Glycan mimicry as a basis for novel anti-infective drugs

Targeted Photodynamic Treatment of Bacterial Biofilms Using Curcumin Encapsulated in Cells and Cell Wall Particles

Efficient inactivation and removal of pathogenic biofilms in food and biomedical environments remain a significant challenge for food safety applications and medical facilities. This research aims to develop food-grade microcarriers for the targeted delivery of a photosensitizer, curcumin, and photodynamic inactivation of a model pathogenic bacterial biofilm. The microcarriers evaluated in this study include alive yeast cell carriers, deactivated yeast cell carriers, and yeast cell wall particles. The microcarriers were evaluated based on the encapsulation yield of a model photosensitizer (curcumin), binding of the microcarriers to biofilms, and inactivation of the bacteria in the biofilms. The results illustrate that the combination of binding affinity, encapsulation yield, and the intracellular composition of the microcarriers influenced the overall inactivation of bacteria in the biofilms. All of the selected compositions achieved more than 93% inactivation of the bacteria in the biofilm using the photodynamic treatment, and the yeast cell wall particles with curcumin achieved over 99% inactivation of the bacteria in the biofilm matrix. In addition, all of the selected compositions demonstrated significant potential to remove the biofilm from the plastic surface, suggesting the role of binding affinity of the microcarriers in removal of the biofilm from surfaces. Overall, this study developed biomaterial formulations for targeted photodynamic inactivation and potential removal of biofilms.

Shiga toxin-glycosphingolipid interaction: Status quo of research with focus on primary human brain and kidney endothelial cells

Shiga toxin (Stx)-mediated injury of the kidneys and the brain represent the major extraintestinal complications in humans upon infection by enterohemorrhagic Escherichia coli (EHEC). Damage of renal and cerebral endothelial cells is the key event in the pathogenesis of the life-threatening hemolytic uremic syndrome (HUS). Stxs are AB₅ toxins and the B-pentamers of the two clinically important Stx subtypes Stx1a and Stx2a preferentially bind to the glycosphingolipid globotriaosylceramide (Gb3Cer, Galα4Galβ4Glcβ1Cer) and to less extent to globotetraosylceramide (Gb4Cer, GalNAcβ3Galα4Galβ4Glcβ1), which are expected to reside in lipid rafts in the plasma membrane of the human endothelium. This review summarizes the current knowledge on the Stx glycosphingolipid receptors and their lipid membrane ensemble in primary human brain microvascular endothelial cells (pHBMECs) and primary human renal glomerular endothelial cells (pHRGECs). Increasing knowledge on the precise initial molecular mechanisms by which Stxs interact with cellular targets will help to develop specific therapeutics and/or preventive measures to combat EHEC-caused diseases.

Glycomaterials for immunomodulation, immunotherapy, and infection prophylaxis

Synthetic carbohydrate-modified materials that can engage the innate and adaptive immune systems are receiving increasing interest to confer protection against onset of future disease, such as pathogen infection, as well as to treat established diseases, such as autoimmunity and cancer.

Inhibition of adhesion of intestinal pathogens (Escherichia coli, Vibrio cholerae, Campylobacter jejuni, and Salmonella Typhimurium) by common oligosaccharides

Inhibition of the binding of pathogenic adhesins to host glycans by suitable oligosaccharides forms the basis of antiadhesion therapies. Experiments were carried out to study the inhibition capability of oligosaccharides on the adhesion of four microorganisms (Escherichia coli, Vibrio cholerae, Campylobacter jejuni, and Salmonella Typhimurium) to HT-29 cells. Results showed that, in the absence of oligosaccharides, all of the four pathogens efficiently adhered to the cells. Cell adhesion with different bacteria was inhibited by distinct oligosaccharides (e.g., the adhesion number relative to control of V. cholerae could be significantly decreased by pectin oligosaccharide and chitooligosaccharide to about 16.1% and 18.9%, respectively). Saturation studies showed that the extent of antiadhesive effect for most of the suitable carbohydrates was dependent on their concentration. The observations from the study suggest that various carbohydrates may have antiadhesive activity and may be useful in future therapeutic study.

Targeting the bacteria-host interface: strategies in anti-adhesion therapy

Bacterial infections are a major cause of morbidity and mortality worldwide and are increasingly problematic to treat due to the rise in antibiotic-resistant strains. It becomes more and more challenging to develop new antimicrobials that are able to withstand the ever-increasing repertoire of bacterial resistance mechanisms. This necessitates the development of alternative approaches to prevent and treat bacterial infections. One of the first steps during bacterial infection is adhesion of the pathogen to host cells. A pathogen’s ability to colonize and invade host tissues strictly depends on this process. Thus, interference with adhesion (anti-adhesion therapy) is an efficient way to prevent or treat bacterial infections. As a basis to present different strategies to interfere with pathogen adhesion, this review briefly introduces general concepts of bacterial attachment to host cells. We further discuss advantages and disadvantages of anti-adhesion treatments and issues that are in need of improvement so as to make anti-adhesion compounds a more broadly applicable alternative to conventional antimicrobials.

Glycopolymers with secondary binding motifs mimic glycan branching and display bacterial lectin selectivity in addition to affinity

Polymer architecture is exploited as an alternative to glycan synthesis to enhance selectivity towards pathogenic lectins.

Carbohydrate-oligonucleotide conjugates

This unit describes a strategy for attaching two mannose and two galactose residues to an oligonucleotide. This conjugation can be performed at the 5'-end of the oligonucleotide sequence, using modified phosphoramidites. First, the oligonucleotide scaffold is synthesized on solid support using a DNA synthesizer, with commercially available and modified phosphoramidites. After the first "click" reaction with a galactosylated azide derivative on solid support, the bromine atoms are replaced with azides and a second click reaction is performed with propargylated mannose either on solid support or in solution. Additionally, using a monoalkynated solid support, the conjugation with carbohydrate residues can be performed at the 3'-end of the oligonucleotide according to a similar protocol. Curr. Protoc. Nucleic Acid Chem. 39:4.38.1-4.38.25.

Carbohydrate-based therapeutics

In recent years there has been a resurgence of interest in the biological roles of carbohydrates and as a result it is now known that carbohydrates are involved in a vast array of disease processes. This review summarises progress in the development of carbohydrate-based therapeutics that involve: inhibition of carbohydrate-lectin interactions; immunisation, using monoclonal antibodies for carbohydrate antigens; inhibition of enzymes that synthesise disease-associated carbohydrates; replacement of carbohydrate-processing enzymes; targeting of drugs to specific disease cells via carbohydrate-lectin interactions; carbohydrate based anti-thrombotic agents.

Evaluation of Fab and F(ab')2 fragments and isotype variants of a recombinant human monoclonal antibody against Shiga toxin 2

5C12 HuMAb is a human monoclonal antibody against the A subunit of Shiga toxin 2 (Stx2). We have previously shown that 5C12 HuMAb effectively neutralizes the cytotoxic effects of this toxin by redirecting its transport within the cell and also by neutralizing the toxin's ability to inhibit protein synthesis. The 5C12 HuMAb and its recombinant IgG1 version protect mice at a dose of 0.6 microg against a lethal challenge of Stx2. The contribution of the Fc region to this observed neutralization activity of the 5C12 antibody against Stx2 was investigated in this study. Using recombinant DNA technology, 5C12 isotype variants (IgG1, IgG2, IgG3, and IgG4) and antibody fragments [Fab, F(ab')(2)] were expressed in Chinese hamster ovary cells and evaluated in vitro and in vivo. All four 5C12 isotype variants showed protection in vitro, with the IgG3 and IgG4 variants showing the highest protection in vivo. The Fab and F(ab')(2) fragments also showed protection in vitro but no protection in the mouse toxicity model. Similar results were obtained for a second HuMAb (5H8) against the B subunit of Stx2. The data suggest the importance of the Fc region for neutralization activity, but it is not clear if this is related to the stability of the full-length antibody or if the Fc region is required for effective elimination of the toxin from the body.

Bioengineered bugs expressing oligosaccharide receptor mimics: toxin-binding probiotics for treatment and prevention of enteric infections

Many microbial pathogens recognize oligosaccharides displayed on the surface of host cells as receptors for toxins and adhesins. These ligand-receptor interactions are critical for disease pathogenesis, making them promising targets for novel anti-infectives. One strategy with particular utility against enteric infections involves expression of molecular mimics of host oligosaccharides on the surface of harmless bacteria capable of surviving in the gut. This can be achieved in Gram-negative bacteria by manipulating the outer core region of the lipopolysaccharide (LPS) through expression of cloned heterologous glycosyltransferases. The resultant chimeric LPS molecules are incorporated into the outer membrane by the normal assembly route and presented as a closely packed 2-D array of receptor mimics. Several such "designer probiotics" have been constructed, and these bind bacterial toxins in the gut lumen with very high avidity, blocking their uptake by host cells and thereby preventing disease.

Synthesis of mannose and galactose oligonucleotide conjugates by bi-click chemistry

Glyco oligonucleotide conjugates, each exhibiting two mannose and two galactose residues, were efficiently synthesized by two successive 1,3-dipolar cycloadditions (click chemistry). Two phosphoramidite derivatives were used: one bearing a bromoalkyl group as a precursor to azide functionalization and another bearing a propargyl group. After a first cycloaddition with a mannosyl-azide derivative, the bromine atoms were substituted with NaN(3) and a second click reaction was performed with a 1'-O-propargyl galactose, affording the heteroglyco oligonucleotide conjugate.

Cloning and characterization of a novel alpha-galactosidase from Bifidobacterium breve 203 capable of synthesizing Gal-alpha-1,4 linkage

A novel alpha-galactosidase gene (aga2) was cloned from Bifidobacterium breve 203. It contained an ORF of 2226-bp nucleotides encoding 741 amino acids with a calculated molecular mass of 81.5 kDa. The recombinant enzyme Aga2 was heterogeneously expressed, purified and characterized. Regarding substrate specificity for hydrolysis, Aga2 was highly active towards p-nitrophenyl-alpha-d-galactopyranoside (pNPG). The Km value for pNPG was estimated to be 0.27 mM and for melibiose it was estimated to be 4.3 mM. Aga2 was capable of catalyzing transglycosylation as well as hydrolysis. The enzyme synthesized a trisaccharide (Gal-alpha-1, 4-Gal-alpha-1, 6-Glc) using melibiose as a substrate. It was a new oligosaccharide produced by glycosidase and contained Gal-alpha-1,4 linkage, a novel galactosidic link formed by microbial alpha-galactosidase. In the presence of pNPG as a donor, Aga2 was able to catalyze glycosyl transfer to various acceptors including monosaccharides, disaccharides and sugar alcohols.

Adherence, anti-adherence, and oligosaccharides preventing pathogens from sticking to the host

For many pathogenic bacteria, infections are initiated only after the organism has first adhered to the host cell surface. If adherence can be inhibited, then the subsequent infection can also be inhibited. This approach forms the basis of anti-adherence strategies, which have been devised to prevent a variety of bacterial infections. In this chapter, the molecular basis by which respiratory, urinary, and gastrointestinal tract pathogens adhere to host cells will be described. The five general types of anti-adherence agents will also be reviewed. The most well-studied are the receptor analogs, which include oligosaccharides produced synthetically or derived from natural sources, including milk, berries, and other plants. Their ability to inhibit pathogen adherence may lead to development of novel, food-grade anti-infective agents that are inexpensive and safe.

Prebiotic galactooligosaccharides reduce adherence of enteropathogenic Escherichia coli to tissue culture cells

Prebiotic oligosaccharides are thought to provide beneficial effects in the gastrointestinal tract of humans and animals by stimulating growth of selected members of the intestinal microflora. Another means by which prebiotic oligosaccharides may confer health benefits is via their antiadhesive activity. Specifically, these oligosaccharides may directly inhibit infections by enteric pathogens due to their ability to act as structural mimics of the pathogen binding sites that coat the surface of gastrointestinal epithelial cells. In this study, the ability of commercial prebiotics to inhibit attachment of microcolony-forming enteropathogenic Escherichia coli (EPEC) was investigated. The adherence of EPEC strain E2348/69 on HEp-2 and Caco-2 cells, in the presence of fructooligosaccharides, inulin, galactooligosaccharides (GOS), lactulose, and raffinose was determined by cultural enumeration and microscopy. Purified GOS exhibited the greatest adherence inhibition on both HEp-2 and Caco-2 cells, reducing the adherence of EPEC by 65 and 70%, respectively. In addition, the average number of bacteria per microcolony was significantly reduced from 14 to 4 when GOS was present. Adherence inhibition by GOS was dose dependent, reaching a maximum at 16 mg/ml. When GOS was added to adhered EPEC cells, no displacement was observed. The expression of BfpA, a bundle-forming-pilus protein involved in localized adherence, was not affected by GOS, indicating that adherence inhibition was not due to the absence of this adherence factor. In addition, GOS did not affect autoaggregation. These observations suggest that some prebiotic oligosaccharides may have antiadhesive activity and directly inhibit the adherence of pathogens to the host epithelial cell surface.

Microbead analysis of cell binding to immobilized lectin: an alternative to microarrays in the development of carbohydrate drugs and diagnostic tests

Microarray technology is currently used in the development of carbohydrate drugs and diagnostic tests. Here we model an inexpensive alternative to microarrays using derivatized microbeads. In this model we examine the binding of mannose-rich yeast to microbeads derivatized with concanavalin A (Con A), a mannose-binding lectin, in the presence of 30 different sugars and 9 different pH conditions. We developed a listing of effective saccharide inhibitors of immobilized Con A based on 3901 replicates. We suggest that this is the most extensive saccharide inhibitor list ever developed for this lectin and it may be useful to use this listing to replace the less extensive lists that have been in the literature for decades. Information is also provided on pH effects on immobilized Con A binding based on 918 trials. Two assays to study binding, one which qualitatively scores more or less binding than control in thousands of replicate samples, and another that quantitatively evaluates binding by counting the number of cells bound to each bead, are also modeled here. We know of no previous studies that provide such extensive information on saccharide inhibition and pH effects on the binding of immobilized Con A. We suggest that this microbead approach, using beads derivatized with lectins or sugars, and the two simple assays presented here, can in some cases substitute for more expensive microarray technology in the development of carbohydrate drugs and diagnostic tests. If, for example, our model Saccharomyces cerevisiae was a pathogen, these studies show that it binds via cell surface mannose residues and drugs to prevent binding could be developed using the inhibitors of binding identified here. The beads could be also used in the development of diagnostic tests that identify the presence of the organism in blood samples, etc. in much the same way as microarray technology is being used today.

Designer probiotics for prevention of enteric infections

Many microbial pathogens, including those responsible for major enteric infections, exploit oligosaccharides that are displayed on the surface of host cells as receptors for toxins and adhesins. Blocking crucial ligand-receptor interactions is therefore a promising therapeutic strategy. One approach is to express molecular mimics of host receptors on the surface of harmless recombinant bacteria that can survive in the gut. These 'designer probiotics' bind bacterial toxins in the gut lumen with very high avidity, thereby preventing disease. This article discusses recent progress with this strategy.

Glycodendriproteins: a synthetic glycoprotein mimic enzyme with branched sugar-display potently inhibits bacterial aggregation

The continuing ability of bacteria to resist current antibiotic treatments highlights the need for alternative strategies for inhibiting their pathogenicity. Bacterial attachment is a major factor in infectivity and virulence. This key binding phase of bacteria to any potential host is mediated by adhesin proteins and so these present an attractive therapeutic target for antiinfective blocking strategies. However, the natural ligands to adhesins are large, typically complex molecules that are difficult to mimic with small molecules. We describe here a method that creates precise synthetic mimics of glycoproteins that are designed to bind adhesins. By using protein-degrading enzymes as the basis for these mimics we have created large-molecule protein ligands that inhibit aggregation of pathogenic bacteria at levels greater than a million-fold higher than small-molecule inhibitors of adhesins.

Large-scale synthesis of globotriose derivatives through recombinant E. coli

The carbohydrate chains decorating cell membranes and secreted proteins participate in a range of important biological processes. However, their ultimate significance and possible therapeutic potential have not been fully explored due to the lack of economical methods for their production. This study is an example of the use of a genetically engineered bacterial strain in the preparation of diverse oligosaccharides. Based on an ex vivo biosynthetic pathway, an artificial gene cluster was constructed by linking the genes of five associated enzymes on a plasmid vector. This plasmid was inserted into the E. coli NM522 strain to form globotriose-producing cells ('superbug' pLDR20-CKTUF). The specific strain was conveniently applied to the synthesis of globotriose trisaccharide and its derivatives, as potential neutralizers for Shiga toxin. This work demonstrates a novel and economical method for generating ligand diversity for carbohydrate drug development.

Anti-adhesion therapy of bacterial diseases: prospects and problems

The alarming increase in drug-resistant bacteria makes a search for novel means of fighting bacterial infections imperative. An attractive approach is the use of agents that interfere with the ability of the bacteria to adhere to tissues of the host, since such adhesion is one of the initial stages of the infectious process. The validity of this approach has been unequivocally demonstrated in experiments performed in a wide variety of animals, from mice to monkeys, and recently also in humans. Here we review various approaches to anti-adhesion therapy, including the use of receptor and adhesin analogs, dietary constituents, sublethal concentrations of antibiotics and adhesin-based vaccines. Because anti-adhesive agents are not bactericidal, the propagation and spread of resistant strains is much less likely to occur than as a result of exposure to bactericidal agents, such as antibiotics. Anti-adhesive drugs, once developed, may, therefore, serve as a new means to fight infectious diseases.