An analytical model describing the <i>E</i>. coli cell wall as a pressurized elastic cylinder

Bacteria have tough shells that allow them to withstand large turgor pressures. In gram-negative bacteria, such as E. coli, shell toughness stems from the ~4 nm thick peptidoglycan layer cushioned between inner and outer lipid membranes. Under physiological conditions, the mechanical situation is analogous to that of a thin-walled pressurized elastic balloon. We here introduce a simplified analytical model to rationalize our experiments that compress E. coli between parallel plates while monitoring shapes, forces and indentation depths. In general –given the molecular structure of the peptidoglycan layer– the elastic response is expected to be non-linear and anisotropic. We provide evidence that deformations due to simple osmotic swelling permit a quantitative reading of the anisotropy. Comparison to experimental data shows consistency and allowed us to fix further parameters of the model.