Distribution of mechanical stress in the Escherichia coli cell envelope

Abstract

The cell envelope in Gram-negative bacteria comprises two distinct membranes with a cell wall between them. There has been a growing interest in 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).

A Minimal Membrane Metal Transport System: Dynamics and Energetics of mer Proteins

“…We first constructed two systems, one with and one without Hg 2+ at the Cys21/Cys22 binding site, in a 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphorylcholine (DMPC) membrane, which was used for reconstitution of the protein in experiments . To investigate the effect of lipid composition on the dynamic properties of the protein, we additionally built two more systems for each with a “Top6” membrane, which contains the top six lipids found in the E. coli inner membrane . Lastly, we modeled MerFt (PDB:2H3O), a truncated 60‐residue construct spanning the integral membrane helix–loop–helix region of MerF, embedded in a DMPC membrane.…”

“…The initial area of the membrane was 30 nm × 30 nm for all the systems, requiring 2517 (CG15), 2401 (CG25), and 2314 (CG35) lipids. An approximately 3:1 ratio of POPE:POPG lipids was used, which is a simple mimic of a bacterial cytoplasmic membrane . The system was solvated with ∼20 Å of water on both sides of the membrane (36–37,000 water beads), and 581 (CG15), 521 (CG25), or 461 (CG35) Na + ions were added to neutralize the systems.…”

Model-Driven Controlled Alteration of Nanopillar Cap Architecture Reveals its Effects on Bactericidal Activity

“…According to our estimation, this surface induced tension in the range of 155 to 592 mN/m at the maximal cover and at the minimum of the free energy, the tension was in the range of 5 to 203 mN/m. The reported critical tension for the bacteria membrane's rupture is between 30 and 75 mN/m [24,33], values which can be exceeded by the induced tension in the envelope for the CS nanopillars. These results are in agreement with our experimental observations and support the hypothesis that stretching of the cell envelope over the gaps leads to mechanical stress, which can cause rupturing of the bacterial membrane in the suspended region and subsequently lead to lysis.…”

“…The bacterial envelope is a complex multilayered visco-elastic anisotropic medium [18,23,24]. The viscous properties of the envelope are intimately linked with bacterial growth; in other words, bacteria and consequently, the cell envelope, behave elastically in the absence of growth or on a short time scale compared to the cell doubling time [25].…”

“…The electron micrographs shown in Figure 2 confirm the absence of biofilm formation. Additionally, Hwang et al [24] reported linear elastic behavior of the bacterial envelope with negligible strain stiffening up to 50% of the area expansion. While it was reported that anisotropy of the rod-like bacteria's envelope, like that of P. aeruginosa, is essential to explaining their observed behavior during growth and cell division [23,26], the reported anisotropy [27] is much smaller in scale than the observed variation of the overall bacterial envelope stiffness [28].…”