The contribution of suilysin to the pathogenesis of Streptococcus suis meningitis

Genomic Characterization and Comparative Analysis of Streptococcus zhangguiae sp. nov. Isolated from the Respiratory Tract of Marmota Himalayana

Two Gram-stain-positive, oxidase-negative, non-motile, facultative anaerobic, α-hemolytic, coccus-shaped bacteria (zg-86T and zg-70) were isolated from the respiratory tracts of marmots (Marmota Himalayana) on the Qinghai-Tibet Plateau of China. Phylogenetic analysis of the 16S rRNA gene and 545 core genes revealed that these two strains belong to the Streptococcus genus. These strains were most closely related to Streptococcus respiraculi HTS25T, Streptococcus cuniculi CCUG 65085T, and Streptococcus marmotae HTS5T. The average nucleotide identity (ANI) and digital DNA‒DNA hybridization (dDDH) were below the threshold for species delineation. The predominant cellular fatty acids (CFAs) in this novel species were C16:0, C18:0, and C18:1ω9c, whereas the primary polar lipids were phosphatidylglycerol (PG), phosphatidylethanolamine (PE) and an unknown phosphoglycolipid (PGL). The optimal growth conditions for the strains were 37 °C, pH 7.0, and 0.5% (w/v) NaCl on brain-heart infusion (BHI) agar supplemented with 5% defibrinated sheep blood. Comparative genomics analyses revealed the potential pathogenicity of strain zg-86T through comparisons with suis subclade strains in terms of virulence factors, pathogen-host interactions (PHIs) and mobile genetic factors (MGEs). Based on the phenotypic characteristics and phylogenetic analyses, we propose that these two isolates represent novel species in the genus Streptococcus, for which the names Streptococcus zhangguiae sp. nov. (the type strain zg-86T=GDMCC 1.1758T=JCM 34273T) is proposed.

Evaluating the Antibacterial and Antivirulence Activities of Floxuridine against Streptococcus suis

Streptococcus suis is an emerging zoonotic pathogen that can cause fatal diseases such as meningitis and sepsis in pigs and human beings. The overuse of antibiotics is leading to an increased level of resistance in S. suis, and novel antimicrobial agents or anti-virulence agents for the treatment of infections caused by S. suis are urgently needed. In the present study, we investigated the antibacterial activity, mode of action and anti-virulence effects of floxuridine against S. suis. Floxuridine showed excessive antibacterial activity against S. suis both in vivo and in vitro; 4 × MIC of floxuridine could kill S. suis within 8 h in a time-kill assay. Meanwhile, floxuridine disrupted the membrane structure and permeability of the cytoplasmic membrane. Molecular docking revealed that floxuridine and SLY can be directly bind to each other. Moreover, floxuridine effectively inhibited the hemolytic capacity and expression levels of the virulence-related genes of S. suis. Collectively, these results indicate that the FDA-approved anticancer drug floxuridine is a promising agent and a potential virulence inhibitor against S. suis.

Canagliflozin Inhibited the Activity of Hemolysin and Reduced the Inflammatory Response Caused by Streptococcus suis

Highly virulent Streptococcus suis (S. suis) infections can cause Streptococcal toxic shock-like syndrome (STSLS) in pigs and humans, in which an excessive inflammatory response causes severe damage. Hemolysin (SLY) is a major virulence factor of S. suis serotype 2 that produces pores in the target cell membrane, leading to cytoplasmic K+ efflux and activation of the NLRP3 inflammasome, ultimately causing STSLS. The critical aspect of hemolysin in the pathogenesis of S. suis type 2 makes it an attractive target for the development of innovative anti-virulence drugs. Here, we use the S. suis toxin protein (SLY) as a target for virtual screening. A compound called canagliflozin, a hypoglycemic agent, was identified through screening. Canagliflozin significantly inhibits the hemolytic activity of hemolysin. The results combined with molecular dynamics simulation, surface plasmon resonance, and nano differential scanning fluorimetry show that canagliflozin inhibits the hemolytic activity of SLY by binding to SLY. In addition, canagliflozin markedly reduced the release of SC19-induced inflammatory factors at the cellular level and in mice. Importantly, the combination of canagliflozin and ampicillin had a 90% success rate in mice, significantly greater than the therapeutic effect of ampicillin. The findings suggest that canagliflozin may be a promising new drug candidate for S. suis infections.

Biofilm modifiers: The disparity in paradigm of oral biofilm ecosystem

A biofilm is a population of sessile microorganisms that has a distinct organized structure and characteristics like channels and projections. Good oral hygiene and reduction in the prevalence of periodontal diseases arise from minimal biofilm accumulation in the mouth, however, studies focusing on modifying the ecology of oral biofilms have not yet been consistently effective. The self-produced matrix of extracellular polymeric substances and greater antibiotic resistance make it difficult to target and eliminate biofilm infections, which lead to serious clinical consequences that are often lethal. Therefore, a better understanding is required to target and modify the ecology of biofilms in order to eradicate the infection, not only in instances of oral disorders but also in terms of nosocomial infections. The review focuses on several biofilm ecology modifiers to prevent biofilm infections, as well as the involvement of biofilm in antibiotic resistance, implants or in-dwelling device contamination, dental caries, and other periodontal disorders. It also discusses recent advances in nanotechnology that may lead to novel strategies for preventing and treating infections caused by biofilms as well as a novel outlook to infection control.

Genomic characterization and virulence of Streptococcus suis serotype 4 clonal complex 94 recovered from human and swine samples

Streptococcus suis is a zoonotic pathogen that causes invasive infections in humans and pigs. Herein, we performed genomic analysis of seven S. suis serotype 4 strains belonging to clonal complex (CC) 94 that were recovered from a human patient or from diseased and clinically healthy pigs. Genomic exploration and comparisons, as well as in vitro cytotoxicity tests, indicated that S. suis CC94 serotype 4 strains are potentially virulent. Genomic analysis revealed that all seven strains clustered within minimum core genome group 3 (MCG-3) and had a high number of virulence-associated genes similar to those of virulent serotype 2 strains. Cytotoxicity assays showed that both the human lung adenocarcinoma cell line and HeLa cells rapidly lost viability following incubation for 4 h with the strains at a concentration of 106 bacterial cells. The human serotype 4 strain (ID36054) decreased cell viability profoundly and similarly to the control serotype 2 strain P1/7. In addition, strain ST1689 (ID34572), isolated from a clinically healthy pig, presented similar behaviour in an adenocarcinoma cell line and HeLa cells. The antimicrobial resistance genes tet(O) and ermB that confer resistance to tetracyclines, macrolides, and lincosamides were commonly found in the strains. However, aminoglycoside and streptothricin resistance genes were found only in certain strains in this study. Our results indicate that S. suis CC94 serotype 4 strains are potentially pathogenic and virulent and should be monitored.

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.

Human Streptococcus suis Infections in Thailand: Epidemiology, Clinical Features, Genotypes, and Susceptibility

Streptococcus suis is a zoonotic pathogen causing substantial economic losses to the pig industry, as well as being a human health burden due to infections worldwide, especially in Southeast Asia. In Thailand, there was high cumulative incidence in humans during 1987-2021, mostly in males. At least five large outbreaks have been documented after the largest outbreak in China in 2005, which was related to the consumption of raw pork or dishes containing pig's blood. The major clinical features are sepsis or meningitis, with hearing loss a major complication of S. suis disease. Thai human S. suis isolates have shown diversity in serotypes and sequence types (STs), with serotype 2 and STs 1 and 104 being major genotypes. β-Lactam antibiotics can be used in empirical treatment for human S. suis infections; however, intermediate resistance to penicillin has been reported. Reducing S. suis incidence in Thailand requires a multidimensional approach, with combined efforts from the government and public health sectors through policy, regulations, education, and active surveillance.

Genomic differences between sequence types 1 and 104 of Streptococcus suis Serotype 2

Background

Streptococcus suis is a zoonotic pathogen that can cause invasive infections in humans who are in close contact with infected pigs or contaminated pork-derived products. S. suis serotype 2 sequence type (ST) 1 strains are mostly associated with meningitis, whereas ST104 strains are mostly recovered from sepsis cases in humans. No data are available for comparison of the ST1 and ST104 strains at the genomic level, particularly concerning virulence-associated genes. Thus, genomic comparison of both STs was performed in this study.

Methods

An ST1 isolate (ID26154) from the cerebrospinal fluid of a patient with meningitis and an ST104 isolate (ID24525) from the blood of a patient with sepsis were subjected to shotgun pyrosequencing using the 454 GS Junior System. Genomic comparison was conducted between the ST1 isolate and the ST104 isolate using the Artemis Comparison Tool (ACT) to identify the region of differences (RDs) between ST1 and ST104.

Results

Fifty-eight RDs were unique to the ST104 genome and were mainly involved in metabolism and cell functional activities, cell wall anchored proteins, bacteriophages and mobile genetic elements, ABC-type transporters, two-component signal transductions, and lantibiotic proteins. Some virulence genes mostly found in ST1 strains were also present in the ST104 genome. Whole-genome comparison is a powerful tool for identifying genomic region differences between different STs of S. suis serotype 2, leading to the identification of the molecular basis of virulence involved in the pathogenesis of the infection.

Metabolomics and proteomics analyses revealed mechanistic insights on the antimicrobial activity of epigallocatechin gallate against Streptococcus suis

Streptococcus suis (S. suis) is a highly virulent zoonotic pathogen and causes severe economic losses to the swine industry worldwide. Public health security is also threatened by the rapidly growing antimicrobial resistance in S. suis. Therefore, there is an urgent need to develop new and safe antibacterial alternatives against S. suis. The green tea polyphenol epigallocatechin gallate (EGCG) with a number of potential health benefits is known for its antibacterial effect; however, the mechanism of its bactericidal action remains unclear. In the present, EGCG at minimal inhibitory concentration (MIC) showed significant inhibitory effects on S. suis growth, hemolytic activity, and biofilm formation, and caused damage to S. suis cells in vitro. EGCG also reduced S. suis pathogenicity in Galleria mellonella larvae in vivo. Metabolomics and proteomics analyses were performed to investigate the underlying mechanism of antibacterial activity of EGCG at MIC. Many differentially expressed proteins involved in DNA replication, synthesis of cell wall, and cell membrane, and virulence were down-regulated after the treatment of S. suis with EGCG. EGCG not only significantly reduced the hemolytic activity of S. suis but also down-regulated the expression of suilysin (Sly). The top three shared KEGG pathways between metabolomics and proteomics analysis were ABC transporters, glycolysis/gluconeogenesis, and aminoacyl-tRNA biosynthesis. Taken together, these data suggest that EGCG could be a potential phytochemical compound for treating S. suis infection.

The VraSR two-component signal transduction system contributes to the damage of blood-brain barrier during Streptococcus suis meningitis

Streptococcus suis (S. suis) is an important zoonotic pathogen that can cause high morbidity and mortality in both humans and swine. As the most important life-threatening infection of the central nervous system (CNS), meningitis is an important syndrome of S. suis infection. The vancomycin resistance associated sensor/regulator (VraSR) is a critical two-component signal transduction system that affects the ability of S. suis to resist the host innate immune system and promotes its ability to adhere to brain microvascular endothelial cells (BMECs). Prior work also found mice infected with ΔvraSR had no obvious neurological symptoms, unlike mice infected with wild-type SC19. Whether and how VraSR participates in the development of S. suis meningitis remains unknown. Here, we found ΔvraSR-infected mice did not show obvious meningitis, compared with wild-type SC19-infected mice. Moreover, the proinflammatory cytokines and chemokines in serum and brains of ΔvraSR-infected mice, including IL-6, TNF-α, MCP-1 and IFN-γ, were significantly lower than wild-type infected group. Besides, blood-brain barrier (BBB) permeability also confirmed that the mutant had lower ability to disrupt BBB. Furthermore, in vivo and in vitro experiments showed that SC19 could increase BBB permeability by downregulating tight junction (TJ) proteins such as ZO-1, β-Catenin, Occludin, and Clauidn-5, compared with mutant ΔvraSR. These findings provide new insight into the influence of S. suis VraSR on BBB disruption during the pathogenic process of streptococcal meningitis, thereby offering potential targets for future preventative and therapeutic strategies against this disease.

Suilyin Disrupts the Blood-Brain Barrier by Activating Group III Secretory Phospholipase A2

Serious diseases caused by Streptococcus suis serotype 2 (S. suis 2) include septicaemia and meningitis, which are associated with high morbidity and mortality. Proliferation in the blood can result in a breach of the blood-brain barrier (BBB) and provide entry into the cerebrospinal fluid (CSF), where bacteria cause inflammation of the meningeal membranes resulting in meningitis. The molecular mechanisms of how this pathogen crosses the BBB remain unclear. Suilysin (SLY) has been identified as an important secreted virulence factor of S. suis 2 and may play a vital role in provoking meningitis. In this investigation, we demonstrate that SLY can increase the paracellular permeability of BBB, both in vivo and in vitro, via the activation of group III secretory phospholipase A2 (PLA2G3). Our results indicate that at lower, sublytic concentrations, the toxin can stimulate cerebral microvascular endothelial cells to release TNF-α, thereby inducing high level expressions of PLA2G3. Abnormal elevations of PLA2G3 might further injure tissues through direct cytolytic effectors or other responses.

Interleukin-17A Contributed to the Damage of Blood-CNS Barriers During Streptococcus suis Meningitis

Streptococcus suis (S. suis) is an emerging zoonotic agent that can cause meningitis in humans with high mortality and morbidity. Meningitic S. suis can induce higher level of IL-17 than non-meningitic S. suis. Besides, IL-17A plays various roles on bacterial clearance or disruption of blood-CNS barriers through the downregulation and reorganization of tight junction (TJ) molecules. However, it remains to be elucidated for the role of IL-17A on the infection with meningitic S. suis. Here, we found that meningitic S. suis infection could not only cause acute death due to the damage of multiple organs, but also cause meningitis and clinical nervous signs since 60 h of post-infection due to the penetration of blood-CNS barriers after lasting bacteremia. In contrast, the mice with deficiency of il17a gene could not significantly change the acute inflammatory response and acute death, but it could not show obvious meningitis and clinical nervous signs caused by the meningitic S. suis infection. In addition, we also found that IL-17A could inhibit the transcription and expression of TJ proteins that facilitated the leakage of blood-CNS barriers since 60 h of post-infection during meningitic S. suis infection. Thus, our findings demonstrated that IL-17A could downregulate TJ proteins, which undoubtedly facilitated the leakage of blood-CNS barriers for bacterial invasion and then caused S. suis meningitis, providing potential targets for future prevention and treatment of this disease.

Acacetin attenuates Streptococcus suis virulence by simultaneously targeting suilysin and inflammation

Streptococcus suis (S. suis), an important zoonotic pathogenic bacterium, can cause multiple diseases and fatal infections in both humans and animals. The emergence of highly virulent and extensively drug-resistant strains of S. suis has raised questions about the efficacy of available therapeutic agents, thereby necessitating novel therapeutic strategies. Suilysin (SLY) is one of the most essential determinants of virulence for the pathogenicity of S. suis capsular type 2 (SS2). In addition, inhibiting the excessive inflammatory response is a strategy to reduce the damage caused by SS2 infection. In this study, we identified acacetin as an effective inhibitor of SLY, which inhibited the oligomerisation of SLY without affecting bacterial growth. Furthermore, the addition of 4-16 μg/ml acacetin to the co-infection system of the cells reduced S. suis-induced inflammation by downregulating the activation of the MAPK signalling pathway, thereby alleviating the S. suis-mediated cell injury. Thus, in addition to the conventional antibiotic therapy, acacetin represent a potential drug candidate and strategy for the treatment of S. suis infections as it simultaneously inhibited the haemolytic activity of SLY and downregulated the inflammatory response.

The biological role of MutT in the pathogenesis of the zoonotic pathogen Streptococcus suis serotype 2

Streptococcus suis (S. suis) is an important rising pathogen that causes serious diseases in humans and pigs. Although some putative virulence factors of S. suis have been identified, its pathogenic mechanisms are largely unclear. Here, we identified a putative virulence-associated factor MutT, which is unique to S. suis serotype 2 (SS2) virulent strains. To investigate the biological roles of MutT in the SS2 virulent strain ZY05719, the mutT knockout mutant (ΔmutT) was generated and used to explore the phenotypic and virulent variations between the parental and ΔmutT strains. We found that the mutT mutation significantly inhibited cell growth ability, shortened the chain length, and displayed a high susceptibility to H2O2-induced oxidative stress. Moreover, this study revealed that MutT induced the adhesion and invasion of SS2 to host cells. Deletion of mutT increased microbial clearance in host tissues of the infected mice. Sequence alignment results suggested that mutT was encoded in a strain-specific manner, in which the detection was strongly linked to bacterial pathogenicity. In both zebrafish and mice infection models, the virulence of ΔmutT was largely reduced compared with that of ZY05719. Overall, this study provides compelling evidence that MutT is indispensable for the virulence of SS2 and highlights the biological role of MutT in bacteria pathogenesis during infection.

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.

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

As an important zoonotic pathogen, Streptococcus suis (S. suis) infection has been reported to be a causative agent for variety of diseases in humans and animals, especially Streptococcal toxic shock-like syndrome (STSLS), which is commonly seen in cases of severe S. suis infection. STSLS is often accompanied by excessive production of inflammatory cytokines, which is the main cause of death. This calls for development of new strategies to avert the damage caused by STSLS. In this study, we found for the first time that Baicalein, combined with ampicillin, effectively improved severe S. suis infection. Further experiments demonstrated that baicalein significantly inhibited the hemolytic activity of SLY by directly binding to SLY and destroying its secondary structure. Cell-based assays revealed that Baicalein did not exert toxic effects and conferred protection in S. suis-infected cells. Interestingly, compared with ampicillin alone, Baicalein combined with ampicillin resulted in a higher survival rate in mice severely infected with S. suis. At the same time, we found that baicalein can be combined with meropenem against MRSA. In conclusion, these results indicate that baicalein has a good application prospect.

Autophagy is a defense mechanism controlling Streptococcus suis serotype 2 infection in murine microglia cells

Streptococcus suis (S. suis) is an important swine and human pathogen, causing severe meningitis with high morbidity and mortality worldwide. Microglial activation and inflammation are responsible for bacterial meningitis. S. suis has been identified to activate microglia, but the role of autophagy following S. suis infection in microglial cells remains elusive. In this study, using western blot, immunofluorescent staining and transmission electron microscopy (TEM), we demonstrated that S. suis serotype 2 (SS2) triggered autophagosome and enhanced autophagic flux in BV2 microglial cells. Autophagy activators, rapamycin, could further promote autophagy in S. suis-infected BV2 cells. Conversely, autophagy inhibitors including siRNA targeting ATG5, Beclin-1, ATG9a and ATG12 attenuated the autophagic process. Consistent with the in vitro results, autophagy was activated following S. suis infection in brain tissue including frontal cortex and hippocampus in a mouse model of meningitis. Further experiment showed that autophagy serves as a cellular defense mechanism to limit invaded bacteria and microglia inflammation in S. suis-infected BV2 cells. This is the first study reporting that the interaction between autophagy and microglia cells in response to S. suis infection. The possible mechanism involved could additionally suggest potential therapeutic approaches for bacterial meningitis.

Effective Antibacterial and Antihemolysin Activities of Ellipticine Hydrochloride against Streptococcus suis in a Mouse Model

Streptococcal toxic shock-like syndrome (STSLS) caused by the epidemic strain of Streptococcus suis leads to severe inflammation and high mortality. The life and health of humans and animals are also threatened by the increasingly severe antimicrobial resistance in Streptococcus suis There is an urgent need to discover novel strategies for the treatment of S. suis infection. Suilysin (SLY) is considered to be an important virulence factor in the pathogenesis of S. suis In this study, ellipticine hydrochloride (EH) was reported as a compound that antagonizes the hemolytic activity of SLY. In vitro, EH was found to effectively inhibit SLY-mediated hemolytic activity. Furthermore, EH had a strong affinity for SLY, thereby directly binding to SLY to interfere with the hemolytic activity. Meanwhile, it was worth noting that EH was also found to have a significant antibacterial activity. In vivo, compared with traditional ampicillin, EH not only significantly improved the survival rate of mice infected with S. suis 2 strain Sc19 but also relieved lung pathological damage. Furthermore, EH effectively decreased the levels of inflammatory cytokines (interleukin-6 [IL-6], tumor necrosis factor alpha [TNF-α]) and blood biochemistry enzymes (alanine transaminase [ALT], aspartate transaminase [AST], creatine kinase [CK]) in Sc19-infected mice. Additionally, EH markedly reduced the bacterial load of tissues in Sc19-infected mice. In conclusion, our findings suggest that EH can be a potential compound for treating S. suis infection in view of its antibacterial and antihemolysin activity.IMPORTANCE In recent years, the inappropriate use of antibiotics has unnecessarily caused the continuous emergence of resistant bacteria. The antimicrobial resistance of Streptococcus suis has also become an increasingly serious problem. Targeting virulence can reduce the selective pressure of bacteria on antibiotics, thereby alleviating the development of bacterial resistance to a certain extent. Meanwhile, the excessive inflammatory response caused by S. suis infection is considered the primary cause of acute death. Here, we found that ellipticine hydrochloride (EH) exhibited effective antibacterial and antihemolysin activities against S. suisin vitroIn vivo, compared with ampicillin, EH had a significant protective effect on S. suis serotype 2 strain Sc19-infected mice. Our results indicated that EH, with dual antibacterial and antivirulence effects, will contribute to treating S. suis infections and alleviating the antimicrobial resistance of S. suis to a certain extent. More importantly, EH may develop into a promising drug for the prevention of acute death caused by excessive inflammation.

Apigenin and Ampicillin as Combined Strategy to Treat Severe Streptococcus suis Infection

As an important zoonotic pathogen, Streptococcus suis (S. suis) can cause a variety of diseases both in human and animals, especially Streptococcal toxic shock-like syndrome (STSLS), which commonly appears in severe S. suis infection. STSLS is often accompanied by excessive production of inflammatory cytokines, which is the main cause of host death. Therefore, it is urgent to find a new strategy to relieve the damage caused by STSLS. In this study, we found, for the first time, that apigenin, as a flavonoid compound, could combine with ampicillin to treat severe S. suis infection. Studies found that apigenin did not affect the growth of S. suis and the secretion of suilysin (SLY), but it could significantly inhibit the hemolytic activity of SLY by directly binding to SLY and destroying its secondary structure. In cell assays, apigenin was found to have no significant toxic effects on effective concentrations, and have a good protective effect on S. suis-infected cells. More importantly, compared with the survival rate of S. suis-infected mice treated with only ampicillin, the survival rate of apigenin combined with an ampicillin-treated group significantly increased to 80%. In conclusion, all results indicate that apigenin in combination with conventional antibiotics can be a potential strategy for treating severe S. suis infection.

Acquiring high expression of suilysin enable non-epidemic Streptococccus suis to cause streptococcal toxic shock-like syndrome (STSLS) through NLRP3 inflammasome hyperactivation

The epidemic Streptococcus suis (S. suis) strain [Sequence type (ST) 7] was gradually evolving from the non-epidemic ST1 strain and got the ability for high expressing of suilysin (SLY). And the high expression of SLY was required for the epidemic strain to cause NLRP3 hyperactivation, which is essential for the induction of cytokines storm, dysfunction of multiple organs, and a high incidence of mortality, the characters of streptococcal toxic shock-like syndrome (STSLS). However, it remains to be elucidated whether acquiring high SLY expression due to genome evolution was sufficient for the non-epidemic strain to cause STSLS. Here, we found that the overexpression of SLY in ST1 strain (P1/7-SLY) could obviously increase the inflammasome activation, which was dependent on NLRP3 signalling. In contrast, the strain (P1/7-mSLY) overexpressing the mutant SLY (protein without hemolytic activity) could not significantly increase the inflammasome activation. Furthermore, similar to the epidemic strain, P1/7-SLY could cause STSLS in nlrp3^+/+ mice but not in nlrp3^−/− mice. In contrast, P1/7-mSLY could not cause STSLS in both nlrp3^+/+ mice and nlrp3^−/− mice. In summary, we demonstrate that genetic evolution enabling S. suis strain to express high level of SLY may be an essential and sufficient condition for NLRP3 inflammasome hyperactivation, which could further cause cytokines storm and STSLS.

Mode of action and structural modelling of the interaction of formononetin with suilysin

AIMS

Suilysin is a critical pore-forming virulence factor of Streptococcus suis that has been demonstrated to substantially contribute to its pathogenicity. We have demonstrated that formononetin alleviates S. suis infection both in vivo and in vitro by targeting suilysin. However, the molecular mechanism of the effect is unclear. Our aim was to determine the molecular mechanism of the effect of formononetin on suilysin.

METHODS AND RESULTS

The mechanism of interaction between formononetin and suilysin was investigated by molecular modelling. The results indicated that formononetin was bound at the junction of domain two and domain four of suilysin. The binding free energy values indicated that the A415, Y412, E414, N413, T61, T62 and G416 residues are critical for this binding, this observation was confirmed by the changes in the flexibility of these residues and the distances between these residues and formononetin. The inhibitory effect of formononetin on the pore-forming activity of suilysin, binding constant and binding free energy were significantly decreased by site-specific mutagenesis of Y412 and N413. Finally, we analysed the spatial configuration of suilysin before and after formononetin binding, the results indicated that the binding changed the conformation of suilysin, especially the angle between domain two and domain four, resulting in the disruption of cholesterol binding to suilysin and in the loss of pore-forming activity.

CONCLUSIONS

Formononetin is located at the junction of domain two and domain four of suilysin, and Y412 and N413 play critical roles in the binding. Formononetin binding changes the angle between domain two and domain four of suilysin, resulting in the loss of the pore-inducing activity of suilysin.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work will promote the application of formononetin to combat S. suis infections and may contribute to the development of new inhibitors or modification of existing inhibitors.

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.

Streptococcus suis Research: Progress and Challenges

Streptococcus suis is considered among the top bacterial pathogens leading to important economic losses to the swine industry, with the incidence of disease increasing as the prophylactic use of antimicrobial is being vanished worldwide. S. suis is also a zoonotic agent afflicting people in close contact with infected pigs or pork meat. Besides, in some Asian countries, it is considered a major public health concern for the general population as well. Antimicrobial resistance is one of the most important global health challenges, and in the absence of preventive measures (such as effective vaccines), S. suis remains a risk for increased antimicrobial resistance and transmission of resistance genes to other bacteria beyond the host animal species. The studies in this Special Issue have evidenced the importance of swine population demographics and management on disease control, progress in molecular tools to better understand the epidemiology of S. suis infections in swine and humans, and the mechanisms involved in different aspects of the immuno-pathogenesis of the disease. The importance of reducing the prophylactic use of antimicrobials in livestock productions and the development of alternative control measures, including vaccination, are herein discussed.

Cryptotanshinone ameliorates the pathogenicity of Streptococcus suis by targeting suilysin and inflammation

AIMS

Streptococcus suis is a highly zoonotic pathogen that is a serious threat to human health and the development of the pig industry worldwide. The virulence factors produced during S. suis infection play an important role, and the pore-forming activity of suilysin is considered an important virulence-related factor, especially in meningitis. Treatment of S. suis infection with traditional antibiotics is becoming increasingly challenging due to bacterial resistance. The purpose of this study is to verify the role of cryptotanshinone in the process of S. suis infection and provide a new drug precursor for the treatment of S. suis infection.

METHODS AND RESULTS

In this study, we used circular dichroism spectroscopy to demonstrate that cryptotanshinone alters the secondary structure of suilysin. The results of the antibacterial activity and haemolysis assays showed cryptotanshinone could inhibit the pore-forming activity of suilysin without affecting bacterial growth or its expression. We also showed that cryptotanshinone reduces bacterial damage and penetration in vitro, reduce the S. suis-induced inflammatory response and provide protection against bacterial infections in vivo and in vitro.

CONCLUSIONS

Cryptotanshinone is a potential compound precursor for treating S. suis infection.

SIGNIFICANCE AND IMPACT OF THE STUDY

Cryptotanshinone may be a promising leading compound for S. suis infection and related diseases.

Complement C3aR/C5aR-binding protein Suilysin of Streptococcus suis contributes to monocyte chemotaxis

Streptococcus suis is an emerging swine and human pathogen causing severe infections and sudden death. During infection, complement C3a and C5a were reported to induce immune cells towards infection and injury sites via their corresponding receptors C3aR and C5aR. However, how S. suis evade immune surveillance mediated by C3aR and C5aR remains unclear. In this study, we analyze and construct an S. suis bacterial two-hybrid prey library containing 39 LPXTG motif anchored proteins and 18 secreted proteins. Two highly possible C3aR-binding proteins: thiol-activated toxin Suilysin, putative RTX family exoprotein A gene and three highly possible C5aR-binding proteins: thiol-activated toxin Suilysin, putative 5'-nucleotidase and subtilisin-like serine protease are identified through bacterial two-hybrid assay. Far-western blot assay confirms that a cholesterol-binding cytolysin Suilysin can interact with both C3aR and C5aR. Chemotaxis assays demonstrate that recombinant and natural Suilysin can inhibit monocyte chemotaxis mediated by C3a and C5a. These findings enlarge our knowledge of suilysin biological significance and provide a new perspective on S. suis complement evasion.

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.

A Streptococcus suis Live Vaccine Suppresses Streptococcal Toxic Shock-Like Syndrome and Provides Sequence Type-Independent Protection

Background

Streptococcus suis is an encapsulated zoonotic pathogen. Increasing antimicrobial resistance invokes the need for effective vaccines. Despite many attempts to develop an effective vaccine, none is currently available.

Methods

A capsular polysaccharide (CPS)-expressing attenuated mutant 2015033 was constructed by deleting 5 virulence-associated factors (sly, scpA, ssnA, fhb, and ssads) in an infective S. suis strain SC19. The safety and immune effect of 2015033 were determined both in vitro and in vivo.

Results

Deletion of 5 genes did not impact the growth ability and CPS generation of 2015033, and the mutant exhibited no cytotoxicity in different cell models. 2015033 was more easily eliminated by innate immunity both in vitro and in vivo. In addition, 2015033 showed a diminished invasive ability in different mouse organs (brain, lung, and liver) and avirulent properties in mice associated with weak inflammation-inducing ability. Immunization with 2015033 triggered T cell-dependent immunity, suppressed streptococcal toxic shock-like syndrome, and conferred sequence type-independent protection to mice during infection.

Conclusions

This study presents the feasibility of the strategy of multigene deletion for the development of promising live vaccines against invasive encapsulated pathogens.

Isorhamnetin attenuates Streptococcus suis virulence by inhibiting the inflammatory response

Streptococcus suis (S. suis) is one of the most common swine pathogens in the swine industry and leads to great harm to the normal progress of the swine industry. S. suis can also infect humans and cause a variety of fatal diseases, such as meningitis and streptococcal toxic shock syndrome, that pose a major threat to the safety of life and health of both humans and animals. In this paper, we found that isorhamnetin, a natural flavonoid compound without activity against S. suis, could significantly reduce the S. suis-stimulated production of the inflammatory cytokines interleukin (IL)-1β, IL-6, and tumor necrosis factor alpha (TNF-α) and down-regulate the inflammatory response by inhibiting the activation of p38 and ERK in tissues infected with S. suis, thereby exerting protection against S. suis infection. The above findings suggest that isorhamnetin is a potential lead compound for the treatment of S. suis infections, thus laying a preliminary theoretical foundation for the further development of isorhamnetin as a candidate drug.

Ginkgetin in vitro and in vivo reduces Streptococcus suis virulence by inhibiting suilysin activity

AIMS

Suilysin (SLY), a crucial virulence-related factor, has multiple cytotoxicities that are regarded as playing a key role in several diseases induced by Streptococcus suis. The aim of this study was to identify an effective inhibitor of SLY and to evaluate the potential inhibitory effect of the inhibitor against S. suis virulence.

METHODS AND RESULTS

Antibacterial activity experiments and haemolysis tests were used to identify the SLY inhibitor ginkgetin, and Western blot analysis and oligomerization inhibition tests were employed to determine the potential mechanism for its inhibition effect. The potential inhibitory effect of ginkgetin against S. suis virulence was then assessed through a cytotoxicity test and a mouse infection model. In this study, we demonstrated that the natural ingredient ginkgetin can significantly reduce the haemolytic activity of SLY to protect against S. suis-mediated cell injury in vitro by directly binding to SLY to block the oligomerization of the protein and reducing the bacterial burden in vivo.

CONCLUSIONS

The results suggest that ginkgetin can start being used as a potential lead drug for the treatment of S. suis infections.

SIGNIFICANCE AND IMPACT OF THE STUDY

The prevention and treatment of S. suis infection might be possible through the targeting of SLY by ginkgetin.

PrsA contributes to Streptococcus suis serotype 2 pathogenicity by modulating secretion of selected virulence factors

Streptococcus suis serotype 2 (S. suis 2) is a major zoonotic pathogen. Parvulin-type peptidyl-prolyl isomerase (PrsA) in S. suis 2 is found surface-associated, pro-inflammatory and cytotoxic. To further explore the roles of PrsA in S. suis 2 infection, we constructed a prsA deletion mutant (ΔprsA) and a complemented strain (CΔprsA). The ΔprsA mutant showed increased length of bacterial chains and decreased growth. Deletion of prsA increased bacterial adhesion to host epithelial cells but with weakened invasion. The ΔprsA mutant had reduced survival in RAW264.7 macrophages and pig whole blood, and significantly attenuated in virulence to mice. All these phenotypes of the mutant could be reversed largely to the levels of its parental strain by gene complementation. Western blotting revealed that suilysin was markedly reduced both in surface-associated (SAP) and secreted fractions (SecP) of ΔprsA, which might be responsible for reduced hemolytic activity. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and enolase were significantly increased in both SAP and SecP fractions as a result of prsA deletion. Increased adhesion of the ΔprsA mutant to bEND.3 cells was prevented using polyclonal antibodies against GAPDH and enolase. Overall, we propose that S. suis 2 deploys PrsA to control translocation of important virulence factors, thereby favoring its survival in the host with enhanced pathogenicity by compromising its interactions with the host cells. Further investigation is required to find out how PrsA modulates protein translocation to benefit S. suis infection and if there are other S. suis 2 substrates of potential virulence regulated by PrsA.

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.

Binding of Fibronectin to SsPepO Facilitates the Development of Streptococcus suis Meningitis

Background

SsPepO is an important virulence in Streptococcus suis.

Methods

In this study, we showed that SsPepO contributes to the human fibronectin-mediated adherence ability of S. suis to human brain microvascular endothelial cells.

Results

The addition of an antifibronectin antibody or an arginine-glycine-aspartic acid peptide that blocks fibronectin binding to integrins significantly reduced adherence of the wild-type but not the SspepO mutant strain, indicating the importance of the SsPepO-fibronectin-integrin interaction for S. suis cellular adherence.

Conclusions

By analyzing Evans blue extravasation in vivo, we showed that the interaction between SsPepO and human fibronectin significantly increased permeability of the blood-brain barrier. Furthermore, the SspepO mutant caused lower bacterial loads in the brain than wild-type S. suis in models of meningitis. These data demonstrate that SsPepO is a fibronectin-binding protein, which plays a contributing role in the development of S. suis meningitis.

An NLRP3 inflammasome-triggered cytokine storm contributes to Streptococcal toxic shock-like syndrome (STSLS)

Infection with the Streptococcus suis (S. suis) epidemic strain can cause Streptococcal toxic shock-like syndrome (STSLS), which is characterized by a cytokine storm, dysfunction of multiple organs and a high incidence of mortality despite adequate treatment. Despite some progress concerning the contribution of the inflammatory response to STSLS, the precise mechanism underlying STSLS development remains elusive. Here, we use a murine model to demonstrate that caspase-1 activity is critical for STSLS development. Furthermore, we show that inflammasome activation by S. suis is mainly dependent on NLRP3 but not on NLRP1, AIM2 or NLRC4. The important role of NLRP3 activation in STSLS is further confirmed in vivo with the NLRP3 inhibitor MCC950 and nlrp3-knockout mice. By comparison of WT strain with isogenic strains with mutation of various virulence genes for inflammasome activation, Suilysin is essential for inflammasome activation, which is dependent on the membrane perforation activity to cause cytosolic K+ efflux. Moreover, the mutant strain msly (P353L) expressing mutagenic SLY without hemolytic activity was unable to activate the inflammasome and does not cause STSLS. In summary, we demonstrate that the high membrane perforation activity of the epidemic strain induces a high level of NLRP3 inflammasome activation, which is essential for the development of the cytokine storm and multi-organ dysfunction in STSLS and suggests NLRP3 inflammasome as an attractive target for the treatment of STSLS.

Quercetin reduces Streptococcus suis virulence by inhibiting suilysin activity and inflammation

Streptococcus suis, a globally distributed bacterial pathogen, is an important zoonotic agent for humans and animals that can lead to multiple deaths and cause major economic losses. Suilysin (SLY), secreted by most pathogenic S. suis strains, is a cytotoxic toxin that belongs to the cholesterol-dependent cytolysin family; this toxin plays a key role in a mouse meningitis model, suggesting that effective interference with the biological activity of SLY may be a potential treatment for S. suis infection. In addition, the inflammatory response induced by S. suis is an important manifestation in infections and is associated with multiple fatal diseases. In this study, we found that the natural compound quercetin can directly inhibit the pore-forming activity of SLY without affecting bacterial growth and SLY secretion at the concentrations tested in our assay. In addition, quercetin treatment significantly alleviated cytotoxicity caused by S. suis infection and effectively reduced the release of the pro-inflammatory cytokines IL-1β, IL-6, and tumor necrosis factor alpha (TNF-α) stimulated by bacteria. Significantly decreased mortality was observed for the S. suis-infected mice that received quercetin. Our results suggested that quercetin may represent a promising therapeutic candidate for S. suis infection by targeting SLY and the subsequent inflammation. The present study provides a new strategy and leading compound for S. suis infection.

Listeriolysin O: A phagosome-specific cytolysin revisited

Listeriolysin O (LLO) is an essential determinant of Listeria monocytogenes pathogenesis that mediates the escape of L. monocytogenes from host cell vacuoles, thereby allowing replication in the cytosol without causing appreciable cell death. As a member of the cholesterol-dependent cytolysin (CDC) family of pore-forming toxins, LLO is unique in that it is secreted by a facultative intracellular pathogen, whereas all other CDCs are produced by pathogens that are largely extracellular. Replacement of LLO with other CDCs results in strains that are extremely cytotoxic and 10,000-fold less virulent in mice. LLO has structural and regulatory features that allow it to function intracellularly without causing cell death, most of which map to a unique N-terminal region of LLO referred to as the proline, glutamic acid, serine, threonine (PEST)-like sequence. Yet, while LLO has unique properties required for its intracellular site of action, extracellular LLO, like other CDCs, affects cells in a myriad of ways. Because all CDCs form pores in cholesterol-containing membranes that lead to rapid Ca²⁺ influx and K⁺ efflux, they consequently trigger a wide range of host cell responses, including mitogen-activated protein kinase activation, histone modification, and caspase-1 activation. There is no debate that extracellular LLO, like all other CDCs, can stimulate multiple cellular activities, but the primary question we wish to address in this perspective is whether these activities contribute to L. monocytogenes pathogenesis.

The XRE Family Transcriptional Regulator SrtR in Streptococcus suis Is Involved in Oxidant Tolerance and Virulence

Streptococcus suis is a zoonotic pathogen that harbors anti-oxidative stress genes, which have been reported to be associated with virulence. Serial passage has been widely used to obtain phenotypic variant strains to investigate the functions of important genes. In the present study, S. suis serotype 9 strain DN13 was serially passaged in mice 30 times. The virulence of a single colony from passage 10 (SS9-P10) was found to increase by at least 140-fold as indicated by LD₅₀ values, and the increased virulence was stable for single colonies from passage 20 (SS0-P20) and 30 (SS0-P30). Compared to the parental strain, the mouse-adapted strains were more tolerant to oxidative and high temperature stress. Genome-wide analysis of nucleotide variations found that reverse mutations occurred in seven genes, as indicated by BLAST analysis. Three of the reverse mutation genes or their homologs in other bacteria were reported to be virulence-associated, including ide_Ssuis in S. suis, a homolog of malR of Streptococcus pneumoniae, and a homolog of the prepilin peptidase-encoding gene in Legionella pneumophila. However, these genes were not involved in the stress response. Another gene, srtR (stress response transcriptional regulator), encoding an XRE family transcriptional regulator, which had an internal stop in the parental strain, was functionally restored in the adapted strains. Further analysis of DN13 and SS9-P10-background srtR-knock-out and complementing strains supported the contribution of this gene to stress tolerance in vitro and virulence in mice. srtR and its homologs are widely distributed in Gram-positive bacteria including several important human pathogens such as Enterococcus faecium and Clostridioides difficile, indicating similar functions in these bacteria. Taken together, our study identified the first member of the XRE family of transcriptional regulators that is involved in stress tolerance and virulence. It also provides insight into the mechanism of enhanced virulence after serial passage in experimental animals.

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

Streptococcus suis, an important zoonotic pathogen, has caused considerable economic losses in the swine industry and severe public health issues worldwide. The development of a novel effective strategy for the prevention and therapy of S. suis is urgently needed. Here, amentoflavone, a natural biflavonoid compound isolated from Chinese herbs that has negligible anti-S. suis activity, was identified as a potent antagonist of suilysin (SLY)-mediated hemolysis without interfering with the expression of SLY. Amentoflavone effectively inhibited SLY oligomerization, which is critical for its pore-forming activity. The treatment with amentoflavone reduced S. suis-induced cytotoxicity in macrophages (J774 cells). Furthermore, S. suis-infected mice that received amentoflavone exhibited lower mortality and bacterial burden. Additionally, amentoflavone significantly decreased the production of tumor necrosis factor alpha (TNF-α), interleukin-1β (IL-1β), and IL-6 in an S. suis-infected cell model. Analyses of signaling pathways demonstrated that amentoflavone reduced S. suis-induced inflammation in S. suis serotype 2 (SS2)-infected cells by regulating the p38, Jun N-terminal protein kinase 1 and 2 (JNK1/2), and NF-κB pathways. The antivirulence and anti-inflammatory properties of amentoflavone against S. suis infection provide the possibility for future pharmaceutical application of amentoflavone in the treatment of S. suis infection.IMPORTANCE The widespread use of antibiotics in therapy and in the prevention of Streptococcus suis infection in the swine industry raises concerns for the emergence of a resistant strain. The use of antivirulence agents has potential benefits, mainly because of the reduced selective pressure for the development of bacterial resistance. In this study, we found that amentoflavone is an effective agent against S. suis serotype 2 (SS2) infection both in vitro and in vivo Our results demonstrated that amentoflavone is a promising anti-infective therapeutic for S. suis infections, due to its antivirulence and anti-inflammatory effects without antibacterial activity, with fewer side effects than conventional antibacterial agents.

Silibinin attenuates Streptococcus suis serotype 2 virulence by targeting suilysin

AIMS

To determine the antivirulence properties of silibinin against suilysin (SLY), a virulence factor of Streptococcus suis serotype 2 (SS2) that plays an important role in the pathogenesis of S. suis infection and its protective effect against SS2 infection in a mouse model.

METHODS AND RESULTS

Susceptibility testing, haemolysis assay and Western blot assays were employed to evaluate the performance of silibinin on SLY pore-forming activity. Cytotoxicity assays and mouse infection tests were also performed to determine the efficacy of silibinin against SS2 infection. The results showed that silibinin, a flavonoid with little anti-S. suis activity, was identified to be a potent antagonist of SLY-mediated haemolysis through the inhibition of its oligomerization. Treatment with silibinin reduced S. suis-induced cytotoxicity in macrophages (J774 cells). In addition, S. suis-infected mice that received silibinin showed a lower bacterial burden.

CONCLUSIONS

Our results demonstrated that silibinin is a promising candidate for the development of antivirulence therapeutic agents to treat S. suis infections.

SIGNIFICANCE AND IMPACT OF THE STUDY

The antivirulent property of silibinin against SS2 by targeting SLY provides the possibility for the future pharmaceutical application of silibinin to prevent and treat S. suis infection.

Fisetin Lowers Streptococcus suis serotype 2 Pathogenicity in Mice by Inhibiting the Hemolytic Activity of Suilysin

Streptococcus suis serotype 2 is a serious zoonotic pathogen and has attracted worldwide attention since the first human case was reported in Denmark in 1968. Some virulence factors have been reported to be involved in the pathogenesis of the infection caused by Streptococcus suis serotype 2, and then novel strategies to identify some anti-virulence compounds which can effectively inhibit the pathogenic bacterial infection have recently been reported. Suilysin is an essential virulence factor for Streptococcus suis serotype 2 since it creates pores in the target cells membranes, which aids bacterial colonization. The important role of suilysin in the virulence of Streptococcus suis serotype 2 renders it an ideal target for designing novel anti-virulence therapeutics. We find that fisetin, as a natural flavonoid, is a potent antagonist against suilysin-mediated hemolysis. The aim of this study is to evaluate the effect of fisetin on the hemolytic activity of suilysin from Streptococcus suis serotype 2. Fisetin is found to significantly inhibit the hemolytic activity of suilysin. Within the range of effective concentrations, fisetin does not influence the growth of Streptococcus suis serotype 2 and the expression of suilysin protein. In vitro, fisetin effectively inhibits the death of macrophages (J774A.1 and RAW264.7) infected with Streptococcus suis serotype 2 by weakening intracellular bacterial multiplication. Animal model experiment shows that fisetin effectively improves the survival rate of animals infected with Streptococcus suis serotype 2. Our findings suggest that fisetin could be used as an antitoxin against suilysin and be developed into a promising therapeutic candidate for treating Streptococcus suis serotype 2 infection.

Intracranial Subarachnoidal Route of Infection for Investigating Roles of Streptococcus suis Biofilms in Meningitis in a Mouse Infection Model

Streptococcus suis is not only a major bacterial pathogen of pigs worldwide but also an emerging zoonotic agent. In humans and pigs, meningitis is a major manifestation of S. suis infections. A suitable infection model is an essential tool to understand the mechanisms of diseases caused by pathogens. Several routes of infection in mice have been developed to study the pathogenesis of S. suis infection. However, the intraperitoneal, intranasal, and intravenous routes of infection are not suitable for studying the roles of S. suis surface components in meningitis directly in the brain, such as the extracellular matrix from biofilms. Although intracisternal inoculation has been used for S. suis infection, the precise injection site has not been described. Here, the intracranial subarachnoidal route of infection was described in a mouse model to investigate the roles of biofilms in S. suis meningitis. S. suis planktonic cells or biofilm state cells were directly injected into the subarachnoid space of mice through the injection site located 3.5 mm rostral from the bregma. Histopathological analysis and increased mRNA expression of TLR2 and cytokines of the brain tissue from mice injected with biofilm state cells clearly indicated that S. suis biofilm plays definitive roles in S. suis meningitis. This route of infection has obvious advantages over other routes of infection, allowing the study of the host-bacterium interaction. Furthermore, it permits the effect of bacterial components on host immune responses directly in the brain to be assessed, and mimics bacterial entrance into the central nervous system. This route of infection can be extended for investigating the mechanisms of meningitis caused by other bacteria. In addition, it can also be used to test the efficacy of drugs against bacterial meningitis.

Genotypic diversity of Streptococcus suis strains isolated from humans in Thailand

The purpose of this study is to characterize Streptococcus suis isolates recovered from human infections regarding serotype distribution, genotypic profile, clinical manifestations, and epidemiology. A total of 668 S. suis isolates recovered from human infections in Thailand were characterized based on serotyping by multiplex PCR and co-agglutination, genotypic profiles by multilocus sequence typing, and PCR for virulence-associated genes, as well as review of medical records. Serotype 2 (94.6%) was predominant, followed by serotype 14 (4.5%), 24 (0.45%), 5 (0.3%), and 4 (0.15%). Multilocus sequence typing analyses revealed seven clonal complexes (CC): CC1 (56.43%), CC104 (31.74%), CC233/379 (5.4%), CC25 (4.5%), CC28 (0.9%), CC221/234 (0.6%), CC94 (0.15%), and two singletons. The CC1 group contained serotype 2 and 14 isolates, while CC25, 28, 104, and 233/379 consisted of serotype 2 isolates only. CC221/234 contained serotype 5 and 24 isolates, whereas the single serotype 4 isolate belonged to CC94. Two singletons contained serotype 5 (ST235) and 2 (ST236) isolates. Our data showed that ST1 isolates were more associated with meningitis than those of other STs (p < 0.001). The major route of infection was shown to be close contact with infected pigs or contaminated raw pork-derived products, including occupational exposure and recent consumption of raw pork products. This study revealed a relatively large number of CCs of S. suis causing human infection in Thailand. Among them, CC1 followed by CC104, with serotype 2 isolates, are predominant. Food safety campaigns and public health interventions would be important for controlling the S. suis infection in humans.

Investigation of the inhibition effect and mechanism of myricetin to Suilysin by molecular modeling

In the present study, the inhibitory effect and mechanism of myricetin, a natural flavonoid compound, in relation to Suilysin (SLY) were investigated through molecular dynamics simulations, mutational analysis and fluorescence-quenching assays. Myricetin is a potential inhibitor that does not exhibit antimicrobial activity but has been shown to inhibit SLY cytotoxicity. Molecular dynamics simulations and mutational analysis revealed that myricetin binds directly to SLY in the gap between domains 2 and 3, an important region for oligomerization and pore formation. The results of principal component analysis (PCA) indicated that the binding of myricetin in this gap region restricts the conformational transition of SLY from a monomer to an oligomer, thereby counteracting the haemolytic activity of SLY. This mechanism was verified using a haemolysis assay. These results demonstrated that myricetin is a strong candidate as a novel therapeutic agent for the treatment of Streptococcus suis infections.

Interaction of factor H-binding protein of Streptococcus suis with globotriaosylceramide promotes the development of meningitis

Streptococcus suis is an important emerging zoonotic agent that causes acute bacterial meningitis in humans with high mortality and morbidity. Our previous work showed that factor H-binding protein (Fhb) contributed to virulence of S. suis, but the role of Fhb in the development of S. suis meningitis remained unclear. In this study, we demonstrated for the first time that Fhb contributed to the traversal of S. suis across the human blood-brain barrier by allelic-exchange mutagenesis, complementation and specific antibody blocking studies. We also showed that globotriaosylceramide (Gb3), the receptor of Fhb, was involved in this process and affected S. suis infection-induced activation of myosin light chain 2 through Rho/ROCK signaling in hCMEC/D3 cells. Using a murine model of S. suis meningitis, we further demonstrated that Gb3-deficiency prevented the mice from developing severe brain inflammation or injury. Our results demonstrate that the Fhb-Gb3 interaction plays an important role in the development of S. suis meningitis and might be a potential therapeutic target against S. suis infection.

Morin Attenuates Streptococcus suis Pathogenicity in Mice by Neutralizing Suilysin Activity

Streptococcus suis, a Gram-positive pathogen, is widely recognized as an important agent of swine infection, and it is also known to cause a variety of zoonoses, such as meningitis, polyarthritis and pneumonia. Suilysin (SLY), an extracellular pore-forming toxin that belongs to the cholesterol-dependent cytolysin family, is an essential virulence factor of S. suis capsular type 2 (SS2). Here, we found that morin hydrate (morin), a natural flavonoid that lacks anti-SS2 activity, inhibits the hemolytic activity of SLY, protects J774 cells from SS2-induced injury and protects mice from SS2 infection. Further, by molecular modeling and mutational analysis, we found that morin binds to the "stem" domain 2 in SLY and hinders its transformation from the monomer form to the oligomer form, which causes the loss of SLY activity. Our study demonstrates that morin hinders the cell lysis activity of SLY through a novel mechanism of interrupting the heptamer formation. These findings may lead to the development of promising therapeutic candidates for the treatment of SS2 infections.

Critical Streptococcus suis Virulence Factors: Are They All Really Critical?

Streptococcus suis is an important swine pathogen that can be transmitted to humans by contact with diseased animals or contaminated raw pork products. This pathogen possesses a coat of capsular polysaccharide (CPS) that confers protection against the immune system. Yet, the CPS is not the only virulence factor enabling this bacterium to successfully colonize, invade, and disseminate in its host leading to severe systemic diseases such as meningitis and toxic shock-like syndrome. Indeed, recent research developments, cautiously inventoried in this review, have revealed over 100 'putative virulence factors or traits' (surface-associated or secreted components, regulatory genes or metabolic pathways), of which at least 37 have been claimed as being 'critical' for virulence. In this review we discuss the current contradictions and controversies raised by this explosion of virulence factors and the future directions that may be conceived to advance and enlighten research on S. suis pathogenesis.

Genotyping and investigating capsular polysaccharide synthesis gene loci of non-serotypeable Streptococcus suis isolated from diseased pigs in Canada

Streptococcus suis (S. suis) is an important swine pathogen and an emerging zoonotic agent. Most clinical S. suis strains express capsular polysaccharides (CPS), which can be typed by antisera using the coagglutination test. In this study, 79 S. suis strains recovered from diseased pigs in Canada and which could not be typed using antisera were further characterized by capsular gene typing and sequencing. Four patterns of cps locus were observed: (1) fifteen strains were grouped into previously reported serotypes but presented several mutations in their cps loci, when compared to available data from reference strains; (2) seven strains presented a complete deletion of the cps locus, which would result in an inability to synthesize capsule; (3) forty-seven strains were classified in recently described novel cps loci (NCLs); and (4) ten strains carried novel NCLs not previously described. Different virulence gene profiles (based on the presence of mrp, epf, and/or sly) were observed in these non-serotypeable strains. This study provides further insight in understanding the genetic characteristics of cps loci in non-serotypeable S. suis strains recovered from diseased animals. When using a combination of the previously described 35 serotypes and the complete NCL system, the number of untypeable strains recovered from diseased animals in Canada would be significantly reduced.

EGFR transactivation contributes to neuroinflammation in Streptococcus suis meningitis

BACKGROUND

Streptococcus suis serotype 2 (SS2) is an important zoonotic bacterial pathogen in both humans and animals, which can cause high morbidity and mortality. Meningitis is one of the major clinical manifestations of SS2 infection. However, the specific process of SS2 meningitis and its molecular mechanisms remain unclear. Epidermal growth factor receptor (EGFR) has been reported to initiate transduction of intracellular signals and regulate host inflammatory responses. Whether and how EGFR contributes to the development of S. suis meningitis are currently unknown.

METHODS

The tyrosine phosphorylation of cellular proteins, the transactivation of EGFR, as well as its dimerization, and the associated signal transduction pathways were investigated by immunoprecipitation and western blotting. Real-time quantitative PCR was used to investigate the transcriptional level of the ErbB family members, EGFR-related ligands, cytokines, and chemokines. The secretion of cytokines and chemokines in the serum and brain were detected by Q-Plex™ Chemiluminescent ELISA.

RESULTS

We found an important role of EGFR in SS2 strain SC19-induced meningitis. SC19 increasingly adhered to human brain microvascular endothelial cells (hBMEC) and caused inflammatory lesions in the brain tissues, with significant induction and secretion of proinflammatory cytokines and chemokines in the serum and brains. SC19 infection of hBMEC induced tyrosine phosphorylation of cellular EGFR in a ligand-dependent manner involving the EGF-like ligand HB-EGF, amphiregulin (AREG), and epiregulin (EREG) and led to heterodimerization of EGFR/ErbB3. The EGFR transactivation did not participate in SS2 strain SC19 adhesion of hBMEC, as well as in bacterial colonization in vivo. However, its transactivation contributed to the bacterial-induced neuroinflammation, via triggering the MAPK-ERK1/2 and NF-κB signaling pathways in hBMEC that promote the production of proinflammatory cytokines and chemokines.

CONCLUSIONS

We investigated for the first time the tyrosine phosphorylation of cellular proteins in response to SS2 strain SC19 infection of hBMEC and demonstrated the contribution of EGFR to SS2-induced neuroinflammation. These observations propose a novel mechanism involving EGFR in SS2-mediated inflammatory responses in the brain, and therefore, EGFR might be an important host target for further investigation and prevention of neuroinflammation caused by SS2 strains.