Construction and characterization of Streptococcus suis-Escherichia coli shuttle cloning vectors

HylS', a fragment of truncated hyaluronidase of Streptococcus suis, contributes to immune evasion by interaction with host complement factor C3b

Pathogenic bacteria have evolved many strategies to evade surveillance and attack by complements. Streptococcus suis is an important zoonotic pathogen that infects humans and pigs. Hyaluronidase (HylA) has been reported to be a potential virulence factor of S. suis. However, in this study, it was discovered that the genomic region encoding HylA of the virulent S. suis strain SC19 and other ST1 strains was truncated into four fragments when aligned with a strain containing intact HylA and possessing hyaluronidase activity. As a result, SC19 had no hyaluronidase activity, but one truncated HylA fragment, designated as HylS,' directly interacted with complement C3b, as confirmed by western ligand blotting, pull-down, and ELISA assays. The deposition of C3b and membrane attack complex (MAC) formation on the surface of a HylS'-deleted mutant (ΔhylS') was significantly increased compared to wild-type SC19. In human sera and whole blood, ΔhylS' survival was significantly reduced compared to that in SC19. The resistance of ΔhylS' to macrophages and human polymorphonuclear neutrophil PMNs also decreased. In a mouse infection model, ΔhylS' showed reduced lethality and lower bacterial load in the organs compared to that of SC19. We conclude that the truncated hyaluronidase HylS' fragment contributes to complement evasion and the pathogenesis of S. suis.

A bifunctional amylopullulanase of Streptococcus suis ApuA contributes to immune evasion by interaction with host complement C3b

The complement system is the first defense line of the immune system. However, pathogens have evolved numerous strategies to evade complement attacks. Streptococcus suis is an important zoonotic bacterium, harmful to both the pig industry and human health. ApuA has been reported as a bifunctional amylopullulanase and also contributed to virulence of S. suis. Herein, we found that ApuA could activate both classical and alternative pathways of the complement system. Furthermore, by using bacterial two-hybrid, far-western blot and ELISA assays, it was confirmed that ApuA could interact with complement C3b. The interaction domain of ApuA with C3b was found to be its α-Amylase domain (ApuA_N). After construction of an apuA mutant (ΔapuA) and its complementary strain, it was found that compared to the wild-type strain (WT), ΔapuA had significantly increased C3b deposition and membrane attack complex formation. Additionally, ΔapuA showed significantly lower survival rates in human serum and blood and was more susceptible to engulfment by neutrophils and macrophages. Mice infected with ΔapuA had significantly higher survival rates and lower bacterial loads in their blood, lung and brains, compared to those infected with WT. In summary, this study identified ApuA as a novel factor involved in the complement evasion of S. suis and suggested its multifunctional role in the pathogenesis of S. suis.

Streptococcus suis Deploys Multiple ATP-Dependent Proteases for Heat Stress Adaptation

Streptococcus suis is an important zoonotic pathogen, causing cytokine storms of Streptococcal toxic shock-like syndrome amongst humans after a wound infection into the bloodstream. To overcome the challenges of fever and leukocyte recruitment, invasive S. suis must deploy multiple stress responses forming a network and utilize proteases to degrade short-lived regulatory and misfolded proteins induced by adverse stresses, thereby adapting and evading host immune responses. In this study, we found that S. suis encodes multiple ATP-dependent proteases, including single-chain FtsH and double-subunit Clp protease complexes ClpAP, ClpBP, ClpCP, and ClpXP, which were activated as the fever of infected mice in vivo. The expression of genes ftsH, clpA/B/C, and clpP, but not clpX, were significantly upregulated in S. suis in response to heat stress, while were not changed notably under the treatments with several other stresses, including oxidative, acidic, and cold stimulation. FtsH and ClpP were required for S. suis survival within host blood under heat stress in vitro and in vivo. Deletion of ftsH or clpP attenuated the tolerance of S. suis to heat, oxidative and acidic stresses, and significantly impaired the bacterial survival within macrophages. Further analysis identified that repressor CtsR directly binds and controls the clpA/B/C and clpP operons and is relieved by heat stress. In summary, the deployments of multiple ATP-dependent proteases form a flexible heat stress response network that appears to allow S. suis to fine-tune the degradation or refolding of the misfolded proteins to maintain cellular homeostasis and optimal survival during infection.

Loss of expression of the glutamate dehydrogenase (gdh) of Streptococcus suis serotype 2 compromises growth and pathogenicity

Streptococcus suis serotype 2 is a zoonotic agent that causes substantial economic losses to the swine industry and threatens human public health. Factors that contribute to its ability to cause disease are not yet fully understood. Glutamate dehydrogenase (GDH) is an enzyme found in living cells and plays vital roles in cellular metabolism. It has also been shown to affect pathogenic potential of certain bacteria. In this study, we constructed a S. suis serotype 2 GDH mutant (Δgdh) by insertional inactivation mediated by a homologous recombination event and confirmed loss of expression of GDH in the mutant by immunoblot and enzyme activity staining assays. Compared with the wild type (WT) strain, Δgdh displayed a different phenotype. It exhibited impaired growth in all conditions evaluated (solid and broth media, increased temperature, varying pH, and salinity) and formed cells of reduced size. Using a swine infection model, pigs inoculated with the WT strain exhibited fever, specific signs of disease, and lesions, and the strain could be re-isolated from the brain, lung, joint fluid, and blood samples collected from the infected pigs. Pigs inoculated with the Δgdh strain did not exhibit any clinical signs of disease nor histologic lesions, and the strain could not be re-isolated from any of the tissues nor body fluid sampled. The Δgdh also showed a decreased level of survival in pig blood. Taken together, these results suggest that the gdh is important in S. suis physiology and its ability to colonize, disseminate, and cause disease.

O-acetyl-homoserine sulfhydrylase deficient Streptococcus suis serotype 2 strain SC19 becomes an avirulent strain and provides immune protection against homotype infection in mice

O-acetyl-homoserine sulfhydrylase (OAHS) is a pyridoxal 5'-phosphate-dependent enzyme involved in microbial methionine biosynthesis, which catalyzes the conversion of o-acetyl-homoserine (OAH) to homocysteine. In our previous study, we found that OAHS of Streptococcus suis serotype 2 (SS2) can interact with the porcine blood-brain barrier (BBB) model, but whether OAHS regulates the penetration of BBB during SS2 infection is still unclear. To explore the role of OAHS in SS2 infection, OAHS-deficient SS2 mutant strain (SC19-ΔOAHS) and gene complemental strain (SC19-cΔOAHS) were constructed. Compared to the parent strain, with the loss of oahs, the chain length of SC19-ΔOAHS was shortened, the virulence was significantly reduced, the survival rate of mice infected with SC19-ΔOAHS was obviously increased accompanied by the relieved clinical symptoms. And the survival ability of SC19-ΔOAHS in whole blood was also remarkably decreased. Interestingly, the adhesion of SC19-ΔOAHS to endothelial cells was markedly increased, but the deficiency of OAHS significantly inhibited the strain penetrating BBB both in vivo and in vitro. Most of these phenomena can be reversed by the complemental strain (SC19-cΔOAHS). Further study showed that the deficiency of OAHS severely reduced SC19-induced endothelial cell apoptosis, tight junctions (TJs) protein impairment and the expression of SS2 virulence factor Enolase (Eno), involved in the destruction of BBB. Additionally, SC19-ΔOAHS immunized mice were able to resist SC19 or JZLQ022 infection. In conclusion, we confirmed that OAHS promoted the pathogenicity by enhancing host's BBB permeability and immune escape, and SC19- ΔOAHS is a potential live vaccine.

Endopeptidase O promotes Streptococcus suis immune evasion by cleaving the host- defence peptide cathelicidins

Streptococcus suis is a zoonotic Gram-positive bacterium that causes invasive infections such as sepsis and meningitis, threatening public health worldwide. For successful establishment of infection, the bacterium should subvert the innate effectors of immune defence, including the cathelicidin family of host-defence peptides that combat pathogenic bacteria by directly disrupting cell membranes and coordinating immune responses. Here, our study shows that an extracellular endopeptidase O (PepO) of S. suis contributes to assisting the bacterium to resist cathelicidin-mediated killing, as the deletion of the pepO gene makes S. suis more sensitive to the human cathelicidin LL-37, as well as its mouse equivalent, mCRAMP. This protease targets and cleaves both LL-37 and mCRAMP, degrading them into shorter peptides with only a few amino acids, thereby abrogating their ability to kill S. suis. By cleaving LL-37 and mCRAMP, PepO impairs their chemotactic properties for neutrophil migration and undermines their anti-apoptosis activity, which is required for prolonging neutrophil lifespan. Also, PepO inhibits the ability of LL-37 and mCRAMP to promote lysosome development in macrophages. Moreover, the loss of PepO attenuates organ injury and decreases bacterial burdens in a murine model of S. suis bacteraemia. Taken together, these data provide novel insights into the role of the intrinsic proteolytic characteristics of PepO in S. suis-host interaction. Our findings demonstrate that S. suis utilizes the PepO protease to cleave cathelicidins, which is an immunosuppressive strategy adopted by this bacterium to facilitate pathogenesis.

Study of the Role of Lipoprotein Maturation Enzymes in the Pathogenesis of the Infection Caused by the Streptococcus suis Serotype 2 Sequence Type 25 North American Prototype Strain

Streptococcus suis serotype 2 is an important swine bacterial pathogen causing sudden death, septic shock, and meningitis. However, serotype 2 strains are phenotypically and genotypically heterogeneous and composed of a multitude of sequence types (STs) whose distributions greatly vary worldwide. It has been previously shown that the lipoprotein (LPP) maturation enzymes diacylglyceryl transferase (Lgt) and signal peptidase (Lsp) significantly modulate the inflammatory host response and play a differential role in virulence depending on the genetic background of the strain. Differently from Eurasian ST1/ST7 strains, the capsular polysaccharide of a North American S. suis serotype 2 ST25 representative strain only partially masks sub-capsular domains and bacterial wall components. Thus, our hypothesis is that since LPPs would be more surface exposed in ST25 strains than in their ST1 or ST7 counterparts, the maturation enzymes would play a more important role in the pathogenesis of the infection caused by the North American strain. Using isogenic Δlgt and Δlsp mutants derived from the wild-type ST25 strain, our studies suggest that these enzymes do not seem to play a role in the interaction between S. suis and epithelial and endothelial cells, regardless of the genetics background of the strain used. However, a role in the formation of biofilms (also independently of the STs) has been demonstrated. Moreover, the involvement of LPP dendritic cell activation in vitro seems to be somehow more pronounced with the ST25 strain. Finally, the Lgt enzyme seems to play a more important role in the virulence of the ST25 strain. Although some differences between STs could be observed, our original hypothesis that LPPs would be significantly more important in ST25 strains due to a better bacterial surface exposition could not be confirmed.

luxS contributes to intramacrophage survival of Streptococcus agalactiae by positively affecting the expression of fruRKI operon

The LuxS quorum sensing system is a widespread system employed by many bacteria for cell-to-cell communication. The luxS gene has been demonstrated to play a crucial role in intramacrophage survival of piscine Streptococcus agalactiae, but the underlying mechanism remains largely unknown. In this study, transcriptome analysis, followed by the luxS gene deletion and subsequent functional studies, confirmed that impaired bacterial survival inside macrophages due to the inactivation of luxS was associated with reduced transcription of the fruRKI operon, encoding the fructose-specific phosphotransferase system. Further, luxS was determined not to enhance the transcription of fruRKI operon by binding its promoter, but to upregulate the expression of this operon via affecting the binding ability of catabolite control protein A (CcpA) to the catabolite responsive element (cre) in the promoter of fruRKI. Collectively, our study identifies a novel and previously unappreciated role for luxS in bacterial intracellular survival, which may give a more thorough understanding of the immune evasion mechanism in S. agalactiae.

Conjugative transfer of streptococcal prophages harboring antibiotic resistance and virulence genes

Prophages play important roles in the transduction of various functional traits, including virulence factors, but remain debatable in harboring and transmitting antimicrobial resistance genes (ARGs). Herein we characterize a prevalent family of prophages in Streptococcus, designated SMphages, which harbor twenty-five ARGs that collectively confer resistance to ten antimicrobial classes, including vanG-type vancomycin resistance locus and oxazolidinone resistance gene optrA. SMphages integrate into four chromosome attachment sites by utilizing three types of integration modules and undergo excision in response to phage induction. Moreover, we characterize four subtypes of Alp-related surface proteins within SMphages, the lethal effects of which are extensively validated in cell and animal models. SMphages transfer via high-frequency conjugation that is facilitated by integrative and conjugative elements from either donors or recipients. Our findings explain the widespread of SMphages and the rapid dissemination of ARGs observed in members of the Streptococcus genus.

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.

DivIVA Interacts with the Cell Wall Hydrolase MltG To Regulate Peptidoglycan Synthesis in Streptococcus suis

Bacterial morphology is largely determined by the spatial and temporal regulation of peptidoglycan (PG) biosynthesis. Ovococci possess a unique pattern of PG synthesis different from the well studied Bacillus, and the mechanism of the coordination of PG synthesis remains poorly understood. Several regulatory proteins have been identified to be involved in the regulation of ovococcal morphogenesis, among which DivIVA is an important one to regulate PG synthesis in streptococci, while its mechanism is largely unknown. Here, the zoonotic pathogen Streptococcus suis was used to investigate the regulation of DivIVA on PG synthesis. Fluorescent d-amino acid probing and 3D-structured illumination microscopy found that DivIVA deletion caused abortive peripheral PG synthesis, resulting in a decreased aspect ratio. The phosphorylation-depleted mutant (DivIVA3A) cells displayed a longer nascent PG and became longer, whereas the phosphorylation-mimicking mutant (DivIVA3E) cells showed a shorter nascent PG and became shorter, suggesting that DivIVA phosphorylation is involved in regulating peripheral PG synthesis. Several DivIVA-interacting proteins were identified, and the interaction was confirmed between DivIVA and MltG, a cell wall hydrolase essential for cell elongation. DivIVA did not affect the PG hydrolysis activity of MltG, while the phosphorylation state of DivIVA affected its interaction with MltG. MltG was mislocalized in the ΔdivIVA and DivIVA3E cells, and both ΔmltG and DivIVA3E cells formed significantly rounder cells, indicating an important role of DivIVA phosphorylation in regulating PG synthesis through MltG. These findings highlight the regulatory mechanism of PG synthesis and morphogenesis of ovococci. IMPORTANCE The peptidoglycan (PG) biosynthesis pathway provides a rich source of novel antimicrobial drug targets. However, bacterial PG synthesis and its regulation is a very complex process involving dozens of proteins. Moreover, unlike the well studied Bacillus, ovococci undergo unusual PG synthesis with unique mechanisms of coordination. DivIVA is an important regulator of PG synthesis in ovococci, while its exact role in regulating PG synthesis remains poorly understood. In this study, we determined the role of DivIVA in regulating lateral PG synthesis of Streptococcus suis and identified a critical interacting partner, MltG, in which DivIVA influenced the subcellular localizations of MltG through its phosphorylation. Our study characterizes the detailed role of DivIVA in regulating bacterial PG synthesis, which is very helpful for understanding the process of PG synthesis in streptococci.

A link between STK signalling and capsular polysaccharide synthesis in Streptococcus suis

Synthesis of capsular polysaccharide (CPS), an important virulence factor of pathogenic bacteria, is modulated by the CpsBCD phosphoregulatory system in Streptococcus. Serine/threonine kinases (STKs, e.g. Stk1) can also regulate CPS synthesis, but the underlying mechanisms are unclear. Here, we identify a protein (CcpS) that is phosphorylated by Stk1 and modulates the activity of phosphatase CpsB in Streptococcus suis, thus linking Stk1 to CPS synthesis. The crystal structure of CcpS shows an intrinsically disordered region at its N-terminus, including two threonine residues that are phosphorylated by Stk1. The activity of phosphatase CpsB is inhibited when bound to non-phosphorylated CcpS. Thus, CcpS modulates the activity of phosphatase CpsB thereby altering CpsD phosphorylation, which in turn modulates the expression of the Wzx-Wzy pathway and thus CPS production.

Thioredoxin A of Streptococcus suis Serotype 2 Contributes to Virulence by Inhibiting the Expression of Pentraxin 3 to Promote Survival Within Macrophages

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen that can infect humans in contact with infected pigs or their byproducts. It can employ different types of genes to defend against oxidative stress and ensure its survival. The thioredoxin (Trx) system is a key antioxidant system that contributes adversity adaptation and pathogenicity. SS2 has been shown to encode putative thioredoxin genes, but the biological roles, coding sequence, and underlying mechanisms remains uncharacterized. Here, we demonstrated that SSU05_0237-ORF, from a clinical SS2 strain, ZJ081101, encodes a protein of 104 amino acids with a canonical CGPC active motif and an identity 70-85% similar to the thioredoxin A (TrxA) in other microorganisms. Recombinant TrxA efficiently catalyzed the thiol-disulfide oxidoreduction of insulin. The deletion of TrxA led to a significantly slow growth and markedly compromised tolerance of the pathogen to temperature stress, as well as impaired adhesion ability to pig intestinal epithelial cells (IPEC-J2). However, it was not involved in H2O2 and paraquat-induced oxidative stress. Compared with the wild-type strain, the ΔTrxA strain was more susceptible to killing by macrophages through increasing NO production. Treatment with TrxA mutant strain also significantly attenuated cytotoxic effects on RAW 264.7 cells by inhibiting inflammatory response and apoptosis. Knockdown of pentraxin 3 in RAW 264.7 cells was more vulnerable to phagocytic activity, and TrxA promoted SS2 survival in phagocytic cells depending on pentraxin 3 activity compared with the wild-type strain. Moreover, a co-inoculation experiment in mice revealed that TrxA mutant strain is far more easily cleared from the body than the wild type strain in the period from 8-24 h, and exhibits significantly attenuated oxidative stress and liver injury. In summary, we reveal the important role of TrxA in the pathogenesis of SS2.

Novel virulence factor Cba induces antibody-dependent enhancement (ADE) of Streptococcus suis Serotype 9 infection in a mouse model

Streptococcus suis (SS) is a zoonotic pathogen that affects the health of humans and the development of the pig industry. The SS Cba protein is a collagen adhesin, and a few of its homologs are related to the enhancement of bacterial adhesion. We compared the phenotypes of SS9-P10, SS9-P10 cba knockout strains and its complementary strains in vitro and in vivo and found that knocking out the cba gene did not affect the growth characteristics of the strain, but it significantly reduced the ability of SS to form biofilms, adhesion to host cells, phagocytic resistance to macrophages and attenuated virulence in a mouse infection model. These results indicated that Cba was a virulence related factor of SS9. In addition, Mice immunized with the Cba protein had higher mortality and more serious organ lesions after challenge, and the same was observed in passive immunization experiments. This phenomenon is similar to the antibody-dependent enhancement of infection by bacteria such as Acinetobacter baumannii and Streptococcus pneumoniae. To our knowledge, this is the first demonstration of antibody-dependent enhancement of SS, and these observations highlight the complexity of antibody-based therapy for SS infection.

Laminin-binding protein of Streptococcus suis serotype 2 influences zinc acquisition and cytokine responses

Streptococcus suis serotype 2 is an important bacterial pathogen of swine, responsible for substantial economic losses to the swine industry worldwide. The knowledge on the pathogenesis of the infection caused by S. suis is still poorly known. It has been previously described that S. suis possesses at least one lipoprotein with double laminin and zinc (Zn)-binding properties, which was described in the literature as either laminin-binding protein (Lmb, as in the current study), lipoprotein 103, CDS 0330 or AdcAII. In the present study, the role of the Lmb in the pathogenesis of the infection caused by S. suis serotype 2 was dissected. Using isogenic mutants, results showed that Lmb does not play an important role in the laminin-binding activity of S. suis, even when clearly exposed at the bacterial surface. In addition, the presence of this lipoprotein does not influence bacterial adhesion to and invasion of porcine respiratory epithelial and brain endothelial cells and it does not increase the susceptibility of S. suis to phagocytosis. On the other hand, the Lmb was shown to play an important role as cytokine activator when tested in vitro with dendritic cells. Finally, this lipoprotein plays a critical role in Zn acquisition from the host environment allowing bacteria to grow in vivo. The significant lower virulence of the Lmb defective mutant may be related to a combination of a lower bacterial survival due to the incapacity to acquire Zn from their surrounding milieu and a reduced cytokine activation.

Identification of the RNA-binding domain-containing protein RbpA that acts as a global regulator of the pathogenicity of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2), an emerging zoonotic pathogen, causes swine diseases and human cases of streptococcal toxic shock syndrome. RNA-binding proteins (RBPs) can modulate gene expression through post-transcriptional regulation. In this study, we identified an RBP harbouring an S1 domain, named RbpA, which facilitated SS2 adhesion to host epithelial cells and contributed to bacterial pathogenicity. Comparative proteomic analysis identified 145 proteins that were expressed differentially between ΔrbpA strain and wild-type strain, including several virulence-associated factors, such as the extracellular protein factor (EF), SrtF pilus, IgA1 protease, SBP2 pilus, and peptidoglycan-binding LysM’ proteins. The mechanisms underlying the regulatory effects of RbpA on their encoding genes were explored, and it was found that RbpA regulates gene expression through diverse mechanisms, including post-transcriptional regulation, and thus acts as a global regulator. These results partly reveal the pathogenic mechanism mediated by RbpA, improving our understanding of the regulatory systems of S. suis and providing new insights into bacterial pathogenicity.

Pathogenic investigations of Streptococcus pasteurianus, an underreported zoonotic pathogen, isolated from a diseased piglet with meningitis

Streptococcus pasteurianus, an underreported opportunistic pathogen, is considered an increasingly recognized cause of meningitis and bacteremia in many animals and humans worldwide. However, except for some epidemiological studies, there is no report about the gene-deletion mutagenesis, virulence factors, reservoir niches or animal infection models for this pathogen. In this study, we first isolated an S. pasteurianus strain from a newly weaned piglet's brain with meningitis. The genomic sequence of this swine isolate WUSP067 shared high homology with that of two human strains. The comparative genome analysis showed that strain WUSP067 contained a fucose utilization cluster absent in human strains, and it shared 91% identity with that of an integrative and conjugative element (ICE) ICEssuZJ20091101-2 from Streptococcus suis, another important swine bacterial pathogen. Strain WUSP067 was resistant to erythromycin, tulathromycin, lincomycin, clindamycin, doxycycline and gentamycin, and ICEs are vehicles for harbouring antimicrobial resistance genes. The infection model was established using the 3-week-old newly weaned ICR mice. The 50% lethal dose value of strain WUSP067 was 4.0 × 107 colony-forming units per mouse. The infected mice showed severe signs of meningitis and pathological changes in brains. Furthermore, the capsule-deficient mutant was generated using natural transformation, and we showed that capsule was an essential virulence factor for S. pasteurianus. In addition, we found that tonsils and hilar lymph nodes of healthy pigs may be reservoir niches for this bacterium. Thus, our study provided valuable information about the pathogenetic characteristics and antimicrobial resistance of S. pasteurianus and paved the way for studying its pathogenesis.

Highly Effective Markerless Genetic Manipulation of Streptococcus suis Using a Mutated PheS-Based Counterselectable Marker

Streptococcus suis is an important zoonotic pathogen, however, an efficient markerless genetic manipulation system is still lacking for further physiological and pathological studies on this bacterium. Several techniques have been developed for markerless genetic manipulation of S. suis utilizing either a temperature-sensitive vector or a counterselectable markers (CSMs), however, at present, the efficiency of these techniques is not very satisfactory. In this study, we developed a strategy for markerless genetic manipulation of S. suis employing a CSM based on a conditionally lethal mutant allele of pheS, which encodes the α-subunit of phenylalanyl-tRNA synthetase (PheS). This mutant pheS, mPheS, was constructed by introducing site-directed mutations for a T261S/A315G double-substitution and a number of silent mutations to decrease its similarity with the endogenous wild type pheS gene (wtPheS). Additionally, five potentially strong promoters from S. suis were screened for their ability to drive high-level expression of mPheS, thus endowing the carrier strain with sufficient sensitivity to the phenylalanine analog p-chloro-phenylalanine (p-Cl-phe). Insertion of these P-mPheS cassettes into a vector or into the chromosomal locus via a linked erythromycin resistance gene revealed that mPheS allele driven by promoters P₀₅₃₀ and P₁₅₀₃ renders S. suis sensitive to as low as 0.01% (or 0.5 mM) of p-Cl-phe. This offers two potential CSMs for S. suis with p-Cl-phe as a counterselective agent. P₁₅₀₃-mPheS was revealed to be 100% efficient for counter-selection in S. suis by application in a precise gene deletion. Using P₁₅₀₃-mPheS as a CSM, a two-step insertion and excision strategy for markerless genetic manipulation of S. suis were developed, supplying a powerful tool for markerless genetic manipulation of S. suis.

Construction of Streptococcus agalactiae sialic acid mutant and evaluation of its potential as a live attenuated vaccine in Nile tilapia (Oreochromis niloticus)

AIMS

This study aimed to develop a live attenuated vaccine as an effective approach to prevent streptococcosis in tilapia (Oreochromis niloticus).

METHODS AND RESULTS

We eliminated the virulence factor, sialic acid (Sia) encoded by the neuA-D gene cluster from the Group B Streptococcus (Streptococcus agalactiae, GBS) strain WC1535, to construct Sia-deficient S. agalactiae (ΔSia) mutant by homologous recombination. Results showed that the ΔSia mutant had higher adherence to HEp-2 cells and lower resistance to RAW264.7 cell phagocytosis than the wild-type S. agalactiae. The virulence of the ΔSia mutant to tilapia dramatically decreased with no virulence recovery. The relative percent survivals (RPSs) were 50.00% and 54.50% at 30 days when challenged at the wild-type WC1535 doses of 1.0 × 10⁷ and 5.0 × 10⁷ CFU fish^-1 , respectively, via intraperitoneal (IP) injection. The tilapia vaccinated via IP injection with the ΔSia mutant induced strong antibody agglutination titers. The expression of IL-1β, TNF-α, MHC-Iα, and MHC-IIβ could be enhanced in the intestine, spleen, and head kidney for tilapia administered with the ΔSia mutant.

CONCLUSIONS

GBS Sia plays a critical role in adherence to HEp-2 cells and resistance to the immune clearance of RAW264.7 cells. Moreover, the ΔSia mutant is a safe, stable, and immunogenic live attenuated vaccine candidate to protect tilapia against GBS infection.

SIGNIFICANCE AND IMPACT OF STUDY

The results offer more evidence of the importance of Sia in GBS and may be instructive in the control of tilapia streptococcosis.

Development and Application of Two Inducible Expression Systems for Streptococcus suis

Streptococcus suis is an important zoonotic bacterial pathogen posing a threat to the pig industry as well as public health, for which the mechanisms of growth and cell division remain largely unknown. Developing convenient genetic tools that can achieve strictly controlled gene expression is of great value for investigating these fundamental physiological processes of S. suis. In this study, we first identified three strong constitutive promoters, P_g, P_t, and P_e, in S. suis. Promoter P_g was used to drive the expression of repressor genes tetR and lacI, and the operator sequences were added within promoters P_t and P_e. By optimizing the insertion sites of the operator sequence, we successfully constructed an anhydrotetracycline (ATc)-inducible expression system and an isopropyl-β-d-thiogalactopyranoside (IPTG)-inducible expression system in S. suis. We showed that these two systems provided inducer-concentration- and induction-time-dependent expression of the reporter gene. By using these tools, we investigated the subcellular localization of a key cell division protein, FtsZ, which showed that it could be correctly localized to the midcell region. In addition, we constructed a conditional knockout strain for the glmS gene, which is an essential gene, and showed that our ATc-inducible promoter could provide strictly controlled expression of glmS in trans, suggesting that our inducible expression systems can be used for deletion of essential genes in S. suis. Therefore, for the first time we developed two inducible expression systems in S. suis and showed their applications in the study of an important cell division protein and an essential gene. These genetic tools will further facilitate the functional study of other important genes of S. suis. IMPORTANCE Streptococcus suis is an important zoonotic bacterial pathogen. Studying the mechanisms of cell growth and division is important for the identification of novel antimicrobial drug targets. Inducible expression systems can provide strictly controlled expression of the protein of interest and are useful tools to study the functions of physiologically important proteins. However, there is a lack of convenient genetic tools that can achieve inducible protein expression in S. suis. In this study, we developed two (ATc-inducible and IPTG-inducible) inducible expression systems and showed their applications in a subcellular localization study of a cell division protein and the construction of conditional knockout of essential genes in S. suis. These systems will be useful for functional studies of important proteins of S. suis.

The AdcR-regulated AdcA and AdcAII contribute additively to zinc acquisition and virulence in Streptococcus suis

Metals are necessary elements for bacteria. Typically, vertebrate hosts restrict invading bacterial pathogens from accessing metals. Therefore, bacteria have evolved high-affinity metal importers to acquire metals. Streptococcus suis is a major swine pathogen and an emerging zoonotic agent that endangers the swine industry and human health worldwide. Herein, we aimed to identify the zinc acquisition systems in S. suis and evaluate their roles in bacterial virulence. Bioinformatic analyses revealed that S. suis encodes homologues of AdcA and AdcAII, two well-characterised Zn-binding lipoproteins in certain streptococci. Quantitative reverse transcription PCR (qRT-PCR) analysis revealed that the expressions of adcA and adcAII were significantly upregulated in response to Zn limitation, with a higher expression level of adcAII than adcA. Gene deletion mutants and complementation strains were constructed; their growth characteristics under Zn-deficient and Zn-replete conditions indicated that AdcA and AdcAII have overlapping functionality in Zn acquisition. A mouse infection model was used to evaluate the roles of AdcA and AdcAII in S. suis virulence. Mice infected with the double mutant ΔadcAΔadcAII exhibited a significantly higher survival rate, decreased bacterial burden, and lower production of inflammatory cytokines compared to those infected with the wild type (WT) strain. Furthermore, ΔadcAΔadcAII showed reduced competitiveness in infection establishment compared with the WT strain. RNA sequencing, qRT-PCR, and electrophoretic mobility shift assays revealed that AdcR negatively regulates the expressions of adcA and adcAII. Collectively, our results demonstrated that AdcA and AdcAII, which are negatively regulated by AdcR, contribute additively to zinc acquisition and virulence in S. suis.

Screening for Virulence-Related Genes via a Transposon Mutant Library of Streptococcus suis Serotype 2 Using a Galleria mellonella Larvae Infection Model

Streptococcus suis (S. suis) is a zoonotic bacterial pathogen causing lethal infections in pigs and humans. Identification of virulence-related genes (VRGs) is of great importance in understanding the pathobiology of a bacterial pathogen. To identify novel VRGs, a transposon (Tn) mutant library of S. suis strain SC19 was constructed in this study. The insertion sites of approximately 1700 mutants were identified by Tn-seq, which involved 417 different genes. A total of 32 attenuated strains were identified from the library by using a Galleria mellonella larvae infection model, and 30 novel VRGs were discovered, including transcription regulators, transporters, hypothetical proteins, etc. An isogenic deletion mutant of hxtR gene (ΔhxtR) and its complementary strain (CΔhxtR) were constructed, and their virulence was compared with the wild-type strain in G. mellonella larvae and mice, which showed that disruption of hxtR significantly attenuated the virulence. Moreover, the ΔhxtR strain displayed a reduced survival ability in whole blood, increased sensitivity to phagocytosis, increased chain length, and growth defect. Taken together, this study performed a high throughput screening for VRGs of S. suis using a G. mellonella larvae model and further characterized a novel critical virulence factor.

Role of Maturation of Lipoproteins in the Pathogenesis of the Infection Caused by Streptococcus suis Serotype 2

Streptococcus suis serotype 2 is an important porcine bacterial pathogen associated with multiple pathologies in piglets. Bacterial lipoproteins (LPPs) have been described as playing important roles in the pathogenesis of the infection of other Gram-positive bacteria as adhesins, pro-inflammatory cell activators and/or virulence factors. In the current study, we aimed to evaluate the role of the prolipoprotein diacylglyceryl transferase (Lgt) and lipoprotein signal peptidase (Lsp) enzymes, which are responsible for LPP maturation, on the pathogenesis of the infection caused by two different sequence types (STs) of S. suis serotype 2 strains (virulent ST1 and highly virulent ST7). Through the use of isogenic Δlgt, Δlsp and double Δlgt/Δlsp mutants, it was shown that lack of these enzymes did not influence S. suis adhesion/invasion to porcine respiratory epithelial cells. However, in the absence of the Lsp and/or Lgt, a significant reduction in the capacity of S. suis to activate phagocytic cells and induce pro-inflammatory mediators (in vitro and in vivo) was observed. In general, results obtained with the double mutant did not differ in comparison to single mutants, indicating lack of an additive effect. Finally, our data suggest that these enzymes play a differential role in virulence, depending on the genetic background of the strain and being more important for the highly virulent ST7 strain.

Role of the Sortase A in the Release of Cell-Wall Proteinase PrtS in the Growth Medium of Streptococcus thermophilus 4F44

Growth of the lactic acid bacterium Streptococcus thermophilus in milk depends on its capacity to hydrolyze proteins of this medium through its surface proteolytic activity. Thus, strains exhibiting the cell envelope proteinase (CEP) PrtS are able to grow in milk at high cellular density. Due to its LPNTG motif, which is possibly the substrate of the sortase A (SrtA), PrtS is anchored to the cell wall in most S. thermophilus strains. Conversely, a soluble extracellular PrtS activity has been reported in the strain 4F44. It corresponds, in fact, to a certain proportion of PrtS that is not anchored to the cell wall but rather is released in the growth medium. The main difference between PrtS of strain 4F44 (PrtS_4F44) and other PrtS concerns the absence of a 32-residue imperfect duplication in the prodomain of the CEP, postulated as being required for the maturation and correct subsequent anchoring of PrtS. In fact, both mature (without the prodomain at the N-terminal extremity) and immature (with the prodomain) forms are found in the soluble PrtS_4F44 form along with an intact LPNTG at their C-terminal extremity. Investigations we present in this work show that (i) the imperfect duplication is not implied in PrtS maturation; (ii) the maturase PrtM is irrelevant in PrtS maturation which is probably automaturated; and (iii) SrtA allows for the PrtS anchoring in S. thermophilus but the SrtA of strain 4F44 (SrtA_4F44) displays an altered activity.

TroR Negatively Regulates the TroABCD System and Is Required for Resistance to Metal Toxicity and Virulence in Streptococcus suis

Streptococcus suis is an emerging zoonotic pathogen that causes severe swine and human infections. Metals are essential nutrients for life; however, excess metals are toxic to bacteria. Therefore, maintenance of intracellular metal homeostasis is important for bacterial survival. Here, we characterize a DtxR family metalloregulator, TroR, in S. suis. TroR is located upstream of the troABCD operon, whose expression was found to be significantly downregulated in response to excess manganese (Mn). Deletion of troR resulted in reduced growth when S. suis was cultured in metal-replete medium supplemented with elevated concentrations of zinc (Zn), copper (Cu), or cobalt (Co). Mn supplementation could alleviate the growth defects of the ΔtroR mutant under Zn and Co excess conditions; however, it impaired the growth of the wild-type (WT) and complemented (CΔtroR) strains under Cu excess conditions. The growth of ΔtroR was also inhibited in metal-depleted medium supplemented with elevated concentrations of Mn. Moreover, the ΔtroR mutant accumulated increased levels of intracellular Mn and Co, rather than Zn and Cu. Deletion of troR in S. suis led to significant upregulation of the troABCD operon. Furthermore, troA expression in the WT strain was induced by ferrous iron [Fe(II)] and Co and repressed by Mn and Cu; the repression of troA was mediated by TroR. Finally, TroR is required for S. suis virulence in an intranasal mouse model. Together, these data suggest that TroR is a negative regulator of the TroABCD system and contributes to resistance to metal toxicity and virulence in S. suis. IMPORTANCE Metals are essential nutrients for life; however, the accumulation of excess metals in cells can be toxic to bacteria. In the present study, we identified a metalloregulator, TroR, in Streptococcus suis, which is an emerging zoonotic pathogen. In contrast to the observations in other species that TroR homologs usually contribute to the maintenance of homeostasis of one or two metals, we demonstrated that TroR is required for resistance to the toxicity conferred by multiple metals in S. suis. We also found that deletion of troR resulted in significant upregulation of the troABCD operon, which has been demonstrated to be involved in manganese acquisition in S. suis. Moreover, we demonstrated that TroR is required for the virulence of S. suis in an intranasal mouse model. Collectively, these results suggest that TroR is a negative regulator of the TroABCD system and contributes to resistance to metal toxicity and virulence in S. suis.

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

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

Transcriptional regulator XtgS is involved in iron transition and attenuates the virulence of Streptococcus agalactiae

Streptococcus agalactiae (GBS) is an important pathogen that has increasingly received attention for its role in invasive infections and its broad host range. Research on the regulation of gene expression could illuminate GBS pathogenesis. We previously identified a novel transcriptional regulator XtgS, which is a negative regulator of GBS pathogenicity. Here, we demonstrate that XtgS overexpression significantly attenuated GBS virulence in zebrafish infection tests, and XtgS indirectly downregulated the transcription of two iron transport systems based on the results of transcriptomic analysis, electrophoretic mobility shift assays (EMSAs) and lacZ fusion assays. Subsequent studies verified that the inactivation of iron transport system 1 resulted in GBS excessive iron accumulation and attenuated virulence. Thus, we infer that the downregulation of iron transport system 1 caused by XtgS overexpression probably attenuates bacterial virulence, which partially clarifies the mechanism by which XtgS alleviates the pathogenesis. These findings provide new insights into the relationship between exogenous transcriptional regulation and bacterial virulence.

A Markerless Gene Deletion System in Streptococcus suis by Using the Copper-Inducible Vibrio parahaemolyticus YoeB Toxin as a Counterselectable Marker

Streptococcus suis is an important zoonotic pathogen causing severe infections in swine and humans. Induction of the Vibrio parahaemolyticus YoeB toxin in Escherichia coli resulted in cell death, leading to the speculation that YoeBVp can be a counterselectable marker. Herein, the counterselection potential of YoeBVp was assessed in S. suis. The yoeBVp gene was placed under the copper-induced promoter PcopA. The PcopA-yoeBVp construct was cloned into the S. suis-E. coli shuttle vector pSET2 and introduced into S. suis to assess the effect of YoeBVp expression on S. suis growth. Reverse transcription quantitative PCR showed that copper induced yoeBVp expression. Growth curve analyses and spot dilution assays showed that YoeBVp expression inhibited S. suis growth both in liquid media and on agar plates, revealing that YoeBVp has the potential to be a counterselectable marker for S. suis. A SCIY cassette comprising the spectinomycin-resistance gene and copper-induced yoeBVp was constructed. Using the SCIY cassette and peptide-induced competence, a novel two-step markerless gene deletion method was established for S. suis. Moreover, using the ΔperR mutant generated by this method, we demonstrated that PmtA, a ferrous iron and cobalt efflux pump in S. suis, was negatively regulated by the PerR regulator.

The phosphorylation of phosphoglucosamine mutase GlmM by Ser/Thr kinase STK mediates cell wall synthesis and virulence in Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen that causes serious economic losses in the pig industry. Phosphorylation is an important mechanism of protein modification. Recent studies have reported that the serine/threonine kinase (STK) gene contributes to the growth and virulence of SS2. However, the mechanism underlying the regulatory functions of STK in SS2 has not been thoroughly elucidated to date. In this study, phosphoproteomic analysis was performed to determine substrates of the STK protein. Twenty-two proteins with different cell functions were identified as potential substrates of STK. Phosphoglucosamine mutase (GlmM) was selected for further investigation among them. In vitro phosphorylation assay and immunoprecipitation assay indicated that GlmM was phosphorylated by STK at the Ser-101 site and the phosphorylation level of GlmM can be affected. We observed that compared to the wild-type strain ZY05719, the glmM-deficient strain (ΔglmM) and the glmM S101A point mutation strain (CΔglmM S101A) showed aberrant cell morphology and attenuated virulence, including enlarged cell volume, absent capsule, decreased resistance, lower survival caused by unusual peptidoglycan synthesis, and significantly attenuated pathogenicity in a mouse infection model. Additionally, compared to ZY05719 and CΔglmM, GlmM enzyme acivities and peptidoglycan concentrations of the stk-deficient strain (Δstk), CΔglmM S101A decreased significantly. These experiments revealed that STK phosphorylates GlmM at the Ser-101 site to impact GlmM enzyme activity and control cell wall peptidoglycan synthesis to affect SS2 pathogenicity.

Screening for phagocytosis resistance-related genes via a transposon mutant library of Streptococcus suis serotype 2

Streptococcus suis

serotype 2 (SS2) is a serious zoonotic pathogen which causes symptoms of streptococcal toxic shock syndrome (STSS) and septicemia; these symptoms suggest that SS2 may have evade innate immunity. Phagocytosis is an important innate immunity process where phagocytosed pathogens are killed by lysosome enzymes, reactive oxygen, and nitrogen species, and acidic environments in macrophages following engulfment. A previously constructed mutant SS2 library was screened, revealing 13 mutant strains with decreased phagocytic resistance. Through inverse PCR, the transposon insertion sites were determined. Through bioinformatic analysis, the 13 disrupted genes were identified as Cps2F, 3 genes belonging to ABC transporters, WalR, TehB, rpiA, S-transferase encoding gene, prs, HsdM, GNAT family N-acetyltransferase encoding gene, proB, and upstream region of DnaK. Except for the capsular polysaccharide biosynthesis associated Cps2F, the other genes had not been linked to a role in anti-phagocytosis. The survival ability in macrophages and whole blood of randomly picked mutant strains were significantly impaired compared with wild-type ZY05719. The virulence of the mutant strains was also attenuated in a mouse infection model. In the WalR mutant, the transcription of HP1065 decreased significantly compared with wild-type strain, indicating WalR might regulated HP1065 expression and contribute to the anti-phagocytosis of SS2. In conclusion, we identified 13 genes that influenced the phagocytosis resistant ability of SS2, and many of these genes have not been reported to be associated with resistance to phagocytosis. Our work provides novel insight into resistance to phagocytosis, and furthers our understanding of the pathogenesis mechanism of SS2.

Capsular polysaccharide switching in Streptococcus suis modulates host cell interactions and virulence

The capsular polysaccharide (CPS) of Streptococcus suis defines various serotypes based on its composition and structure. Though serotype switching has been suggested to occur between S. suis strains, its impact on pathogenicity and virulence remains unknown. Herein, we experimentally generated S. suis serotype-switched mutants from a serotype 2 strain that express the serotype 3, 4, 7, 8, 9, or 14 CPS. The effects of serotype switching were then investigated with regards to classical properties conferred by presence of the serotype 2 CPS, including adhesion to/invasion of epithelial cells, resistance to phagocytosis by macrophages, killing by whole blood, dendritic cell-derived pro-inflammatory mediator production and virulence using mouse and porcine infection models. Results demonstrated that these properties on host cell interactions were differentially modulated depending on the switched serotypes, although some different mutations other than loci of CPS-related genes were found in each the serotype-switched mutant. Among the serotype-switched mutants, the mutant expressing the serotype 8 CPS was hyper-virulent, whereas mutants expressing the serotype 3 or 4 CPSs had reduced virulence. By contrast, switching to serotype 7, 9, or 14 CPSs had little to no effect. These findings suggest that serotype switching can drastically alter S. suis virulence and host cell interactions.

The otc gene of Streptococcus suis plays an important role in biofilm formation, adhesion, and virulence in a murine model

Streptococcus suis (S. suis) is an emerging zoonotic pathogen that can cause meningitis, arthritis, pneumonia, and sepsis. It poses a serious threat to the swine industry and public health worldwide. Ornithine carbamoyltransferase (OTC) is involved in the arginine deiminase system. OTC, which is a widely distributed enzyme in microorganisms, mammals, and higher plants, catalyzes the conversion of ornithine to citrulline. The present study showed that the otc gene plays an important role in the pathogenesis of S. suis infections. The ability of an otc-deficient mutant (Δotc) to form a biofilm was significantly reduced compared to the wild-type (WT) strain, as determined by crystal violet staining. Confocal laser scanning microscopy and scanning electron microscopy observations showed that the weakening of biofilm formation by the Δotc strain is related to a decrease in the extracellular matrix. In addition, compared to the WT strain, the Δotc strain had a reduced capacity to adhere to human laryngeal epidermoid carcinoma (HEp-2) cells compared to the WT strain. A real-time PCR analysis showed that the expression of adhesion-related genes by the Δotc strain was also lower than that of the WT strain. The virulence of the Δotc strain was significantly lower than that of the WT strain in a murine infection model. In addition, a histological analysis showed that the pathogenicity of the Δotc strain was lower than that of the WT strain, causing only slight inflammatory lesions in lung, liver, spleen, and kidney tissues. No significant differences were observed between the complemented mutant (CΔotc) and WT strains with respect to biofilm formation, adhesion, gene expression, and virulence. The present study provided evidence that the otc gene plays a pivotal role in the regulation of S. suis adhesion and biofilm formation. It also suggested that the otc gene is indirectly involved in the pathogenesis of S. suis serotype 2 infections.

The Novel Streptococcal Transcriptional Regulator XtgS Negatively Regulates Bacterial Virulence and Directly Represses PseP Transcription

Streptococcus agalactiae (group B streptococcus [GBS]) has received continuous attention for its involvement in invasive infections and its broad host range. Transcriptional regulators have an important impact on bacterial adaptation to various environments. Research on transcriptional regulators will shed new light on GBS pathogenesis. In this study, we identified a novel XRE-family transcriptional regulator encoded on the GBS genome, designated XtgS. Our data demonstrate that XtgS inactivation significantly increases bacterial survival in host blood and animal challenge test, suggesting that it is a negative regulator of GBS pathogenicity. Further transcriptomic analysis and quantitative reverse transcription-PCR (qRT-PCR) mainly indicated that XtgS significantly repressed transcription of its upstream gene pseP Based on electrophoretic mobility shift and lacZ fusion assays, we found that an XtgS homodimer directly binds a palindromic sequence in the pseP promoter region. Meanwhile, the PseP and XtgS combination naturally coexists in diverse Streptococcus genomes and has a strong association with sequence type, serotype diversification and host adaptation of GBS. Therefore, this study reveals that XtgS functions as a transcriptional regulator that negatively affects GBS virulence and directly represses PseP expression, and it provides new insights into the relationships between transcriptional regulator and genome evolution.

Inactivation of the htpsA gene affects capsule development and pathogenicity of Streptococcus suis

Streptococcus suis

serotype 2 (S. suis 2) is an important swine pathogen and also an emerging zoonotic agent. HtpsA has been reported as an immunogenic cell surface protein on the bacterium. In the present study, we constructed an isogenic mutant strain of htpsA, namely ΔhtpsA, to study its role in the development and virulence of S. suis 2. Our results showed that the mutant strain lost its typical encapsulated structure with decreased concentrations of sialic acid. Furthermore, the survival rate in whole blood, the anti-phagocytosis by RAW264.7 murine macrophage, and the adherence ability to HEp-2 cells were all significantly affected in the ΔhtpsA. In addition, the deletion of htpsA sharply attenuated the virulence of S. suis 2 in an infection model of mouse. RNA-seq analysis revealed that 126 genes were differentially expressed between the ΔhtpsA and the wild-type strains, including 28 upregulated and 98 downregulated genes. Among the downregulated genes, many were involved in carbohydrate metabolism and synthesis of virulence-associated factors. Taken together, htpsA was demonstrated to play a role in the morphological development and pathogenesis of the highly virulent S. suis 2 05ZYH33 strain.

M-like protein SrM is not crucial to the virulence of a novel isolate of Streptococcus equi subsp. ruminatorum from Macaca mulatta

In this study, a Streptococcus strainnamed FJ1804, was isolated from a blood sample collected from a dead Macaca mulatta in China and, was subsequently classified as Streptococcus equi subsp. ruminatorum (S.e. ruminatorum) through 16S rRNA gene sequence analysis. After whole genome sequencing and analysis, an M-like protein encoding gene that encodes an SrM protein that is homologous to the crucial S.e. zooepidemicus crucial virulence factor SzP, was identified in the genome of FJ1804. To determinethe function of SrM in this bacterium, a strain deleted of srm as well as a complement strain were constructed. The results of in vitro cell adherence, invasion and phagocytosis assays and in vivo animal challenge and histopathology showed that the anti-phagocytosis was decreased and the adherence rate was increased in the srm deletion strain, whereas the invasion rate, pathological features and LD₅₀ values inboth zebrafish and BALB/c mice model showed no difference compared to that observed for the WT strain. To the best of our knowledge, this is first of an infection caused by S.e. ruminatorum, which is a newly identified zoonotic pathogen, in Macaca mulatta, and our data suggest that, compared with other S.e. zooepidemicus strains, the SzP homologous protein is not crucial to the virulence of this bacterium.

Role of luxS in immune evasion and pathogenicity of piscine Streptococcus agalactiae is not dependent on autoinducer-2

luxS-mediated autoinducer 2 (AI-2)-dependent quorum sensing (QS) has been demonstrated to affect many bacterial phenotypes, including virulence. Streptococcus agalactiae harbors a functional luxS gene required for the biosynthesis of AI-2. In this study, we investigated the regulation effect and mechanism of the luxS/AI-2 QS system in the pathogenicity of the piscine S. agalactiae strain GD201008-001. We found that inactivation of luxS caused a marked decrease in biofilm formation, hemolytic activity, antiphagocytosis and intracellular survival of S. agalactiae. Except for hemolytic activity, the altered phenotypes due to the luxS deletion were AI-2-independent. Further investigation indicated that high levels of the proinflammatory cytokines IL-1β and IL-6 could be induced in macrophages co-incubated with the luxS deletion mutant and synthetic AI-2, single or combined. Also, the results of tilapia infection showed that inactivation of luxS significantly decreased the virulence of S. agalactiae but upregulated the expression of cytokines in spleens and brains. Increased proinflammatory effects of the luxS mutant were restored in the luxS complemented strain but could not be restored by AI-2 addition. All the findings suggest that luxS is involved in virulence-associated phenotypes and immunological evasion of S. agalactiae, and furthermore, this involvement is mostly AI-2-independent. This study will provide valuable insights into our understanding of the role of the LuxS/AI-2 QS system in the pathogenesis of S. agalactiae.

PmtA functions as a ferrous iron and cobalt efflux pump in Streptococcus suis

Transition metals are nutrients essential for life. However, an excess of metals can be toxic to cells, and host-imposed metal toxicity is an important mechanism for controlling bacterial infection. Accordingly, bacteria have evolved metal efflux systems to maintain metal homeostasis. Here, we established that PmtA functions as a ferrous iron [Fe(II)] and cobalt [Co(II)] efflux pump in Streptococcus suis, an emerging zoonotic pathogen responsible for severe infections in both humans and pigs. pmtA expression is induced by Fe(II), Co(II), and nickel [Ni(II)], whereas PmtA protects S. suis against Fe(II) and ferric iron [Fe(III)]-induced bactericidal effect, as well as Co(II) and zinc [Zn(II)]-induced bacteriostatic effect. In the presence of elevated concentrations of Fe(II) and Co(II), ΔpmtA accumulates high levels of intracellular iron and cobalt, respectively. ΔpmtA is also more sensitive to streptonigrin, a Fe(II)-activated antibiotic. Furthermore, growth defects of ΔpmtA under Fe(II) or Co(II) excess conditions can be alleviated by manganese [Mn(II)] supplementation. Finally, PmtA plays a role in tolerance to H₂O₂-induced oxidative stress, yet is not involved in the virulence of S. suis in mice. Together, these data demonstrate that S. suis PmtA acts as a Fe(II) and Co(II) efflux pump, and contributes to oxidative stress resistance.

The cysteine protease ApdS from Streptococcus suis promotes evasion of innate immune defenses by cleaving the antimicrobial peptide cathelicidin LL-37

Streptococcus suis is a globally distributed zoonotic pathogen associated with meningitis and septicemia in humans, posing a serious threat to public health. To successfully invade and disseminate within its host, this bacterium must overcome the innate immune system. The antimicrobial peptide LL-37 impedes invading pathogens by directly perforating bacterial membranes and stimulating the immune function of neutrophils, which are the major effector cells against S. suis However, little is known about the biological relationship between S. suis and LL-37 and how this bacterium adapts to and evades LL-37-mediated immune responses. In this study by using an array of approaches, including enzyme, chemotaxis, cytokine assays, quantitative RT-PCR, and CD spectroscopy, we found that the cysteine protease ApdS from S. suis cleaves LL-37 and thereby plays a key role in the interaction between S. suis and human neutrophils. S. suis infection stimulated LL-37 production in human neutrophils, and S. suis exposure to LL-37 up-regulated ApdS protease expression in the bacterium. We observed that ApdS targets and rapidly cleaves LL-37, impairing its bactericidal activity against S. suis We attributed this effect to the decreased helical content of the secondary structure in the truncated peptide. Moreover, ApdS rescued S. suis from killing by human neutrophils and neutrophil extracellular traps because LL-37 truncation attenuated neutrophil chemotaxis and inhibited the formation of extracellular traps and the production of reactive oxygen species. Altogether, our findings reveal an immunosuppressive strategy of S. suis whereby the bacterium blunts the innate host defenses via ApdS protease-mediated LL-37 cleavage.

A Conserved Streptococcal Virulence Regulator Controls the Expression of a Distinct Class of M-Like Proteins

Streptococcus equi subspecies zooepidemicus (SEZ) are group C streptococci that are important pathogens of economically valuable animals such as horses and pigs. Here, we found that many SEZ isolates bind to a monoclonal antibody that recognizes poly-N-acetylglucosamine (PNAG), a polymer that is found as a surface capsule-like structure on diverse microbes. A fluorescence-activated cell sorting-based transposon insertion sequencing (Tn-seq) screen, coupled with whole-genome sequencing, was used to search for genes for PNAG biosynthesis. Surprisingly, mutations in a gene encoding an M-like protein, szM, and the adjacent transcription factor, designated sezV, rendered strains PNAG negative. SezV was required for szM expression and transcriptome analysis showed that SezV has a small regulon. SEZ strains with inactivating mutations in either sezV or szM were highly attenuated in a mouse model of infection. Comparative genomic analyses revealed that linked sezV and szM homologues are present in all SEZ, S. equi subspecies equi (SEE), and M18 group A streptococcal (GAS) genomes in the database, but not in other streptococci. The antibody to PNAG bound to a wide range of SEZ, SEE, and M18 GAS strains. Immunochemical studies suggest that the SzM protein may be decorated with a PNAG-like oligosaccharide although an intact oligosaccharide substituent could not be isolated. Collectively, our findings suggest that the szM and sezV loci define a subtype of virulent streptococci and that an antibody to PNAG may have therapeutic applications in animal and human diseases caused by streptococci bearing SzM-like proteins.IMPORTANCE M proteins are surface-anchored virulence factors in group A streptococci, human pathogens. Here, we identified an M-like protein, SzM, and its positive regulator, SezV, in Streptococcus equi subspecies zooepidemicus (SEZ), an important group of pathogens for domesticated animals, including horses and pigs. SzM and SezV homologues were found in the genomes of all SEZ and S. equi subspecies equi and M18 group A streptococcal strains analyzed but not in other streptococci. Mutant SEZ strains lacking either sezV or szM were highly attenuated in a mouse model of infection. Collectively, our findings suggest that SezV-related regulators and the linked SzM family of M-like proteins define a new subset of virulent streptococci.

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.

Interleukin-1 signaling induced by Streptococcus suis serotype 2 is strain-dependent and contributes to bacterial clearance and inflammation during systemic disease in a mouse model of infection

Streptococcus suis serotype 2 is an important porcine pathogen and zoonotic agent causing sudden death, septic shock and meningitis, with exacerbated inflammation being a hallmark of the infection. A rapid, effective and balanced innate immune response against S. suis is critical to control bacterial growth without causing excessive inflammation. Even though interleukin (IL)-1 is one of the most potent and earliest pro-inflammatory mediators produced, its role in the S. suis pathogenesis has not been studied. We demonstrated that a classical virulent European sequence type (ST) 1 strain and the highly virulent ST7 strain induce important levels of IL-1 in systemic organs. Moreover, bone marrow-derived dendritic cells and macrophages contribute to its production, with the ST7 strain inducing higher levels. To better understand the underlying mechanisms involved, different cellular pathways were studied. Independently of the strain, IL-1β production required MyD88 and involved recognition via TLR2 and possibly TLR7 and TLR9. This suggests that the recognized bacterial components are similar and conserved between strains. However, very high levels of the pore-forming toxin suilysin, produced only by the ST7 strain, are required for efficient maturation of pro-IL-1β via activation of different inflammasomes resulting from pore formation and ion efflux. Using IL-1R^−/− mice, we demonstrated that IL-1 signaling plays a beneficial role during S. suis systemic infection by modulating the inflammation required to control and clear bacterial burden, thus promoting host survival. Beyond a certain threshold, however, S. suis-induced inflammation cannot be counterbalanced by this signaling, making it difficult to discriminate its role.

Identification of novel genes associated with anti-phagocytic functions in Streptococcus equi subsp. zooepidemicus

The anti-phagocytic abilities of bacteria often affect bacterial pathogenicity. Here, random mutant library of Streptococcus equi subsp. zooepidemicus (SEZ) was constructed using transposon mutagenesis. After careful screening, 30 transposon mutants with different transposon insertion sites were identified by conducting quantitative phagocytosis and insertion-site confirmation assays, whose anti-phagocytic abilities were significantly reduced relative to the wild-type strain. Insertion sites of 19 strains were monocistronic, including genes coding membrane proteins, transporters, and enzymes with unknown pathological function, such as sadM, adhP, purD, guaA, alpha-galactosidase coding gene, ABC transporter permease coding gene, metallo-beta-lactamase coding gene, and three secreted enzyme coding genes spuZ, slaB, and endoS, as well as known virulence factor coding genes, such as hasA and szM. The insertion sites of another 11 strains were polycistronic. We focused on four monocistronic-mutant strains: MhtpZ, MspuZ, MslaB, and MendoS. The anti-phagocytic abilities of not only the mutants that were precoincubated with the recombinant proteins, but also the complement strains were significantly more pronounced than those of all four corresponding mutants. The polyclonal antiserum against SlaB or EndoS also significantly decreased the anti-phagocytic capacity of wild-type SEZ. All four mutants exhibited significantly decreased viability in whole blood and reduced lethality in mice relative to the wild-type strain. Thus, we identified a variety of new anti-phagocytic factors, particularly multiple SEZ secreted enzymes. These factors are instrumental in the phagocytic resistance of SEZ in the absence of opsonin. Our results provide a framework for further studies of SEZ pathogenesis and relevant vaccine development for novel potential targets.

Role of ClpX and ClpP in Streptococcus suis serotype 2 stress tolerance and virulence

Streptococcus suis has received increasing attention for its involvement in severe infections in pigs and humans; however, their pathogenesis remains unclear. ClpX and ClpP, two subunits of the ATP-dependent caseinolytic protease Clp, play key roles in bacterial adaptation to various environmental stresses. In this study, a virulent S. suis serotype 2 strain, ZY05719, was employed to construct clpX and clpP deletion mutants (ΔclpX and ΔclpP, respectively) and their complementation strains. Both ΔclpX and ΔclpP displayed significantly reduced adaptability compared with the wild-type strain, evident through several altered phenotypes: formation of long cell chains, tendency to aggregate in culture, and reduced growth under acidic pH and H₂O₂-induced oxidative stress. ClpP and ClpX were required for the optimal growth during heat and cold stress, respectively. An in vitro experiment on RAW264.7 macrophage cells showed significantly increased sensitivity of ΔclpX and ΔclpP to phagocytosis compared with the wild-type strain. Mouse infection assays verified the deletion of clpX and clpP led to not only fewer clinical symptoms and lower mortality but also to a marked attenuation in bacterial colonization. These virulence-related phenotypes were restored by genetic complementation. Furthermore, the deletion of clpX or clpP caused a significant decrease in the expression of sodA, tpx, and apuA compared with the wild-type strain, suggesting that these genes may be regulated by ClpX and ClpP as downstream response factors to facilitate the bacterial tolerance against various environmental stresses. Taken together, these results suggest that ClpX and ClpP play important roles in stress tolerance for achieving the full virulence of S. suis serotype 2 during infection.

Utilization of the ComRS system for the rapid markerless deletion of chromosomal genes in Streptococcus suis

Aim: To develop a markerless gene deletion strategy in Streptococcus suis to solve the problem that several serotypes against electrotransformation of foreign DNA. Materials & methods: Bioinformatics retrieval was performed to identified ComRS systems functioning for natural transformation. A sacB-spc cassette with the upper and lower homologous fragments was amplification by fusion-PCR for spectinomycin-positive and sucrose-negative selection during gene deletion. Results & conclusion: Three phylogenetic clusters of ComR were identified to function for natural transformation by specific recognition to competence pheromone in S. suis. Thus, they were employed to establish gene deletion method. Its efficiency for genetic replacement was dependent on the length of homologs fragment and the concentration of donor DNA. This rapid gene-editing technique may greatly facilitate molecular studies on S. suis.

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.

Virulence regulation of cel-EIIB protein mediated PTS system in Streptococcus agalactiae in Nile tilapia

Streptococcus agalactiae is a major pathogen of tilapia causing significant economic losses for the global aquatic industry yearly. To elucidate the role of cel-EIIB protein-mediated phosphotransferase systems (PTS) in the virulence regulation of S. agalactiae, cel-EIIB gene deletion in a virulent strain THN0901 was achieved by homologous recombination. The cellobiose utilization of △cel-EIIB strain was significantly decreased relative to S.a.THN0901 strain incubating in LB with 10 mg/ml cellobiose (p < 0.05). The biofilm formation ability of △cel-EIIB strain was also significantly decreased when cultured in BHI medium (p < 0.05). Under a lower infection dose, the accumulative mortality of tilapia caused by △cel-EIIB strain was dramatically decreased (20%), of which S.a.THN0901 strain and △cel-EIIB::i strain were 53.33% and 50%, respectively. The competition experience using tilapia model indicated the invasion and colonization ability of △cel-EIIB strain was significantly weaker than that of S.a.THN0901 strain (p < 0.05). Compared to △cel-EIIB::i strain, the mRNA expression of csrS, csrR, rgfA, rgfC, bgrR and bgrS was significantly downregulated in △cel-EIIB strain (p < 0.05). In conclusion, cel-EIIB protein-mediated cel-PTS not only contributes to biofilm formation and virulence regulation, but also plays an important role in the invasion and colonization of S. agalactiae.

The Streptococcos suis sortases SrtB and SrtF are essential for disease in pigs

The porcine pathogen Streptococcus suis colonizes the upper respiratory tracts of pigs, potentially causing septicaemia, meningitis and death, thus placing a severe burden on the agricultural industry worldwide. It is also a zoonotic pathogen that is known to cause systemic infections and meningitis in humans. Understanding how S. suis colonizes and interacts with its hosts is relevant for future strategies of drug and vaccine development. As with other Gram-positive bacteria, S. suis utilizes enzymes known as sortases to attach specific proteins bearing cell wall sorting signals to its surface, where they can play a role in host-pathogen interactions. The surface proteins of bacteria are often important in adhesion to and invasion of host cells. In this study, markerless in-frame deletion mutants of the housekeeping sortase srtA and the two pilus-associated sortases, srtB and srtF, were generated and their importance in S. suis infections was investigated. We found that all three of these sortases are essential to disease in pigs, concluding that their cognate-sorted proteins may also be useful in protecting pigs against infection.

Interaction of Group B Streptococcus sialylated capsular polysaccharides with host Siglec-like molecules dampens the inflammatory response in tilapia

Group B Streptococcus (GBS, S. agalactiae) infection in tilapia (Oreochromis niloticus) causes widespread death of this species and is a significant issue for the aquaculture industry. The major virulence factor for GBS is its sialylated capsular polysaccharides (CPs). These CPs interact with sialic acid-binding immunoglobulin-like lectins (Siglecs) on the host immune cells to regulate the downstream inflammatory response and evade detection. Previously, we cloned multiple Siglec-like molecules from an O. niloticus cDNA library, all of which were shown to interact with the sialylated CPs of GBS. In the present study, we investigated the effects of GBS infection on the expression of pro- and anti-inflammatory cytokines in O. niloticus as well as OnSiglec-like-transfected macrophage cells. Eukaryotic expression vectors containing full-length OnSiglec-1-like, -4b-like, -14-like were constructed and used to transfect RAW264 macrophages in vitro as well as live tilapia in vivo prior to GBS infection. The expression of the anti-inflammatory cytokine interleukin (IL)-10 and the pro-inflammatory cytokines tumor necrosis factor (TNF)-α, IL-6, and interferon (INF)-β were then analyzed by qPCR. Our results indicate that as infection progressed, IL-10 expression was significantly upregulated, while that of TNF-α and IL-6 were significantly downregulated in the OnSiglec-like-transfected cells. INF-β expression was also downregulated in cells transfected with OnSiglec-1-like and -4b-like, but was not significantly effected in OnSiglec-14-like-transfected cells. Notably, the magnitude of these cytokine expression changes was greatly decreased when a ΔneuA GBS mutant was used to infect the OnSiglec-1-like-transfected cells. In GBS-infected tilapia, IL-10 expression was significantly upregulated in all tissues, whereas INF-β expression in the spleen, kidney, and gills was significantly downregulated at 12 hpi. While the expression of TNF-α was slightly upregulated, this change was not significant. In GBS ΔneuA mutant-infected O. niloticus, IL-10 expression in all of the tissues was significantly lower than that observed for the wild-type GBS group, while TNF-α expression was higher in the mutant infected group. There was no significant difference in INF-β expression between the two groups. Taken together, sialylated CPs on GBS appear to interact with host OnSiglec-like molecules to transmit negative regulatory signals via enhanced anti-inflammatory cytokine IL-10 production and reduced pro-inflammatory cytokine production, ultimately leading to dampening of the host immune response. The results of this study further elucidate the molecular mechanism underlying GBS infection in tilapia and also provide candidate drug target molecules.

The Redox-Sensing Regulator Rex Contributes to the Virulence and Oxidative Stress Response of Streptococcus suis Serotype 2

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen responsible for septicemia and meningitis. The redox-sensing regulator Rex has been reported to play critical roles in the metabolism regulation, oxidative stress response, and virulence of various pathogens. In this study, we identified and characterized a Rex ortholog in the SS2 virulent strain SS2-1 that is involved in bacterial pathogenicity and stress environment susceptibility. Our data show that the Rex-knockout mutant strain Δrex exhibited impaired growth in medium with hydrogen peroxide or a low pH compared with the wildtype strain SS2-1 and the complementary strain CΔrex. In addition, Δrex showed a decreased level of survival in whole blood and in RAW264.7 macrophages. Further analyses revealed that Rex deficiency significantly attenuated bacterial virulence in an animal model. A comparative proteome analysis found that the expression levels of several proteins involved in virulence and oxidative stress were significantly different in Δrex compared with SS2-1. Electrophoretic mobility shift assays revealed that recombinant Rex specifically bound to the promoters of target genes in a manner that was modulated by NADH and NAD⁺. Taken together, our data suggest that Rex plays critical roles in the virulence and oxidative stress response of SS2.

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.