Progress toward developing a carbohydrate-conjugate vaccine against Clostridium difficile ribotype 027: synthesis of the cell-surface polysaccharide PS-I repeating unit

Synthesis of a glycan hairpin

The primary sequence of a biopolymer encodes the essential information for folding, permitting to carry out sophisticated functions. Inspired by natural biopolymers, peptide and nucleic acid sequences have been designed to adopt particular three-dimensional (3D) shapes and programmed to exert specific functions. In contrast, synthetic glycans capable of autonomously folding into defined 3D conformations have so far not been explored owing to their structural complexity and lack of design rules. Here we generate a glycan that adopts a stable secondary structure not present in nature, a glycan hairpin, by combining natural glycan motifs, stabilized by a non-conventional hydrogen bond and hydrophobic interactions. Automated glycan assembly enabled rapid access to synthetic analogues, including site-specific 13C-labelled ones, for nuclear magnetic resonance conformational analysis. Long-range inter-residue nuclear Overhauser effects unequivocally confirmed the folded conformation of the synthetic glycan hairpin. The capacity to control the 3D shape across the pool of available monosaccharides has the potential to afford more foldamer scaffolds with programmable properties and functions.

Synthesis of oligosaccharides to identify an immunologically active epitope against Candida auris infection

Candida auris (C. auris) is an emerging multidrug-resistant fungal pathogen that represents a significant public health challenge as it can spread rapidly and result in high mortality rates. The mannans on the C. auris cell surface are potent immunogens and attractive targets for developing a glycoconjugate vaccine. We synthesized the oligosaccharides resembling cell surface mannans of C. auris and printed them onto microarray slides that were used to screen plasma from mice infected with C. auris. IgM antibodies in mouse plasma recognize the β-1,2 linkage present in C. auris surface mannans. Disaccharide 19 emerged from glycan array screening as a lead for developing a vaccine against C. auris, as the majority of patient plasma samples showed antibodies against this glycan. The synthetic oligosaccharides can be used for the early detection of C. auris infections.

Glycosaminoglycans from the Starfish Lethasterias fusca: Structures and Influence on Hematopoiesis

Crude anionic polysaccharides extracted from the Pacific starfish Lethasterias fusca were purified by anion-exchange chromatography. The main fraction LF, having MW 14.5 kDa and dispersity 1.28 (data of gel-permeation chromatography), was solvolytically desulfated and giving rise to preparation LF-deS with a structure of dermatan core [→3)-β-d-GalNAc-(1→4)-α-l-IdoA-(1→]n, which was identified according to NMR spectroscopy data. Analysis of the NMR spectra of the parent fraction LF led to identification of the main component as dermatan sulfate LF-Derm →3)-β-d-GalNAc4R-(1→4)-α-l-IdoA2R3S-(1→ (where R was SO3 or H), bearing sulfate groups at O-3 or both at O-2 and O-3 of α-l-iduronic acid, as well as at O-4 of some N-acetyl-d-galactosamine residues. The minor signals in NMR spectra of LF were assigned as resonances of heparinoid LF-Hep composed of the fragments →4)-α-d-GlcNS3S6S-(1→4)-α-l-IdoA2S3S-(1→. The 3-O-sulfated and 2,3-di-O-sulfated iduronic acid residues are very unusual for natural glycosaminoglycans, and further studies are needed to elucidate their possible specific influence on the biological activity of the corresponding polysaccharides. To confirm the presence of these units in LF-Derm and LF-Hep, a series of variously sulfated model 3-aminopropyl iduronosides were synthesized and their NMR spectra were compared with those of the polysaccharides. Preparations LF and LF-deS were studied as stimulators of hematopoiesis in vitro. Surprisingly, it was found that both preparations were active in these tests, and hence, the high level of sulfation is not necessary for hematopoiesis stimulation in this particular case.

Discovery of Semi- and Fully-Synthetic Carbohydrate Vaccines Against Bacterial Infections Using a Medicinal Chemistry Approach

The glycocalyx, a thick layer of carbohydrates, surrounds the cell wall of most bacterial and parasitic pathogens. Recognition of these unique glycans by the human immune system results in destruction of the invaders. To elicit a protective immune response, polysaccharides either isolated from the bacterial cell surface or conjugated with a carrier protein, for T-cell help, are administered. Conjugate vaccines based on isolated carbohydrates currently protect millions of people against Streptococcus pneumoniae, Haemophilus influenzae type b, and Neisseria meningitides infections. Active pharmaceutical ingredients (APIs) are increasingly discovered by medicinal chemistry and synthetic in origin, rather than isolated from natural sources. Converting vaccines from biologicals to pharmaceuticals requires a fundamental understanding of how the human immune system recognizes carbohydrates and could now be realized. To illustrate the chemistry-based approach to vaccine discovery, I summarize efforts focusing on synthetic glycan-based medicinal chemistry to understand the mammalian antiglycan immune response and define glycan epitopes for novel synthetic glycoconjugate vaccines against Streptococcus pneumoniae, Clostridium difficile, Klebsiella pneumoniae, and other bacteria. The chemical tools described here help us gain fundamental insights into how the human system recognizes carbohydrates and drive the discovery of carbohydrate vaccines.

NIS/TMSOTf-Promoted Glycosidation of Glycosyl ortho-Hexynylbenzoates for Versatile Synthesis of O-Glycosides and Nucleosides

Glycosidation plays a pivotal role in the synthesis of O-glycosides and nucleosides that mediate a diverse range of biological processes. However, efficient glycosidation approach for the synthesis of both O-glycosides and nucleosides remains challenging in terms of glycosidation yields, mild reaction conditions, readily available glycosyl donors, and cheap promoters. Here, we report a versatile N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf)-promoted glycosidation approach with glycosyl ortho-hexynylbenzoates as donors for the highly efficient synthesis of O-glycosides and nucleosides. The glycosidation approach highlights the merits of mild reaction conditions, cheap promoters, extremely wide substrate scope, and good to excellent yields. Notably, the glycosidation approach performs very well in the construction of a series of challenging O- and N-glycosidic linkages. The glycosidation approach is then applied to the efficient synthesis of oligosaccharides via the one-pot strategy and the stepwise strategy. On the basis of the isolation and characterization of the departure species derived from the leaving group, a plausible mechanism of NIS/TMSOTf-promoted glycosidation of glycosyl ortho-hexynylbenzoates is proposed.

Chemical Synthesis Elucidates the Key Antigenic Epitope of the Autism-Related Bacterium Clostridium bolteae Capsular Octadecasaccharide

The gut pathogen Clostridium bolteae has been associated with the onset of autism spectrum disorder (ASD). To create vaccines against C. bolteae, it is important to identify exact protective epitopes of the immunologically active capsular polysaccharide (CPS). Here, a series of C. bolteae CPS glycans, up to an octadecasaccharide, was prepared. Key to achieving the total syntheses is a [2+2] coupling strategy based on a β-d-Rhap-(1→3)-α-d-Manp repeating unit that in turn was accessed by a stereoselective β-d-rhamnosylation. The 4,6-O-benzylidene-induced conformational locking is a powerful strategy for forming a β-d-mannose-type glycoside. An indirect strategy based on C2 epimerization of β-d-quinovoside was efficiently achieved by Swern oxidation and borohydride reduction. Sequential glycosylation, and regioselective and global deprotection produced the disaccharide and tetrasaccharide, up to the octadecasaccharide. Glycan microarray analysis of sera from rabbits immunized with inactivated C. bolteae bacteria revealed a humoral immune response to the di- and tetrasaccharide, but none of the longer sequences. The tetrasaccharide may be a key motif for designing glycoconjugate vaccines against C. bolteae.

Synthetic Oligosaccharide-Based Vaccines Protect Mice from Clostridioides difficile Infections

Infections with Clostridioides difficile (formerly Clostridium difficile) have increased in incidence, morbidity, and mortality over the past decade. Preventing infections is becoming increasingly important, as frontline antibiotics become less effective and frequently induce recurrence by disrupting intestinal microbiota. The clinically most advanced vaccine approaches prevent symptoms once C. difficile infection is established by inducing immunity to secreted clostridial cytotoxins. However, they do not inhibit bacterial colonization and thereby favor asymptomatic carriage. Synthetic oligosaccharides resembling the C. difficile surface glycans PS-I, PS-II, and PS-III are immunogenic and serve as basis for colonization-preventing vaccines. Here, we demonstrate that glycoconjugate vaccine candidates based on synthetic oligosaccharides protected mice from infections with two different C. difficile strains. Four synthetic antigens, ranging in size from disaccharides to hexasaccharides, were conjugated to CRM₁₉₇, which is a carrier protein used in commercial vaccines. The vaccine candidates induced glycan-specific antibodies in mice and substantially limited C. difficile colonization and colitis after experimental infection. The glycoconjugates ameliorated intestinal pathology more substantially than a toxin-targeting vaccine. Colonization of the gut by C. difficile was selectively inhibited while intestinal microbiota remained preserved. Passive transfer experiments with anti-PS-I serum revealed that protection is mediated by specific antiglycan antibodies; however, cell-mediated immunity likely also contributed to protection in vivo. Thus, glycoconjugate vaccines against C. difficile are a complementary approach to toxin-targeting strategies and are advancing through preclinical work.

Potential targets for next generation antimicrobial glycoconjugate vaccines

Cell surface carbohydrates have been proven optimal targets for vaccine development. Conjugation of polysaccharides to a carrier protein triggers a T-cell-dependent immune response to the glycan moiety. Licensed glycoconjugate vaccines are produced by chemical conjugation of capsular polysaccharides to prevent meningitis caused by meningococcus, pneumococcus and Haemophilus influenzae type b. However, other classes of carbohydrates (O-antigens, exopolysaccharides, wall/teichoic acids) represent attractive targets for developing vaccines. Recent analysis from WHO/CHO underpins alarming concern toward antibiotic-resistant bacteria, such as the so called ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp.) and additional pathogens such as Clostridium difficile and Group A Streptococcus. Fungal infections are also becoming increasingly invasive for immunocompromised patients or hospitalized individuals. Other emergencies could derive from bacteria which spread during environmental calamities (Vibrio cholerae) or with potential as bioterrorism weapons (Burkholderia pseudomallei and mallei, Francisella tularensis). Vaccination could aid reducing the use of broad-spectrum antibiotics and provide protection by herd immunity also to individuals who are not vaccinated.

This review analyzes structural and functional differences of the polysaccharides exposed on the surface of emerging pathogenic bacteria, combined with medical need and technological feasibility of corresponding glycoconjugate vaccines.

Cyanopivaloyl Ester in the Automated Solid-Phase Synthesis of Oligorhamnans

The development of effective protecting group chemistry is an important driving force behind the progress in the synthesis of complex oligosaccharides. Automated solid-phase synthesis is an attractive technique for the rapid assembly of oligosaccharides, built up of repetitive elements. The fact that (harsh) reagents are used in excess in multiple reaction cycles makes this technique extra demanding on the protecting groups used. Here, the synthesis of a set of oligorhamnan fragments is reported using the cyanopivaloyl (PivCN) ester to ensure effective neighboring group participation during the glycosylation events. The PivCN group combines the favorable characteristics of the parent pivaloyl (Piv) ester, stability, minimal migratory aptitude, minimal orthoester formation, while it can be cleaved under mild conditions. We show that the remote CN group in the PivCN renders the PivCN carbonyl more electropositive and thus susceptible to nucleophilic cleavage. This feature is built upon in the automated solid-phase assembly of the oligorhamnan fragments. Where the use of a Piv-protected building block failed because of incomplete cleavage, PivCN-protected synthons performed well and allowed the generation of oligorhamnans, up to 16 monosaccharides in length.

Identification and Design of Synthetic B Cell Epitopes for Carbohydrate-Based Vaccines

Synthetic oligosaccharide-based vaccines are promising alternatives to conventional antibacterial carbohydrate vaccines prepared with isolated polysaccharides. Unlike polysaccharides, synthetic glycans are well defined, contaminant-free, and accessible even for pathogens that cannot be fermented or show limited carbohydrate biosynthesis in vitro. However, identifying synthetic glycan B cell epitopes that induce protective immunity has traditionally been a time-consuming trial-and-error process, as predicting the immunogenicity of an oligosaccharide by means of structure alone is not straightforward. We here describe how synthetic oligosaccharide epitopes for candidate vaccines can be rationally identified prior to preclinical immunogenicity studies. Epitopes are selected on the basis of their recognition by antibodies associated with protection from disease in humans or small animals. In addition, we show how murine antibody responses to a large oligosaccharide can inform the identification of a minimal B cell epitope that may help designing easy to synthesize vaccine candidates. The procedures, exemplified with a surface carbohydrate of Clostridium difficile, may serve as a guideline for selecting protective oligosaccharide epitopes for vaccines against infectious and malignant diseases.

Synthesis of Defined and Functionalized Glycans of Lipoteichoic Acid: A Cell Surface Polysaccharide from Clostridium difficile

Two structurally defined, functionalized glycans of lipoteichoic acid (LTA, also known as PS-III) from C. difficile, which have one or two repeating units of LTA linked to the core trisaccharide, were efficiently synthesized via a convergent [2 + 3] or [2 + 2 + 3] strategy. The α-linkage of both N-acetylglucosamine residues in the repeating unit were constructed with glycosyl imidates of azidosugars as donors, while the phosphodiester bridges between the oligosaccharides were fashioned using H-phosphonate chemistry. Both synthetic targets contained a 3-aminopropyl group at the core trisaccharide reducing end, facilitating their conjugation to other biomolecules to afford conjugates useful for various biological studies and applications.

Synthetic Glycan Microarrays

Structurally diverse glycans are expressed by all animate beings and exert diverse biological functions through specific interactions with glycan binding proteins (GBPs). In humans, glycan-GBP interactions are implicated in many disease-relevant processes in development, infection and immune response to bacterial and viral pathogens. Recent progress in chemical synthesis, including automated glycan assembly, has facilitated access to complex glycans that cannot be isolated from biological material. Glycan immobilization on microarrays allows rapid, multiplexed glycan-GBP interaction studies to reveal biological functions. Synthetic glycan microarrays have enabled, for instance, the identification of glycan ligands for lectins, the definition of vaccine antigens, revealed viral glycan receptors and can serve as diagnostic tools for human disease. Here, we describe the methods to fabricate custom glycan microarrays that are used to examine glycan-GBP binding specificities. Conjugation-ready synthetic glycans are covalently attached to microarray surfaces through nucleophilic linker moieties. Microarrays are incubated with GBPs, and binding events are quantitatively detected by fluorescent signals. These methods are readily adaptable to a multitude of purposes from basic research to biomedical applications.

Multivalent display of minimal Clostridium difficile glycan epitopes mimics antigenic properties of larger glycans

Synthetic cell-surface glycans are promising vaccine candidates against Clostridium difficile. The complexity of large, highly antigenic and immunogenic glycans is a synthetic challenge. Less complex antigens providing similar immune responses are desirable for vaccine development. Based on molecular-level glycan–antibody interaction analyses, we here demonstrate that the C. difficile surface polysaccharide-I (PS-I) can be resembled by multivalent display of minimal disaccharide epitopes on a synthetic scaffold that does not participate in binding. We show that antibody avidity as a measure of antigenicity increases by about five orders of magnitude when disaccharides are compared with constructs containing five disaccharides. The synthetic, pentavalent vaccine candidate containing a peptide T-cell epitope elicits weak but highly specific antibody responses to larger PS-I glycans in mice. This study highlights the potential of multivalently displaying small oligosaccharides to achieve antigenicity characteristic of larger glycans. The approach may result in more cost-efficient carbohydrate vaccines with reduced synthetic effort.

Immunologically-active glycans are promising vaccine candidates but can be difficult to synthesize. Here, the authors show that pentavalent display of a minimal disaccharde epitope on a chemical scaffold can mimic a native C. difficile glycan antigen, representing a simple approach to synthetic vaccine production.

Cross Reactive Material 197 glycoconjugate vaccines contain privileged conjugation sites

Production of glycoconjugate vaccines involves the chemical conjugation of glycans to an immunogenic carrier protein such as Cross-Reactive-Material-197 (CRM197). Instead of using glycans from natural sources recent vaccine development has been focusing on the use of synthetically defined minimal epitopes. While the glycan is structurally defined, the attachment sites on the protein are not. Fully characterized conjugates and batch-to-batch comparisons are the key to eventually create completely defined conjugates. A variety of glycoconjugates consisting of CRM197 and synthetic oligosaccharide epitopes was characterised using mass spectrometry techniques. The primary structure was assessed by combining intact protein MALDI-TOF-MS, LC-MALDI-TOF-MS middle-down and LC-ESI-MS bottom-up approaches. The middle-down approach on CNBr cleaved glycopeptides provided almost complete sequence coverage, facilitating rapid batch-to-batch comparisons, resolving glycan loading and identification of side products. Regions close to the N- and C-termini were most efficiently conjugated.

The Design of a Clostridium difficile Carbohydrate-Based Vaccine

Clostridium difficile vaccines composed of surface polysaccharides (PSs) have the potential to simultaneously control infection and colonization levels in humans. Hot water-phenol treatment of C. difficile biomass can extricate water-soluble PS-I and PS-II; and water- and phenol-soluble PS-III. C. difficile vaccines based on PS-II have attracted the most attention due its facile purification and ubiquitous expression by C. difficile ribotypes. Anti PS-II antibodies recognize both C. difficile vegetative cell and sporulating preparations and confer protection against C. difficile infection in a mouse model. The design of such an efficacious C. difficile PS-II conjugate vaccine is described here.

Identification of carbohydrate anomers using ion mobility-mass spectrometry

Carbohydrates are ubiquitous biological polymers that are important in a broad range of biological processes. However, owing to their branched structures and the presence of stereogenic centres at each glycosidic linkage between monomers, carbohydrates are harder to characterize than are peptides and oligonucleotides. Methods such as nuclear magnetic resonance spectroscopy can be used to characterize glycosidic linkages, but this technique requires milligram amounts of material and cannot detect small amounts of coexisting isomers. Mass spectrometry, on the other hand, can provide information on carbohydrate composition and connectivity for even small amounts of sample, but it cannot be used to distinguish between stereoisomers. Here, we demonstrate that ion mobility-mass spectrometry--a method that separates molecules according to their mass, charge, size, and shape--can unambiguously identify carbohydrate linkage-isomers and stereoisomers. We analysed six synthetic carbohydrate isomers that differ in composition, connectivity, or configuration. Our data show that coexisting carbohydrate isomers can be identified, and relative concentrations of the minor isomer as low as 0.1 per cent can be detected. In addition, the analysis is rapid, and requires no derivatization and only small amounts of sample. These results indicate that ion mobility-mass spectrometry is an effective tool for the analysis of complex carbohydrates. This method could have an impact on the field of carbohydrate synthesis similar to that of the advent of high-performance liquid chromatography on the field of peptide assembly in the late 1970s.

Chemical biology approaches to designing defined carbohydrate vaccines

Carbohydrate antigens have shown promise as important targets for developing effective vaccines and pathogen detection strategies. Modifying purified microbial glycans through synthetic routes or completely synthesizing antigenic motifs are attractive options to advance carbohydrate vaccine development. However, limited knowledge on structure-property correlates hampers the discovery of immunoprotective carbohydrate epitopes. Recent advancements in tools for glycan modification, high-throughput screening of biological samples, and 3D structural analysis may facilitate antigen discovery process. This review focuses on advances that accelerate carbohydrate-based vaccine development and various technologies that are driving these efforts. Herein we provide a critical overview of approaches and resources available for rational design of better carbohydrate antigens. Structurally defined and fully synthetic oligosaccharides, designed based on molecular understanding of antigen-antibody interactions, offer a promising alternative for developing future carbohydrate vaccines.

The roles of host and pathogen factors and the innate immune response in the pathogenesis of Clostridium difficile infection

Clostridium difficile (C. difficile) is the most common cause of nosocomial antibiotic-associated diarrhea and the etiologic agent of pseudomembranous colitis. The clinical manifestation of C. difficile infection (CDI) is highly variable, from asymptomatic carriage, to mild self-limiting diarrhea, to the more severe pseudomembranous colitis. Furthermore, in extreme cases, colonic inflammation and tissue damage can lead to toxic megacolon, a condition requiring surgical intervention. C. difficile expresses two key virulence factors; the exotoxins, toxin A (TcdA) and toxin B (TcdB), which are glucosyltransferases that target host-cell monomeric GTPases. In addition, some hypervirulent strains produce a third toxin, binary toxin or C. difficile transferase (CDT), which may contribute to the pathogenesis of CDI. More recently, other factors such as surface layer proteins (SLPs) and flagellin have also been linked to the inflammatory responses observed in CDI. Although the adaptive immune response can influence the severity of CDI, the innate immune responses to C. difficile and its toxins play crucial roles in CDI onset, progression, and overall prognosis. Despite this, the innate immune responses in CDI have drawn relatively little attention from clinical researchers. Targeting these responses may prove useful clinically as adjuvant therapies, especially in refractory and/or recurrent CDI. This review will focus on recent advances in our understanding of how C. difficile and its toxins modulate innate immune responses that contribute to CDI pathogenesis.

Synthesis of conjugation-ready zwitterionic oligosaccharides by chemoselective thioglycoside activation

A method to chemoselectively activate thioglycosides in the presence of thioethers is developed and applied in the total synthesis of repeating units of S. pneumoniae Sp1 and B. fragilis PS A1. Biochemical evaluation of these glycans is performed after conjugation to reporter moieties.

Carbohydrate-based Clostridium difficile vaccines

Clostridium difficile is responsible for thousands of deaths each year and a vaccine would be welcomed, especially one that would disrupt bacterial maintenance, colonization and persistence in carriers and convalescent patients. Structural explorations at the University of Guelph (ON, Canada) discovered that C. difficile may express three phosphorylated polysaccharides, named PSI, PSII and PSIII; this review captures our recent efforts to create vaccines based on these glycans, especially PSII, the common antigen that has precipitated immediate attention. The authors describe the design and immunogenicity of vaccines composed of raw polysaccharides and conjugates thereof. So far, it has been observed that anti-PSII antibodies can be raised in farm animals, mice and hamster models; humans and horses carry anti-PSII IgA and IgG antibodies from natural exposure to C. difficile, respectively; phosphate is an indispensable immunogenic epitope and vaccine-induced PSII antibodies recognize PSII on C. difficile outer surface.

Immunological evaluation of a synthetic Clostridium difficile oligosaccharide conjugate vaccine candidate and identification of a minimal epitope

Clostridium difficile is the cause of emerging nosocomial infections that result in abundant morbidity and mortality worldwide. Thus, the development of a vaccine to kill the bacteria to prevent this disease is highly desirable. Several recently identified bacterial surface glycans, such as PS-I and PS-II, are promising vaccine candidates to preclude C. difficile infection. To circumvent difficulties with the generation of natural PS-I due to its low expression levels in bacterial cultures, improved chemical synthesis protocols for the pentasaccharide repeating unit of PS-I and oligosaccharide substructures were utilized to produce large quantities of well-defined PS-I related glycans. The analysis of stool and serum samples obtained from C. difficile patients using glycan microarrays of synthetic oligosaccharide epitopes revealed humoral immune responses to the PS-I related glycan epitopes. Two different vaccine candidates were evaluated in the mouse model. A synthetic PS-I repeating unit CRM197 conjugate was immunogenic in mice and induced immunoglobulin class switching as well as affinity maturation. Microarray screening employing PS-I repeating unit substructures revealed the disaccharide Rha-(1→3)-Glc as a minimal epitope. A CRM197-Rha-(1→3)-Glc disaccharide conjugate was able to elicit antibodies recognizing the C. difficile PS-I pentasaccharide. We herein demonstrate that glycan microarrays exposing defined oligosaccharide epitopes help to determine the minimal immunogenic epitopes of complex oligosaccharide antigens. The synthetic PS-I pentasaccharide repeating unit as well as the Rha-(1→3)-Glc disaccharide are promising novel vaccine candidates against C. difficile that are currently in preclinical evaluation.

Clostridium difficile PSI polysaccharide: synthesis of pentasaccharide repeating block, conjugation to exotoxin B subunit, and detection of natural anti-PSI IgG antibodies in horse serum

Clostridium difficile is the most common cause of antimicrobial-associated diarrhea in humans and may cause death. Previously, we discovered that C. difficile expresses three polysaccharides, named PSI, PSII, and PSIII. It has now been established that PSII is a conserved antigen abundantly present on the cell-surface and biofilm of C. difficile. In contrast, the expression of PSI and PSIII appears to be stochastic processes. In this work, the total chemical synthesis of the PSI pentasaccharide repeating unit carrying a linker at the reducing end, α-l-Rhap-(1→3)-β-d-Glcp-(1→4)-[α-l-Rhap-(1→3)]-α-d-Glcp-(1→2)-α-d-Glcp-(1→O(CH2)5NH2, was achieved by a linear synthesis strategy from four monosaccharide building blocks. The synthesized PSI pentasaccharide was conjugated to a subunit of C. difficile exotoxin B yielding a potential dual C. difficile vaccine. More significantly, sera from healthy horses were shown to contain natural anti-PSI IgG antibodies that detected both the synthetic non-phosphorylated PSI repeat and the native PSI polysaccharide, with a slightly higher recognition of the native PSI polysaccharide.

De novo synthesis of D- and L-fucosamine containing disaccharides

The availability of rare monosaccharides that cannot be isolated from natural sources is currently limiting the access to the synthesis and the biological evaluation of complex bacterial cell-surface glycans. Here, we report the synthesis of D- and L-fucosamine building blocks by a de novo approach from L- and D-Garner aldehydes. These differentially protected monosaccharide building blocks were utilized to prepare disaccharides present on the surface of Pseudomonas aeruginosa bacteria.

Clostridium difficile infection: toxins and non-toxin virulence factors, and their contributions to disease establishment and host response

Clostridium difficile infection is the leading cause of antibiotic- and healthcare-associated diarrhea, and its containment and treatment imposes a significant financial burden, estimated to be over $3 billion in the USA alone. Since the year 2000, CDI epidemics/outbreaks have occurred in North America, Europe and Asia. These outbreaks have been variously associated with, or attributed to, the emergence of Clostridium difficile strains with increased virulence, an increase in resistance to commonly used antimicrobials such as the fluoroquinolones, or host susceptibilities, including the use of gastric acid suppressants, to name a few. Efforts to elucidate C. difficile pathogenic mechanisms have been hampered by a lack of molecular tools, manipulatable animal models, and genetic intractability of clinical C. difficile isolates. However, in the past 5 y, painstaking efforts have resulted in the unraveling of multiple C. difficile virulence-associated pathways and mechanisms. We have recently reviewed the disease, its associated risk factors, transmission and interventions (Viswanathan, Gut Microbes 2010). This article summarizes genetics, non-toxin virulence factors, and host-cell biology associated with C. difficile pathogenesis as of 2011, and highlights those findings/factors that may be of interest as future intervention targets.