Lipopolysaccharide endotoxins

Synthesis and proinflammatory properties of muramyl tripeptides containing lysine and diaminopimelic acid moieties

The unusual amino acid diaminopimelic acid (DAP) was prepared by cross metathesis of appropriately protected vinyl glycine and allyl glycine derivatives. Catalytic hydrogenation of the cross-coupling product resulted in reduction of the double bond and the removal of protecting groups. The resulting compounds were appropriately protected for the polymer-supported and solution-phase synthesis of muramyl tripeptides 2 and 3, which differ in the amidation of the alpha-carboxylic acids of the isoglutamine and DAP moieties. Muramyl dipeptide (1, MDP), the DAP-containing muramyl tripeptide 3, and the lysine-containing muramyl tripeptides 4 and 5 induced TNF-alpha gene expression without TNF-alpha protein production in a human monocytic cell line. The observed block in translation could be removed by co-incubation with LPS, resulting in an apparent synergistic effect. Compound 2 did not induce TNF-alpha gene expression, neither did it exhibit a synergistic effect with LPS; this indicates that amidation of the alpha-carboxylic acids of the isoglutamine and DAP moieties results in a loss of biological activity. It is proposed that amidation of alpha-carboxylic acids is a strategy that may be used by pathogens to avoid detection by the innate immune system. Furthermore, the pattern recognition receptors Nod1 and Nod2 have been implicated in the possible induction of a synergistic effect of muropeptides with LPS.

Kdo2-Lipid A of Escherichia coli, a defined endotoxin that activates macrophages via TLR-4

The LIPID MAPS Consortium (www.lipidmaps.org) is developing comprehensive procedures for identifying all lipids of the macrophage, following activation by endotoxin. The goal is to quantify temporal and spatial changes in lipids that occur with cellular metabolism and to develop bioinformatic approaches that establish dynamic lipid networks. To achieve these aims, an endotoxin of the highest possible analytical specification is crucial. We now report a large-scale preparation of 3-deoxy-D-manno-octulosonic acid (Kdo)(2)-Lipid A, a nearly homogeneous Re lipopolysaccharide (LPS) sub-structure with endotoxin activity equal to LPS. Kdo(2)-Lipid A was extracted from 2 kg cell paste of a heptose-deficient Escherichia coli mutant. It was purified by chromatography on silica, DEAE-cellulose, and C18 reverse-phase resin. Structure and purity were evaluated by electrospray ionization/mass spectrometry, liquid chromatography/mass spectrometry and (1)H-NMR. Its bioactivity was compared with LPS in RAW 264.7 cells and bone marrow macrophages from wild-type and toll-like receptor 4 (TLR-4)-deficient mice. Cytokine and eicosanoid production, in conjunction with gene expression profiling, were employed as readouts. Kdo(2)-Lipid A is comparable to LPS by these criteria. Its activity is reduced by >10(3) in cells from TLR-4-deficient mice. The purity of Kdo(2)-Lipid A should facilitate structural analysis of complexes with receptors like TLR-4/MD2.

Overexpression and characterization of Wzz of Escherichia coli O86:H2

O-Antigen plays a critical role in the bacterium-host interplay, the chain length is an important factor in O-antigen functions. Wzz protein is responsible for O-antigen chain length regulation, but the mechanism is still unknown. Here, we overexpressed the Wzz of Escherichia coli O86:H2 in wzz mutant O86:H2 strain, the yield can achieve 15 mg/L. The recombinant Wzz was purified to 99% purity in dodecylmaltoside by sequential Ni-affinity chromatography and anion-exchange. Size exclusion chromatography and in vivo cross-linking experiments both showed that Wzz formed tetramer. Furthermore, analysis with circular dichroism revealed that the predominant structural composition in Wzz is alpha-helices, and incubation with O-antigen significantly changed Wzz conformation. The results suggested that Wzz protein can interact with O-antigen.

An enzymatic ruler modulates Lewis antigen glycosylation of Helicobacter pylori LPS during persistent infection

Helicobacter pylori persistently colonizes about half the human population and contributes to the development of peptic ulcer disease and gastric cancer. This organism has evolved means to structurally alter its surface characteristics to evade innate and adaptive immune responses. H. pylori produces LPS O-antigen units that can be posttranslationally fucosylated to generate Lewis antigens, structures also found on human epithelial cells. We demonstrate an extensive diversity of Lewis x and Lewis y expression in LPS O-antigen units, occurring over time and in different regions of the human stomach. Lewis expression patterns were correlated with the on/off status of the three fucosyltransferases (FucT), FutA, FutB, and FutC, which are regulated via slipped-strand mispairing in intragenic polyC tract regions of the corresponding genes. The alpha1,3-FucT, FutA and FutB, each contain a C-terminal heptad repeat region, consisting of a variable number of DD/NLRV/INY tandem repeats. Variations in the number of heptad repeats correlated to the sizes of O-antigen polymers to become decorated by fucose residues. Our data support a molecular ruler mechanism for how H. pylori varies its LPS fucosylation pattern, where one heptad repeat in the enzyme corresponds to one N-acetyl-beta-lactosamine unit in the O-antigen polysaccharide.

Molecular analysis of the O-antigen gene cluster of Escherichia coli O86:B7 and characterization of the chain length determinant gene (wzz)

Escherichia coli O86:B7 has long been used as a model bacterial strain to study the generation of natural blood group antibody in humans, and it has been shown to possess high human blood B activity. The O-antigen structure of O86:B7 was solved recently in our laboratory. Comparison with the published structure of O86:H2 showed that both O86 subtypes shared the same O unit, yet each of the O antigens is polymerized from a different terminal sugar in a different glycosidic linkage. To determine the genetic basis for the O-antigen differences between the two O86 strains, we report the complete sequence of O86:B7 O-antigen gene cluster between galF and hisI, each gene was identified based on homology to other genes in the GenBank databases. Comparison of the two O86 O-antigen gene clusters revealed that the encoding regions between galF and gnd are identical, including wzy genes. However, deletion of the two wzy genes revealed that wzy in O86:B7 is responsible for the polymerization of the O antigen, while the deletion of wzy in O86:H2 has no effect on O-antigen biosynthesis. Therefore, we proposed that there must be another functional wzy gene outside the O86:H2 O-antigen gene cluster. Wzz proteins determine the degree of polymerization of the O antigen. When separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the lipopolysaccharide (LPS) of O86:B7 exhibited a modal distribution of LPS bands with relatively short O units attached to lipid A-core, which differs from the LPS pattern of O86:H2. We proved that the wzz genes are responsible for the different LPS patterns found in the two O86 subtypes, and we also showed that the very short type of LPS is responsible for the serum sensitivity of the O86:B7 strain.

The UDP N-acetylgalactosamine 4-epimerase gene is essential for mesophilic Aeromonas hydrophila serotype O34 virulence

Mesophilic Aeromonas hydrophila strains of serotype O34 typically express smooth lipopolysaccharide (LPS) on their surface. A single mutation in the gene that codes for UDP N-acetylgalactosamine 4-epimerase (gne) confers the O(-) phenotype (LPS without O-antigen molecules) on a strain in serotypes O18 and O34, but not in serotypes O1 and O2. The gne gene is present in all the mesophilic Aeromonas strains tested. No changes were observed for the LPS core in a gne mutant from A. hydrophila strain AH-3 (serotype O34). O34 antigen LPS contains N-acetylgalactosamine, while no such sugar residue forms part of the LPS core from A. hydrophila AH-3. Some of the pathogenic features of A. hydrophila AH-3 gne mutants are drastically reduced (serum resistance or adhesion to Hep-2 cells), and the gne mutants are less virulent for fish and mice compared to the wild-type strain. Strain AH-3, like other mesophilic Aeromonas strains, possess two kinds of flagella, and the absence of O34 antigen molecules by gne mutation in this strain reduced motility without any effect on the biogenesis of both polar and lateral flagella. The reintroduction of the single wild-type gne gene in the corresponding mutants completely restored the wild-type phenotype (presence of smooth LPS) independently of the O wild-type serotype, restored the virulence of the wild-type strain, and restored motility (either swimming or swarming).

LPS priming potentiates and prolongs proinflammatory cytokine response to the trichothecene deoxynivalenol in the mouse

Simultaneous exposure to lipopolysaccharide (LPS) markedly amplifies induction of proinflammatory cytokine expression as well as IL-1-driven lymphocyte apoptosis by trichothecene deoxynivalenol (DON) in the mouse. The purpose of this research was to test the hypothesis that LPS priming will sensitize a host to DON-induced proinflammatory cytokine induction and apoptosis. In mice primed with LPS (1 mg/kg bw) ip. and treated 8 h later with DON po., the minimum DON doses for inducing IL-1alpha, IL-1beta, IL-6 and TNF-alpha serum proteins and splenic mRNAs were significantly lower than the DON doses required for vehicle-primed mice. LPS priming also decreased onset time and dramatically increased magnitude and duration of cytokine responses. LPS-primed mice maintained heightened sensitivity to DON for up to 24 h. LPS priming doses as low as 50 microg/kg bw evoked sensitization. DNA fragmentation analysis and flow cytometry also revealed that mice primed with LPS (1 mg/kg) for 8 h and exposed to DON (12.5 mg/kg) exhibited massive thymocyte loss by apoptosis 12 h later compared to mice exposed to DON or LPS alone. LPS priming decreased DON-induced p38 and ERK 1/2 phosphorylation suggesting that enhanced mitogen-activated protein kinase activation was not involved in increased cytokine responses. Taken together, exposure to LPS rendered mice highly susceptible to DON induction of cytokine expression and this correlated with increased apoptosis in the thymus.

Periplasmic protein-protein contacts in the inner membrane protein Wzc form a tetrameric complex required for the assembly of Escherichia coli group 1 capsules

The K antigenic capsular polysaccharide forms a structural layer, the capsule, on the surfaces of Escherichia coli cells. The capsule provides an important protective covering that helps protect encapsulated bacteria from host immune defenses. The assembly and translocation of the capsule requires proteins in the inner and outer membranes. The inner membrane protein Wzc is a tyrosine autokinase that plays an essential role in what is believed to be a coordinated biosynthesis and secretion process. Mutants lacking Wzc can form K antigen oligosaccharides but are unable to polymerize high molecular weight capsular polymers. Wzc homologs have been identified in exopolymer biosynthesis systems in many different Gram-negative and -positive bacteria. Using single particle averaging on cryo-negatively stained samples, we have produced the first three-dimensional structure of this type of membrane protein in its phosphorylated state at approximately 14 A resolution. Perfluoro-octanoate-PAGE analysis of detergent-solubilized oligomeric Wzc and symmetry analysis of the transmission electron microscopy data clearly demonstrated that Wzc forms a tetrameric complex with C4 rotational symmetry. Viewed from the top of the complex, the oligomer is square with a diameter of approximately 100 A and can be divided into four separate densities. From the side, Wzc is approximately 110 A high and has a distinctive appearance similar to an extracted molar tooth. The upper "crown" region is approximately 55 A high and forms a continuous ring of density. Four unconnected "roots" ( approximately 65 A high) emerge from the underside of the crown. We propose that the crown is formed by protein-protein contacts from the four Wzc periplasmic domains, while each root represents an individual cytoplasmic tyrosine autokinase domain.

Advances in understanding bacterial outer-membrane biogenesis

The outer membrane of gram-negative bacteria such as Escherichia coli serves as a protective barrier that controls the influx and efflux of solutes. This allows the bacteria to inhabit several different, and often hostile, environments. The assembly of the E. coli outer membrane has been difficult to study using traditional genetic and biochemical methods, and how all its components reach the outer membrane after being synthesized in the cytoplasm and cytoplasmic membrane, how they are assembled in an environment that is devoid of an obvious energy source, and how assembly proceeds without disrupting the integrity of this essential cellular structure are all fundamental questions that remain unanswered. Here, we review the new approaches that have led to the recent discovery of components of the machinery involved in the biogenesis of this distinctive cellular organelle.

Study on the antiinflammatory activity of methanol extract from seagrass Zostera japonica

Methanolic extracts from the seagrass Zostera japonica were extracted successively using n-hexane (n-C(6)H(14)), dichloromethane (CH(2)Cl(2)), ethyl acetate (EtOAc), and water to give the n-C(6)H(14) (16.8%), CH(2)Cl(2) (40.6%), EtOAc (34.1%), and H(2)O (8.5%) soluble fractions, respectively. We have demonstrated that the hexane fraction has the highest capacity to inhibit proIL-1beta expression as compared to other fractions in lipopolysaccharides (LPS)-stimulated J774A.1 murine macrophages. Further analysis of the composition and antiinflammatory activity of the subfraction H5 from hexane fraction showed that it had the best antiinflammatory capacity and that it's major constituents were fatty acids, including palmitic acid methyl ester (21.5%), palmitic acid (24.02%), linoleic acid methyl ester (13.09%), oleic acid methyl ester (8.41%), and linoleic acid (7.93%), respectively. H5 inhibited LPS-induced TNFalpha, IL-1beta, and IL-6 in a dose-dependent manner, suggesting that H5 is bioactive in antiinflammation in vitro. This study is the first to report the antiinflammatory activity of extracts obtained from the seagrass Z. japonica.

Determination of critical micelle concentrations and aggregation numbers by fluorescence correlation spectroscopy: Aggregation of a lipopolysaccharide

Fluorescence correlation spectroscopy (FCS) is often used to determine the mass or radius of a particle by using the dependence of the diffusion coefficient on the mass and shape. In this article we discuss how the particle size of aggregates can be measured by using the concentration dependence of the amplitude of the autocorrelation function (ACF) instead of the temporal decay. We titrate a solution of aggregates or micelles with a fluorescent label that possesses a high affinity for these structures and measure the changes in the amplitude of the ACF. We develop the theory describing the change of the ACF amplitude with increasing concentrations of labels and use it to fit experimental data. It is shown how this method can determine the aggregation number and critical micelle concentration of a standard detergent nonaethylene glycol monododecyl ether (C12E9) and a lipopolysaccharide (LPS: Escherichia coli 0111:B4).

Expression of a Porphyromonas gingivalis lipid A palmitylacyltransferase in Escherichia coli yields a chimeric lipid A with altered ability to stimulate interleukin-8 secretion

In Escherichia coli the gene htrB codes for an acyltransferase that catalyses the incorporation of laurate into lipopolysaccharide (LPS) as a lipid A substituent. We describe the cloning, expression and characterization of a Porphyromonas gingivalis htrB homologue. When the htrB homologue was expressed in wild-type E. coli or a mutant strain deficient in htrB, a chimeric LPS with altered lipid A structure was produced. Compared with wild-type E. coli lipid A, the new lipid A species contained a palmitate (C16) in the position normally occupied by laurate (C12) suggesting that the cloned gene performs the same function as E. coli htrB but preferentially transfers the longer-chain palmitic acid that is known to be present in P. gingivalis LPS. LPS was purified from wild-type E. coli, the E. coli htrB mutant strain and the htrB mutant strain expressing the P. gingivalis acyltransferase. LPS from the palmitate bearing chimeric LPS as well as the htrB mutant exhibited a reduced ability to activate human embryonic kidney 293 (HEK293) cells transfected with TLR4/MD2. LPS from the htrB mutant also had a greatly reduced ability to stimulate interleukin-8 (IL-8) secretion in both endothelial cells and monocytes. In contrast, the activity of LPS from the htrB mutant bacteria expressing the P. gingivalis gene displayed wild-type activity to stimulate IL-8 production from endothelial cells but a reduced ability to stimulate IL-8 secretion from monocytes. The intermediate activation observed in monocytes for the chimeric LPS was similar to the pattern seen in HEK293 cells expressing TLR4/MD2 and CD14. Thus, the presence of a longer-chain fatty acid on E. coli lipid A altered the activity of the LPS in monocytes but not endothelial cell assays and the difference in recognition does not appear to be related to differences in Toll-like receptor utilization.

A matrix-assisted laser desorption/ionization mass spectrometry approach to the lipid A from Mesorhizobium loti

The isolation, purification and analysis of the lipid A obtained from Mesorhizobium loti Ayac 1 BII strain is presented. Analysis of the carbohydrate moiety after acid hydrolysis by high-pH anion-exchange chromatography with pulse amperometric detection (HPAEC-PAD) showed the presence of glucosamine and galacturonic acid as the only sugar components. Gas chromatographic (GC) and GC/mass spectrometric (MS) analysis of the fatty acids revealed the presence of 3-OH-C12:0; 3-OH-C13:0; 3-OH-C20:0 and 27-OH-C28:0 among the major hydroxylated species. In addition, C16:0, C17:0, C18:0 and C 20:0 were shown as main saturated fatty acids. Different polyacylated species were evidenced by thin layer chromatography of lipid A, allowing the purification of two fractions. Ultraviolet matrix-assisted laser desorption/ionization time-of-flight (UV-MALDI-TOF) MS analysis with different matrices, in the positive- and negative-ion mode, was performed. The fast moving component revealed the presence of hexa-acylated species, varying in the fatty acid composition. Species containing three 3-OH fatty acids and a 27-OH-C28:0 fatty acid were observed. Individual ions within this family differ by +/-14 mass units. The slow moving component was enriched mainly in penta-acylated species. Among them, three subgroups were detected: the major one compatible with the sugar core bearing two 3-OH 20:0 fatty acids, a 3-OH 13:0 or a 3-OH 12:0 fatty acid, a 27-OH 28:0 fatty acid and one saturated fatty acid. Each signal differs in a C18:0 acyl unit from the corresponding hexa-acylated family. On the other hand, a subgroup bearing one 3-OH 20:0 fatty acid, one 27-OH 28:0 fatty acid and two non-polar fatty acids was shown. A minor subgroup compatible with structures containing two hydroxylated and three non-polar fatty acids was also detected. The results obtained showed that nor-harmane was an excellent matrix for charged lipid A structural studies in both, positive and negative ion modes.

Two different O-polysaccharides from Escherichia coli O86 are produced by different polymerization of the same O-repeating unit

The structure of a new O-polysaccharide from Escherichia coli O86:K62:B7 was determined using NMR and methylation analysis. The structure is as follows: [carbohydrate: see text]. Comparison with the previously published structure from E. coli O86:K2:H2 revealed that the O-polysaccharides from these two E. coli O86 serotypes share the same branched pentasaccharide repeating unit. However, they differ in the anomeric configuration of the linkage, the linkage position, and the identity of the residue through which polymerization occurs. The immunochemical activity of these two forms of LPS toward anti-B antibody was studied and compared. The results showed that LPS from E. coli O86:K2:H2 strain possesses higher blood group B reactivity. The immunoreactivity difference was explained by modeling of the O-repeating unit tetrasaccharide fragments. This finding provides a good system for the further study of O-polysaccharide biosynthesis especially the repeating unit polymerization mechanism.

Chlorella viruses

Chlorella viruses or chloroviruses are large, icosahedral, plaque-forming, double-stranded-DNA-containing viruses that replicate in certain strains of the unicellular green alga Chlorella. DNA sequence analysis of the 330-kbp genome of Paramecium bursaria chlorella virus 1 (PBCV-1), the prototype of this virus family (Phycodnaviridae), predict approximately 366 protein-encoding genes and 11 tRNA genes. The predicted gene products of approximately 50% of these genes resemble proteins of known function, including many that are completely unexpected for a virus. In addition, the chlorella viruses have several features and encode many gene products that distinguish them from most viruses. These products include: (1) multiple DNA methyltransferases and DNA site-specific endonucleases, (2) the enzymes required to glycosylate their proteins and synthesize polysaccharides such as hyaluronan and chitin, (3) a virus-encoded K(+) channel (called Kcv) located in the internal membrane of the virions, (4) a SET domain containing protein (referred to as vSET) that dimethylates Lys27 in histone 3, and (5) PBCV-1 has three types of introns; a self-splicing intron, a spliceosomal processed intron, and a small tRNA intron. Accumulating evidence indicates that the chlorella viruses have a very long evolutionary history. This review mainly deals with research on the virion structure, genome rearrangements, gene expression, cell wall degradation, polysaccharide synthesis, and evolution of PBCV-1 as well as other related viruses.

Endotoxin (Lipopolysaccharide) Neutralization by Innate Immunity Host-Defense Peptides

Binding of lipopolysaccharide (LPS) to macrophages results in proinflammatory cytokine secretion. In extreme cases it leads to endotoxic shock. A few innate immunity antimicrobial peptides (AMPs) neutralize LPS activity. However, the underlying mechanism and properties of the peptides are not yet clear. Toward meeting this goal we investigated four AMPs and their fluorescently labeled analogs. These AMPs varied in composition, length, structure, and selectivity toward cells. The list included human LL-37 (37-mer), magainin (24-mer), a 15-mer amphipathic alpha-helix, and its D,L-amino acid structurally altered analog. The peptides were investigated for their ability to inhibit LPS-mediated cytokine release from RAW264.7 and bone marrow-derived primary macrophages, to bind LPS in solution, and when LPS is already bound to macrophages (fluorescence spectroscopy and confocal microscopy), to compete with LPS for its binding site on the CD14 receptor (flow cytometry) and affect LPS oligomerization. We conclude that a strong binding of a peptide to LPS aggregates accompanied by aggregate dissociation prevents LPS from binding to the carrier protein lipopolysaccharide-binding protein, or alternatively to its receptor, and hence inhibits cytokine secretion.

Inhibition of Lipopolysaccharide Activity by a Bacterial Cyclic Lipopeptide Surfactin

Compounds that inactivate lipopolysaccharide (LPS) activity have the potential of being new anti-inflammatory agents. Therefore, we searched among microbial secondary metabolites for compounds that inhibited LPS-stimulated adhesion between human umbilical vein endothelial cells (HUVEC) and HL-60 cells. By this screening, we found a cyclic lipopeptide surfactin from the culture broth of Bacillus sp. BML752-121F2 to be inhibitory. The addition of the surfactin prior to the LPS stimulation decreased HL-60 cell-HUVEC adhesion without showing any cytotoxicity. We confirmed that surfactin inhibited LPS-induced expression of ICAM-1 and VCAM-1 in HUVEC. It also inhibited the cellular adhesion induced by lipid A, the active component of LPS; but it did not inhibit TNF-alpha or IL-1 beta-induced cell adhesion. Then, surfactin was shown to suppress the interaction of lipid A with LPS-binding protein (LBP) that mediates the transport of LPS to its receptors. Finally, surface plasmon resonance (SPR) analysis revealed the surfactin to interact reversibly with lipid A. Thus, this Bacillus surfactin was shown to be an inhibitor of LPS-induced signal transduction, directly interacting with LPS.

The Pseudomonas aeruginosa proteome during anaerobic growth

Isotope-coded affinity tag analysis and two-dimensional gel electrophoresis followed by tandem mass spectrometry were used to identify Pseudomonas aeruginosa proteins expressed during anaerobic growth. Out of the 617 proteins identified, 158 were changed in abundance during anaerobic growth compared to during aerobic growth, including proteins whose increased expression was expected based on their role in anaerobic metabolism. These results form the basis for future analyses of alterations in bacterial protein content during growth in various environments, including the cystic fibrosis airway.

A comparative categorization of protein function encoded in bacterial or archeal genomic islands

Genomes of prokaryotes harbor genomic islands (GIs), which are frequently acquired via horizontal gene transfer (HGT). Here I present an analysis of GIs with respect to gene-encoded functions. GIs were identified by statistical analysis of codon usage and clustering. Genes classified as putatively alien (pA) or putatively native (pN) were categorized according to the COG database. Among pA and pN genes, the distribution of COG functions and classes were studied for different groupings of prokaryotes. Groups were formed according to taxonomical relation or habitats. In all groups, genes related to class L (replication, recombination, and repair) were statistically significantly overrepresented in GIs. GIs of bacteria and archaea showed a distinct pattern of preferences. In archeal GIs, genes belonging to COG class M (cell wall/membrane/envelope biogenesis) or Q (secondary metabolites biosynthesis, transport, and catabolism) were more frequent. In bacterial GIs, genes of classes U (intracellular trafficking, secretion, and vesicular transport), N (cell motility), and V (defense mechanisms) were predominant. Underrepresentation was strongest for genes belonging to class J (translation, ribosomal structure, and biogenesis). Among single COG functions overrepresented in GIs were transferases and transporters. In both superkingdoms, HGT enhances genomic content by meeting demands that are independent of the studied habitats. These findings are in agreement with the complexity theory, which predicts the preferential import of operational genes. However, only specific subsets of operational genes were enriched in GIs. Modification of the cell envelope, cell motility, secretion, and protection of cellular DNA are major issues in HGT.

Moraxella catarrhalis bacterium without endotoxin, a potential vaccine candidate

Lipooligosaccharide (LOS) is a major surface component of Moraxella catarrhalis and a possible virulence factor in the pathogenesis of human infections caused by this organism. The presence of LOS on the bacterium is an obstacle to the development of vaccines derived from whole cells or outer membrane components of the bacterium. An lpxA gene encoding UDP-N-acetylglucosamine acyltransferase responsible for the first step of lipid A biosynthesis was identified by the construction and characterization of an isogenic M. catarrhalis lpxA mutant in strain O35E. The resulting mutant was viable despite the complete loss of LOS. The mutant strain showed significantly decreased toxicity by the Limulus amebocyte lysate assay, reduced resistance to normal human serum, reduced adherence to human epithelial cells, and enhanced clearance in lungs and nasopharynx in a mouse aerosol challenge model. Importantly, the mutant elicited high levels of antibodies with bactericidal activity and provided protection against a challenge with the wild-type strain. These data suggest that the null LOS mutant is attenuated and may be a potential vaccine candidate against M. catarrhalis.

Interactions between folding factors and bacterial outer membrane proteins

The outer membrane is the first line of contact between Gram-negative bacteria and their external environment. Embedded in the outer membrane are integral outer membrane proteins (OMPs) that perform a diverse range of tasks. OMPs are synthesized in the cytoplasm and are translocated across the inner membrane and probably diffuse through the periplasm before they are inserted into the outer membrane in a folded and biologically active form. Passage through the periplasm presents a number of challenges, due to the hydrophobic nature of the OMPs and the choice of membranes into which they can insert. Recently, a number of periplasmic proteins and one OMP have been shown to play a role in OMP biogenesis. In this review, we describe what is known about these folding factors and how they function in a biological context. In particular, we focus on how they interact with the OMPs at the molecular level and present a comprehensive overview of data relating to a possible effect on OMP folding yield and kinetics. Furthermore, we discuss the role of lipo-chaperones, i.e. lipopolysaccharide and phospholipids, in OMP folding. Important advances have clearly been made in the field, but much work remains to be done, particularly in terms of describing the biophysical basis for the chaperone-OMP interactions which so intricately regulate OMP biogenesis.

The lipid A palmitoyltransferase PagP: molecular mechanisms and role in bacterial pathogenesis

Palmitoylated lipid A can both protect pathogenic bacteria from host immune defences and attenuate the activation of those same defences through the TLR4 signal transduction pathway. A palmitate chain from a phospholipid is incorporated into lipid A by an outer membrane enzyme PagP, which is an 8-stranded antiparallel beta-barrel preceded by an amino-terminal amphipathic alpha-helix. The PagP barrel axis is tilted by 25 degrees with respect to the membrane normal. An interior hydrophobic pocket in the outer leaflet-exposed half of the molecule functions as a hydrocarbon ruler that allows the enzyme to distinguish palmitate from other acyl chains found in phospholipids. Internalization of a phospholipid palmitoyl group within the barrel appears to occur by lateral diffusion from the outer leaflet through non-hydrogen-bonded regions between beta-strands. The MsbA-dependent trafficking of lipids from the inner membrane to the outer membrane outer leaflet is necessary for lipid A palmitoylation in vivo. The mechanisms by which bacteria regulate pagP gene expression strikingly reflect the corresponding pathogenic lifestyle of the bacterium. Variations on PagP structure and function can be illustrated with the known homologues from Gram-negative bacteria, which include pathogens of humans and other mammals in addition to pathogens of insects and plants. The PagP enzyme is potentially a target for the development of anti-infective agents, a probe of outer membrane lipid asymmetry, and a tool for the synthesis of lipid A-based vaccine adjuvants and endotoxin antagonists.

Identification of GutQ from Escherichia coli as a D-arabinose 5-phosphate isomerase

The glucitol operon (gutAEBDMRQ) of Escherichia coli encodes a phosphoenolpyruvate:sugar phosphotransferase system that metabolizes the hexitol D-glucitol (sorbitol). The functions for all but the last gene, gutQ, have been previously assigned. The high sequence similarity between GutQ and KdsD, a D-arabinose 5-phosphate isomerase (API) from the 3-deoxy-D-manno-octulosonate (KDO)-lipopolysaccharide (LPS) biosynthetic pathway, suggested a putative activity, but its role within the context of the gut operon remained unclear. Accordingly, the enzyme was cloned, overexpressed, and characterized. Recombinant GutQ was shown to indeed be a second copy of API from the E. coli K-12 genome with biochemical properties similar to those of KdsD, catalyzing the reversible aldol-ketol isomerization between D-ribulose 5-phosphate (Ru5P) and D-arabinose 5-phosphate (A5P). Genomic disruptions of each API gene were constructed in E. coli K-12. TCM11[(deltakdsD)] was capable of sustaining essential LPS synthesis at wild-type levels, indicating that GutQ functions as an API inside the cell. The gut operon remained inducible in TCM7[(deltagutQ)], suggesting that GutQ is not directly involved in d-glucitol catabolism. The conditional mutant TCM15[(deltagutQdeltakdsD)] was dependent on exogenous A5P both for LPS synthesis/growth and for upregulation of the gut operon. The phenotype was suppressed by complementation in trans with a plasmid encoding a functional copy of GutQ or by increasing the amount of A5P in the medium. As there is no obvious obligatory role for GutQ in the metabolism of d-glucitol and there is no readily apparent link between D-glucitol metabolism and LPS biosynthesis, it is suggested that A5P is not only a building block for KDO biosynthesis but also may be a regulatory molecule involved in expression of the gut operon.

The catalytic and conformational cycle of Aquifex aeolicus KDO8P synthase: role of the L7 loop

KDO8P synthase catalyzes the condensation of arabinose 5-phosphate (A5P) and phosphoenolpyruvate (PEP) to form the 8-carbon sugar KDO8P and inorganic phosphate (P(i)). The X-ray structure of the wild-type enzyme shows that when both PEP and A5P bind, the active site becomes isolated from the environment due to a conformational change of the L7 loop. The structures of the R106G mutant, without substrates, and with PEP and PEP plus A5P bound, were determined and reveal that in R106G closure of the L7 loop is impaired. The structural perturbations originating from the loss of the Arg(106) side chain point to a role of the L2 loop in stabilizing the closed conformation of the L7 loop. Despite the increased exposure of the R106G active site, no abnormal reaction of PEP with water was observed, ruling out the hypothesis that the primary function of the L7 loop is to shield the active site from bulk solvent during the condensation reaction. However, the R106G enzyme displays several kinetic abnormalities on both the substrate side (smaller K(m)(PEP), larger K(i)(A5P) and K(m)(A5P)) and the product side (smaller K(i)(Pi) and K(i)(KDO8P)) of the reaction. As a consequence, the mutant enzyme is less severely inhibited by A5P and more severely inhibited by P(i) and KDO8P. Simulations of the flux of KDO8P synthesis under metabolic steady-state conditions (constant concentration of reactants and products over time) suggest that in vivo R106G is expected to perform optimally in a narrower range of substrate and product concentrations than the wild-type enzyme.

Endotoxin recognition: in fish or not in fish?

The interaction between pathogens and their multicellular hosts is initiated by activation of pathogen recognition receptors (PRRs). These receptors, that include most notably members of the toll-like receptor (TLR) family, recognize specific pathogen-associated molecular patterns (PAMPs). TLR4 is a central part of the receptor complex that is involved in the activation of the immune system by lipopolysaccharide (LPS) through the specific recognition of its endotoxic moiety (Lipid A). This is a critical event that is essential for the immune response to Gram-negative bacteria as well as the etiology of endotoxic shock. Interestingly, compared to mammals, fish are resistant to endotoxic shock. This in vivo resistance concurs with in vitro studies demonstrating significantly lowered sensitivity of fish leukocytes to LPS activation. Further, our in vitro analyses demonstrate that in trout mononuclear phagocytes, LPS fails to induce antiviral genes, an event that occurs downstream of TLR4 and is required for the development of endotoxic shock. Finally, an in silico approach that includes mining of different piscine genomic and EST databases, reveals the presence in fish of all of the major TLR signaling elements except for the molecules specifically involved in TLR4-mediated endotoxin recognition and signaling in mammals. Collectively, our analysis questions the existence of TLR4-mediated cellular responses to LPS in fish. We further speculate that other receptors, in particular beta-2 integrins, may play a primary role in the activation of piscine leukocytes by LPS.

Identification of novel Salmonella enterica serovar Typhimurium DT104-specific prophage and nonprophage chromosomal sequences among serovar Typhimurium isolates by genomic subtractive hybridization

Genomic subtractive hybridization was performed between Salmonella enterica serovar Typhimurium LT2 and DT104 to search for novel Salmonella serovar Typhimurium DT104-specific sequences. The subtraction resulted mainly in the isolation of DNA fragments with sequence similarity to phages. Two fragments identified were associated with possible virulence factors. One fragment was identical to irsA of Salmonella serovar Typhimurium ATCC 14028, which is suggested to be involved in macrophage survival. The other fragment was homologous to HldD, an Escherichia coli O157:H7 lipopolysaccharide assembly-related protein. Five selected DNA fragments-irsA, the HldD homologue, and three fragments with sequence similarity to prophages-were tested for their presence in 17 Salmonella serovar Typhimurium DT104 isolates and 27 non-DT104 isolates by PCR. All five selected DNA fragments were Salmonella serovar Typhimurium DT104 specific among the serovar Typhimurium isolates tested. These DNA fragments can be useful for better detection and typing of Salmonella serovar Typhimurium DT104.

The Incorporation of Glucosamine into Enterobacterial Core Lipopolysaccharide

The core lipopolysaccharide (LPS) of Klebsiella pneumoniae is characterized by the presence of disaccharide alphaGlcN-(1,4)-alphaGalA attached by an alpha1,3 linkage to l-glycero-d-manno-heptopyranose II (ld-HeppII). Previously it has been shown that the WabH enzyme catalyzes the incorporation of GlcNAc from UDP-GlcNAc to outer core LPS. The presence of GlcNAc instead of GlcN and the lack of UDP-GlcN in bacteria indicate that an additional enzymatic step is required. In this work we identified a new gene (wabN) in the K. pneumoniae core LPS biosynthetic cluster. Chemical and structural analysis of K. pneumoniae non-polar wabN mutants showed truncated core LPS with GlcNAc instead of GlcN. In vitro assays using LPS truncated at the level of d-galacturonic acid (GalA) and cell-free extract containing WabH and WabN together led to the incorporation of GlcN, whereas none of them alone were able to do it. This result suggests that the later enzyme (WabN) catalyzes the deacetylation of the core LPS containing the GlcNAc residue. Thus, the incorporation of the GlcN residue to core LPS in K. pneumoniae requires two distinct enzymatic steps. WabN homologues are found in Serratia marcescens and some Proteus strains that show the same disaccharide alphaGlcN-(1,4)-alphaGalA attached by an alpha1,3 linkage to ld-HeppII.

Viability of a capsule- and lipopolysaccharide-deficient mutant of Neisseria meningitidis

Neisseria meningitidis is the only lipopolysaccharide (LPS)-producing gram-negative bacterial species shown to be viable also without LPS. It was thought that the presence of capsular polysaccharide is necessary for this unusual feature. However, we show now that no part of the capsule gene cluster is required for maintaining LPS deficiency in N. meningitidis.

Assessment of TNFα contribution to the functional up-regulation of kinin B1 receptors in the mouse paw after treatment with LPS

It has been widely demonstrated that LPS is able to induce kinin B(1) receptor up-regulation throughout several models of inflammation. Using an in-vivo system in which LPS was administered systemically, we assessed the participation of the pro-inflammatory cytokine TNFalpha in the functional up-regulation of B(1) receptors in the mouse paw. Systemic treatment with LPS (10 microg/animal, i.v. 24 h before) resulted in a marked increase (about 5-fold) in the mouse paw edema induced by the selective B(1) receptor agonist des-Arg(9)-BK (50 nmol/paw) in both Swiss and C57/BL6 mice. The up-regulation of des-Arg(9)-BK-caused edema following LPS treatment was found to be greatly diminished in TNFalpha p55(-/-) receptor knockout mice. In addition, the paw edema evoked by des-Arg(9)-BK was significantly reduced when mice received the anti-TNFalpha antibody (100 [corrected] microg/kg, i.v.) 5 min before the LPS treatment. A similar inhibition of B(1) receptor-mediated paw edema was observed when mice were treated with thalidomide (30 mg/kg, s.c.) [corrected] a drug known for reducing TNFalpha synthesis, 5 min prior to LPS administration. ELISA experiment [corrected] revealed that TNFalpha serum levels were maximal at 1 h following LPS systemic treatment. Taken together, the present results suggest that the early production of the pro-inflammatory cytokine TNFalpha is probably responsible for driving the sequence of events involved in the functional up-regulation of B(1) receptors in the mouse paw.

MsbA Is Not Required for Phospholipid Transport in Neisseria meningitidis

The outer membrane of Gram-negative bacteria contains phospholipids and lipopolysaccharide (LPS) in the inner and outer leaflet, respectively. Little is known about the transport of the phospholipids from their site of synthesis to the outer membrane. The inner membrane protein MsbA of Escherichia coli, which is involved in the transport of LPS across the inner membrane, has been reported to be involved in phospholipid transport as well. Here, we have reported the construction and the characterization of a Neisseria meningitidis msbA mutant. The mutant was viable, and it showed a retarded growth phenotype and contained very low amounts of LPS. However, it produced an outer membrane, demonstrating that phospholipid transport was not affected by the mutation. Notably, higher amounts of phospholipids were produced in the msbA mutant than in its isogenic parental strain, provided that capsular biosynthesis was also disrupted. Although these results confirmed that MsbA functions in LPS transport, they also demonstrated that it is not required for phospholipid transport, at least not in N. meningitidis.

Stereospecific deuteration of 2-deoxyerythrose 4-phosphate using 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase

Racemic 2-deoxyerythrose 4-phosphate was synthesized and one enantiomer of this compound was found to be a substrate for Escherichia coli 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase, the first enzyme of the shikimate pathway. When the reaction was carried out in deuterium oxide, an enzyme-catalyzed regio- and stereoselective incorporation of deuterium into the product was observed.

Eukaryotic UDP-galactopyranose mutase (GLF gene) in microbial and metazoal pathogens

Galactofuranose (Gal(f)) is a novel sugar absent in mammals but present in a variety of pathogenic microbes, often within glycoconjugates that play critical roles in cell surface formation and the infectious cycle. In prokaryotes, Gal(f) is synthesized as the nucleotide sugar UDP-Gal(f) by UDP-galactopyranose mutase (UGM) (gene GLF). Here we used a combinatorial bioinformatics screen to identify a family of candidate eukaryotic GLFs that had previously escaped detection. GLFs from three pathogens, two protozoa (Leishmania major and Trypanosoma cruzi) and one fungus (Cryptococcus neoformans), had UGM activity when expressed in Escherichia coli and assayed in vivo and/or in vitro. Eukaryotic GLFs are closely related to each other but distantly related to prokaryotic GLFs, showing limited conservation of core residues around the substrate-binding site and flavin adenine dinucleotide binding domain. Several eukaryotes not previously investigated for Gal(f) synthesis also showed strong GLF homologs with conservation of key residues. These included other fungi, the alga Chlamydomonas and the algal phleovirus Feldmannia irregularis, parasitic nematodes (Brugia, Onchocerca, and Strongyloides) and Caenorhabditis elegans, and the urochordates Halocynthia and Cionia. The C. elegans open reading frame was shown to encode UGM activity. The GLF phylogenetic distribution suggests that Gal(f) synthesis may occur more broadly in eukaryotes than previously supposed. Overall, GLF/Gal(f) synthesis in eukaryotes appears to occur with a disjunct distribution and often in pathogenic species, similar to what is seen in prokaryotes. Thus, UGM inhibition may provide an attractive drug target in those eukaryotes where Gal(f) plays critical roles in cellular viability and virulence.

Toll-like receptor 4 signaling regulates cytosolic phospholipase A2 activation and lipid generation in lipopolysaccharide-stimulated macrophages

Inflammatory lipid mediators such as prostaglandins and leukotrienes play crucial roles in the pathogenesis of bacterial lipopolysaccharide (LPS)-induced inflammation. Cytosolic phospholipase A(2) (cPLA(2)) is a key enzyme in the generation of pro-inflammatory lipid mediators. Here, we found that Toll-like receptor 4 (TLR4) is essential for LPS-induced cPLA(2) activation and lipid release. Inhibition of TLR4 protein expression by TLR4 small interfering RNA or neutralization of TLR4 by the specific antibody against TLR4/MD2 blocked cPLA(2) phosphorylation and cPLA(2)-hydrolyzed arachidonic acid release. Furthermore, activation of the TLR4 signaling pathway by LPS regulated cPLA(2) activation and lipid release. cPLA(2) phosphorylation and cPLA(2)-hydrolyzed lipid release were significantly impaired when TLR4 adaptor protein, either MyD88 or TRIF, was knocked down in LPS-stimulated macrophages. Similarly, LPS-induced arachidonate release was inhibited in cells transfected with a dominant-negative MyD88 or TRIF construct. Subsequently, cPLA(2) activation could be suppressed by inhibition of the TLR4 adaptor protein-directed p38 and ERK MAPK pathways. These findings suggest that, in LPS-induced inflammation, the TLR4-mediated MyD88- and TRIF-dependent MAPK pathways result in cPLA(2) activation and production of pro-inflammatory lipid mediators.

Functional genome annotation through phylogenomic mapping

Accurate determination of functional interactions among proteins at the genome level remains a challenge for genomic research. Here we introduce a genome-scale approach to functional protein annotation--phylogenomic mapping--that requires only sequence data, can be applied equally well to both finished and unfinished genomes, and can be extended beyond single genomes to annotate multiple genomes simultaneously. We have developed and applied it to more than 200 sequenced bacterial genomes. Proteins with similar evolutionary histories were grouped together, placed on a three dimensional map and visualized as a topographical landscape. The resulting phylogenomic maps display thousands of proteins clustered in mountains on the basis of coinheritance, a strong indicator of shared function. In addition to systematic computational validation, we have experimentally confirmed the ability of phylogenomic maps to predict both mutant phenotype and gene function in the delta proteobacterium Myxococcus xanthus.

Structural characterization of lipid A from nontypeable and type f Haemophilus influenzae: variability of fatty acid substitution

Lipid A isolated by mild acid hydrolysis from lipopolysaccharides of 22 nontypeable and 2 type f Haemophilus influenzae strains was investigated using electrospray ionization coupled to quadrupole ion trap mass spectrometry. The lengths, positions, and number of acyl chains in the lipid A molecule were determined using multiple-step tandem mass spectrometry (MSn). All of the analyzed strains showed a major lipid A molecule comprising beta-2-amino-2-deoxy-D-glucopyranose-(1-->6)-alpha-2-amino-2-deoxy-D-glucopyranose phosphorylated at the C4' and C1 positions. The C2/C2' and C3/C3' positions were substituted by amide-linked and ester-linked 3-hydroxytetradecanoic acid chains, respectively. The fatty acid chains on C3' and C2' were further esterified by tetradecanoic acid chains. In all strains, minor amounts of lipid A molecules with different acylation patterns were identified. Thus, structures comprising the hexaacylated lipid A with the C2 or C3 position being substituted by 3-hydroxydecanoic acid, and hexaacylated lipid A with the C3 and C3' positions being substituted by 3-hydroxydodecanoic or dodecanoyloxytetradecanoic acid, respectively, were found. In addition, lipid A with an acetyl group attached to the 3-hydroxytetradecanoic acid groups attached to the C2 or C3 position was detected in two nontypeable H. influenzae strains.

Study on the antiinflammatory activity of essential oil from leaves of Cinnamomum osmophloeum

The leaf essential oil from indigenous cinnamon (Cinnamomum osmophloeum Kaneh.) was investigated by gas chromatography-mass spectrometry, and 21 compounds were identified. The major constituents of leaf essential oil were the monoterpenes 1,8-cineole (17.0%) and santolina triene (14.2%) and the sesquiterpenes spathulenol (15.7%) and caryophyllene oxide (11.2%). In the antiinflammatory activity assay, we demonstrated that the essential oil has a higher capacity to inhibit proIL-1beta protein expression induced by LPS-treated J774A.1 murine macrophage. At dosages of 60 microg/mL, essential oil clearly inhibited proIL-1beta protein expression. Furthermore, a dose of 60 microg/mL of essential oil was effectively inhibitory for IL-1beta and IL-6 production but not for TNF-alpha, suggesting that essential oil was bioactive in antiinflammation in vitro. This study is the first to report antiinflammatory activity of extracts obtained from the leaf essential oil of C. osmophloeum.

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.

Recent advances in studies on structure and symbiosis-related function of rhizobial K-antigens and lipopolysaccharides

Exopolysaccharides (EPSs) and K polysaccharides (K-antigens, capsular polysaccharides, or KPSs) are important for the recognition of the symbiotic partner and the infection process, whereas lipopolysaccharides (LPSs) may function at a later stage of symbiosis. Recently, considerable progress has been made in the structural investigation of rhizobial K-antigens and LPSs. This structural data, together with the availability of more and more mutant data, allows new insights into the structure-function relationships of surface polysaccharides and the mode of their action on host cells. This review focuses on rhizobial LPSs and K-antigens. It gives a condensed overview of the recent developments in analysis of their structures and roles during symbiosis.

Mass spectrometry advances in lipidomica: collision-induced decomposition of Kdo2-lipid A

Mass spectrometry has made significant advances in the analysis of lipid substances, both simple and complex present in extracts of eukaryotic and prokaryotic cells. The development of the ionization techniques of electrospray ionization and matrix-assisted laser desorption ionization (MALDI) have both been applied to the analysis of lipids. The example of the types of structural information that can be obtained from MALDI-TOF tandem mass spectrometry is exemplified by the analysis of Kdo2-lipid A, a complex lipopolysaccharide known to activate toll-like 4 receptors on mammalian cells. Analysis of Kdo2-lipid A obtained from an Escherichia coli WBB06 was found to generate an abundant [M-H]- ion at m/z 2236.4 and a more abundant carbon-13 isotope at m/z 2237.4. Furthermore, collisional activation of the lipid A portion of the molecule at m/z 1796.3 resulted in a series of ions corresponding to the loss of all four fatty acyl groups as neutral carboxylic acids. An altogether different challenge of mass spectrometry applied to the area of lipid analysis is that of quantitative analysis. Two rather different requirements have emerged. One with high precision and accuracy for the measurement of relatively few lipid species that are produced at very low concentrations and typically interact with specific receptor proteins. A rather different challenge is that for the analysis of abundant lipid classes, which are composed of multiple molecular species that can approach several hundred under certain circumstances.

Lipopolysaccharide heterogeneity: innate host responses to bacterial modification of lipid a structure

The innate host response system is composed of various mechanisms designed to detect and facilitate host responses to microbial components, such as lipopolysaccharides (LPS). To enable this to occur, innate systems contain multiple pattern recognition receptors (i.e., LBP, CD14, and TLRs), which identify certain features within bacterial LPS that are foreign to the host, as well as essential and uniquely specific for bacteria. Innate host identification of unique bacterial components or patterns, therefore, relies on the inability of bacteria to alter these essential or critical components dramatically. Historically, LPS have been viewed as essential outer-membrane molecules containing both a highly variable outer region (O-segment) as well as a relatively conserved inner region (lipid A). However, over the last decade, new evidence has emerged, revealing that increased natural diversity or heterogeneity within specific components of LPS, such as lipid A-resulting in minor to moderate changes in lipid A structure-can produce dramatic host responses. Therefore, examples of natural lipid A heterogeneity, and the mechanisms that control it, represent a novel approach in which bacteria modulate host responses and may thereby confer specific advantages to certain bacterial species under changing environmental host conditions.

Importance of unusually modified lipid A in Sinorhizobium stress resistance and legume symbiosis

Sinorhizobium meliloti, a legume symbiont and Brucella abortus, a phylogenetically related mammalian pathogen, both require their BacA proteins to establish chronic intracellular infections in their respective hosts. The lipid A molecules of S. meliloti and B. abortus are unusually modified with a very-long-chain fatty acid (VLCFA; C > or = 28) and we discovered that BacA is involved in this unusual modification. This observation raised the possibility that the unusual lipid A modification could be crucial for the chronic infection of both S. meliloti and B. abortus. We investigated this by constructing and characterizing S. meliloti mutants in the lpxXL and acpXL genes, which encode an acyl transferase and acyl carrier protein directly involved in the biosynthesis of VLCFA-modified lipid A. Our analysis revealed that the unusually modified lipid A is important, but not crucial, for S. meliloti chronic infection and that BacA must have an additional function, which in combination with its observed effect on the lipid A in the free-living form of S. meliloti, is essential for the chronic infection. Additionally, we discovered that in the absence of VLCFAs, S. meliloti produces novel pentaacylated lipid A species, modified with unhydroxylated fatty acids, which are important for stress resistance.

Identification of an Escherichia coli operon required for formation of the O-antigen capsule

Escherichia coli produces polysaccharide capsules that, based on their mechanisms of synthesis and assembly, have been classified into four groups. The group 4 capsule (G4C) polysaccharide is frequently identical to that of the cognate lipopolysaccharide O side chain and has, therefore, also been termed the O-antigen capsule. The genes involved in the assembly of the group 1, 2, and 3 capsules have been described, but those required for G4C assembly remained obscure. We found that enteropathogenic E. coli (EPEC) produces G4C, and we identified an operon containing seven genes, ymcD, ymcC, ymcB, ymcA, yccZ, etp, and etk, which are required for formation of the capsule. The encoded proteins appear to constitute a polysaccharide secretion system. The G4C operon is absent from the genomes of enteroaggregative E. coli and uropathogenic E. coli. E. coli K-12 contains the G4C operon but does not express it, because of the presence of IS1 at its promoter region. In contrast, EPEC, enterohemorrhagic E. coli, and Shigella species possess an intact G4C operon.

Functional analysis of the glycero-manno-heptose 7-phosphate kinase domain from the bifunctional HldE protein, which is involved in ADP-L-glycero-D-manno-heptose biosynthesis

The core oligosaccharide component of the lipopolysaccharide can be subdivided into inner and outer core regions. In Escherichia coli, the inner core consists of two 3-deoxy-d-manno-octulosonic acid and three glycero-manno-heptose residues. The HldE protein participates in the biosynthesis of ADP-glycero-manno-heptose precursors used in the assembly of the inner core. HldE comprises two functional domains: an N-terminal region with homology to the ribokinase superfamily (HldE1 domain) and a C-terminal region with homology to the cytidylyltransferase superfamily (HldE2 domain). We have employed the structure of the E. coli ribokinase as a template to model the HldE1 domain and predict critical amino acids required for enzyme activity. Mutation of these residues renders the protein inactive as determined in vivo by functional complementation analysis. However, these mutations did not affect the secondary or tertiary structure of purified HldE1, as judged by fluorescence spectroscopy and circular dichroism. Furthermore, in vivo coexpression of wild-type, chromosomally encoded HldE and mutant HldE1 proteins with amino acid substitutions in the predicted ATP binding site caused a dominant negative phenotype as revealed by increased bacterial sensitivity to novobiocin. Copurification experiments demonstrated that HldE and HldE1 form a complex in vivo. Gel filtration chromatography resulted in the detection of a dimer as the predominant form of the native HldE1 protein. Altogether, our data support the notions that the HldE functional unit is a dimer and that structural components present in each HldE1 monomer are required for enzymatic activity.

functional analysis of conserved gene products involved in assembly of Escherichia coli capsules and exopolysaccharides: evidence for molecular recognition between Wza and Wzc for colanic acid biosynthesis

Group 1 capsular polysaccharides (CPSs) of Escherichia coli and some loosely cell-associated exopolysaccharides (EPSs), such as colanic acid, are assembled by a Wzy-dependent polymerization system. In this biosynthesis pathway, Wza, Wzb, and Wzc homologues are required for surface expression of wild-type CPS or EPS. Multimeric complexes of Wza in the outer membrane are believed to provide a channel for polymer export; Wzc is an inner membrane tyrosine autokinase and Wzb is its cognate phosphatase. This study was performed to determine whether the Wza, Wzb, and Wzc proteins for colanic acid expression in E. coli K-12 could function in the E. coli K30 prototype group 1 capsule system. When expressed together, colanic acid Wza, Wzb, and Wzc could complement a wza-wzb-wzc defect in E. coli K30, suggesting conservation in their collective function in Wzy-dependent CPS and EPS systems. Expressed individually, colanic acid Wza and Wzb could also function in K30 CPS expression. In contrast, the structural requirements for Wzc function were more stringent because colanic acid Wzc could restore translocation of K30 CPS to the cell surface only when expressed with its cognate Wza protein. Chimeric colanic acid-K30 Wzc proteins were constructed to further study this interaction. These proteins could restore K30 biosynthesis but were unable to couple synthesis to export. The chimeric protein comprising the periplasmic domain of colanic acid Wzc was functional for effective K30 CPS surface expression only when coexpressed with colanic acid Wza. These data highlight the importance of Wza-Wzc interactions in group 1 CPS assembly.

The enigmatic acyl carrier protein phosphodiesterase of Escherichia coli: genetic and enzymological characterization

The acyl carrier proteins (ACPs) of fatty acid synthesis are functional only when modified by attachment of the prosthetic group, 4'-phosphopantetheine (4'-PP), which is transferred from CoA to the hydroxyl group of a specific serine residue. Almost 40 years ago Vagelos and Larrabee reported an enzyme from Escherichia coli that removed the prosthetic group. We report that this enzyme, called ACP hydrolyase or ACP phosphodiesterase, is encoded by a gene (yajB) of previously unknown function that we have renamed acpH. A mutant E. coli strain having a total deletion of the acpH gene has been constructed that grows normally, showing that phosphodiesterase activity is not essential for growth, although it is required for turnover of the ACP prosthetic group in vivo. ACP phosphodiesterase (AcpH) has been purified to homogeneity for the first time and is a soluble protein that very readily aggregates upon overexpression in vivo or concentration in vitro. The purified enzyme has been shown to cleave acyl-ACP species with acyl chains of 6-16 carbon atoms and is active on some, but not all, non-native ACP species tested. Possible physiological roles for AcpH are discussed.

Cationic antimicrobial peptide resistance in Neisseria meningitidis

Cationic antimicrobial peptides (CAMPs) are important components of the innate host defense system against microbial infections and microbial products. However, the human pathogen Neisseria meningitidis is intrinsically highly resistant to CAMPs, such as polymyxin B (PxB) (MIC > or = 512 microg/ml). To ascertain the mechanisms by which meningococci resist PxB, mutants that displayed increased sensitivity (> or =4-fold) to PxB were identified from a library of mariner transposon mutants generated in a meningococcal strain, NMB. Surprisingly, more than half of the initial PxB-sensitive mutants had insertions within the mtrCDE operon, which encodes proteins forming a multidrug efflux pump. Additional PxB-sensitive mariner mutants were identified from a second round of transposon mutagenesis performed in an mtr efflux pump-deficient background. Further, a mutation in lptA, the phosphoethanolamine (PEA) transferase responsible for modification of the lipid A head groups, was identified to cause the highest sensitivity to PxB. Mutations within the mtrD or lptA genes also increased meningococcal susceptibility to two structurally unrelated CAMPs, human LL-37 and protegrin-1. Consistently, PxB neutralized inflammatory responses elicited by the lptA mutant lipooligosaccharide more efficiently than those induced by wild-type lipooligosaccharide. mariner mutants with increased resistance to PxB were also identified in NMB background and found to contain insertions within the pilMNOPQ operon involved in pilin biogenesis. Taken together, these data indicated that meningococci utilize multiple mechanisms including the action of the MtrC-MtrD-MtrE efflux pump and lipid A modification as well as the type IV pilin secretion system to modulate levels of CAMP resistance. The modification of meningococcal lipid A head groups with PEA also prevents neutralization of the biological effects of endotoxin by CAMP.

Structures of the biological repeating units in the O-chain polysaccharides ofHafnia alveistrains having a typical lipopolysaccharide outer core region

Earlier, the structures of the O-chain polysaccharides of the lipopolysaccharides (LPS) of a number of Hafnia alvei strains have been established. However, it remained unknown, which is the first and the last monosaccharide of the O-chain. This is defined by the structure of the so-called biological repeating unit (O-unit), which is pre-assembled and then polymerised in the course of biosynthesis of bacterial polysaccharides by the Wzy-dependent pathway. Now we report on the structures of the O-units in 10 H. alvei strains. The LPS were cleaved by mild acid hydrolysis and oligosaccharide fractions IIIa and IIIb were isolated by gel chromatography subsequently on Sephadex G-50 and BioGel P-2 and studied by methylation analysis and NMR spectroscopy. Fraction IIIb was found to represent the core oligosaccharide containing a terminal upstream alpha-d-Glc-(1-->3)-alpha-d-Glc or alpha-d-Gal-(1-->3)-alpha-d-Glc disaccharide in the outer region that is typical of H. alvei. Fraction IIIa consists of the LPS core with one O-unit linked by a 3-substituted beta-d-GalNAc residue (in strains PCM 1189 and PCM 1546) or a 3-substituted beta-d-GlcNAc residue (in the other strains studied). In most strains examined the beta-configuration of the d-GlcNAc linkage in the first O-unit attached to the core is the same and in some strains is opposite to that found in the interior O-units of the O-chain polysaccharide. Various monosaccharides, including d-Glc, d-Gal, d-GlcA and acyl derivatives of 3-amino-3,6-dideoxy-d-glucose or 4-amino-4,6-dideoxy-d-glucose, occupy the non-reducing end of the O-unit.

Polymyxin B: an ode to an old antidote for endotoxic shock

Endotoxic shock, a syndrome characterized by deranged hemodynamics, coagulation abnormalities, and multiple system organ failure is caused by the release into the circulation of lipopolysaccharide (LPS), the structurally diverse component of Gram-negative bacterial outer membranes, and is responsible for 60% mortality in humans. Polymyxin B (PMB), a cyclic, cationic peptide antibiotic, neutralizes endotoxin but induces severe side effects in the process. The potent endotoxin neutralizing ability of PMB, however, offers possibilities for designing non-toxic therapeutic agents for combating endotoxicosis. Amongst the numerous approaches for combating endotoxic shock, peptide mediated neutralization of LPS seems to be the most attractive one. The precise mode of binding of PMB to LPS and the structural features involved therein have been elucidated only recently using a variety of biophysical approaches. These suggest that efficient neutralization of endotoxin by PMB is not achieved by mere binding to LPS but requires its sequestration from the membrane. Incorporation of this feature into the design of endotoxin neutralizing peptides should lead to the development of effective antidotes for endotoxic shock.

The elicitation of plant innate immunity by lipooligosaccharide of Xanthomonas campestris

Lipopolysaccharides (LPSs) and lipooligosaccharides (LOSs) are major components of the cell surface of Gram-negative bacteria with diverse roles in bacterial pathogenesis of animals and plants that include elicitation of host defenses. Little is known about the mechanisms of perception of these molecules by plants and about the associated signal transduction pathways that trigger plant immunity. Here we address the issue of the molecular basis of elicitation of plant defenses through the structural determination of the LOS of the plant pathogen Xanthomonas campestris pv. campestris strain 8004 and examination of the effects of LOS and fragments obtained by chemical treatments on the immune response in Arabidopsis thaliana. The structure shows a strong accumulation of negatively charged groups in the lipid A-inner core region and has a number of novel features, including a galacturonyl phosphate attached at a 3-deoxy-D-manno-oct-2-ulosonic acid residue and a unique phosphoramide group in the inner core region. Intact LOS and the lipid A and core oligosaccharides derived from it were all able to induce the defense-related genes PR1 and PR2 in Arabidopsis and to prevent the hypersensitive response caused by avirulent bacteria. Although LOS induced defense-related gene transcription in two temporal phases, the core oligosaccharide induced only the earlier phase, and lipid A induced only the later phase. These findings suggest that plant cells can recognize lipid A and core oligosaccharide structures within LOS to trigger defensive cellular responses and that this may occur via two distinct recognition events.