The PG layer comprises long polymers of the repeating disaccharide N-acetylglucosamine– N-acetylmuramic acid (NAG–NAM) that are linked via peptide bridges: both traditional 4–3 ( D-Ala– meso-diaminopimelic acid (mDAP)) crosslinks and non-traditional 3–3 (mDAP–mDAP) crosslinks. In between the two membranes is the periplasmic space, which contains the peptidoglycan (PG) layer and periplasmic proteins. The cytoplasmic membrane is composed of a phospholipid bilayer, whereas the outer membrane comprises an interior leaflet of phospholipids and an exterior leaflet of lipopolysaccharide (LPS) LPS is composed of lipid A, the core oligosaccharide and O antigen. The cell envelope of Gram-negative bacteria consists of two membranes, the outer membrane and the cytoplasmic membrane. Envelope proteins are either soluble (periplasmic proteins), membrane-associated, integral or anchored into the leaflet of either membrane via covalently attached lipid appendages (lipoproteins) ( Fig. The net-like PG layer within the periplasm gives bacteria their shape and imparts protection from osmotic changes and sheer stress. The periplasm is an oxidative environment that promotes protein folding but does not contain nucleotide sources of energy, such as ATP or GTP 12. The cytoplasmic membrane consists of a typical phospholipid bilayer that serves as an electrochemical barrier 11. The outer membrane is a fairly unusual outermost cell barrier, being composed of an interior leaflet of phospholipids and an exterior leaflet of lipopolysaccharide (LPS also known as endotoxin). The Gram-negative envelope consists of two membranes, the outer membrane and the cytoplasmic membrane, and the periplasmic space in between, which contains a layer of peptidoglycan (PG) 11. Furthermore, it is becoming clear that multiple mechanisms can lead to the production of OMVs in bacteria.Īs OMVs are derived from the cell envelope of Gram-negative bacteria, it is important to consider the unique architecture of this bacterial component in order to understand the mechanisms that are involved in OMV budding and detachment ( Fig. Notably, OMVs from non-pathogenic bacteria mediate functions similar to those mediated by other extracellular vesicles, such as cellular communication, surface modifications and the elimination of undesired components 10. Only recently have genetic and biochemical analyses led researchers to begin to elucidate mechanistic aspects of OMV production, as well as to appreciate aspects of OMV production by non-pathogenic bacteria. Over the years, the study of OMVs has generally focused on the function of these vesicles, particularly as it relates to bacterial pathogenesis. All types of Gram-negative bacteria have been seen to produce OMVs 1, 2, 3 in a variety of environments, including planktonic cultures, fresh and salt water, biofilms, inside eukaryotic cells and within mammalian hosts 4, 5, 6, 7, 8, 9. Outer-membrane vesicles (OMVs), which are derived from the cell envelope of Gram-negative bacteria, have been observed and studied for decades. In all domains of life - Eukarya, Archaea and Bacteria - cells produce and release membrane-bound material, often termed membrane vesicles, microvesicles, exosomes, tolerasomes, agrosomes and virus-like particles. The versatile characteristics of OMVs and their immunomodulatory properties can be exploited for bioengineering applications and vaccine development. OMVs can serve in bacterial communities as 'public goods' by distributing enzymes that break down extracellular material into nutrients, by recruiting iron, by acting as decoys for bacteriophages or antibiotics and by transferring DNA between cells. Well-characterized cargoes include virulence factors, antibiotic-degrading enzymes, surface adherence factors, proteases and enzymes that are important for nutrient acquisition. OMV cargo may be enriched or excluded compared with its abundance in the bacterial envelope, suggesting that cargo recruitment is a regulated rather than stochastic process. Lipopolysaccharide (LPS) subtypes also affect the levels of OMV production, as well as OMV cargo recruitment. Mutations that subtly affect envelope crosslinking affect OMV production, whereas bacterial mutants that are unable to crosslink the envelope are typically unstable and form lysis products instead of OMVs. The difficulty in finding a single molecular or genetic basis for OMV production is probably due to species-dependent differences in envelope architecture, environmental influences on envelope composition and redundancy of OMV-producing pathways. Vesicles derived from the outer membrane of Gram-negative bacteria, or outer-membrane vesicles (OMVs), are heterogeneous in size and composition, encapsulate soluble periplasmic content and are ubiquitously produced.
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