The conformation of core oligosaccharides from Escherichia coli and Salmonella typhimurium lipopolysaccharides as predicted by semi-empirical calculations

Bacteriophage PhiX174's ecological niche and the flexibility of its Escherichia coli lipopolysaccharide receptor

To determine bacteriophage PhiX174's ecological niche, 783 Escherichia coli isolates were screened for susceptibility. Sensitive strains are diverse regarding their phylogenies and core lipopolysaccharides (LPS), but all have rough phenotypes. Further analysis of E. coli K-12 LPS mutants revealed that PhiX174 can use a wide diversity of LPS structures to initiate its infectious process.

Mutation in the LPS outer core biosynthesis gene, galU, affects LPS interaction with the RTX toxins ApxI and ApxII and cytolytic activity of Actinobacillus pleuropneumoniae serotype 1

Lipopolysaccharides (LPS) and Apx toxins are major virulence factors of Actinobacillus pleuropneumoniae, a pathogen of the respiratory tract of pigs. Here, we evaluated the effect of LPS core truncation in haemolytic and cytotoxic activities of this microorganism. We previously generated a highly attenuated galU mutant of A. pleuropneumoniae serotype 1 that has an LPS molecule lacking the GalNAc-Gal II-Gal I outer core residues. Our results demonstrate that this mutant exhibits wild-type haemolytic activity but is significantly less cytotoxic to porcine alveolar macrophages. However, no differences were found in gene expression and secretion of the haemolytic and cytotoxic toxins ApxI and ApxII, both secreted by A. pleuropneumoniae serotype 1. This suggests that the outer core truncation mediated by the galU mutation affects the toxins in their cytotoxic activities. Using both ELISA and surface plasmon resonance binding assays, we demonstrate a novel interaction between LPS and the ApxI and ApxII toxins via the core oligosaccharide. Our results indicate that the GalNAc-Gal II-Gal I trisaccharide of the outer core is fundamental to mediating LPS/Apx interactions. The present study suggests that a lack of binding between LPS and ApxI/II affects the cytotoxicity and virulence of A. pleuropneumoniae.

Similarities and disparities between core-specific and O-side-chain-specific antilipopolysaccharide monoclonal antibodies in models of endotoxemia and bacteremia in mice

We have previously described cross-reactive antilipopolysaccharide (anti-LPS), or anti-endotoxin, monoclonal antibodies (MAbs) which provide cross-protection in several systems of endotoxin bioactivity. The protective effects of the murine cross-reactive MAb WN1 222-5 (immunoglobulin G2a(kappa) [IgG/2a(kappa)]) and of its chimerized version, SDZ 219-800 [human IgG1(kappa)], have now been evaluated in lethality models against LPS from three different serotypes and in bacterial infection models. We confirmed the protective activity of the two MAbs in D-galactosamine-sensitized mice challenged with LPS of other E. coli serotypes (O18, O127, and O111). The protective effect correlated with the suppression of tumor necrosis factor formation. Furthermore, WN1 222-5 enhanced bacterial clearance of intravenously administered E. coli O111 bacteria, thus protecting mice from death. However, the MAbs were unable to provide protection in a peritonitis model (intraperitoneal inoculation). Our study, therefore, shows that LPS cross-reactive antibodies are capable of mediating cross-protection against LPS and bacteria but that the selected models have a clear influence on the results.

Comparison of X-ray powder-diffraction data of various bacterial lipopolysaccharide structures with theoretical model conformations

X-ray powder-diffraction experiments have been performed on dry samples of lipid A and various rough-mutant lipopolysaccharides (LPS) of Salmonella minnesota, Salmonella typhimurium and Escherichia coli. The diffraction patterns obtained indicated exclusively lamellar, bilayered arrangements in all samples. The periodicities were found to be in the range 4.5 nm for lipid A to 8.8 nm for Ra-LPS. Upon treatment with water-saturated air, swelling of the lamellar structures was achieved, as indicated by shifts of reflections. The increase in bilayer dimensions normally was about 0.3 nm. X-ray intensities were used for the determination of the inner bilayer structure, i.e. for calculation of the one-dimensional electron-density distribution across the bilayer. For lipid A and several Re-LPS, Rd2-LPS, Rd1-LPS and Rc-LPS samples, a striking coincidence of the electron-density distributions in the lipid-A domain was found, suggesting that in all these structures the lipid-A portion is similarly arranged. For Rb1 and Ra-LPS the lipid-A domain could not be resolved due to the limited number of observed reflections. For other Re-mutant lipopolysaccharide samples, quite different X-ray patterns were obtained. Some samples yielded diffraction patterns indicating a very high state of order in the lipid-A domain, whereas, in others, a significantly reduced order in the lipid-A domain was inferred. Comparison of the X-ray data with features of a calculated three-dimensional molecular model of lipopolysaccharide revealed reasonable agreement in molecular dimensions and bilayer structure.

A broadly cross-protective monoclonal antibody binding to Escherichia coli and Salmonella lipopolysaccharides

During the last decade, episodes of sepsis have increased and Escherichia coli has remained the most frequent clinical isolate. Lipopolysaccharides (LPS; endotoxin) are the major toxic and antigenic components of gram-negative bacteria and qualify as targets for therapeutic interventions. Molecules that neutralize the toxic effects of LPS are actively investigated. In this paper, we describe a murine monoclonal antibody (MAb; WN1 222-5), broadly cross-reactive and cross-protective for smooth (S)-form and rough (R)-form LPS. As shown in enzyme-linked immunosorbent assay and the passive hemolysis assay, WN1 222-5 binds to the five known E. coli core chemotypes, to Salmonella core, and to S-form LPS having these core structures. In immunoblots, it is shown to react with both the nonsubstituted core LPS and with LPS carrying O-side chains, indicating the exposure of the epitope in both S-form and R-form LPS. This MAb of the immunoglobulin G2a class is not lipid A reactive but binds to E. coli J5, an RcP+ mutant which carries an inner core structure common to many members of the family Enterobacteriaceae. Phosphate groups present in the inner core contribute to the epitope but are not essential for the binding of WN1 222-5 to complete core LPS. Cross-reactivity for clinical bacterial isolates is broad. WN1 222-5 binds to all E. coli clinical isolates tested so far (79 blood isolates, 80 urinary isolates, and 21 fecal isolates) and to some Citrobacter, Enterobacter, and Klebsiella isolates. This pattern of reactivity indicates that its binding epitope is widespread among members of the Enterobacteriaceae. WN1 222-5 exhibits biologically relevant activities. In vitro, it inhibits the Limulus amoebocyte lysate assay activity of S-form and R-form LPS in a dose-dependent manner and it neutralizes the LPS-induced release of clinically relevant monokines (interleukin 6 and tumor necrosis factor). In vivo, WN1 222-5 blocks endotoxin-induced pyrogenicity in rabbits and lethality in galactosamine-sensitized mice. The discovery of WN1 222-5 settles the long-lasting controversy over the existence of anti-core LPS MAbs with both cross-reactive and cross-protective activity, opening new possibilities for the immunotherapy of sepsis caused by gram-negative bacteria.

Molecular modelling of the three-dimensional structure and conformational flexibility of bacterial lipopolysaccharide

Molecular modelling techniques have been applied to calculate the three-dimensional architecture and the conformational flexibility of a complete bacterial S-form lipopolysaccharide (LPS) consisting of a hexaacyl lipid A identical to Escherichia coli lipid A, a complete Salmonella typhimurium core oligosaccharide portion, and four repeating units of the Salmonella serogroup B O-specific chain. X-ray powder diffraction experiments on dried samples of LPS were carried out to obtain information on the dimensions of the various LPS partial structures. Up to the Ra-LPS structure, the calculated model dimensions were in good agreement with experimental data and were 2.4 nm for lipid A, 2.8 nm for Re-LPS, 3.5 nm for Rd-LPS, and 4.4 nm for Ra-LPS. The maximum length of a stretched S-form LPS model bearing four repeating units was evaluated to be 9.6 nm; however, energetically favored LPS conformations showed the O-specific chain bent with respect to the Ra-LPS portion and significantly smaller dimensions (about 5.0 to 5.5 nm). According to the calculations, the Ra-LPS moiety has an approximately cylindrical shape and is conformationally well defined, in contrast to the O-specific chain, which was found to be the most flexible portion within the molecule.

Studies of the binding activity of phage G13 to synthetic trisaccharides analogous to binding structures in Salmonella typhimurium and Escherichia coli C core saccharide. Correlation between conformation and binding activity

Phage G13 binds to the carbohydrate part of lipopolysaccharides from rough mutants of Salmonella and Escherichia coli as the first event of infection. Equilibrium dialysis inhibition studies with native and synthetic trisaccharides as inhibitors suggested that phage G13 recognizes branched oligosaccharides having 6-O-alpha- or 7-O-alpha-glycosyl groups with alpha-Man(1----3) [alpha-Man(1----6)]Man (Man[Man]Man) and alpha-Glc(1----3)-[alpha-Hep(1----7)] alpha-Hep(1----3) alpha-Hep(1----5)Kdo as the smallest saccharides with inhibitory activity (Wollin et al., 1989). Of four synthetic analogues to Man[Man]Man only Man(1----3)[alpha-Gal(1----6)]alpha-Man-OMe (Man[Gal]-Man) and alpha-Glc(1----3)[alpha-Hep(1----7)]alpha-Hep-OMe (Glc[Hep]Hep) inhibited the binding of labelled E. coli C core nonasaccharide ligand to G13 with activities which were 10- and 15-fold lower than Man[Man]Man. The trisaccharides alpha-Man(1----3)[alpha-Glc(1----6)[alpha-Man-OMe (Man[Glc]Mann) and alpha-Man(1---3)[alpha-Tal(1----6)]alpha-Man-OMe (Man[Tal]Man) showed no inhibition at concentrations 75-fold higher than Man[Man]Man. Minimum energy conformation calculations of the saccharides using the GESA method showed that the 6-O-alpha-Man group in Man[Man]Man and the 7-O-alpha-Hep group SL805 pentasaccharide expose their OH-2 and OH-3 groups in a similar way and these are postulated to be key structural features for binding activity. The importance of hydroxy groups at certain positions is implied from the fact that both manno- and galacto-isomers are active. We also conclude that the O6-C6-C5-O5-C1 region of the 3-O-alpha-glycosyl group in the Man[Man]Man trisaccharide, or part of it, is important for the G13 binding activity.

Definition of the phage G13 receptor as structural domains of trisaccharides in Salmonella and Escherichia coli core oligosaccharides

The interaction between phage G13 and different bacterial and synthetic oligosaccharides has been studied using equilibrium dialysis inhibition. The results, and conformational analysis of the oligosaccharides, make us conclude that the phage G13 carbohydrate receptor is a conformational domain involving three sugar residues. The following trisaccharide elements contain the domain: alpha-D-Galp-(1----3)-[alpha-D-Galp-(1----6)]-alpha-D-Glcp, alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)-alpha-D-Manp , and alpha-D-Glcp-(1----3)-[L-gly-alpha-D-man-Hepp-(1----7)]-L-gly-alph a-D- man-Hepp. Thus two structures, either a hexose substituted with alpha-D-glycopyranosyl groups in the 3- and 6-positions, or a heptose substituted with such groups in the 3- and 7-positions are functional G13 binding sites. Such domains are present in several cores of lipopolysaccharides from Salmonella and Escherichia coli species. Some cores, e.g. those from S. typhimurium chemotypes Ra, Rb1 and Rb2, contain two such domains. The identification of two G13 receptor domains within different core saccharides could explain the broad host range of this phage.

Interaction between phage G13 and its oligosaccharide receptor studied by equilibrium dialysis

The reversible binding of phage G13, a phi X174-like single-strand DNA phage, to a 3H-labelled nonasaccharide from the lipopolysaccharide of its natural host Escherichia coli C was studied with equilibrium dialysis. The binding constant (Ka) was determined to 1.3 x 10(7) M-1 in Scatchard and Lineweaver-Burk plots. Approximately one saccharide bound per G13 phage particle which suggests that only one of the 12 spikes in each G13 virion was engaged in the phage/receptor saccharide interaction. Equilibrium dialysis inhibition experiments with saccharides from lipopolysaccharides of an isogenic series of Salmonella typhimurium mutants showed that hepta- and pentasaccharides from two G13-sensitive bacteria, i.e., with efficiencies of plating of 0.1-1.0 compared to E. coli C, were efficient inhibitors with Ka-values greater than or equal to 1.2 x 10(7) M-1. The octa- and hexasaccharides from two G13 resistant strains, with efficiency of plating less than or equal to x 10(-4), were either greater than 1000-fold or greater than 15-fold less efficient as inhibitors with Ka-values less than or equal to 8.8 x 10(5) M-1. The results show that phage G13 binds in a specific and reversible way to penta-, hepta-, and nonasaccharides from G13 sensitive bacteria with the specificity residing in the hexose and heptose region of the core lipopolysaccharide.