Coarse-grained simulations of bacterial cell-wall mechanics and failure under extreme conditions

Bacterial cell walls have to contain high internal turgor pressures of ~1atm in gram-negative bacteria and >10 atm in gram-positive bacteria. At the same time the wall has to be continuously expanded while a bacterium grows. For most bacterial cell walls a covalently crosslinked polymer network, the peptidoglycan (PG) layer, provides mechanical toughness. The PG layer is a thin porous polymer network - made of rigid glycan strands crosslinked by flexible oligopeptides. Bacteria achieve mechanical toughness while the wall is growing by careful control of defect generation, material insertion and network repair mechanisms. Many antibiotics act by interfering with these mechanisms. In order to understand the mechanisms of mechanical wall failure under extreme challenges, we model the PG layer as an anisotropic elastic network composed of two types of nonlinear springs (glycans and oligopeptides) using parameters from E-coli. The model assigns different structural, linear and non-linear elastic properties to the network constituents: Glycan strands are rigid and long, while peptides are flexible and short. We characterize stress-strain relationships, anisotropy, pore size distributions, and failure susceptibility and geometry as a function of the crosslink density, length distribution of glycan strands and angular alignment.