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

Abstract

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.

Pseudomonas fluorescensMFE01 delivers a putative type VI secretion amidase that confers biocontrol against the soft‐rot pathogen Pectobacterium atrosepticum

“…To better visualize the cellular consequences of MFE01 on target cells, we performed fluorescence microscopy experiments using a strain of E. coli harbouring plasmid pTHV037dsbA‐SP‐msfTq2O_sol‐Cytomcherry as recipient. This strain produces mCherry in the cytoplasm and sfTurquoise in the periplasmic space, allowing to differentiate the two compartments (Deghelt et al, 2023). Time‐lapse fluorescence microscopy recordings showed that upon MFE01 T6SS firing, the periplasm of the attacked cell collapsed at one pole of the cell, eventually reached the other pole, before cell rounding (Figure 3E, Videos S5 and S6).…”

Thicket and Mesh: How the Outer Membrane Can Resist Tension Imposed by the Cell Wall

“…Combining these suggests an expected K A of 535 dyn/cm for the CW 1.0× system, 775 dyn/cm for the CW 2.0× system, and 1575 dyn/cm for the CW 2.5× system, all lower than what we measured here by 300−800 dyn/cm albeit with a similar upward trend. However, we note that Lpp, which covalently links OM and CW together and was included here, also plays a role in stiffening the cell envelope, 18,20,29 and its contribution may not be purely additive as it affects both the OM and CW. 18 The OM has become recognized as a major contributor to the bacterial cell's mechanical stability under the load imposed by turgor pressure.…”

“…Furthermore, in Escherichia coli and related bacteria, the OM and CW are covalently linked by Braun's lipoprotein (Lpp), 21,27,28 which contributes to the stiffness of the combined system as well. 18,20,29 In the present work, to account for the combination of OM, CW, and Lpp, we have constructed and simulated multiple OM and CW systems linked by Lpp. In particular, we initialized the CW under different degrees of strain to quantify the competition between it and the OM.…”

“…This conclusion gives more context to the finding that OM plays an essential role in load-bearing, yet also supports the classical notion that the CW is the dominant element, at least in extreme circumstances. Furthermore, in Escherichia coli and related bacteria, the OM and CW are covalently linked by Braun’s lipoprotein (Lpp), ,, which contributes to the stiffness of the combined system as well. ,, …”

Meeting report ‘Microbiology 2023: from single cell to microbiome and host’, an international interacademy conference in Würzburg

“…He highlighted that while peptidoglycan is essential, it is not sufficient for survival under osmotic stress. Collet proposed that attaching peptidoglycan to the outer membrane forms a mechanical unit that allows periplasmic pressure buildup, thus balancing turgor pressure across the inner membrane and preventing cell lysis (Asmar et al 2017 , Deghelt et al 2023 ).…”