The orphan response regulator CovR: a globally negative modulator of virulence in Streptococcus suis serotype 2

Streptococcal arginine deiminase system defences macrophage bactericidal effect mediated by XRE family protein XtrSs

The arginine deiminase system (ADS) has been identified in various bacteria and functions to supplement energy production and enhance biological adaptability. The current understanding of the regulatory mechanism of ADS and its effect on bacterial pathogenesis is still limited. Here, we found that the XRE family transcriptional regulator XtrSs negatively affected Streptococcus suis virulence and significantly repressed ADS transcription when the bacteria were incubated in blood. Electrophoretic mobility shift (EMSA) and lacZ fusion assays further showed that XtrSs directly bind to the promoter of ArgR, an acknowledged positive regulator of bacterial ADS, to repress ArgR transcription. Moreover, we provided compelling evidence that S. suis could utilize arginine via ADS to adapt to acid stress, while ΔxtrSs enhanced this acid resistance by upregulating the ADS operon. Moreover, whole ADS-knockout S. suis increased arginine and antimicrobial NO in the infected macrophage cells, decreased intracellular survival, and even caused significant attenuation of bacterial virulence in a mouse infection model, while ΔxtrSs consistently presented the opposite results. Our experiments identified a novel ADS regulatory mechanism in S. suis, whereby XtrSs regulated ADS to modulate NO content in macrophages, promoting S. suis intracellular survival. Meanwhile, our findings provide a new perspective on how Streptococci evade the host's innate immune system.

Evolution of orphan and atypical histidine kinases and response regulators for microbial signaling diversity

Two-component signaling systems (TCS) are the predominant means of microbes for sensing and responding to environmental stimuli. Typically, TCS is comprised of a sensor histidine kinase (HK) and a cognate response regulator (RR), which might have coevolved together. They usually involve the phosphoryl transfer signaling mechanism. However, there are also some orphan and atypical HK and RR homologs, and their evolutionary origins are still not very clear. They are not associated with cognate pairs or lack the conserved residues for phosphoryl transfer, but they could receive or respond to signals from other regulators. The objective of this study is to reveal the evolutionary history of these orphan and atypical HK and RR homologs. Structural, domain, sequence, and phylogenetic analyses indicated that their evolution process might undergo gene duplication, divergence, and domain shuffling. Meanwhile, lateral gene transfer might also be involved for their gene distribution. Evolution of orphan and atypical HK and RR homologs have increased their signaling diversity, which could be helpful for microbial adaption in complex environments.

Encapsulated Streptococcus suis impairs optimal neutrophil functions which are not rescued by priming with colony-stimulating factors

The porcine pathogen and zoonotic agent Streptococcus suis induces an exacerbated inflammation in the infected hosts that leads to sepsis, meningitis, and sudden death. Several virulence factors were described for S. suis of which the capsular polysaccharide (CPS) conceals it from the immune system, and the suilysin exhibits cytotoxic activity. Although neutrophils are recruited rapidly upon S. suis infection, their microbicidal functions appear to be poorly activated against the bacteria. However, during disease, the inflammatory environment could promote neutrophil activation as mediators such as the granulocyte colony-stimulating factor granulocyte (G-CSF) and the granulocyte-macrophages colony-stimulating factor (GM-CSF) prime neutrophils and enhance their responsiveness to bacterial detection. Thus, we hypothesized that CPS and suilysin prevent an efficient activation of neutrophils by S. suis, but that G-CSF and GM-CSF rescue neutrophil activation, leading to S. suis elimination. We evaluated the functions of porcine neutrophils in vitro in response to S. suis and investigated the role of the CPS and suilysin on cell activation using isogenic mutants of the bacteria. We also studied the influence of G-CSF and GM-CSF on neutrophil response to S. suis by priming the cells with recombinant proteins. Our study confirmed that CPS prevents S. suis-induced activation of most neutrophil functions but participates in the release of neutrophil-extracellular traps (NETs). Priming with G-CSF did not influence cell activation, but GM-CSF strongly promote IL-8 release, indicating its involvement in immunomodulation. However, priming did not enhance microbicidal functions. Studying the interaction between S. suis and neutrophils-first responders in host defense-remains fundamental to understand the immunopathogenesis of the infection and to develop therapeutical strategies related to neutrophils' defense against this bacterium.

Orphan response regulator CovR plays positive regulative functions in the survivability and pathogenicity of Streptococcus suis serotype 2 isolated from a pig

BACKGROUND

Streptococcus suis serotype 2 (S. suis 2) is an important zoonotic pathogen. Orphan response regulator CovR plays crucial regulative functions in the survivability and pathogenicity of S. suis 2. However, research on the CovR in S. suis 2 is limited.

RESULTS

In this study, the regulative functions of CovR in the survivability and pathogenicity were investigated in S. suis 2 isolated from a diseased pig. The deletion of CovR significantly weakened the survivability and pathogenicity of S. suis 2. Compared with the wild-type strain, ΔcovR showed slower growth rates and thinner capsular polysaccharides. Moreover, ΔcovR showed reduced adhesion and invasion to Hep-2 cells as well as anti-phagocytosis and anti-killing ability to 3D4 cells and anti-serum killing ability. In addition, the deletion of CovR significantly reduced the colonisation ability of S. suis 2 in mice. The survival rate of mice infected with ΔcovR was increased by 16.7% compared with that of mice infected with S. suis 2. Further, the deletion of CovR led to dramatic changes in metabolism-related pathways in S. suis 2, five of those, including fructose and mannose metabolism, glycerolipid metabolism, ABC transporters, amino sugar and nucleotide sugar metabolism and phosphotransferase system, were significantly down-regulated.

CONCLUSIONS

Based on the results, CovR plays positive regulative functions in the survivability and pathogenicity of S. suis 2 SC19 strain isolated from a pig.

Transcriptome and metabolome profiling to elucidate the mechanism underlying the poor growth of Streptococcus suis serotype 2 after orphan response regulator CovR deletion

The deletion of orphan response regulator CovR reduces the growth rate of Streptococcus suis serotype 2 (S. suis 2). In this study, metabolome and transcriptome profiling were performed to study the mechanisms underlying the poor growth of S. suis 2 caused by the deletion of orphan response regulator CovR. By comparing S. suis 2 (ΔcovR) and S. suis 2 (SC19), 146 differentially accumulated metabolites (upregulated: 83 and downregulated: 63) and 141 differentially expressed genes (upregulated: 86 and downregulated: 55) were identified. Metabolome and functional annotation analysis revealed that the growth of ΔcovR was inhibited by the imbalance aminoacyl tRNA biosynthesis (the low contents of L-lysine, L-aspartic acid, L-glutamine, and L-glutamic acid, and the high content of L-methionine). These results provide a new insight into the underlying poor growth of S. suis 2 caused by the deletion of orphan response regulator CovR. Metabolites and candidate genes regulated by the orphan response regulator CovR and involved in the growth of S. suis 2 were reported in this study.

A critical review on experimental Streptococcus suis infection in pigs with a focus on clinical monitoring and refinement strategies

Streptococcus suis (S. suis) is a major pig pathogen worldwide with zoonotic potential. Though different research groups have contributed to a better understanding of the pathogenesis of S. suis infections in recent years, there are still numerous neglected research topics requiring animal infection trials. Of note, animal experiments are crucial to develop a cross-protective vaccine which is highly needed in the field. Due to the severe clinical signs associated with S. suis pathologies such as meningitis and arthritis, implementation of refinement is very important to reduce pain and distress of experimentally infected pigs. This review highlights the great diversity of clinical signs and courses of disease after experimental S. suis pig infections. We review clinical read out parameters and refinement strategies in experimental S. suis pig infections published between 2000 and 2021. Currently, substantial differences exist in describing clinical monitoring and humane endpoints. Most of the reviewed studies set the body temperature threshold of fever as high as 40.5°C. Monitoring intervals vary mainly between daily, twice a day and three times a day. Only a few studies apply scoring systems. Published scoring systems are inconsistent in their inclusion of parameters such as body temperature, feeding behavior, and respiratory signs. Locomotion and central nervous system signs are more common clinical scoring parameters in different studies by various research groups. As the heterogenicity in clinical monitoring limits the comparability between studies we hope to initiate a discussion with this review leading to an agreement on clinical read out parameters and monitoring intervals among S. suis research groups.

The Threat of Potentially Pathogenic Bacteria in the Feces of Bats

Bats have attracted global attention because of their zoonotic association with severe acute respiratory syndrome coronavirus (SARS-CoV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Previous and ongoing studies have predominantly focused on bat-borne viruses; however, the prevalence or abundance of bat-borne pathogenic bacteria and their potential public health significance have largely been neglected. For the first time, this study used both metataxonomics (16S rRNA marker gene sequencing) and culturomics (traditional culture methods) to systematically evaluate the potential public health significance of bat fecal pathogenic bacteria. To this end, fecal samples were obtained from five bat species across different locations in China, and their microbiota composition was analyzed. The results revealed that the bat microbiome was most commonly dominated by Proteobacteria, while the strictly anaerobic phylum Bacteroidetes occupied 35.3% of the relative abundance in Rousettus spp. and 36.3% in Hipposideros spp., but less than 2.7% in the other three bat species (Taphozous spp., Rhinolophus spp., and Myotis spp.). We detected 480 species-level phylotypes (SLPs) with PacBio sequencing, including 89 known species, 330 potentially new species, and 61 potentially higher taxa. In addition, a total of 325 species were identified by culturomics, and these were classified into 242 named species and 83 potentially novel species. Of note, 32 of the 89 (36.0%) known species revealed by PacBio sequencing were found to be pathogenic bacteria, and 69 of the 242 (28.5%) known species isolated by culturomics were harmful to people, animals, or plants. Additionally, nearly 40 potential novel species which may be potential bacterial pathogens were identified. IMPORTANCE Bats are one of the most diverse and widely distributed groups of mammals living in close proximity to humans. In recent years, bat-borne viruses and the viral zoonotic diseases associated with bats have been studied in great detail. However, the prevalence and abundance of pathogenic bacteria in bats have been largely ignored. This study used high-throughput sequencing techniques (metataxonomics) in combination with traditional culture methods (culturomics) to analyze the bacterial flora in bat feces from different species of bats in China, revealing that bats are natural hosts of pathogenic bacteria and carry many unknown bacteria. The results of this study can be used as guidance for future investigations of bacterial pathogens in bats.

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.

Neutrophils in Streptococcus suis Infection: From Host Defense to Pathology

Streptococcus suis is a swine pathogen and zoonotic agent responsible for economic losses to the porcine industry. Infected animals may develop meningitis, arthritis, endocarditis, sepsis and/or sudden death. The pathogenesis of the infection implies that bacteria breach mucosal host barriers and reach the bloodstream, where they escape immune-surveillance mechanisms and spread throughout the organism. The clinical manifestations are mainly the consequence of an exacerbated inflammation, defined by an exaggerated production of cytokines and recruitment of immune cells. Among them, neutrophils arrive first in contact with the pathogens to combat the infection. Neutrophils initiate and maintain inflammation, by producing cytokines and deploying their arsenal of antimicrobial mechanisms. Furthermore, neutrophilic leukocytosis characterizes S. suis infection, and lesions of infected subjects contain a large number of neutrophils. Therefore, this cell type may play a role in host defense and/or in the exacerbated inflammation. Nevertheless, a limited number of studies addressed the role or functions of neutrophils in the context of S. suis infection. In this review, we will explore the literature about S. suis and neutrophils, from their interaction at a cellular level, to the roles and behaviors of neutrophils in the infected host in vivo.

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.

A novel small RNA contributes to restrain cellular chain length and anti-phagocytic ability in Streptococcus suis 2

Streptococcus suis serotype 2 (SS2) is an important porcine and human pathogen. Regulatory small non-coding RNAs (sRNAs) play an essential role in diverse physiological processes, although they remain poorly understood in SS2. In this study, we identified eight novel sRNAs through a combination of computational strategies and experimental identification. To explore roles of these novel sRNAs, sRNA34 was preferentially selected to assess phenotypes of the deletion strain in vitro and in vivo. The inactivation of sRNA34 significantly elongated the cellular chain, remarkably increased sensitivity to phagocytosis by RAW264.7, and attenuated virulence in a mouse infection model. Transcriptomic analysis revealed that inactivation of sRNA34 altered expression of multiple genes contributing to cellular chain formation and elongation, indicating a potential mechanism of sRNA34 in maintaining proper bacterial chain length to resist phagocytosis by the host cell. In summary, sRNA34 is a novel sRNA that contributes to cellular chain regulation and the anti-phagocytosis ability of SS2.

Identification of an Autorepressing Two-Component Signaling System That Modulates Virulence in Streptococcus suis Serotype 2

Streptococcus suis is one of the most important pathogens affecting the swine industry and is also an emerging zoonotic agent for humans. Two-component signaling systems (TCSs) play important roles in the adaptation of pathogenic bacteria to host environments. In this study, we identified a novel TCS, named TCS09HKRR, which facilitated Streptococcus suis serotype 2 (SS2) resistance to clearance by the host immune system and contributed to bacterial pathogenicity. Furthermore, RNA-sequencing analyses identified 79 genes that were differentially expressed between the wild-type (WT) and ΔTCS09HKRR strains, among which half of the 39 downregulated genes belonged to the capsular biosynthesis clusters. Transmission electron microscopy confirmed that the capsule of the ΔTCS09HKRR strain was thinner than that of the WT strain. Electrophoretic mobility shift assays (EMSA) showed that the regulator of TCS09HKRR (TCS09RR) could not bind the promoter regions of cps and neu clusters, which suggested that TCS09HKRR regulates capsule biosynthesis by indirect pathways. Unexpectedly, the TCS09HKRR operon was upregulated when TCS09HKRR was deleted. A specific region, ATGACATTTGTCAC, which extends from positions -193 to -206 upstream of the TCS09HKRR operon, was further identified as the TCS09RR-binding site using EMSA. These results suggested the involvement of a negative feedback loop in this regulation. In addition, TCS09RR was significantly upregulated by more than 18-fold when coincubated with RAW264.7 macrophages. Our data suggested that autorepression renders TCS09HKRR more sensitive to host stimuli, which optimizes the regulatory network of capsular biosynthesis in SS2.

Pdh is involved in the cell division and Normal septation of Streptococcus suis

Streptococcus suis (S. suis) is an important zoonotic pathogen that causes major economic losses in the pig industry worldwide. The S. suis cell division process is an integral part of its growth and reproduction, which is controlled by a complex regulatory network. Pyruvate dehydrogenase (PDH), which catalyzes the oxidative decarboxylation of pyruvate to form acetyl-CoA, while reducing NAD + to NADH, plays an important role in energy metabolism. Recently, we reported that pdh regulates virulence by reducing stress tolerance and biofilm formation in S. suis serotype 2. In this study, we found that deletion of the pdh gene in S. suis resulted in abnormal cell chains, plump morphology and abnormal localization of the Z rings, indicating that the knockout mutant is impaired in its ability to divide. In addition, the interaction between FtsZ and PDH in vitro was confirmed by ELISA, and qRT-PCR analysis revealed that the deletion of the pdh gene results in differential expression of the division-related genes ftsZ, ftsK, ftsl, zapA, divIC, pbp1a, rodA, mreD, and sepF. These results indicate that pdh is involved in the normal formation of Z rings and cell morphology during S. suis cell division.

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.

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.

Role of two-component regulatory systems in the virulence of Streptococcus suis

Streptococcus suis is an important zoonotic pathogen that causes severe infections and great economic losses worldwide. Understanding how this pathogen senses and responds to environmental signals during the infectious process can offer insight into its pathogenesis and may be helpful in the development of drug targets. Two-component regulatory systems (TCSs) play an essential role in this environmental response. In S. suis, at least 15 groups of TCSs have been predicted. Among them, several have been demonstrated to be involved in virulence and/or stress response. In this review, we discuss the progress in the study of TCSs in S. suis, focusing on the role of these systems in the virulence of this bacterium.

The Two-Component Signaling System VraSRss Is Critical for Multidrug Resistance and Full Virulence in Streptococcus suis Serotype 2

Streptococcus suis has received increasing attention for its involvement in severe human infections worldwide as well as in multidrug resistance. Two-component signaling systems (TCSSs) play important roles in bacterial adaptation to various environmental stimuli. In this study, we identified a novel TCSS located in S. suis serotype 2 (SS2), designated VraSR_SS, which is involved in bacterial pathogenicity and susceptibility to antimicrobials. Our data demonstrated that the yvqF_SS gene, located upstream of vraSR_SS , shared the same promoter with the TCSS genes, which was directly regulated by VraSR_SS, as shown in electrophoretic mobility shift assays. Notably, YvqF_SS and VraSR_SS constitute a novel multidrug resistance module of SS2 that participates in resistance to certain groups of antimicrobials. Further analyses showed that VraSR_SS inactivation significantly attenuated bacterial virulence in animal models, which, coupled with the significant activation of VraSR_SS expression observed in host blood, strongly suggested that VraSR_SS is an important regulator of SS2 pathogenicity. Indeed, RNA-sequencing analyses identified 106 genes that were differentially expressed between the wild-type and ΔvraSR_SS strains, including genes involved in capsular polysaccharide (CPS) biosynthesis. Subsequent studies confirmed that VraSR_SS indirectly regulated the transcription of CPS gene clusters and, thus, controlled the CPS thickness shown by transmission electron microscopy. Decreased CPS biosynthesis caused by vraSR_SS deletion subsequently increased bacterial adhesion to epithelial cells and attenuated antiphagocytosis against macrophages, which partially clarified the pathogenic mechanism mediated by VraSR_SS Taken together, our data suggest that the novel TCSS, VraSR_SS, plays critical roles for multidrug resistance and full virulence in SS2.

RitR is an archetype for a novel family of redox sensors in the streptococci that has evolved from two-component response regulators and is required for pneumococcal colonization

To survive diverse host environments, the human pathogen Streptococcus pneumoniae must prevent its self-produced, extremely high levels of peroxide from reacting with intracellular iron. However, the regulatory mechanism(s) by which the pneumococcus accomplishes this balance remains largely enigmatic, as this pathogen and other related streptococci lack all known redox-sensing transcription factors. Here we describe a two-component-derived response regulator, RitR, as the archetype for a novel family of redox sensors in a subset of streptococcal species. We show that RitR works to both repress iron transport and enable nasopharyngeal colonization through a mechanism that exploits a single cysteine (Cys128) redox switch located within its linker domain. Biochemical experiments and phylogenetics reveal that RitR has diverged from the canonical two-component virulence regulator CovR to instead dimerize and bind DNA only upon Cys128 oxidation in air-rich environments. Atomic structures show that Cys128 oxidation initiates a "helical unravelling" of the RitR linker region, suggesting a mechanism by which the DNA-binding domain is then released to interact with its cognate regulatory DNA. Expanded computational studies indicate this mechanism could be shared by many microbial species outside the streptococcus genus.

Capsular Sialyltransferase Specificity Mediates Different Phenotypes in Streptococcus suis and Group B Streptococcus

The capsular polysaccharide (CPS) represents a key virulence factor for most encapsulated streptococci. Streptococcus suis and Group B Streptococcus (GBS) are both well-encapsulated pathogens of clinical importance in veterinary and/or human medicine and responsible for invasive systemic diseases. S. suis and GBS are the only Gram-positive bacteria which express a sialylated CPS at their surface. An important difference between these two sialylated CPSs is the linkage between the side-chain terminal galactose and sialic acid, being α-2,6 for S. suis but α-2,3 for GBS. It is still unclear how sialic acid may affect CPS production and, consequently, the pathogenesis of the disease caused by these two bacterial pathogens. Here, we investigated the role of sialic acid and the putative effect of sialic acid linkage modification in CPS synthesis using inter-species allelic exchange mutagenesis. To this aim, a new molecular biogenetic approach to express CPS with modified sialic acid linkage was developed. We showed that sialic acid (and its α-2,6 linkage) is crucial for S. suis CPS synthesis, whereas for GBS, CPS synthesis may occur in presence of an α-2,6 sialyltransferase or in absence of sialic acid moiety. To evaluate the effect of the CPS composition/structure on sialyltransferase activity, two distinct capsular serotypes within each bacterial species were compared (S. suis serotypes 2 and 14 and GBS serotypes III and V). It was demonstrated that the observed differences in sialyltransferase activity and specificity between S. suis and GBS were serotype unrestricted. This is the first time that a study investigates the interspecies exchange of capsular sialyltransferase genes in Gram-positive bacteria. The obtained mutants represent novel tools that could be used to further investigate the immunomodulatory properties of sialylated CPSs. Finally, in spite of common CPS structural characteristics and similarities in the cps loci, sialic acid exerts differential control of CPS expression by S. suis and GBS.

Streptococcus suis DivIVA Protein Is a Substrate of Ser/Thr Kinase STK and Involved in Cell Division Regulation

Streptococcus suis serotype 2 is an important swine pathogen and an emerging zoonotic agent that causes severe infections. Recent studies have reported a eukaryotic-like Ser/Thr protein kinase (STK) gene and characterized its role in the growth and virulence of different S. suis 2 strains. In the present study, phosphoproteomic analysis was adopted to identify substrates of the STK protein. Seven proteins that were annotated to participate in different cell processes were identified as potential substrates, which suggests the pleiotropic effects of stk on S. suis 2 by targeting multiple pathways. Among them, a protein characterized as cell division initiation protein (DivIVA) was further investigated. In vitro analysis demonstrated that the recombinant STK protein directly phosphorylates threonine at amino acid position 199 (Thr-199) of DivIVA. This effect could be completely abolished by the T199A mutation. To determine the specific role of DivIVA in growth and division, a divIVA mutant was constructed. The ΔdivIVA strain exhibited impaired growth and division, including lower viability, enlarged cell mass, asymmetrical division caused by aberrant septum, and extremely weak pathogenicity in a mouse infection model. Collectively, our results reveal that STK regulates the cell growth and virulence of S. suis 2 by targeting substrates that are involved in different biological pathways. The inactivation of DivIVA leads to severe defects in cell division and strongly attenuates pathogenicity, thereby indicating its potential as a molecular drug target against S. suis.

Genome-wide analysis of an avirulent strain that induces protective immunity against challenge with virulent Streptococcus suis serotype 2

BACKGROUND

It was previously reported in China that two recent large-scale outbreaks of Streptococcus suis serotype 2 (S. suis 2) infections in human were caused by two highly virulent S. suis 2 strains, from which a novel genomic island (GEI), associated with disease onset and progression and designated 89 K, was identified. Here, an avirulent strain, 05HAS68, was isolated from a clinically healthy pig.

RESULTS

By comparing the genomes of this avirulent strain with virulent strains, it was found that massive genomic rearrangements occurred, resulting in alterations in gene expression that caused enormous single gene gain and loss. Important virulent genes were lost, such as extracellular protein factor (ef) and suilysin (sly) and larger mutants, such as muramidase-released protein (mrp). Piglets vaccinated with the avirulent strain, 05HAS68, had increased TNF-α and IFN-γ levels in the peripheral blood and were fully protected from challenge infection with the most virulent S. suis 2 strain, 05ZYH33. Transfusion of T cells and plasma from vaccinated pigs resulted in protection of recipient animals against the 05ZYH33 challenge.

CONCLUSION

These results suggest that analysis genome of the avirulent strains are instrumental in the development of vaccines and for the functional characterization of important of genetic determinants.

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.

Initial steps of the pathogenesis of the infection caused by Streptococcus suis: fighting against nonspecific defenses

Interactions between a bacterial pathogen and its potentially susceptible host are initiated with the colonization step. During respiratory/oral infection, the pathogens must compete with the normal microflora, resist defense mechanisms of the local mucosal immunity, and finally reach, adhere, and breach the mucosal epithelial cell barrier in order to induce invasive disease. This is the case during infection by the swine and zoonotic pathogen Streptococcus suis, which is able to counteract mucosal barriers to induce severe meningitis and sepsis in swine and in humans. The initial steps of the pathogenesis of S. suis infection has been a neglected area of research, overshadowed by studies on the systemic and central nervous phases of the disease. In this Review article, we provide for the first time, an exclusive focus on S. suis colonization and the potential mechanisms involved in S. suis establishment at the mucosa, as well as the mechanisms regulating mucosal barrier breakdown. The role of mucosal immunity is also addressed. Finally, we demystify the extensive list of putative adhesins and virulence factors reported to be involved in the initial steps of pathogenesis by S. suis.

Formate-tetrahydrofolate ligase is involved in the virulence of Streptococcus suis serotype 2

Streptococcus suis is an emerging zoonotic pathogen that causes severe infections in pigs and humans. However, the pathogenesis of S. suis remains unclear. The present study targeted a putative virulence-associated factor (fhs, encoding the formate-tetrahydrofolate ligase) of S. suis. To investigate the role of fhs in the virulence potential of S. suis serotype 2, an fhs deletion mutant (Δfhs) and the corresponding complementation strain (CΔfhs) were generated. The Δfhs mutant displayed similar growth compared to that of the wild-type and complementation strains. Using murine and pig infection models, we demonstrated for the first time that the formate-tetrahydrofolate ligase is required for the full virulence of S. suis 2. Our findings provide a new insight into the pathogenesis of S. suis 2.

Effect of the glycosyltransferases on the capsular polysaccharide synthesis of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (S. suis 2) is a serious zoonotic pathogen causing septicemia and meningitis in piglets and humans. The capsular polysaccharide (CPS) is an essential virulence factor for S. suis 2 to infect the host. The synthesis of CPS repeating units involves multiple glycosyltransferases. In this study, four genes (cps2E, cps2G, cps2J and cps2L) encoding different glycosyltransferases involved in CPS synthesis were researched in S. suis 2. Four deletion mutants (Δcps2E, Δcps2G, Δcps2J and Δcps2L) with their CPS incomplete and their sialic acid content significantly decreased were constructed in S. suis 2 SC19. All these four mutant strains showed enhanced adhesion to Hep-2 cells and increased sensitivity to phagocytosis. Flow cytometric analysis also revealed that these four mutants were more susceptible to the attack by the complement system. In a mouse model of infection, the mutant strains were rapidly cleared by the immune system, compared with the wild-type strain. In summary, this study characterized four genes (cps2E, cps2G, cps2J and cps2L) involved in CPS synthesis of S. suis 2 SC19 and it revealed that these genes were all crucial for SC19 to invade and survive in the host.

First human case report of sepsis due to infection with Streptococcus suis serotype 31 in Thailand

Background

Streptococcus suis is a zoonotic pathogen that causes invasive infections in humans and pigs. It has been reported that S. suis infection in humans is mostly caused by serotype 2. However, human cases caused by other serotypes have rarely been reported. This is the first report of a human case of infection with S. suis serotype 31 in Thailand.

Case presentation

A 55-year-old male alcohol misuser with liver cirrhosis was admitted with sepsis to a hospital in the Central Region of Thailand. He had consumed a homemade, raw pork product prior to the onset of illness. He was alive after treatment with ceftriaxone and no complication occurred. An isolate from blood culture at the hospital was suspected as viridans group Streptococcus. It was confirmed at a reference laboratory as S. suis serotype 31 by biochemical tests, 16S rDNA sequencing, and multiplex polymerase chain reaction for serotyping, but it was untypable by the co-agglutination test with antisera against recognized S. suis serotypes, suggesting loss of capsular material. The absence of a capsule was confirmed by transmission electron microscopy. The isolate was confirmed to be sequence type 221, with 13 putative virulence genes that are usually found in serotype 2 strains.

Conclusion

We should be aware of the emergence of S. suis infections caused by uncommon serotypes in patients with predisposing conditions. Laboratory capacity to identify S. suis in the hospital is needed in developing countries, which can contribute to enhanced surveillance, epidemiological control, and prevention strategies in the prevalent area.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-015-1136-0) contains supplementary material, which is available to authorized users.

Isolation and characterization of a native avirulent strain of Streptococcus suis serotype 2: a perspective for vaccine development

Streptococcus suis, an emerging infectious pathogen, is the cause of two large-scale outbreaks of human streptococcal toxic shock syndrome in China, and has attracted much attention from the scientific community. The genetic basis of its pathogenesis remains enigmatic, and no effective prevention measures have been established. To better understand the virulence differentiation of S. suis and develop a promising vaccine, we isolated and sequenced a native avirulent S. suis strain (05HAS68). Animal experiments revealed that 05HAS68 is an avirulent strain and could protect piglets from the attack of virulent strains. Comparative genomics analyses demonstrated the genetic basis for the lack of virulence in 05HAS68, which is characterized by the absence of some important virulence-associated factors and the intact 89K pathogenicity island. Lack of virulence was also illustrated by reduced survival of 05HAS68 compared to a virulent strain in pig whole blood. Further investigations revealed a large-scale genomic rearrangement in 05HAS68, which was proposed to be mediated by transposase genes and/or prophages. This genomic rearrangement may have caused the genomic diversity of S. suis, and resulted in biological discrepancies between 05HAS68 and highly virulent S. suis strains.

Characterization of the pivotal carbon metabolism of Streptococcus suis serotype 2 under ex vivo and chemically defined in vitro conditions by isotopologue profiling

Streptococcus suis is a neglected zoonotic pathogen that has to adapt to the nutritional requirements in the different host niches encountered during infection and establishment of invasive diseases. To dissect the central metabolic activity of S. suis under different conditions of nutrient availability, we performed labeling experiments starting from [(13)C]glucose specimens and analyzed the resulting isotopologue patterns in amino acids of S. suis grown under in vitro and ex vivo conditions. In combination with classical growth experiments, we found that S. suis is auxotrophic for Arg, Gln/Glu, His, Leu, and Trp in chemically defined medium. De novo biosynthesis was shown for Ala, Asp, Ser, and Thr at high rates and for Gly, Lys, Phe, Tyr, and Val at moderate or low rates, respectively. Glucose degradation occurred mainly by glycolysis and to a minor extent by the pentose phosphate pathway. Furthermore, the exclusive formation of oxaloacetate by phosphoenolpyruvate (PEP) carboxylation became evident from the patterns in de novo synthesized amino acids. Labeling experiments with S. suis grown ex vivo in blood or cerebrospinal fluid reflected the metabolic adaptation to these host niches with different nutrient availability; however, similar key metabolic activities were identified under these conditions. This points at the robustness of the core metabolic pathways in S. suis during the infection process. The crucial role of PEP carboxylation for growth of S. suis in the host was supported by experiments with a PEP carboxylase-deficient mutant strain in blood and cerebrospinal fluid.

Two Spx regulators modulate stress tolerance and virulence in Streptococcus suis serotype 2

Streptococcus suis serotype 2 is an important zoonotic pathogen causing severe infections in pigs and humans. The pathogenesis of S. suis 2 infections, however, is still poorly understood. Spx proteins are a group of global regulators involved in stress tolerance and virulence. In this study, we characterized two orthologs of the Spx regulator, SpxA1 and SpxA2 in S. suis 2. Two mutant strains (ΔspxA1 and ΔspxA2) lacking the spx genes were constructed. The ΔspxA1 and ΔspxA2 mutants displayed different phenotypes. ΔspxA1 exhibited impaired growth in the presence of hydrogen peroxide, while ΔspxA2 exhibited impaired growth in the presence of SDS and NaCl. Both mutants were defective in medium lacking newborn bovine serum. Using a murine infection model, we demonstrated that the abilities of the mutant strains to colonize the tissues were significantly reduced compared to that of the wild-type strain. The mutant strains also showed a decreased level of survival in pig blood. Microarray analysis revealed a global regulatory role for SpxA1 and SpxA2. Furthermore, we demonstrated for the first time that Spx is involved in triggering the host inflammatory response. Collectively, our data suggest that SpxA1 and SpxA2 are global regulators that are implicated in stress tolerance and virulence in S. suis 2.

Assessment of the pathogenesis of Streptococcus suis type 2 infection in piglets for understanding streptococcal toxic shock-like syndrome, meningitis, and sequelae

Streptococcus suis type 2 (SS2) is an zoonotic pathogen that had caused outbreaks in 1998 and 2005 in China. It is still not very clear how the disease progresses into the streptococcal toxic shock-like syndrome (STSLS) or meningitis, as well as the sequelae from the survivals. The present study used piglets as infection model to systematically investigate the pathogenesis of the infection caused by the SS2 strain 05ZYH33. The infected piglets showed joint swelling, lameness, and crouch at beginning, then developed into septic-like shock syndrome (SLSS) or prostration syndrome, at last the survivals showed physical activity impairment. The morbidity and mortality were 100% (71% for SLSS, 29% for prostration syndrome) and 29%, respectively. The pigs exhibiting SLSS had deep invasive infections in tissues and organs, and displayed more severe bacteremia and cytokine secretion in the bloodstream and organs than pigs with prostration syndrome. Moreover, the polymorphisms in the toll-like receptor 1 (TLR1) and TLR2 genes varied between the pigs affected with SLSS and prostration syndrome. Several lines of evidence indicated that SS2 infection progression into SLSS or relatively lighter prostration syndrome in pigs is closely related to the degrees of bacteremia and cytokine storm, which may be inherently determined by the diversity of innate immunity-associated genes. Furthermore, brain lesions, such as venous thrombosis, may directly contribute to the sequelae in human cases, were identified in the pigs. These results might help us to further understand the pathogenesis of SS2 in humans.

The Streptococcus suis transcriptional landscape reveals adaptation mechanisms in pig blood and cerebrospinal fluid

Streptococcus suis (SS) is an important pathogen of pigs, and it is also recognized as a zoonotic agent for humans. SS infection may result in septicemia or meningitis in the host. However, little is known about genes that contribute to the virulence process and survival within host blood or cerebrospinal fluid (CSF). Small RNAs (sRNA) have emerged as key regulators of virulence in several bacteria, but they have not been investigated in SS. Here, using a differential RNA-sequencing approach and RNAs from SS strain P1/7 grown in rich medium, pig blood, or CSF, we present the SS genome-wide map of 793 transcriptional start sites and 370 operons. In addition to identifying 29 sRNAs, we show that five sRNA deletion mutants attenuate SS virulence in a zebrafish infection model. Homology searches revealed that 10 sRNAs were predicted to be present in other pathogenic Streptococcus species. Compared with wild-type strain P1/7, sRNAs rss03, rss05, and rss06 deletion mutants were significantly more sensitive to killing by pig blood. It is possible that rss06 contributes to SS virulence by indirectly activating expression of SSU0308, a virulence gene encoding a zinc-binding lipoprotein. In blood, genes involved in the synthesis of capsular polysaccharide (CPS) and subversion of host defenses were up-regulated. In contrast, in CSF, genes for CPS synthesis were down-regulated. Our study is the first analysis of SS sRNAs involved in virulence and has both improved our understanding of SS pathogenesis and increased the number of sRNAs known to play definitive roles in bacterial virulence.

Streptococcus suis infection: an emerging/reemerging challenge of bacterial infectious diseases?

Streptococcus suis (S. suis) is a family of pathogenic gram-positive bacterial strains that represents a primary health problem in the swine industry worldwide. S. suis is also an emerging zoonotic pathogen that causes severe human infections clinically featuring with varied diseases/syndromes (such as meningitis, septicemia, and arthritis). Over the past few decades, continued efforts have made significant progress toward better understanding this zoonotic infectious entity, contributing in part to the elucidation of the molecular mechanism underlying its high pathogenicity. This review is aimed at presenting an updated overview of this pathogen from the perspective of molecular epidemiology, clinical diagnosis and typing, virulence mechanism, and protective antigens contributing to its zoonosis.

Carbohydrate availability regulates virulence gene expression in Streptococcus suis

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

Contribution of eukaryotic-type serine/threonine kinase to stress response and virulence of Streptococcus suis

Streptococcus suis serotype 2 (SS2) is an important swine and human pathogen responsible for septicemia and meningitis. The bacterial homologues of eukaryotic-type serine/threonine kinases (ESTKs) have been reported to play critical roles in various cellular processes. To investigate the role of STK in SS2, an isogenic stk mutant strain (Δstk) and a complemented strain (CΔstk) were constructed. The Δstk showed a significant decrease in adherence to HEp-2 cells, compared with the wild-type strain, and a reduced survival ratio in whole blood. In addition, the Δstk exhibited a notable reduced tolerance of environmental stresses including high temperature, acidic pH, oxidative stress, and high osmolarity. More importantly, the Δstk was attenuated in both the CD1 mouse and piglet models of infection. The results of quantitative reverse transcription-PCR (qRT-PCR) analysis indicated that the expressions of a few genes involving in adherence, stress response and virulence were clearly decreased in the Δstk mutant strain. Our data suggest that SsSTK is required for virulence and stress response in SS2.

The two-component system NisK/NisR contributes to the virulence of Streptococcus suis serotype 2

Two-component signal-transduction systems (TCSTSs) may regulate some virulence factors in response to external stimuli, and thus allowing Streptococcus suis serotype 2 to interact with the host, promote survival, and cause disease. Here, a mutant of the NisKR TCSTS had attenuated virulence in vitro, as exemplified by lowered hemolytic activity, reduced adherence to epithelial cells, increased elimination by macrophages, and decreased resistance to killing by neutrophils. Results also showed that this system is important for the ability of S. suis serotype 2 to survive and proliferate in an in vivo mouse model. Thus, the NisKR system plays a significant role in pathogenesis, both in colonization and invasive disease.

Bacterial Adhesion of Streptococcus suis to Host Cells and Its Inhibition by Carbohydrate Ligands

Streptococcus suis is a Gram-positive bacterium, which causes sepsis and meningitis in pigs and humans. This review examines the role of known S. suis virulence factors in adhesion and S. suis carbohydrate-based adhesion mechanisms, as well as the inhibition of S. suis adhesion by anti-adhesion compounds in in vitro assays. Carbohydrate-binding specificities of S. suis have been identified, and these studies have shown that many strains recognize Galα1-4Gal-containing oligosaccharides present in host glycolipids. In the era of increasing antibiotic resistance, new means to treat infections are needed. Since microbial adhesion to carbohydrates is important to establish disease, compounds blocking adhesion could be an alternative to antibiotics. The use of oligosaccharides as drugs is generally hampered by their relatively low affinity (micromolar) to compete with multivalent binding to host receptors. However, screening of a library of chemically modified Galα1-4Gal derivatives has identified compounds that inhibit S. suis adhesion in nanomolar range. Also, design of multivalent Galα1-4Gal-containing dendrimers has resulted in a significant increase of the inhibitory potency of the disaccharide. The S. suis adhesin binding to Galα1-4Gal-oligosaccharides, Streptococcal adhesin P (SadP), was recently identified. It has a Galα1-4Gal-binding N-terminal domain and a C-terminal LPNTG-motif for cell wall anchoring. The carbohydrate-binding domain has no homology to E. coli P fimbrial adhesin, which suggests that these Gram-positive and Gram-negative bacterial adhesins recognizing the same receptor have evolved by convergent evolution. SadP adhesin may represent a promising target for the design of anti-adhesion ligands for the prevention and treatment of S. suis infections.

Genetic analysis of Streptococcus suis isolates from wild rabbits

This work aims to investigate the presence of Streptococcus suis in wild rabbits. A total of 65 S. suis isolates were recovered from 33.3% of the wild rabbits examined. Most isolates (86.2%) belong to genotype cps9. These isolates were further characterized by pulsed field gel electrophoresis (PFGE), multilocus sequence typing (MLST) and virulence genotyping. Overall, S. suis exhibited a low genetic diversity. Only 5 genetic profiles were obtained by PFGE and most isolates (71.4%) were included in two pulsotypes that were also widely distributed among the wild rabbit population. MLST analysis assigned all cps9 isolates into three new singlestones (ST216, ST217 and ST284), which were not genetically related to the European ST87 and Spanish ST61 widespread swine clones, indicating a different genetic background for the S. suis isolates from wild rabbits and pigs. Wild rabbit isolates exhibited the genotype mrp-/epf-/sly-, different from those showed by most of the swine S. suis isolates of the ST87 and ST61 clones. None of the S. suis isolated from wild rabbits exhibited the genotype cps2/mrp+/epf+/sly+ associated with human infections. These results indicate that S. suis isolates from wild rabbits are not genetically related with prevalent clones usually associated with infections in pigs or humans in Europe and do not exhibit either their virulence genotypes. Therefore, although wild rabbits could represent an unknown reservoir of this pathogen, they could not represent a potential risk for pigs or humans.

HP0197 contributes to CPS synthesis and the virulence of Streptococcus suis via CcpA

Streptococcus suis serotype 2 (SS2), a major swine pathogen and an emerging zoonotic agent, has greatly challenged global public health. The encoding proteins with unknown functions the bacterium encodes are an obstruction to studies of the pathogenesis. A novel surface protective antigen HP0197 is one of these proteins which have no sequence homology to any known protein. In the present study, the protein was determined to be involved in bacterial virulence through an evaluation of the isogenic mutant (Δhp0197) in both mice and pigs. The experimental infection also indicated that Δhp0197 could be cleared easily during infection, which could be attributed to the reduced thickness of the capsular polysaccharides (CPS) and the significantly reduced phagocytotic resistance. Microarrays-based comparative transcriptome analysis suggested that the suppressed expression of the operon responsible for CPS synthesis might be reversed by CcpA activity, which controlled global regulation of carbon catabolite through the binding of the CcpA and HPr-Ser-46-P to the catabolite-responsive elements (cre) of the target operons. The hypothesis was approved by the fact that the purified FLAG-tagged HPr from WT stain exhibited a higher binding activity to cre with CcpA compared to the Δhp0197 by the Electrophoretic Mobility Shift Assay, suggesting lower level of phosphorylation of the phosphocarrier protein HPr at residue Ser-46 (HPr-Ser-46P) in Δhp0197. These indicated that HP0197 could enhance CcpA activity to control the expression of genes involved in carbohydrate utilization and CPS synthesis, thus contributing to the virulence of S. suis.

Attenuation of Streptococcus suis virulence by the alteration of bacterial surface architecture

NeuB, a sialic acid synthase catalyzes the last committed step of the de novo biosynthetic pathway of sialic acid, a major element of bacterial surface structure. Here we report a functional NeuB homologue of Streptococcus suis, a zoonotic agent, and systematically address its molecular and immunological role in bacterial virulence. Disruption of neuB led to thinner capsules and more susceptibility to pH, and cps2B inactivation resulted in complete absence of capsular polysaccharides. These two mutants both exhibited increased adhesion and invasion to Hep-2 cells and improved sensibility to phagocytosis. Not only do they retain the capability of inducing the release of host pro-inflammatory cytokines, but also result in the faster secretion of IL-8. Easier cleaning up of the mutant strains in whole blood is consistent with virulence attenuation seen with experimental infections of both mice and SPF-piglets. Therefore we concluded that altered architecture of S. suis surface attenuates its virulence.

An orphan chemotaxis sensor regulates virulence and antibiotic tolerance in the human pathogen Pseudomonas aeruginosa

The synthesis of virulence factors by pathogenic bacteria is highly regulated and occurs in response to diverse environmental cues. An array of two component systems (TCSs) serves to link perception of different cues to specific changes in gene expression and/or bacterial behaviour. Those TCSs that regulate functions associated with virulence represent attractive targets for interference in anti-infective strategies for disease control. We have previously identified PA2572 as a putative response regulator required for full virulence of Pseudomonas aeruginosa, the opportunistic human pathogen, to Galleria mellonella (Wax moth) larvae. Here we have investigated the involvement of candidate sensors for signal transduction involving PA2572. Mutation of PA2573, encoding a probable methyl-accepting chemotaxis protein, gave rise to alterations in motility, virulence, and antibiotic resistance, functions which are also controlled by PA2572. Comparative transcriptome profiling of mutants revealed that PA2572 and PA2573 regulate expression of a common set of 49 genes that are involved in a range of biological functions including virulence and antibiotic resistance. Bacterial two-hybrid analysis indicated a REC-dependent interaction between PA2572 and PA2573 proteins. Finally expression of PA2572 in the PA2573 mutant background restored virulence to G. mellonella towards wild-type levels. The findings indicate a role for the orphan chemotaxis sensor PA2573 in the regulation of virulence and antibiotic tolerance in P. aeruginosa and indicate that these effects are exerted in part through signal transduction involving PA2572.

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

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

Common regulators of virulence in streptococci

Streptococcal species are a diverse group of bacteria which can be found in animals and humans. Their interactions with host organisms can vary from commensal to pathogenic. Many of the pathogenic species are causative agents of severe, invasive infections in their hosts, accounting for a high burden of morbidity and mortality, associated with high economic costs in industry and health care. Among them, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumoniae, and Streptococcus suis are discussed here. An environmentally stimulated and tightly controlled expression of their virulence factors is of utmost importance for their pathogenic potential. Thus, the most universal and widespread regulators from the classes of stand-alone transcriptional regulators, two-component signal transduction systems (TCS), eukaryotic-like serine/threonine kinases, and small noncoding RNAs are the topic of this chapter. The regulatory levels are reviewed with respect to function, activity, and their role in pathogenesis. Understanding of and interfering with transcriptional regulation mechanisms and networks is a promising basis for the development of novel anti-infective therapies.

Functional definition of LuxS, an autoinducer-2 (AI-2) synthase and its role in full virulence of Streptococcus suis serotype 2

Quorum sensing is a widespread chemical communication in response to fluctuation of bacterial population density, and has been implicated into bacterial biofilm formation and regulation of expression of virulence factors. The luxS gene product, S-ribosylhomocysteinase, catalizes the last committed step in biosynthetic pathway of autoinducer 2 (AI-2), a signaling molecule for inter-species quorum sensing. We found a luxS homologue in 05ZYH33, an epidemic strain of Streptococcus suis serotype 2 (SS2) in China. A luxS null mutant (ΔluxS) of 05ZYH33 strain was obtained using an approach of homologous recombination. LuxS was determined to be required for AI-2 production in 05ZYH33 strain of S. suis 2. Inactivation of luxS gene led to a wide range of phenotypic changes including thinner capsular walls, increased tolerance to H(2)O(2), reduced adherence capacity to epithelial cells, etc. In particular, loss of LuxS impaired dramatically its full virulence of SS2 in experimental model of piglets, and functional complementation restored it nearly to the level of parent strain. Genome-wide transcriptome analyses suggested that some known virulence factors such as CPS are down-regulated in the ΔluxS mutant, which might in part explain virulence attenuation by luxS deletion. Similarly, 29 of 71 genes with different expression level were proposed to be targets candidate regulated by LuxS/AI-2-dependent quorum sensing.