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Nonlinear Mechanics of the Bacterial Cell Wall


Despite the relentless study of the Gram-negative bacterium Escherichia coli that began many decades ago and has continued to the present, the understanding of its mechanics remains weak. Technological advances in microfluidic devices and the use of atomic force microscopy (AFM) as a tool to measure mechanical properties of nanometer scale objects have pushed the field forward. However, in the latter case it remains difficult to translate force-deflection curves acquired through AFM into meaningful properties when the material is anisotropic, which is the case for the peptidoglycan (PG) cell wall in E. coli. The situation is further complicated by the cell's turgor pressure, which has not been reliable measured and may frequently undergo changes on the order of several atmospheres. Nevertheless, these difficulties provide opportunities for theoretical and computational mechanics researchers to help close the gaps in understanding. In this thesis we start by developing analytical and computational composite thin shell models that recognize the inner membrane as an additional stress-bearing structure within the cell envelope. These models are also utilized to investigate the anisotropic material properties of the cell wall. Next, an osmotic transport model is constructed to search for reasons why cell lysis occurs on short time scales during a rapid change in external osmolarity known as osmotic shock. Finally, we investigate the active mechanics of a growing cell wall by developing a finite growth kinematics model. New PG material is inserted into the cell wall in thin strips, which leads to a highly disordered stress field littered with stress concentrations. We study the interactions of these defects in the cell wall and demonstrate how geometric and material nonlinearity affect them.

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