Isabella Carter
The outer membrane is a defining feature of Gram-negative bacteria, which is a category of bacteria that does not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. The outer membrane of Gram-negative bacteria contains lipopolysaccharides (LPS) in its outer leaflet and phospholipids in the inner leaflet. The LPS molecule is toxic and acts as an endotoxin that elicits a strong immune response when the bacteria infect animals. The outer membrane also contains porins, which act like pores for specific molecules. The outer membrane is an essential load-bearing element in Gram-negative bacteria, providing rigidity to the cell and protecting it from external insults like antibiotics.
| Characteristics | Values |
|---|---|
| Composition | Lipopolysaccharides (LPS), proteins, phospholipids, and Braun's lipoprotein |
| Structure | Asymmetric bilayer with phospholipids in the inner leaflet and lipopolysaccharide in the outer leaflet |
| Function | Selective permeability barrier, protection from antibiotics, and maintenance of cell shape |
| Clinical Significance | Endotoxin that triggers a toxic reaction and septic shock when bacteria are lysed by immune cells |
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What You'll Learn
Lipopolysaccharides (LPS)
The O-antigen is a repeating hydrophilic oligosaccharide side chain that is specific to the bacterial serotype. It is involved in the adhesion of bacteria to animal cells and the subsequent infection of the host. The O-antigen is highly variable in structure, as well as in the number of repeating units, resulting in significant heterogeneity in LPS preparations.
The LPS molecule is toxic and is classified as an endotoxin. It elicits a strong immune response when the bacteria infect animals, which can lead to septic shock, resulting in low blood pressure, respiratory failure, reduced oxygen delivery, and lactic acidosis. The lipid A moiety has been identified as critical to the endotoxin activity of LPS, triggering a toxic reaction when the bacteria are lysed by immune cells.
The outer membrane of Gram-negative bacteria, including the LPS layer, acts as a protective barrier against numerous antibiotics, detergents, and the antimicrobial enzyme lysozyme produced by animals as part of their innate immune system. LPS contributes to the outer membrane's integrity and provides an effective permeability barrier against deleterious molecules such as antibiotics and cationic antimicrobial peptides.
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Phospholipids
Gram-negative bacteria are characterised by a three-layered cell envelope, with a thin peptidoglycan cell wall sandwiched between an inner (cytoplasmic) membrane and an outer membrane. The outer membrane is a hallmark of Gram-negative bacteria and is essential for their survival in diverse environments.
The outer membrane is an asymmetric lipid bilayer, with phospholipids occurring almost exclusively in the inner leaflet, and the outer leaflet composed of lipopolysaccharides (LPS). Phospholipids are a key component of the outer membrane, with their arrangement contributing to the asymmetry of the membrane. The outer membrane proteins (OMPs) have a unique beta-barrel structure that spans the membrane. The beta barrels fold to expose a hydrophobic surface before their insertion into the outer membrane.
The phospholipids in the inner leaflet of the outer membrane are critical to its function as a selective permeability barrier. This barrier prevents the entry of many toxic molecules into the cell, while still allowing the uptake of nutrients and the excretion of toxic waste products. The outer membrane also plays a role in resisting internal turgor pressure.
The biogenesis of the outer membrane is not yet fully understood. It requires that the individual components are transported from the site of synthesis to their final destination outside the inner membrane by crossing both hydrophilic and hydrophobic compartments.
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Braun's lipoprotein
Gram-negative bacteria are characterised by their cell envelope, which consists of a thin peptidoglycan cell wall sandwiched between an inner (cytoplasmic) membrane and an outer membrane. The outer membrane of these bacteria contains lipopolysaccharides (LPS) in its outer leaflet and phospholipids in the inner leaflet.
The outer membrane of Gram-negative bacteria is distinct from the inner membrane in terms of composition and structure. It contains outer membrane proteins (OMPs) with a unique beta-barrel structure that spans the membrane. Braun's lipoprotein, also known as Lpp, is one such OMP. It was first discovered in 1969 and is present in large amounts in E. coli, more so than any other protein.
Lpp is a lipoprotein that is anchored to the outer membrane by its amino-terminal lipid group. It forms a peptide bond with the peptidoglycan layer, providing the only known covalent connection between the outer membrane and the peptidoglycan layer in E. coli. This covalent bond is essential for stabilising the integrity of the cell envelope. Lpp also plays a role in virulence and has been linked to the plague-causing bacterium Yersinia pestis.
The discovery of Lpp led to the definition of a new class of bacterial proteins, the lipoproteins, which are extracytoplasmic, globular proteins anchored to a membrane by a lipid moiety. Lpp has been studied for its role in the cell division process, where it works alongside another cell envelope-associated protein, Pal.
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Periplasm
Gram-negative bacteria are characterised by the presence of both a cytoplasmic membrane and an outer cell membrane, with a thin layer of peptidoglycan between them. This space between the inner and outer membranes is called the periplasmic space or periplasmic compartment, and it contains the periplasm. The periplasm is a concentrated gel-like matrix, which can occupy up to 40% of the total volume of Gram-negative bacteria.
The periplasm is not just a structural component but also serves several important functions. It contains binding proteins for amino acids, sugars, vitamins, and ions, as well as degradative and detoxifying enzymes essential for bacterial nutrition. It also plays a role in cell signalling, with the lipoprotein RcsF acting as a stress sensor. The periplasm houses motility organelles such as the flagellum, which spans both membranes enclosing the periplasm.
The periplasm is also the site of O-antigen repeat unit polymerisation by the Wzy polymerase. These units are then ligated to the lipid A-core moiety by the WaaL ligase. The periplasm is an important component of the gram-negative envelope, which includes the plasma membrane, the cell wall, the peptidoglycan layer, and the periplasm.
The periplasm is a defining feature of Gram-negative bacteria, which are distinguished from Gram-positive bacteria by the presence of an outer lipid membrane. Gram-negative bacteria are generally more antimicrobial resistant than Gram-positive bacteria. The outer membrane of Gram-negative bacteria acts as a selective permeability barrier, allowing the passage of molecules with a molecular weight of less than 600.
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Outer membrane proteins
Gram-negative bacteria are characterised by their double membrane structure, with an inner membrane (IM) and an outer membrane (OM). The outer membrane is unique in its composition and asymmetrical lipid distribution, with the inner leaflet containing phospholipids and the outer leaflet composed of lipopolysaccharides (LPS). This outer leaflet of LPS is a defining feature of gram-negative bacteria and is responsible for the toxic reaction triggered when these bacteria are lysed by immune cells, which can lead to septic shock.
The outer membrane of gram-negative bacteria contains a variety of fully integrated membrane proteins, known as outer membrane proteins (OMPs). OMPs have a unique structure, folding into beta barrels that span the membrane. These beta barrels vary in sequence and size, ranging from 8 to 36 beta strands. The biogenesis of OMPs involves translocating the protein across the inner membrane through 'Sec' machinery and inserting it into the outer membrane using a barrel assembly machinery complex. This process requires the protein to cross both hydrophilic and hydrophobic compartments, and the exact machinery and energy source driving it are not yet fully understood.
OMPs can be classified into two major groups: transmembrane proteins and lipoproteins. Some specific examples of OMPs include OmpA, OmpX, phospholipase A, general porins (OmpF, PhoE), substrate-specific porins (LamB, ScrY), and the TonB-dependent iron siderophore transporters FhuA and FepA.
Outer-membrane vesicles (OMVs), produced by gram-negative bacteria, are spherical buds of the outer membrane filled with periplasmic content. OMVs play a role in bacterial survival, pathogenesis, and interaction with their environment. They can also serve as 'public goods' in bacterial communities by distributing enzymes, recruiting iron, acting as decoys, and transferring DNA between cells. The functional versatility of OMVs has sparked interest in bioengineering applications and vaccine development.
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Frequently asked questions
The outer membrane of gram-negative bacteria is composed of lipopolysaccharides (LPS), proteins, phospholipids, and Braun's lipoprotein.
Lipopolysaccharides (LPS) are a unique component of the outer membrane of gram-negative bacteria. They are classified as endotoxins that elicit a strong immune response when the bacteria infect animals.
The outer membrane is a hallmark of gram-negative bacteria and serves as a selective barrier, preventing the entry of toxic molecules. It also plays a vital role in bacterial survival in diverse environments and protects against antibiotics.
Gram-positive bacteria do not possess an outer membrane. Instead, they have a membrane bilayer surrounded by a thick layer of peptidoglycan. The outer membrane of gram-negative bacteria is an asymmetrical lipid bilayer with phospholipids in the inner leaflet and lipopolysaccharides (LPS) in the outer leaflet.
