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

ARF6 promotes Streptococcus suis suilysin induced apoptosis in HBMECs

Streptococcus suis (S. suis) is a significant zoonotic microorganism that causes a severe illness in both pigs and humans and is characterized by severe meningitis and septicemia. Suilysin (SLY), which is secreted by S. suis, plays a crucial role as a virulence factor in the disease. To date, the interaction between SLY and host cells is not fully understood. In this study, we identified the interacting proteins between SLY and human brain microvascular endothelial cells (HBMECs) using the TurboID-mediated proximity labeling method. 251 unique proteins were identified in TurboID-SLY treated group, of which six plasma membrane proteins including ARF6, GRK6, EPB41L5, DSC1, TJP2, and PNN were identified. We found that the proteins capable of interacting with SLY are ARF6 and PNN. Subsequent investigations revealed that ARF6 substantially increased the invasive ability of S. suis in HBMECs. Furthermore, ARF6 promoted SLY-induced the activation of p38 MAPK signaling pathway in HBMECs. Moreover, ARF6 promoted the apoptosis in HBMECs through the activation of p38 MAPK signaling pathway induced by SLY. Finally, we confirmed that ARF6 could increase the virulence of SLY in C57BL/6 mice. These findings offer valuable insights that contribute to a deeper understanding of the pathogenic mechanism of SLY.

Theaflavin Ameliorates Streptococcus suis-Induced Infection In Vitro and In Vivo

Streptococcus suis (S. suis) is one of the most important zoonotic pathogens that threaten the lives of pigs and humans. Even worse, the increasingly severe antimicrobial resistance in S. suis is becoming a global issue. Therefore, there is an urgent need to discover novel antibacterial alternatives for the treatment of S. suis infection. In this study, we investigated theaflavin (TF1), a benzoaphenone compound extracted from black tea, as a potential phytochemical compound against S. suis. TF1 at MIC showed significant inhibitory effects on S. suis growth, hemolytic activity, and biofilm formation, and caused damage to S. suis cells in vitro. TF1 had no cytotoxicity and decreased adherent activity of S. suis to the epithelial cell Nptr. Furthermore, TF1 not only improved the survival rate of S. suis-infected mice but also reduced the bacterial load and the production of IL-6 and TNF-α. A hemolysis test revealed the direct interaction between TF1 and Sly, while molecular docking showed TF1 had a good binding activity with the Glu198, Lys190, Asp111, and Ser374 of Sly. Moreover, virulence-related genes were downregulated in the TF1-treated group. Collectively, our findings suggested that TF1 can be used as a potential inhibitor for treating S. suis infection in view of its antibacterial and antihemolytic activity.

Role of Metabolic Adaptation of Streptococcus suis to Host Niches in Bacterial Fitness and Virulence

Streptococcus suis, both a common colonizer of the porcine upper respiratory tract and an invasive pig pathogen, successfully adapts to different host environments encountered during infection. Whereas the initial infection mainly occurs via the respiratory tract, in a second step, the pathogen can breach the epithelial barrier and disseminate within the whole body. Thereby, the pathogen reaches other organs such as the heart, the joints, or the brain. In this review, we focus on the role of S. suis metabolism for adaptation to these different in vivo host niches to encounter changes in nutrient availability, host defense mechanisms and competing microbiota. Furthermore, we highlight the close link between S. suis metabolism and virulence. Mutants deficient in metabolic regulators often show an attenuation in infection experiments possibly due to downregulation of virulence factors, reduced resistance to nutritive or oxidative stress and to phagocytic activity. Finally, metabolic pathways as potential targets for new therapeutic strategies are discussed. As antimicrobial resistance in S. suis isolates has increased over the last years, the development of new antibiotics is of utmost importance to successfully fight infections in the future.

Identification of Streptococcus suis putative zoonotic virulence factors: A systematic review and genomic meta-analysis

Streptococcus suis is an emerging zoonotic pathogen. Over 100 putative virulence factors have been described, but it is unclear to what extent these virulence factors could contribute to zoonotic potential of S. suis. We identified all S. suis virulence factors studied in experimental models of human origin in a systematic review and assessed their contribution to zoonotic potential in a subsequent genomic meta-analysis. PubMed and Scopus were searched for English-language articles that studied S. suis virulence published until 31 March 2021. Articles that analyzed a virulence factor by knockout mutation, purified protein, and/or recombinant protein in a model of human origin, were included. Data on virulence factor, strain characteristics, used human models and experimental outcomes were extracted. All publicly available S. suis genomes with available metadata on host, disease status and country of origin, were included in a genomic meta-analysis. We calculated the ratio of the prevalence of each virulence factor in human and pig isolates. We included 130 articles and 1703 S. suis genomes in the analysis. We identified 53 putative virulence factors that were encoded by genes which are part of the S. suis core genome and 26 factors that were at least twice as prevalent in human isolates as in pig isolates. Hhly3 and NisK/R were particularly enriched in human isolates, after stratification by genetic lineage and country of isolation. This systematic review and genomic meta-analysis have identified virulence factors that are likely to contribute to the zoonotic potential of S. suis.

The type II histidine triad protein HtpsC facilitates invasion of epithelial cells by highly virulent Streptococcus suis serotype 2

Streptococcus suis serotype 2 (S. suis 2) is an important zoonotic pathogen that presents a significant threat both to pigs and to workers in the pork industry. The initial steps of S. suis 2 pathogenesis are unclear. In this study, we found that the type II histidine triad protein HtpsC from the highly virulent Chinese isolate 05ZYH33 is structurally similar to internalin A (InlA) from Listeria monocytogenes, which plays an important role in mediating listerial invasion of epithelial cells. To determine if HtpsC and InlA function similarly, an isogenic htpsC mutant (ΔhtpsC) was generated in S. suis by homologous recombination. The htpsC deletion strain exhibited a diminished ability to adhere to and invade epithelial cells from different sources. Double immunofluorescence microscopy also revealed reduced survival of the ΔhtpsC mutant after co-cultivation with epithelium. Adhesion to epithelium and invasion by the wild type strain was inhibited by a monoclonal antibody against E-cadherin. In contrast, the htpsC-deficient mutant was unaffected by the same treatment, suggesting that E-cadherin is the host-cell receptor that interacts with HtpsC and facilitates bacterial internalization. Based on these results, we propose that HtpsC is involved in the process by which S. suis 2 penetrates host epithelial cells, and that this protein is an important virulence factor associated with cell adhesion and invasion.

Streptococcus suis pathogenesis-A diverse array of virulence factors for a zoonotic lifestyle

Streptococcus suis is a major cause of respiratory tract and invasive infections in pigs and is responsible for a substantial disease burden in the pig industry. S. suis is also a significant cause of bacterial meningitis in humans, particularly in South East Asia. S. suis expresses a wide array of virulence factors, and although many are described as being required for disease, no single factor has been demonstrated to be absolutely required. The lack of uniform distribution of known virulence factors among individual strains and lack of evidence that any particular virulence factor is essential for disease makes the development of vaccines and treatments challenging. Here we review the current understanding of S. suis virulence factors and their role in the pathogenesis of this important zoonotic pathogen.

Bordetella bronchiseptica promotes adherence, colonization, and cytotoxicity of Streptococcus suis in a porcine precision-cut lung slice model

Bordetella (B.) bronchiseptica and Streptococcus (S.) suis are major pathogens in pigs, which are frequently isolated from co-infections in the respiratory tract and contribute to the porcine respiratory disease complex (PRDC). Despite the high impact of co-infections on respiratory diseases of swine (and other hosts), very little is known about pathogen-pathogen-host interactions and the mechanisms of pathogenesis. In the present study, we established a porcine precision-cut lung slice (PCLS) model to analyze the effects of B. bronchiseptica infection on adherence, colonization, and cytotoxic effects of S. suis. We hypothesized that induction of ciliostasis by a clinical isolate of B. bronchiseptica may promote subsequent infection with a virulent S. suis serotype 2 strain. To investigate this theory, we monitored the ciliary activity by light microscopy, measured the release of lactate dehydrogenase, and calculated the number of PCLS-associated bacteria. To study the role of the pore-forming toxin suilysin (SLY) in S. suis-induced cytotoxicity, we included a SLY-negative isogenic mutant and the complemented mutant strain. Furthermore, we analyzed infected PCLS by histopathology, immunofluorescence microscopy, and field emission scanning electron microscopy. Our results showed that pre-infection with B. bronchiseptica promoted adherence, colonization, and, as a consequence of the increased colonization, the cytotoxic effects of S. suis, probably by reduction of the ciliary activity. Moreover, cytotoxicity induced by S. suis is strictly dependent on the presence of SLY. Though the underlying molecular mechanisms remain to be fully clarified, our results clearly support the hypothesis that B. bronchiseptica paves the way for S. suis infection.

Inhibitory Effect of Piceatannol on Streptococcus suis Infection Both in vitro and in vivo

Suilysin (SLY) plays a critical role in Streptococcus suis infections making it an ideal target to the combat infection caused by this pathogen. In the present study, we found that piceatannol (PN), a natural compound, inhibits pore-formation by blocking the oligomerization of SLY without affecting the growth of S. suis and the expression of SLY. Furthermore, PN alleviated the J774 cell damage and the expression of the inflammatory cytokine tumor necrosis factor-α (TNF-α) and interleukin-1α (IL-1β) induced by S. suis in vitro. The computational biology and biochemistry results indicated that PN binds to the joint region of D2 and D4 in SLY, and Asn57, Pro58, Pro59, Glu76, Ile379, Glu380, and Glu418 were critical residues involved in the binding. The binding effect between PN and SLY hindered the SLY monomers from forming the oligomers, thereby weakening the hemolytic activity of SLY. This mechanism was also verified by hemolysis analysis and analysis of K_A formation after site-specific mutagenesis. Furthermore, PN protected mice from S. suis infections by reducing bacterial colony formation and the inflammatory response in target organs in vivo. These results indicate that PN is a feasible drug candidate to combat S. suis infections.

Update on Streptococcus suis Research and Prevention in the Era of Antimicrobial Restriction: 4th International Workshop on S. suis

Streptococcus suis is a swine pathogen and a zoonotic agent afflicting people in close contact with infected pigs or pork meat. Sporadic cases of human infections have been reported worldwide. In addition, S. suis outbreaks emerged in Asia, making this bacterium a primary health concern in this part of the globe. In pigs, S. suis disease results in decreased performance and increased mortality, which have a significant economic impact on swine production worldwide. Facing the new regulations in preventive use of antimicrobials in livestock and lack of effective vaccines, control of S. suis infections is worrisome. Increasing and sharing of knowledge on this pathogen is of utmost importance. As such, the pathogenesis and epidemiology of the infection, antimicrobial resistance, progress on diagnosis, prevention, and control were among the topics discussed during the 4th International Workshop on Streptococcus suis (held in Montreal, Canada, June 2019). This review gathers together recent findings on this important pathogen from lectures performed by lead researchers from several countries including Australia, Canada, France, Germany, Japan, Spain, Thailand, The Netherlands, UK, and USA. Finally, policies and recommendations for the manufacture, quality control, and use of inactivated autogenous vaccines are addressed to advance this important field in veterinary medicine.

Membrane Binding, Cellular Cholesterol Content and Resealing Capacity Contribute to Epithelial Cell Damage Induced by Suilysin of Streptococcus suis

Streptococcus (S.) suis is a major cause of economic losses in the pig industry worldwide and is an emerging zoonotic pathogen. One important virulence-associated factor is suilysin (SLY), a toxin that belongs to the family of cholesterol-dependent pore-forming cytolysins (CDC). However, the precise role of SLY in host–pathogen interactions is still unclear. Here, we investigated the susceptibility of different respiratory epithelial cells to SLY, including immortalized cell lines (HEp-2 and NPTr cells), which are frequently used in in vitro studies on S. suis virulence mechanisms, as well as primary porcine respiratory cells, which represent the first line of barrier during S. suis infections. SLY-induced cell damage was determined by measuring the release of lactate dehydrogenase after infection with a virulent S. suis serotype 2 strain, its isogenic SLY-deficient mutant strain, or treatment with the recombinant protein. HEp-2 cells were most susceptible, whereas primary epithelial cells were hardly affected by the toxin. This prompted us to study possible explanations for these differences. We first investigated the binding capacity of SLY using flow cytometry analysis. Since binding and pore-formation of CDC is dependent on the membrane composition, we also determined the cellular cholesterol content of the different cell types using TLC and HPLC. Finally, we examined the ability of those cells to reseal SLY-induced pores using flow cytometry analysis. Our results indicated that the amount of membrane-bound SLY, the cholesterol content of the cells, as well as their resealing capacity all affect the susceptibility of the different cells regarding the effects of SLY. These findings underline the differences of in vitro pathogenicity models and may further help to dissect the biological role of SLY during S. suis infections.

Viral Coinfection Replaces Effects of Suilysin on Streptococcus suis Adherence to and Invasion of Respiratory Epithelial Cells Grown under Air-Liquid Interface Conditions

Streptococcus suis is an important zoonotic pathogen which can infect humans and pigs worldwide, posing a potential risk to global public health. Suilysin, a pore-forming cholesterol-dependent cytolysin, is considered to play an important role in the pathogenesis of S. suis infections. It is known that infection with influenza A viruses may favor susceptibility to secondary bacterial infection, resulting in more severe disease and increased mortality. However, the molecular mechanisms underlying these coinfections are incompletely understood. Applying highly differentiated primary porcine respiratory epithelial cells grown under air-liquid interface (ALI) conditions, we analyzed the contribution of swine influenza viruses (SIV) to the virulence of S. suis, with a special focus on its cytolytic toxin, suilysin. We found that during secondary bacterial infection, suilysin of S. suis contributed to the damage of well-differentiated respiratory epithelial cells in the early stage of infection, whereas the cytotoxic effects induced by SIV became prominent at later stages of infection. Prior infection by SIV enhanced the adherence to and colonization of porcine airway epithelial cells by a wild-type (wt) S. suis strain and a suilysin-negative S. suis mutant in a sialic acid-dependent manner. A striking difference was observed with respect to bacterial invasion. After bacterial monoinfection, only the wt S. suis strain showed an invasive phenotype, whereas the mutant remained adherent. When the epithelial cells were preinfected with SIV, the suilysin-negative mutant also showed an invasion capacity. Therefore, we propose that coinfection with SIV may compensate for the lack of suilysin in the adherence and invasion process of suilysin-negative S. suis.

Neutrophil extracellular trap formation in the Streptococcus suis-infected cerebrospinal fluid compartment

Streptococcus suis is an important meningitis-causing pathogen in pigs and humans. Neutrophil extracellular traps (NETs) have been identified as host defense mechanism against different pathogens. Here, NETs were detected in the cerebrospinal fluid (CSF) of S. suis-infected piglets despite the presence of active nucleases. To study NET-formation and NET-degradation after transmigration of S. suis and neutrophils through the choroid plexus epithelial cell barrier, a previously described model of the human blood-CSF barrier was used. NETs and respective entrapment of streptococci were recorded in the "CSF compartment" despite the presence of active nucleases. Comparative analysis of S. suis wildtype and different S. suis nuclease mutants did not reveal significant differences in NET-formation or bacterial survival. Interestingly, transcript expression of the human cathelicidin LL-37, a NET-stabilizing factor, increased after transmigration of neutrophils through the choroid plexus epithelial cell barrier. In good accordance, the porcine cathelicidin PR-39 was significantly increased in CSF of piglets with meningitis. Furthermore, we confirmed that PR-39 is associated with NETs in infected CSF and inhibits neutrophil DNA degradation by bacterial nucleases. In conclusion, neutrophils form NETs after breaching the infected choroid plexus epithelium, and those NETs may be protected by antimicrobial peptides against bacterial nucleases.

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.

Astrocytes Enhance Streptococcus suis-Glial Cell Interaction in Primary Astrocyte-Microglial Cell Co-Cultures

Streptococcus (S.) suis infections are the most common cause of meningitis in pigs. Moreover, S. suis is a zoonotic pathogen, which can lead to meningitis in humans, mainly in adults. We assume that glial cells may play a crucial role in host-pathogen interactions during S. suis infection of the central nervous system. Glial cells are considered to possess important functions during inflammation and injury of the brain in bacterial meningitis. In the present study, we established primary astrocyte-microglial cell co-cultures to investigate interactions of S. suis with glial cells. For this purpose, microglial cells and astrocytes were isolated from new-born mouse brains and characterized by flow cytometry, followed by the establishment of astrocyte and microglial cell mono-cultures as well as astrocyte-microglial cell co-cultures. In addition, we prepared microglial cell mono-cultures co-incubated with uninfected astrocyte mono-culture supernatants and astrocyte mono-cultures co-incubated with uninfected microglial cell mono-culture supernatants. After infection of the different cell cultures with S. suis, bacteria-cell association was mainly observed with microglial cells and most prominently with a non-encapsulated mutant of S. suis. A time-dependent induction of NO release was found only in the co-cultures and after co-incubation of microglial cells with uninfected supernatants of astrocyte mono-cultures mainly after infection with the capsular mutant. Only moderate cytotoxic effects were found in co-cultured glial cells after infection with S. suis. Taken together, astrocytes and astrocyte supernatants increased interaction of microglial cells with S. suis. Astrocyte-microglial cell co-cultures are suitable to study S. suis infections and bacteria-cell association as well as NO release by microglial cells was enhanced in the presence of astrocytes.

Efficient suilysin-mediated invasion and apoptosis in porcine respiratory epithelial cells after streptococcal infection under air-liquid interface conditions

Streptococci may colonize the epithelium in the airways and other entry sites. While local infection often remains asymptomatic, severe or even fatal diseases occur when streptococci become invasive and spread to different sites in the infected host. We have established porcine respiratory air-liquid interface cultures (ALI) from the porcine lung to analyze the interaction of streptococci with their primary target cells. As representative of the streptococcal family we chose Streptococcus suis (S. suis) that is not only a major swine respiratory pathogen but can also infect humans. Suilysin, a cholesterol-dependent cytolysin (CDC), is an important virulence factor. By comparing a S. suis wt strain with a suilysin-deficient mutant, we demonstrate that suilysin contributes to (i) adherence to airway cells (ii) loss of ciliated cells (iii) apoptosis, and (iv) invasion. Furthermore, we show that cytolytic activity of suilysin is crucial for these effects. A striking result of our analysis was the high efficiency of S. suis-induced apoptosis and invasion upon infection under ALI conditions. These properties have been reported to be less efficient when analyzed with immortalized cells. We hypothesize that soluble effectors such as suilysin are present at higher concentrations in cells kept at ALI conditions and thus more effective. These results should be relevant also for infection of the respiratory tract by other respiratory pathogens.

Identification and Characterization of msf, a Novel Virulence Factor in Haemophilus influenzae

Haemophilus influenzae is an opportunistic pathogen. The emergence of virulent, non-typeable strains (NTHi) emphasizes the importance of developing new interventional targets. We screened the NTHi supragenome for genes encoding surface-exposed proteins suggestive of immune evasion, identifying a large family containing Sel1-like repeats (SLRs). Clustering identified ten SLR-containing gene subfamilies, each with various numbers of SLRs per gene. Individual strains also had varying numbers of SLR-containing genes from one or more of the subfamilies. Statistical genetic analyses of gene possession among 210 NTHi strains typed as either disease or carriage found a significant association between possession of the SlrVA subfamily (which we have termed, macrophage survival factor, msf) and the disease isolates. The PittII strain contains four chromosomally contiguous msf genes. Deleting all four of these genes (msfA1-4) (KO) resulted in a highly significant decrease in phagocytosis and survival in macrophages; which was fully complemented by a single copy of the msfA1 gene. Using the chinchilla model of otitis media and invasive disease, the KO strain displayed a significant decrease in fitness compared to the WT in co-infections; and in single infections, the KO lost its ability to invade the brain. The singly complemented strain showed only a partial ability to compete with the WT suggesting gene dosage is important in vivo. The transcriptional profiles of the KO and WT in planktonic growth were compared using the NTHi supragenome array, which revealed highly significant changes in the expression of operons involved in virulence and anaerobiosis. These findings demonstrate that the msfA1-4 genes are virulence factors for phagocytosis, persistence, and trafficking to non-mucosal sites.

Dynamic Virus-Bacterium Interactions in a Porcine Precision-Cut Lung Slice Coinfection Model: Swine Influenza Virus Paves the Way for Streptococcus suis Infection in a Two-Step Process

Swine influenza virus (SIV) and Streptococcus suis are common pathogens of the respiratory tract in pigs, with both being associated with pneumonia. The interactions of both pathogens and their contribution to copathogenesis are only poorly understood. In the present study, we established a porcine precision-cut lung slice (PCLS) coinfection model and analyzed the effects of a primary SIV infection on secondary infection by S. suis at different time points. We found that SIV promoted adherence, colonization, and invasion of S. suis in a two-step process. First, in the initial stages, these effects were dependent on bacterial encapsulation, as shown by selective adherence of encapsulated, but not unencapsulated, S. suis to SIV-infected cells. Second, at a later stage of infection, SIV promoted S. suis adherence and invasion of deeper tissues by damaging ciliated epithelial cells. This effect was seen with a highly virulent SIV subtype H3N2 strain but not with a low-virulence subtype H1N1 strain, and it was independent of the bacterial capsule, since an unencapsulated S. suis mutant behaved in a way similar to that of the encapsulated wild-type strain. In conclusion, the PCLS coinfection model established here revealed novel insights into the dynamic interactions between SIV and S. suis during infection of the respiratory tract. It showed that at least two different mechanisms contribute to the beneficial effects of SIV for S. suis, including capsule-mediated bacterial attachment to SIV-infected cells and capsule-independent effects involving virus-mediated damage of ciliated epithelial cells.

Suicin 3908, a new lantibiotic produced by a strain of Streptococcus suis serotype 2 isolated from a healthy carrier pig

While Streptococcus suis serotype 2 is known to cause severe infections in pigs, it can also be isolated from the tonsils of healthy animals that do not develop infections. We hypothesized that S. suis strains in healthy carrier pigs may have the ability to produce bacteriocins, which may contribute to preventing infections by pathogenic S. suis strains. Two of ten S. suis serotype 2 strains isolated from healthy carrier pigs exhibited antibacterial activity against pathogenic S. suis isolates. The bacteriocin produced by S. suis 3908 was purified to homogeneity using a three-step procedure: ammonium sulfate precipitation, cationic exchange HPLC, and reversed-phase HPLC. The bacteriocin, called suicin 3908, had a low molecular mass; was resistant to heat, pH, and protease treatments; and possessed membrane permeabilization activity. Additive effects were obtained when suicin 3908 was used in combination with penicillin G or amoxicillin. The amino acid sequence of suicin 3908 suggested that it is lantibiotic-related and made it possible to identify a bacteriocin locus in the genome of S. suis D12. The putative gene cluster involved in suicin production by S. suis 3908 was amplified by PCR, and the sequence analysis revealed the presence of nine open reading frames (ORFs), including the structural gene and those required for the modification of amino acids, export, regulation, and immunity. Suicin 3908, which is encoded by the suiA gene, exhibited approximately 50% identity with bovicin HJ50 (Streptococcus bovis), thermophilin 1277 (Streptococcus thermophilus), and macedovicin (Streptococcus macedonicus). Given that S. suis 3908 cannot cause infections in animal models, that it is susceptible to conventional antibiotics, and that it produces a bacteriocin with antibacterial activity against all pathogenic S. suis strains tested, it could potentially be used to prevent infections and to reduce antibiotic use by the swine industry.

Host-pathogen Interaction at the Intestinal Mucosa Correlates With Zoonotic Potential of Streptococcus suis

BACKGROUND

Streptococcus suis has emerged as an important cause of bacterial meningitis in adults. The ingestion of undercooked pork is a risk factor for human S. suis serotype 2 (SS2) infection. Here we provide experimental evidence indicating that the gastrointestinal tract is an entry site of SS2 infection.

METHODS

We developed a noninvasive in vivo model to study oral SS2 infection in piglets. We compared in vitro interaction of S. suis with human and porcine intestinal epithelial cells (IEC).

RESULTS

Two out of 15 piglets showed clinical symptoms compatible with S. suis infection 24-48 hours after ingestion of SS2. SS2 was detected in mesenteric lymph nodes of 40% of challenged piglets. SS2 strains isolated from patients showed significantly higher adhesion to human IEC compared to invasive strains isolated from pigs. In contrast, invasive SS9 strains showed significantly higher adhesion to porcine IEC. Translocation across human IEC, which occurred predominately via a paracellular route, was significantly associated with clonal complex 1, the predominant zoonotic genotype. Adhesion and translocation were dependent on capsular polysaccharide production.

CONCLUSIONS

SS2 should be considered a food-borne pathogen. S. suis interaction with human and pig IEC correlates with S. suis serotype and genotype, which can explain the zoonotic potential of SS2.

Carbohydrate availability regulates virulence gene expression in Streptococcus suis

Streptococcus suis is a major bacterial pathogen of young pigs causing worldwide economic problems for the pig industry. S. suis is also an emerging pathogen of humans. Colonization of porcine oropharynx by S. suis is considered to be a high risk factor for invasive disease. In the oropharyngeal cavity, where glucose is rapidly absorbed but dietary α-glucans persist, there is a profound effect of carbohydrate availability on the expression of virulence genes. Nineteen predicted or confirmed S. suis virulence genes that promote adhesion to and invasion of epithelial cells were expressed at higher levels when S. suis was supplied with the α-glucan starch/pullulan compared to glucose as the single carbon source. Additionally the production of suilysin, a toxin that damages epithelial cells, was increased more than ten-fold when glucose levels were low and S. suis was growing on pullulan. Based on biochemical, bioinformatics and in vitro and in vivo gene expression studies, we developed a biological model that postulates the effect of carbon catabolite repression on expression of virulence genes in the mucosa, organs and blood. This research increases our understanding of S. suis virulence mechanisms and has important implications for the design of future control strategies including the development of anti-infective strategies by modulating animal feed composition.

Effect of licochalcone A on growth and properties of Streptococcus suis

Streptococcus suis (S.suis) is an important emerging worldwide pig pathogen and zoonotic agent with rapid evolution of virulence and drug resistance. In this study, we wanted to investigate the effect of licochalcone A on growth and properties of Streptococcus suis. The antimicrobial activity of licochalcone A was tested by growth inhibition assay and the minimal inhibitory concentrations (MICs) also were determined. The effect of licochalcone A on S.suis biofilm formation was characterized by crystal violet staining. The effect of licochalcone A on suilysin secretion was evaluated by titration of hemolytic activity. To understand the antimicrobial effect, gene expression profile of S.suis treated by licochalcone A was analyzed by DNA microarray. Our results demonstrated that licochalcone A showed antimicrobial activity on S.suis with MICs of 4 µg/ml for S.suis serotype 2 strains and 8 µg/ml for S.suis serotype 7 strains. Biofilm formation was inhibited by 30–40% in the presence of licochalcone A (3 µg/ml) and suilysin secretion was also significantly inhibited in the presence of licochalcone A (1.5 µg/ml). The gene expression profile of S.suis in the presence of licochalcone A showed that 132 genes were differentially regulated, and we analyzed the regulated genes in the aspect of the bacterial cell cycle control. Among the deregulated genes, the genes responsible for the mass doubling was increased expression, but the genes responsible for DNA replication and cell division were inhibited the expression. So, we think the regulation of the cell cycle genes might provide a mechanistic understanding of licochalcone A mediated antimicrobial effect against S.suis.

A novel fibronectin-binding protein of Streptococcus suis serotype 2 contributes to epithelial cell invasion and in vivo dissemination

Streptococcus suis serotype 2 (SS2) is an important pathogen with zoonotic potential. In this study, a novel in vivo induced protein Ssa, encoded by the functionally unknown gene SSU05_1311, was identified as a surface anchored fibronectin-binding protein. The recombinant Ssa as well as its truncated proteins harboring the N-terminal domain (residues 33-153) could bind to human fibronectin with high-affinity. Isogenic knockout of ssa in SS2 led to decrease of bacterial binding to immobilized fibronectin. SS2 Δssa mutant showed reduced adherence and invasion of human pharyngeal epithelial (HEp-2) cells compared to the wild type strain. Heterologous surface display of Ssa fibronectin-binding domain in Escherichia coli enhanced the bacterial attachment and entry to HEp-2 cells. Less SS2 Δssa mutants than the wild type strain were recovered from the blood and brains of the Balb/c mice infected intranasally. But there was no significant difference between the wild type and the mutant on phagocytosis by macrophages (RAW264.7) and bacterial killing by murine PMNs under opsonizing condition. Our data suggest that Ssa is an important virulence factor for SS2 crossing the mucosal epithelia to disseminate in vivo.

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.

Adherence and invasion of streptococci to eukaryotic cells and their role in disease pathogenesis

Streptococcal adhesion, invasion, intracellular trafficking, dissemination, and persistence in eukaryotic cells have a variety of implications in the infection pathogenesis. While cell adhesion establishes the initial host contact, adhering bacteria exploit the host cell for their own benefit. Internalization into the host cell is an essential step for bacterial survival and subsequent dissemination and persistence, thus playing a key role in the course of infection. This chapter summarizes the current knowledge about the diverse mechanisms of streptococcal adhesion to and invasion into different eukaryotic cells and the impact on dissemination and persistence which is reflected by consequences for the pathogenesis of streptococcal infections.

Understanding Streptococcus suis serotype 2 infection in pigs through a transcriptional approach

Background

Streptococcus suis serotype 2 (S. suis 2) is an important pathogen of pigs. S suis 2 infections have high mortality rates and are characterized by meningitis, septicemia and pneumonia. S. suis 2 is also an emerging zoonotic agent and can infect humans that are exposed to pigs or their by-products. To increase our knowledge of the pathogenesis of meningitis, septicemia and pneumonia in pigs caused by S. suis 2, we profiled the response of peripheral blood mononuclear cells (PBMC), brain and lung tissues to infection with S. suis 2 strain SC19 using the Affymetrix Porcine Genome Array.

Results

A total of 3,002 differentially expressed transcripts were identified in the three tissues, including 417 unique genes in brain, 210 in lung and 213 in PBMC. These genes showed differential expression (DE) patterns on analysis by visualization and integrated discovery (DAVID). The DE genes involved in the immune response included genes related to the inflammatory response (CD163), the innate immune response (TLR2, TLR4, MYD88, TIRAP), cell adhesion (CD34, SELE, SELL, SELP, ICAM-1, ICAM-2, VCAM-1), antigen processing and presentation (MHC protein complex) and angiogenesis (VEGF), together with genes encoding cytokines (interleukins). Five selected genes were validated by qRT-PCR analysis.

Conclusions

We studied the response to infection with S. suis 2 strain SC19 by microarray analysis. Our findings confirmed some genes identified in previous studies and discovered numerous additional genes that potentially function in S. suis 2 infections in vivo. This new information will form the foundation of future investigations into the pathogenesis of S. suis.

Identification of Streptococcus suis serotype 2 genes preferentially expressed in the natural host

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen for swine and humans. Previous research about the mechanism of SS2 infection was largely established on in vitro or ex vivo models. In this study, we focused on the identification of SS2 genes preferentially expressed in vivo during natural infection in pigs. Eighty SS2 genes were identified to be up-regulated in the porcine brains and lungs by selective capture of transcribed sequences (SCOTS) and comparative dot blot analysis, followed by quantitative RT-PCR validation. These genes could be classified into 5 functional categories: metabolism, cell wall associated proteins, transporters, cell replication, and function unknown. Some of these genes may contribute to the survival and pathogenesis of SS2 in the host via the following strategies. First, SS2 evades the host innate immune clearance through modifying its metabolism and cell wall composition as indicated by the up-regulation of the corresponding gene ldh and pbp2A, respectively. Secondly, SS2 adapts to the in vivo conditions by inducing the expression of the two-component signal transduction system VicKR which may function on the target genes such as pcsB involved in stress response and cell wall biosynthesis. Thirdly, SS2 enhances its virulence in vivo by up-regulating the virulence genes, such as sly, pdgA, ssp, gidA, gcp and hp1311. Further study of these in vivo up-regulated genes will contribute to understanding the in vivo survival mechanism and pathogenesis of SS2.

Streptococcus suis: a new emerging or an old neglected zoonotic pathogen?

Infections caused by Streptococcus suis are considered a global and an economical problem in the swine industry. Moreover, S. suis is an agent of zoonosis that afflicts people in close contact with infected pigs or pork-derived products. Although sporadic cases of S. suis infections in humans (mainly meningitis) have been reported during the last 40 years, a large outbreak due to this pathogen emerged in the summer of 2005 in China. The severity of the infection in humans during the outbreak, such as a shorter incubation time, more rapid disease progression and higher rate of mortality, attracted a lot of attention from the scientific community and the general press. In fact, the number of publications on S. suis (including the number of reported human cases) has significantly increased during recent years. In this article we critically review the present knowledge on S. suis infection in humans, we discuss the hypotheses that may explain the 2005 outbreak and the repercussion of such an episode on the scientific community.

RpoS- and Crp-dependent transcriptional control of Salmonella Typhi taiA and hlyE genes: role of environmental conditions

A novel pathogenicity island, SPI-18, carries the taiA-hlyE operon, encoding virulence factors in Salmonella Typhi. To determine the effects of certain environmental conditions on the expression of these genes, beta-galactosidase assays, RT-PCR reactions, western blot analyses and measurement of hemolytic activity were performed. The conditions studied are those likely found by S. Typhi during infection in the human host. We found RpoS-dependent transcriptional upregulation in low pH and high osmolarity for both genes. Our results show that oxygen depletion apparently did not affect transcription of the taiA-hlyE operon. On the other hand, the transcriptional regulator Crp, previously described as an activator of hlyE transcription in Escherichia coli, is involved in transcriptional repression of hlyE in S. Typhi. Moreover, addition of glucose to the growth medium results in decreasing the hlyE mRNA, suggesting that there is another factor related to catabolite repression different from Crp and involved in downregulation of hlyE in S. Typhi.

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.

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 infections in humans: the Chinese experience and the situation in North America

Infections caused by Streptococcus suis are considered a global problem in the swine industry. In this animal species, S. suis is associated with septicemia, meningitis, endocarditis, arthritis and, occasionally, other infections. Moreover, it is an agent of zoonosis that afflicts people in close contact with infected pigs or pork-derived products. Although sporadic cases of S. suis infection in humans have been reported, a large outbreak due to S. suis serotype 2 emerged in the summer of 2005 in Sichuan, China. A similar outbreak was observed in another Chinese province in 1998. Symptoms reported in these two outbreaks include high fever, malaise, nausea and vomiting, followed by nervous symptoms, subcutaneous hemorrhage, septic shock and coma in severe cases. The increased severity of S. suis infections in humans, such as a shorter incubation time, more rapid disease progression and higher rate of mortality, underscores the critical need to better understand the factors associated with pathogenesis of S. suis infection. From the 35 capsular serotypes currently known, serotype 2 is considered the most virulent and frequently isolated in both swine and humans. Here, we review the epidemiological, clinical and immunopathological features of S. suis infection in humans.

Streptococcus suis: an emerging zoonotic pathogen

Streptococcus suis is a major porcine pathogen worldwide, and can be transmitted to human beings by close contact with sick or carrier pigs. S suis causes meningitis, septicaemia, endocarditis, arthritis, and septic shock in both pigs and human beings, and mortality is high. Human infection with S suis occurs mainly among certain risk groups that have frequent exposure to pigs or pork. Outbreaks of human S suis infection are uncommon, although several outbreaks have occurred in China in recent years. In July, 2005, the largest outbreak of human S suis infection occurred in Sichuan province, China, where 204 people were infected and 38 of them died. There have been 409 cases of human S suis infection worldwide, most of which have occurred in China, Thailand, and the Netherlands, and these infections have led to 73 deaths. This review provides background information on the biology and molecular characteristics of this Gram-positive bacterium, and describes the clinical signs, pathology, epidemiology, diagnosis, and treatment of human infection with S suis.

Amplified fragment length polymorphism of Streptococcus suis strains correlates with their profile of virulence-associated genes and clinical background

Amplified fragment length polymorphism (AFLP) typing was applied to 116 Streptococcus suis isolates with different clinical backgrounds (invasive/pneumonia/carrier/human) and with known profiles of virulence-associated genes (cps1, -2, -7 and -9, as well as mrp, epf and sly). A dendrogram was generated that allowed identification of two clusters (A and C) with different subclusters (A1, A2, C1 and C2) and two heterogeneous groups of strains (B and D). For comparison, three strains from each AFLP subcluster and group were subjected to multilocus sequence typing (MLST) analysis. The closest relationship and lowest diversity were found for patterns clustering within AFLP subcluster A1, which corresponded with sequence type (ST) complex 1. Strains within subcluster A1 were mainly invasive cps1 and mrp+ epf+ (or epf*) sly+ cps2+ strains of porcine or human origin. A new finding of this study was the clustering of invasive mrp* cps9 isolates within subcluster A2. MLST analysis suggested that A2 correlates with a single ST complex (ST87). In contrast to A1 and A2, subclusters C1 and C2 contained mainly pneumonia isolates of genotype cps7 or cps2 and epf- sly-. In conclusion, this study demonstrates that AFLP allows identification of clusters of S. suis strains with clinical relevance.

Identification of a novel virulence determinant with serum opacification activity in Streptococcus suis

Streptococcus suis serotype 2 is a porcine and human pathogen with adhesive and invasive properties. In other streptococci, large surface-associated proteins (>100 kDa) of the MSCRAMM family (microbial surface components recognizing adhesive matrix molecules) are key players in interactions with host tissue. In this study, we identified a novel opacity factor of S. suis (OFS) with structural homology to members of the MSCRAMM family. The N-terminal region of OFS is homologous to the respective regions of fibronectin-binding protein A (FnBA) of Streptococcus dysgalactiae and the serum opacity factor (SOF) of Streptococcus pyogenes. Similar to these two proteins, the N-terminal domain of OFS opacified horse serum. Serum opacification activity was detectable in sodium dodecyl sulfate extracts of wild-type S. suis but not in extracts of isogenic ofs knockout mutants. Heterologous expression of OFS in Lactococcus lactis demonstrated that a high level of expression of OFS is sufficient to provide surface-associated serum opacification activity. Furthermore, serum opacification could be inhibited by an antiserum against recombinant OFS. The C-terminal repetitive sequence elements of OFS differed significantly from the respective repeat regions of FnBA and SOF as well as from the consensus sequence of the fibronectin-binding repeats of MSCRAMMs. Accordingly, fibronectin binding was not detectable in recombinant OFS. To investigate the putative function of OFS in the pathogenesis of invasive S. suis diseases, piglets were experimentally infected with an isogenic mutant strain in which the ofs gene had been knocked out by an in-frame deletion. The mutant was severely attenuated in virulence but not in colonization, demonstrating that OFS represents a novel virulence determinant of S. suis.

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.

Streptococcus suisserotype 2 binding to extracellular matrix proteins

Streptococcus suis serotype 2 is a major swine and human pathogen that causes septicemia and meningitis. The ability of S. suis serotype 2 to bind to different extracellular matrix (ECM) proteins was evaluated by ELISA. All 23 strains tested bound to plasma and cellular fibronectin and collagen types I, III, and V, some to fibrin, vitronectin, and laminin, and none to the other ECM proteins tested. An unencapsulated isogenic mutant bound to ECM proteins better than its parental encapsulated strain, suggesting that the polysaccharide capsule interfered with binding. Cross-inhibition was observed between soluble plasma fibronectin and collagens in the ECM adherence assay, indicating that binding domains for both proteins exist on the same or nearby bacterial surface molecules. On the other hand, pre-incubation with plasma fibronectin increased binding to collagen IV, suggesting that S. suis might use fibronectin as a bridging molecule. The results of heat treatment and proteolytic digestion suggest that adhesins for these ECM proteins are proteinaceous in nature.

Putative mannose-specific phosphotransferase system component IID represses expression of suilysin in serotype 2 Streptococcus suis

In this study, we generated a genomic mutant library from a North American strain of serotype 2 Streptococcus suis using the pGh9:ISS1 transposition vector. Suilysin is the hemolysin made by S. suis. A hyper-hemolytic mutant was identified by screening for hemolytic phenotype using media with human blood. The hyper-hemolytic phenotype was characterised by a quantitative hemolysis microplate method. The use of green fluorescent protein (GFP) as a reporter also showed that suilysin gene expression was greater in the mutant. DNA sequence analysis of 3.8 kb surrounding the ISS1 insertion site revealed four open reading frames (ORFs) with three consecutive ORFs that belong to a putative mannose-specific phosphotransferase system (PTS). The S. suis gene homologous to mannose permease IID, manN, was interrupted by the transposon. A complementation test showed that manN repressed the expression of suilysin and the absence of manN was responsible for the hyper-hemolytic phenotype. However, both wild type and isogenic hyper-hemolytic mutant S. suis fermented mannose, glucose and lactose. Thus, despite its potential roles in carbohydrate transport, phosphorylation and metabolism, the manN homologue in the putative mannose-specific PTS regulates gene expression in S. suis.

Non-encapsulated strains reveal novel insights in invasion and survival of Streptococcus suis in epithelial cells

Streptococcus suis is a porcine and human pathogen causing invasive diseases, such as meningitis or septicaemia. Host cell interactions of S. suis have been studied mainly with serotype 2 strains, but multiple capsular serotypes as well as non-typeable strains exist with diverse virulence features. At present, S. suis is considered an extracellular pathogen. However, whether or not it can also invade host cells is a matter of controversial discussions. We have assessed adherence and invasion of S. suis for HEp-2 epithelial cells by comparing 10 serotype 2 strains and four non-typeable (NT) strains. Only the NT strains and a non-encapsulated serotype 2 mutant strain, but none of the serotype 2 strains, adhered strongly and were invasive. Invasion seemed to be affected by environmental signals, as suggested from comparison of strains grown in different media. Further phenotypic and genotypic characterization revealed a high diversity among the different strains. Electron microscopic analysis of invasion of selected invasive NT strains indicated different uptake mechanisms. One strain induced large invaginations comparable to those seen in 'caveolae' mediated uptake, whereas invasion of the other strains was accompanied by formation of filipodia-like membrane protrusions. Invasion of all strains, however, was similarly susceptible to hypertonic sucrose, which inhibits receptor-mediated endocytosis. Irrespective of the uptake pathway, streptococci resided in acidified phago-lysosome like vacuoles. All strains, except one, survived intracellularly as well as extracellular acidic conditions. Survival seemed to be associated with the AdiS protein, an environmentally regulated arginine deiminase of S. suis. Concluding, invasion and survival of NT strains of S. suis in epithelial cells revealed novel evidence that S. suis exhibits a broad variety of virulence-associated features depending on genetic variation and regulation.

Invasion of porcine brain microvascular endothelial cells by Streptococcus suis serotype 2

Streptococcus suis is an important swine pathogen that mainly causes meningitis and occasionally causes other infections, such as endocarditis, arthritis, and pneumonia. The pathogenesis of S. suis infection has not been completely defined. However, in order to cause meningitis, S. suis has to cross the blood-brain barrier (BBB) made up of brain microvascular endothelial cells. The objective of this work was to study the interactions of S. suis serotype 2 with porcine brain microvascular endothelial cells (PBMEC). The ability of North American and European S. suis serotype 2 strains to adhere to PBMEC and, most importantly, to invade PBMEC was demonstrated by using an antibiotic protection assay and was confirmed by electron microscopy. The polysaccharide capsule of S. suis seemed to partially interfere with the adhesion and invasion abilities of the bacterium. Our results showed that intracellular viable S. suis could be found in PBMEC up to 7 h after antibiotic treatment. Inhibition studies demonstrated that invasion of PBMEC by S. suis required actin microfilaments but not microtubular cytoskeletal elements or active bacterial RNA or protein synthesis. At high bacterial doses, suilysin-positive strains were toxic for PBMEC. The role of suilysin in cytotoxicity was confirmed by using purified suilysin, electron microscopy, and the lack of toxicity of a suilysin-negative mutant. In swine, the invasion of endothelial cells of the BBB could play an important role in the pathogenesis of the meningitis caused by S. suis.

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.

Development of a multilocus sequence typing scheme for the pig pathogen Streptococcus suis: identification of virulent clones and potential capsular serotype exchange

Streptococcus suis is an important pathogen of pigs and occasionally causes serious human disease. However, little is known about the S. suis population structure, the clonal relationships between strains, the potential of particular clones to cause disease, and the relevance of serotype as a marker for epidemiology. Here we describe a multilocus sequence typing (MLST) scheme for S. suis developed in order to begin to address these issues. Seven housekeeping gene fragments from each of 294 S. suis isolates obtained from various S. suis diseases and from asymptomatic carriage representing 28 serotypes and nine distinct countries of origin were sequenced. Between 32 and 46 alleles per locus were identified, giving the ability to distinguish >1.6 x 10(11) sequence types (STs). However only 92 STs were identified in this study. Of the 92 STs 18 contained multiple isolates, the most common of which, ST1, was identified on 141 occasions from six countries. Assignment of the STs to lineages resulted in 37 being identified as unique and unrelated STs while the remaining 55 were assigned to 10 complexes. ST complexes ST1, ST27, and ST87 dominate the population; while the ST1 complex was strongly associated with isolates from septicemia, meningitis, and arthritis, the ST87 and ST27 complexes were found to contain significantly higher numbers of lung isolates. In agreement with the observed distribution of disease-causing isolates of S. suis, most isolates previously characterized as of high virulence in porcine infection models belong to ST1, while isolates belonging to other STs appear to be less virulent in general. Finally nine STs were found to contain isolates of multiple serotypes, and many isolates belonging to the same serotypes were found to have very disparate genetic backgrounds. As well as highlighting that the serotype can often be a poor indicator of genetic relatedness between S. suis isolates, these findings suggest that capsular genes may be moving horizontally through the S. suis population.

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

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