The outer membrane and peptidoglycan layer form a single mechanical device balancing turgor

Bacteria are subject to a substantial concentration differential of osmolytes between the interior and exterior of the cell, which results in cytoplasmic turgor pressure. Failure to mechanically balance turgor pressure causes cells to burst. Here, we show that in Gram-negative bacteria, the outer membrane and peptidoglycan layer function together to resist turgor: when attached to each other, these two layers form a robust mechanical unit that allows pressure build-up in the periplasmic compartment, which in turn balances cytoplasmic turgor across the inner membrane, preventing cell death. Thus, the peptidoglycan layer is necessary but not sufficient to maintain turgor, which challenges the general view that protecting cells from bursting is the specific task of the peptidoglycan cell wall. ONE-SENTENCE SUMMARY The peptidoglycan and outer membrane are interconnected layers that cooperate to balance cytoplasmic turgor. ### Competing Interest Statement The authors have declared no competing interest.