Gram-negative bacteria possess a complex cell envelope that consists of a plasma membrane, a peptidoglycan cell wall and an outer membrane. The envelope is a selective chemical barrier1 that defines cell shape2 and allows the cell to sustain large mechanical loads such as turgor pressure3. It is widely believed that the covalently cross-linked cell wall underpins the mechanical properties of the envelope4,5. Here we show that the stiffness and strength of Escherichia coli cells are largely due to the outer membrane. Compromising the outer membrane, either chemically or genetically, greatly increased deformation of the cell envelope in response to stretching, bending and indentation forces, and induced increased levels of cell lysis upon mechanical perturbation and during L-form proliferation. Both lipopolysaccharides and proteins contributed to the stiffness of the outer membrane. 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 stiffer than the cell wall, and that mechanical loads are often balanced between these structures.
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The authors thank members of the Huang, Theriot, and Weibel laboratories for discussions, K. Amberg-Johnson for assistance with immunoblotting, and T. Silhavy, T. Bernhardt, A. Maurelli, B. Hammer and H. Arjes for feedback, antibodies and strains. This work was supported by National Institutes of Health (NIH) Director’s New Innovator Awards DP2OD006466 (to K.C.H.) and DP2OD008735 (to D.B.W.), National Science Foundation (NSF) CAREER Award MCB-1149328 (to K.C.H.), the Stanford Systems Biology Center funded by NIH grant P50 GM107615 (to K.C.H. and J.A.T.), NIH Grant R37-AI036929 (to J.A.T.), the Howard Hughes Medical Institute (to J.A.T.), and NSF Grant DMR-1121288 (to D.B.W.). K.C.H. is a Chan Zuckerberg Biohub Investigator. E.R.R. was supported by a postdoctoral fellowship from the Simbios Center for Physics Based Computation at Stanford University under NIH Grant U54 GM072970. P.D.O. was supported by a postdoctoral fellowship from the Swiss National Science Foundation under Grant P2ELP3_172318. This work was also supported in part by the NSF under Grant PHYS-1066293, the hospitality of the Aspen Center for Physics and by the Allen Discovery Center program through The Paul G. Allen Frontiers Group.
Nature thanks J.-F. Collet, W. Margolin, N. Minc and A. Rutenberg for their contribution to the peer review of this work.