞壁。人们越来越感兴趣的是了解这种细胞包膜如何适应渗透压（或膨压），渗透压是由细胞质和外部环境之间溶质浓度的差异产生的。然而，细胞壁、内膜（IM）和外膜（OM）如何通过承受由此产生的表面张力来有效保护细胞免受这种压力，从而防止内膜膨出、细胞形态异常、原生质体和细胞裂解，这一点仍未得到探索。在这项研究中，我们使用分子动力学（MD）模拟结合实验来解决 IM、OM 和细胞壁模型如何以及在多大程度上响应表面张力变化。我们从张力-面积等温线上计算了所有三个组件的面积压缩模量。还使用模拟 IM 和 OM 单个小叶的单层实验来表征它们的可压缩性。虽然膜在膨胀时会变得更软，但细胞壁在中等至高张力下会表现出明显的应变硬化。我们将这些结果整合到一个细胞包膜模型中，其中 OM 和细胞壁在低膨压（0.3 atm）下分担张力，但细胞壁中的张力在高值（>1 atm）下占主导地位。
