Streptococcus suis serotype 2 interactions with human brain microvascular endothelial cells

Genomic analysis and virulence of human Streptococcus suis serotype 14

PURPOSE

Streptococcus suis serotype 14 is the second most prevalent serotype being highly prevalent in Southeast Asia. This study aimed to characterize genetic background, population structure, virulent genes, antimicrobial-resistant genes, and virulence of human S. suis serotype 14.

METHODS

Genomes of 11 S. suis serotype 14 were sequenced by short- and long-read sequencing platforms. The genomes were analyzed for genetic relationship, virulence-associated genes, and antimicrobial-resistant genes. Antimicrobial susceptibility was conducted and the virulence was tested based on cell assay.

RESULTS

All isolates belonged to clonal complex (CC) 1, with nine sequence type (ST) 105 isolates and each isolate of ST1 and ST237. They were susceptible to penicillin, whereas tetracycline and macrolide were resistance due to tetO and ermB. Genomic analysis revealed that the serotype 14-ST105 isolates were closely related to zoonotic serotype 14-ST105 isolates from Vietnam and the serotype 1-ST105 Thai strain. The serotype 14-ST1 isolate was closely related to pig-diseased serotype 1-ST1 isolates from UK and USA, whereas the serotype 14-ST237 isolate was related to serotype 1-ST237 strains recovered from healthy pig from Thailand. Of 150 virulence-associated genes, 13 were absent from the serotype 14 isolates, including atl1, atlAss, hhly3, nisK, nisR, pnuC, salK, salR, sp1, srtG, virB4, virD4, and zmp. The virulence of strain 32481, a representative S. suis serotype 14-ST105 isolate showed reduced adhesion and invasion of two epithelial cell lines (A549 and HeLa) when compared to the serotype 2-ST1 strain P1/7, whereas apoptosis was similar.

CONCLUSION

This study highlighted the pathogenic potential of virulent serotype 14-ST105 strains and the need for increased monitoring of S. suis serotypes other than for serotype 2.

Identification of plasminogen-binding sites in Streptococcus suis enolase that contribute to bacterial translocation across the blood-brain barrier

Streptococcus suis is an emerging zoonotic pathogen that can cause invasive disease commonly associated with meningitis in pigs and humans. To cause meningitis, S. suis must cross the blood-brain barrier (BBB) comprising blood vessels that vascularize the central nervous system (CNS). The BBB is highly selective due to interactions with other cell types in the brain and the composition of the extracellular matrix (ECM). Purified streptococcal surface enolase, an essential enzyme participating in glycolysis, can bind human plasminogen (Plg) and plasmin (Pln). Plg has been proposed to increase bacterial traversal across the BBB via conversion to Pln, a protease which cleaves host proteins in the ECM and monocyte chemoattractant protein 1 (MCP1) to disrupt tight junctions. The essentiality of enolase has made it challenging to unequivocally demonstrate its role in binding Plg/Pln on the bacterial surface and confirm its predicted role in facilitating translocation of the BBB. Here, we report on the CRISPR/Cas9 engineering of S. suis enolase mutants eno261, eno252/253/255, eno252/261, and eno434/435 possessing amino acid substitutions at in silico predicted binding sites for Plg. As expected, amino acid substitutions in the predicted Plg binding sites reduced Plg and Pln binding to S. suis but did not affect bacterial growth in vitro compared to the wild-type strain. The binding of Plg to wild-type S. suis enhanced translocation across the human cerebral microvascular endothelial cell line hCMEC/D3 but not for the eno mutant strains tested. To our knowledge, this is the first study where predicted Plg-binding sites of enolase have been mutated to show altered Plg and Pln binding to the surface of S. suis and attenuation of translocation across an endothelial cell monolayer in vitro.

Cholesterol dependent cytolysins and the brain: Revealing a potential therapeutic avenue for bacterial meningitis

Bacterial meningitis is a catastrophic nervous system disorder with high mortality and wide range of morbidities. Some of the meningitis-causing bacteria occupy cholesterol dependent cytolysins (CDCs) to increase their pathogenicity and arrange immune-evasion strategy. Studies have observed that the relationship between CDCs and pathogenicity in these meningitides is complex and involves interactions between CDC, blood-brain barrier (BBB), glial cells and neurons. In BBB, these CDCs acts on capillary endothelium, tight junction (TJ) proteins and neurovascular unit (NVU). CDCs also observed to elicit intriguing effects on brain inflammation which involves microglia and astrocyte activations, along with neuronal damage as the end-point of pathological pathways in bacterial meningitis. As some studies mentioned potential advantage of CDC-targeted therapeutic mechanisms to combat CNS infections, it might be a fruitful avenue to deepen our understanding of CDC as a candidate for adjuvant therapy to combat bacterial meningitis.

The adcA and lmb Genes Play an Important Role in Drug Resistance and Full Virulence of Streptococcus suis

Streptococcus suis is an recognized zoonotic pathogen of swine and severely threatens human health. Zinc is the second most abundant transition metal in biological systems. Here, we investigated the contribution of zinc to the drug resistance and pathogenesis of S. suis. We knocked out the genes of AdcACB and Lmb, two Zn-binding lipoproteins. Compared to the wild-type strain, we found that the survival rate of this double-mutant strain (ΔadcAΔlmb) was reduced in Zinc-limited medium, but not in Zinc-supplemented medium. Additionally, phenotypic experiments showed that the ΔadcAΔlmb strain displayed impaired adhesion to and invasion of cells, biofilm formation, and tolerance of cell envelope-targeting antibiotics. In a murine infection model, deletion of the adcA and lmb genes in S. suis resulted in a significant decrease in strain virulence, including survival rate, tissue bacterial load, inflammatory cytokine levels, and histopathological damage. These findings show that AdcA and Lmb are important for biofilm formation, drug resistance, and virulence in S. suis. IMPORTANCE Transition metals are important micronutrients for bacterial growth. Zn is necessary for the catalytic activity and structural integrity of various metalloproteins involved in bacterial pathogenic processes. However, how these invaders adapt to host-imposed metal starvation and overcome nutritional immunity remains unknown. Thus, pathogenic bacteria must acquire Zn during infection in order to successfully survive and multiply. The host uses nutritional immunity to limit the uptake of Zn by the invading bacteria. The bacterium uses a set of high-affinity Zn uptake systems to overcome this host metal restriction. Here, we identified two Zn uptake transporters in S. suis, AdcA and Lmb, by bioinformatics analysis and found that an adcA and lmb double-mutant strain could not grow in Zn-deficient medium and was more sensitive to cell envelope-targeting antibiotics. It is worth noting that the Zn uptake system is essential for biofilm formation, drug resistance, and virulence in S. suis. The Zn uptake system is expected to be a target for the development of novel antimicrobial therapies.

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.

Host HSPD1 Translocation from Mitochondria to the Cytoplasm Induced by Streptococcus suis Serovar 2 Enolase Mediates Apoptosis and Loss of Blood–Brain Barrier Integrity

Streptococcus suis serovar 2 (S. suis serovar 2) is a zoonotic pathogen that causes meningitis in pigs and humans, and is a serious threat to the swine industry and public health. Understanding the mechanism(s) by which S. suis serovar 2 penetrates the blood–brain barrier (BBB) is crucial to elucidating the pathogenesis of meningitis. In a previous study, we found that expression of the virulence factor enolase (Eno) by S. suis serovar 2 promotes the expression of host heat shock protein family D member 1 (HSPD1) in brain tissue, which leads to the apoptosis of porcine brain microvascular endothelial cells (PBMECs) and increased BBB permeability, which in turn promotes bacterial translocation across the BBB. However, the mechanism by which HSPD1 mediates Eno-induced apoptosis remains unclear. In this study, we demonstrate that Eno promotes the translocation of HSPD1 from mitochondria to the cytoplasm, where HSPD1 binds to β-actin (ACTB), the translocated HSPD1, and its interaction with ACTB led to adverse changes in cell morphology and promoted the expression of apoptosis-related proteins, second mitochondria-derived activator of caspases (Smac), and cleaved caspase-3; inhibited the expression of X-linked inhibitor of apoptosis protein (XIAP); and finally promoted cell apoptosis. These results further elucidate the role of HSPD1 in the process of Eno-induced apoptosis and increased BBB permeability, increasing our understanding of the pathogenic mechanisms of meningitis, and providing a framework for novel therapeutic strategies.

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.

Bacteria elevate extracellular adenosine to exploit host signaling for blood-brain barrier disruption

Bacterial meningitis remains a substantial cause of mortality worldwide and survivors may have severe lifelong disability. Although we know that meningeal bacterial pathogens must cross blood-central nervous system (CNS) barriers, the mechanisms which facilitate the virulence of these pathogens are poorly understood. Here, we show that adenosine from a surface enzyme (Ssads) of Streptococcus suis facilitates this pathogen’s entry into mouse brains. Monolayer translocation assays (from the human cerebrovascular endothelium) and experiments using diverse inhibitors and agonists together demonstrate that activation of the A1 adenosine receptor signaling cascade in hosts, as well as attendant cytoskeleton remodeling, promote S. suis penetration across blood-CNS barriers. Importantly, our additional findings showing that Ssads orthologs from other bacterial species also promote their translocation across barriers suggest that exploitation of A1 AR signaling may be a general mechanism of bacterial virulence.

A Comparative Transcriptome Analysis of Human and Porcine Choroid Plexus Cells in Response to Streptococcus suis Serotype 2 Infection Points to a Role of Hypoxia

Streptococcus suis (S. suis) is an important opportunistic pathogen, which can cause septicemia and meningitis in pigs and humans. Previous in vivo observations in S. suis-infected pigs revealed lesions at the choroid plexus (CP). In vitro experiments with primary porcine CP epithelial cells (PCPEC) and human CP epithelial papilloma (HIBCPP) cells demonstrated that S. suis can invade and traverse the CP epithelium, and that the CP contributes to the inflammatory response via cytokine expression. Here, next generation sequencing (RNA-seq) was used to compare global transcriptome profiles of PCPEC and HIBCPP cells challenged with S. suis serotype (ST) 2 infected in vitro, and of pigs infected in vivo. Identified differentially expressed genes (DEGs) were, amongst others, involved in inflammatory responses and hypoxia. The RNA-seq data were validated via quantitative PCR of selected DEGs. Employing Gene Set Enrichment Analysis (GSEA), 18, 28, and 21 enriched hallmark gene sets (GSs) were identified for infected HIBCPP cells, PCPEC, and in the CP of pigs suffering from S. suis ST2 meningitis, respectively, of which eight GSs overlapped between the three different sample sets. The majority of these GSs are involved in cellular signaling and pathways, immune response, and development, including inflammatory response and hypoxia. In contrast, suppressed GSs observed during in vitro and in vivo S. suis ST2 infections included those, which were involved in cellular proliferation and metabolic processes. This study suggests that similar cellular processes occur in infected human and porcine CP epithelial cells, especially in terms of inflammatory response.

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.

Streptococcus suis serotype 2 enolase interaction with host brain microvascular endothelial cells and RPSA-induced apoptosis lead to loss of BBB integrity

Host proteins interacting with pathogens are receiving more attention as potential therapeutic targets in molecular medicine. Streptococcus suis serotype 2 (SS2) is an important cause of meningitis in both humans and pigs worldwide. SS2 Enolase (Eno) has previously been identified as a virulence factor with a role in altering blood brain barrier (BBB) integrity, but the host cell membrane receptor of Eno and The mechanism(s) involved are unclear. This study identified that SS2 Eno binds to 40S ribosomal protein SA (RPSA) on the surface of porcine brain microvascular endothelial cells leading to activation of intracellular p38/ERK-eIF4E signalling, which promotes intracellular expression of HSPD1 (heat-shock protein family D member 1), and initiation of host-cell apoptosis, and increased BBB permeability facilitating bacterial invasion. This study reveals novel functions for the host-interactional molecules RPSA and HSPD1 in BBB integrity, and provides insight for new therapeutic strategies in meningitis.

Streptococcus suis Induces Expression of Cyclooxygenase-2 in Porcine Lung Tissue

Streptococcus suis is a common pathogen colonising the respiratory tract of pigs. It can cause meningitis, sepsis and pneumonia leading to economic losses in the pig industry worldwide. Cyclooxygenase-2 (COX-2) and its metabolites play an important regulatory role in different biological processes like inflammation modulation and immune activation. In this report we analysed the induction of COX-2 and the production of its metabolite prostaglandin E₂ (PGE₂) in a porcine precision-cut lung slice (PCLS) model. Using Western blot analysis, we found a time-dependent induction of COX-2 in the infected tissue resulting in increased PGE₂ levels. Immunohistological analysis revealed a strong COX-2 expression in the proximity of the bronchioles between the ciliated epithelial cells and the adjacent alveolar tissue. The morphology, location and vimentin staining suggested that these cells are subepithelial bronchial fibroblasts. Furthermore, we showed that COX-2 expression as well as PGE₂ production was detected following infection with two prevalent S. suis serotypes and that the pore-forming toxin suilysin played an important role in this process. Therefore, this study provides new insights in the response of porcine lung cells to S. suis infections and serves as a basis for further studies to define the role of COX-2 and its metabolites in the inflammatory response in porcine lung tissue during infections with S. suis.

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

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

FliC of Vibrio splendidus-related strain involved in adhesion to Apostichopus japonicus

Vibrio splendidus-related strains are important opportunistic marine pathogens, and they can infect many important marine animals, including the sea cucumber Apostichopus japonicus. In this study, one gene coding flagellin was cloned and a V. splendidus-related strain AJ01/GFPFliC with the overexpression of fliC gene was constructed to explore the function of FliC. AJ01/GFPFliC showed a 3-4 h delay in the initial growth phase and then its growth was faster than that of the wild type strain AJ01. The abilities of swarming motility and biofilm formation ability of AJ01/GFPFliC were also higher than that of AJ01. The adhesion rate of AJ01/GFPFliC to the slide and the coelomocytes of A. japonicus increased from 1% to 5%, and 25% to 40%, respectively, and the adhered AJ01/GFPFliC cells in intestinal tissue of A. japonicus reached 8.0 × 10⁶ CFU/g, which was 2.5-fold higher than that of the control strain AJ01/GFP. Concluded from all the data suggested that FliC was an adhesion factor of V. splendidus-related strain AJ01 that could also contribute to bacterial swarming motility and biofilm formation.

Dihydrolipoamide dehydrogenase of Vibrio splendidus is involved in adhesion to Apostichopus japonicus

Vibrio splendidus is one of the most opportunistic marine pathogens and infects many important marine animals, including the sea cucumber Apostichopus japonicus. In this study, two genes named DLD1 and DLD2, encoding dihydrolipoamide dehydrogenase (DLD) homologues in pathogenic V. splendidus, were cloned, and conditionally expressed in Escherichia coli BL21 (DE3). The enzymatic activities of DLD1 and DLD2 showed that they both belonged to the NADH oxidase family. Both DLD1 and DLD2 were located on the outer membrane of V. splendidus as detected by whole-cell ELISA. To study the adhesion function of DLD1 and DLD2, polyclonal antibodies were prepared, and antibody block assay was performed to detect the normal function of the two proteins. DLD1 and DLD2 were determined to play important roles in adhesion to different matrices and the adhesive ability of V. splendidus reduced more than 50% when DLD1 or DLD2 was defective.

Intranasal Vaccination With Multiple Virulence Factors Promotes Mucosal Clearance of Streptococcus suis Across Serotypes and Protects Against Meningitis in Mice

BACKGROUND

Streptococcus suis is an emerging zoonotic agent. Its natural habitat is the tonsils, which are the main portals of S. suis entry into the bloodstream of pigs. The remarkable variability of the bacteria and complex pathogenic mechanisms make the development of a vaccine a difficult task.

METHOD

Five conserved virulence factors involved in critical events of S. suis pathogenesis were combined and used as an intranasal vaccine (V5). The effect of V5 was investigated with intranasal and systemic challenge models.

RESULTS

V5 induced antibody and T-cell responses at the mucosal site and systemically. The immunity promoted clearance of S. suis from the nasopharynx independent of S. suis serotypes and reduced lethality after systemic challenge with S. suis serotype 2. Moreover, mice that survived sepsis from intravenous infection developed meningitis, whereas none of these mice showed neuropathological symptoms after V5 receipt.

CONCLUSION

Intranasal immunization with multiple conserved virulence factors decreases S. suis colonization at the nasopharynx across serotypes and inhibits the dissemination of the bacteria in the host. The protective mucosal immunity effects would potentially reduce the S. suis reservoir and prevent S. suis disease in pigs.

pdh modulate virulence through reducing stress tolerance and biofilm formation of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (S. suis 2) is a zoonotic pathogen. It causes meningitis, arthritis, pneumonia and sepsis in pigs, leading to extremely high mortality, which seriously affects public health and the development of the pig industry. Pyruvate dehydrogenase (PDH) is an important sugar metabolism enzyme that is widely present in microorganisms, mammals and higher plants. It catalyzes the irreversible oxidative decarboxylation of pyruvate to acetyl-CoA and reduces NAD+ to NADH. In this study, we found that the virulence of the S. suis ZY05719 sequence type 7 pdh deletion strain (Δpdh) was significantly lower than the wild-type strain (WT) in the mouse infection model. The distribution of viable bacteria in the blood and organs of mice infected with the Δpdh was significantly lower than those infected with WT. Bacterial survival rates were reduced in response to temperature stress, salt stress and oxidative stress. Additionally, compared to WT, the ability to adhere to and invade PK15 cells, biofilm formation and stress resistance of Δpdh were significantly reduced. Moreover, real-time PCR results showed that pdh deletion reduced the expression of multiple adhesion-related genes. However, there was no significant difference in the correlation biological analysis between the complemented strain (CΔpdh) and WT. Moreover, the survival rate of Δpdh in RAW264.7 macrophages was significantly lower than that of the WT strain. This study shows that PDH is involved in the pathogenesis of S. suis 2 and reduction in virulence of Δpdh may be related to the decreased ability to resist stress of the strain.

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.

Virulence Factors of Meningitis-Causing Bacteria: Enabling Brain Entry across the Blood-Brain Barrier

Infections of the central nervous system (CNS) are still a major cause of morbidity and mortality worldwide. Traversal of the barriers protecting the brain by pathogens is a prerequisite for the development of meningitis. Bacteria have developed a variety of different strategies to cross these barriers and reach the CNS. To this end, they use a variety of different virulence factors that enable them to attach to and traverse these barriers. These virulence factors mediate adhesion to and invasion into host cells, intracellular survival, induction of host cell signaling and inflammatory response, and affect barrier function. While some of these mechanisms differ, others are shared by multiple pathogens. Further understanding of these processes, with special emphasis on the difference between the blood-brain barrier and the blood-cerebrospinal fluid barrier, as well as virulence factors used by the pathogens, is still needed.

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.

Streptococcus suis Meningitis: Epidemiology, Clinical Presentation and Treatment

Streptococcus suis, a prevalent porcine pathogen, may sporadically cause infections in humans, and has recently emerged as a cause of zoonoses in some professionals. The aim of this article was to review available data on epidemiology, etiopathogenesis, diagnostics, and management of the most common form of S. suis infection, purulent meningitis. Literature data show that S. suis is an important etiological factor of purulent meningitis, especially in subjects being occupationally exposed to contact with pigs and/or pork meat. Owing to growing incidence of S. suis meningitis, a history of such exposure should be verified in each patient presenting with typical meningeal symptoms. Whenever S. suis was confirmed as the etiological factor of purulent meningitis, therapeutic protocol should be adjusted appropriately, to avoid patient's exposure to potentially ototoxic antimicrobial agents and corticosteroids. Considering the biphasic character of S. suis meningitis and its frequently atypical outcome, all individuals with this condition should be optimally supervised by a multidisciplinary team, including an ENT specialist.

Molecular typing of Streptococcus suis strains isolated from diseased and healthy pigs between 1996-2016

Streptococcus suis is an economically important pathogen of pigs as well as a zoonotic cause of human disease. Serotyping is used for further characterization of isolates; some serotypes seem to be more virulent and more widely spread than others. This study characterizes a collection of German field isolates of Streptococcus suis from pigs dating from 1996 to 2016 with respect to capsular genes (cps) specific for individual serotypes and pathotype by multiplex PCR and relates results to the clinical background of these isolates. The most prominent finding was the reduction in prevalence of serotype-2/serotype-1/2 among invasive isolates during this sampling period, which might be attributed to widely implemented autogenous vaccination programs in swine against serotype 2 in Germany. In diseased pigs (systemically ill; respiratory disease) isolates of serotype-1/serotype-14, serotype-2/serotype-1/2, serotype 3 to 5 and 7 to 9 were most frequent while in carrier isolates a greater variety of cps types was found. Serotype-1/serotype-14 seemed to be preferentially located in joints, serotype 4 and serotype 3 in the central nervous system, respectively. The virulence associated extracellular protein factor was almost exclusively associated with invasive serotype-1/serotype-14 and serotype-2/serotype-1/2 isolates. In contrast, lung isolates of serotype-2/serotype-1/2 mainly harbored the gene for muramidase-released protein. Serotype 4 and serotype 9 isolates from clinically diseased pigs most frequently carried the muramidase-released protein gene and the suilysin gene. When examined by transmission electron microscopy all but one of the isolates which were non-typable by molecular and serological methods showed various amounts of capsular material indicating potentially new serotypes among these isolates. Given the variety of cps types/serotypes detected in pigs, not only veterinarians but also medical doctors should consider other serotypes than just serotype 2 when investigating potential human cases of Streptococcus suis infection.

The VraSR regulatory system contributes to virulence in Streptococcus suis via resistance to innate immune defenses

Streptococcus suis is a highly invasive pathogen that can cause sepsis and meningitis in pigs and humans. However, we have limited understanding of the mechanisms S. suis uses to evade innate immunity. To investigate the involvement of the two-component signal transduction system of S. suis in host immune defense, we examined the expression of 15 response regulators of S. suis following stimulation with polymorphonuclear leukocytes (PMNs). We found that several response regulators were significantly up-regulated including vraR. Thus, we constructed an isogenic deletion mutant of vraSR genes in S. suis and demonstrated VraSR promotes both bacterial survival in human blood and resistance to human PMN-mediated killing. The VraSR mutant was more susceptible to phagocytosis by human PMNs and had greater sensitivity to oxidant and lysozyme than wild-type S. suis. Furthermore, in vitro findings and in vivo evidence from a mouse infection model together strongly demonstrate that ΔvraSR had greatly attenuated virulence compared with wild-type S. suis. Collectively, our data reveal that VraSR is a critical regulatory system that contributes to the survival of S. suis and its ability to defend against host innate immunity.

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.

The serine/threonine protein kinase of Streptococcus suis serotype 2 affects the ability of the pathogen to penetrate the blood-brain barrier

Streptococcus suis serotype 2 (SS2) is a zoonotic agent that causes meningitis in humans and pigs. However, the mechanism whereby SS2 crosses the microvasculature endothelium of the brain is not understood. In this study, transposon (TnYLB-1) mutagenesis was used to identify virulence factors potentially associated with invasive ability in pathogenic SS2. A poorly invasive mutant was identified and was found to contain a TnYLB-1 insertion in the serine/threonine kinase (stk) gene. Transwell chambers containing hBMECs were used to model the blood-brain barrier (BBB). We observed that the SS2 wild-type ZY05719 strain crossed the BBB model more readily than the mutant strain. Hence, we speculated that STK is associated with the ability of crossing blood-brain barrier in SS2. In vitro, compared with ZY05719, the ability of the stk-deficient strain (Δstk) to adhere to and invade both hBMECs and bEnd.3 cells, as well as to cross the BBB, was significantly attenuated. Immunocytochemistry using antibodies against claudin-5 in bEnd.3 cells showed that infection by ZY05719 disrupted BBB tight junction proteins to a greater extent than in infection by Δstk. The studies revealed that SS2 initially binds at or near intercellular junctions and crosses the BBB via paracellular traversal. Claudin-5 mRNA levels were indistinguishable in ZY05719- and Δstk-infected cells. This result indicated that the decrease of claudin-5 was maybe induced by protein degradation. Cells infected by ZY05719 exhibited higher ubiquitination levels than cells infected by Δstk. This result indicated that ubiquitination was involved in the degradation of claudin-5. Differential proteomic analysis showed that E3 ubiquitin protein ligase HECTD1 decreased by 1.5-fold in Δstk-infected bEnd.3 cells relative to ZY05719-infected cells. Together, the results suggested that STK may affect the expression of E3 ubiquitin ligase HECTD1 and subsequently increase the degradation of claudin-5, thus enabling SS2 to traverse the BBB.

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.

The non-conserved region of MRP is involved in the virulence of Streptococcus suis serotype 2

Muramidase-released protein (MRP) of Streptococcus suis serotype 2 (SS2) is an important epidemic virulence marker with an unclear role in bacterial infection. To investigate the biologic functions of MRP, 3 mutants named Δmrp, Δmrp domain 1 (Δmrp-d1), and Δmrp domain 2 (Δmrp-d2) were constructed to assess the phenotypic changes between the parental strain and the mutant strains. The results indicated that MRP domain 1 (MRP-D1, the non-conserved region of MRP from a virulent strain, a.a. 242-596) played a critical role in adherence of SS2 to host cells, compared with MRP domain 1* (MRP-D1*, the non-conserved region of MRP from a low virulent strain, a.a. 239-598) or MRP domain 2 (MRP-D2, the conserved region of MRP, a.a. 848-1222). We found that MRP-D1 but not MRP-D2, could bind specifically to fibronectin (FN), factor H (FH), fibrinogen (FG), and immunoglobulin G (IgG). Additionally, we confirmed that mrp-d1 mutation significantly inhibited bacteremia and brain invasion in a mouse infection model. The mrp-d1 mutation also attenuated the intracellular survival of SS2 in RAW246.7 macrophages, shortened the growth ability in pig blood and decreased the virulence of SS2 in BALB/c mice. Furthermore, antiserum against MRP-D1 was found to dramatically impede SS2 survival in pig blood. Finally, immunization with recombinant MRP-D1 efficiently enhanced murine viability after SS2 challenge, indicating its potential use in vaccination strategies. Collectively, these results indicated that MRP-D1 is involved in SS2 virulence and eloquently demonstrate the function of MRP in pathogenesis of 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.

Development and Evaluation of an Immunochromatographic Strip for Detection of Streptococcus Suis Type 2 Antibody

In this study, an immunochromatographic strip (ICS) was developed for the detection of antibody against Streptococcus suis serotype 2 (SS2). Colloidal gold particles labeled with staphylococcal protein A (SPA), which can bind to the FC fragment of mammalian immunoglobulin, were used as the detector reagent. The capsular polysaccharide (CPS) of SS2 and affinity-purified IgG from a healthy naive pig were immobilized on test and control regions of a nitrocellulose membrane, respectively. The ICS was used to 1) detect anti-CPS antibody in 14 sera taken from 4 SS2-infected pigs, 24 sera from pigs hyperimmunized with SS2, and 68 sera from pigs inoculated or infected with bacteria other than SS2; 2) determine anti-CPS antibody titers of 20 positive sera for comparison with enzyme-linked immunosorbent assay (ELISA); and 3) detect anti-CPS antibody in 226 clinical sera taken from diseased pigs also for comparison with ELISA. An ELISA used as a reference test determined the specificity and sensitivity of the ICS to be 97.1% and 86.3%, respectively. There was excellent agreement between the results obtained by ELISA and the ICS (kappa = 0.843). Additionally, there was strong agreement between the results of bacterial isolation from pig tonsils and ICS test (kappa = 0.658). Because it is rapid and easy to use, the test is suitable for the serological surveillance of SS2 at farms.

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.

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

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

Recruitment of Factor H to the Streptococcus suis Cell Surface is Multifactorial

Streptococcus suis is an important bacterial swine pathogen and a zoonotic agent. Recently, two surface proteins of S. suis, Fhb and Fhbp, have been described for their capacity to bind factor H—a soluble complement regulatory protein that protects host cells from complement-mediated damages. Results obtained in this study showed an important role of host factor H in the adhesion of S. suis to epithelial and endothelial cells. Both Fhb and Fhbp play, to a certain extent, a role in such increased factor H-dependent adhesion. The capsular polysaccharide (CPS) of S. suis, independently of the presence of its sialic acid moiety, was also shown to be involved in the recruitment of factor H. However, a triple mutant lacking Fhb, Fhbp and CPS was still able to recruit factor H resulting in the degradation of C3b in the presence of factor I. In the presence of complement factors, the double mutant lacking Fhb and Fhbp was similarly phagocytosed by human macrophages and killed by pig blood when compared to the wild-type strain. In conclusion, this study suggests that recruitment of factor H to the S. suis cell surface is multifactorial and redundant.

Biological activities of suilysin: role in Streptococcus suis pathogenesis

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

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

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

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

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

GidA, a tRNA Modification Enzyme, Contributes to the Growth, and Virulence of Streptococcus suis Serotype 2

Glucose-inhibited division protein (GidA), is a tRNA modification enzyme functioning together with MnmE in the addition of a carboxymethylaminomethyl group to position 5 of the anticodon wobble uridine of tRNA. Here, we report a GidA homolog from a Chinese isolate SC-19 of the zoonotic Streptococcus suis serotype 2 (SS2). gidA disruption led to a defective growth, increased capsule thickness, and reduced hemolytic activity. Moreover, the gidA deletion mutant (ΔgidA) displayed reduced mortality and bacterial loads in mice, reduced ability of adhesion to and invasion in epithelial cells, and increased sensitivity to phagocytosis. The iTRAQ analysis identified 372 differentially expressed (182 up- and 190 down-regulated) proteins in ΔgidA and SC-19. Numerous DNA replication, cell division, and virulence associated proteins were downregulated, whereas many capsule synthesis enzymes were upregulated by gidA disruption. This is consistent with the phenotypes of the mutant. Thus, GidA is a translational regulator that plays an important role in the growth, cell division, capsule biosynthesis, and virulence of SS2. Our findings provide new insight into the regulatory function of GidA in bacterial pathogens.

The CodY regulator is essential for virulence in Streptococcus suis serotype 2

The main role of CodY, a global regulatory protein in most low G + C gram-positive bacteria, is in transcriptional repression. To study the functions of CodY in Streptococcus suis serotype 2 (S. suis 2), a mutant codY clone named ∆codY was constructed to explore the phenotypic variation between ∆codY and the wild-type strain. The result showed that the codY mutation significantly inhibited cell growth, adherence and invasion ability of S. suis 2 to HEp-2 cells. The codY mutation led to decreased binding of the pathogen to the host cells, easier clearance by RAW264.7 macrophages and decreased growth ability in fresh blood of Cavia porcellus. The codY mutation also attenuated the virulence of S. suis 2 in BALB/c mice. Morphological analysis revealed that the codY mutation decreased the thickness of the capsule of S. suis 2 and changed the surface structures analylized by SDS-PAGE. Finally, the codY mutation altered the expressions of many virulence related genes, including sialic acid synthesis genes, leading to a decreased sialic acid content in capsule. Overall, mutation of codY modulated bacterial virulence by affecting the growth and colonization of S. suis 2, and at least via regulating sialic acid synthesis and capsule thickness.

Host-pathogen interactions in bacterial meningitis

Bacterial meningitis is a devastating disease occurring worldwide with up to half of the survivors left with permanent neurological sequelae. Due to intrinsic properties of the meningeal pathogens and the host responses they induce, infection can cause relatively specific lesions and clinical syndromes that result from interference with the function of the affected nervous system tissue. Pathogenesis is based on complex host–pathogen interactions, some of which are specific for certain bacteria, whereas others are shared among different pathogens. In this review, we summarize the recent progress made in understanding the molecular and cellular events involved in these interactions. We focus on selected major pathogens, Streptococcus pneumonia, S. agalactiae (Group B Streptococcus), Neisseria meningitidis, and Escherichia coli K1, and also include a neglected zoonotic pathogen, Streptococcus suis. These neuroinvasive pathogens represent common themes of host–pathogen interactions, such as colonization and invasion of mucosal barriers, survival in the blood stream, entry into the central nervous system by translocation of the blood–brain and blood–cerebrospinal fluid barrier, and induction of meningeal inflammation, affecting pia mater, the arachnoid and subarachnoid spaces.

Hyaluronate lyase activity of Streptococcus suis serotype 2 and modulatory effects of hyaluronic acid on the bacterium's virulence properties

Background

Streptococcus suis serotype 2 is a major swine pathogen and zoonotic agent worldwide causing mainly meningitis and septicemia. Hyaluronate lyases are enzymes that degrade hyaluronic acid, a major constituent of animal tissues, and have been reported as virulence factors in various bacterial species. Since the hyaluronate lyase of S. suis has been considered ambiguously as a virulence factor, we screened 50 isolates from the three major clonal complexes found in North America (sequence type [ST] 1, ST25, and ST28) known to differ in their degree of virulence in order to link the presence or absence of this activity with the degree of virulence. Moreover, the effect of exogenous hyaluronic acid on S. suis virulence factor gene expression and the pro-inflammatory response of brain macrovascular endothelial cells (BMEC) was also investigated.

Results

We found that all but one ST1 isolates (high virulence) were devoid of hyaluronate lyase activity whereas all ST25 (intermediate virulence) and ST28 (low virulence) isolates possessed the activity. A 2 bp insertion was responsible for the lack of activity in ST1 strains. Since the most virulent isolates did not degrade hyaluronic acid, this tissue component may be found during the infectious process. Therefore, we investigated its effect on S. suis and host cells. Hyaluronic acid was found to modulate S. suis adhesion to BMEC, to increase S. suis virulence factor expression, and to enhance pro-inflammatory cytokine secretion by BMEC.

Conclusions

These findings suggest that S. suis hyaluronate lyase does not represent a critical virulence factor in its active form. However, exogenous hyaluronic acid that is likely to interact with S. suis and host cells during the course of infection appears to modulate several virulence determinants of the bacterium, in addition to promote inflammation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-015-1692-9) contains supplementary material, which is available to authorized users.

Streptococcal toxins: role in pathogenesis and disease

Group A Streptococcus (Streptococcus pyogenes), group B Streptococcus (Streptococcus agalactiae) and Streptococcus pneumoniae (pneumococcus) are host-adapted bacterial pathogens among the leading infectious causes of human morbidity and mortality. These microbes and related members of the genus Streptococcus produce an array of toxins that act against human cells or tissues, resulting in impaired immune responses and subversion of host physiological processes to benefit the invading microorganism. This toxin repertoire includes haemolysins, proteases, superantigens and other agents that ultimately enhance colonization and survival within the host and promote dissemination of the pathogen.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Interaction of fibrinogen and muramidase-released protein promotes the development of Streptococcus suis meningitis

Muramidase-released protein (MRP) is as an important virulence marker of Streptococcus suis (S. suis) serotype 2. Our previous works have shown that MRP can bind human fibrinogen (hFg); however, the function of this interaction in S. suis meningitis is not known. In this study, we found that the deletion of mrp significantly impairs the hFg-mediated adherence and traversal ability of S. suis across human cerebral microvascular endothelial cells (hCMEC/D3). Measurement of the permeability to Lucifer yellow in vitro and Evans blue extravasation in vivo show that the MRP-hFg interaction significantly increases the permeability of the blood–brain barrier (BBB). In the mouse meningitis model, wild type S. suis caused higher bacterial loads in the brain and more severe histopathological signs of meningitis than the mrp mutant at day 3 post-infection. Western blot analysis and immunofluorescence observations reveal that the MRP-hFg interaction can destroy the cell adherens junction protein p120-catenin of hCMEC/D3. These results indicate that the MRP-hFg interaction is important in the development of S. suis meningitis.

FbsC, a novel fibrinogen-binding protein, promotes Streptococcus agalactiae-host cell interactions

Streptococcus agalactiae (group B Streptococcus or GBS) is a common cause of invasive infections in newborn infants and adults. The ability of GBS to bind human fibrinogen is of crucial importance in promoting colonization and invasion of host barriers. We characterized here a novel fibrinogen-binding protein of GBS, designated FbsC (Gbs0791), which is encoded by the prototype GBS strain NEM316. FbsC, which bears two bacterial immunoglobulin-like tandem repeat domains and a C-terminal cell wall-anchoring motif (LPXTG), was found to be covalently linked to the cell wall by the housekeeping sortase A. Studies using recombinant FbsC indicated that it binds fibrinogen in a dose-dependent and saturable manner, and with moderate affinity. Expression of FbsC was detected in all clinical GBS isolates, except those belonging to the hypervirulent lineage ST17. Deletion of fbsC decreases NEM316 abilities to adhere to and invade human epithelial and endothelial cells, and to form biofilm in vitro. Notably, bacterial adhesion to fibrinogen and fibrinogen binding to bacterial cells were abolished following fbsC deletion in NEM316. Moreover, the virulence of the fbsC deletion mutant and its ability to colonize the brain were impaired in murine models of infection. Finally, immunization with recombinant FbsC significantly protected mice from lethal GBS challenge. In conclusion, FbsC is a novel fibrinogen-binding protein expressed by most GBS isolates that functions as a virulence factor by promoting invasion of epithelial and endothelial barriers. In addition, the protein has significant immunoprotective activity and may be a useful component of an anti-GBS vaccine.

Pathogens penetrating the central nervous system: infection pathways and the cellular and molecular mechanisms of invasion

The brain is well protected against microbial invasion by cellular barriers, such as the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). In addition, cells within the central nervous system (CNS) are capable of producing an immune response against invading pathogens. Nonetheless, a range of pathogenic microbes make their way to the CNS, and the resulting infections can cause significant morbidity and mortality. Bacteria, amoebae, fungi, and viruses are capable of CNS invasion, with the latter using axonal transport as a common route of infection. In this review, we compare the mechanisms by which bacterial pathogens reach the CNS and infect the brain. In particular, we focus on recent data regarding mechanisms of bacterial translocation from the nasal mucosa to the brain, which represents a little explored pathway of bacterial invasion but has been proposed as being particularly important in explaining how infection with Burkholderia pseudomallei can result in melioidosis encephalomyelitis.