Purification of a galactosyl-alpha 1-4-galactose-binding adhesin from the gram-positive meningitis-associated bacterium Streptococcus suis

Streptococcus suis - The "Two Faces" of a Pathobiont in the Porcine Respiratory Tract

Streptococcus (S.) suis is a frequent early colonizer of the upper respiratory tract of pigs. In fact, it is difficult to find S. suis-free animals under natural conditions, showing the successful adaptation of this pathogen to its porcine reservoir host. On the other hand, S. suis can cause life-threatening diseases and represents the most important bacterial cause of meningitis in pigs worldwide. Notably, S. suis can also cause zoonotic infections, such as meningitis, septicemia, endocarditis, and other diseases in humans. In Asia, it is classified as an emerging zoonotic pathogen and currently considered as one of the most important causes of bacterial meningitis in adults. The “two faces” of S. suis, one of a colonizing microbe and the other of a highly invasive pathogen, have raised many questions concerning the interpretation of diagnostic detection and the definition of virulence. Thus, one major research challenge is the identification of virulence-markers which allow differentiation of commensal and virulent strains. This is complicated by the high phenotypic and genotypic diversity of S. suis, as reflected by the occurrence of (at least) 33 capsular serotypes. In this review, we present current knowledge in the context of S. suis as a highly diverse pathobiont in the porcine respiratory tract that can exploit disrupted host homeostasis to flourish and promote inflammatory processes and invasive diseases in pigs and humans.

Bacterial Adhesion of Streptococcus suis to Host Cells and Its Inhibition by Carbohydrate Ligands

Streptococcus suis is a Gram-positive bacterium, which causes sepsis and meningitis in pigs and humans. This review examines the role of known S. suis virulence factors in adhesion and S. suis carbohydrate-based adhesion mechanisms, as well as the inhibition of S. suis adhesion by anti-adhesion compounds in in vitro assays. Carbohydrate-binding specificities of S. suis have been identified, and these studies have shown that many strains recognize Galα1-4Gal-containing oligosaccharides present in host glycolipids. In the era of increasing antibiotic resistance, new means to treat infections are needed. Since microbial adhesion to carbohydrates is important to establish disease, compounds blocking adhesion could be an alternative to antibiotics. The use of oligosaccharides as drugs is generally hampered by their relatively low affinity (micromolar) to compete with multivalent binding to host receptors. However, screening of a library of chemically modified Galα1-4Gal derivatives has identified compounds that inhibit S. suis adhesion in nanomolar range. Also, design of multivalent Galα1-4Gal-containing dendrimers has resulted in a significant increase of the inhibitory potency of the disaccharide. The S. suis adhesin binding to Galα1-4Gal-oligosaccharides, Streptococcal adhesin P (SadP), was recently identified. It has a Galα1-4Gal-binding N-terminal domain and a C-terminal LPNTG-motif for cell wall anchoring. The carbohydrate-binding domain has no homology to E. coli P fimbrial adhesin, which suggests that these Gram-positive and Gram-negative bacterial adhesins recognizing the same receptor have evolved by convergent evolution. SadP adhesin may represent a promising target for the design of anti-adhesion ligands for the prevention and treatment of S. suis infections.

HP0197 contributes to CPS synthesis and the virulence of Streptococcus suis via CcpA

Streptococcus suis serotype 2 (SS2), a major swine pathogen and an emerging zoonotic agent, has greatly challenged global public health. The encoding proteins with unknown functions the bacterium encodes are an obstruction to studies of the pathogenesis. A novel surface protective antigen HP0197 is one of these proteins which have no sequence homology to any known protein. In the present study, the protein was determined to be involved in bacterial virulence through an evaluation of the isogenic mutant (Δhp0197) in both mice and pigs. The experimental infection also indicated that Δhp0197 could be cleared easily during infection, which could be attributed to the reduced thickness of the capsular polysaccharides (CPS) and the significantly reduced phagocytotic resistance. Microarrays-based comparative transcriptome analysis suggested that the suppressed expression of the operon responsible for CPS synthesis might be reversed by CcpA activity, which controlled global regulation of carbon catabolite through the binding of the CcpA and HPr-Ser-46-P to the catabolite-responsive elements (cre) of the target operons. The hypothesis was approved by the fact that the purified FLAG-tagged HPr from WT stain exhibited a higher binding activity to cre with CcpA compared to the Δhp0197 by the Electrophoretic Mobility Shift Assay, suggesting lower level of phosphorylation of the phosphocarrier protein HPr at residue Ser-46 (HPr-Ser-46P) in Δhp0197. These indicated that HP0197 could enhance CcpA activity to control the expression of genes involved in carbohydrate utilization and CPS synthesis, thus contributing to the virulence of S. suis.

Identification of a novel streptococcal adhesin P (SadP) protein recognizing galactosyl-α1-4-galactose-containing glycoconjugates: convergent evolution of bacterial pathogens to binding of the same host receptor

Bacterial adhesion is often a prerequisite for infection, and host cell surface carbohydrates play a major role as adhesion receptors. Streptococci are a leading cause of infectious diseases. However, only few carbohydrate-specific streptococcal adhesins are known. Streptococcus suis is an important pig pathogen and a zoonotic agent causing meningitis in pigs and humans. In this study, we have identified an adhesin that mediates the binding of S. suis to galactosyl-α1-4-galactose (Galα1-4Gal)-containing host receptors. A functionally unknown S. suis cell wall protein (SSU0253), designated here as SadP (streptococcal adhesin P), was identified using a Galα1-4Gal-containing affinity matrix and LC-ESI mass spectrometry. Although the function of the protein was not previously known, it was recently identified as an immunogenic cell wall protein in a proteomic study. Insertional inactivation of the sadP gene abolished S. suis Galα1-4Gal-dependent binding. The adhesin gene sadP was cloned and expressed in Escherichia coli. Characterization of its binding specificity showed that SadP recognizes Galα1-4Gal-oligosaccharides and binds its natural glycolipid receptor, GbO(3) (CD77). The N terminus of SadP was shown to contain a Galα1-Gal-binding site and not to have apparent sequence similarity to other bacterial adhesins, including the E. coli P fimbrial adhesins, or to E. coli verotoxin or Pseudomonas aeruginosa lectin I also recognizing the same Galα1-4Gal disaccharide. The SadP and E. coli P adhesins represent a unique example of convergent evolution toward binding to the same host receptor structure.

Crystal structure of the functional region of Uro-adherence factor A from Staphylococcus saprophyticus reveals participation of the B domain in ligand binding

Staphylococci use cell wall-anchored proteins as adhesins to attach to host tissues. Staphylococcus saprophyticus, a uropathogenic species, has a unique cell wall-anchored protein, uro-adherence factor A (UafA), which shows erythrocyte binding activity. To investigate the mechanism of adhesion by UafA, we determined the crystal structure of the functional region of UafA at 1.5 Å resolution. The structure was composed of three domains, designated as the N2, N3, and B domains, arranged in a triangular relative configuration. Hemagglutination inhibition assay with domain-truncated mutants indicated that both N and B domains were necessary for erythrocyte binding. Based on these results, a novel manner of ligand binding in which the B domain acts as a functional domain was proposed as the adhesion mechanism of S. saprophyticus.

Response of swine spleen to Streptococcus suis infection revealed by transcription analysis

Astract

Adhesion determinants of the Streptococcus species

Streptococci are clinically important Gram-positive bacteria that are capable to cause a wide variety of diseases in humans and animals. Phylogenetic analyses based on 16S rRNA sequences of the streptococcal species reveal a clustering pattern, reflecting, with a few exceptions, their pathogenic potential and ecological preferences. Microbial adhesion to host tissues is the initial critical event in the pathogenesis of most infections. Streptococci use multiple adhesins to attach to the epithelium, and their expression is regulated in response to environmental and growth conditions. Bacterial adhesins recognize and bind cell surface molecules and extracellular matrix components through specific domains that for certain adhesin families have been well defined and found conserved across the streptococcal species. In this review, we present the different streptococcal adhesin families categorized on the basis of their adhesive properties and structural characteristics, and, when available, we focus the attention on conserved functional domains.

Surface-associated and secreted factors ofStreptococcus suisin epidemiology, pathogenesis and vaccine development

Streptococcus suisis an invasive porcine pathogen associated with meningitis, arthritis, bronchopneumonia and other diseases. The pathogen constitutes a major health problem in the swine industry worldwide. Furthermore,S. suisis an important zoonotic agent causing meningitis and other diseases in humans exposed to pigs or pork. Current knowledge on pathogenesis is limited, despite the enormous amount of data generated by ‘omics’ research. Accordingly, immunprophylaxis (in pigs) is hampered by lack of a cross-protective vaccine against virulent strains of this diverse species. This review focuses on bacterial factors, both surface-associated and secreted ones, which are considered to contribute toS. suisinteraction(s) with host factors and cells. Factors are presented with respect to (i) their identification and features, (ii) their distribution amongS. suisand (iii) their significance for virulence, immune response and vaccination. This review also shows the enormous progress made in research onS. suisover the last few years, and it emphasizes the numerous challenging questions remaining to be answered in the future.

Tracing the history of Galalpha1-4Gal on glycoproteins in modern birds

Galalpha1-4Gal is typically found in mammalian glycolipids in small quantities, and recognized by some pathogens, such as uropathogenic Escherichia coli. In contrast, glycoproteins containing Galalpha1-4Gal were rarely found in vertebrates except in a few species of birds and amphibians until recently. However, we had previously reported that pigeon (Columba livia) egg white and serum glycoproteins are rich in N-glycans with Galalpha1-4Gal at non-reducing termini. Our investigation with egg white glycoproteins from 181 avian species also revealed that the distribution of (Galalpha1-4Gal)-containing glycoproteins was not rare among avians, and is correlated with the phylogeny of birds. The differentiated expression was most likely emerged at earlier stage of diversification of modern birds, but some birds might have lost the facility for the expression relatively recently.

Crystal Structure of Streptococcus suis Dps-like Peroxide Resistance Protein Dpr: Implications for Iron Incorporation

The Dps-like peroxide resistance protein (Dpr) is an aerotolerance and hydrogen peroxide resistance agent found in the meningitis-associated pathogen Streptococcus suis. Dpr is believed to act by binding free intracellular iron to prevent Fenton chemistry-catalysed formation of toxic hydroxyl radicals. The crystal structure of Dpr has been determined to 1.95 A resolution. The final model has an Rcyst value of 18.5% (Rfree = 22.4%) and consists of 12 identical monomers (each of them comprising a four alpha-helix bundle) that form a hollow sphere obeying 23 symmetry. Structural features show that Dpr belongs to the Dps family of bacterial proteins. Twelve putative ferroxidase centers, each formed at the interface of neighboring monomer pairs, were identified in the Dpr structure with structural similarities to those found in other Dps family members. Dpr was crystallized in the absence of iron, hence no bound iron was found in the structure in contrast to other Dps family members. A novel metal-binding site approximately 6A from the ferroxidase centre was identified and assigned to a bound calcium ion. Two residues from the ferroxidase centre (Asp63 and Asp74) were found to be involved in calcium binding. Structural comparison with other family members revealed that Asp63 and Asp74 adopt different conformation in the Dpr structure. The structure of Dpr presented here shows potential local conformational changes that may occur during iron incorporation. A role for the metal-binding site in iron uptake is proposed.

Molecular basis of H2O2 resistance mediated by Streptococcal Dpr. Demonstration of the functional involvement of the putative ferroxidase center by site-directed mutagenesis in Streptococcus suis

H(2)O(2) is an unavoidable cytotoxic by-product of aerobic life. Dpr, a recently discovered member of the Dps protein family, provides a means for catalase-negative bacteria to tolerate H(2)O(2). Potentially, Dpr could bind free intracellular iron and thus inhibit the Fenton chemistry-catalyzed formation of toxic hydroxyl radicals (H(2)O(2) + Fe(2+) --> (.)OH + (-)OH + Fe(3+)). We explored the in vivo function of Dpr in the catalase- and NADH peroxidase-negative pig and human pathogen Streptococcus suis. We show that: (i) a Dpr allelic exchange knockout mutant was hypersensitive ( approximately 10(6)-fold) to H(2)O(2), (ii) Dpr incorporated iron in vivo, (iii) a putative ferroxidase center was present in Dpr, (iv) single amino acid substitutions D74A or E78A to the putative ferroxidase center abolished the in vivo iron incorporation, and (v) the H(2)O(2) hypersensitive phenotype was complemented by wild-type Dpr or by a membrane-permeating iron chelator, but not by the site-mutated forms of Dpr. These results demonstrate that the putative ferroxidase center of Dpr is functionally active in iron incorporation and that the H(2)O(2) resistance is mediated by Dpr in vivo by its iron binding activity.

Purification of Shiga-like toxin 1 by pigeon egg white glycoproteins immobilized on Sepharose gels

The galabiose structure Galalpha1-4Gal is rarely found in natural glycoproteins, but is abundantly present in pigeon egg white proteins as Galalpha(1-4)Galbeta(1-4)GlcNAc termini. Pigeon ovalbumin, ovomucoid, or the whole egg white were immobilized on periodate-oxidized Sepharose CL-6B gels by reductive amination. These gels were found to bind Shiga-like toxin type 1 (SLT-1) specifically and efficiently. SLT-1 was eluted from the gel beads with 0.5 M melibiose, which was more efficient and milder than elution with 4.5 M MgCl(2). SLT-1 was purified to homogeneity from the crude extract of Escherichia coli SLT100 expressing SLT-1 by a single affinity chromatographic step in 83-88% yield. The capacity of the gel was estimated to be ca. 1mg toxin/ml gel. Interestingly, SLT-2 was not bound by these affinity gels containing Galalpha1-4Galbeta1-4GlcNAc termini. Since SLT-2 has been shown to bind to Galalpha1-4Galbeta1-4Glc-terminating compounds, our results suggest that Glc in globotriose moiety is important for binding SLT-2, and replacing the Glc with GlcNAc in this triose renders it ineffective for binding SLT-2.

Streptococcus suis interactions with the murine macrophage cell line J774: adhesion and cytotoxicity

Streptococcus suis capsular type 2 is an important etiological agent of swine meningitis, and it is also a zoonotic agent. Since one hypothesis of the pathogenesis of S. suis infection is that bacteria enter the bloodstream and invade the meninges and other tissues in close association with mononuclear phagocytes, the objective of the present study was to evaluate the capacity of S. suis type 2 to adhere to macrophages. An enzyme-linked immunosorbent assay technique was standardized to simply and accurately measure the rate of bacterial attachment to phagocytic cells. Results were confirmed by plate counting. Adhesion was dependent on bacterial concentration and incubation time and was not affected by cytochalasin pretreatment of macrophages. Inhibition studies showed that the sialic acid moiety of the S. suis capsule would be, at least in part, responsible for bacterial recognition by macrophages. Serum preopsonization of bacteria increased adhesion levels. Complement would be partially implicated in the serum-enhanced binding of S. suis to cells. Adhesion varied among different S. suis type 2 isolates. However, high bacterial concentrations of several isolates were cytotoxic for cells, and these cytotoxic effects correlated with suilysin production. Indeed, hemolytic strain supernatants, as well as purified suilysin, reproduced cytotoxic effects observed with live bacteria, and these effects were inhibited by cholesterol pretreatment. Bacterial adhesion and cytotoxicity were confirmed by scanning and transmission electron microscopy. We hypothesize that attachment of bacteria to phagocytes could play an important role in the pathogenesis of S. suis infection by allowing bacterial dissemination and causing a bacteremia and/or septicemia. This interaction could also be related to the activation of the host inflammatory response observed during meningitis.

Isolation and characterization of major glycoproteins of pigeon egg white: ubiquitous presence of unique N-glycans containing Galalpha1-4Gal

Ovotransferrin (POT), two ovalbumins (POA(hi) and POA(lo)), and ovomucoid (POM) were isolated from pigeon egg white (PEW). Unlike their chicken egg white counterparts, PEW glycoproteins contain terminal Galalpha1-4Gal, as evidenced by GS-I lectin (specific for terminal alpha-Gal), anti-P(1) (Galalpha1-4Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glcbeta1-1Cer) monoclonal antibody, and P fimbriae on uropathogenic Escherichia coli (specific for Galalpha1-4Gal). Galalpha1-4Gal on PEW glycoproteins were found in N-glycans releasable by treatment with glycoamidase F. The respective contents of N-glycans in each glycoprotein were 3.5%, POT; 17%, POA(hi); and 31-37%, POM. POA(hi) has four N-glycosylation sites, in contrast to chicken ovalbumin, which has only one. High performance liquid chromatography analysis showed that N-glycans on POA(hi) were highly heterogeneous. Mass spectrometric analysis revealed that the major N-glycans were monosialylated tri-, tetra-, and penta-antennary oligosaccharides containing terminal Galalpha1-4Gal with or without bisecting N-acetylglucosamine. Oligosaccharide chains terminating in Galalpha1-4Gal are rare among N-glycans from the mammals and avians that have been studied, and our finding is the first predominant presence of (Galalpha1-4Gal)-terminated N-glycans.

Glycoproteins and their relationship to human disease

Glycoproteins are proteins that carry N- and O-glycosidically-linked carbohydrate chains of complex structures and functions. N-glycan chains are assembled in the endoplasmic reticulum and the Golgi by a controlled sequence of glycosyltransferase and glycosidase processing reactions involving dolichol intermediates. The assembly of O-glycans occurs in the Golgi and does not involve dolichol. For most reactions, families of glycosyltransferases exist; the expression of the individual enzymes within a family is often subject to complex regulation. The biosynthesis of N- and O-glycan is controlled at the level of gene expression, mRNA, enzyme protein activity and localization, and through substrate and cofactor concentrations at the site of synthesis. This complex regulation results in many hundreds of structures, the range of which varies in different species, cell types, tissue types, states of development and differentiation. In diseased cells, the relative proportions of these structures are often characteristically different from normal, and may be useful for the assessment of the stage of the disease and for diagnosis. Knowledge of disease-specific glycoprotein structures and their functions may be used therapeutically, in immunotherapy, in blocking cell adhesion or interfering with other binding or biological processes. Recently, some of the mechanisms underlying glycoprotein alterations in disease have been elucidated. This opens the possibility of an active interference in the disease process. The functions of glycans in diseased cells will become more clear with the tools of molecular biology and transgenic animal models.

Epithelial invasion and cell lysis by virulent strains of Streptococcus suis is enhanced by the presence of suilysin

Streptococcus suis is an important pathogen of pigs causing arthritis, pneumonia and meningitis and is an occupational disease of farmers and those in the meat industry. As with other streptococci, both virulent and avirulent strains of S. suis are frequently carried asymptomatically in the tonsillar crypts and nasal cavities. Little is known about the process by which virulent strains cross the mucosal epithelia to generate systemic disease and whether this process requires expression of specific bacterial virulence factors. Although putative virulence factors have been postulated, no specific role in the disease process has yet been demonstrated for these factors. This study is the first demonstration that virulent strains of S. suis both invade and lyse HEp-2 cells, a continuous laryngeal epithelial cell line, and that at least one bacterial virulence factor, suilysin, is involved in this process.

Determination of the cell adhesion specificity of Streptococcus suis with the complete set of monodeoxy analogues of globotriose

Streptococcus suis causes meningitis and other serious infections in pigs and humans, and binds to host cell globotriosylceramide. In order to determine the essential hydroxyls involved in binding, the complete set of monodeoxy derivatives of the receptor trisaccharide Gal alpha1-Gal beta1-4Glc were tested as inhibitors of bacterial hemagglutination. Removal of the 4''-, 6'', 2' or 3'-hydroxyls abolished inhibitory activity, which indicated that they were critically involved in binding. The same results were obtained using synthetic lipid-linked monodeoxy derivatives of the trisaccharides in a thin-layer overlay assay. The P(N) and P(O) subtypes of the S. suis adhesin showed similar binding patterns. The hydroxyls of the glucose moiety were not critical for binding, although the adhesin binds better to the trisaccharide Gal alpha1-4Gal beta1-4Glc than the disaccharide Gal alpha1-4Gal.

Streptococcus suis: past and present

Steptococcus suis is a Gram-positive, facultatively anaerobic coccus that has been implicated as the cause of a wide range of clinical disease syndromes in swine and other domestic animals. In swine, the disease has spread worldwide but is more prevalent in countries with intensive swine management practices. The disease syndromes caused by S. suis in swine include arthritis, meningitis, pneumonia, septicaemia, endocarditis, polyserositis, abortions and abscesses. S. suis has also been implicated in disease in humans, especially among abattoir workers and swine and pork handlers. In humans, S. suis type 2 can cause meningitis, which may result in permanent hearing loss, septicaemia, endocarditis and death. The pathogenic mechanism of S. suis is not well defined. Several virulence factors have been identified, but their roles in pathogenesis and disease have not been well elucidated. Much work is in progress on characterization of virulence factors and mechanisms, with emphasis on the control of the disease. Because of the non-availability of suitable immunoprophylaxis, control of S. suis infection has depended mainly on the use of antimicrobials.

The galactosyl-(alpha 1-4)-galactose-binding adhesin of Streptococcus suis: occurrence in strains of different hemagglutination activities and induction of opsonic antibodies

The occurrence of the galactose-(alpha 1-4)-galactose-specific adhesin in Streptococcus suis, a pig and human pathogen causing sepsis, meningitis, and other serious infections, was studied. Poly- and monoclonal anti-bodies to the purified adhesin, as well as pigeon ovomucoid, a specific probe for the adhesin activity, detected one single protein band in extracts of S. suis. The adhesin was detected in all 23 strains studied, representing pathogenic serotypes (1, 2, 4, 5, 7, 8, and nontypeable) and including several weakly hemagglutinating or nonhemagglutinating strains and phase variants. The amount of adhesin detected was not correlated with the hemagglutination activity of the intact bacteria. Extraction of cells showing no binding of pigeon ovomucoid by ultrasonic treatment resulted in extracts with pigeon ovomucoid binding activity, suggesting that the adhesin was not accessible to the probe on the intact cells. Analysis of the amount of capsular polysaccharide revealed an inverse relationship between the hemagglutination activity and expression of capsular polysaccharide, thus suggesting a factor influencing adhesin accessibility. The purified adhesin was highly immunogenic and induced in preliminary experiments bactericidal activity in mice. Thus, the adhesin, with its specific binding mechanism to host cells and a proposed pathogenic role, is widely expressed among strains of different serotypes and therefore appears to represent a novel promising candidate for the development of a vaccine against S. suis.