The chemical structure of the lipid A component of lipopolysaccharides from Vibrio cholerae

The outer membrane is an essential load-bearing element in Gram-negative bacteria

Gram-negative bacteria possess a complex cell envelope that consists of a plasma membrane, a peptidoglycan cell wall and an outer membrane. The envelope is a selective chemical barrier1 that defines cell shape2 and allows the cell to sustain large mechanical loads such as turgor pressure3. It is widely believed that the covalently cross-linked cell wall underpins the mechanical properties of the envelope4,5. Here we show that the stiffness and strength of Escherichia coli cells are largely due to the outer membrane. Compromising the outer membrane, either chemically or genetically, greatly increased deformation of the cell envelope in response to stretching, bending and indentation forces, and induced increased levels of cell lysis upon mechanical perturbation and during L-form proliferation. Both lipopolysaccharides and proteins contributed to the stiffness of the outer membrane. These findings overturn the prevailing dogma that the cell wall is the dominant mechanical element within Gram-negative bacteria, instead demonstrating that the outer membrane can be stiffer than the cell wall, and that mechanical loads are often balanced between these structures.

Endotoxin evaluation of eleven lipopolysaccharides by whole blood assay does not always correlate with Limulus amebocyte lysate assay

More than 90% of all publications on endotoxin were carried out with endotoxins (lipopolysaccharide, LPS) from enterobacteriaceae. We compared the immune stimulatory potency of 11 different LPSs using human whole blood incubations. While the majority of LPSs induced cytokine release equipotently, a 1000-fold more LPS from Pseudomonas aeruginosa or Vibrio cholerae was still less potent in inducing TNF, IL-1β, IL-10 and IFN-γ though it potently induced nanogram quantities IL-8. All LPSs tested, regardless of the micro-organism, showed Toll-like receptor (TLR)4-dependence, except for the LPSs from P. aeruginosa and V. cholerae, which were both TLR4- and TLR2-dependent. Interestingly, UV-inactivated P. aeruginosa bacteria, although Gram-negative, also showed TLR2- and TLR4-dependence. Repurification of commercial LPS preparations by phenol re-extraction led to a complete loss of the TLR2 dependency, indicating contamination with lipoproteins. In the Limulus amebocyte lysate assay, often performed to exclude contamination in purified water likely to originate from P. aeruginosa, P. aeruginosa LPS was only 2-fold less potent than LPS from S. abortus equi or the assay standard LPS from E. coli. This results in an overestimation of pyrogenic burden by a factor of 500 in the sample when compared with the biological activity of highly purified P. aeruginosa LPS in human whole blood.

Temperature-Dependence of Lipid A Acyl Structure in Psychrobacter cryohalolentis and Arctic Isolates of Colwellia hornerae and Colwellia piezophila

Lipid A is a fundamental Gram-negative outer membrane component and the essential element of lipopolysaccharide (endotoxin), a potent immunostimulatory molecule. This work describes the metabolic adaptation of the lipid A acyl structure by Psychrobacter cryohalolentis at various temperatures in its facultative psychrophilic growth range, as characterized by MALDI-TOF MS and FAME GC-MS. It also presents the first elucidation of lipid A structure from the Colwellia genus, describing lipid A from strains of Colwellia hornerae and Colwellia piezophila, which were isolated as primary cultures from Arctic fast sea ice and identified by 16S rDNA sequencing. The Colwellia strains are obligate psychrophiles, with a growth range restricted to 15 °C or less. As such, these organisms have less need for fluidity adaptation in the acyl moiety of the outer membrane, and they do not display alterations in lipid A based on growth temperature. Both Psychrobacter and Colwellia make use of extensive single-methylene variation in the size of their lipid A molecules. Such single-carbon variations in acyl size were thought to be restricted to psychrotolerant (facultative) species, but its presence in these Colwellia species shows that odd-chain acyl units and a single-carbon variation in lipid A structure are present in obligate psychrophiles, as well.

Elucidation of a novel Vibrio cholerae lipid A secondary hydroxy-acyltransferase and its role in innate immune recognition

Similar to most Gram-negative bacteria, the outer leaflet of the outer membrane of Vibrio cholerae is comprised of lipopolysaccharide. Previous reports have proposed that V. cholerae serogroups O1 and O139 synthesize structurally different lipid A domains, which anchor lipopolysaccharide within the outer membrane. In the current study, intact lipid A species of V. cholerae O1 and O139 were analysed by mass spectrometry. We demonstrate that V. cholerae serogroups associated with human disease synthesize a similar asymmetrical hexa-acylated lipid A species, bearing a myristate (C14:0) and 3-hydroxylaurate (3-OH C12:0) at the 2'- and 3'-positions respectively. A previous report from our laboratory characterized the V. cholerae LpxL homologue Vc0213, which transfers a C14:0 to the 2'-position of the glucosamine disaccharide. Our current findings identify V. cholerae Vc0212 as a novel lipid A secondary hydroxy-acyltransferase, termed LpxN, responsible for transferring the 3-hydroxylaurate (3-OH C12:0) to the V. cholerae lipid A domain. Importantly, the presence of a 3-hydroxyl group on the 3'-linked secondary acyl chain was found to promote antimicrobial peptide resistance in V. cholerae; however, this functional group was not required for activation of the innate immune response.

The lipid A from Vibrio fischeri lipopolysaccharide: a unique structure bearing a phosphoglycerol moiety

Vibrio fischeri, a bioluminescent marine bacterium, exists in an exclusive symbiotic relationship with the Hawaiian bobtail squid, Euprymna scolopes, whose light organ it colonizes. Previously, it has been shown that the lipopolysaccharide (LPS) or free lipid A of V. fischeri can trigger morphological changes in the juvenile squid's light organ that occur upon colonization. To investigate the structural features that might be responsible for this phenomenon, the lipid A from V. fischeri ES114 LPS was isolated and characterized by multistage mass spectrometry (MS(n)). A microheterogeneous mixture of mono- and diphosphorylated diglucosamine disaccharides was observed with variable states of acylation ranging from tetra- to octaacylated forms. All lipid A species, however, contained a set of conserved primary acyl chains consisting of an N-linked C14:0(3-OH) at the 2-position, an unusual N-linked C14:1(3-OH) at the 2'-position, and two O-linked C12:0(3-OH) fatty acids at the 3- and 3'-positions. The fatty acids found in secondary acylation were considerably more variable, with either a C12:0 or C16:1 at the 2-position, C14:0 or C14:0(3-OH) at the 2'-position, and C12:0 or no substituent at the 3'-position. Most surprising was the presence of an unusual set of modifications at the secondary acylation site of the 3-position consisting of phosphoglycerol (GroP), lysophosphatidic acid (GroP bearing C12:0, C16:0, or C16:1), or phosphatidic acid (GroP bearing either C16:0 + C12:0 or C16:0 + C16:1). Given their unusual nature, it is possible that these features of the V. fischeri lipid A may underlie the ability of E. scolopes to recognize its symbiotic partner.

Secondary acylation of Vibrio cholerae lipopolysaccharide requires phosphorylation of Kdo

The lipopolysaccharide of Vibrio cholerae has been reported to contain a single 3-deoxy-d-manno-octulosonic acid (Kdo) residue that is phosphorylated. The phosphorylated Kdo sugar further links the hexa-acylated V. cholerae lipid A domain to the core oliogosaccharide and O-antigen. In this report, we confirm that V. cholerae possesses the enzymatic machinery to synthesize a phosphorylated Kdo residue. Further, we have determined that the presence of the phosphate group on the Kdo residue is necessary for secondary acylation in V. cholerae. The requirement for a secondary substituent on the Kdo residue (either an additional Kdo sugar or a phosphate group) was also found to be critical for secondary acylation catalyzed by LpxL proteins from Bordetella pertussis, Escherichia coli, and Haemophilus influenzae. Although three putative late acyltransferase orthologs have been identified in the V. cholerae genome (Vc0212, Vc0213, and Vc1577), only Vc0213 appears to be functional. Vc0213 functions as a myristoyl transferase acylating lipid A at the 2'-position of the glucosamine disaccharide. Generally acyl-ACPs serve as fatty acyl donors for the acyltransferases required for lipopolysaccharide biosynthesis; however, in vitro assays indicate that Vc0213 preferentially utilizes myristoyl-CoA as an acyl donor. This is the first report to biochemically characterize enzymes involved in the biosynthesis of the V. cholerae Kdo-lipid A domain.

Lipopolysaccharides of Vibrio cholerae: III. Biological functions

This review presents the salient features of the biological functions including the (i) endotoxic activities, (ii) antigenic properties, (iii) immunological responses to and (iv) phage receptor activities of the Vibrio cholerae lipopolysaccharides (LPS). The biological functions of the capsular polysaccharide (CPS) of V. cholerae have also been discussed briefly as a relevant topic. The roles of LPS and other extracellular polysaccharides in the (i) intestinal adherence and virulence of the vibrios and (ii) the biofilm formation by the organisms have been analysed on the basis of the available data. Every effort has been made to bring out, wherever applicable, the lacunae in our knowledge. The need for the continuous serogroup surveillance and monitoring of the environmental waters and the role of LPS in the designing of newer cholera vaccines has been discussed briefly in conclusion.

Lipopolysaccharides of Vibrio cholerae. I. Physical and chemical characterization

Vibrio cholerae is the causative organism of the disease cholera. The lipopolysaccharide (LPS) of V. cholerae plays an important role in eliciting the antibacterial immune response of the host and in classifying the vibrios into some 200 or more serogroups. This review presents an account of our up-to-date knowledge of the physical and chemical characteristics of the three constituents, lipid-A, core-polysaccharide (core-PS) and O-antigen polysaccharide (O-PS), of the LPS of V. cholerae of different serogroups including the disease-causing ones, O1 and O139. The structure and occurrence of the capsular polysaccharide (CPS) on V. cholerae O139 have been discussed as a relevant topic. Similarity and dissimilarity between the structures of LPS of different serogroups, and particularly between O22 and O139, have been analysed with a view to learning their role in the causation of the epidemic form of the disease by avoiding the host defence mechanism and in the evolution of the newer pathogenic strains in future. An idea of the emerging trends of research involving the use of immunogens prepared from synthetic oligosaccharides that mimic terminal epitopes of the O-PS of V. cholerae O1 in the development of a conjugate anti cholera vaccine is also discussed.

Structure and activity of persicomycins, toxins produced by a Pseudomonas syringae pv. persicae/Prunus persica isolate

A toxigenic property has been demonstrated in a Pseudomonas syringae pv. persicae/Prunus persica isolate. Several substances, which are named persicomycins, have been purified in variable quantities from cultures. The structures of four of them were established by NMR and chemical ionization mass spectrometry. These compounds are 3-(3'-hydroxy)hydroxy fatty acids and thus represent a new family among the phytobacterial toxins. Other minor substances have also been isolated and have been shown to belong to the same family on the basis of their 7H-NMR spectra. All of them cause necrosis of peach tree tissues, a symptom similar to the one obtained after bacterial infection and antibiosis of microorganisms such as Bacillus thuringiensis. These results provide evidence that necrosis-inducing toxins are not restricted to the pathovar syringae. Furthermore, similar substances were purified from necrosed tissues of inoculated and diseased peach trees. 3-(3'-Hydroxydecanoyloxy)hexadecenoic acid was isolated from both such tissues and from cultures, which strongly suggests a similar toxigenesis in vivo and in vitro. The involvement of persicomycins in the die-back disease of peach trees is now clearly established, which demonstrates that the toxigenic property of the bacterium participates in the disease. The phytotoxicity of the persicomycins is discussed in comparison with the lipodepsipeptide necrotic toxins of the syringae pathovar.

Lipid A mutants of Vibrio cholerae: isolation and partial characterization

Vibrio cholerae mutants resistant to common antibiotics and neutral and anionic detergents were isolated. Analysis of isolated outer membranes revealed a significant deficiency in the acylation of lipid A in the resistant strains. The content of amide-linked and ester-bound fatty acids in the lipid A of the mutant strains compared to that of the wild type was about 50-56% and 29-37% respectively. This defect was specific for lipid A as there was no change in the acylation of phospholipids. The reduction in fatty acid content of lipid A was reflected in the altered endotoxic properties in the mutant strains.

Genetics of Vibrio cholerae and its bacteriophages

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The effect of removal of D-fructose on the antigenicity of the lipopolysaccharide from a rough mutant of Vibrio cholerae Ogawa

The lipopolysaccharide (LPS) of a rough mutant (95R) of Vibrio cholerae Ogawa has been investigated chemically and serologically. D-Fructose was released from LPS under conditions (10mM trifluoroacetic acid, 60 degrees, 1 h) that liberated no other sugar constituent of the LPS (2-amino-2-deoxy-D-glucose, D-glucose, L-glycero-D-manno-heptose). Upon periodate oxidation, D-fructose and D-glucose were oxidised quantitatively, whereas approximately 50% of heptose was periodate-resistant. The data indicate that D-fructose does not link the polysaccharide and lipid A portion as proposed earlier, and suggest that D-fructose is present as a branch. By passive hemolysis inhibition, it was shown that the release of D-fructose paralleled the exposure of an antigenic determinant cryptic in LPS.

Nature and location of amide-bound (R)-3-acyloxyacyl groups in lipid A of lipopolysaccharides from various gram-negative bacteria

It has previously been demonstrated [Eur. J. Biochem. 124, 191-198 (1982) and 137, 15-22 (1983)] that the lipid A component of Salmonella and Proteus lipopolysaccharides contains amide-linked (R)-3-acyloxyacyl residues. In the present study lipid A of other gram-negative bacteria was analysed for the presence of amide-bound 3-acyloxyacyl residues. It was found that such residues are constituents of all lipid A tested (Agrobacterium tumefaciens, Chromobacterium violaceum, Pseudomonas aeruginosa, Xanthomonas sinensis, Bacteroides fragilis, Vibrio cholerae, Fusobacterium nucleatum, Rhodospirillum tenue, Acinetobacter calcoaceticus, and Escherichia coli). Amide-linked (R)-3-acyloxyacyl groups, therefore, represent common and ubiquitous structural elements of bacterial lipid A. The composition of 3-acyloxyacyl groups differed considerably among different bacteria. As amide-bound (R)-3-hydroxy fatty acids straight chain and isobranched acyl groups with 10-17 carbon atoms were identified. The most frequently encountered fatty acids, substituting the 3-hydroxyl group of 3-hydroxy fatty acids, were nonhydroxylated straight chain and isobranched acyl residues with 10-17 carbon atoms as well as (S)-2-hydroxy fatty acids with 12 carbon atoms. In some cases, using laser desorption mass spectrometry, the distribution of 3-acyloxyacyl residues over the two available glucosamine amino groups of the lipid A backbone was investigated.

Structural studies on the phosphate-free lipid A of Rhodomicrobium vannielii ATCC 17100

The structure of the free lipid A from Rhodomicrobium vannielii ATCC 17100 was elucidated. It consists of a central beta-1',6-linked glucosamine disaccharide which is not substituted by phosphate. About 30% of the disaccharide molecules are substituted with mannopyranose in beta-1,4'-linkage to the non-reducing glucosamine. The reducing glucosamine can be directly reduced with NaBH4, indicating either that this glucosamine is not substituted at C1 or its substituent has been removed during the preparation of free lipid A or is removed during reduction with NaBH4. The following formula shows the 'backbone' structure of the free lipid A from Rm. vannielii ATCC 17100: beta-Manp(1-- leads to 4)-beta-GlcpN(1 leads to 6)GlcpN. 3-(R)-Hydroxyhexadecanoic acid is linked to the amino group of the reducing glucosamine. The residue at the amino group of the non-reducing glucosamine has not been identified. The hydroxyl groups of the central disaccharide are acylated with 3-(tetradecanoyloxy)-tetradecanoic acid, 3-hydroxytetradecanoic acid, delta 14-docosenoic acid (delta 14-C22:1) and acetyl groups. The hydroxyl groups of the mannose are not substituted.

The lipopolysaccharide of a chloridazon-degrading bacterium

Lipopolysaccharide of a chloridazon-degrading bacterium was obtained by a two-stage extraction procedure with phenol/EDTA in a yield of 0.3% of dried bacteria. The carbohydrate moiety consisted of heptose, 3-deoxyoctulosonic acid and D-glucose in a molar ratio of 1:2:2 X 3. Lipid A was composed of 1 mol 2,3-diamino-2,3-dideoxy-D-glucose, 2 mol amide-bound and 2.6 mol ester-bound fatty acids/mol. Amide-bound fatty acids were 3-hydroxydodecanoic acid and 3-hydroxyhexadecanoic acid; dodecanoic acid and R-(-)-3-hydroxydodec-5-cis-enoic acid were found to be present in ester linkage. Under conditions of acidic hydrolysis, the latter was converted into the cis and trans isomers of 5-hexyltetrahydrofuran-2-acetic acid. Dodecanoic acid was demonstrated to be linked with the hydroxy groups of the amide-bound fatty acids. The taxonomic significance of these results, especially the demonstration of 2,3-diamino-2, 3-dideoxy-D-glucose, is discussed.

Chemical structure of the lipid A component of the lipopolysaccharide from a Proteus mirabilis Re-mutant

The chemical structure of the lipid A component from the lipopolysaccharide of a Proteus mirabilis Re-mutant (strain R45) was analysed. It consists of a beta(1-6)-linked D-glucosamine disaccharide which carries two phosphate groups, one being ester-linked to position 4' of the nonreducing glucosaminyl residue and the other being bound to the glycosidic hydroxyl group of the reducing glucosaminyl residue. The ester-bound phosphate group is quantitatively substituted by a 4-amino-4-deoxy-L-arabinopyranosyl residue, the glycosidic phosphoryl group appears to be unsubstituted. Two available hydroxyl groups of the disaccharadide (probably at positions 3 and 3') are acylated by approximately 1 mol each of (R)-3-tetradecanoyloxytetradecanoic and (R)-3-hydroxytetradecanoic acid/mol. The amino group of the nonreducing glucosaminyl residue carries (R)-3-tetradecanoyloxytetradecanoic and that of the reducing residue (R)-3-hydroxytetradecanoic acid. In addition smaller amounts of (R)-3-hexadecanoyloxytetradecanoic acid are present in amide linkage. The attachment site of the oligosaccharide portion to lipid A was also investigated. It was found that the hydroxyl group at position 6' of the nonreducing glucosaminyl residue carries 3-deoxy-D-manno-octulosonic acid. This indicates that the saccharide portion in this Proteus lipopolysaccharide is linked to lipid A via the primary hydroxyl group in position 6'.

Structural studies on the non-toxic lipid A from Rhodopseudomonas sphaeroides ATCC 17023

The lipid A isolated from Rhodopseudomonas sphaeroides ATCC 17023 has been investigated. A sequence of analyses indicated beta-D-glucosaminyl-1,6-D-glucosamine as the sugar backbone carrying phosphate groups at C-1 of the reducing glucosamine and C-4 of the non-reducing glucosamine. Both 3-(7-tetradecenoyl)oxytetradecanoic and 3-oxotetradecanoic acid are linked to the NH2 groups. Two residues of 3-hydroxydecanoic acid are linked to hydroxy groups at C-3 of both reducing and non-reducing glucosamine, and the hydroxy group of 3-hydroxydecanoic acid is free. In free lipid A, the hydroxy group at C-4 of the reducing glucosamine and at C-6 of the non-reducing glucosamine are unsubstituted. The latter probably arises from a 3-deoxy-D-manno-octulosonate linkage. The structural similarities and dissimilarities existing between lipid A of R. sphaeroides, which is non-toxic, and lipid A types such as those from Salmonella, which are toxic, are discussed.

Further studies on bacterial lipopolysaccharide-induced protection against experimental allergic encephalomyelitis in guinea pigs. Effects of chemical modifications

Bacterial lipopolysaccharide (LPS)-induced protection against experimental allergic encephalomyelitis (EAE) was studied in guinea pigs using chemically modified derivatives. Hydroxylaminolysis or alkaline hydrolysis of LPS, by which ester-linked fatty acids are removed from LPS, resulted in total loss of its mitogenic activity for B lymphocytes, and EAE-suppressive activity was simultaneously reduced. Similar diminished activity was observed in delayed-type skin reactions to myelin basic protein (BP) and anti-BP antibody production detected by passive hemagglutination and enzyme-linked immunosorbent assay (ELISA). These results indicate that the active site of LPS in suppressing EAE is in the lipid portion and that there exists a good correlation between the capacity of LPS to suppress EAE and its mitogenic activity.

Structural studies of the Vibrio cholerae O-antigen

The dominant part of the O-antigen of Vibrio cholerae is a homopolysaccharide composed of (1 leads to 2)-linked 4-amino-4,6-dideoxy-alpha-D-mannopyranosyl (perosaminyl) residues, in amino groups of which are acylated by 3-deoxy-L-glycero-tetronic acid. Most of the amino sugar is decomposed during acid hydrolysis. Treatment of the polymer with anhydrous hydrogen fluoride, which cleaves the glycosidic linkages but does not cause N-deacylation, followed by acid hydrolysis under mild conditions, produced the monomer in good yield. Treatment of the N-deacylated polysaccharide with nitrous acid caused deamination with concomitant rearrangements, typical of 4-amino-4-deoxyhexopyranosyl residues in which the amino group occupies an equatorial position.