Lipopolysaccharide

Last updated: 2025-08-05 00:24:23

Lipopolysaccharide

Lipopolysaccharide (LPS), now more commonly known as endotoxin,^[1] is a collective term for components of the outermost membrane of the cell envelope of gram-negative bacteria,^[2] such as E. coli and Salmonella^[3] with a common structural architecture. Lipopolysaccharides are large molecules consisting of three parts: an outer core polysaccharide termed the O-antigen, an inner core oligosaccharide and Lipid A (from which toxicity is largely derived), all covalently linked. In current terminology, the term endotoxin is often used synonymously with LPS, although there are a few endotoxins (in the original sense of toxins that are inside the bacterial cell that are released when the cell disintegrates) that are not related to LPS, such as the so-called delta endotoxin proteins produced by Bacillus thuringiensis.^[4]

Lipopolysaccharides can have substantial impacts on human health, primarily through interactions with the immune system. LPS is a potent activator of the immune system and is a pyrogen (agent that causes fever).^[5] In severe cases, LPS can trigger a brisk host response and multiple types of acute organ failure ^[6] which can lead to septic shock.^[7] In lower levels and over a longer time period, there is evidence LPS may play an important and harmful role in autoimmunity, obesity, depression, and cellular senescence.^[8]^[9]^[10]^[11]

Discovery

The toxic activity of LPS was first discovered and termed endotoxin by Richard Friedrich Johannes Pfeiffer. He distinguished between exotoxins, toxins that are released by bacteria into the surrounding environment, and endotoxins, which are toxins "within" the bacterial cell and released only after destruction of the bacterial outer membrane.^[12] Subsequent work showed that release of LPS from Gram negative microbes does not necessarily require the destruction of the bacterial cell wall, but rather, LPS is secreted as part of the normal physiological activity of membrane vesicle trafficking in the form of bacterial outer membrane vesicles (OMVs), which may also contain other virulence factors and proteins.^[13]^[3]

Functions in bacteria

LPS is a major component of the outer cell membrane of gram-negative bacteria, contributing greatly to the structural integrity of the bacteria and protecting the membrane from certain kinds of chemical attack. LPS is the most abundant antigen on the cell surface of most gram-negative bacteria, contributing up to 80% of the outer membrane of E. coli and Salmonella.^[3] LPS increases the negative charge of the cell membrane and helps stabilize the overall membrane structure. It is of crucial importance to many gram-negative bacteria, which die if the genes coding for it are mutated or removed. However, it appears that LPS is nonessential in at least some gram-negative bacteria, such as Neisseria meningitidis, Moraxella catarrhalis, and Acinetobacter baumannii.^[14] It has also been implicated in non-pathogenic aspects of bacterial ecology, including surface adhesion, bacteriophage sensitivity, and interactions with predators such as amoebae. LPS is also required for the functioning of omptins, a class of bacterial protease.^[15]

Composition

The saccharolipid Kdo₂-Lipid A. Kdo residues in red (core), glucosamine residues in blue, acyl chains in black and phosphate groups in green.

LPS are amphipathic and composed of three parts: the O antigen (or O polysaccharide) which is hydrophilic, the core oligosaccharide (also hydrophilic), and Lipid A, the hydrophobic domain.

O-antigen

The repetitive glycan polymer contained within an LPS is referred to as the O antigen, O polysaccharide, or O side-chain of the bacteria. The O antigen is attached to the core oligosaccharide, and comprises the outermost domain of the LPS molecule. The structure and composition of the O chain is highly variable from strain to strain, determining the serological specificity of the parent bacterial strain;^[16] there are over 160 different O antigen structures produced by different E. coli strains.^[17] The presence or absence of O chains determines whether the LPS is considered "rough" or "smooth". Full-length O-chains would render the LPS smooth, whereas the absence or reduction of O-chains would make the LPS rough.^[18] Bacteria with rough LPS usually have more penetrable cell membranes to hydrophobic antibiotics, since a rough LPS is more hydrophobic.^[19] O antigen is exposed on the very outer surface of the bacterial cell, and, as a consequence, is a target for recognition by host antibodies.

Core

The core domain always contains an oligosaccharide component that attaches directly to lipid A and commonly contains sugars such as heptose and 3-Deoxy-D-manno-oct-2-ulosonic acid (also known as KDO, keto-deoxyoctulosonate).^[20] The core oligosaccharide is less variable in its structure and composition, a given core structure being common to large groups of bacteria.^[16] The LPS cores of many bacteria also contain non-carbohydrate components, such as phosphate, amino acids, and ethanolamine substituents.

Lipid A

Lipid A is, in normal circumstances, a phosphorylated glucosamine disaccharide decorated with multiple fatty acids. These hydrophobic fatty acid chains anchor the LPS into the bacterial membrane, and the rest of the LPS projects from the cell surface. The lipid A domain is the most bioactive and responsible for much of the toxicity of Gram-negative bacteria. When bacterial cells are lysed by the immune system, fragments of membrane containing lipid A may be released into the circulation, causing fever, diarrhea, and possible fatal endotoxic septic shock (a form of septic shock). The Lipid A moiety is a very conserved component of the LPS.^[21] However Lipid A structure varies among bacterial species. Lipid A structure largely defines the degree and nature of the overall host immune activation.^[22]

Lipooligosaccharides

The "rough form" of LPS has a lower molecular weight due to the absence of the O polysaccharide. In its place is a short oligosaccharide: this form is known as Lipooligosaccharide (LOS), and is a glycolipid found in the outer membrane of some types of Gram-negative bacteria, such as Neisseria spp. and Haemophilus spp.^[8]^[23] LOS plays a central role in maintaining the integrity and functionality of the outer membrane of the Gram negative cell envelope. LOS play an important role in the pathogenesis of certain bacterial infections because they are capable of acting as immunostimulators and immunomodulators.^[8] Furthermore, LOS molecules are responsible for the ability of some bacterial strains to display molecular mimicry and antigenic diversity, aiding in the evasion of host immune defenses and thus contributing to the virulence of these bacterial strains. In the case of Neisseria meningitidis, the lipid A portion of the molecule has a symmetrical structure and the inner core is composed of 3-deoxy-D-manno-2-octulosonic acid (KDO) and heptose (Hep) moieties. The outer core oligosaccharide chain varies depending on the bacterial strain.^[8]^[23]

LPS detoxification

A highly conserved host enzyme called acyloxyacyl hydrolase (AOAH) may detoxify LPS when it enters, or is produced in, animal tissues. It may also convert LPS in the intestine into an LPS inhibitor. Neutrophils, macrophages and dendritic cells produce this lipase, which inactivates LPS by removing the two secondary acyl chains from lipid A to produce tetraacyl LPS. If mice are given LPS parenterally, those that lack AOAH develop high titers of non-specific antibodies, develop prolonged hepatomegaly, and experience prolonged endotoxin tolerance. LPS inactivation may be required for animals to restore homeostasis after parenteral LPS exposure.^[24] Although mice have many other mechanisms for inhibiting LPS signaling, none is able to prevent these changes in animals that lack AOAH.

Dephosphorylation of LPS by intestinal alkaline phosphatase can reduce the severity of Salmonella tryphimurium and Clostridioides difficile infection restoring normal gut microbiota.^[25] Alkaline phosphatase prevents intestinal inflammation (and "leaky gut") from bacteria by dephosphorylating the Lipid A portion of LPS.^[26]^[27]^[28]

Biosynthesis and transport

**LPS final assembly:** O-antigen subunits are translocated across the inner membrane (by Wzx) where they are polymerized (by Wzy, chain length determined by Wzz) and ligated (by WaaL) on to complete Core-Lipid A molecules (which were translocated by MsbA).^[29]

**LPS transport:** Completed LPS molecules are transported across the periplasm and outer membrane by the lipopolysaccharide transport (Lpt) proteins A, B, C, D, E, F, and G.^[30]

The entire process of making LPS starts with a molecule called lipid A-Kdo2, which is first created on the surface of the bacterial cell's inner membrane. Then, additional sugars are added to this molecule on the inner membrane before it's moved to the space between the inner and outer membranes (periplasmic space) with the help of a protein called MsbA. The O-antigen, another part of LPS, is made by special enzyme complexes on the inner membrane. It is then moved to the outer membrane through three different systems: one is Wzy-dependent, another relies on ABC transporters, and the third involves a synthase-dependent process.^[31]

Ultimately, LPS is transported to the outer membrane by a membrane-to-membrane bridge of lipolysaccharide transport (Lpt) proteins.^[30]^[32] This transporter is a potential antibiotic target.^[33]^[34]

Biological effects on hosts infected with Gram-negative bacteria

LPS storage in the body

The human body carries endogenous stores of LPS.^[35] The epithelial surfaces are colonized by a complex microbial flora (including gram-negative bacteria). Gram-negative bacterial will shed endotoxins. This host-microbial interaction is a symbiotic relationship which plays a critical role in systemic immunologic homeostasis. When this is disrupted, it can lead to disease such as endotoxemia and endotoxic septic shock.

Immune response

LPS acts as the prototypical endotoxin because it binds the CD14/TLR4/MD2 receptor complex in many cell types, but especially in monocytes, dendritic cells, macrophages and B cells, which promotes the secretion of pro-inflammatory cytokines, nitric oxide, and eicosanoids.^[36] Bruce Beutler was awarded a portion of the 2011 Nobel Prize in Physiology or Medicine for his work demonstrating that TLR4 is the LPS receptor.^[37]^[38]

As part of the cellular stress response, superoxide is one of the major reactive oxygen species induced by LPS in various cell types that express TLR (toll-like receptor).^[39] LPS is also an exogenous pyrogen (fever-inducing substance).^[5]

LPS function has been under experimental research for several years due to its role in activating many transcription factors. LPS also produces many types of mediators involved in septic shock. Of mammals, humans are much more sensitive to LPS than other primates,^[40] and other animals as well (e.g., mice). A dose of 1 μg/kg induces shock in humans, but mice will tolerate a dose up to a thousand times higher.^[41] This may relate to differences in the level of circulating natural antibodies between the two species.^[42]^[43] It may also be linked to multiple immune tactics against pathogens, and part of a multi-faceted anti-microbial strategy that has been informed by human behavioral changes over our species' evolution (e.g., meat eating, agricultural practices, and smoking).^[40] Said et al. showed that LPS causes an IL-10-dependent inhibition of CD4 T-cell expansion and function by up-regulating PD-1 levels on monocytes which leads to IL-10 production by monocytes after binding of PD-1 by PD-L1.^[44]

Endotoxins are in large part responsible for the dramatic clinical manifestations of infections with pathogenic Gram-negative bacteria, such as Neisseria meningitidis, the pathogens that causes meningococcal disease, including meningococcemia, Waterhouse–Friderichsen syndrome, and meningitis.

Portions of the LPS from several bacterial strains have been shown to be chemically similar to human host cell surface molecules; the ability of some bacteria to present molecules on their surface which are chemically identical or similar to the surface molecules of some types of host cells is termed molecular mimicry.^[45] For example, in Neisseria meningitidis L2,3,5,7,9, the terminal tetrasaccharide portion of the oligosaccharide (lacto-N-neotetraose) is the same tetrasaccharide as that found in paragloboside, a precursor for ABH glycolipid antigens found on human erythrocytes.^[8] In another example, the terminal trisaccharide portion (lactotriaose) of the oligosaccharide from pathogenic Neisseria spp. LOS is also found in lactoneoseries glycosphingolipids from human cells.^[8] Most meningococci from groups B and C, as well as gonococci, have been shown to have this trisaccharide as part of their LOS structure.^[8] The presence of these human cell surface 'mimics' may, in addition to acting as a 'camouflage' from the immune system, play a role in the abolishment of immune tolerance when infecting hosts with certain human leukocyte antigen (HLA) genotypes, such as HLA-B35.^[8]

LPS can be sensed directly by hematopoietic stem cells (HSCs) through the bonding with TLR4, causing them to proliferate in reaction to a systemic infection. This response activate the TLR4-TRIF-ROS-p38 signaling within the HSCs and through a sustained TLR4 activation can cause a proliferative stress, leading to impair their competitive repopulating ability.^[46] Infection in mice using S. typhimurium showed similar results, validating the experimental model also in vivo.

Effect of variability on immune response

O-antigens (the outer carbohydrates) are the most variable portion of the LPS molecule, imparting antigenic specificity. In contrast, lipid A is the most conserved part. However, lipid A composition also may vary (e.g., in number and nature of acyl chains even within or between genera). Some of these variations may impart antagonistic properties to these LPS. For example, diphosphoryl lipid A of Rhodobacter sphaeroides (RsDPLA) is a potent antagonist of LPS in human cells, but is an agonist in hamster and equine cells.^[47]

It has been speculated that conical lipid A (e.g., from E. coli) is more agonistic, while less conical lipid A like that of Porphyromonas gingivalis may activate a different signal (TLR2 instead of TLR4), and completely cylindrical lipid A like that of Rhodobacter sphaeroides is antagonistic to TLRs.^[48]^[49] In general, LPS gene clusters are highly variable between different strains, subspecies, species of bacterial pathogens of plants and animals.^[50]^[51]

Normal human blood serum contains anti-LOS antibodies that are bactericidal and patients that have infections caused by serotypically distinct strains possess anti-LOS antibodies that differ in their specificity compared with normal serum.

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