Lipopolysaccharide structures from Agrobacterium and Rhizobiaceae species

Emerging Research Topics in the Vibrionaceae and the Squid-Vibrio Symbiosis

The Vibrionaceae encompasses a cosmopolitan group that is mostly aquatic and possesses tremendous metabolic and genetic diversity. Given the importance of this taxon, it deserves continued and deeper research in a multitude of areas. This review outlines emerging topics of interest within the Vibrionaceae. Moreover, previously understudied research areas are highlighted that merit further exploration, including affiliations with marine plants (seagrasses), microbial predators, intracellular niches, and resistance to heavy metal toxicity. Agarases, phototrophy, phage shock protein response, and microbial experimental evolution are also fields discussed. The squid-Vibrio symbiosis is a stellar model system, which can be a useful guiding light on deeper expeditions and voyages traversing these "seas of interest". Where appropriate, the squid-Vibrio mutualism is mentioned in how it has or could facilitate the illumination of these various subjects. Additional research is warranted on the topics specified herein, since they have critical relevance for biomedical science, pharmaceuticals, and health care. There are also practical applications in agriculture, zymology, food science, and culinary use. The tractability of microbial experimental evolution is explained. Examples are given of how microbial selection studies can be used to examine the roles of chance, contingency, and determinism (natural selection) in shaping Earth's natural history.

Variation in blood microbial lipopolysaccharide (LPS) contributes to immune reconstitution in response to suppressive antiretroviral therapy in HIV

BACKGROUND

In HIV infection, even under long-term antiretroviral therapy (ART), up to 20% of HIV-infected individuals fail to restore CD4+ T cell counts to the levels similar to those of healthy controls. The mechanisms of poor CD4+ T cell reconstitution on suppressive ART are not fully understood.

METHODS

Here, we tested the hypothesis that lipopolysaccharide (LPS) from bacteria enriched in the plasma from immune non-responders (INRs) contributes to blunted CD4+ T cell recovery on suppressive ART in HIV. We characterized plasma microbiome in HIV INRs (aviremic, CD4+ T cell counts < 350 cells/μl), immune responders (IRs, CD4+ T cell counts > 500 cells/μl), and healthy controls. Next, we analyzed the structure of the lipid A domain of three bacterial species identified by mass spectrometry (MS) and evaluated the LPS function through LPS induced proinflammatory responses and CD4+ T cell apoptosis in PBMCs. In comparison, we also evaluated plasma levels of proinflammatory cytokine and chemokine patterns in these three groups. At last, to study the causality of microbiome-blunted CD4+ T cell recovery in HIV, B6 mice were intraperitoneally (i.p.) injected with heat-killed Burkholderia fungorum, Serratia marcescens, or Phyllobacterium myrsinacearum, twice per week for total of eight weeks.

FINDINGS

INRs exhibited elevated plasma levels of total microbial translocation compared to the IRs and healthy controls. The most enriched bacteria were Burkholderia and Serratia in INRs and were Phyllobacterium in IRs. Further, unlike P. myrsinacearum LPS, B. fungorum and S. marcescens LPS induced proinflammatory responses and CD4+ T cell apoptosis in PBMCs, and gene profiles of bacteria-mediated cell activation pathways in THP-1 cells in vitro. Notably, LPS structural analysis by mass spectrometry revealed that lipid A from P. myrsinacearum exhibited a divergent structure consistent with weak toll-like receptor (TLR) 4 agonism, similar to the biological profile of probiotic bacteria. In contrast, lipid A from B. fungorum and S. marcescens showed structures more consistent with canonical TLR4 agonists stemming from proinflammatory bacterial strains. Finally, intraperitoneal (i.p.) injection of inactivated B. fungorum and S. marcescens but not P. myrsinacearum resulted in cell apoptosis in mesenteric lymph nodes of C57BL/6 mice in vivo.

INTERPRETATION

These results suggest that the microbial products are causally associated with INR phenotype. In summary, variation in blood microbial LPS immunogenicity may contribute to immune reconstitution in response to suppressive ART. Collectively, this work is consistent with immunologically silencing microbiome being causal and targetable with therapy in HIV.

FUNDING

This work was supported by the National Institute of Allergy and Infectious Diseases (NIAID; R01 AI128864, Jiang) (NIAID; P30 AI027767, Saag/Health), the Medical Research Service at the Ralph H. Johnson VA Medical Center (merit grant VA CSRD MERIT I01 CX-002422, Jiang), and the National Institute of Aging (R21 AG074331, Scott). The SCOPE cohort was supported by the UCSF/Gladstone Institute of Virology & Immunology CFAR (P30 AI027763, Gandhi) and the CFAR Network of Integrated Clinical Systems (R24 AI067039, Saag). The National Center for Advancing Translational Sciences of the National Institutes of Health under Award Number UL1TR001450 (the pilot grant, Jiang). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Caulobacter lipid A is conditionally dispensable in the absence of fur and in the presence of anionic sphingolipids

Lipid A, the membrane-anchored portion of lipopolysaccharide (LPS), is an essential component of the outer membrane (OM) of nearly all Gram-negative bacteria. Here we identify regulatory and structural factors that together render lipid A nonessential in Caulobacter crescentus. Mutations in the ferric uptake regulator fur allow Caulobacter to survive in the absence of either LpxC, which catalyzes an early step of lipid A synthesis, or CtpA, a tyrosine phosphatase homolog we find is needed for wild-type lipid A structure and abundance. Alterations in Fur-regulated processes, rather than iron status per se, underlie the ability to survive when lipid A synthesis is blocked. Fitness of lipid A-deficient Caulobacter requires an anionic sphingolipid, ceramide phosphoglycerate (CPG), which also mediates sensitivity to the antibiotic colistin. Our results demonstrate that, in an altered regulatory landscape, anionic sphingolipids can support the integrity of a lipid A-deficient OM.

Lipid A, the membrane-anchoring segment of lipopolysaccharide, is generally considered to be an essential component of the Gram-negative bacterial outer membrane. Zik et al. show that deletion of the transcriptional regulator fur and synthesis of the anionic sphingolipid ceramide phosphoglycerate enable Caulobacter crescentus to survive without lipid A.

Dual adhesive unipolar polysaccharides synthesized by overlapping biosynthetic pathways in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a member of the Alphaproteobacteria that pathogenises plants and associates with biotic and abiotic surfaces via a single cellular pole. A. tumefaciens produces the unipolar polysaccharide (UPP) at the site of surface contact. UPP production is normally surface-contact inducible, but elevated levels of the second messenger cyclic diguanylate monophosphate (cdGMP) bypass this requirement. Multiple lines of evidence suggest that the UPP has a central polysaccharide component. Using an A. tumefaciens derivative with elevated cdGMP and mutationally disabled for other dispensable polysaccharides, a series of related genetic screens have identified a large number of genes involved in UPP biosynthesis, most of which are Wzx-Wzy-type polysaccharide biosynthetic components. Extensive analyses of UPP production in these mutants have revealed that the UPP is composed of two genetically, chemically, and spatially discrete forms of polysaccharide, and that each requires a specific Wzy-type polymerase. Other important biosynthetic, processing, and regulatory functions for UPP production are also revealed, some of which are common to both polysaccharides, and a subset of which are specific to each type. Many of the UPP genes identified are conserved among diverse rhizobia, whereas others are more lineage specific.

Dissecting Lipopolysaccharide Composition and Structure by GC-MS and MALDI Spectrometry

Lipopolysaccharides (LPSs) are the main components of the external leaflet of the outer membrane of Gram-negative bacteria. They exert multiple functions, starting from conferring stability to the bacterial membrane to mediating the interaction of the microbe with the external environment. The composition and the structure of LPSs present tremendous diversity even within bacteria of the same species, and for this reason, the determination of the structure of these molecules is crucial because it can provide information on the motifs key for the virulence of a pathogen or that are associated to a bacterium of the commensal or beneficial microbiota. In addition, structural data disclose the effects triggered from a mutation or from the use of an antibiotic, or they can be used as tools to check the quality of adjuvants and/or medications, as vaccines, that make use of LPS.The structural study of LPSs is complex, and it can be achieved with the right combination of different techniques. In this frame, this chapter focuses on the two MS-based approaches, the gas chromatography-mass spectrometry (GC-MS) and the matrix-assisted laser desorption/ionization (MALDI).

Freeze-Drying To Produce Efficacious CPMV Virus-like Particles

In situ cancer vaccination that uses immune stimulating agents is revolutionizing the way that cancer is treated. In this realm, viruses and noninfectious virus-like particles have gained significant traction in reprogramming the immune system to recognize and eliminate malignancies. Recently, cowpea mosaic virus-like particles (VLPs) have shown exceptional promise in their ability to fight a variety of cancers. However, the current methods used to produce CPMV VLPs rely on agroinfiltration in plants. These protocols remain complicated and labor intensive and have the potential to introduce unwanted immunostimulatory agents, like lipopolysaccharides. This Letter describes a simple "post-processing" method to remove RNA from wild-type CPMV, while retaining the structure and function of the capsid. Lyophilization was able to eject encapsulated RNA to form lyo-eCPMV and, when purified, eliminated nearly all traces of encapsulated RNA. Lyo-eCPMV was characterized by cryo-electron microscopy single particle reconstruction to confirm the structural integrity of the viral capsid. Finally, lyo-eCPMV showed  equivalent anticancer efficacy as eCPMV, produced by agroinfiltration, when using an invasive melanoma model. These results describe a straightforward method to prepare CPMV VLPs from infectious virions.

A new factor controlling cell envelope integrity in Alphaproteobacteria in the context of cell cycle, stress response and infection

The bacterial envelope is a remarkable and complex compartment of the prokaryotic cell in which many essential functions take place. The article by Herrou and collaborators (Herrou et al., in press), by a clever combination of structural analysis, genetics and functional characterization in free-living bacterial cells and during infection in animal models, elucidates a new factor, named EipA, that plays a major role in Brucella spp envelope biogenesis and cell division. The authors demonstrate a genetic connection between eipA and lipopolysaccharide synthesis, specifically genes involved in the synthesis of the O-antigen portion of lipopolysaccharide (LPS). Beyond its crucial role in Brucella physiology, the conservation of EipA in the class Alphaproteobacteria urges microbiologists to pursue future investigation of its homologs in other species belonging to this important group of bacteria.

Complex gene response of herbicide-resistant Enterobacter strain NRS-1 under different glyphosate stresses

Knowledge of biological evolution and genetic mechanisms is gained by studying the adaptation of bacteria to survive in adverse environmental conditions. In this regard, transcriptomic profiling of a glyphosate-tolerant Enterobacter strain NRS-1 was studied under four different treatments to investigate the gene-regulatory system for glyphosate tolerance. A total of 83, 83, 60 and 74 genes were up-regulated and 108, 87, 178 and 117 genes down-regulated under 60-NPG, 110-NPG, NaCl (355 mM) and HCl (pH 4.46) stress treatments, respectively. Complex gene network was identified to be involved in regulating tolerance to glyphosate. This study revealed that NRS-1 has gained glyphosate tolerance at the cost of osmotic and acidic resistance. The 25 differentially expressed genes are reported to may have partly changed the function for providing resistance to glyphosate directly, among them genes metK, mtbK, fdnG and wzb that might detoxify/degrade the glyphosate. However, under 110-NPG condition, NRS-1 might have utilized economical and efficient ways by depressing its metabolism and activity to pass through this stress. Hence, the present study provides insights into the genes involved in glyphosate tolerance, which can be effectively utilized to engineer herbicide-resistant crop varieties after their proper validation to manage weed growth.

More than Rotating Flagella: Lipopolysaccharide as a Secondary Receptor for Flagellotropic Phage 7-7-1

Bacteriophage 7-7-1, a member of the family Myoviridae, infects the soil bacterium Agrobacterium sp. strain H13-3. Infection requires attachment to actively rotating bacterial flagellar filaments, with flagellar number, length, and rotation speed being important determinants for infection efficiency. To identify the secondary receptor(s) on the cell surface, we isolated motile, phage-resistant Agrobacterium sp. H13-3 transposon mutants. Transposon insertion sites were pinpointed using arbitrary primed PCR and bioinformatics analyses. Three genes were recognized, whose corresponding proteins had the following computationally predicted functions: AGROH133_07337, a glycosyltransferase; AGROH133_13050, a UDP-glucose 4-epimerase; and AGROH133_08824, an integral cytoplasmic membrane protein. The first two gene products are part of the lipopolysaccharide (LPS) synthesis pathway, while the last is predicted to be a relatively small (13.4-kDa) cytosolic membrane protein with up to four transmembrane helices. The phenotypes of the transposon mutants were verified by complementation and site-directed mutagenesis. Additional characterization of motile, phage-resistant mutants is also described. Given these findings, we propose a model for Agrobacterium sp. H13-3 infection by bacteriophage 7-7-1 where the phage initially attaches to the flagellar filament and is propelled down toward the cell surface by clockwise flagellar rotation. The phage then attaches to and degrades the LPS to reach the outer membrane and ejects its DNA into the host using its syringe-like contractile tail. We hypothesize that the integral membrane protein plays an important role in events following viral DNA ejection or in LPS processing and/or deployment. The proposed two-step attachment mechanism may be conserved among other flagellotropic phages infecting Gram-negative bacteria.IMPORTANCE Flagellotropic bacteriophages belong to the tailed-phage order Caudovirales, the most abundant phages in the virome. While it is known that these viruses adhere to the bacterial flagellum and use flagellar rotation to reach the cell surface, their infection mechanisms are poorly understood. Characterizing flagellotropic-phage-host interactions is crucial to understanding how microbial communities are shaped. Using a transposon mutagenesis approach combined with a screen for motile, phage-resistant mutants, we identified lipopolysaccharides as the secondary cell surface receptor for phage 7-7-1. This is the first cell surface receptor identified for flagellotropic phages. One hypothetical membrane protein was also recognized as essential for infection. These new findings, together with previous results, culminated in an infection model for phage 7-7-1.

Leaf-Encapsulated Vaccines: Agroinfiltration and Transient Expression of the Antigen Staphylococcal Endotoxin B in Radish Leaves

Transgene introgression is a major concern associated with transgenic plant-based vaccines. Agroinfiltration can be used to selectively transform nonreproductive organs and avoid introgression. Here, we introduce a new vaccine modality in which Staphylococcal enterotoxin B (SEB) genes are agroinfiltrated into radishes (Raphanw sativus L.), resulting in transient expression and accumulation of SEB in planta. This approach can simultaneously express multiple antigens in a single leaf. Furthermore, the potential of high-throughput vaccine production was demonstrated by simultaneously agroinfiltrating multiple radish leaves using a multichannel pipette. The expression of SEB was detectable in two leaf cell types (epidermal and guard cells) in agroinfiltrated leaves. ICR mice intranasally immunized with homogenized leaves agroinfiltrated with SEB elicited detectable antibody to SEB and displayed protection against SEB-induced interferon-gamma (IFN-γ) production. The concept of encapsulating antigens in leaves rather than purifying them for immunization may facilitate rapid vaccine production during an epidemic disease.

Synthesis of a Pentasaccharide Fragment Related to the Inner Core Region of Rhizobial and Agrobacterial Lipopolysaccharides

The pentasaccharide fragment α-d-Man-(1 → 5)-[α-d-Kdo-(2 → 4)-]α-d-Kdo-(2 → 6)-β-d-GlcNAc-(1 → 6)-α-d-GlcNAc equipped with a 3-aminopropyl spacer moiety was prepared by a sequential assembly of monosaccharide building blocks. The glucosamine disaccharide—as a backbone surrogate of the bacterial lipid A region—was synthesized using an 1,3-oxazoline donor, which was followed by coupling with an isopropylidene-protected Kdo-fluoride donor to afford a protected tetrasaccharide intermediate. Eventually, an orthogonally protected manno-configured trichloroacetimidate donor was used to achieve the sterically demanding glycosylation of the 5-OH group of Kdo in good yield. The resulting pentasaccharide is suitably protected for further chain elongation at positions 3, 4, and 6 of the terminal mannose. Global deprotection afforded the target pentasaccharide to be used for the conversion into neoglycoconjugates and “clickable” ligands.

Structure of the Lipopolysaccharide from the Bradyrhizobium sp. ORS285 rfaL Mutant Strain

The importance of the outer membrane and of its main constituent, lipopolysaccharide, in the symbiosis between rhizobia and leguminous host plants has been well studied. Here, the first complete structural characterization of the entire lipopolysaccharide from an O‐chain‐deficient Bradyrhizobium ORS285 rfaL mutant is achieved by a combination of chemical analysis, NMR spectroscopy, MALDI MS and MS/MS. The lipid A structure is shown to be consistent with previously reported Bradyrhizobium lipid A, that is, a heterogeneous blend of penta‐ to hepta‐acylated species carrying a nonstoichiometric hopanoid unit and possessing very‐long‐chain fatty acids ranging from 26:0(25‐OH) to 32:0(31‐OH). The structure of the core oligosaccharide region, fully characterized for the first time here, is revealed to be a nonphosphorylated linear chain with methylated sugar residues, with a heptose residue exclusively present in the outer core region, and with the presence of two singly substituted 3‐deoxy‐d‐manno‐oct‐2‐ulosonic acid (Kdo) residues, one of which is located in the outer core region. The lipid A moiety is linked to the core moiety through an uncommon 4‐substituted Kdo unit.

Structure, biosynthesis and function of unusual lipids A from nodule-inducing and N2-fixing bacteria

This review focuses on the chemistry and structures of (Brady)rhizobium lipids A, indispensable parts of lipopolysaccharides. These lipids contain unusual (ω-1) hydroxylated very long chain fatty acids, which are synthesized by a very limited group of bacteria, besides rhizobia. The significance and requirement of the very long chain fatty acids for outer membrane stability as well as the genetics of the synthesis pathway are discussed. The biological role of these fatty acids for bacterial life in extremely different environments (soil and intracellular space within nodules) is also considered.

Bacterial Molecular Signals in the Sinorhizobium fredii-Soybean Symbiosis

Sinorhizobium (Ensifer) fredii (S. fredii) is a rhizobial species exhibiting a remarkably broad nodulation host-range. Thus, S. fredii is able to effectively nodulate dozens of different legumes, including plants forming determinate nodules, such as the important crops soybean and cowpea, and plants forming indeterminate nodules, such as Glycyrrhiza uralensis and pigeon-pea. This capacity of adaptation to different symbioses makes the study of the molecular signals produced by S. fredii strains of increasing interest since it allows the analysis of their symbiotic role in different types of nodule. In this review, we analyze in depth different S. fredii molecules that act as signals in symbiosis, including nodulation factors, different surface polysaccharides (exopolysaccharides, lipopolysaccharides, cyclic glucans, and K-antigen capsular polysaccharides), and effectors delivered to the interior of the host cells through a symbiotic type 3 secretion system.

Lipopolysaccharides as Microbe-associated Molecular Patterns: A Structural Perspective

The lipopolysaccharide (LPS) macromolecule is the major constituent of the external leaflet of the Gram-negative outer membrane, exerting a plethora of biological activities in animals and plants. Among all, it represents a defensive barrier which helps bacteria to resist antimicrobial compounds and external stress factors and is involved in most aspects of host–bacterium interactions such as recognition, adhesion and colonization. One of the most interesting and studied LPS features is its key role in the pathogenesis of Gram-negative infections potentially causing fever or circulatory shock. On the other hand, the LPS acts as a beneficial factor for the host since it is recognized by specific receptors of the host innate immune system; this recognition activates the host defenses culminating, in most cases, in destruction of the pathogen. Most of the biological roles of the LPS are strictly related to its primary structure; thus knowledge of the structural architecture of such a macromolecule, which is different even among bacterial strains belonging to the same species, is a first step but is essential in order to understand the molecular bases of the wide variety of biological activities exerted by LPSs.

Lipopolysaccharide O-chain core region required for cellular cohesion and compaction of in vitro and root biofilms developed by Rhizobium leguminosarum

The formation of biofilms is an important survival strategy allowing rhizobia to live on soil particles and plant roots. Within the microcolonies of the biofilm developed by Rhizobium leguminosarum, rhizobial cells interact tightly through lateral and polar connections, forming organized and compact cell aggregates. These microcolonies are embedded in a biofilm matrix, whose main component is the acidic exopolysaccharide (EPS). Our work shows that the O-chain core region of the R. leguminosarum lipopolysaccharide (LPS) (which stretches out of the cell surface) strongly influences bacterial adhesive properties and cell-cell cohesion. Mutants defective in the O chain or O-chain core moiety developed premature microcolonies in which lateral bacterial contacts were greatly reduced. Furthermore, cell-cell interactions within the microcolonies of the LPS mutants were mediated mostly through their poles, resulting in a biofilm with an altered three-dimensional structure and increased thickness. In addition, on the root epidermis and on root hairs, O-antigen core-defective strains showed altered biofilm patterns with the typical microcolony compaction impaired. Taken together, these results indicate that the surface-exposed moiety of the LPS is crucial for proper cell-to-cell interactions and for the formation of robust biofilms on different surfaces.

Crystal structure of the HIV neutralizing antibody 2G12 in complex with a bacterial oligosaccharide analog of mammalian oligomannose

Human immunodeficiency virus-1 (HIV-1) is a major public health threat that continues to infect millions of people worldwide each year. A prophylactic vaccine remains the most cost-effective way of globally reducing and eliminating the spread of the virus. The HIV envelope spike, which is the target of many vaccine design efforts, is densely mantled with carbohydrate and several potent broadly neutralizing antibodies to HIV-1 recognize carbohydrate on the envelope spike as a major part of their epitope. However, immunizing with recombinant forms of the envelope glycoprotein does not typically elicit anti-carbohydrate antibodies. Thus, studies of alternative antigens that may serve as a starting point for carbohydrate-based immunogens are of interest. Here, we present the crystal structure of one such anti-carbohydrate HIV neutralizing antibody (2G12) in complex with the carbohydrate backbone of the lipooligosaccharide from Rhizobium radiobacter strain Rv3, which exhibits a chemical structure that naturally mimics the core high-mannose carbohydrate epitope of 2G12 on HIV-1 gp120. The structure described here provides molecular evidence of the structural homology between the Rv3 oligosaccharide and highly abundant carbohydrates on the surface of HIV-1 and raises the potential for the design of novel glycoconjugates that may find utility in efforts to develop immunogens for eliciting carbohydrate-specific neutralizing antibodies to HIV.

Covalently linked hopanoid-lipid A improves outer-membrane resistance of a Bradyrhizobium symbiont of legumes

Lipopolysaccharides (LPSs) are major components of the outer membrane of Gram-negative bacteria and are essential for their growth and survival. They act as a structural barrier and play an important role in the interaction with eukaryotic hosts. Here we demonstrate that a photosynthetic Bradyrhizobium strain, symbiont of Aeschynomene legumes, synthesizes a unique LPS bearing a hopanoid covalently attached to lipid A. Biophysical analyses of reconstituted liposomes indicate that this hopanoid-lipid A structure reinforces the stability and rigidity of the outer membrane. In addition, the bacterium produces other hopanoid molecules not linked to LPS. A hopanoid-deficient strain, lacking a squalene hopene cyclase, displays increased sensitivity to stressful conditions and reduced ability to survive intracellularly in the host plant. This unusual combination of hopanoid and LPS molecules may represent an adaptation to optimize bacterial survival in both free-living and symbiotic states.

Chemistry of lipid A: at the heart of innate immunity

In many Gram-negative bacteria, lipopolysaccharide (LPS) and its lipid A moiety are pivotal for bacterial survival. Depending on its structure, lipid A carries the toxic properties of the LPS and acts as a potent elicitor of the host innate immune system via the Toll-like receptor 4/myeloid differentiation factor 2 (TLR4/MD-2) receptor complex. It often causes a wide variety of biological effects ranging from a remarkable enhancement of the resistance to the infection to an uncontrolled and massive immune response resulting in sepsis and septic shock. Since the bioactivity of lipid A is strongly influenced by its primary structure, a broad range of chemical syntheses of lipid A derivatives have made an enormous contribution to the characterization of lipid A bioactivity, providing novel pharmacological targets for the development of new biomedical therapies. Here, we describe and discuss the chemical aspects regarding lipid A and its role in innate immunity, from the (bio)synthesis, isolation and characterization to the molecular recognition at the atomic level.

Inactivation of the lpcC gene alters surface-related properties and symbiotic capability of Bradyrhizobium japonicum

We investigated the role of the Bradyrhizobium japonicum lpcC gene, encoding a mannosyl transferase, involved in the lipopolysaccharide (LPS) biosynthesis. The inactivation of the lpcC gene considerably altered the LPS structure and the cell surface properties. LPS analysis showed that the lpcC mutant JS715 had an abnormal LPS structure deficient in O-antigen. The cell surface hydrophobicity increased approximately threefold in JS715 compared to the wild type. The increased cell surface hydrophobicity is likely to be related with cell aggregation in the mutant culture. For the growth comparison, JS715 showed slower growth rate than the wild type. The motility of JS715 decreased in soft agar plates, but it showed enhanced biofilm-forming ability. Interestingly, JS715 was not able to nodulate the host legume soybean (Glycine max). This study shows not only that lpcC is involved in the biosynthesis of O-antigen in the B. japonicum LPS, but also that inactivation of the lpcC gene affects symbiotic capability of B. japonicum and surface-related properties such as cell hydrophobicity, biofilm formation and motility.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrates the role of the B. japonicum lpcC in nodulation with soybean and importance of cell surface hydrophobicity. The results also highlight that intact LPS is required for successful symbiosis between B. japonicum and soybeans. Our findings not only support previous studies emphasizing the necessity of LPS on the interaction between the two symbiotic partners, but also contribute to a better understanding of the symbiotic mechanisms.

A bifunctional glycosyltransferase from Agrobacterium tumefaciens synthesizes monoglucosyl and glucuronosyl diacylglycerol under phosphate deprivation

Glycolipids are mainly found in phototrophic organisms (like plants and cyanobacteria), in Gram-positive bacteria, and a few other bacterial phyla. Besides the function as bulk membrane lipids, they often play a role under phosphate deprivation as surrogates for phospholipids. The Gram-negative Agrobacterium tumefaciens accumulates four different glycolipids under phosphate deficiency, including digalactosyl diacylglycerol and glucosylgalactosyl diacylglycerol synthesized by a processive glycosyltransferase. The other two glycolipids have now been identified by mass spectrometry and nuclear magnetic resonance spectroscopy as monoglucosyl diacylglycerol and glucuronosyl diacylglycerol. These two lipids are synthesized by a single promiscuous glycosyltransferase encoded by the ORF atu2297, with UDP-glucose or UDP-glucuronic acid as sugar donors. The transfer of sugars differing in their chemistry is a novel feature not observed before for lipid glycosyltransferases. Furthermore, this enzyme is the first glucuronosyl diacylglycerol synthase isolated. Deletion mutants of Agrobacterium lacking monoglucosyl diacylglycerol and glucuronosyl diacylglycerol or all glycolipids are not impaired in growth or virulence during infection of tobacco leaf discs. Our data suggest that the four glycolipids and the nonphospholipid diacylglyceryl trimethylhomoserine can mutually replace each other during phosphate deprivation. This redundancy of different nonphospholipids may represent an adaptation mechanism to enhance the competitiveness in nature.

Immunosuppression during Rhizobium-legume symbiosis

Rhizobium infects host legumes to elicit new plant organs, nodules where dinitrogen is fixed as ammonia that can be directly utilized by plants. The nodulation factor (NF) produced by Rhizobium is one of the determinant signals for rhizobial infection and nodule development. Recently, it was found to suppress the innate immunity on host and nonhost plants as well as its analogs, chitins. Therefore, NF can be recognized as a microbe/pathogen-associated molecular pattern (M/PAMP) like chitin to induce the M/PAMP triggered susceptibility (M/PTS) of host plants to rhizobia. Whether the NF signaling pathway is directly associated with the innate immunity is not clear till now. In fact, other MAMPs such as lipopolysaccharide (LPS), exopolysaccharide (EPS) and cyclic-β-glucan, together with type III secretion system (T3SS) effectors are also required for rhizobial infection or survival in leguminous nodule cells. Interestingly, most of them play similarly negative roles in the innate immunity of host plants, though their signaling is not completely elucidated. Taken together, we believe that the local immunosuppression on host plants induced by Rhizobium is essential for the establishment of their symbiosis.

The Sinorhizobium fredii HH103 lipopolysaccharide is not only relevant at early soybean nodulation stages but also for symbiosome stability in mature nodules

In this work we have characterised the Sinorhizobium fredii HH103 greA lpsB lpsCDE genetic region and analysed for the first time the symbiotic performance of Sinorhizobium fredii lps mutants on soybean. The organization of the S. fredii HH103 greA, lpsB, and lpsCDE genes was equal to that of Sinorhizobium meliloti 1021. S. fredii HH103 greA, lpsB, and lpsE mutant derivatives produced altered LPS profiles that were characteristic of the gene mutated. In addition, S. fredii HH103 greA mutants showed a reduction in bacterial mobility and an increase of auto-agglutination in liquid cultures. RT-PCR and qPCR experiments demonstrated that the HH103 greA gene has a positive effect on the transcription of lpsB. Soybean plants inoculated with HH103 greA, lpsB or lpsE mutants formed numerous ineffective pseudonodules and showed severe symptoms of nitrogen starvation. However, HH103 greA and lps mutants were also able to induce the formation of a reduced number of soybean nodules of normal external morphology, allowing the possibility of studying the importance of bacterial LPS in later stages of the S. fredii HH103-soybean symbiosis. The infected cells of these nodules showed signs of early termination of symbiosis and lytical clearance of bacteroids. These cells also had very thick walls and accumulation of phenolic-like compounds, pointing to induced defense reactions. Our results show the importance of bacterial LPS in later stages of the S. fredii HH103-soybean symbiosis and their role in preventing host cell defense reactions. S. fredii HH103 lpsB mutants also showed reduced nodulation with Vigna unguiculata, although the symbiotic impairment was less pronounced than in soybean.

Effect of leguminous lectins on the growth of Rhizobium tropici CIAT899

Rhizobium tropici is a Gram-negative bacterium that induces nodules and fixed atmospheric nitrogen in symbiotic association with Phaseolus vulgaris (common bean) and some other leguminous species. Lectins are proteins that specifically bind to carbohydrates and, consequently, modulate different biological functions. In this study, the d-glucose/d-mannose-binding lectins (from seeds of Dioclea megacarpa, D. rostrata and D. violacea) and d-galactose-binding lectins (from seeds of Bauhinia variegata, Erythina velutina and Vatairea macrocarpa) were purified using chromatographic techniques and evaluated for their effect on the growth of R. tropici CIAT899. All lectins were assayed with a satisfactory degree of purity according to SDS-PAGE analysis, and stimulated bacterial growth; in particular, the Dioclea rostrata lectin was the most active among all tested proteins. As confirmed in the present study, both d-galactose- and d-glucose/d-mannose-binding lectins purified from the seeds of leguminous plants may be powerful biotechnological tools to stimulate the growth of R. tropici CIAT99, thus improving symbiotic interaction between rhizobia and common bean and, hence, the production of this field crop.

Characterization of nitric oxide-inducing lipid A derived from Mesorhizobium loti lipopolysaccharide

Mesorhizobium loti is a member of the rhizobia and forms nitrogen-fixing symbioses with several Lotus species. Recently, it was reported that M. loti bacterial cells and their lipopolysaccharide (LPS) preparations transiently induced nitric oxide (NO) production in the roots of L. japonicus. We subsequently found that polysaccharides and the lipid A moiety were responsible for this NO induction. In this study, we elucidated the chemical structure of M. loti lipid A and characterized its NO-inducing activity in response to structural modifications. M. loti LPS were partially hydrolyzed with hydrazine or aqueous hydrofluoric acid to obtain O-deacylated or dephosphorylated LPS, respectively. The untreated and treated LPS fractions were subjected to weak acid hydrolysis to obtain lipid A fractions. The chemical structure of M. loti lipid A was elucidated by chemical composition analysis, MALDI-TOF-MS, and NMR spectra to be P-4-β-GlcNN(1-6)α-GlcNN(1-1)α-GalA, in which positions 2 and 3 of β-GlcNN are substituted for 3-acyloxy-fatty amides, and positions 2 and 3 of α-GlcNN are substituted for 3OH-fatty amides. The partial hydrolysis of lipid A appeared to reduce its NO-inducing activity. These results suggest that L. japonicus root cells recognize the lipid A structure as a means of controlling NO production.

A bacterial lipooligosaccharide that naturally mimics the epitope of the HIV-neutralizing antibody 2G12 as a template for vaccine design

The broadly neutralizing antibody 2G12 binds a fairly conserved cluster of oligomannose sugars on the HIV surface glycoprotein gp120, which has led to the hypothesis that these sugars pose potential vaccine targets. Here, we present the chemical analysis, antigenicity, and immunogenicity of a bacterial lipooligosaccharide (LOS) comprised of a manno-oligosaccharide sequence analogous to the 2G12 epitope. Antigenic similarity of the LOS to oligomannose was evidenced by 2G12 binding to the LOS and the inability of sera elicited against synthetic oligomannosides, but incapable of binding natural oligomannose, to bind the LOS. Immunization with heat-killed bacteria yielded epitope-specific serum antibodies with the capacity to bind soluble gp120. Although these sera did not exhibit specific anti-HIV activity, our data suggest that this LOS may find utility as a template for the design of glycoconjugates to target HIV.

Biological activity of (lipo)polysaccharides of the exopolysaccharide-deficient mutant Rt120 derived from Rhizobium leguminosarum bv. trifolii strain TA1

Lipopolysaccharides (LPS) from Rhizobium leguminosarum biovar trifolii TA1 (RtTA1) and its mutant Rt120 in the pssBpssA intergenic region as well as degraded polysaccharides (DPS) derived from the LPS were elucidated in terms of their chemical composition and biological activities. The polysaccharide portions were examined by methylation analysis, MALDI-TOF mass spectrometry, and (1)H NMR spectroscopy. A high molecular mass carbohydrate fraction obtained from Rt120 DPS by Sephadex G-50 gel chromatography was composed mainly of L-rhamnose, 6-deoxy-L-talose, D-galactose, and D-galacturonic acid, whereas that from RtTA1 DPS contained L-fucose, 2-acetamido-2,6-dideoxy-D-glucose, D-galacturonic acid, 3-deoxy-3-methylaminofucose, D-glucose, D-glucuronic acid, and heptose. Relative intensities of the major (1)H NMR signals for O-acetyl and N-acetyl groups were 1 : 0.8 and 1 : 1.24 in DPS of Rt120 and RtTA1, respectively. The intact mutant LPS exhibited a twice higher lethal toxicity than the wild type LPS. A higher in vivo production of TNFα and IL-6 after induction of mice with Rt120 LPS correlated with the toxicity, although the mutant LPS induced the secretion of IL-1β and IFNγ more weakly than RtTA1 LPS. A polysaccharide obtained by gel chromatography on Bio-Gel P-4 of the high molecular mass material from Rt120 had a toxic effect on tumor HeLa cells but was inactive against the normal human skin fibroblast cell line. The polysaccharide from RtTA1 was inactive against either cell line. The potent inhibitory effect of the mutant DPS on tumor HeLa cells seems to be related with the differences in sugar composition.

Poke weed mitogen requires Toll-like receptor ligands for proliferative activity in human and murine B lymphocytes

Poke weed mitogen (PWM), a lectin purified from Phytolacca americana is frequently used as a B cell-specific stimulus to trigger proliferation and immunoglobulin secretion. In the present study we investigated the mechanisms underlying the B cell stimulatory capacity of PWM. Strikingly, we observed that highly purified PWM preparations failed to induce B cell proliferation. By contrast, commercially available PWM preparations with B cell activity contained Toll-like receptor (TLR) ligands such as TLR2-active lipoproteins, lipopolysaccharide and DNA of bacterial origin. We show that these microbial substances contribute to the stimulatory activity of PWM. Additional experimental data highlight the capacity of PWM to enable B cell activation by immunostimulatory DNA. Based on these findings we propose that the lectin sensitizes B cells for TLR stimulation as described for B cell receptor ligation and that B cell mitogenicity of PWM preparations results from synergistic activity of the poke weed lectin and microbial TLR ligands present in the PWM preparations.

Low endotoxic activity of lipopolysaccharides isolated from Bradyrhizobium, Mesorhizobium, and Azospirillum strains

The endotoxic activities of lipopolysaccharides (LPS) isolated from different strains of rhizobia and rhizobacteria (Bradyrhizobium, Mesorhizobium, and Azospirillum) were compared to those of Salmonella enterica sv. Typhimurium LPS. The biological activity of all the examined preparations, measured as Limulus lysate gelation, production of tumor necrosis factor (TNF), interleukin-1β (IL-1β), and interleukin-6 (IL-6), and nitrogen oxide (NO) induction in human myelomonocytic cells (line THP-1), was considerably lower than that of the reference enterobacterial endotoxin. Among the rhizobial lipopolysaccharides, the activities of Mesorhizobium huakuii and Azospirillum lipoferum LPSs were higher than those of the LPS preparations from five strains of Bradyrhizobium. The weak endotoxic activity of the examined preparations was correlated with differences in lipid A structure compared to Salmonella.

Comparison of TNFα to lipopolysaccharide as an inflammagen to characterize the idiosyncratic hepatotoxicity potential of drugs: Trovafloxacin as an example

Idiosyncratic drug reactions (IDRs) are poorly understood, unpredictable, and not detected in preclinical studies. Although the cause of these reactions is likely multi-factorial, one hypothesis is that an underlying inflammatory state lowers the tolerance to a xenobiotic. Previously used in an inflammation IDR model, bacterial lipopolysaccharide (LPS) is heterogeneous in nature, making development of standardized testing protocols difficult. Here, the use of rat tumor necrosis factor-α (TNFα) to replace LPS as an inflammatory stimulus was investigated. Sprague-Dawley rats were treated with separate preparations of LPS or TNFα, and hepatic transcriptomic effects were compared. TNFα showed enhanced consistency at the transcriptomic level compared to LPS. TNFα and LPS regulated similar biochemical pathways, although LPS was associated with more robust inflammatory signaling than TNFα. Rats were then codosed with TNFα and trovafloxacin (TVX), an IDR-associated drug, and evaluated by liver histopathology, clinical chemistry, and gene expression analysis. TNFα/TVX induced unique gene expression changes that clustered separately from TNFα/levofloxacin, a drug not associated with IDRs. TNFα/TVX cotreatment led to autoinduction of TNFα resulting in potentiation of underlying gene expression stress signals. Comparison of TNFα/TVX and LPS/TVX gene expression profiles revealed similarities in the regulation of biochemical pathways. In conclusion, TNFα could be used in lieu of LPS as an inflammatory stimulus in this model of IDRs.

Structures of the lipopolysaccharides from Rhizobium leguminosarum RBL5523 and its UDP-glucose dehydrogenase mutant (exo5)

Rhizobial lipopolysaccharide (LPS) is required to establish an effective symbiosis with its host plant. An exo5 mutant of Rhizobium leguminosarum RBL5523, strain RBL5808, is defective in UDP-glucose (Glc) dehydrogenase that converts UDP-Glc to UDP-glucuronic acid (GlcA). This mutant is unable to synthesize either UDP-GlcA or UDP-galacturonic acid (GalA) and is unable to synthesize extracellular and capsular polysaccharides, lacks GalA in its LPS and is defective in symbiosis (Laus MC, Logman TJ, van Brussel AAN, Carlson RW, Azadi P, Gao MY, Kijne JW. 2004. Involvement of exo5 in production of surface polysaccharides in Rhizobium leguminosarum and its role in nodulation of Vicia sativa subsp. nigra. J Bacteriol. 186:6617-6625). Here, we determined and compared the structures of the RBL5523 parent and RBL5808 mutant LPSs. The parent LPS core oligosaccharide (OS), as with other R. leguminosarum and Rhizobium etli strains, is a Gal(1)Man(1)GalA(3)Kdo(3) octasaccharide in, which each of the GalA residues is terminally linked. The core OS from the mutant lacks all three GalA residues. Also, the parent lipid A consists of a fatty acylated GlcNGlcNonate or GlcNGlcN disaccharide that has a GalA residue at the 4'-position, typical of other R. leguminosarum and R. etli lipids A. The mutant lipid A lacks the 4'-GalA residue, and the proximal glycosyl residue was only present as GlcNonate. In spite of these alterations to the lipid A and core OSs, the mutant was still able to synthesize an LPS containing a normal O-chain polysaccharide (OPS), but at reduced levels. The structure of the OPS of the mutant LPS was identical to that of the parent and consists of an O-acetylated →4)-α-d-Glcp-(1→3)-α-d-QuipNAc-(1→ repeating unit.

Chemical composition of lipopolysaccharides isolated from various endophytic nitrogen-fixing bacteria of the genus Herbaspirillum

Bacteria from the genus Herbaspirillum are endophytes responsible for nitrogen fixation in gramineous plants of economic importance such as maize, sugarcane, sorghum, rice, and wheat. Some species are known to produce plant growth substances. In contrast, Herbaspirillum rubrisubalbicans strains are known to be mild plant pathogens. The molecular communication between the plant and the microbes might involve lipopolysaccharides present in the outer membrane of these gram-negative bacteria. Phenol-water extraction was used to obtain lipopolysaccharides from 7 strains of Herbaspirillum seropedicae (SmR1, Z67, Z78, ZA95, and M2) and H. rubrisubalbicans (M1 and M4). The electrophoretic profiles and chemical composition of the lipopolysaccharides obtained in the phenol and aqueous extracts were shown herein.

Characterization of a gene family of outer membrane proteins (ropB) in Rhizobium leguminosarum bv. viciae VF39SM and the role of the sensor kinase ChvG in their regulation

The outer membrane of Gram-negative bacteria represents the interface between the bacterium and its external environment. It has a critical role as a protective barrier against harmful substances and is also important in host-bacteria interactions representing the initial physical point of contact between the host cell and bacterial cell. RopB is a previously identified outer membrane protein from Rhizobium leguminosarum bv. viciae that is present in free-living cells but absent in bacteroids (H. P. Roest, I. H. Mulders, C. A. Wijffelman, and B. J. Lugtenberg, Mol. Plant Microbe Interact. 8:576-583, 1995). The functions of RopB and the molecular mechanisms of ropB gene regulation have remained unknown. We identified and cloned ropB and two homologs (ropB2 and ropB3) from the R. leguminosarum VF39SM genome. Reporter gene fusions indicated that the expression of ropB was 8-fold higher when cells were grown in complex media than when they were grown in minimal media, while ropB3 expression was constitutively expressed at low levels in both complex and minimal media. Expression of ropB2 was negligible under all conditions tested. The use of minimal media supplemented with various sources of peptides resulted in a 5-fold increase in ropB expression. An increase in ropB expression in the presence of peptides was not observed in a chvG mutant background, indicating a role for the sensor kinase in regulating ropB expression. Each member of the ropB gene family was mutated using insertional mutagenesis, and the mutants were assayed for susceptibility to antimicrobial agents and symbiotic phenotypes. All mutants formed effective nodules on pea plants, and gene expression for each rop gene in bacteroids was negligible. The functions of ropB2 and ropB3 remain cryptic, while the ropB mutant had an increased sensitivity to detergents, hydrophobic antibiotics, and weak organic acids, suggesting a role for RopB in outer membrane stability.

Lipopolysaccharides in Rhizobium-legume symbioses

The establishment of nitrogen-fixing symbiosis between a legume plant and its rhizobial symbiont requires that the bacterium adapt to changing conditions that occur with the host plant that both promotes and allows infection of the host root nodule cell, regulates and resists the host defense response, permits the exchange of metabolites, and contributes to the overall health of the host. This adaptive process involves changes to the bacterial cell surface and, therefore, structural modifications to the lipopolysaccharide (LPS). In this chapter, we describe the structures of the LPSs from symbiont members of the Rhizobiales, the genetics and mechanism of their biosynthesis, the modifications that occur during symbiosis, and their possible functions.

The diversity of the core oligosaccharide in lipopolysaccharides

Bacterial lipopolysaccharides (LPSs) are the major component of the outer membrane of Gram-negative bacteria. They have a structural role since they contribute to the cellular rigidity by increasing the strength of cell wall and mediating contacts with the external environment that can induce structural changes to allow life in different conditions. Furthermore, the low permeability of the outer membrane acts as a barrier to protect bacteria from host-derived antimicrobial compounds. Lipopolysaccharides are amphiphilic macromolecules generally comprising three defined regions distinguished by their genetics, structures and function: the lipid A, the core oligosaccharide and a polysaccharide portion, the O-chain. In some Gram-negative bacteria LPS can terminate with the core portion to form rough type LPS (R-LPS, LOS). The core oligosaccharide is an often branched and phosphorylated heterooligosaccharide with less than fifteen sugars, more conserved in the inner region, proximal to the lipid A, and often carrying non-stoichiometric substitutions leading to variation and micro-heterogeneity. The core oligosaccharide contributes to the bacterial viability and stability of the outer membrane, can assure the serological specificity and possesses antigenic properties.

Structure of the O-polysaccharides of the lipopolysaccharides of Mesorhizobium loti HAMBI 1148 and Mesorhizobium amorphae ATCC 19655 containing two O-methylated monosaccharides

The O-polysaccharide of Mesorhizobium loti HAMBI 1148 was obtained by mild acid degradation of the lipopolysaccharide and studied by sugar and methylation analyses, Smith degradation, and (1)H and (13)C NMR spectroscopies, including 2D (1)H/(1)H COSY, TOCSY, ROESY, and H-detected (1)H/(13)C HSQC experiments. The O-polysaccharide was found to have a branched hexasaccharide-repeating unit of the following structure: [Formula: see text] where 2-acetamido-2-deoxy-4-O-methyl-D-glucose (D-GlcNAc4Me) and methyl group on 2-substituted D-rhamnose (Me) shown in italics are present in approximately 80% and approximately 40% repeating units, respectively. Similar studies of the O-polysaccharide from Mesorhizobium amorphae ATCC 19655 by sugar analysis and NMR spectroscopy revealed essentially the same structure but a higher content of 3-O-methyl-D-rhamnose ( approximately 70%).