Lipid A and Bacterial Lipopolysaccharides
1. Introduction to the Cell Wall of Gram-negative Bacteria and its Special Lipids
The cell wall or envelope of Gram-negative bacteria, including those of human pathogens such as Escherichia coli and Salmonella enterica, is composed of two distinct lipid membranes, an inner membrane, containing glycerophospholipids and integral membrane proteins, and a highly distinctive outer membrane. The outer membrane is an asymmetric bilayer, the outer leaflet of which in most species consists predominantly of lipopolysaccharides (~75%) of which lipid A, a complex glycolipid, is a key component that serves as a hydrophobic anchor for the macromolecules. The term lipid A was first coined by Westphal and Luderitz, while 'lipid B' was the name given to the conventional phospholipids.
|Schematic representation of the cell wall of Gram-negative cell bacteria. © Jeff Dahl, CC BY-SA 4.0 via Wikimedia Commons|
The aqueous compartment between the inner and outer membranes is termed the periplasm and contains peptidoglycan molecules in a thin layer arranged parallel to the cell surface; this is an elastic heteropolymer of glycan strands interconnected by short peptide chains that protects bacterial cells from lysis by internal osmotic pressure and from external stress conditions. This layer is linked covalently to a proteolipid (such as Braun's lipoprotein) in the inner leaflet of the outer membrane. The exchange process between the bacterial cell and its environment is controlled through porins with size-exclusion properties in the outer membranes. The inner leaflet contains mainly conventional glycerophospholipids, i.e. phosphatidylethanolamine, phosphatidylglycerol and cardiolipin, with proteolipids. Both leaflets of the inner membrane contain conventional phospholipids.
Cyanobacteria produce lipopolysaccharides with related structures, while other biologically relevant lipopolysaccharides include lipochito-oligosaccharides (Nod factors), which are produced by nitrogen-fixing rhizobia. Lipopolysaccharides are indispensable macromolecules for the growth and survival of the Gram-negative bacteria, providing an effective permeability barrier at the environmental interface to cationic antimicrobial peptides and antibiotics used in clinical practice, while enhancing intracellular survival and contributing to the evasion of the immune defenses by mimicry of host molecules. Such lipopolysaccharides and especially the lipid A components are of particular pharmacological interest in that they can act as toxins and stimulate strongly the innate immune system in eukaryotic host species.
2. Structure and Occurrence of Lipid A and Lipopolysaccharides
Early attempts to determine the structures of lipid A and lipopolysaccharides were greatly hindered by their amphipathic nature and their strong tendency to form aggregates by hydrophobic bonding or via cross-linking through ionic species. However, improved extraction methods and the discovery that the lipid component could be cleaved from the rest of the molecule by mild acidic hydrolysis lead to the unravelling of the detailed structures. Modern mass spectrometric methods, especially with matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization, together with NMR spectroscopy have been invaluable aids. Thus, lipopolysaccharides derived from different groups of Gram-negative bacteria are now known have a common basic structure comprising two parts - a covalently bound lipid component, termed lipid A, and a hydrophilic hetero-polysaccharide. While the polysaccharide component interacts with the external environment, including the defenses of the animal or plant host species, lipid A provides the anchor that secures the molecule in the bacterial outer membrane. It is always attached to two units of 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo), which is considered as a marker of lipopolysaccharides, and the complete entity is best described as Kdo2-lipid A. This is the minimal lipopolysaccharide entity that is essential to sustain the growth of most Gram-negative bacteria.
Lipid A is a unique and distinctive phosphoglycolipid, the structure of which is highly conserved among species. All contain two glucosamine (2,3-diamino-2,3-dideoxy-D-glucose) residues, which are present as a β(1→6)-linked dimer. This disaccharide contains α-glycosidic and non-glycosidic phosphoryl groups in the 1 and 4’ positions, and with four to seven saturated acyl chains, i.e. (R)-3-hydroxy fatty acids at positions O-2, O-3, O-2' and O-3' in ester and amide linkages, of which two are usually further acylated at their 3-hydroxyl group. However, variations in the fine structure can arise from the type of hexosamine present, the degree of phosphorylation, the presence of phosphate substituents, and importantly in the nature, chain length, number, and position of the acyl groups. In the lipid A illustrated from the most studied organism E. coli, there are six fatty acids in total. The four hydroxy fatty acids are C14 in chain length, and the hydroxyl groups of the two (R)-3-hydroxy fatty acids of the distal GlcN-residue (GlcN II), and not those of the GlcN-residue at the reducing side (GlcN I), are acylated by non-hydroxy fatty acids (12:0 and 14:0). Some molecular species contain an additional fatty acid attached to the amide-linked 3-hydroxy acid and the phosphate group may be substituted with ethanolamine-phosphate (of GlcN I). In E. coli, there can be more than 106 lipid A residues in each cell.
There are a few important exceptions to this type of fatty acid pattern, and bacterial species show variability in the chain-length and number of fatty acid residues (2 to 9), or with amino-sugars replacing phosphate residues or with methyl substituents. For example, in the lipid A of Helicobacter pylori in comparison to that of E. coli, there are four rather than six fatty acids with a longer average chain-length (16-18). In Rhodobacter sphaeroides, the amide-linked fatty acid of the disaccharide backbone is 3-oxo-tetradecanoate, while some species contain 2-hydroxy acids. Agrobacterium and Rhizobiaceae species, which are plant pathogens and symbionts, respectively, tend to have pentacyl units with four C12 to C20 3-hydroxy acids and one very-long-chain (ω-1)-hydroxy acid such as 27‑hydroxyoctacosanoic acid (sometimes with 3-hydroxy-butyric acid linked in turn) attached to one of the 3-hydroxyl groups. Bradyrhizobium (slow-growing nodulating rhizobia) has at least one molecule of a hopanoid, carboxyl-bacteriohopanediol or its 2-methyl derivative, linked covalently to the (ω-1)-hydroxy group of a very-long-chain fatty acid (often C30), which reinforces the stability and rigidity of the outer membrane and thereby facilitates their dual life cycle, outside and inside the plant; they also have a 25‑hydroxy-C26 fatty acid that is not linked to the hopanoid.
The large number of fully saturated fatty acyl groups in each molecule of lipid is believed to create a gel-like lipid interior of low fluidity that inhibits the penetration of hydrophobic solutes into the membrane. Also, it is believed that the external barrier is stabilized by lipopolysaccharide-associated divalent cations, e.g. Mg2+ and Ca2+, linking adjacent molecules through salt bridges that neutralize the repulsive forces. The result is an oriented and tightly cross-linked leaflet that protects bacteria from a variety of hydrophobic and large hydrophilic host-defence molecules including some antimicrobial peptides. It permits growth and survival of bacteria in harsh environments including those within eukaryotic hosts.
In Rhizobiaceae species, the basic structure is a little more variable, and the phosphates residues may be absent or substituted with glucuronic acid. Together with the distinctive fatty acyl pattern, this may be a strategy of the organism to weaken or evade the response of the plant and so enable symbiosis. The lipopolysaccharide of Francisella species have a number of unusual features, not least in that the lipid A exists partly in the free form, i.e. not linked to Kdo, core sugars and O-specific chain. In comparison to the lipid A from E. coli, the phosphate group at the 1‑position of the β-(1–6)-linked diglucosamine unit is replaced by α-linked galactosamine and there is no phosphate at the 4'-position, while the fatty acid components are C18 and C16 in chain-length; some remodelling of this can occur in response to acid stress. In some species, the disaccharide unit can have a different composition.
Lipopolysaccharides from marine cyanobacteria such as the genus Synechococcus (illustrated) differ significantly from those of all other species in that the lipid moieties consist of tri- and tetraacylated structures with hydroxy- (odd-chain) and nonhydroxy-fatty acids connected to the diglucosamine backbone. They lack heptose, Kdo and phosphate residues and instead have a single galacturonic acid attached to glucosamine. Other species of marine cyanobacteria can have differing carbohydrate compositions. Whether these represent primitive structures or an adaptation to the marine environment is a matter of speculation.
Polysaccharide components: The hetero-polysaccharide chains of the intact lipopolysaccharides extend outwards for a distance of about 10 nm from the surface of the outer membrane, which has an important function in nutrient uptake but also provides the organisms with a remarkable permeability barrier that confers resistance to many different detergents, cationic antimicrobial peptides and antibiotics.
In each bacterial species, the hetero-saccharide is in two parts - a core unit with inner and outer parts, and an outer 'O-specific' chain consisting of a complex polymer of oligosaccharides, which determines the serological or antigenic specificity of the lipopolysaccharide. The presence or absence of the latter determines the appearance – ‘smooth’ or ‘rough’ – of a bacterial colony. Thus, the ‘rough’ type lipopolysaccharides lack the O-specific chain, while a ‘semi-rough’ or short-chain type contains only one O-chain repeating unit attached to the core oligosaccharide-lipid A. For example, in a few important pathogens, including Bordetella pertussis, Neisseria meningitidis and Haemophilus influenzae, the lipopolysaccharides lack an O-chain and are sometimes termed 'lipooligosaccharides'. These species tend to have more complex and highly decorated core structures that are responsible for bacterial specificity.
In most species, the core polysaccharide is structurally more uniform than the O-chain. The inner part of the core region tends to be more conserved and is composed of the characteristic components heptose, mainly in the L-glycero-D-manno configuration, and 3-deoxy-D-manno-octulosonic (or 2-keto-3-deoxyoctonic) acid (Kdo). The Kdo residue is located at the reducing end of the oligosaccharide chain and is essential for its biological activity. These saccharide units are usually substituted by charged phosphate groups, resulting in an accumulation of charge in this inner region. For some time it was thought that the minimum structure for cell viability in E. coli had the di-Kdo moiety, but viable mutants lacking Kdo and with the basic tetra-acyl form of lipid A, i.e. lacking the two secondary acyl groups (and termed 'lipid IVA'), have recently been produced. Indeed, lipid IVA may be the minimum structure required for the viability of the organism. During the biosynthesis of lipid A, intermediates are formed with a pyrophosphate residue in position 1 and a monophosphate in position 4’ of the disaccharide unit, and it is now recognized that some species retain the pyrophosphate substituent. Other structural modifications sometimes observed include phosphorylation and phosphoethanolamine addition.
The O-specific chains are distinctive and characteristic. Smooth-type Gram-negative bacteria synthesise lipopolysaccharides that differ in the length, branching and fine structure of this part of the molecule. The polysaccharide chains consist of repetitive subunits that extend out from the bacteria, and they can include from one to 25 chemically identical repeating oligosaccharide units, which in turn contain from 2 to 7 monosaccharide residues. As a result of diversity in the nature of the monosaccharides, their alternative configurations and the innumerable types of glycosidic linkage, the O-chain in most bacterial species is unique. While each repeating unit may only contain a limited number of monosaccharide residues, there are more than a hundred types that can be selected in addition to many kinds of non-carbohydrate substituents. For example, in some phytophathogens, most of the O-specific chains have a backbone of rhamnose residues, which may be of the D or L configuration and in α or β anomeric forms, often in the same structure. The references cited below afford more detailed information.
Again, lipopolysaccharides from marine cyanobacteria such as the genus Synechococcus differ appreciably in the core region in that they lack heptose and Kdo and instead have a chain of 4-linked glucose units, with one rhamnose in some strains, but no phosphate residues. Some of these structures include long fatty acyl chains with varying degrees of unsaturation. Similarly, the lipid A components in bacteria from extreme environments can differ appreciably in structure from the norm, and they together with their biosynthetic enzymes are being studied for their potential therapeutic value.
In pathogenic bacteria, it is the O-chains that come in contact with the host organism during infection, and as they are antigenic, they form the basis for serotype classification of bacterial genera and so are also termed 'O-antigens'. They protect the bacterium against the lytic action of the host defenses as well as from the effects of antibiotics. On the other hand, when separated from the lipid A component, the O-antigens do not display endotoxic activity. Lipid A is the membrane anchor for capsular polysaccharides in Bacteroides fragilis in the human gut microbiome. In this organism, the polysaccharide A is zwitterionic and contains more than 100 repeating units of a tetrasaccharide consisting of D-galactopyranose, 2,4-dideoxy-4-amino-D-FucNAc, D‑N‑acetylgalactosamine, and D-galactofuranose with 4,6‑pyruvate. This is considered to be the model symbiotic immunomodulatory molecule that confers benefits to the host in respect of autoimmune, inflammatory and infectious diseases.
3. Biosynthesis of Kdo2-Lipid A
Lipid A is synthesised in the cytoplasmic compartment of Gram-negative bacteria, and the essential details of the process are now known. In brief in E. coli, lipid A is synthesised on the cytoplasmic surface of the inner membrane by a conserved pathway of nine distinct enzymes (sometimes termed the 'Raetz' pathway), the first six of which are required for bacterial growth (and are targets for the development of new antibiotics). In addition, there are three protein-bound acyl donor substrates together with UDP-N-acetylglucosamines (UDP-GlcNAc), ATP, and CMP-3-deoxy-D-manno-octulosonic acid (CMP-Kdo). While some variation exists between species, especially in relation to acylation, the enzymes of this pathway are highly conserved in Gram-negative bacteria. It is only possible to give a brief outline of the various steps here, but more detailed information is available in the reading lists below.
Biosynthesis begins with the transfer of one molecule of 3-hydroxy-14:0 (in E. coli) from its linkage to acyl carrier protein to position O-3 of UDP‑N‑acetylglucosamine by means of the enzyme LpxA. This is deacetylated by LpxC (the committed and rate-limiting step in the pathway), followed by the transfer of a second 3-hydroxy-14:0 to the free amino group by the enzyme LpxD. LpxA and LpxD act in effect as 'hydrocarbon rulers' and determine the length of hydroxyacyl chains incorporated, ensuring that all are the same. UDP-diacyl-GlcN is then cleaved by the pyrophosphatase LpxH to form the phosphorylated intermediate termed lipid X (a key molecule in the discovery of the pathway), which is in turn condensed with a further molecule of UDP-diacyl-GlcN by LpxB (the disaccharide synthase) to produce a β(1→6)-linked disaccharide, which carries 3-hydroxy-residues at positions 2, 3, 2' and 3' together with an α-linked phosphate at O-1.
The next reactions in the sequence are catalysed sequentially by the integral membrane proteins LpxK, KdtA, LpxL and LpxM. Kinase LpxK phosphorylates the 4'‑position of the disaccharide-1-P to form lipid IVA, a critical biosynthetic step and regulatory node. KdtA then transfers two Kdo residues from CMP-Kdo to the non-reducing GlcN of lipid IVA before the remaining acyl groups are added (LpxL and LpxM) to produce the basic Kdo2-lipid A molecule. Thus, while acyl transferases in some organisms have strict substrate requirements, others are more tolerant both of the fatty acid and acyl acceptor and result in a heterogeneous lipid A composition of the outer membrane.
In addition, some Gram-negative bacteria express multiple tailoring enzymes that work sequentially to further modify lipid A structures during transport of the intact lipopolysaccharide molecule from the outer surface of the cytoplasmic membrane to the inner surface of the outer membrane. For example, enzymes in the periplasmic leaflet can modify the phosphate moieties on the disaccharide backbone by hydrolysis or by the addition of cationic sugars or phosphoethanolamine that enable the organisms to circumvent the host immune system responses, influencing the virulence of some pathogens and their resistance to antibiotics. Such enzymes are seen as potential therapeutic targets.
The O-antigen is also synthesised by membrane-associated enzyme complexes in the cytoplasm and requires C55-undecaprenyl phosphate as an acceptor for assembly of the oligosaccharide chain via four different routes and the action of many different glycosyltransferases, but this is a topic for carbohydrate experts. The synthetic mechanism and the mode of transport across the inner membrane differs according to species. After the completion of the Raetz pathway to Kdo2-lipid A, the core oligosaccharide is attached at the cytoplasmic side of the inner membrane, when the nascent core-lipid A is flipped to the periplasmic surface of the inner membrane by a specific transporter (MsbA). The O-antigen polymer, which must also be transported to the periplasmic side of the inner membrane, is then attached. Transport of the mature lipopolysaccharide from the periplasmic side of the inner membrane to its final destination, the outer leaflet of the outer membrane, is carried out by the Lpt (Lipopolysaccharide transport) machinery, which consists of seven proteins (LptA to LptG) that link together in a complex or protein bridge (in two sub-assemblies) spanning the entire cell wall, i.e. cytoplasm to inner membrane to periplasm to outer membrane. Cardiolipin is reported to aid this process. These transport proteins are also seen as potential targets for the development of novel antibiotics against Gram-negative pathogens.
Glycerophospholipids: The conventional phospholipids, phosphatidylethanolamine (~80%), phosphatidylglycerol (~15%) and cardiolipin (~5%), are synthesised at the inner leaflet of the inner membrane, and must be flipped across to the outer leaflet, before some are transported across the periplasm to the inner leaflet of the outer membrane. The transport mechanisms are poorly understood, but it is evident that transport can occur in both directions, possibly via contact sites between the inner and outer membranes or by protein-mediated systems.
4. Lipid A as an Endotoxin in Bacterial Infections
As the bacterial lipopolysaccharide lipid A is one of the most conserved structures within all Gram negative bacterial species, it is an important pathogen associated molecule that is recognized by innate immune systems across all kingdoms of life, and this can result in the clearance of a bacterial infection in a timely manner. Thus, when bacteria multiply and then die and break up, a heat-stable lipopolysaccharide is liberated, and this functions as a powerful bacterial toxin that has been termed an endotoxin to differentiate it from the heat-labile exotoxins released by live bacteria. This was first demonstrated in humans towards the end of the 19th century when it was shown that heat-killed cholera bacteria were themselves toxic.
The lipid A component, in particular, is known to be responsible for many of the toxic effects of infections with Gram-negative bacteria. Because of its conserved structure in diverse pathogens of this kind, it is recognized as a pathogen-associated molecule by many different receptors, e.g. toll-like receptor 4 (TLR4), on immune cells (monocytes, macrophages, neutrophils and dendritic cells especially), and stimulates a robust inflammatory response by activation of caspase-4 and caspase-5 in humans and thence secretion of pro-inflammatory cytokines. As one example and rather simplistically, a lipopolysaccharide binds to a large hydrophobic pocket in TLR4 via its lipid chains, while its phosphate group with its two negative charges can interact directly with the myeloid differentiation factor 2 (MD-2) leading to formation of a heterodimer complex that recognizes a common 'pattern' in structurally diverse lipopolysaccharide molecules and is active in immune signalling.
TLR4 is unique in that it exists as a transmembrane protein that enables the transmission of information on lipopolysaccharide detection to the cytosol, where the dimerized TLR4 domains are detected by a protein TIRAP, which binds to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) in the plasma membrane. This leads to the formation of molecular complexes in the cell that initiate the TLR4 signalling. As part of the mechanism, lipid A induces S-palmitoylation and thence activation of phosphatidylinositol 4-kinase, which generates phosphatidylinositol 4-monophosphate, the precursor of PI(4,5)P2. The strength of these effects is dependent on the precise structure of the lipid A, but the result is recruitment of immune cells to the site of infection to attack the foreign pathogen, a response that is beneficial for clearing minor bacterial infections. On occasion, however, lipid A can trigger systemic inflammation to cause tissue damage and in the worst cases this can lead to septic shock and death of the host by lung or kidney failure. While the response to lipopolysaccharide exposure is highly complex, and the rates of transcription of hundreds of genes are affected, one relatively constant factor appears to the involvement of phosphatidylinositol 4,5-bisphosphate in the process.
There are exceptions to this general mechanism. For example, the TLR4 receptor does not recognize the Francisella novicida endotoxin, which is thus able to evade the host innate immune system. Instead, this stimulates the cyclooxygenase-2-dependent inflammatory pathway and is responsible for the lethality of such infections through overproduction of proinflammatory effectors including prostaglandin E2. There is also a suggestion that lipopolysaccharides can affect Parkinson's disease by binding to the α-synuclein protein.
There is concern that neurodegeneration and Parkinson's disease especially could be affected by lipopolysaccharide derived from Gram-negative bacteria in the intestines if this is transported to the central nervous system via the vagus nerve when bound to proteins such as alpha-synuclein. Together they could initiate amyloidogenesis.
The observed biological effects are partly due to the primary structure of the lipid A moiety, but also to the fact that it adopts a specific conformation that enhances the activity by enabling binding to specific host molecules. It is evident that the number, positions, and chain-lengths of the fatty acid constituents have a determinant role in the toxicity and biological activity of the molecule, and the secondary (estolide) fatty acid constituents appear especially important in this context. Indeed, quite subtle changes in fatty acid composition can lead to profound changes in toxicity and in the immune response. As a generality, lipid A forms with shorter chain-fatty acids tend to be less toxic than those with longer-chain substituents, while penta-acyl lipid A forms in some bacterial species show highly variably potency depending on the precise composition. The pathogen Yersinia pestis normally synthesises lipid A with six fatty acid chains in the fleas that act as carriers at 21 to 27°C, but in a human host at 37°C it produces lipid A with only four fatty acyl chains because of a single-nucleotide polymorphism that results in a premature stop in translation of a specific lipid A acyltransferase. This lipid A form escapes attack by the immune system as it does not activate the TLR4 receptor, and is crucial for the infectivity and pathogenesis of the organism. Lipid A from invasive strains of Neisseria meningitides is hexa-acylated, whereas lipid A of carrier strains is penta-acylated. Together with the addition of positively charged substituents to the lipid A phosphate groups, increasing acylation reduces the permeability of the bacterial membranes to antibacterial agents.
Full recognition of lipopolysaccharides by pattern-recognition receptors requires the complete complement of six acyl chains that is normally present in the lipid portion. To counter the direct toxic effects, there is an endogenous lipase or acyloxyacyl hydrolase in the liver and spleen of the host that selectively removes the secondary fatty acyl chains from bacterial lipopolysaccharides and prevents their recognition by the mammalian signalling receptors. This is conserved evolutionarily among animal species and reduces substantially the risk of prolonged inflammatory reactions during infections by Gram-negative bacteria. By transforming lipopolysaccharides from stimulants to inhibitors, the enzyme reduces tissue injury, prevents prolonged immunosuppression after infection, and limits the entry of stimulatory forms into the bloodstream.
In contrast, as potent stimulators of the innate immune system, endotoxins can sometimes bring about a significant enhancement of resistance to infection that is beneficial to the host, so chemically modified endotoxins have therapeutic potential. There are also some reports that certain lipopolysaccharide molecules show antagonistic activity against toxic lipopolysaccharides, including the total lipopolysaccharides from the human gut microbiome. By competing with them for the binding to the TLR4/MD-2 complex, they prevent the transmission of the downstream signal responsible for eliciting inflammatory responses. Thus, the structural diversity of lipid A has been harnessed to create a vaccine adjuvant ('MPL®') that enhances a beneficial adaptive immune response safely against a co-inoculated antigen. From all standpoints, these interactions continue to be the subject of intensive study in humans.
While the O-antigen component may be less important to toxicity, it aids the solubility and transport of the lipopolysaccharide molecule and enhances the ability of the bacteria to establish infection; its antigenicity is of course of great importance and it promotes adhesion to epithelial cells. The phosphate groups and other polar moieties of the core polysaccharide are also important to toxicity, as they are involved in binding to receptor molecules, and they can inhibit the action of antimicrobial agents. For example, the substituents at the 4'-phosphate of glucosamine II are responsible for the bacterial resistance to polycationic antibiotics such as polymyxins. If the hydroxyl group at the 4'-phosphate of glucosamine II is not substituted, polymyxin will attach to it and the bacteria will be susceptible to the antibiotic. On the other hand, if the hydroxyl group carries a substituent, polymyxin cannot join and the bacteria will be resistant. Inhibition of the lipid A phosphoethanolamine transferase is therefore believed to have potential as a therapeutic approach. Many organisms produce antimicrobial peptides akin to polymyxins that possess dual functions in that they kill bacteria and neutralize the endotoxic effect of lipopolysaccharides, and this is the subject of much research.
Similarly, lipopolysaccharides are important molecules in the interactions between bacteria and plants, both in relation to symbiosis and to pathogenesis. They protect bacteria from plant-derived antimicrobial substances, and conversely they trigger defence responses following challenge by pathogens. Although the mechanisms are not fully understood, it is apparent that both lipid A and the core oligosaccharide from pathogenic bacteria are potent inducers of immune responses, including a rapid increase in the concentration of cytosolic calcium, the induction of antibacterial oxygen species and the expression of antimicrobial peptides, presumably by different signalling pathways. The receptor kinase Lipooligosaccharide-Specific Reduced Elicitation (LORE) has been shown to mediate plant immune responses to bacterial lipopolysaccharide medium-chain 3-hydroxy fatty acids in a chain length- and hydroxylation-specific manner, with free (R)-3-hydroxydecanoic acid as the strongest immune elicitor.
It should be recognized that the existence of lipid A-containing lipopolysaccharide in the most ancient and primitive Gram-negative bacteria demonstrates that it is absolutely required for their survival, shielding them from a variety of aggressive conditions. It is not produced simply to aggravate humans. On the other hand, because of the increasing emergence of multidrug-resistant bacteria, there is a critical need for the development of novel antibiotics, and the detailed knowledge that has been gained of the biosynthetic pathway for Kdo2-lipid A is seen as providing a target for pharmaceutical intervention. Antagonists of the TLR4 receptor are also being sought, including synthetic lipid A analogues and the less toxic forms from marine bacteria, extremophiles and the intestinal microbiome.
Analysis: While analysis of lipid A per se is a daunting technical challenge, requiring advanced mass spectrometric and other spectroscopic techniques allied to liquid chromatography, it is relatively a much easier task to detect and quantify the 3-hydroxy-fatty acid constituents, especially 3-hydroxytetradecanoic acid, in plasma and other tissues as a means of evaluating endotoxin levels in patients infected with Gram-negative bacteria.
5. Other Bacterial Lipopolysaccharides
Lipo-chitooligosaccharides are a class of signalling molecules produced by nitrogen-fixing rhizobia that play important roles in plant-microbe interactions. They are nodulation factors (Nod factors), which are essential for establishment of the nitrogen-fixing root nodule symbiosis with legume plants. In general, they consist of a carbohydrate (chitin) backbone consisting of three to five N-acetylglucosamine (GlcNAc) units, sulfated on O6 of the reducing residue and with a non-reducing terminal glucosamine unit linked by an amide bond to a fatty acid. In different bacterial species, the number of GlcNAc units, the degree of acetylation, the presence or absence of a sulfate group and the number of double bonds in the unsaturated fatty acid can vary. In that secreted by the bacterium Sinorhizobium meliloti (illustrated), which interacts with plants of the genus Medicago, the fatty acid component is mainly 2E,9Z-hexadecadienoate, but small amounts of palmitate or palmitoleate may be found also.
Similar compounds have now been isolated from one species of mycorrhizal fungi (Rhizophagus irregularis). These are less diverse structurally, but like Nod factors they stimulate root development and induce calcium spiking and transcriptional change in leguminous plants. There is evidence that two complementary receptor systems operate in the host plants, one triggering a signalling cascade that leads to cell division while the second controls the intracellular entry of rhizobia. As many different microbial and fungal species may be involved in the interactions, these receptors must be able to accommodate a wide range of Nod factor structures.
Mycobacteria synthesize intracellular 6-O-methylglucose–containing lipopolysaccharides, and that from M. tuberculosis has acetyl, isobutyryl, succinyl, and octanoyl groups attached to glucose in the terminal region of the molecule; it induces protective T cell formation in host animals. There are many other classes of bacterial lipopolysaccharides, but for reasons of practical convenience, these are discussed elsewhere on this website in relation to glycolipid surfactants, phosphoglycolipids (lipoteichoic acids) and glycophospholipids (capsular lipopolysaccharides).
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