Mitogenic effect contributes to increased virulence of Streptococcus suis sequence type 7 to cause streptococcal toxic shock-like syndrome

Investigation of the Genomic and Pathogenic Features of the Potentially Zoonotic Streptococcus parasuis

Recently, Streptococcus suis reference strains of serotype 20, 22, and 26 were reclassified as Streptococcus parasuis. The public health significance of S. parasuis is underestimated due to the lack of clinical isolates. In the present study, we first reported two sporadic S. parasuis infections in humans, after using full-length 16S rRNA and housekeeping genes' phylogeny and ANI values of genome sequence comparisons to determine the species of their isolates BS26 and BS27. Compared to highly pathogenic S. suis strain P1/7, S. parasuis strains BS26 and BS27 possessed a delayed capacity to initiate lethal infection, which may attribute to the later production of higher level of pro-inflammatory cytokines. Differed to S. suis strain P1/7, S. parasuis strains did not induce significant inflammatory response in the brain of mice. Histopathological changes in liver and lungs were widely present in mice infected with S. parasuis strains. Our data indicated that the pathogenic mechanism of S. parasuis may be different from that of S. suis. Three lineages in the core-genome phylogenetic tree and ten types of cps gene cluster were found in 13 S. parasuis genomes, indicating high heterogeneity of this species. The similarity of CPS structure and antibiotic-resistant genes relative to S. suis indicated the evolutionary affinity between the two species. Our data suggested S. parasuis is a potential zoonotic pathogen and poses severe threat to health of susceptible people. Further study on the epidemiology and public health significance of S. parasuis is urgently necessary.

HP1330 Contributes to Streptococcus suis Virulence by Inducing Toll-Like Receptor 2- and ERK1/2-Dependent Pro-inflammatory Responses and Influencing In Vivo S. suis Loads

Streptococcus suis 2 (SS2) has evolved into a highly invasive pathogen responsible for two large-scale outbreaks of streptococcal toxic shock-like syndrome (STSLS) in China. Excessive inflammation stimulated by SS2 is considered a hallmark of STSLS, even it also plays important roles in other clinical symptoms of SS2-related disease, including meningitis, septicemia, and sudden death. However, the mechanism of SS2-caused excessive inflammation remains poorly understood. Here, a novel pro-inflammatory protein was identified (HP1330), which could induce robust expression of pro-inflammatory cytokines (TNF-α, MCP-1, and IL-1β) in RAW264.7 macrophages. To evaluate the role of HP1330 in SS2 virulence, an hp1330-deletion mutant (Δhp1330) was constructed. In vitro, hp1330 disruption led to a decreased pro-inflammatory ability of SS2 in RAW 264.7 macrophages. In vivo, Δhp1330 showed reduced lethality, pro-inflammatory activity, and bacterial loads in mice. To further elucidate the mechanism of HP1330-induced pro-inflammatory cytokine production, antibody blocking and gene-deletion experiments with macrophages were performed. The results revealed that the pro-inflammatory activity of HP1330 depended on the recognition of toll-like receptor 2 (TLR2). Furthermore, a specific inhibitor of the extracellular signal-regulated kinase 1/2 (ERK1/2) pathways could significantly decrease HP1330-induced pro-inflammatory cytokine production, and western blot analysis showed that HP1330 could induce activation of the ERK1/2 pathway. Taken together, our findings demonstrate that HP1330 contributes to SS2 virulence by inducing TLR2- and ERK1/2-dependent pro-inflammatory cytokine production and influencing in vivo bacterial loads, implying that HP1330 may be associated with STSLS caused by SS2.

A novel pro-inflammatory protein of Streptococcus suis 2 induces the Toll-like receptor 2-dependent expression of pro-inflammatory cytokines in RAW 264.7 macrophages via activation of ERK1/2 pathway

Streptococcus suis 2 is an important swine pathogen and an emergent zoonotic pathogen. Excessive inflammation caused by S. suis is responsible for the high levels of early mortality observed in septic shock-like syndrome cases. However, the mechanisms through which S. suis 2 (SS2) causes excessive inflammation remain unclear. Thus, this study aimed to identify novel pro-inflammatory mediators that play important roles in the development of therapies against SS2 infection. In this study, the novel pro-inflammatory protein HP0459, which was encoded by the SSUSC84_0459 gene, was discovered. The stimulation of RAW 264.7 macrophages with recombinant HP0459 protein induced the expression of pro-inflammatory cytokines (IL-1β, MCP-1 and TNF-α). Compared with the wild-type (WT) strain, the isogenic knockout of HP0459 in SS2 led to reduced production of pro-inflammatory cytokines in RAW264.7 macrophages and in vivo. The pro-inflammatory activity of HP0459 was significantly reduced by an antibody against Toll-like receptor 2 (TLR2) in RAW264.7 macrophages and was lower in TLR2-deficient (TLR2-/-) macrophages than in WT macrophages. Furthermore, specific inhibitors of the extracellular signal-regulated kinase 1/2 (ERK1/2) pathways significantly decreased the HP0459-induced pro-inflammatory cytokine production, and a western blot assay showed that HP0459 stimulation induced the activation of the ERK1/2 pathway. Taken together, our data indicate that HP0459 is a novel pro-inflammatory mediator of SS2 and induces TLR2-dependent pro-inflammatory activity in RAW264.7 macrophages through the ERK1/2 pathway.

Deregulated balance of omega-6 and omega-3 polyunsaturated fatty acids following infection by the zoonotic pathogen Streptococcus suis

Streptococcus suis is an important swine pathogen and an emergent zoonotic pathogen. Excessive inflammation caused by S. suis is responsible for early high mortality in septic shock-like syndrome cases. Polyunsaturated fatty acids (PUFAs) may contribute to regulating inflammatory processes. This study shows that mouse infection by S. suis is accompanied by an increase of arachidonic acid, a proinflammatory omega-6 (ω-6) PUFA, and by a decrease of docosahexaenoic acid, an anti-inflammatory ω-3 PUFA. Macrophages infected with S. suis showed activation of mitogen-activated protein kinase pathways and cyclooxygenase-2 upregulation. Fenretinide, a synthetic vitamin A analog, reduced in vitro expression of inflammatory mediators. Pretreatment of mice with fenretinide significantly improved their survival by reducing systemic proinflammatory cytokines during the acute phase of an S. suis infection. These findings indicate a beneficial effect of fenretinide in diminishing the expression of inflammation and improving survival during an acute infection by a virulent S. suis strain.

Comparative genomic hybridization identifies virulence differences in Streptococcus suis

Streptococcus suis is an important zoonotic pathogen. However, identification of virulent S. suis strains is complicated because of the high diversity of the species. Here we evaluated the genetic difference among S. suis strains using comparative genomic hybridization (CGH) and virulence variation in vivo and in vitro. We showed that different clades differed in their ability to activate TLR2/6 in vitro and their capacity to induce cytokine production in vivo as well as their resistance to phagocytosis and survival in vivo. Our data showed the S. suis strains tested can be classified into three groups having differing levels of virulence: epidemic and highly virulent strains were clustered into clade Ia (epidemic and highly virulent group, E/HV group), virulent strains were clustered into clade Ib (virulent group, V group), and intermediately or weakly virulent strains were clustered into other clades (intermediately or weakly virulent group, I/WV group). Our study provided further insight into the genomic and virulence variation of S. suis.

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

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

Toll-like receptor 2-independent host innate immune response against an epidemic strain of Streptococcus suis that causes a toxic shock-like syndrome in humans

Streptococcus suis is an emerging zoonotic agent causing meningitis and septicemia. Outbreaks in humans in China with atypical cases of streptococcal toxic shock-like syndrome have been described to be caused by a clonal epidemic S. suis strain characterized as sequence type (ST) 7 by multilocus sequence typing, different from the classical ST1 usually isolated in Europe. Previous in vitro studies showed that Toll-like receptor (TLR) 2 plays a major role in S. suis ST1 interactions with host cells. In the present study, the in vivo role of TLR2 in systemic infections caused by S. suis ST1 or ST7 strains using TLR2 deficient (TLR2(-/-)) mice was evaluated. TLR2-mediated recognition significantly contributes to the acute disease caused by the highly virulent S. suis ST1 strain, since the TLR2(-/-) mice remained unaffected when compared to wild type (WT) mice. The lack of mortality could not be associated with a lower bacterial burden; however, a significant decrease in the induction of pro-inflammatory mediators, as evaluated by microarray, real-time PCR and protein assays, was observed. On the other hand, TLR2(-/-) mice infected with the epidemic ST7 strain presented no significant differences regarding survival and expression of pro-inflammatory mediators when compared to the WT mice. Together, these results show a TLR2-independent host innate immune response to S. suis that depends on the strain.

Exacerbated type II interferon response drives hypervirulence and toxic shock by an emergent epidemic strain of Streptococcus suis

Streptococcus suis, a major porcine pathogen, can be transmitted to humans and cause severe symptoms. A large human outbreak associated with an unusual streptococcal toxic shock-like syndrome (STSLS) was described in China. Albeit an early burst of proinflammatory cytokines following Chinese S. suis infection was suggested to be responsible for STSLS case severity, the mechanisms involved are still poorly understood. Using a mouse model, the host response to S. suis infection with a North American intermediately pathogenic strain, a European highly pathogenic strain, and the Chinese epidemic strain was investigated by a whole-genome microarray approach. Proinflammatory genes were expressed at higher levels in mice infected with the Chinese strain than those infected with the European strain. The Chinese strain induced a fast and strong gamma interferon (IFN-γ) response by natural killer (NK) cells. In fact, IFN-γ-knockout mice infected with the Chinese strain showed significantly better survival than wild-type mice. Conversely, infection with the less virulent North American strain resulted in an IFN-β-subjugated, low inflammatory response that might be beneficial for the host to clear the infection. Overall, our data suggest that a highly virulent epidemic strain has evolved to massively activate IFN-γ production, mainly by NK cells, leading to a rapid and lethal STSLS.

The role of toll-like receptors in the pathogenesis of Streptococcus suis

Streptococcus suis is an important agent of swine and human meningitis. Sequence type (ST) 7 emerged in China and was responsible for the human epidemic caused by S. suis in 2005. The virulence of S. suis ST7 is greater than the wild type pathogenic S. suis, ST1; however, the mechanisms for this increased pathogenicity are unknown. The aim of this study was to determine the role of different toll-like receptors (TLRs) involved in regulating the host response to the S. suis infection and to speculate on differing mechanisms used by ST7 strains to induce disease. Here we compared two ST7 strains isolated in the 2005 Sichuan outbreak to two ST1 strains. Our data show TLR2, 6 and 9 are involved in the recognition of heat-killed S. suis independent of the ST type. We found the TLR-dependent cytokine production differed between the two types of strains using whole cell lysate proteins. TLR6 played a greater role in cytokine production induced by the whole cell lysate proteins from the ST7 strain than in that induced by the ST1 strain lysates. The data suggest that mechanisms of inflammation induced by S. suis strains differ where this will be useful in designing efficient strategies in combating streptococcal toxic shock-like syndrome caused by the S. suis ST7 strains.

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.

Uncovering newly emerging variants of Streptococcus suis, an important zoonotic agent

Streptococcus suis is recognized as a major swine pathogen and an emerging zoonotic agent. Two large-scale outbreaks of severe S. suis epidemics occurred in China in 1998 and 2005 that posed serious concerns to public health and challenged the conventional conception that opportunistic infections of S. suis serotype 2 (SS2) in humans were only sporadic cases. An extensive, collaborative study on Chinese SS2 variants, which exhibit strong invasiveness and high pathogenicity, has resulted in the description of a new disease form of streptococcal toxic shock syndrome (STSS) and a putative pathogenicity island (termed 89K). The abbreviation of STSS is used for the severe disease caused by both Staphylococci and Streptococci. The main virulence factors involved in STSS caused by either Staphylococcus aureus or Streptococcus pyogenes consist of so-called superantigens or molecules that trigger a nonspecific, uncontrolled activation of T cells and massive cytokine release. However, although a collection of new virulence factors have been described, no superantigen candidates have been found for SS2 strains, implying that a different mechanism could be involved in the STSS form caused by SS2 variants.

Avian influenza virus, Streptococcus suis serotype 2, severe acute respiratory syndrome-coronavirus and beyond: molecular epidemiology, ecology and the situation in China

The outbreak and spread of severe acute respiratory syndrome-associated coronavirus and the subsequent identification of its animal origin study have heightened the world's awareness of animal-borne or zoonotic pathogens. In addition to SARS, the highly pathogenic avian influenza virus (AIV), H5N1, and the lower pathogenicity H9N2 AIV have expanded their host ranges to infect human beings and other mammalian species as well as birds. Even the ‘well-known’ reservoir animals for influenza virus, migratory birds, became victims of the highly pathogenic H5N1 virus. Not only the viruses, but bacteria can also expand their host range: a new disease, streptococcal toxic shock syndrome, caused by human Streptococcus suis serotype 2 infection, has been observed in China with 52 human fatalities in two separate outbreaks (1998 and 2005, respectively). Additionally, enterohaemorrhagic Escherichia coli O157:H7 infection has increased worldwide with severe disease. Several outbreaks and sporadic isolations of this pathogen in China have made it an important target for disease control. A new highly pathogenic variant of porcine reproductive and respiratory syndrome virus (PRRSV) has been isolated in both China and Vietnam recently; although PRRSV is not a zoonotic human pathogen, its severe outbreaks have implications for food safety. All of these pathogens occur in Southeast Asia, including China, with severe consequences; therefore, we discuss the issues in this article by addressing the situation of the zoonotic threat in China.