MreB mechanically couples the cytoplasmic membrane to the cell wall in Escherichia coli

MreB is a prokaryotic homolog of actin that is present in many rod-shaped bacteria and plays a key role in cell shape maintenance. MreB filaments orient the insertion of cell-wall precursors via circumferential movement along with the cell-wall synthesis machinery. How mechanical forces are involved in the localization and function is largely unknown. In this study, we probed the mechanical contribution of MreB to the cell wall with a microfluidic hyperosmotic shock assay combined with treatment of A22, an MreB-targeting antibiotic. Interestingly, disruption of MreB filaments did not alter cell wall deformation under moderate osmotic shocks, suggesting that MreB does not directly strengthen the cell wall. In the absence of A22, repeated large osmotic shocks led to gradual reduction in cytoplasmic contraction under hyperosmolar conditions, whereas depolymerizing MreB allowed cells to freely plasmolyze, which together indicate that MreB mechanically couples the cytoplasmic membrane and the cell wall. GFP labeling of MreB confirmed that relocation of MreB to the cell poles was key to such phenotypical differences. Based on these data, we propose a physical model in which MreB, together with the other components of the cell-wall synthesis machinery, serves as the major linkage between the cytoplasmic membrane and the cell wall.