The outer membrane is an essential load-bearing element in Gram-negative bacteria

Gram-negative bacteria possess a complex cell envelope consisting of a plasma membrane, a peptidoglycan cell wall, and an outer membrane. The envelope is a selective chemical barrier(1) that defines cell shape(2) and allows the cell to sustain large mechanical loads such as turgor pressure(3). It is widely believed that the covalently cross-linked cell wall grants the envelope its mechanical properties(4,5). Here, we demonstrate that the stiffness and strength of Escherichia coli cells are largely due to the outer membrane. Compromising the outer membrane, chemically or genetically, greatly increased deformation of the cell envelope in response to stretching, bending, and indentation forces, and induced elevated levels of cell lysis upon mechanical perturbation and L-form proliferation. Both lipopolysaccharides and proteins contributed to outer membrane stiffness. These findings overturn the prevailing dogma that the cell wall is the dominant mechanical element within Gram-negative bacteria, instead demonstrating that the outer membrane can be more stiff than the cell wall and that mechanical loads are often balanced between these structures.