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Lipopolysaccharide
Lipopolysaccharide
Lipopolysaccharide
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Structure of a lipopolysaccharide (LPS)

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

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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

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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

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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

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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

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Main article: Core oligosaccharide

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

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Main article: 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

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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

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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

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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

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LPS storage in the body

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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

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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

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Toll-like receptors of the innate immune system recognize LPS and trigger an 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.[52] These differences in humoral immune response to different LOS types can be attributed to the structure of the LOS molecule, primarily within the structure of the oligosaccharide portion of the LOS molecule.[52] In Neisseria gonorrhoeae it has been demonstrated that the antigenicity of LOS molecules can change during an infection due to the ability of these bacteria to synthesize more than one type of LOS,[52] a characteristic known as phase variation. Additionally, Neisseria gonorrhoeae, as well as Neisseria meningitidis and Haemophilus influenzae,[8] are capable of further modifying their LOS in vitro, for example through sialylation (modification with sialic acid residues), and as a result are able to increase their resistance to complement-mediated killing [52] or even down-regulate complement activation[8] or evade the effects of bactericidal antibodies.[8] Sialylation may also contribute to hindered neutrophil attachment and phagocytosis by immune system cells as well as a reduced oxidative burst.[8] Haemophilus somnus, a pathogen of cattle, has also been shown to display LOS phase variation, a characteristic which may help in the evasion of bovine host immune defenses.[53] Taken together, these observations suggest that variations in bacterial surface molecules such as LOS can help the pathogen evade both the humoral (antibody and complement-mediated) and the cell-mediated (killing by neutrophils, for example) host immune defenses.

Non-canonical pathways of LPS recognition

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Recently, it was shown that in addition to TLR4 mediated pathways, certain members of the family of the transient receptor potential ion channels recognize LPS.[54] LPS-mediated activation of TRPA1 was shown in mice[55] and Drosophila melanogaster flies.[56] At higher concentrations, LPS activates other members of the sensory TRP channel family as well, such as TRPV1, TRPM3 and to some extent TRPM8.[57] LPS is recognized by TRPV4 on epithelial cells. TRPV4 activation by LPS was necessary and sufficient to induce nitric oxide production with a bactericidal effect.[58]

Testing

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Lipopolysaccharide is a significant factor that makes bacteria harmful, and it helps categorize them into different groups based on their structure and function. This makes LPS a useful marker for telling apart various Gram-negative bacteria. Swiftly identifying and understanding the types of pathogens involved is crucial for promptly managing and treating infections. Since LPS is the main trigger for the immune response in our cells, it acts as an early signal of an acute infection. Therefore, LPS testing is more specific and meaningful than many other serological tests.[59]

The current methods for testing LPS are quite sensitive, but many of them struggle to differentiate between different LPS groups. Additionally, the nature of LPS, which has both water-attracting and water-repelling properties (amphiphilic), makes it challenging to develop sensitive and user-friendly tests.[59]

The typical detection methods rely on identifying the lipid A part of LPS because Lipid A is very similar among different bacterial species and serotypes. LPS testing techniques fall into six categories, and they often overlap: in vivo tests, in vitro tests, modified immunoassays, biological assays, and chemical assays.[59]

Endotoxin Activity Assay

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Because the LPS is very difficult to measure in whole blood and because most LPS is bound to proteins and complement, the Endotoxin Activity Assay (EAA™) was developed and cleared by the US FDA in 2003. EAA is a rapid in vitro chemiluminescent immunodiagnostic test. It utilizes a specific monoclonal antibody to measure the endotoxin activity in EDTA whole blood specimens. This assay uses the biological response of the neutrophils in a patient's blood to an immunological complex of endotoxin and exogenous antibody – the chemiluminescent reaction formed creates an emission of light. The amount of chemiluminescence is proportional to the logarithmic concentration of LPS in the sample and is a measure of the endotoxin activity in the blood.[60] The assay reacts specifically with the Lipid A moiety of LPS of Gram-negative bacteria and does not cross-react with cell wall constituents of Gram-positive bacteria and other microorganisms.

Pathophysiology

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LPS is a powerful toxin that, when in the body, triggers inflammation by binding to cell receptors. Excessive LPS in the blood, endotoxemia, may cause a highly lethal form of sepsis known as endotoxic septic shock.[6] This condition includes symptoms that fall along a continuum of pathophysiologic states, starting with a systemic inflammatory response syndrome (SIRS) and ending in multiorgan dysfunction syndrome (MODS) before death. Early symptoms include rapid heart rate, quick breathing, temperature changes, and blood clotting issues, resulting in blood vessels widening and reduced blood volume, leading to cellular dysfunction.[59]

Recent research indicates that even small LPS exposure is associated with autoimmune diseases and allergies. High levels of LPS in the blood can lead to metabolic syndrome, increasing the risk of conditions like diabetes, heart disease, and liver problems.[59]

LPS also plays a crucial role in symptoms caused by infections from harmful bacteria, including severe conditions like Waterhouse-Friderichsen syndrome, meningococcemia, and meningitis. Certain bacteria can adapt their LPS to cause long-lasting infections in the respiratory and digestive systems.[59]

Recent studies have shown that LPS disrupts cell membrane lipids, affecting cholesterol and metabolism, potentially leading to high cholesterol, abnormal blood lipid levels, and non-alcoholic fatty liver disease. In some cases, LPS can interfere with toxin clearance, which may be linked to neurological issues.[59]

Health effects

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In general the health effects of LPS are due to its abilities as a potent activator and modulator of the immune system, especially its inducement of inflammation. LPS is directly cytoxic and is highly immunostimulatory – as host immune cells recognize LPS, the complement system is strongly activated. Complement activation and a rising anti-inflammatory response can lead to immune cell dysfunction, immunosuppression, widespread coagulopathy, and serious tissue damage, and can progress to multi-system organ failure and death.[40]

Endotoxemia

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The presence of endotoxins in the blood is called endotoxemia. High level of endotoxemia can lead to septic shock,[61] or more specifically endotoxic septic shock,[6] while lower concentration of endotoxins in the bloodstream is called metabolic endotoxemia.[62] Endotoxemia is associated with obesity, diet,[63] cardiovascular diseases,[63] and diabetes,[62] while also host genetics might have an effect.[64]

Moreover, endotoxemia of intestinal origin, especially, at the host-pathogen interface, is considered to be an important factor in the development of alcoholic hepatitis,[65] which is likely to develop on the basis of the small bowel bacterial overgrowth syndrome and an increased intestinal permeability.[66]

Lipid A may cause uncontrolled activation of mammalian immune systems with production of inflammatory mediators that may lead to endotoxic septic shock.[23][6] This inflammatory reaction is primarily mediated by Toll-like receptor 4 which is responsible for immune system cell activation.[23] Damage to the endothelial layer of blood vessels caused by these inflammatory mediators can lead to capillary leak syndrome, dilation of blood vessels and a decrease in cardiac function and can further worsen shock.[67] LPS is also a potent activator of complemen.[67] Uncontrolled complement activation may trigger destructive endothelial damage leading to disseminated intravascular coagulation (DIC), or atypical hemolytic uremic syndrome (aHUS) with injury to various organs such as including kidneys and lungs.[68] The skin can show the effects of vascular damage often coupled with depletion of coagulation factors in the form of petechiae, purpura and ecchymoses. The limbs can also be affected, sometimes with devastating consequences such as the development of gangrene, requiring subsequent amputation.[67] Loss of function of the adrenal glands can cause adrenal insufficiency and additional hemorrhage into the adrenals causes Waterhouse-Friderichsen syndrome, both of which can be life-threatening.

It has also been reported that gonococcal LOS can cause damage to human fallopian tubes.[52]

Treatment of Endotoxemia

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Toraymyxin is a widely used extracorporeal endotoxin removal therapy through direct hemoadsorption (also referred to as hemoperfusion). It is a polystyrene-derived cartridge with molecules of polymyxin B (PMX-B) covalently bound to mesh fibers contained within it. Polymyxins are cyclic cationic polypeptide antibiotics derived from Bacillus polymyxa with an effective antimicrobial activity against Gram-negative bacteria, but their intravenous clinical use has been limited due to their nephrotoxicity and neurotoxicity side effects.[69] The extracorporeal use of the Toraymyxin cartridge allows PMX-B to bind lipid A with a very stable interaction with its hydrophobic residues thereby neutralizing endotoxins as the blood is filtered through the extracorporeal circuit inside the cartridge, thus reversing endotoxemia and avoiding its toxic systemic effects.[70]

Auto-immune disease

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The molecular mimicry of some LOS molecules is thought to cause autoimmune-based host responses, such as flareups of multiple sclerosis.[8][45] Other examples of bacterial mimicry of host structures via LOS are found with the bacteria Helicobacter pylori and Campylobacter jejuni, organisms which cause gastrointestinal disease in humans, and Haemophilus ducreyi which causes chancroid. Certain C. jejuni LPS serotypes (attributed to certain tetra- and pentasaccharide moieties of the core oligosaccharide) have also been implicated with Guillain–Barré syndrome and a variant of Guillain–Barré called Miller-Fisher syndrome.[8]

[edit]

Epidemiological studies have shown that increased endotoxin load, which can be a result of increased populations of endotoxin-producing bacteria in the intestinal tract, is associated with certain obesity-related patient groups.[9][71][72] Other studies have shown that purified endotoxin from Escherichia coli can induce obesity and insulin-resistance when injected into germ-free mouse models.[73] A more recent study has uncovered a potentially contributing role for Enterobacter cloacae B29 toward obesity and insulin resistance in a human patient.[74] The presumed mechanism for the association of endotoxin with obesity is that endotoxin induces an inflammation-mediated pathway accounting for the observed obesity and insulin resistance.[73] Bacterial genera associated with endotoxin-related obesity effects include Escherichia and Enterobacter.

Depression

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There is experimental and observational evidence that LPS might play a role in depression. Administration of LPS in mice can lead to depressive symptoms, and there seem to be elevated levels of LPS in some people with depression. Inflammation may sometimes play a role in the development of depression, and LPS is pro-inflammatory.[10]

Cellular senescence

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Inflammation induced by LPS can induce cellular senescence, as has been shown for the lung epithelial cells and microglial cells (the latter leading to neurodegeneration).[11]

Role as contaminant in biotechnology and research

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Lipopolysaccharides are frequent contaminants in plasmid DNA prepared from bacteria or proteins expressed from bacteria, and must be removed from the DNA or protein to avoid contaminating experiments and to avoid toxicity of products manufactured using industrial fermentation.[75]

Ovalbumin is frequently contaminated with endotoxins. Ovalbumin is one of the extensively studied proteins in animal models and also an established model allergen for airway hyper-responsiveness (AHR). Commercially available ovalbumin that is contaminated with LPS can falsify research results, as it does not accurately reflect the effect of the protein antigen on animal physiology.[76]

In pharmaceutical production, it is necessary to remove all traces of endotoxin from drug product containers, as even small amounts of endotoxin will cause illness in humans. A depyrogenation oven is used for this purpose. Temperatures in excess of 300 °C are required to fully break down LPS.[77]

The standard assay for detecting presence of endotoxin is the Limulus Amebocyte Lysate (LAL) assay, utilizing blood from the Horseshoe crab (Limulus polyphemus).[78] Very low levels of LPS can cause coagulation of the limulus lysate due to a powerful amplification through an enzymatic cascade. However, due to the dwindling population of horseshoe crabs, and the fact that there are factors that interfere with the LAL assay, efforts have been made to develop alternative assays, with the most promising ones being ELISA tests using a recombinant version of a protein in the LAL assay, Factor C.[79]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Lipopolysaccharide

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from Grokipedia
Lipopolysaccharide (LPS) is a glycolipid complex that constitutes the major component of the outer leaflet of the outer membrane in Gram-negative bacteria, functioning as both a structural element and a potent endotoxin. It is composed of three distinct regions: a hydrophobic lipid A moiety, which anchors the molecule in the membrane and is responsible for its toxicity; a heterogeneous core oligosaccharide that links lipid A to the outer portion; and an optional, highly variable O-antigen polysaccharide chain that extends into the extracellular environment. This tripartite structure enables LPS to contribute to bacterial integrity while also serving as a pathogen-associated molecular pattern (PAMP) recognized by the host immune system. The structural role of LPS is essential for maintaining the impermeability of the Gram-negative bacterial envelope, protecting the cell from environmental threats such as antibiotics, detergents, and host . , typically a phosphorylated acylated with fatty acids, exhibits amphipathic properties that stabilize the bilayer. Variations in the core and O-antigen regions confer species-specific diversity, influencing bacterial serotyping and evasion of host defenses. Biologically, LPS elicits profound immune responses in mammals through (TLR4) activation, leading to the production of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6). This endotoxic activity underlies conditions like and chronic inflammatory diseases when LPS dissociates from during or in the gut microbiota. Research continues to explore LPS modifications for therapeutic applications, including adjuvants and agents.

Introduction and History

Overview and Definition

Lipopolysaccharide (LPS), also known as endotoxin, is a complex molecule that forms a critical component of the outer membrane in . It is composed of three primary regions: a hydrophobic anchor embedded in the membrane, a core linking the lipid to the outer , and a variable O-antigen chain extending into the . This structure makes LPS unique to the outer leaflet of the Gram-negative bacterial envelope, where it contributes to the membrane's barrier function against environmental stresses. LPS is prevalent in nearly all , where it is essential for maintaining outer membrane integrity and permeability, preventing the influx of harmful substances while allowing nutrient uptake. Without LPS, these bacteria exhibit compromised viability and increased susceptibility to antibiotics and host defenses. As an endotoxin, LPS elicits potent inflammatory responses in mammalian hosts upon its release, typically during bacterial or , leading to systemic effects such as fever and . This contrasts with exotoxins, which are secreted protein toxins produced by both Gram-positive and Gram-negative bacteria, whereas endotoxins like LPS are integral to the outer membrane and heat-stable. Chemically, LPS is classified as an amphipathic molecule, with its lipid A portion providing hydrophobicity and the polysaccharide components conferring hydrophilicity, enabling its self-assembly into stable membrane bilayers.

Discovery

The discovery of lipopolysaccharide (LPS) as the primary component of bacterial endotoxins began in the late 19th century with observations of heat-stable toxic substances associated with Gram-negative bacteria. In 1892, Richard Pfeiffer, working in Robert Koch's laboratory, identified these toxins in cell-free filtrates of Vibrio cholerae, noting their ability to induce severe symptoms in guinea pigs even after heating to 60°C, distinguishing them from heat-labile exotoxins produced by other bacteria like those causing diphtheria or tetanus. Pfeiffer termed these heat-stable poisons "endotoxins," marking the first recognition of a cell-bound toxic principle in Gram-negative organisms, which he linked to cholera pathogenesis. Advancements in the 1940s solidified LPS as the key toxic moiety through isolation efforts from species. French biochemist André Boivin, along with Lydia Mesrobeanu, using extraction, obtained a purified polysaccharide-lipid complex from typhi in 1935, which retained full endotoxic activity and elicited specific immune responses. Building on this, British biochemist W. T. J. Morgan refined purification techniques in the early 1940s, isolating LPS from paratyphi and Shigella dysenteriae, confirming its role as the heat-stable, non-protein component responsible for and in animal models. These milestones shifted focus from crude bacterial extracts to defined chemical entities, enabling further biochemical characterization. In the 1950s and , German scientists Otto Westphal and Otto Lüderitz developed superior purification methods that established LPS as the universal "endotoxin" in . Their hot phenol-water extraction protocol, introduced in 1952, yielded highly pure LPS from and strains, free of contaminating proteins and nucleic acids, while preserving . This technique, still widely used today, facilitated detailed chemical analyses revealing LPS's composition as a lipid-polysaccharide conjugate. By the , structural studies confirmed the portion as the toxic core, prompting a evolution from the broad "endotoxin" to "lipopolysaccharide" to reflect its defined chemical nature.

Structure and Composition

Overall Architecture

Lipopolysaccharide (LPS) is a tripartite molecule composed of three distinct domains: , the core , and the O-antigen . The lipid A portion serves as the hydrophobic anchor, embedding into the outer leaflet of the bacterial outer membrane, while the core oligosaccharide extends into the periplasmic space, and the O-antigen projects outward into the extracellular environment as a hydrophilic polysaccharide chain. This modular organization allows LPS to form a densely packed on the outer membrane surface, contributing to the overall asymmetry of the Gram-negative bacterial envelope. LPS exhibits significant heterogeneity in its composition and length, primarily due to variations in the O-antigen region. producing LPS with a complete O-antigen are classified as smooth-form (S-form), resulting in a long, repeating chain that confers a smooth colony morphology. In contrast, rough-form (R-form) LPS lacks the O-antigen, often arising from mutations in biosynthetic genes, leading to truncated structures that expose the core oligosaccharide and produce rough colonies. This variability in O-antigen presence and length influences the overall molecular architecture and surface properties of the bacterial cell. In the outer membrane, LPS molecules integrate to create a selective permeability barrier that protects against hydrophobic antibiotics, detergents, and host . The O-antigen chains typically consist of 20 to 40 repeating units, extending up to approximately 30 nm from the surface, which helps shield underlying structures and modulates interactions with the environment. anchors these assemblies tightly, with each LPS molecule occupying a defined area that maintains integrity. Evolutionarily, the and core oligosaccharide domains of LPS are highly conserved across , reflecting their essential roles in stability and viability. Conversely, the O-antigen is hypervariable, enabling serotype diversity that drives immune evasion and adaptation to specific ecological niches. This conserved core with variable periphery underscores LPS's dual function as a structural cornerstone and an antigenic determinant.

Lipid A

Lipid A constitutes the hydrophobic anchor and bioactive core of lipopolysaccharide (LPS), embedding the molecule within the outer of . Its structure is highly conserved across species, serving as the primary determinant of LPS endotoxicity. The canonical Lipid A molecule features a β(1→6)-linked D-glucosamine backbone, phosphorylated at the 1- and 4'-positions with or groups, which confer a net negative charge essential for stabilization and . Attached to this backbone are four to seven acyl chains, predominantly β-hydroxylated fatty acids such as 3-hydroxymyristate, linked via bonds at the 2- and 2'-positions and bonds at the 3- and 3'-positions. In the prototypical form found in , is hexa-acylated, with two primary -linked (R)-3-hydroxymyristoyl chains and four secondary -linked acyl groups (including laurate, myristate, and palmitate), resulting in a highly amphipathic structure that aggregates into micelles and integrates into bilayers. This hexa-acylated configuration exemplifies the symmetric bisphosphorylated form that predominates in . Significant heterogeneity exists in Lipid A acylation patterns, influencing toxicity and host interactions. For instance, tetra-acylated variants, common in certain environmental or pathogenic bacteria like Bacteroides species, feature only four acyl chains and exhibit reduced endotoxic potency compared to hexa-acylated forms. Such variations arise from differences in the length, saturation, and number of fatty acids, with penta-acylated structures observed in species like Burkholderia cenocepacia or hepta-acylated forms in Acinetobacter baumannii under certain conditions, modulating the molecule's conformational flexibility and receptor affinity. These structural differences underscore why E. coli Lipid A serves as the reference for high endotoxicity in biomedical studies. The endotoxic activity of LPS is intrinsically tied to , independent of the attached core oligosaccharide or O-antigen chains, as demonstrated by isolated eliciting inflammatory responses comparable to intact LPS. This toxicity stems from the lipid's ability to disrupt eukaryotic cell membranes and activate innate immune signaling upon recognition, with the hexa-acylated, bisphosphorylated form being the most potent . is synthesized through intermediates of the Raetz pathway, linking it covalently to the core oligosaccharide at the 6'-position to form the complete LPS.

Core Oligosaccharide

The core of lipopolysaccharide (LPS) serves as a critical bridging region between the hydrophobic moiety and the distal O-antigen, forming a short, branched chain that contributes to the overall amphipathic architecture of the molecule. This region typically comprises 8 to 10 residues, enabling proper spacing and orientation within the bacterial outer . Structurally, the core oligosaccharide is subdivided into an inner core proximal to lipid A and a more distal outer core. The inner core is characterized by its high degree of conservation across Gram-negative bacteria and consists primarily of 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) and L-glycero-D-manno-heptose (Hep) residues. In species such as Escherichia coli and Salmonella enterica, the linkage to lipid A occurs via two Kdo units: the first Kdo is attached to the 6' position of the reducing glucosamine of lipid A through an α-(2→6) ketosidic bond, while the second Kdo is linked to the first via an α-(2→4) bond and serves as the attachment point for heptoses and further extensions. This conserved inner core motif, often including 1 to 3 Hep residues, provides essential structural rigidity and is vital for viability, as mutations disrupting it lead to deep rough phenotypes with compromised membrane integrity. The outer core extends from the inner core and exhibits greater structural variability, incorporating neutral hexoses such as D-glucose, D-galactose, and N-acetyl-D-glucosamine in branched arrangements specific to bacterial genera. Despite this variability, the outer core maintains a scaffold-like role, typically adding 3 to 7 additional sugars to the inner core framework. This region enhances the hydrophilic properties of LPS without the repetitive polymeric nature of the O-antigen. Functionally, the core oligosaccharide imparts negative charge through its Kdo residues and associated phosphates, facilitating electrostatic repulsion that bolsters outer membrane stability and acts as a permeability barrier against environmental stresses. By providing physical separation between the toxic and surface-exposed O-antigen, it supports membrane asymmetry while exhibiting relatively low in wild-type smooth strains, where the O-antigen predominates as the primary antigenic determinant.

O-Antigen

The O-antigen, also known as the O-polysaccharide (O-PS), is the most variable and immunodominant region of lipopolysaccharide (LPS) in , consisting of long chains of repeating units. Each repeat unit typically contains 2 to 8 residues, including common examples such as , , , and glucose, linked by glycosidic bonds in a strain-specific configuration. These units are polymerized to form chains of 10 to 40 repeats, with the exact length regulated by accessory proteins like Wzz to optimize bacterial fitness. This modular structure allows for extensive chemical diversity in composition, linkages, and non-carbohydrate modifications, distinguishing O-antigens across bacterial . The serological diversity of O-antigens is a cornerstone of bacterial classification and pathogenicity, particularly in , where over 100 distinct O-serogroups have been identified based on unique structures and antigenicity. This variability arises from differences in the O-antigen clusters (O-AGCs), which glycosyltransferases and other enzymes responsible for synthesizing specific variants. Such serogroups serve as the basis for serological typing in clinical diagnostics and are prime targets for glycoconjugate against extraintestinal pathogenic E. coli (ExPEC) infections, as demonstrated in preclinical studies showing protective of O-antigen-based immunogens. Biosynthesis of the O-antigen occurs via two primary pathways in . In the Wzy-dependent pathway, individual repeat units are assembled on undecaprenyl phosphate carriers in the , flipped across the inner membrane by the Wzx flippase, and then polymerized by the Wzy polymerase before ligation to the core . Alternatively, the ABC-transporter-dependent pathway synthesizes complete O-antigen chains in the , which are exported directly across the inner membrane by an ATP-binding cassette ( complex, such as Wzm/Wzt, for subsequent attachment to the A-core module. These mechanisms ensure efficient production and tailored to the bacterial . On the outer membrane surface, the O-antigen extends outward as a dense, brush-like layer, shielding underlying structures and modulating interactions with the environment. In smooth bacterial strains possessing full-length O-antigen, this domain constitutes the primary structural component of LPS, accounting for the majority of its mass and conferring the characteristic "smooth" phenotype observed in electrophoretic analyses.

Lipooligosaccharides

Lipooligosaccharides (LOS) represent a truncated variant of lipopolysaccharides (LPS), distinguished by the absence of the repeating O-antigen polysaccharide chain and comprising only the moiety anchored in the outer membrane along with a short core . This core typically consists of 8 to 12 sugar units, including inner core elements like 3-deoxy-D-manno-oct-2-ulosonic acid (KDO) and heptoses, and an outer core with additional neutral and amino sugars. In contrast to the long O-chain of standard smooth-form LPS, LOS adopts a rough-like with a more compact structure. LOS are predominantly found in Gram-negative bacteria that inhabit mucosal surfaces, including key human pathogens such as , , , and . These structures are also present in some commensal and environmental species, where they contribute to surface diversity. The portion of LOS in these organisms is notably heterogeneous, with phase-variable expression allowing to host environments. Structurally, the exposed outer core of LOS, lacking the shielding O-antigen, renders the molecule more accessible to host serum factors, potentially increasing bactericidal sensitivity unless modified by additions like or phosphorylcholine, which can enhance resistance in pathogens like . This exposure also amplifies the endotoxic potential of , as the absence of O-chain dilutes the inflammatory impact less than in full LPS. LOS from mucosal pathogens exhibits higher per unit mass compared to O-antigen-bearing LPS, due to unmasked core epitopes that strongly interact with host receptors. In terms of , the unmodified core structures of LOS promote potent inflammatory responses, contributing to tissue damage and in infections caused by and Haemophilus species. For instance, LOS variations in N. gonorrhoeae influence adherence and invasion of epithelial cells, while in H. influenzae, they modulate formation and persistence in the . These properties underscore LOS as a critical , with its epitopes driving exaggerated release and endothelial activation during disseminated disease.

Other LPS Variants

Deep-rough mutants, such as those in designated as R mutants (e.g., chemotype Re), exhibit severely truncated LPS structures lacking the outer core oligosaccharides due to mutations in genes like those in the rfa operon. These mutants produce minimal LPS consisting primarily of lipid A disaccharide with two Kdo residues and no additional sugars, resulting in highly permeable outer membranes that confer hypersensitivity to hydrophobic antibiotics, bile salts, and detergents. Experimental studies with E. coli deep-rough strains, such as D31m4, have been instrumental in elucidating LPS assembly and membrane stability, revealing pleiotropic effects including reduced outer membrane protein insertion and enhanced biofilm formation in some cases. Modified forms of lipid A, including hypo- and hyper-acylated variants, occur in certain pathogens and alter the standard hexa-acylated structure. In Yersinia pestis, temperature-dependent modifications produce tetra-acylated (hypo-acylated) lipid A at 37°C due to repressed expression of late acyltransferases such as LpxL and LpxM, contrasting with the hexa-acylated form at lower temperatures. Hyper-acylated lipid A, incorporating additional fatty acids like palmitate via PagP in other Gram-negative bacteria, has also been observed, though less common in Yersinia. Capsular LPS hybrids integrate K-antigens (capsular ) with LPS in some , forming low-molecular-weight variants linked to the lipid A-core. In Escherichia coli strains like O9:K30, group I K-antigens such as K30 are expressed as KLPS, a form covalently attached to the LPS core, which requires intact A-core for surface presentation and differs from the high-molecular-weight soluble capsule. This hybrid structure enhances capsule retention on the cell surface, as seen in Klebsiella pneumoniae where O-antigen interactions stabilize group 2 capsules. Natural variants of LPS include penta-acylated in , resulting from incomplete secondary by LpxL1 and LpxL2 enzymes, yielding a structure with five acyl chains instead of six. This modification, often combined with , distinguishes H. pylori LPS from forms and has been characterized through biosynthetic . Unlike lipooligosaccharides, which are truncated at the core without O-antigen, these variants retain core elements but feature unique tailoring.

Functions in Bacteria

Structural Role

Lipopolysaccharide (LPS) plays a critical structural role in the outer (OM) of , contributing to the overall architecture and stability of the cell envelope. By forming the primary component of the outer leaflet, LPS establishes membrane asymmetry, with phospholipids predominantly occupying the inner leaflet, which together create a highly ordered, low-permeability barrier that protects the bacterium from environmental stresses. This asymmetry is maintained by the amphipathic nature of LPS, where the portion anchors deeply into the membrane, promoting tight packing and rigidity essential for cellular integrity. A typical Gram-negative bacterium, such as , contains approximately 2–3 × 10^6 LPS molecules per cell, covering over 75% of the surface. These molecules self-assemble into a bilayer-like primarily through the hydrophobic interactions of their anchors, which consist of six fatty acyl chains that interdigitate to form a densely packed lattice resistant to disruption. This high packing density enhances membrane stability and minimizes fluidity, ensuring the OM functions as a robust scaffold for embedded proteins and other components. The structural arrangement of LPS confers intrinsic resistance to many antibiotics by limiting the diffusion of hydrophobic compounds across the . For instance, the tightly packed LPS layer effectively excludes large hydrophobic molecules like , preventing their access to the periplasmic space and inner membrane targets. This barrier function is a direct consequence of LPS's architectural role, as alterations in its density or composition can compromise membrane impermeability. LPS is indispensable for the viability of most , as demonstrated by the lethality of mutations in genes encoding its biosynthetic enzymes, such as the lpx (e.g., lpxA, lpxB, and lpxC). Null mutants in these genes fail to produce functional , leading to defective OM assembly and rapid due to loss of structural integrity and increased permeability. This essentiality underscores LPS's foundational contribution to bacterial survival in diverse environments.

Protective and Regulatory Functions

Lipopolysaccharide (LPS) plays crucial protective roles in by shielding the outer membrane from host defenses and environmental stresses, while also enabling adaptive regulatory mechanisms for . The O-antigen component, in particular, acts as a molecular shield that masks underlying structures, reducing vulnerability to immune effectors. Additionally, LPS modifications facilitate development and osmotic , and phase-variable expression allows to toggle surface properties in response to changing conditions. These functions collectively enhance bacterial in diverse niches, from host tissues to abiotic surfaces. One key protective function of LPS is resistance to , primarily mediated by the O-antigen, which sterically hinders complement activation and opsonization. In bacteria such as and , the O-antigen covalently linked to the core prevents engulfment by masking core regions that would otherwise bind complement proteins like C1q. This masking reduces initiation, limiting membrane attack complex formation and subsequent bacterial lysis. Similarly, in , long O-antigen chains block C1q and binding to surface epitopes, conferring serum resistance and evading phagocytic uptake. These mechanisms underscore the O-antigen's role in promoting intracellular survival during infection. LPS also contributes to biofilm formation, a multicellular that protects bacteria from antibiotics and host immunity, with modifications enhancing adhesion and matrix stability. In Pseudomonas aeruginosa, alterations in LPS core capping, such as addition of capping sugars, modulate properties that promote initial attachment and microcolony development in biofilms. Cyclic di-GMP signaling further regulates these LPS changes, linking second-messenger levels to enhanced biofilm architecture and immune evasion during chronic infections. O-polysaccharide length influences outer membrane vesicle production, which supports biofilm maturation by facilitating intercellular communication and nutrient exchange. Such dynamic LPS adaptations are essential for P. aeruginosa's persistence in lungs. The core oligosaccharide of LPS regulates permeability and by modulating the outer 's charge and cation binding, maintaining cellular turgor under osmotic stress. Phosphate groups in the core bind divalent cations like Mg²⁺, stabilizing the and restricting passive influx that could disrupt osmotic balance. Under low Mg²⁺ conditions, the PhoP/PhoQ two-component system induces core modifications, such as aminoarabinose addition to phosphates, which reduces negative charge and permeability to monovalent s while enhancing Mg²⁺ retention. This adaptation prevents osmotic in hypotonic environments and supports in Mg²⁺-limited host sites, as seen in . Core truncation mutants exhibit increased leakage, highlighting the 's role in osmotic . Phase variation in O-antigen expression provides a regulatory mechanism for bacterial adaptation, allowing reversible switching between phenotypes to optimize survival in fluctuating host environments. In Salmonella enterica serovar Typhi, O-antigen acetylation undergoes phase-variable changes that alter surface antigenicity without affecting serum resistance, enabling evasion of adaptive immunity while maintaining fitness. This on-off switching, often mediated by slipped-strand mispairing in biosynthetic genes, generates heterogeneous populations where O-antigen-positive cells resist complement, and variants fine-tune interactions with host epithelia. In Escherichia coli, phase-variable O-antigen length influences adhesion and invasion, promoting persistence in the gut or urinary tract. Such variability ensures population-level resilience against immune pressures.

Biosynthesis and Membrane Assembly

Biosynthetic Pathways

Lipid A, the hydrophobic anchor of lipopolysaccharide, is synthesized in the bacterial cytoplasm through the Raetz pathway, a sequence of nine enzymatic reactions beginning with the activated sugar nucleotide UDP-N-acetylglucosamine (UDP-GlcNAc). The pathway commences with LpxA, a UDP-N-acetylglucosamine acyltransferase, which catalyzes the acylation of UDP-GlcNAc at the 3-position with (R)-3-hydroxymyristoyl-acyl carrier protein (ACP) to yield UDP-3-O-[(R)-3-hydroxymyristoyl]-N-acetylglucosamine. This is followed by LpxC, a zinc-dependent deacetylase, which removes the acetyl group to produce UDP-3-O-[(R)-3-hydroxymyristoyl]glucosamine, representing the committed step in the pathway due to LpxC's essentiality and tight regulation. LpxD then adds a second (R)-3-hydroxymyristoyl chain at the 2-position, yielding UDP-2,3-bis[(R)-3-hydroxymyristoyl]glucosamine. LpxH (or LpxB in some bacteria) next hydrolyzes the UDP to form lipid X, the 2,3-bis[(R)-3-hydroxymyristoyl]-D-glucosamine 1-phosphate. LpxB then ligates a second glucosamine unit to lipid X, forming the tetra-acylated disaccharide 1-phosphate (DSMP). LpxK phosphorylates the 4'-position of DSMP to yield lipid IVA, the tetra-acylated, bis-phosphorylated disaccharide precursor. After transport to the periplasm and attachment of two Kdo residues, secondary acylation occurs, with LpxL adding a lauroyl (C12:0) chain from lauroyl-ACP to the 3-hydroxy group of the primary acyl chain at the 2-position, and LpxM adding a myristoyl (C14:0) chain from myristoyl-ACP to the 3-hydroxy group of the primary acyl chain at the 2'-position, resulting in the hexa-acylated lipid A ready for attachment to the core oligosaccharide. Recent studies have elucidated key regulatory mechanisms controlling biosynthesis rates. The stringent response alarmone ppGpp inhibits LpxA activity to coordinate LPS synthesis with nutrient availability. The ObgE interacts with LpxA to modulate its function, while the chaperone LapB promotes degradation of LpxC by the FtsH protease, ensuring balanced production and preventing toxic accumulation of intermediates. These regulators maintain cell envelope , with species-specific variations such as in LapB structure. The core oligosaccharide is assembled sequentially on the lipid A precursor primarily in the cytoplasm, with some extensions occurring in the periplasm, involving a series of glycosyltransferases that add specific sugars to form the inner and outer core regions. The initial step attaches two 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) residues to the 6'-position of lipid A via the glycosyltransferase WaaA (also known as KdtA), utilizing CMP-Kdo as the donor, which is generated from D-arabinose 5-phosphate by the isomerase KdsD, phosphoenolpyruvate synthetase KdsA, and CMP-Kdo synthetase KdsB. Following Kdo2-lipid A formation, the inner core is extended with L-glycero-D-manno-heptose residues by heptosyltransferases such as WaaC (adding the first Hep) and WaaF (adding the second), using ADP-heptose produced via the HldE (or GmhA/B/C/D) pathway from sedoheptulose 7-phosphate. The outer core, which varies between bacterial species but in Escherichia coli typically includes hexoses like glucose and galactose, is added by additional glycosyltransferases (e.g., WaaO, WaaR), often non-stoichiometrically modified with phosphate or amino sugars, completing the conserved yet variable core structure. O-antigen biosynthesis occurs on the lipid carrier undecaprenol pyrophosphate (Und-PP) in the , where individual repeating units (O-units) are assembled by sequential glycosyltransferases before polymerization. In the predominant Wzy/Wzz-dependent pathway, used by most including E. coli, the O-units are flipped across the inner by the flippase Wzx and then polymerized processively at the periplasmic side by the integral Wzy, which adds new O-units to the reducing end of the growing chain, with chain length regulated by the modulator Wzz to achieve modal distributions of 10-20 units. Alternatively, in the ABC-transporter-dependent pathway found in some species like , multiple O-units are polymerized in the to form longer blocks before export via an ATP-binding cassette transporter, offering a blockwise assembly mode that differs in efficiency and regulation from the processive mechanism. The completed O-antigen is subsequently ligated to the terminal sugar of the core by the O-antigen ligase WaaL, though this occurs post-polymerization. In , the genes encoding these biosynthetic enzymes are organized into distinct genetic clusters reflecting the modular nature of LPS assembly. The lipid A genes, including lpxA through lpxM, are distributed across multiple loci, with core components like lpxA, lpxC, lpxD, and lpxH forming an at approximately 4 minutes on the , while others such as lpxB and lpxK are elsewhere, ensuring coordinated expression under regulatory control. The core genes are clustered in the rfa (now renamed waa) locus at 13 minutes, comprising three operons that encode the glycosyltransferases and modifying enzymes for Kdo and addition, with mutations leading to deep rough phenotypes. O-antigen synthesis genes reside in the rfb cluster at around 70 minutes, containing the pathway-specific glycosyltransferases, wzy, wzz, and wzx for and export, allowing serotype-specific variation while maintaining essential functions.

Transport and Insertion

The transport of lipopolysaccharide (LPS) across the inner of begins with the extraction of the lipid A-core precursor from the inner leaflet. This process is driven by the LptB₂FGC complex, an ATP-binding cassette ( embedded in the inner , which harnesses to initiate translocation. LptB forms the domain, while LptF and LptG serve as transmembrane subunits that interact with the lipid A moiety; LptC acts as an adaptor protein bridging the inner to the . The extracted lipid A-core is then handed off sequentially along a periplasmic bridge composed of LptC and the soluble LptA protein, which maintains a continuous pathway to the outer translocon without exposing the hydrophobic to the aqueous . In the , the O-antigen , synthesized independently on undecaprenyl carriers and polymerized by Wzy-dependent mechanisms, is ligated to the non-reducing terminus of the core oligosaccharide on the lipid A-core. This critical joining step is catalyzed by the WaaL , an integral inner with its facing the , which recognizes specific terminal sugars on both the O-antigen and core for precise transfer. WaaL operates without requiring additional cofactors beyond its association, ensuring efficient assembly of the full LPS prior to outer delivery; in waaL disrupt this ligation, leading to truncated LPS forms. The assembled LPS is then inserted into the outer leaflet of the outer membrane via the LptDEFG translocon, augmented by the accessory lipoprotein LptM, which promotes oxidative maturation of LptD through disulfide bond rearrangement and stabilizes the complex for efficient transport. LptD forms a β-barrel channel that accommodates the hydrophilic polysaccharide portions, while the lipoprotein LptE plugs the barrel and facilitates the "flipping" of lipid A from the periplasmic side to the outer leaflet, driven by conformational changes in the complex. This mechanism preserves outer membrane asymmetry by selectively placing LPS externally and excluding phospholipids. Structural studies reveal that LptD-E interacts directly with incoming LPS via electrostatic and hydrophobic contacts, ensuring unidirectional transport, with LptM aiding assembly via the BAM complex. The LptBFG subcomplex at the inner membrane further supports this by providing vectorial delivery, with recent cryo-EM data showing dynamic subunit rearrangements during handover. Quality control during LPS transport and insertion is overseen by the RpoE (σᴱ) sigma factor, an extracytoplasmic function regulator that senses envelope stress from defective assembly intermediates. Upon detection of misfolded outer membrane proteins or aberrant LPS, the anti-sigma factor RseA is degraded, freeing RpoE to transcribe genes encoding chaperones (e.g., Skp, DegP), proteases, and LPS-modifying enzymes like ArnT for aminoarabinose addition. This response mitigates toxicity from exposed and restores integrity; in rpoE mutants, LPS defects accumulate, causing lethality under stress. RpoE-dependent small RNAs further fine-tune expression, integrating transport fidelity with broader homeostasis.

Bacterial and Environmental Detoxification

Mechanisms in Bacteria

Bacteria employ several enzymatic and regulatory mechanisms to modify or degrade (LPS) internally, enabling to environmental stresses, resistance to agents, and modulation of . These strategies primarily target the moiety of LPS, altering its , , or overall membrane integration to mitigate toxicity or enhance survival. Such modifications are crucial during limitation or host interactions, where unmodified LPS can accumulate and impair bacterial fitness. One key mechanism involves the enzymes PagP and PagL, which catalyze and deacylation of , respectively, in Gram-negative pathogens like . PagL, a PhoP/PhoQ-regulated outer , removes the 3-O-linked β-hydroxymyristoyl chain from mature , producing a hypoacylated form with reduced endotoxic potential. This deacylation decreases the ability of to activate host (TLR4), thereby lowering immunogenicity while maintaining integrity. In Salmonella typhimurium, PagL activity is induced under conditions mimicking the host , such as low magnesium, contributing to evasion of innate immune detection. Complementing this, PagP, another outer , transfers a palmitate chain from bilayers to the 2-position of , generating hexa-acylated species that further attenuate inflammatory signaling in host cells. These PagP- and PagL-mediated modifications collectively produce heterogeneous populations, enhancing bacterial resistance to cationic by altering hydrophobicity and charge distribution. The PhoP/PhoQ two-component regulatory system orchestrates broader alterations to lipid A charges in response to environmental cues like magnesium limitation or acidic , which are prevalent in host niches. of PhoQ, the , leads to and of the PhoP response regulator, which transcriptionally upregulates genes encoding modifying enzymes such as ArnT and EptA. ArnT adds 4-amino-4-deoxy-L-arabinose (L-Ara4N) to the 4'- of , while EptA attaches phosphoethanolamine (pEtN) to the 1-, both introducing positive charges that neutralize the native negative phosphates. These modifications reduce electrostatic attraction to polycationic antimicrobials like polymyxin B, conferring resistance without compromising LPS anchoring in the outer membrane. In , PhoP/PhoQ-dependent charge alterations are stimulus-specific; low Mg²⁺ primarily drives L-Ara4N addition, whereas mild favors pEtN incorporation, allowing fine-tuned adaptation to phosphate-scarce environments within the host. This regulatory cascade ensures that heterogeneity supports while preventing self-toxicity from unmodified, highly charged forms. During stationary phase, bacteria utilize controlled autolysis to shed excess LPS via outer membrane vesicles (OMVs), preventing intracellular accumulation that could disrupt membrane homeostasis under nutrient stress. Autolysis, triggered by endogenous hydrolases like endolysins or amidases, leads to localized cell wall degradation and bulging of the outer membrane, facilitating OMV release enriched in LPS. In species such as Escherichia cloacae and Pseudomonas aeruginosa, OMV production peaks in late log and stationary phases, correlating with autolytic events that remodel LPS composition by selectively packaging modified or damaged molecules. This shedding mechanism alleviates toxicity from hypoacylated or aggregated LPS precursors, which might otherwise permeabilize the inner membrane, while recycling lipids for new synthesis. In Salmonella, autolysis-linked OMV extrusion during stationary growth enhances survival by reducing surface-exposed immunogenic LPS, thereby modulating interactions with surrounding cells or the environment. Certain mutants in LPS transport machinery, such as those affecting the ABC transporter MsbA, demonstrate tolerance to modified precursors by altering export dynamics, highlighting adaptive strategies for handling aberrant LPS. MsbA flips newly synthesized lipid A precursors from the cytoplasmic to the periplasmic leaflet of the inner membrane; temperature-sensitive msbA mutants in E. coli accumulate tetra-acylated precursors intracellularly at non-permissive temperatures, leading to toxicity. However, suppressor mutations in msbA, such as single amino acid substitutions (e.g., msbA52 or msbA148), relax substrate specificity, enabling export of these modified precursors to the outer membrane and restoring viability. In Salmonella and E. coli strains with secondary LPS defects (e.g., lpxL mutants producing penta-acylated lipid A), such msbA variants confer tolerance to colistin and other cationic peptides by facilitating the incorporation of hypoacylated forms that reduce membrane charge. This mutant tolerance underscores MsbA's role in quality control, where altered transport prevents lethal buildup of immature LPS, promoting bacterial resilience under biosynthetic stress.

Environmental Degradation

Lipopolysaccharide (LPS) in the environment is subject to abiotic under acidic or basic conditions, which primarily cleaves the acid-labile glycosidic bonds linking the to the 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) residue and within the chains. This process disrupts the overall structure, releasing the core and O-antigen components from the more stable anchor. In natural settings like and sediments, such contributes to LPS breakdown, with the portion degrading more readily than the moiety due to its susceptibility to and bond cleavage. When incubated with estuarine beach mud at 20–22°C for 3 weeks, LPS undergoes extensive , with showing faster rates than ; this process is influenced by pH fluctuations, moisture, and temperature. These chemical processes complement microbial activities but occur independently in abiotic microenvironments. Enzymatic degradation represents a key biotic pathway for LPS breakdown outside bacterial cells, driven by microorganisms and bacteriophages equipped with specific glycosidases and depolymerases. , such as those isolated from estuarine sediments, produce enzymes that hydrolyze the O-specific chains, facilitating . Bacteriophages, including those infecting Gram-negative hosts, often encode tailspike proteins or that target LPS outer membrane components during host attachment and , cleaving glycosidic linkages in the O-antigen to enable viral entry. These enzymes enhance LPS turnover in microbe-rich , preventing accumulation and supporting carbon flux. The component of LPS demonstrates greater stability in the environment than the regions, resisting due to its acylated core.

Host Recognition and Immune Response

Canonical Recognition Pathways

Lipopolysaccharide (LPS), particularly its moiety, is primarily recognized by the host through the (TLR4) complex on innate immune cells such as macrophages. The receptor complex consists of TLR4, MD-2, and ; LPS first binds to MD-2, a soluble accessory protein that cradles the portion, inducing a conformational change that promotes TLR4 dimerization and activation. This binding event is facilitated by lipopolysaccharide-binding protein (LBP), an acute-phase serum protein that solubilizes LPS aggregates in the bloodstream by extracting monomeric LPS from micelles or multimers, thereby enhancing delivery to the CD14-TLR4-MD2 complex on the cell surface. Upon dimerization, the TLR4-MD2-LPS complex recruits adaptor proteins to initiate intracellular signaling through two main pathways: the MyD88-dependent pathway and the TRIF-dependent pathway. In the MyD88-dependent pathway, MyD88 associates with the Toll/interleukin-1 receptor (TIR) domain of TLR4, leading to the assembly of a signaling complex that activates IRAK kinases and TRAF6, ultimately resulting in the nuclear translocation of and the transcription of pro-inflammatory genes. This cascade culminates in the release of cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6), which amplify the inflammatory response to bacterial . The TRIF-dependent pathway, activated following TLR4 into endosomes with the adaptor , leads to activation and production of type I interferons, contributing to antiviral responses and modulation of . The canonical LPS recognition machinery exhibits evolutionary conservation across mammals, with TLR4 orthologs present in diverse species including , mice, and , underscoring its fundamental role in innate immunity. Genetic polymorphisms in the human TLR4 , such as Asp299Gly and Thr399Ile substitutions, have been associated with altered LPS responsiveness and increased susceptibility to in certain populations, though meta-analyses have questioned the strength of these associations.

Non-Canonical Recognition Pathways

In addition to the primary recognition of lipopolysaccharide (LPS) by (TLR4) at the cell surface, host cells employ alternative intracellular pathways to detect cytosolic LPS, enabling responses that bypass surface signaling. These non-canonical pathways are crucial for sensing LPS that has been internalized or released into the during bacterial or , triggering distinct inflammatory outcomes such as rather than traditional production. A key non-canonical sensor is caspase-11 in mice (or the orthologs caspase-4 and caspase-5 in humans), which directly binds free LPS in the to initiate a non-canonical pathway. Upon binding, caspase-11 oligomerizes and auto-processes, becoming activated without requiring an upstream sensor like TLR4 or the adaptor protein ASC. The activated caspase-11 then cleaves gasdermin D (GSDMD), releasing its N-terminal fragment (GSDMD-NT), which forms pores in the plasma membrane and mitochondrial inner membrane. This pore formation disrupts ion homeostasis, leading to —a lytic form of that amplifies by releasing damage-associated molecular patterns (DAMPs) and facilitating IL-1β release via secondary NLRP3 activation. Unlike the canonical TLR4 pathway, which promotes NF-κB-driven transcription of pro-inflammatory genes, the caspase-11/GSDMD axis primarily induces rapid and is independent of MyD88 or TRIF adaptors, providing a mechanism against that evade extracellular detection. Studies in caspase-11-deficient mice demonstrate heightened susceptibility to intracellular Gram-negative pathogens like rodentium, underscoring its role in host defense. Guanylate-binding proteins (GBPs), interferon-inducible effectors, further enhance this pathway by targeting bacterial outer membranes to liberate LPS into the for caspase-11 detection. Intracellular NOD-like receptors (NLRs), such as NOD1 and NOD2, primarily detect cytosolic fragments from bacterial cell walls but contribute to broader responses against carrying LPS through synergistic interactions. Although NOD1 and NOD2 do not directly bind LPS, their activation by muramyl peptides enhances host defense following initial TLR4 engagement with LPS, amplifying signaling and antimicrobial peptide production in the . This cross-talk allows NOD1/2 to support responses in scenarios where bacterial debris, including LPS-associated components, reaches the via or outer membrane vesicle delivery. Scavenger receptor class B type I (SR-BI), expressed on macrophages and hepatocytes, mediates non-inflammatory uptake and clearance of LPS, often in association with (HDL), promoting detoxification in lysosomes without triggering pro-inflammatory signaling. This pathway reduces circulating endotoxin levels and mitigates excessive during endotoxemia, exemplifying a regulatory mechanism distinct from recognition pathways. In SR-BI knockout models, LPS clearance is impaired, leading to exacerbated inflammatory responses and increased mortality in models.

Variability and Immune Evasion

Lipopolysaccharides (LPS) exhibit significant structural variability across bacterial species and even within the same strain, enabling pathogens to modulate host immune recognition and facilitate evasion of innate defenses. This diversity primarily arises in the O-antigen, core oligosaccharide, and regions, allowing bacteria to alter surface exposure of immunogenic epitopes and reduce activation of (TLR4). Such adaptations are crucial for persistent infections, as they hinder rapid detection by host receptors. The O-antigen, a chain extending from the LPS core, serves as a physical barrier that shields underlying core epitopes from host immune surveillance. In like Salmonella enterica serovar Typhimurium, the presence of O-antigen delays LPS internalization by host epithelial cells and impairs TLR4-mediated signaling, resulting in retarded activation of monocytes and reduced proinflammatory production. This masking effect limits access to the moiety, which is the primary TLR4 ligand, thereby dampening early immune responses and allowing bacterial colonization. Experimental evidence from human cell models demonstrates that O-antigen-deficient mutants elicit stronger TLR4 responses compared to wild-type strains, underscoring the shielding role in immune evasion. Modifications to the lipid A anchor, such as underacylation, further contribute to immune evasion by decreasing binding affinity to the MD-2 co-receptor of TLR4. In Francisella tularensis, the causative agent of tularemia, lipid A is predominantly tetra-acylated with atypical hydroxylated fatty acids, which results in minimal stimulation of human and murine TLR4-MD2 complexes and low endotoxic activity. This structural alteration reduces the conformational fit into the hydrophobic pocket of MD-2, preventing effective dimerization of TLR4 and subsequent NF-κB activation, thus allowing the bacterium to replicate intracellularly with limited inflammatory detection. Studies on purified F. tularensis lipid A confirm its 100- to 1,000-fold lower potency in inducing cytokine release compared to canonical hexa-acylated Escherichia coli lipid A. Phase variation mechanisms enable dynamic changes in O-antigen structure during infection, further promoting evasion by altering specificity. In bacteria such as , gene conversion events involving silent cassettes recombine with the active O-antigen locus, leading to high-frequency switching of O-serotypes mid-infection. This , mediated by site-specific recombinases like those encoded by prophage gtr genes, generates phenotypic heterogeneity in bacterial populations, allowing subpopulations to escape existing host antibodies while others persist. Such variation has been observed during murine gut colonization, where phase-variable expression of O1 antigens correlates with prolonged bacterial shedding and reduced clearance. The structural variability of LPS poses significant challenges for vaccine development, as immunity is predominantly serotype-specific and fails to confer broad protection against diverse strains. Antibodies elicited against O-antigen target specific repeating units, providing effective but narrow opsonic activity that does not cross-react with heterologous serotypes, as seen in where LPS vaccines protect against matched O-antigens but not mismatched ones. This limitation necessitates multivalent formulations incorporating multiple O-serotypes for pathogens like or , yet even these struggle to cover the full antigenic diversity, highlighting the need for conserved core or targets to achieve wider efficacy.

Pathophysiological Effects

Endotoxemia and Sepsis

Endotoxemia refers to the presence of lipopolysaccharide (LPS) in the bloodstream, typically resulting from the of during infections. This release is triggered by host immune defenses, such as complement activation and , or by treatment that disrupts bacterial cell walls, leading to the shedding of LPS into circulation. Once in the , LPS binds to LPS-binding protein and initiates a systemic inflammatory response through signaling, inducing a characterized by the overproduction of pro-inflammatory cytokines like interleukin-1 (IL-1) and (TNF). This cascade results in , , and increased , culminating in , , and multi-organ failure. Clinically, approximately 50% of sepsis cases are attributed to Gram-negative bacteria, with LPS playing a central role in endotoxemia-driven pathology; overall mortality ranges from 20% to 50%, depending on severity and factors. Diagnostic markers include elevated endotoxin activity, typically exceeding 0.4 EA units (where EA is the assay's on a 0-1 scale), as measured by the endotoxin activity assay, which correlates with disease progression and risk of severe .

Storage and Long-Term Effects in Hosts

Lipopolysaccharide (LPS) in the bloodstream is primarily sequestered by (HDL) and , which bind and neutralize its bioactivity to prevent excessive immune activation. HDL, particularly the HDL3 subclass, forms complexes with LPS via lipopolysaccharide-binding protein (LBP), facilitating its transport and detoxification in the liver while inhibiting interactions with (TLR4) on immune cells. also contributes to LPS sequestration through nonspecific binding, aiding in its delivery to LBP and soluble for further processing, thereby modulating the inflammatory response during circulation. These binding mechanisms ensure that a significant portion of LPS is rendered inert before reaching target tissues. Following sequestration, LPS accumulates in host tissues such as the liver and , where it can persist beyond acute clearance phases. In the liver, sinusoidal endothelial cells rapidly uptake and eliminate most circulating LPS, but residual or chronically translocated LPS from the gut deposits in hepatocytes and Kupffer cells, contributing to ongoing low-grade . serves as a depot for LPS, particularly in conditions of metabolic endotoxemia, where it associates with droplets in adipocytes and promotes infiltration. The persistence of LPS in these tissues, influenced by binding to lipoproteins and slow release from stores, allows for prolonged exposure compared to its minutes-to-hours circulation . This tissue persistence enables chronic low-level activation of TLR4 signaling in resident macrophages, sustaining proinflammatory cytokine production without overt . In macrophages within adipose and hepatic tissues, internalized LPS triggers sustained nuclear factor kappa B () pathway activation, leading to persistent expression of interleukin-6 and tumor necrosis factor-alpha at subacute levels. Such signaling fosters a state of immune priming, where macrophages exhibit heightened responsiveness to subsequent stimuli, contributing to long-term inflammatory disruption. Animal models demonstrate that even single or intermittent LPS injections induce metabolic changes persisting for months. For instance, in mice subjected to low-dose LPS, alterations in and insulin sensitivity endure up to 5 months post-exposure, linked to remodeling and hepatic dysregulation. These models highlight how initial LPS exposure reprograms metabolic pathways, resulting in sustained shifts in energy storage and utilization without resolving fully after clearance.

Broader Health Implications

Low-grade endotoxemia, characterized by elevated circulating levels of gut-derived lipopolysaccharide (LPS), has been linked to the pathogenesis of . This condition arises primarily from impaired intestinal barrier integrity, often termed "leaky gut," which facilitates LPS translocation from the into the systemic circulation. Once in the bloodstream, LPS binds to (TLR4) on immune cells, promoting chronic low-grade inflammation that drives and metabolic dysfunction. Studies have shown that high-fat diets exacerbate this process by altering composition, further increasing LPS leakage and contributing to obesity-associated metabolic derangements. Human evidence supports these associations, with plasma LPS concentrations reported to be 1.5- to 2-fold higher in individuals with or compared to lean controls, often correlating positively with (BMI). For instance, one study found LPS levels 57% higher in women with and versus those without, alongside elevated markers of like interleukin-6. Another investigation in patients revealed 76% elevated circulating LPS, which correlated with insulin levels in healthy controls but was disrupted in disease states. These findings suggest metabolic endotoxemia as a potential mediator linking gut-derived LPS to and broader features. In neurological contexts, LPS exposure mimics aspects of depression through microglial activation in the , inducing and depressive-like behaviors in animal models. Systemic LPS administration triggers microglial hyperactivation, leading to increased pro-inflammatory release in regions like the , which parallels symptoms of . Research from the 2020s emphasizes the role of in amplifying LPS translocation, thereby heightening vulnerability to such neurological effects via sustained low-grade endotoxemia and gut-brain axis dysregulation.

Role in Autoimmunity and Metabolic Disorders

Lipopolysaccharide (LPS) contributes to through mechanisms including molecular mimicry and chronic , where bacterial components trigger cross-reactive immune responses. In (RA), antibodies in patient sera bind to LPS from bacteria like , including both smooth and rough forms, potentially contributing to aberrant immune activation against joint tissues. While molecular mimicry in RA primarily involves bacterial proteins such as sharing epitopes with self-antigens like , elevated anti-LPS antibodies may play a supportive role in breaking and exacerbating . Administration of bacterial LPS in animal models induces autoantibodies and autoimmune phenotypes, supporting its role in disease initiation. In metabolic disorders, LPS drives chronic via (TLR4) activation in , promoting and that impair metabolic . Elevated circulating LPS levels, known as metabolic endotoxemia, correlate with , where gut-derived LPS translocates into the bloodstream and stimulates TLR4 on adipocytes and macrophages, leading to pro-inflammatory release and remodeling. This process fosters , reducing expandability and contributing to ; for instance, TLR4-deficient mice exhibit attenuated and improved glucose tolerance in high-fat diet models. In , heightened LPS-TLR4 signaling sustains hyperglycemia-induced , with TLR4 knockouts or antagonists reducing and pancreatic beta-cell dysfunction in models. Recent post-2020 research highlights LPS's involvement in pathways that accelerate aging-related . LPS stimulation induces senescence in macrophages via activation and redistribution, amplifying inflammaging and immune dysregulation in aged tissues. Altered in aging increases LPS production, activating and promoting , which heightens susceptibility to autoimmune conditions like through persistent low-grade from chronically stored LPS in host tissues.

Applications and Contaminants

Use in Research and Biotechnology

Lipopolysaccharide (LPS) serves as a critical experimental tool in immunological research, particularly as a purified agonist for (TLR4) to study innate immune activation and signaling pathways. Purified LPS from is routinely used to stimulate TLR4/MD-2 complexes in cellular assays, enabling researchers to investigate downstream effects such as activation, cytokine production, and endotoxin tolerance in macrophages and dendritic cells. For instance, studies employ low-dose purified LPS to mimic bacterial and dissect TLR4-dependent adaptive immune responses without inducing severe toxicity. Additionally, LPS acts as an adjuvant in vaccine formulations by enhancing antigen presentation and T-cell priming, as demonstrated in preclinical models where it boosts humoral and cellular immunity against viral and bacterial pathogens. In , recombinant E. coli strains have been engineered for the production of (GMP)-grade LPS and its derivatives, facilitating scalable and controlled synthesis for therapeutic applications. These strains enable the biosynthetic modification of LPS structures to reduce while preserving immunostimulatory properties, yielding high-purity material suitable for clinical-grade reagents. A notable example is the detoxified form, monophosphoryl (MPL), produced via recombinant pathways in E. coli and incorporated as a key component in the Shingrix adjuvant system (AS01B), where it synergizes with QS-21 to elicit robust responses against zoster. This approach has enabled GMP-compliant manufacturing of MPL at yields sufficient for large-scale vaccination campaigns, minimizing endotoxin-related risks in human use. Historically, LPS detection has relied on the (LAL) assay, derived from the blood of horseshoe crabs (Limulus polyphemus), which was developed in the by Frederik Bang and Jack Levin as a sensitive method for endotoxin testing in pharmaceuticals and medical devices. The LAL test exploits the clotting cascade triggered by LPS in crab amebocytes, detecting femtogram levels of endotoxin and becoming the FDA-approved standard by 1977 for ensuring sterility in injectable drugs and biologics. This assay has screened billions of medical products annually, preventing pyrogenic reactions, though it raises ecological concerns due to horseshoe crab harvesting. Emerging advancements in the 2020s focus on synthetic Lipid A analogs as refined TLR4 agonists for targeted therapies, offering improved safety profiles over native LPS for and infectious disease treatment. These small-molecule mimics, such as glucosamine-based compounds (e.g., FP20 series), activate TLR4 with reduced pyrotoxicity and enhanced specificity, promoting antitumor immune responses in preclinical models when conjugated to peptides or formulated in nanoparticles. For example, analogs like CRX-527 have shown promise in enhancement by selectively biasing TRIF-dependent signaling for sustained immunity without excessive . Recent structural studies of synthetic LPS variants reveal novel binding modes to TLR4/MD-2, paving the way for species-independent agonists in precision medicine applications.

Contamination Risks and Detection

Lipopolysaccharide (LPS), commonly referred to as endotoxin, represents a major contamination risk in and pharmaceutical environments, primarily arising from the overgrowth of in water purification systems, cell culture media, and reagents. These sources introduce LPS into biological preparations, such as proteins, vaccines, and injectables, where even trace amounts can compromise product safety and experimental validity. In , contaminated water or equipment can lead to pyrogenic risks in parenteral drugs, necessitating stringent controls to prevent batch failures. Health risks associated with LPS contamination are profound, as concentrations as low as 1 ng/mL can induce pyrogenic reactions, including fever, , and in severe cases, or organ failure in sensitive individuals or animal models. Intravenous exposure to such levels triggers systemic immune activation via , potentially leading to life-threatening endotoxemia in injectable therapeutics. In laboratory assays, undetected LPS at nanogram per milliliter levels can confound results by mimicking or exacerbating inflammatory responses, underscoring the need for vigilant monitoring in biotech applications. Detection of LPS contamination relies primarily on the Limulus Amebocyte Lysate (LAL) assay, which exploits the clotting cascade from horseshoe crab amebocytes to identify endotoxins with high sensitivity. The gel-clot variant provides a qualitative readout, forming a solid clot in the presence of LPS above a threshold (typically 0.03-0.5 EU/mL), while the chromogenic method quantifies endotoxin levels through colorimetric changes measured at 405 nm, enabling precise detection down to 0.005 EU/mL. As an ethical and sustainable alternative to LAL, the recombinant Factor C (rFC) assay uses a synthetically produced version of the key enzyme, offering equivalent sensitivity and specificity without relying on animal-derived reagents. The United States Pharmacopeia (USP) approved Chapter <86> Bacterial Endotoxins Test Using Recombinant Reagents in July 2024, with the chapter becoming official in May 2025, allowing rFC for routine pharmaceutical testing. In Europe, significant regulatory changes effective in 2025 promote non-animal pyrogen detection methods, including rFC, to reduce reliance on horseshoe crabs. Mitigation strategies for LPS contamination emphasize prevention through endotoxin-free protocols, including the use of certified pyrogen-free water, reagents, and glassware, alongside rigorous cleaning of equipment to avoid bacterial biofilms. Deactivation methods include treatment with polymyxin B, a cationic peptide that binds lipid A and neutralizes LPS bioactivity at concentrations of 10-50 μg/mL, effectively removing up to 99% of endotoxin from protein solutions without altering biological function. Heat inactivation, particularly dry heat at 250°C for 30 minutes or moist heat at 121°C for 20 minutes, destroys LPS structure in non-protein contexts, though care must be taken to preserve sample integrity in biotech workflows.

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  153. https://www.researchgate.net/publication/19009108_Inactivation_of_Endotoxin_by_Polymyxin_B
  154. https://academic.oup.com/jleukbio/article/80/2/359/6922544
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