Streptococcus suis is an important swine pathogen that causes meningitis, endocarditis, arthritis and septicaemia. As a zoonotic agent, S. suis also causes similar diseases in humans. Binding of pathogenic bacteria to extracellular matrix components enhances their adhesion to and invasion of host cells. In the present study we isolated and identified a novel fibronectin-binding protein from S. suis. The native protein (designated SsEno) possessed not only high homology with other bacterial enolases but also enolase activity. We cloned, expressed and purified SsEno and showed that it is ubiquitously expressed by all S. suis serotypes and we identified its surface localization using immunoelectron microscopy. ELISA demonstrated that SsEno binds specifically to fibronectin and plasminogen in a lysine-dependent manner. Additional surface plasmon resonance assays demonstrated that SsEno binds to fibronectin or plasminogen with low nanomolar affinity. Inhibition experiments with anti-SsEno antibodies also showed that bacterial SsEno is important for the adhesion to and invasion of brain microvascular endothelial cells by S. suis. Overall, the present work is the first study, to our knowledge, to demonstrate a fibronectinbinding activity of a bacterial enolase, and shows that, similar to other bacterial fibronectin-binding proteins, SsEno may contribute to the virulence of S. suis. INTRODUCTIONStreptococcus suis is a major swine pathogen that causes septicaemia, meningitis, endocarditis and arthritis (Higgins & Gottschalk, 2005). Of the 35 known serotypes, serotype 2 is the most frequently isolated and associated with disease (Higgins & Gottschalk, 2005). It has been proposed that two serotypes (serotypes 32 and 34) be excluded from S. suis and redesignated Streptococcus orisratti (Hill et al., 2005). S. suis, especially serotype 2, has also been described as an important zoonotic agent that affects people in close contact with infected pigs or pork-derived products (Lun et al., 2007). Indeed, an important number of cases of human disease with a high rate of mortality in China were linked directly to a concurrent outbreak of S. suis infection in pigs (Ye et al., 2006).Little is known about S. suis virulence factors. The capsule polysaccharide (CPS) is a critical virulence factor, given that unencapsulated isogenic mutants are completely avirulent and rapidly cleared from the circulation in pig and mouse infection models (Charland et al., 2000;Smith et al., 1999). However, non-virulent strains are also encapsulated, indicating that the virulence of this pathogen is a multifactorial process . Other potential virulence factors have also been described in S. suis, including a haemolysin (suilysin), a 136 kDa muramidase-released protein (MRP), a 110 kDa extracellular factor (EF) protein, a hyaluronidase, a superoxide dismutase, various proteases, a serum opacity factor and different adhesins (Baums et al., 2006;.The pathogenesis of S. suis infection is not fully understood and likely involves many steps . Binding between b...

A Streptococcus suis surface protein reacting with convalescent-phase sera from pigs clinically infected by S. suis type 2 was identified. The apparent 110-kDa protein, designated Sao, exhibits typical features of membraneanchored surface proteins of gram-positive bacteria, such as a signal sequence and an LPVTG membrane anchor motif. In spite of high identity with the partially sequenced genomes of S. suis Canadian strain 89/1591 and European strain P1/7, Sao does not share significant homology with other known sequences. However, a conserved avirulence domain that is often found in plant pathogens has been detected. Electron microscopy using an Sao-specific antiserum has confirmed the surface location of the Sao protein on S. suis. The Sao-specific antibody reacts with cell lysates of 28 of 33 S. suis serotypes and 25 of 26 serotype 2 isolates in immunoblots, suggesting its high conservation in S. suis species. The immunization of piglets with recombinant Sao elicits a significant humoral antibody response. However, the antibody response is not reflected in protection of pigs that are intratracheally challenged with a virulent strain in our conventional vaccination model.Streptococcus suis is an important swine pathogen that causes many pathological conditions, such as arthritis, endocarditis, meningitis, pneumonia, and septicemia (19,21). It is also an important zoonotic agent for humans in contact with colonized, otherwise healthy pigs or their by-products, causing meningitis and endocarditis (1, 53). Thirty-three serotypes (types 1 to 31, 33, and 1/2) based on capsular antigens are currently known (15-17, 22, 24, 43). Type 2 is considered the most virulent and prevalent type in diseased pigs. The mechanisms involved in the pathogenesis and virulence of S. suis are not completely understood (19), and attempts to control the infection are hampered by the lack of an effective vaccine.Several approaches have been used to develop vaccines for S. suis. However, little success was achieved because the protection was either serotype or strain dependent, and results in most instances were equivocal (23, 42). For example, some protection with killed whole cells or live avirulent vaccines was reported, but this required repeated immunization, and the protection against heterologous challenges was not evaluated (25, 56). Exposure of young pigs to live virulent strains showed a positive effect in reducing clinical signs characteristics of S. suis infection (52). Since the S. suis capsule plays an important role in virulence, attempts have been made to develop a vaccine based on capsular material. However, this vaccination approach was unsatisfactory because the capsular polysaccharide is poorly immunogenic (9). More recently, interest has shifted toward protein antigens of S. suis as vaccine candidates. Subunit vaccines using suilysin (27) or muramidase-released protein and extracellular protein factor (57) have been shown to protect pigs from homologous and heterologous serotype 2 strains, but their use is hindered by ...

Sao is a Streptococcus suis surface protein recently identified as a potential vaccine candidate. In this study, recombinant Sao in combination with Quil A provided cross-protection against S. suis serotype 2 disease in mouse and pig vaccination protocols. Subcutaneous immunization of mice elicited strong immunoglobulin G (IgG) antibody responses. All four IgG subclasses were induced, with the IgG2a titer being the highest, followed by those of IgG1, IgG2b, and IgG3. Challenge of the mice with S. suis strain 31533 resulted in a mortality rate of 80% for the control group, which received Quil A only. In contrast, all of the mice immunized with Sao survived. In a pig vaccination protocol, intramuscular immunization with Sao also elicited significant humoral antibody responses, and both the IgG1 and IgG2 subclasses were induced, with a predominance of IgG2 production. In vitro assay showed that Sao-induced antibodies significantly promoted the ability of porcine neutrophils in opsonophagocytic killing of S. suis. An aerosol challenge of the pigs with S. suis strain 166 resulted in clinical signs characteristic of S. suis infection in diseased pigs. The vaccine group showed significantly better survival, lower clinical scores, and less S. suis recovery from postmortem tissue samples than did the control group. Furthermore, this study also revealed that although challenge S. suis strains express Sao size variants, recombinant Sao conferred cross-protection. These data demonstrate that recombinant Sao formulated with Quil A triggers strong opsonizing antibody responses which confer efficient immunity against challenge infection with heterologous S. suis type 2.

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