Protection of experimentally infected pigs by suilysin, the thiol-activated haemolysin of Streptococcus suis

Bovine Herpesvirus-4 Based Vaccine Provides Protective Immunity against Streptococcus suis Disease in a Rabbit Model

Streptococcus suis (S. suis) is a bacterial pathogen of pigs that has a major animal health and economic impact on the pig industry. Bovine herpesvirus-4 (BoHV-4) is a new virus-based vaccine vector that has been used for the immunogenic delivery of antigens from a variety of pathogens. In the present study, two recombinant BoHV-4-based vectors were evaluated for their ability to induce immunity and protection against S. suis in a rabbit model. The GMD protein is a fusion protein consisting of multiple dominant B-cell epitopes ((B-cell dominant epitopes of GAPDH, MRP, and DLDH antigens) (BoHV-4/GMD)) and the second suilysin (SLY) (BoHV-4/SLY) from S. suis serotype 2 (SS2). Both GMD and SLY delivered by the BoHV-4 vectors were recognized by sera from SS2-infected rabbits. The vaccination of rabbits with the BoHV-4 vectors induced antibodies against SS2, as well as against additional S. suis serotypes, SS7 and SS9. However, sera from BoHV-4/GMD-vaccinated animals promoted a significant level of phagocytic activity by pulmonary alveolar macrophages (PAMs) against SS2, SS7, and SS9. In contrast, sera from rabbits immunized with BoHV-4/SLY induced PAM phagocytic activity against only SS2. In addition, BoHV-4 vaccines differed in the associated level of protection against lethal SS2 challenge, which ranged from high (71.4%) to low (12.5%) for BoHV-4/GMD and BoHV-4/SLY, respectively. These data suggest BoHV-4/GMD as a promising vaccine candidate against S. suis disease.

Recombinant suilysin of Streptococcus suis enhances the protective efficacy of an engineered Pasteurella multocida toxin protein

Suilysin (Sly) from Streptococcus suis has been shown to elicit strong immune responses and may act as a vaccine adjuvant. In the present study, we tested the adjuvant effect of Sly using an engineered Pasteurella multocida toxin, rPMT-NC, as the antigen. The antigen was also formulated with other conventional adjuvants (aluminum hydroxide, water-in-oil-in-water) for comparison. The efficacy of these vaccine formulations were evaluated in mice. The optimal dosage of purified rSly for enhancing immune responses in mice was first determined to be 40 μg/ml based on significantly (p < 0.05) increased serum antibody titers, expression of cytokines, including interleukin (IL)-4, IL-12, and interferon (IFN)-γ and the survival rate after challenge with P. multocida. Mice immunized with rPMT-NC + rSly had augmented antibody production and cellular immunity compare to those immunized with rPMT-NC plus other adjuvants. In addition, the survival rate of mice immunized with rPMT-NC + rSly was the highest (70% v.s. 30% of mice immunized with rPMT-NC alone) among all groups. In conclusion, rSly has the potential to be used as a biological adjuvant to enhance immune responses and protective efficacy of protein-based vaccines.

An Engineered Outer Membrane-Defective Escherichia coli Secreting Protective Antigens against Streptococcus suis via the Twin-Arginine Translocation Pathway as a Vaccine

Live bacterial vector vaccines are one of the most promising vaccine types and have the advantages of low cost, flexibility, and good safety. Meanwhile, protein secretion systems have been reported as useful tools to facilitate the release of heterologous antigen proteins from bacterial vectors. The twin-arginine translocation (Tat) system is an important protein export system that transports fully folded proteins in a signal peptide-dependent manner. In this study, we constructed a live vector vaccine using an engineered commensal Escherichia coli strain in which amiA and amiC genes were deleted, resulting in a leaky outer membrane that allows the release of periplasmic proteins to the extracellular environment. The protective antigen proteins SLY, enolase, and Sbp against Streptococcus suis were targeted to the Tat pathway by fusing a Tat signal peptide. Our results showed that by exploiting the Tat pathway and the outer membrane-defective E. coli strain, the antigen proteins were successfully secreted. The strains secreting the antigen proteins were used to vaccinate mice. After S. suis challenge, the vaccinated group showed significantly higher survival and milder clinical symptoms compared with the vector group. Further analysis showed that the mice in the vaccinated group had lower burdens of bacteria load and slighter pathological changes. Our study reports a novel live bacterial vector vaccine that uses the Tat system and provides a new alternative for developing S. suis vaccine.

A novel bicyclic 2,4-diaminopyrimidine inhibitor of Streptococcus suis dihydrofolate reductase

Streptococcus suis is a Gram-positive bacterial pathogen of pigs and an emerging zoonotic pathogen. It has become increasingly resistant to multiple classes of antibiotics. New drug candidates and knowledge of their targets are needed to combat antibiotic-resistant S. suis. In this study, the open-source Pathogen Box compound library was screened. Thirty hits that effectively inhibited S. suis growth at 10 µM were identified. Among the most potent hits, MMV675968 (a diaminoquinazoline analog) was shown to target S. suis dihydrofolate reductase (SsDHFR) via (1) growth inhibition of an E. coli surrogate whose growth is dependent on exogenously expressed SsDHFR and (2) inhibition of in vitro SsDHFR activity. Thymidine supplement is able to reverse growth inhibition by MMV675968 in both E. coli surrogate and S. suis, indicating that a thymidine-related pathway is a major target of MMV675968. Comparison of MMV675968 with seven DHFR inhibitors representing different core structures revealed that bicyclic 2,4-diaminopyrimidines with long and flexible side chains are highly effective in inhibiting SsDHFR and S. suis growth. MMV675968 and related compounds thus may serve as starting points for developing antibiotics against drug resistant S. suis.

Live attenuated Salmonella enterica serovar Choleraesuis vector delivering a conserved surface protein enolase induces high and broad protection against Streptococcus suis serotypes 2, 7, and 9 in mice

Streptococcus suis, a major zoonotic pathogen in swine, can be classified into 35 serotypes. However, no universal vaccine against the multiple serotypes of S. suis is available, though some studies have shown homologous protection. Hence, developing an effective universal vaccine to protect pigs against multiple S. suis serotypes is necessary, or at the very least, to protect pigs against diseases caused by the dominant pathogenic serotypes. Enolase, a highly conserved surface protein, is present in all of the described S. suis serotypes. rSC0016 is an improved recombinant attenuated S. Choleraesuis vaccine vector, combining a sopB mutation with regulated delayed systems, achieving an adequate balance between host safety and immunogenicity. In order to develop a universal vaccine against the multiple serotypes of S. suis, a novel recombinant vaccine strain rSC0016 that carries a heterologous antigen enolase was developed in this study. According, it was found that the recombinant vaccine strain rSC0016(pS-Enolase) exhibited better colonization compared to the vaccine control strain rSC0018(pYA3493). In addition, a mouse model immunized with the strain rSC0016(pS-Enolase) elicited significant IgG antibody responses against both enolase and Salmonella antigens, while inducing good mucosal, humoral, and cellular immune responses against enolase. Finally, immunization with rSC0016(pS-Enolase) was shown to confer 100%, 80%, and 100% protection against the serotypes of SS2, SS7, and SS9, respectively, and significantly reduced histopathological lesions in mice. Overall, this study provides a promising universal vaccine candidate for use against the multiple serotypes of S. suis.

Interaction of Macrophages and Cholesterol-Dependent Cytolysins: The Impact on Immune Response and Cellular Survival

Cholesterol-dependent cytolysins (CDCs) are key virulence factors involved in many lethal bacterial infections, including pneumonia, necrotizing soft tissue infections, bacterial meningitis, and miscarriage. Host responses to these diseases involve myeloid cells, especially macrophages. Macrophages use several systems to detect and respond to cholesterol-dependent cytolysins, including membrane repair, mitogen-activated protein (MAP) kinase signaling, phagocytosis, cytokine production, and activation of the adaptive immune system. However, CDCs also promote immune evasion by silencing and/or destroying myeloid cells. While there are many common themes between the various CDCs, each CDC also possesses specific features to optimally benefit the pathogen producing it. This review highlights host responses to CDC pathogenesis with a focus on macrophages. Due to their robust plasticity, macrophages play key roles in the outcome of bacterial infections. Understanding the unique features and differences within the common theme of CDCs bolsters new tools for research and therapy.

Contribution of Nlrp3 Inflammasome Activation Mediated by Suilysin to Streptococcal Toxic Shock-like Syndrome

Objective: The aim of this study was to investigate the molecular mechanism of inflammasome activation in response to Streptococcus suis serotype 2 (SS2) infection and its contribution to the development of streptococcal toxic shock-like syndrome (STSS).

Methods: To verify the role of suilysin (SLY) in STSS, we infected bone-marrow-derived macrophages (BMDMs) in vitro and C57BL/6J mice intraperitoneally (IP) with the SS2 wild-type (WT) strain or isogenic sly mutant (∆SLY) to measure the interleukin (IL)-1β release and survival rate. To determine the role of inflammasome activation and pyroptosis in STSS, we infected BMDMs from WT and various deficient mice, including Nlrp3-deficient (Nlrp3^−/−), Nlrc4-deficient (Nlrc4^−/−), Asc-deficient (Asc^−/−), Aim2-deficient (Aim2^−/−), Caspase-1/11-deficient (Caspase-1/11^−/−), and Gsdmd-deficient (Gsdmd^−/−) ex vivo, and IP injected WT, Nlrp3^−/−, Caspase-1/11^−/−, and Gsdmd^−/− mice with SS2, to compare the IL-1β releases and survival rate in vivo.

Results: The SS2-induced IL-1β production in mouse macrophages is mediated by SLY ex vivo. The survival rate of WT mice infected with SS2 was significantly lower than that of mice infected with the ∆SLY strain in vivo. Furthermore, SS2-triggered IL-1β releases, and the cytotoxicity in the BMDMs required the activation of the NOD-Like Receptors Family Pyrin Domain Containing 3 (Nlrp3), Caspase-1/11, and gasdermin D (Gsdmd) inflammasomes, but not the Nlrc4 and Aim2 inflammasomes ex vivo. The IL-1β production and survival rate of WT mice infected with SS2 were significantly lower than those of the Nlrp3^−/−, Caspase-1/11^−/−, and Gsdmd^−/− mice in vivo. Finally, the inhibitor of the Nlrp3 inflammasome could reduce the IL-1β release and cytotoxicity of SS2-infected macrophages ex vivo and protect SS2-infected mice from death in vivo.

Conclusion: Nlrp3 inflammasome activation triggered by SLY in macrophages played an important role in the pathogenesis of STSS.

Identification of novel pig and human immunoglobulin G-binding proteins and characterization of the binding regions of enolase from Streptococcus suis serotype 2

Streptococcus suis, a major emerging pathogen in swine and humans, expresses immunoglobulin G (IgG)-binding proteins (IBPs), which contribute to the ability of organism to evasion of host immune system. The objective of this study was to identify novel pig IgG (pIgG) and human IgG (hIgG)-binding proteins and characterize the binding regions of enolase from Streptococcus suis serotype 2 (S. suis 2). Here, four pIgG-binding proteins (pIBPs) and five hIgG-binding proteins (hIBPs) were identified from S. suis 2 surface proteins by 2D-Far-western blot assays. All the newly captured proteins were expressed and further confirmed their binding activity to pIgG or hIgG by Far-western blot and dot blot. In addition to previously identified factor H, fibronectin, collagen, fibrinogen, plasminogen and laminin, we also found that both pIgG and hIgG can specifically interact with enolase. Binding assays indicated that interactions of S. suis 2 enolase with pIgG and hIgG is primarily mediated by the enolase C-terminal portion (Enolase-C, a.a. 142–432). We found that hIgG exhibited stronger binding ability to Enolase-C than pIgG. Further analysis of the C-terminal regions of enolase (Enolase-C1 and Enolase-C2) suggested that the C-terminus possessed two different binding domains with distinct host IgG proteins. Strikingly, we confirmed that pIgG interacted with the Enolase-C1 (a.a. 142–271) and hIgG interacted with the Enolase-C2 (a.a. 271–432). These observations of enolase provide interesting insights in the pathogenesis of S. suis infection.

Proteomic and Bioinformatic Analysis of Streptococcus suis Human Isolates: Combined Prediction of Potential Vaccine Candidates

Streptococcus suis is a Gram-positive bacterium responsible for major infections in pigs and economic losses in the livestock industry, but also an emerging zoonotic pathogen causing serious diseases in humans. No vaccine is available so far against this microorganism. Conserved surface proteins are among the most promising candidates for new and effective vaccines. Until now, research on this pathogen has focused on swine isolates, but there is a lack of studies to identify and characterize surface proteins from human clinical isolates. In this work, we performed a comparative proteomic analysis of six clinical isolates from human patients, all belonging to the major serotype 2, by “shaving” the live bacterial cells with trypsin, followed by LC-MS/MS analysis. We identified 131 predicted surface proteins and carried out a label-free semi-quantitative analysis of protein abundances within the six strains. Then, we combined our proteomics results with bioinformatic tools to help improving the selection of novel antigens that can enter the pipeline of vaccine candidate testing. Our work is then a complement to the reverse vaccinology concept.

Streptococcus suis 2 Transcriptional Regulator TstS Stimulates Cytokine Production and Bacteremia to Promote Streptococcal Toxic Shock-Like Syndrome

Two large-scale outbreaks of streptococcal toxic shock-like syndrome (STSLS) have revealed Streptococcus suis 2 to be a severe and evolving human pathogen. We investigated the mechanism by which S. suis 2 causes STSLS. The transcript abundance of the transcriptional regulator gene tstS was found to be upregulated during experimental infection. Compared with the wild-type 05ZY strain, a tstS deletion mutant (ΔtstS) elicited reduced cytokine secretion in macrophages. In a murine infection model, tstS deletion resulted in decreased virulence and bacterial load, and affected cytokine production. Moreover, TstS expression in the P1/7 strain of S. suis led to the induction of STSLS in the infected mice. This is noteworthy because, although it is virulent, the P1/7 strain does not normally induce STSLS. Through a microarray-based comparative transcriptomics analysis, we found that TstS regulates multiple metabolism-related genes and several virulence-related genes associated with immune evasion.

Comparative immunosecretome analysis of prevalent Streptococcus suis serotypes

Streptococcus suis is a major Gram-positive swine pathogen associated with a wide variety of diseases in pigs. The efforts made to develop vaccines against this pathogen have failed because of lack of common cross-reactive antigens against different serotypes. Nowadays the interest has moved to surface and secreted proteins, as they have the highest chances to raise an effective immune response because they are in direct contact with host cells and are really exposed and accessible to antibodies. In this work, we have performed a comparative immunosecretomic approach to identify a set of immunoreactive secreted proteins common to the most prevalent serotypes of S. suis. Among the 67 proteins identified, three (SSU0020, SSU0934, and SSU0215) were those predicted extracellular proteins most widely found within the studied serotypes. These immunoreactive proteins may be interesting targets for future vaccine development as they could provide possible cross-reactivity among different serotypes of this pathogen.

Immunogenicity and protective capacity of EF-Tu and FtsZ of Streptococcus suis serotype 2 against lethal infection

Vaccine development efforts against Streptococcus suis serotype 2 (S. suis 2) are often constrained by strain/serotype antigen variability. Bioinformatics analyses revealed two highly conserved S. suis 2 factors, EF-Tu and FtsZ. Murine immunization with recombinant proteins emulsified in white oil adjuvant or eukaryotic DNA vaccine vectors provided significant protection against lethal S. suis 2 challenge. Immune responses elicited by recombinant protein immunization revealed the robust generation of humoral immune responses, with a mixed induction of Th1-type and Th2-type responses. Furthermore, the antiserum from mice immunized with recombinant proteins significantly inhibited the growth of S. suis 2 in healthy pig whole blood, suggesting the triggering of a strong opsonizing response. Histological examination found that immunizing mice with purified recombinant proteins reduced neutrophil and macrophage accumulation in brain and lung tissues after challenge with virulent S. suis. Taken together, these findings reveal that EF-Tu and FtsZ may be promising targets for subunit and DNA vaccine candidates against S. suis 2 infection.

Streptococcus suis DivIVA Protein Is a Substrate of Ser/Thr Kinase STK and Involved in Cell Division Regulation

Streptococcus suis serotype 2 is an important swine pathogen and an emerging zoonotic agent that causes severe infections. Recent studies have reported a eukaryotic-like Ser/Thr protein kinase (STK) gene and characterized its role in the growth and virulence of different S. suis 2 strains. In the present study, phosphoproteomic analysis was adopted to identify substrates of the STK protein. Seven proteins that were annotated to participate in different cell processes were identified as potential substrates, which suggests the pleiotropic effects of stk on S. suis 2 by targeting multiple pathways. Among them, a protein characterized as cell division initiation protein (DivIVA) was further investigated. In vitro analysis demonstrated that the recombinant STK protein directly phosphorylates threonine at amino acid position 199 (Thr-199) of DivIVA. This effect could be completely abolished by the T199A mutation. To determine the specific role of DivIVA in growth and division, a divIVA mutant was constructed. The ΔdivIVA strain exhibited impaired growth and division, including lower viability, enlarged cell mass, asymmetrical division caused by aberrant septum, and extremely weak pathogenicity in a mouse infection model. Collectively, our results reveal that STK regulates the cell growth and virulence of S. suis 2 by targeting substrates that are involved in different biological pathways. The inactivation of DivIVA leads to severe defects in cell division and strongly attenuates pathogenicity, thereby indicating its potential as a molecular drug target against S. suis.

Salmonella enterica serovar Choleraesuis vector delivering SaoA antigen confers protection against Streptococcus suis serotypes 2 and 7 in mice and pigs

Streptococcus suis is one of the major pathogens that cause economic losses in the swine industry worldwide. However, current bacterins only provide limited prophylactic protection in the field. An ideal vaccine against S. suis should protect pigs against the clinical diseases caused by multiple serotypes, or at least protect against the dominant serotype in a given geographic region. A new recombinant Salmonella enterica serotype Choleraesuis vaccine vector, rSC0011, that is based on the regulated delayed attenuation system and regulated delayed antigen synthesis system, was developed recently. In this study, an improved recombinant attenuated Salmonella Choleraesuis vector, rSC0016, was developed by incorporating a sopB mutation to ensure adequate safety and maximal immunogenicity. In the spleens of mice, rSC0016 colonized less than rSC0011. rSC0016 and rSC0011 colonized similarly in Peyer's patches of mice. The recombinant vaccine rSC0016(pS-SaoA) induced stronger cellular, humoral, and mucosal immune responses in mice and swine against SaoA, a conserved surface protein that is present in many S. suis serotypes, than did rSC0011(pS-SaoA) without sopB or rSC0018(pS-SaoA), which is an avirulent, chemically attenuated vaccine strain. rSC0016(pS-SaoA) provided 100% protection against S. suis serotype 2 in mice and pigs, and full cross-protection against SS7 in pigs. This new vaccine vector provides a foundation for the development of a universal vaccine against multiple serotypes of S. suis in pigs.

Suilysin Stimulates the Release of Heparin Binding Protein from Neutrophils and Increases Vascular Permeability in Mice

Most of the deaths that occurred during two large outbreaks of Streptococcus suis infections in 1998 and 2005 in China were caused by streptococcal toxic shock syndrome (STSS), which is characterized by increased vascular permeability. Heparin-binding protein (HBP) is thought to mediate the vascular leakage. The purpose of this study was to investigate the detailed mechanism underlying the release of HBP and the vascular leakage induced by S. suis. Significantly higher serum levels of HBP were detected in Chinese patients with STSS than in patients with meningitis or healthy controls. Suilysin (SLY) is an exotoxin secreted by the highly virulent strain 05ZYH33, and it stimulated the release of HBP from the polymorphonuclear neutrophils and mediated vascular leakage in mice. The release of HBP induced by SLY was caused by a calcium influx-dependent degranulation. Analyses using a pharmacological approach revealed that the release of HBP induced by SLY was related to Toll-like receptor 4, p38 mitogen-activated protein kinase, and the 1-phosphatidylinositol 3-kinase pathway. It was also dependent on a G protein-coupled seven-membrane spanning receptor. The results of this study provide new insights into the vascular leakage in STSS associated with non-Group A streptococci, which could lead to the discovery of potential therapeutic targets for STSS associated with S. suis.

Biological activities of suilysin: role in Streptococcus suis pathogenesis

Streptococcus suis is an important swine and zoonotic pathogen equipped with several virulence factors. The pore-forming toxins are the most abundant bacterial toxins and classified as critical virulence (associated) factors of several pathogens. The role of suilysin (SLY), a pore-forming cholesterol-dependent cytolysin of S. suis, as a true virulence factor is under debate. Most of the bacterial toxins have been reported to modulate the host immune system to facilitate invasion and subsequent replication of bacteria within respective host cells. SLY has been demonstrated to play an important role in the pathogenesis of S. suis infection and inflammatory response in vitro and in vivo. This review highlights the contributions of SLY to the pathogenicity of S. suis. It will address its role during the development of S. suis meningitis in pigs, as well as humans, and discuss SLY as a potential vaccine candidate.

Characterization of IgA1 protease as a surface protective antigen of Streptococcus suis serotype 2

IgA1 protease of Streptococcus suis serotype 2 (SS2) has been proven to be relative with virulence and immunogenicity, however, its protective efficacy remained to be evaluated. The present study found evidence that immunization with purified recombinant IgA1 protease (600-1926aa) could induce high IgG antibody titers and could confer complete protection against a challenge with a lethal dose of SS2 in a mouse model. In addition, our findings confirmed that the IgA1 protease distributes on the surface of SS2. Therefore, the present study identified the virulence-associated protein, IgA1 protease, as a novel surface protective antigen of SS2.

Two novel regulators of N-acetyl-galactosamine utilization pathway and distinct roles in bacterial infections

Bacterial pathogens can exploit metabolic pathways to facilitate their successful infection cycles, but little is known about roles of d‐galactosamine (GalN)/N‐acetyl‐d‐galactosamine (GalNAc) catabolism pathway in bacterial pathogenesis. Here, we report the genomic reconstruction of GalN/GalNAc utilization pathway in Streptococci and the diversified aga regulons. We delineated two new paralogous AgaR regulators for the GalN/GalNAc catabolism pathway. The electrophoretic mobility shift assays experiment demonstrated that AgaR2 (AgaR1) binds the predicted palindromes, and the combined in vivo data from reverse transcription quantitative polymerase chain reaction and RNA‐seq suggested that AgaR2 (not AgaR1) can effectively repress the transcription of the target genes. Removal of agaR2 (not agaR1) from Streptococcus suis 05ZYH33 augments significantly the abilities of both adherence to Hep‐2 cells and anti‐phagocytosis against RAW264.7 macrophage. As anticipated, the dysfunction in AgaR2‐mediated regulation of S. suis impairs its pathogenicity in experimental models of both mice and piglets. Our finding discovered two novel regulators specific for GalN/GalNAc catabolism and assigned them distinct roles into bacterial infections. To the best of our knowledge, it might represent a first paradigm that links the GalN/GalNAc catabolism pathway to bacterial pathogenesis.

Streptococcus suis vaccines: candidate antigens and progress

Streptococcus suis is a major swine pathogen and an emerging zoonotic agent of human meningitis and streptococcal toxic shock-like syndrome. S. suis is a well-encapsulated pathogen and multiple serotypes have been described based on the capsular polysaccharide antigenic diversity. In addition, high genotypic, phenotypic and geographic variability exits among strains within the same serotype. Besides, S. suis uses an arsenal of virulence factors to evade the host immune system. Together, these characteristics have challenged the development of efficacious vaccines to fight this important pathogen. In this careful and comprehensive review, clinical field information and experimental data have been compiled and compared for the first time to give a precise overview of the current status of vaccine development against S. suis. The candidate antigens and vaccine formulations under research are examined and the feasibility of reaching the goal of a "universal" cross-protective S. suis vaccine discussed.

Biology and Diseases of Swine

Swine are used in biomedical research as models for biomedical research and for teaching. This chapter covers normative biology and behavior along with common and emerging swine diseases. Xenotransplantation is discussed along with similarities and differences of swine immunology.

Increased production of suilysin contributes to invasive infection of the Streptococcus suis strain 05ZYH33

Streptococcus suis serotype 2 (SS2) is widely recognized in the veterinary world as the cause of rapidly progressive and fatal sepsis in infant pigs, manifested with meningitis, polyarthritis and pneumonia. It has evolved into a highly infectious strain, and caused two large-scale outbreaks of human epidemic in China, characterized bytypical toxic-shock syndrome and invasive infection. However, the molecular basis of virulence of this emerging zoonotic pathogen is still largely unknown. The present study shows that the sequence type (ST)7 epidemic strain S. suis 05ZYH33 causes higher mortality, higher necrosis of polymorphonuclear neutrophils and a significantly higher damage to human umbilical vein endothelial cells compared to the non-epidemic strain S. suis 1940. These differences appear to associate with the enhanced secretion of suilysin (sly) by S. suis 05ZYH33 compared to the non-epidemic strain 1940. Inclusion of additional strains confirmed that the epidemic ST7 strains produce more sly protein (mean, 1.49 g/ml; range, 0.76–1.91 g/ml) than non-epidemic strains (mean, 0.33 g/ml; range, 0.07–0.94 g/ml), and this difference is significant (P<0.001). The nonpolar mutant strain S. suis Δsly, constructed from the epidemic ST7 strain S. suis 05ZYH33 confirmed the role of sly on the enhanced virulence of S. suis ST7 strains. These findings suggest that increased sly production in S. suis 05ZYH33 facilitates penetration to the epithelium and its survival in the bloodstream, thereby contributing to the invasive infection.

Streptococcus suis infection: an emerging/reemerging challenge of bacterial infectious diseases?

Streptococcus suis (S. suis) is a family of pathogenic gram-positive bacterial strains that represents a primary health problem in the swine industry worldwide. S. suis is also an emerging zoonotic pathogen that causes severe human infections clinically featuring with varied diseases/syndromes (such as meningitis, septicemia, and arthritis). Over the past few decades, continued efforts have made significant progress toward better understanding this zoonotic infectious entity, contributing in part to the elucidation of the molecular mechanism underlying its high pathogenicity. This review is aimed at presenting an updated overview of this pathogen from the perspective of molecular epidemiology, clinical diagnosis and typing, virulence mechanism, and protective antigens contributing to its zoonosis.

Characterization and proteome analysis of inosine 5-monophosphate dehydrogenase in epidemic Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen that causes severe disease symptoms in pigs and humans. In the present study, we found one isogenic mutant lacking inosine 5-monophosphate dehydrogenase (IMPDH) ΔZY05719 was attenuated in pigs compared with the wild-type SS2 strain ZY05719. Comparative proteome analysis of the secreted proteins expression profiles between ZY05719 and ΔZY05719 allowed us to identify Triosephosphate isomerase (TPI) and glyceraldehyde phosphate dehydrogenase (GAPDH), which were down expressed in the absence of the IMPDH. Both of them are glycolytic enzymes participating in the glycolytic pathway. Compared with ZY05719, ΔZY05719 lost the ability of utilize mannose, which might relate to down expression of TPI and GAPDH. In addition, GAPDH is a well-known factor that involved in adhesion to host cells, and we demonstrated ability of adhesion to HEp-2 and PK15 by ΔZY05719 was significantly weakened, in contrast to ZY05719. The adhesion to host cells is the crucial step to cause infection for pathogen, and the reduction adhesion of ΔZY05719, to some extent illustrates the attenuated virulence of ΔZY05719.

The contribution of suilysin to the pathogenesis of Streptococcus suis meningitis

BACKGROUND

Streptococcus suis is an emerging zoonotic pathogen, and causes sepsis and meningitis in humans. Although sequence type (ST) 1 and ST104 strains are capable of causing sepsis, ST1 strains commonly cause meningitis. In this study, we investigated the role of suilysin, a member of cholesterol-dependent cytolysins, in differential pathogenicity between ST1 and ST104 strains.

METHODS

The levels of transcription and translation of the sly gene and messenger RNA of both ST strains were compared by means of quantitative polymerase chain reaction and Western blotting. Survival rates and bacterial densities in brain were compared between mice infected with wild-type and sly-knockout ST1 strain. ST104 infections with or without complementation of suilysin were also assessed.

RESULTS

The amounts of suilysin produced by ST1 strains were much higher than those produced by ST104 strains. Lower production of suilysin by ST104 strains were attributed to the attenuated sly gene expression, which seemed to be associated with 2 nucleotide insertions in sly promoter region. Furthermore, suilysin contributed to the higher bacterial density and enhanced inflammation in brain and increased mortality.

CONCLUSIONS

Our data may explain why ST1 strains, but not ST104 strains, commonly cause meningitis and also suggest the contribution of suilysin to the pathogenesis of meningitis in humans.

The genetically modified suilysin, rSLY(P353L), provides a candidate vaccine that suppresses proinflammatory response and reduces fatality following infection with Streptococcus suis

Streptococcus suis is a persistent global hazard in the swine industry and an emerging threat to public health. The high mortality in China following outbreaks of streptococcal toxic shock syndrome (STSS) underscores the urgency for effective prevention. A limited understanding of the pathogenesis of S. suis in STSS may explain the lack of biological products for prevention. Suilysin (SLY) is an important virulence factor in the pathogenesis of S. suis. To identify a candidate vaccine for S. suis-induced STSS, we constructed a recombinant non-hemolytic mutant of SLY that has hemagglutination activity, rSLY(P353L), and evaluated its ability to induce inflammatory response and prevent fatal S. suis infection in mice. The rSLY(P353L) mutant, as compared with hemolytic rSLY, elicited lower levels of IL-6, KC and IL-10 at 3h and 5h post-treatment (p<0.05), indicating that hemolytic activity is associated with rSLY-mediated inflammation. Furthermore, passive immunization with anti-SLY(P353L) antisera protected mice from acute death after infection with S. suis SC84 (p<0.05). Effects were not due to protection against tissue damage, as S. suis SC84 caused no detectable histopathological lesions in mice within 24h. However, immunization with rSLY(P353L) caused significantly reduced levels of KC and IL-1β at 6 and 9h post-challenge and IL-6 at 9h post-challenge (p<0.05). In conclusion, rSLY(P353L) may provide a potential vaccine for protection against S. suis-induced STSS due to its reduction in proinflammatory response early in S. suis infection.

Antimicrobial resistance and prudent drug use forStreptococcus suis

This paper reviews information on antimicrobial resistance patterns and prudent use of antimicrobials to reduce the impact and spread of resistantStreptococcus suisstrains.S. suisis an important pathogen in swine, which can cause significant economic loss. Prudent use of antimicrobials forS. suisis essential to preserve the therapeutic efficacy of broad-spectrum antimicrobials and to minimize selection of resistantS. suisstrains. Resistance ofS. suisto antimicrobials commonly used in swine, including lincosamides, macrolides, sulphonamides, and tetracycline, has been documented worldwide, with resistance in up to 85% of strains. Among antimicrobials examined, resistance ofS. suishas been demonstrated to be relatively low for penicillin (0–27%), ampicillin (0.6–23%), and ceftiofur (0–23%). For penicillin, this result may be due in part to the unique mechanism by which resistance is acquired through modifications in the structure of penicillin-binding proteins. Recommendations to controlS. suisinfection include focused and careful choice and appropriate use of antimicrobials, together with preventive measures intended to improve swine management.

Streptococcus suis serotype 9 bacterin immunogenicity and protective efficacy

Streptococcus suis diseases in pigs, most importantly meningitis, are worldwide responsible for major economic losses in the pig industry. About one fourth of invasive S. suis diseases are caused by S. suis serotype 9 strains in Europe. However, little is known about serotype 9 since most studies were performed with serotype 2. The objective of this study was to determine the immunogenicity and protective efficacy of a serotype 9 bacterin in piglets. Challenge was conducted with a reference serotype 9 strain, belonging to the same clonal complex but to a different sequence type as the bacterin strain. The bacterin induced protection against mortality but not morbidity. Eleven days post infection, 3 of 7 vaccinated survivors were not fully convalescent and had not eliminated the challenge strain from inner organs completely. In accordance with the clinical findings, the majority of piglets showed fibrinous-suppurative lesions in at least one inner organ or tissue. In contrast to the placebo group such lesions were not detected in one third of bacterin-vaccinated piglets. Determination of specific serum IgG titers revealed that the bacterin elicited seroconversion against muramidase-released protein and basic membrane lipoprotein. Furthermore, vaccination was associated with induction of opsonizing antibodies against the serotype 9 challenge strain. However, titers of opsonizing antibodies were rather low in comparison to those found in our previous serotype 2 vaccination trial. Piglets developed substantially higher titers of opsonizing antibodies after challenge. Opsonizing antibodies were absorbable with the serotype 9 challenge strain but not with an unencapsulated isogenic mutant of a serotype 2 strain indicating their specificity. The results indicate that a serotype 9 bacterin is less protective than a serotype 2 bacterin, most likely due to inducing only low titers of opsonizing antibodies. This might contribute to emergence of serotype 9 strains, in particular strains of this clonal complex, in Europe.

Homologous whole bacterin vaccination is not able to reduce Streptococcus suis serotype 9 strain 7997 transmission among pigs or colonization

Streptococcus suis (S. suis) is an important porcine pathogen worldwide, and antibiotics are often applied to treat or prevent clinical signs. Vaccination could be an alternative measure to reduce the abundant use of antimicrobials. The aim of this study was to determine the effect of vaccination with homologues whole bacterin vaccine containing S. suis serotype 9 strain 7997 on transmission of this serotype among pigs and on mucosal colonization. Caesarean derived, colostrum deprived pigs (N=50) were housed pair wise. Thirteen pairs were vaccinated intramuscularly with 2-3×10(9) colony forming units (CFU) inactivated S. suis serotype 9 per dose and α-tocopherolactetaat as adjuvant at 3 and 5 weeks of age; twelve pairs served as non-vaccinated controls. At 7 weeks of age, one pig of each pair was intranasally inoculated with 1-2×10(9)CFU of the homologues strain, whereas the other pig of each pair was contact-exposed. Tonsil brushings and saliva swabs were collected for 4 weeks, and tested for the presence of S. suis by bacteriological culture. No differences in number of S. suis in the tonsils or saliva samples or in clinical signs were observed between vaccinated and control pigs. In all pairs, transmission between inoculated and contact exposed pigs occurred, and no difference was observed in rate at which this occurred. The estimated transmission rate parameter β between vaccinated pigs was β(v)=5.27/day, and for non-vaccinated pigs β(nv)=2.77/day (P=0.18). It was concluded that vaccination against S. suis serotype 9 did not reduce transmission, nor colonization and that there were no indications that protection against clinical signs was induced.

Evaluation of the immunogenicity and the protective efficacy of a novel identified immunogenic protein, SsPepO, of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (S. suis 2) is an important porcine and human pathogen. Some proteins secreted by S. suis 2 are thought to play important roles in the pathogenesis of this organism and in its induced immune response. SsPepO has been previously identified as a secretary immunogenic protein using immunoproteomic techniques. In this study, we confirmed that the sequence of this protein is highly conserved in S. suis 2 and compared it with its homologues in other pathogens. To test the protective efficacy of SsPepO in animal models, the recombinant SsPepO protein was used to immunize mice and pigs. The results demonstrated that it could elicit a strong humoral antibody response and confer significant protection against challenge with a lethal dose of S. suis 2 in mice and pig models. In addition, the antisera against rSsPepO could efficiently inhibit bacterial growth in a whole blood assay and conferred significant protection against S. suis 2 infection in passive immunization experiments. Our findings suggest that SsPepO plays an important role in the pathogenesis of S. suis 2 and would be a promising subunit vaccine candidate.

Evaluation of recombinant proteins of Haemophilus parasuis strain SH0165 as vaccine candidates in a mouse model

The recently completed genome sequence of Haemophilus parasuis strain SH0165 allowed us to screen putative OMPs for the development of recombinant vaccines. The objective of this study was to evaluate the immunogenicity and protective efficacy of three OMPs of H. parasuis. Three putative OMPs (SmpA, YgiW and FOG) were cloned, expressed and purified by Ni affinity chromatography using nitriloacetic acid resin. Mice were immunized either individually (individual protein, IP) or synergistically (synergistic protein, SP) with the recombinant proteins. A significant increase in IgG titer was detected in all protein-immunized mice. Isotyping studies revealed that the antibodies produced were predominantly IgG2a-type, indicating a predominant Th1 response. A significant increase was observed in IL-2, IL-4 and IFN-γ levels in the culture supernatants of splenocytes isolated from immunized mice. Furthermore, mice were challenged intraperitoneally with 6×10(9)CFU (5×LD(50)) of highly virulent homologous serovar 5 strain (SH0165) or 7.0×10(9) CFU (5×LD(50)) of the heterologous serovar 4 strain (MD0322) at fourteen days after the last immunization. All of the recombinant proteins enhanced survival and reduced histopathological lesions. Our results indicated that the three OMPs showed protection both individually and synergistically against infection with the highly virulent H. parasuis in mice.

Identification of a novel angiogenin inhibitor 1 and its association with hyaluronidase of Streptococcus suis serotype 2

To characterize the role of hyaluronidase (Hyl) of Streptococcus suis serotype 2 (S. suis 2), the hylA gene that encodes Hyl was cloned, expressed and purified. A murine brain cDNA library was used to screen for interacting proteins of Hyl. Employing the yeast two-hybrid system, a novel murine ribonuclease, angiogenin inhibitor 1 (AI1), which shares 93% homology with porcine AI1, was shown to interact with Hyl in yeast. Through co-immunoprecipitation assays, the interaction between AI1 and Hyl was further confirmed. Transcription and translation products of the identified cDNA were detected in both normal cells and S. suis 2-infected cells. AI1 was found to localize mainly in the cytoplasm of 293T cells. These results suggested that the identified protein AI1 might be involved in the pathogenesis of meningitis through interaction with Hyl of S. suis 2.

Rapid evolution of virulence and drug resistance in the emerging zoonotic pathogen Streptococcus suis

BACKGROUND

Streptococcus suis is a zoonotic pathogen that infects pigs and can occasionally cause serious infections in humans. S. suis infections occur sporadically in human Europe and North America, but a recent major outbreak has been described in China with high levels of mortality. The mechanisms of S. suis pathogenesis in humans and pigs are poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

The sequencing of whole genomes of S. suis isolates provides opportunities to investigate the genetic basis of infection. Here we describe whole genome sequences of three S. suis strains from the same lineage: one from European pigs, and two from human cases from China and Vietnam. Comparative genomic analysis was used to investigate the variability of these strains. S. suis is phylogenetically distinct from other Streptococcus species for which genome sequences are currently available. Accordingly, approximately 40% of the approximately 2 Mb genome is unique in comparison to other Streptococcus species. Finer genomic comparisons within the species showed a high level of sequence conservation; virtually all of the genome is common to the S. suis strains. The only exceptions are three approximately 90 kb regions, present in the two isolates from humans, composed of integrative conjugative elements and transposons. Carried in these regions are coding sequences associated with drug resistance. In addition, small-scale sequence variation has generated pseudogenes in putative virulence and colonization factors.

CONCLUSIONS/SIGNIFICANCE

The genomic inventories of genetically related S. suis strains, isolated from distinct hosts and diseases, exhibit high levels of conservation. However, the genomes provide evidence that horizontal gene transfer has contributed to the evolution of drug resistance.

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.

Intranasal immunization with a live Streptococcus suis isogenic ofs mutant elicited suilysin-neutralization titers but failed to induce opsonizing antibodies and protection

In Europe, Streptococcus suis serotypes 2 and 9 are very important causative agents of meningitis, arthritis and septicemia in piglets. So far, no vaccine has been described which elicits protection against both serotypes. The working hypothesis of this study was that an isogenic serotype 2 mutant attenuated in virulence but not in colonization of the respiratory tract might induce protective immunity against both serotypes. Piglets were immunized with the attenuated S. suis serotype 2 strain 10Deltaofs2/12, which is deficient in expression of the serum opacity factor OFS. Three weeks after intranasal application of 10Deltaofs2/12 piglets were challenged intravenously with two strains representing the most important S. suis pathotypes in Europe, the homologous MRP+ EF+ SLY+ serotype 2 strain and a heterologous MRP* SLY+ serotype 9 strain. Application of the live vaccine elicited significant serum IgG responses against muramidase-released protein (MRP), extracellular factor (EF) and, most prominently, suilysin. Seroconversion against suilysin was accompanied with an increase of suilysin-neutralization titers in the vaccinated group. Though mortality was lower in the vaccinated groups, significant protection was not observed, neither against the homologous nor against the heterologous challenge. This was in agreement with the finding that the vaccinated piglets had low opsonizing antibody titers insufficient to mediate elimination of the two challenge strains by porcine neutrophils. In conclusion, a single intranasal application of the S. suis serotype 2 strain 10Deltaofs2/12 elicited humoral immune responses against different S. suis antigens but failed to induce sufficient opsonizing antibody titers and protective immunity against systemic serotypes 2 and 9 challenges.

Pathologic Analysis of the Brain from Streptococcus suis Type 2 Experimentally Infected Pigs

Streptococcus suis type 2 ( S. suis 2) is known as a major porcine pathogen worldwide and causes meningitis, septicemia, endocarditis, arthritis, and septic shock in pigs. Suilysin, a secreted protein of S. suis 2, is considered to be an important S. suis 2 virulence-associated factor. In this paper, the cerebellar lesions following experimental infection of pigs with S. suis 2 were studied. An immunohistochemical technique was applied to compare the distribution of bacteria and secreted suilysin protein in brain. The infected pigs developed histologic lesions of meningoencephalitis. Immunohistochemistry identified bacteria within the cytoplasm of neutrophils and macrophages localized in meningeal lesions. The secreted suilysin protein showed a similar localization within the cytoplasm of inflammatory cells, indicating that suilysin had high expression in vivo and may contribute to the pathogenesis of streptococcal meningoencephalitis.

Streptococcus suis bacterin and subunit vaccine immunogenicities and protective efficacies against serotypes 2 and 9

Streptococcus suis causes numerous diseases in pigs, most importantly, meningitis, arthritis, septicemia, and bronchopneumonia. One of the major problems in modern swine production is the lack of a vaccine protecting against more than one S. suis serotype. The objective of this study was to determine the protective efficacy of a serotype 2 murein-associated protein (MAP) fraction subunit vaccine in comparison to that of a bacterin against experimental challenge with serotype 2 (containing muramidase-released protein [MRP], extracellular factor, and suilysin [SLY]) and serotype 9 (containing MRP variant MRP* and SLY) strains. MAP was shown to include different surface-associated proteins, such as the MRP and surface antigen one (SAO) expressed by both pathotypes used for challenge. The results of this study demonstrated that the serotype 2 bacterin induced protective immunity against homologous challenge. In contrast, the protective efficacy of the MAP subunit vaccine was low, though MAP immunization resulted in high serum immunoglobulin G2 titers against MRP and SAO. Importantly, immunization with bacterin but not with MAP induced opsonizing antibody titers against the serotype 2 strain, and these antibody titers were found to correlate with protection. However, after absorption with a nonencapsulated isogenic mutant, the sera from bacterin-immunized piglets failed to facilitate neutrophil killing, indicating that antibodies directed against capsule may not have been essential for opsonophagocytosis. Furthermore, induction of opsonizing antibodies against serotype 9 was not detectable in the group receiving bacterin or in the group receiving the MAP vaccine. In agreement, protection against the heterologous serotype 9 strain was low in both groups. Thus, identification of an antigen protecting against these two important S. suis pathotypes remains an important goal of future studies.

Identification and characterization of novel immunogenic proteins of Streptococcus suis serotype 2

Streptococcus suis, a zoonotic pathogen, caused serious outbreaks in humans with high mortality rates in the past decade. To develop safer and more effective vaccines, particularly for human protection, cell wall and extracellular proteins of S. suis serotype 2 were analyzed by an immunoproteomic approach in this study. Thirty-two proteins with high immunogenicity were identified and 22 of them were newly identified. Further analyses of 9 selected proteins revealed that (1) these 9 proteins were expressed in all tested virulent S. suis serotype 2 isolates, (2) antisera against 6 of the selected proteins efficiently killed the bacteria by opsonized phagocytosis in human blood, and (3) significantly higher levels of serum antibodies against 3 proteins were detected in both patients and infected swines. Therefore, our results suggest the 3 proteins (SSU98_0197, SSU98_1094 and SSU1664) have strong potential to be vaccine candidates.

Immunoproteomic Assay of Membrane-associated Proteins of Streptococcus suis Type 2 China Vaccine Strain HA9801

Immunoproteomic approaches were undertaken to study the immunogenicity of the membrane-associated proteins of the Streptococcus suis type 2 (SS2) China vaccine strain HA9801. The membrane-associated proteins were enriched using the Triton X-114 extraction protocol and were analysed by two-dimensional gel electrophoresis (2-DE) and subsequent immunoblotting using the hyperimmune serum of SS2-HA9801-immunized specific pathogen free (SPF) minipigs. A total of 11 proteins were recognized, and the corresponding spots on a duplicate gel were excised and identified by MALDI-TOF MS.

Immunization with recombinant Sao protein confers protection against Streptococcus suis infection

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.

Potential use of an unencapsulated and aromatic amino acid-auxotrophic Streptococcus suis mutant as a live attenuated vaccine in swine

Streptococcus suis is responsible for severe economic losses to the swine industry. Prevention of the diseases caused by this pathogen is hampered by the inability of available vaccines to generate a protective response in pigs. A non-virulent, aromatic amino acid-auxotrophic and unencapsulated mutant strain of S. suis was generated in this study and a preliminary evaluation of its protective capacities was conducted in swine. Deletion of the cognate promoter of the aro operon in S. suis virulent strain S735 resulted in the abolishment of the expression of the four members of the operon, aroA, aroK, pheA and orf10. The resulting mutant strain BD101 was auxotrophic for aromatic amino acids as demonstrated by its failure to grow in a chemically defined medium unless it was supplemented with these compounds. In addition, as a result of the deletion of the cognate promoter of the aro operon, mutant BD101 lost its encapsulated phenotype. A protection assay was performed by immunisation of pigs with live strain BD101. Vaccination resulted in minor clinical signs but did not substantially impair the growth of vaccinated animals. Immunisation of animals with live mutant BD101 induced a considerable antibody response against S. suis. Vaccinated pigs presented only minor clinical signs and a survival rate of 100%, while 57% of non-vaccinated animals died, after a challenge with the virulent parent strain S735. In order to prevent S. suis infections in swine, it may be useful to further evaluate strain BD101 as a vaccine candidate.