Gram-negative bacteria live in virtually all ecosystems on earth and comprise a wide range of important environmental and host associated species. Their evolutionary success is based upon the robustness of their cell envelope, which contains an outer membrane and a thin, single-layered cell wall layer made of peptidoglycan, located in the periplasm around the cytoplasmic membrane. E. coli uses some of its most abundant proteins to tightly connect the outer membrane with the peptidoglycan layer, which is crucial to maintain the permeability barrier function of the outer membrane. Mutants lacking a strong outer membrane-peptidoglycan linkage shed vesicles into the environment and have a 'leaky' outer membrane. The peptidoglycan-outer membrane 'superstructure' protects the cell from bursting due to its internal osmotic pressure (turgor, 0.3-7 atm; species/condition dependent).
Growing and dividing cells remodel their peptidoglycan by the coordinated activities of synthases and hydrolases, which insert nascent peptidoglycan into the existing layer and remove old material. At the same time, lipopolysaccharide (LPS), outer membrane proteins (OMPs, porins) and phospholipids are transported through the meshes in the peptidoglycan net for outer membrane biogenesis. We poorly understand how the cell expands its cell envelope layers together during growth, how it coordinates the multiple biosynthetic activities and transport processes, and how it maintains the robustness of the cell envelope under stress conditions.
We have recently discovered that E. coli inserts OMPs into the outer membrane near peptidoglycan growth sites, providing the first mechanism that spatially coordinates outer membrane biogenesis with peptidoglycan growth. In another line of research, we have investigated how outer membrane assembly stress affects the integrity and structure of the peptidoglycan layer, leading to the discovery of a peptidoglycan repair mechanism and several novel peptidoglycan-related enzymes that become important under stress conditions. Our research revealed that Gram-negative bacteria fortify their cell envelope during growth and under stress. The newly discovered mechanisms might offer target sites for novel antibiotics to address the global challenge of antimicrobial resistance.