Curvature-Dependent Localization of the Bacterial Cytoskeleton Drives De Novo Morphogenesis in Escherichia Coli

Abstract

A peptidoglycan cell wall determines the shape of nearly all bacteria. The cell wall, along with the shape that it adopts, is crucial to cellular physiology. The mechanical integrity and morphology of the cell wall is determined by the spatiotemporal patterning of cell wall synthesis; therefore a rod-shaped cell such as Escherichia coli faces the challenge of coordinating the nanoscale proteins responsible for peptidoglycan synthesis to construct a micron-scale sacculus. What are the principles that allow cell wall synthesis proteins to establish order over a range of length scales spanning nearly three orders of magnitude? We approached this question by examining the process of reversion in cell-wall-deficient ‘L-forms’ of E. coli, in which cell-wall synthesis has disrupted by beta-lactam antibiotics. An L-form undergoing reversion begins in a spherical shape without an intact cell wall. When cell wall synthesis inhibiting antibiotics are removed, the cell generates newrod-shaped protrusions, which eventually undergo septation and adopt the normal rod morphology of E. coli. The reversion of L-forms thus provides on opportunity to study morphogenesis in bacteria lacking an intact cell wall. We therefore investigated the morphological dynamics of the reversion process in L-forms of E. coli, and simultaneously imaged the localization of MreB, a bacterial actin required for the rod-like morphology of many bacteria. We found that MreB localizes to negatively curved regions of the cell (e.g. invaginations), and furthermore targets cell wall synthesis to those regions. Our results therefore suggest a model in which the localization of MreB in response to geometric cues is crucial to morphogenesis in E. coli.