In Silico Measurement of the Mechanical Properties of Realistic Bacterial Inner and Outer Membranes

Abstract

The cell envelope in Gram-negative bacteria is made of two distinct membranes and a cell wall between them. From a mechanics point of view, the cell maintains a higher concentration of solutes in the cytoplasm than in the external environment and the difference in osmotic pressure, known as the turgor pressure, places a great stress on the envelope, although which element bears the most is unclear. Now, we have used molecular dynamics (MD) simulations of membranes to resolve how lipid membranes respond to changes in lateral tension resulting from the turgor pressure. In this study, we used two models of the inner, cytoplasmic membrane; the first membrane is modeled as a mixed 75%POPE/25% POPG bilayer while the second membrane model consists of saturated, unsaturated, and cycle-containing lipids that more accurately reflect the diverse population of lipids within the E. coli cytoplasmic membrane. Additionally, we looked at the bacterial outer membrane, which has an outer leaflet of lipopolysaccharides that stiffen it. We applied surface tensions of values from 10 to 200 dynes/cm, in increments of 10 dynes/cm, and measured a variety of properties of membranes (area per lipid, thickness, etc.), thus providing a quantitative description of the requirements of membrane rupture. More general mechanical properties of membranes were also characterized, namely the elastic area compressibility modulus, the energy change, and Young's modulus, in order to describe the elasticity of membrane. Our work demonstrates that differences in lipid composition result in a differential response to lateral tension.