Outer Membrane Lipid Secretion and the Innate Immune Response to Gram-Negative Bacteria

Abstract

The outer membrane (OM) of Gram-negative bacteria is an asymmetric lipid bilayer that consists of inner leaflet phospholipids and outer leaflet lipopolysaccharides (LPS). The asymmetric character and unique biochemistry of LPS molecules contribute to the OM’s ability to function as a molecular permeability barrier that protects the bacterium against hazards in the environment. Assembly and regulation of the OM have been extensively studied for understanding mechanisms of antibiotic resistance and bacterial defense against host immunity; however, there is little knowledge on how Gram-negative bacteria release their OMs into their environment to manipulate their hosts. Discoveries in bacterial lipid trafficking, OM lipid homeostasis, and host recognition of microbial patterns have shed new light on how microbes secrete OM vesicles (OMVs) to influence inflammation, cell death, and disease pathogenesis. Pathogens release OMVs that contain phospholipids, like cardiolipins, and components of LPS molecules, like lipid A endotoxins. These multiacylated lipid amphiphiles are molecular patterns that are differentially detected by host receptors like the Toll-like receptor 4/myeloid differentiation factor 2 complex (TLR4/MD-2), mouse caspase-11, and human caspases 4 and 5. We discuss how lipid ligands on OMVs engage these pattern recognition receptors on the membranes and in the cytosol of mammalian cells. We then detail how bacteria regulate OM lipid asymmetry, negative membrane curvature, and the phospholipid-to-LPS ratio to control OMV formation. The goal is to highlight intersections between OM lipid regulation and host immunity and to provide working models for how bacterial lipids influence vesicle formation.

Bacterial-Derived Outer Membrane Vesicles are Potent Adjuvants that Drive Humoral and Cellular Immune Responses

“…It is possible that OMVs presented specific PAMPs, such as lipopolysaccharide or surface lipoproteins, to the DCs in a more concentrated manner or that OMVs were better able to penetrate the cellular membrane, triggering both extracellular and cytosolic sensors, which enhanced DC activation. OMVs have been shown to signal through both TLR4 and caspase-11 [18,28], affirming the ability of OMV nanoparticles to access and stimulate both external and internal sensing pathways. Examination of the pathways responsible for OMV-mediated adjuvanticity was beyond the scope of the current study, and further work is needed to elucidate which pathways are essential for adjuvant function.…”

The composition and function ofEnterococcus faecalismembrane vesicles

“…In addition, it is possible that MVassociated unmethylated prokaryotic DNA is recognized by TLR9 receptors on host immune system cells leading to NF-kB activation [58,59]. Finally, bacterial lipids can engage pattern recognition receptors on host cell membranes to control inflammation and immunity by interacting with TLR2 and TLR4 receptors [60,61]. Whether lipids present within enterococcal MVs play a role in immune modulation remains unclear.…”

Conserved tandem arginines for PbgA/YejM allowSalmonellato regulate LpxC and control lipopolysaccharide biogenesis during infection

“…In the resource allocation model, the ability to negatively regulate LpxC and lipid A-core biogenesis serves as a mechanism by which enterobacteriaceae free up fatty acid and N-acetyl glucosamine substrates for use in other metabolic pathways (30,39,40,42,43). Instead of a strategy to preserve key metabolites, enterobacterial downregulation of lipid A-core biosynthesis might alternatively serve an offensive purpose (6). High-resolution crystal structures of PbgA reveal contacts between lipid A-core and the backbone of R215, as well as the side chain of R216 (Fig.…”

“…S. Typhimurium pbgAΔ191-586 suppressor mutants that encode the LpxC Y113C substitution persist in mice without causing lethality (5). The mechanism involves Toll-like receptor 4 (Tlr4), which is the innate immune receptor that binds to the lipid A-core moiety of LPS and controls immune activation and antimicrobial defenses in response to gram-negative bacteria (5,6). Therefore, we sought to test whether the two pairs of dual arginines were necessary for the ability of S. Typhimurium to kill wild type and Tlr4-deficient mice.…”

“…Typhimurium to increase the activity of a multitude of regulatory proteins and mechanisms, many of which control the glycerophospholipid (GPL) and lipopolysaccharide (LPS) content of the outer membrane (OM) (4). Maintaining and regulating OM-lipid content is critical to nearly every aspect of S. Typhimurium pathogenesis and contributes to intrinsic antimicrobial resistance (4)(5)(6).…”