Modeling the peptidoglycan layer of gram-negative bacteria as an anisotropic, elastic network composed of two types of nonlinear springs

Bacteria mechanically protect themselves by covalently linked peptidoglycan (PG) cell walls that preserve cellular morphology and contain high osmotic pressures. As a bacterium grows, the cell wall undergoes continuous expansion. Despite a good understanding of the molecular components and the assembly machineries of the cell wall, it remains largely unknown how the mesoscopic mechanical properties of the cell wall emerge from the properties and arrangement of molecular components. Here, we introduce a quantitative physical model of the bacterial cell wall, based on molecular details, that predicts the mesoscopic mechanical response of the cell wall for the Gram-negative bacterium Escherichia coli. We modeled the PG layer as an anisotropic elastic network composed of two types of nonlinear springs (glycans and oligopeptides). We vary structural properties such as glycan length distribution, angular distribution, and cross-link density (pore size distribution) to accurately reproduce observed mechanical properties such as stress-strain relationships (elastic moduli), strain and stress ratios between axial and hoop directions.