Production and characterization of two Streptococcus suis capsular type 2 mutants

Study of the Role of Lipoprotein Maturation Enzymes in the Pathogenesis of the Infection Caused by the Streptococcus suis Serotype 2 Sequence Type 25 North American Prototype Strain

Streptococcus suis serotype 2 is an important swine bacterial pathogen causing sudden death, septic shock, and meningitis. However, serotype 2 strains are phenotypically and genotypically heterogeneous and composed of a multitude of sequence types (STs) whose distributions greatly vary worldwide. It has been previously shown that the lipoprotein (LPP) maturation enzymes diacylglyceryl transferase (Lgt) and signal peptidase (Lsp) significantly modulate the inflammatory host response and play a differential role in virulence depending on the genetic background of the strain. Differently from Eurasian ST1/ST7 strains, the capsular polysaccharide of a North American S. suis serotype 2 ST25 representative strain only partially masks sub-capsular domains and bacterial wall components. Thus, our hypothesis is that since LPPs would be more surface exposed in ST25 strains than in their ST1 or ST7 counterparts, the maturation enzymes would play a more important role in the pathogenesis of the infection caused by the North American strain. Using isogenic Δlgt and Δlsp mutants derived from the wild-type ST25 strain, our studies suggest that these enzymes do not seem to play a role in the interaction between S. suis and epithelial and endothelial cells, regardless of the genetics background of the strain used. However, a role in the formation of biofilms (also independently of the STs) has been demonstrated. Moreover, the involvement of LPP dendritic cell activation in vitro seems to be somehow more pronounced with the ST25 strain. Finally, the Lgt enzyme seems to play a more important role in the virulence of the ST25 strain. Although some differences between STs could be observed, our original hypothesis that LPPs would be significantly more important in ST25 strains due to a better bacterial surface exposition could not be confirmed.

Differential role of MyD88 signaling in Streptococcus suis serotype 2-induced systemic and central nervous system diseases

Streptococcus suis serotype 2 is an important porcine bacterial pathogen and a zoonotic agent responsible for sudden death, septic shock and meningitis, with exacerbated inflammation being a hallmark of the systemic and central nervous system (CNS) infections. However, S. suis serotype 2 strains are genetically and phenotypically heterogeneous, being composed of a multitude of sequence types (STs) whose virulence greatly varies. Yet, most studies have used 'classical' virulent Eurasian ST1 or ST7 strains, even though ST25 and ST28 strains account for most isolates in North America. While recognition of S. suis by innate immune cells has been associated with the myeloid differentiation primary response 88 (MyD88)-dependent Toll-like receptor (TLR) pathway in vitro, particularly surface-associated TLR2, little information is available regarding its role in vivo. This study demonstrates for the first time a differential role of MyD88 signaling in S. suis-induced systemic and CNS diseases, regardless of strain background diversity. The MyD88-dependent pathway is critical for the development of systemic disease via its role in inflammation, which subsequently controls bacterial burden. However, and differently from what has been described in vitro, TLR2 and TLR4 individually do not contribute to systemic disease, suggesting possible compensation in their absence and/or a collaborative role with other MyD88-dependent TLRs. On the other hand, CNS disease does not necessarily require MyD88 signaling and, consequently, neither TLR2 nor TLR4, suggesting a partial implication of other pathways. Finally, regardless of its notable heterogeneity, recognition of S. suis serotype 2 appears to be similar, indicating that recognized components are conserved motifs.

Recognition of Lipoproteins by Toll-like Receptor 2 and DNA by the AIM2 Inflammasome Is Responsible for Production of Interleukin-1β by Virulent Suilysin-negative Streptococcus suis Serotype 2

Streptococcus suis serotype 2 is an important porcine bacterial pathogen and zoonotic agent causing sudden death, septic shock and meningitis. These pathologies are the consequence of an exacerbated inflammatory response composed of various mediators including interleukin (IL)-1β. Elevated levels of the toxin suilysin (SLY) were demonstrated to play a key role in S. suis-induced IL-1β production. However, 95% of serotype 2 strains isolated from diseased pigs in North America, many of which are virulent, do not produce SLY. In this study, we demonstrated that SLY-negative S. suis induces elevated levels of IL-1β in systemic organs, with dendritic cells contributing to this production. SLY-negative S. suis-induced IL-1β production requires MyD88 and TLR2 following recognition of lipoproteins. However, the higher internalization rate of the SLY-negative strain results in intracellularly located DNA being recognized by the AIM2 inflammasome, which promotes IL-1β production. Finally, the role of IL-1 in host survival during the S. suis systemic infection is beneficial and conserved, regardless of SLY production, via modulation of the inflammation required to control bacterial burden. In conclusion, this study demonstrates that SLY is not required for S. suis-induced IL-1β production.

Rapid high resolution melting assay to differentiate Streptococcus suis serotypes 2, 1/2, 1, and 14

This rapid high resolution melting (HRM) assay allows distinguishing between Streptococcus suis serotype pairs 2 and 1/2 as well as 1 and 14, respectively, based on a single‐nucleotide polymorphism within capsular polysaccharide synthesis gene cluster K. This assay is easy to implement and identifies potential zoonotic serotypes.

Structural analysis and immunostimulatory potency of lipoteichoic acids isolated from three Streptococcus suis serotype 2 strains

Streptococcus suis serotype 2 is an important porcine and human pathogen. Lipoteichoic acid (LTA) from S. suis has been suggested to contribute to its virulence, and absence of d-alanylation from the S. suis LTA is associated with increased susceptibility to cationic antimicrobial peptides. Here, using high-resolution NMR spectroscopy and MS analyses, we characterized the LTA structures from three S. suis serotype 2 strains differing in virulence, sequence type (ST), and geographical origin. Our analyses revealed that these strains possess-in addition to the typical type I LTA present in other streptococci-a second, mixed-type series of LTA molecules of high complexity. We observed a ST-specific difference in the incorporation of glycosyl residues into these mixed-type LTAs. We found that strains P1/7 (ST1, high virulence) and SC84 (ST7, very high virulence) can attach a 1,2-linked α-d-Glcp residue as branching substituent to an α-d-Glcp that is 1,3-linked to glycerol phosphate moieties and that is not present in strain 89-1591 (ST25, intermediate virulence). In contrast, the latter strain could glycosylate its LTA at the glycerol O-2 position, which was not observed in the other two strains. Using LTA preparations from WT strains and from mutants with an inactivated prolipoprotein diacylglyceryl transferase, resulting in deficient lipoprotein acylation, we show that S. suis LTAs alone do not induce Toll-like receptor 2-dependent pro-inflammatory mediator production from dendritic cells. In summary, our study reveals an unexpected complexity of LTAs present in three S. suis serotype 2 strains differing in genetic background and virulence.

Role of the Streptococcus suis serotype 2 capsular polysaccharide in the interactions with dendritic cells is strain-dependent but remains critical for virulence

Streptococcus suis serotype 2 is an important porcine bacterial pathogen and zoonotic agent responsible for sudden death, septic shock, and meningitis. However, serotype 2 strains are heterogeneous, composed of a multitude of sequence types (STs) whose distribution greatly varies worldwide. Of the virulence factors presently described for S. suis, the capsular polysaccharide (CPS) is a critical factor implicated in a multitude of functions, including in impairment of phagocytosis and innate immune cell activation by masking underlying bacterial components. However, these roles have been described using Eurasian ST1 and ST7 strains, which greatly differ from North American ST25 strains. Consequently, the capacity of the CPS to mask surface antigens and putative virulence factors in non-Eurasian strains remains unknown. Herein, the role of the S. suis serotype 2 CPS of a prototype intermediate virulent North American ST25 strain, in comparison with that of a virulent European ST1 strain, with regards to interactions with dendritic cells, as well as virulence during the systemic phase of infection, was evaluated. Results demonstrated that the CPS remains critical for virulence and development of clinical disease regardless of strain background, due to its requirement for survival in blood. However, its role in the interactions with dendritic cells is strain-dependent. Consequently, certain key characteristics associated with the CPS are not necessarily applicable to all S. suis serotype 2 strains. This indicates that though certain factors may be important for S. suis serotype 2 virulence, strain background could be as determining, reiterating the need in using strains from varying backgrounds in order to better characterize the S. suis pathogenesis.

Type I Interferon Induced by Streptococcus suis Serotype 2 is Strain-Dependent and May Be Beneficial for Host Survival

Streptococcus suis serotype 2 is an important porcine bacterial pathogen and emerging zoonotic agent mainly responsible for sudden death, septic shock, and meningitis, with exacerbated inflammation being a hallmark of the infection. However, serotype 2 strains are genotypically and phenotypically heterogeneous, being composed of a multitude of sequence types (STs) whose virulence greatly varies: the virulent ST1 (Eurasia), highly virulent ST7 (responsible for the human outbreaks in China), and intermediate virulent ST25 (North America) are the most important worldwide. Even though type I interferons (IFNs) are traditionally associated with important antiviral functions, recent studies have demonstrated that they may also play an important role during infections with extracellular bacteria. Upregulation of IFN-β levels was previously observed in mice following infection with this pathogen. Consequently, the implication of IFN-β in the S. suis serotype 2 pathogenesis, which has always been considered a strict extracellular bacterium, was evaluated using strains of varying virulence. This study demonstrates that intermediate virulent strains are significantly more susceptible to phagocytosis than virulent strains. Hence, subsequent localization of these strains within the phagosome results in recognition of bacterial nucleic acids by Toll-like receptors 7 and 9, leading to activation of the interferon regulatory factors 1, 3, and 7 and production of IFN-β. Type I IFN, whose implication depends on the virulence level of the S. suis strain, is involved in host defense by participating in the modulation of systemic inflammation, which is responsible for the clearance of blood bacterial burden. As such, when induced by intermediate, and to a lesser extent, virulent S. suis strains, type I IFN plays a beneficial role in host survival. The highly virulent ST7 strain, however, hastily induces a septic shock that cannot be controlled by type I IFN, leading to rapid death of the host. A better understanding of the underlying mechanisms involved in the control of inflammation and subsequent bacterial burden could help to develop control measures for this important porcine and zoonotic agent.

The bias of experimental design, including strain background, in the determination of critical Streptococcus suis serotype 2 virulence factors

Streptococcus suis serotype 2 is an important porcine bacterial pathogen and emerging zoonotic agent mainly responsible for sudden death, septic shock, and meningitis. However, serotype 2 strains are genotypically and phenotypically heterogeneous. Though a multitude of virulence factors have been described for S. suis serotype 2, the lack of a clear definition regarding which ones are truly “critical” has created inconsistencies that have only recently been highlighted. Herein, the involvement of two factors previously described as being critical for S. suis serotype 2 virulence, whether the dipeptidyl peptidase IV and autolysin, were evaluated with regards to different ascribed functions using prototype strains belonging to important sequence types. Results demonstrate a lack of reproducibility with previously published data. In fact, the role of the dipeptidyl peptidase IV and autolysin as critical virulence factors could not be confirmed. Though certain in vitro functions may be ascribed to these factors, their roles are not unique for S. suis, probably due to compensation by other factors. As such, variations and discrepancies in experimental design, including in vitro assays, cell lines, and animal models, are an important source of differences between results. Moreover, the use of different sequence types in this study demonstrates that the role attributed to a virulence factor may vary according to the S. suis serotype 2 strain background. Consequently, it is necessary to establish standard experimental designs according to the experiment and purpose in order to facilitate comparison between laboratories. Alongside, studies should include strains of diverse origins in order to prevent erroneous and biased conclusions that could affect future studies.

Virulence Studies of Different Sequence Types and Geographical Origins of Streptococcus suis Serotype 2 in a Mouse Model of Infection

Multilocus sequence typing previously identified three predominant sequence types (STs) of Streptococcus suis serotype 2: ST1 strains predominate in Eurasia while North American (NA) strains are generally ST25 and ST28. However, ST25/ST28 and ST1 strains have also been isolated in Asia and NA, respectively. Using a well-standardized mouse model of infection, the virulence of strains belonging to different STs and different geographical origins was evaluated. Results demonstrated that although a certain tendency may be observed, S. suis serotype 2 virulence is difficult to predict based on ST and geographical origin alone; strains belonging to the same ST presented important differences of virulence and did not always correlate with origin. The only exception appears to be NA ST28 strains, which were generally less virulent in both systemic and central nervous system (CNS) infection models. Persistent and high levels of bacteremia accompanied by elevated CNS inflammation are required to cause meningitis. Although widely used, in vitro tests such as phagocytosis and killing assays require further standardization in order to be used as predictive tests for evaluating virulence of strains. The use of strains other than archetypal strains has increased our knowledge and understanding of the S. suis serotype 2 population dynamics.

Interactions of Streptococcus suis serotype 2 with human meningeal cells and astrocytes

BACKGROUND

Streptococcus suis serotype 2 is an important porcine pathogen and emerging zoonotic agent responsible for meningitis, of which different sequence types predominate worldwide. Though bacterial meningitis is defined as an exacerbated inflammation of the meninges, the underlying astrocytes of the glia limitans superficialis may also be implicated. However, the interactions between this pathogen and human meningeal cells or astrocytes remain unknown. Furthermore, the roles of well-described virulence factors (capsular polysaccharide, suilysin and cell wall modifications) in these interactions have yet to be studied. Consequently, the interactions between S. suis serotype 2 and human meningeal cells or astrocytes were evaluated for the first time in order to better understand their involvement during meningitis in humans.

RESULTS

Streptococcus suis serotype 2 adhered to human meningeal cells and astrocytes; invasion of meningeal cells was rare however, whereas invasion of astrocytes was generally more frequent. Regardless of the interaction or cell type, differences were not observed between sequence types. Though the capsular polysaccharide modulated the adhesion to and invasion of meningeal cells and astrocytes, the suilysin and cell wall modifications only influenced astrocyte invasion. Surprising, S. suis serotype 2 induced little or no inflammatory response from both cell types, but this absence of inflammatory response was probably not due to S. suis-induced cell death.

CONCLUSIONS

Though S. suis serotype 2 interacted with human meningeal cells and astrocytes, there was no correlation between sequence type and interaction. Consequently, the adhesion to and invasion of human meningeal cells and astrocytes are strain-specific characteristics. As such, the meningeal cells of the leptomeninges and the astrocytes of the glia limitans superficialis may not be directly implicated in the inflammatory response observed during meningitis in humans.

Antibody response specific to the capsular polysaccharide is impaired in Streptococcus suis serotype 2-infected animals

Streptococcus suis serotype 2 is an extracellular encapsulated bacterium that causes severe septicemia and meningitis in swine and humans. Albeit crucial in the fight against encapsulated bacteria, the nature of the capsular polysaccharide (CPS)-specific antibody (Ab) response during S. suis type 2 infection is unknown. We compared for the first time the features of CPS-specific versus protein-specific Ab responses during experimental infections with live virulent S. suis type 2 in mice. The primary protein-specific Ab response was dominated by both type 1 and 2 IgG subclasses, whereas IgM titers were more modest. The secondary protein-specific Ab response showed all of the features of a memory response with faster kinetics and boosted the titers of all Ig isotypes. In contrast, the primary CPS-specific Ab response was either inexistent or had titers only slightly higher than those in noninfected animals and was essentially composed of IgM. A poor CPS-specific memory response was observed, with only a moderate boost in IgM titers and no IgG. Both protein- and CPS-specific Ab responses were Toll-like receptor 2 independent. By using S. suis type 2 strains of European or North American origin, the poor CPS-specific Ab response was demonstrated to be independent of the genotypic/phenotypic diversity of the strain within serotype 2. Finally, the CPS-specific Ab response was also impaired and lacked isotype switching in S. suis-infected pigs, the natural host of the bacterium. The better resistance of preinfected animals to reinfection with the same strain of S. suis type 2 might thus more likely be related to the development of a protein rather than CPS Ab response.

Virulence factors involved in the pathogenesis of the infection caused by the swine pathogen and zoonotic agent Streptococcus suis

Streptococcus suis is a major swine pathogen responsible for important economic losses to the swine industry worldwide. It is also an emerging zoonotic agent of meningitis and streptococcal toxic shock-like syndrome. Since the recent recognition of the high prevalence of S. suis human disease in southeast and east Asia, the interest of the scientific community in this pathogen has significantly increased. In the last few years, as a direct consequence of these intensified research efforts, large amounts of data on putative virulence factors have appeared in the literature. Although the presence of some proposed virulence factors does not necessarily define a S. suis strain as being virulent, several cell-associated or secreted factors are clearly important for the pathogenesis of the S. suis infection. In order to cause disease, S. suis must colonize the host, breach epithelial barriers, reach and survive in the bloodstream, invade different organs, and cause exaggerated inflammation. In this review, we discuss the potential contribution of different described S. suis virulence factors at each step of the pathogenesis of the infection. Finally, we briefly discuss other described virulence factors, virulence factor candidates and virulence markers for which a precise role at specific steps of the pathogenesis of the S. suis infection has not yet been clearly established.

Phagocytosis and killing of Streptococcus suis by porcine neutrophils

Streptococcus suis serotype 2 is an important swine pathogen responsible for diverse infections, mainly meningitis. Virulence factors and the pathogenesis of infection are not well understood. Neutrophils may play an important role in the pathogenesis of infection given that infiltration by neutrophils and mononuclear cells are frequently observed in lesions caused by S. suis. The objective of this work was to study the interactions between S. suis serotype 2 and porcine neutrophils. Results showed that suilysin is toxic to neutrophils and this could help S. suis evade innate immunity. Moreover, suilysin appears to affect complement-dependent killing by decreasing the opsonization of S. suis and the bactericidal capacity of neutrophils. Our results confirm that capsule polysaccharide protects S. suis against killing and phagocytosis by neutrophils. We also showed that the presence of specific IgG against S. suis serotype 2 promoted killing by neutrophils, indicating that the induction of a strong humoral response is beneficial for clearance of this pathogen.

Expression of green fluorescent protein and its application in pathogenesis studies of serotype 2 Streptococcus suis

We investigated the interaction between type 2 Streptococcus suis and swine phagocytes during infection of the natural host, by using green fluorescent protein (GFP) as a specific marker to observe the challenge organism. We compared the strength of the S. suis sly promoter (SP332) and the synthetic promoter (CP25) in driving GFP expression. Two GFP alleles, gfpP11 and gfpmut3*, were also compared. The two promoters and two alleles were efficiently compared using three different promoter-GFP gene combinations on a shuttle vector, which were transformed into S. suis strains SX332, SX932 or M2. Plasmid pSL6.81 has SP332 with gfpP11, pSL5.24 has SP332 with gfpmut3*, and pSL5.28 has CP25 with gfpmut3*. The transformants were fluorescent with green light when viewed with an epifluorescence microscope or during flow cytometry. The signal was also detected using a laser scanning confocal microscope. The GFP expression level varied and CP25 with gfpmut3* led to greatest expression. For optimizing GFP detection, fluorescence-based cell sorting was applied to SX332(pSL5.28) and the mean fluorescence intensity increased 25.9% after optimization. Fluorescence activated cell sorting (FACS)-based phagocytosis assay showed that, without opsonization, phagocytosis rates of SX332, SX932 and M2 by both neutrophils and monocytes were similar and low. After opsonization, the phagocytosis of M2 increased 10-fold while phagocytosis of SX332 and SX932 did not change. GFP-labeled S. suis was identified in fresh pig tonsil tissue 18 h after infection. The results of this study indicated that GFP was expressed in type 2 S. suis and GFP labeled S. suis could be used in phagocytosis and pathogenesis studies.

Role of suilysin in pathogenesis of Streptococcus suis capsular serotype 2

Three suilysin (SLY) knockout mutant strains of Streptococcus suis serotype 2 were generated by allelic replacement from one North American and two European wild type strains. The mutants were characterized by Southern blot, Western blot and phenotyping. In vitro bactericidal testing showed that both wild type and SLY mutants were resistant to bactericidal factors in whole pig blood. To demonstrate the role of SLY during S. suis infection, four animal trials were carried out using young pigs. Either high dose (4 x 10(6)CFU/ml/pig) or low dose (0.5 x 10(6)CFU/ml/pig) live cell aerosol was applied to the pharynx. In one trial, a low challenge dose of North American strain SX332 and its isogenic sly(-) mutant strain (SX932) resulted in acute disease in 3/5 of pigs exposed to the wild type strain, while 5/5 of pigs exposed to the mutant strain survived the trial. In the repeat trial, 1/8 of pigs in wild type group and 6/8 of pigs in mutant group developed disease. The high dose trial with 332/932 pair showed that 4/8 pigs challenged with wild type and 5/8 of pigs challenged with mutant strain developed disease respectively. The third low dose trial, using European strain 31533 and its isogenic sly(-) mutant strain SX911, showed that 1/8 of pigs challenged with the wild type strain and 4/8 of pigs challenged with the corresponding mutant strain developed disease. All the diseased pigs showed fever, clinical signs and developed septicemia. S. suis was isolated from tissue samples such as brain, submandibular lymph node, lung, spleen, liver, heart or joint. Serum antibody titer against cell surface proteins changed little while the antibody titer against SLY increased only in the wild type group after challenge. sly gene was cloned and expressed in E. coli. The recombinant SLY (rSLY) protein showed 800 hemolysin units per microg protein. In vitro study showed that rSLY triggered TNFalpha production by human monocytes and IL-6 production by pig pulmonary alveolar macrophages and monocytes. Thus, the results of this study suggest that SLY does not seem to be a critical virulence factor for S. suis serotype 2 respiratory infection, but by stimulating cytokine release it may play a role in innate immunity.

Associations of Streptococcus suis serotype 2 ribotype profiles with clinical disease and antimicrobial resistance

A total of 122 Streptococcus suis serotype 2 strains were characterized thoroughly by comparing clinical and pathological observations, ribotype profiles, and antimicrobial resistance. Twenty-one different ribotype profiles were found and compared by cluster analysis, resulting in the identification of three ribotype clusters. A total of 58% of all strains investigated were of two ribotypes belonging to different ribotype clusters. A remarkable relationship existed between the observed ribotype profiles and the clinical-pathological observations because strains of one of the two dominant ribotypes were almost exclusively isolated from pigs with meningitis, while strains of the other dominant ribotype were never associated with meningitis. This second ribotype was isolated only from pigs with pneumonia, endocarditis, pericarditis, or septicemia. Cluster analysis revealed that strains belonging to the same ribotype cluster as one of the dominant ribotypes came from pigs that showed clinical signs similar to those of pigs infected with strains with the respective dominant ribotype profiles. Furthermore, strains belonging to different ribotype clusters had totally different patterns of resistance to antibiotics because strains isolated from pigs with meningitis were resistant to sulfamethazoxazole and strains isolated from pigs with pneumonia, endocarditis, pericarditis, or septicemia were resistant to tetracycline.

Streptococcus suis and group B Streptococcus differ in their interactions with murine macrophages

Streptococcus suis type 2 and group B Streptococcus type III (GBS) are important encapsulated bacterial species causing meningitis. In the present study we compared quantitatively the uptake and intracellular survival of S. suis type 2 and GBS type III with murine macrophages in non-opsonic conditions. The role of the capsule of both pathogens was also studied using previously obtained unencapsulated isogenic mutants. Encapsulated S. suis wild-type strain was practically not phagocytosed, while the unencapsulated mutant was easily ingested by macrophages. On the other hand, the well encapsulated GBS strain and its unencapsulated mutant were both phagocytosed in large numbers. Even if S. suis unencapsulated mutant showed a higher uptake rate than the parental strain, this value was always markedly lower than the numbers of ingested GBS strains. In addition, the intracellular survival of encapsulated and unencapsulated GBS strains was significantly higher than that of S. suis strains. These results suggest that interactions between GBS type III and S. suis type 2 with murine macrophages as well as the role of the capsule as an antiphagocytic factor are different for the two bacterial pathogens.

Immunochemical characterization of an IgG-binding protein of Streptococcus suis

Several bacterial species express surface proteins with affinity for the constant region (Fc) of immunoglobulin (Ig) of different animal species. Previous studies from our group have reported the presence of an IgG-binding protein in various serotypes of Streptococcus suis. This molecule was also shown to bind in a non-immune fashion chicken IgY and to our knowledge this characteristic is unique. In the present study, by dot-blotting, we showed that the native protein, obtained by affinity chromatography, reacted more strongly with IgG from various animal species than the denatured material. Using a competitive enzyme-linked immunosorbent assay the affinity of the native 60-kDa protein (previously identified as a 52-kDa protein) towards IgG of various animal species was compared to pig IgG. Bovine, goat and human IgG were able to compete effectively with pig IgG whereas chicken IgY constituted a poor competitor. Peptide mapping analysis using denatured protein indicated that pig and bovine IgG recognized the same proteolytic fragment whereas chicken IgY did not. The smallest proteolytic fragment that retained the binding activity towards the IgG of the different animal species tested had a molecular mass of approximately 40 kDa. Fragments with Mr < 40 kDa showed specific binding activities. That is, the smallest fragment binding pig and bovine IgG had a Mr of 30 kDa whereas for goat and human IgG a fragment of less than 16 kDa still showed binding activity. Finally, we observed that antisera raised against a heat-shock protein of Pseudomonas aeruginosa reacted with the 60-kDa S. suis protein indicating that the S. suis 60-kDa protein is a member of the 60-kDa hsp family that possesses the characteristic of binding in a non-immune way mammalian IgG and chicken IgY.

Streptococcus suis serotype 2 mutants deficient in capsular expression

Streptococcus suis serotype 2 is responsible for a wide variety of porcine infections. In addition, it is considered a zoonotic agent. Knowledge about the virulence factors for this bacterium is limited but its polysaccharide capsule is thought to be one of the most important. Transposon mutagenesis with the self-conjugative transposon Tn916 was used to obtain acapsular mutants from the virulent S. suis type 2 reference strain S735. Clones were screened by colony-dot ELISA with a monoclonal antibody specific for a type 2 capsular epitope and clones that failed to react with the antibody were characterized. Two mutants, 2A and 79, having one and two Tn916 insertions respectively, were chosen for further characterization. Absence of capsule was confirmed by coagglutination, capillary precipitation and capsular reaction tests and by transmission electron microscopy. Absence of capsular polysaccharides correlated with increased hydrophobicity and phagocytosis by both murine macrophages and porcine monocytes compared to the wild-type strain. Furthermore, both mutants were shown to be avirulent in murine and pig models of infection. Finally, mutant 2A was readily eliminated from circulation in mice compared to the wild-type strain, which persisted more than 48 h in blood. Thus, isogenic mutants defective in capsule production demonstrate the importance of capsular polysaccharides as a virulence factor for S. suis type 2.

Characterization and protective activity of a monoclonal antibody against a capsular epitope shared by Streptococcus suis serotypes 1, 2 and 1/2

A monoclonal antibody (mAb Z3) was produced using BALB/c mice immunized with whole cells of Streptococcus suis serotype 2 reference strain S735. Screening by dot-ELISA showed that mAb Z3, of isotype IgG2b, reacted only with reference strains and field isolates of S. suis serotypes 1, 2 and 1/2. The recognized epitope was demonstrated to be polysaccharide in nature by periodate oxidation, and located in the capsule, since mAb Z3 reacted with purified capsular material by immunoblotting and was able to stabilize the capsule as shown by electron microscopy. Further characterization indicated that mAb Z3 may react specifically with the sialic acid moiety of the capsule, a common constituent of the polysaccharidic capsular material of the three capsular types, since sialidase-treated cells did not react with mAb Z3 in immunoblotting or indirect ELISA. Purified mAb Z3 was shown to significantly increase the rate of phagocytosis of S. suis cells by porcine monocytes and to activate the clearance of bacteria from the circulation in experimentally infected mice. However, mAb Z3 only offered partial protection to mice challenged with a minimal lethal dose. Thus, even though the capsule of S. suis seems to be an important virulence factor, the epitope recognized by mAb Z3 does not appear to be involved in complete protection against infection.

Identification of a Streptococcus suis 60-kDa heat-shock protein using western blotting

This study was initiated to investigate the presence of stress or heat shock proteins in Streptococcus suis. SDS-PAGE and Western blotting using polyclonal and monoclonal antibodies directed against different bacterial heat shock proteins demonstrated cross-reactivity with a protein with an apparent molecular mass of 60 kDa in all S. suis serotypes tested. The 60-kDa cross-reactive protein was present in virulent and avirulent strains of S. suis serotype 2 tested. A rabbit antiserum raised against the 60-kDa S. suis protein recognized the 60-65-kDa heat shock proteins in different Gram-positive and Gram-negative bacteria. Finally, the 60-kDa heat shock protein of S. suis was shown to be mostly secreted into the culture supernatant and, to a lesser extent, cell-associated. Growth under heat stress conditions (42 degrees C) increased the expression of the 60-kDa S. suis protein. This protein is, to our knowledge, the first common antigen found in different serotypes of S. suis.

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.

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

Three groups of three pigs were vaccinated either with vaccine VAC-SLY, containing purified suilysin derived from Streptococcus suis strain P1/7 (serotype 2), or with vaccine VAC-SCF, containing most of the other extracellular antigens produced by strain P1/7 (but essentially free from suilysin), or with a placebo vaccine. The pigs were vaccinated twice at four weeks and six weeks of age and were challenged intravenously with S suis strain P1/7 at eight weeks of age. On the day of challenge, only the VAC-SLY vaccinated pigs showed an increase in haemolysin neutralisation titre. After challenge the placebo vaccinated pigs developed severe clinical signs characterised by lameness involving several joints, a depressed appearance, high temperatures and/or neurological signs. The VAC-SCF vaccinated pigs showed the same clinical signs but less severely. The VAC-SLY vaccinated pigs were the least affected and showed only mild signs which subsided more quickly than those of the other groups. A post mortem investigation and histology of brain tissue samples confirmed the clinical findings; fibrinous arthritis was less severe and less frequently observed in the VAC-SLY vaccinated pigs than in the VAC-SCF or placebo vaccinated pigs, and none of the VAC-SLY vaccinated pigs had meningitis whereas two of the VAC-SCF and two of the placebo vaccinated pigs did so. All the samples of brain, lung and tarsus taken from the VAC-SLY vaccinated pigs were sterile whereas S suis was reisolated from most of these tissues from the other groups.

Role of capsular sialic acid in virulence and resistance to phagocytosis of Streptococcus suis capsular type 2

Streptococcus suis capsular type 2 has a capsule rich in sialic acid (NANA). Sialic acid, known to be an antiphagocytic factor for many bacterial species, inhibits the activation of the alternative complement pathway. The role of capsular NANA in virulence, resistance to phagocytosis and intracellular survival of S. suis capsular type 2 was evaluated. In general, a low concentration of NANA was observed for all the S. suis strains tested. In addition, no difference could be found in NANA concentrations between strains of different virulence degrees. Sialic acid concentration increased in the virulent strain 89-1591 and the avirulent strain 90-1330 after in vivo growth with an increased capsular material thickness compared to growth in vitro. No significant difference could be found in the phagocytosis rate by porcine blood monocytes of either strain and strain 89-1591 treated with sialidase or the sialic acid-binding lectin from Sambucus nigra (SNA I). Intracellular survival of strain 89-1591 decreased after treatments with sialidase or lectin, becoming comparable to that of strain 90-1330. Finally, no difference could be seen in virulence using a murine model, even if strain 89-1591 was treated with the enzyme or the lectin. Thus, NANA does not seem to be a critical virulence factor for S. suis capsular type 2.

In vitro phagocytosis and survival of Streptococcus suis capsular type 2 inside murine macrophages

In this study, data on phagocytosis of Streptococcus suis and its survival inside macrophages are presented. Mouse peritoneal macrophages were incubated in the presence of one of five different strains of S. suis capsular type 2: a virulent wild-type strain (1591), a non-capsulated non-virulent mutant strain (M2), a poorly capsulated non-virulent mutant strain (M42), a non-virulent capsulated strain (1330), and the wild-type reference (virulent) strain S735. Opsonized or non-opsonized bacteria were incubated with macrophages in vitro and samples were obtained after 1 and 3 h incubation. Phagocytosis as well as live and dead intracellular organisms were determined by acridine orange and crystal violet staining. After 1 h incubation, non-opsonized virulent and non-virulent capsulated bacteria were poorly phagocytosed (by less than 7% of the macrophages), whereas the non-capsulated non-virulent mutant strain was highly phagocytosed (by more than 68% of the macrophages). The M42 mutant strain was more phagocytosed than the capsulated strains but less than the non-capsulated M2 mutant strain (35%). In contrast, a higher percentage of live bacteria was observed inside macrophages for the capsulated strains (1591 and S735) than for the non- or poorly capsulated mutant strains (M2 and M42). Opsonization of bacteria with rabbit serum or heat-inactivated rabbit serum significantly increased phagocytosis. For every opsonized strain, after 3 h incubation, the percentage of live bacteria within macrophages was considerably lower than the corresponding non-opsonized strains. In conclusion, the capsule of S. suis type 2 appears to act as an important anti-phagocytic factor. However, virulent capsulated non-opsonized strains can be phagocytosed by mouse peritoneal macrophages within which they appear to survive for at least 3 h. Serum factors other than complement increase not only phagocytosis but also intracellular killing of S. suis of both capsulated and non-capsulated strains.

Streptococcus pneumoniae types 19A and 19F and Streptococcus suis capsular type 8 share common capsular epitopes

Two monoclonal antibodies (MAbs) to Streptococcus pneumoniae types 19A and 19F were tested with the 35 reference strains and 334 field strains of Streptococcus suis by dot blotting. Both MAbs reacted with the capsular type 8 reference strain, and one reacted with 69% and one reacted with 100% of 81 S. suis capsular type 8 field strains tested. Epitopes recognized by both MAbs are capsular in origin.

Agglutination of Streptococcus suis by sialic acid-binding lectins

The 35 Streptococcus suis capsular-type reference strains as well as 45 field strains of type 2 were tested with sialic acid-binding lectins from Sambucus nigra (SNA I), Triticum vulgaris, Maackia amurensis, Homarus americanus, and Limax flavus. Only types 1, 1/2, 2, 14, 15, and 16 agglutinated with SNA I and/or the T. vulgaris lectin. All field strains agglutinated only with SNA I. Reaction with SNA I was probably due to the sialic acid moiety since it disappeared after sialidase treatment. These results confirm the presence of sialic acid in S. suis with the possible terminal sequence N-acetylneuraminic acid-alpha(2,6)GalNAc.

Properties of a Streptococcus suis isolate of serotype 2 and two capsular mutants

Encapsulation is thought to be a critical virulence factor of Streptococcus suis. In the present study two capsular type 2 mutants of S. suis (M42 and M2) and their S. suis parent strain (89-1591) were further characterized. All three cultures reacted with group D specific antiserum whereas parent strain 89-1591 and mutant M42 but not mutant M2 reacted with specific antiserum against capsular type 2. Both mutants had higher surface hydrophobicity and showed an increased adherence to human epithelial cells and to lung macrophages of rabbits as compared to the parent strain. In phagocytosis experiments with polymorphonuclear leucocytes the encapsulated parent strain was more resistant to phagocytosis than both mutant strains. These findings might help to understand the role of encapsulation of S. suis in the process of infection.

Purification and characterization of a 52-kilodalton immunoglobulin G-binding protein from Streptococcus suis capsular type 2

We previously reported that group D streptococci exhibited immunoglobulin G (IgG)-binding activity and that a 52-kDa IgG-binding protein was present in all Streptococcus suis strains examined (B. Serhir, R. Higgins, B. Foiry, and M. Jacques, J. Gen. Microbiol. 139:2953-2958, 1993). The objective of the present study was to purify and characterize this protein. Pig IgG were immobilized through their Fab fragments to ECH-Sepharose 4B, and the protein was purified by affinity chromatography. Electron microscopy observations of the purified material showed filamentous structures with a diameter of approximately 4 nm; these structures were not observed when the material was treated with either urea or ethanolamine. Electrophoretic and Western immunoblot analyses showed that the 52-kDa protein constituted the bulk of the recovered material. This protein was stained with either Coomassie brilliant blue or silver nitrate; it reacted with a large variety of mammalian IgG, human IgG (Fc) fragments, human IgA, and other human plasma proteins. The 52-kDa protein exhibited lower IgG-binding affinities than protein A and protein G. However, it was able to compete with protein A and protein G for binding to human IgG. In addition, it bound chicken IgG with high affinity. This last property differentiated the 52-kDa protein of S. suis from the six IgG-binding proteins described to date. The 52-kDa protein displayed similar affinities for untreated and deglycosylated pig IgG. The N-terminal amino acid sequence (SIITDVYAXEVLDSXGNPTLEV) revealed no homology with any bacterial proteins in the Swiss-Prot database. Its isoelectric point of approximately 4.6 and its amino acid composition, rich in aspartic and glutamic acids, showed that it had some similarities with other IgG-binding proteins. In this report, we have purified and characterized a 52-kDa IgG-binding protein from S. suis capsular type 2. Although this protein shares some similarities with other IgG- and/or IgA-binding proteins, it is unique in reacting with chicken IgG.

Discrimination of virulent and avirulent Streptococcus suis capsular type 2 isolates from different geographical origins

In an effort to relate the protein profile to virulence, proteins from the cellular fractions and from culture supernatants of Streptococcus suis capsular type 2 strains from different geographical origins were compared by using Western blots (immunoblots). The protein profiles of the cellular fractions were similar for the majority of virulent and avirulent isolates studied, with the exception of three virulent Canadian strains for which a 135-kDa protein was not detected. Examination of the culture supernatants revealed the presence of a 135-kDa protein in all strains except the same three virulent Canadian isolates. In addition, a 110-kDa protein was present in 14 of 16 virulent strains and not in avirulent isolates. When injected into mice, the 110-kDa protein induced an immunoglobulin G response and protected against infection with homologous and heterologous virulent strains. Four strains (1330, 0891, TD10, and R75/S2) that were avirulent in the mouse model of infection and four other strains (1591, 999, JL590, and AAH4) that were virulent in the mouse model were injected into pigs. All virulent strains reproduced the disease, and all avirulent strains failed to reproduce the disease (with the exception of transient lameness in one case and fever in another case). The 110-kDa protein therefore appears to be a reliable virulence marker and a good candidate for a subunit vaccine.

Relation between encapsulation and various properties of Streptococcus suis

Encapsulation is thought to be a critical virulence factor for Streptococcus suis. In this study, encapsulation of a selected S. suis culture could be stimulated by cultivation conditions. After cultivation of the S. suis culture in fluid media supplemented with serum, the culture grew with short chains and a uniform turbidity of the growth medium. Autoclave extracts of this culture reacted with capsular type-1-specific antiserum. After cultivation of the S. suis culture in fluid media without serum, the bacteria generally formed longer chains, grew in fluid medium as granular sediment with clear supernatant, and were non-typeable. In addition, the unencapsulated variant appeared to have a more hydrophobic surface and adhered significantly more to HeLa cells. In contrast to the unencapsulated variant, the encapsulated S. suis was phagocytosed to a lesser extent by polymorphonuclear leucocytes. These findings might help to elucidate the role of encapsulation in infections with this bacterial organism.

Identification, purification, and characterization of a thiol-activated hemolysin (suilysin) of Streptococcus suis

The present report describes the identification, purification, and characterization of a hemolysin produced by Streptococcus suis type 2. The hemolysin was purified from the culture supernatant by using different filtration steps, Superose-12 column chromatography, and selective (NH4)2SO4 precipitation. The purified hemolysin, designated suilysin, had an apparent molecular mass of 54,000 Da and exhibited a specific activity of 0.7 x 10(6) hemolytic units per mg. Suilysin appeared to belong to a family of toxins known as the thiol-activated toxins, with which it had several characteristics in common: loss of activity upon oxidation, reactivation upon reduction, and inhibition of activity by small amounts of cholesterol. The N-terminal amino acid sequence of suilysin showed many similarities with parts of the deduced N-terminal amino acid sequences of perfringolysin O, streptolysin O, listeriolysin O, alveolysin, and pneumolysin. Mice immunized with a vaccine containing purified suilysin appeared to be completely protected against a lethal S. suis type 2 challenge, indicating that suilysin is an important factor and that the neutralization of this single factor is sufficient to protect mice against the detrimental effects of an S. suis type 2 infection. Most of the different (serotype) strains appeared to secrete hemolytic activity which was biochemically and immunologically indistinguishable from suilysin into the culture supernatant in vitro, indicating that suilysin might be a cross-protection factor.