Thermosensitive suicide vectors for gene replacement in Streptococcus suis

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.

Thioredoxin C of Streptococcus suis serotype 2 contributes to virulence by inducing antioxidative stress and inhibiting autophagy via the MSR1/PI3K-Akt-mTOR pathway in macrophages

The thioredoxin (Trx) system plays a vital role in protecting against oxidative stress and ensures correct disulfide bonding to maintain protein function. Our previous research demonstrated that TrxA of Streptococcus suis Serotype 2 (SS2), a clinical strain from the lung of a diseased pig, contributes to virulence but is not involved in antioxidative stress. In this study, we identified another gene in the Trx family, TrxC, which encodes a protein of 104 amino acids with a CGDC active motif and 22.4 % amino acid sequence homology with TrxA. Unlike the TrxA, TrxC mutant strains were more susceptible to oxidative stresses induced by hydrogen peroxide and paraquat. In vitro experiments, the survival rate of the TrxC deletion mutant in RAW264.7 macrophages was only one-eighth of that of TrxA mutant strains. Transcriptome analysis revealed that autophagy-related genes were significantly upregulated in the TrxC mutant compared to those in the wild-type or TrxA mutant strains. Co-localization of LC3 puncta with TrxC was confirmed using laser confocal microscopy, and autophagy-related indicators were quantified using western blotting. Autophagy deficiency induced by ATG5 knockout significantly increased SS2 survival rate, especially in TrxC mutant strains. For the upstream signal regulation pathways, we found ΔTrxC strains regulate autophagy by activation of PI3K/Akt/mTOR signaling in RAW264.7 macrophages. In the Akt1-overexpressing cell line, ΔTrxC infection significantly decreased the autophagic response and promoted ΔTrxC mutant strain survival, while inhibition of Akt with MK2206 resulted in reduced ΔTrxC mutant strain survival and enhance the autophagic response. Furthermore, loss of TrxC increased the activity of MSR1, thereby inducing cellular autophagy and phagocytosis. Our data demonstrate that TrxC of SS2 contributes to virulence by inducing antioxidative stress and inhibits autophagy via the PI3K-Akt-mTOR pathway in macrophages, with MSR1 acting as a key factor in controlling infection.

Genetic modification of Streptococcus dysgalactiae by natural transformation

Streptococcus dysgalactiae is an emerging human and animal pathogen. Functional studies of genes involved in virulence of S. dysgalactiae and other pyogenic group streptococci are often hampered by limited genetic tractability. It is known that pyogenic streptococci carry genes required for competence for natural transformation; however, in contrast to other streptococcal subgroups, there is limited evidence for gene transfer by natural transformation in these bacteria. In this study, we systematically assessed the genomes of 179 S. dysgalactiae strains of both human and animal origins (subsp. equisimilis and dysgalactiae, respectively) for the presence of genes required for natural transformation. While a considerable fraction of the strains contained inactive genes, the majority (64.2%) of the strains had an intact gene set. In selected strains, we examined the dynamics of competence activation after addition of competence-inducing pheromones using transcriptional reporter assays and exploratory RNA-seq. Based on these findings, we were able to establish a protocol allowing us to utilize natural transformation to construct deletion mutants by allelic exchange in several S. dysgalactiae strains of both subspecies. As part of the work, we deleted putative lactose utilization genes to study their role in growth on lactose. The data presented here provide new knowledge on the potential of horizonal gene transfer by natural transformation in S. dysgalactiae and, importantly, demonstrates the possibility to exploit natural transformation for genetic engineering in these bacteria.

IMPORTANCE

Numerous Streptococcus spp. exchange genes horizontally through natural transformation, which also facilitates efficient genetic engineering in these organisms. However, for the pyogenic group of streptococci, including the emerging pathogen Streptococcus dysgalactiae, there is limited experimental evidence for natural transformation. In this study, we demonstrate that natural transformation in vitro indeed is possible in S. dysgalactiae strains under optimal conditions. We utilized this method to perform gene deletion through allelic exchange in several strains, thereby paving the way for more efficient gene engineering methods in pyogenic streptococci.

Protease SfpB plays an important role in cell membrane stability and immune system evasion in Streptococcus agalactiae

Bacteria possess the ability to develop diverse and ingenious strategies to outwit the host immune system, and proteases are one of the many weapons employed by bacteria. This study sought to identify S. agalactiae additional serine protease and determine its role in virulence. The S. agalactiae THN0901 genome features one S8 family serine peptidase B (SfpB), acting as a secreted and externally exposed entity. A S8 family serine peptidase mutant strain (ΔsfpB) and complement strain (CΔsfpB) were generated through homologous recombination. Compared to the wild-type strain THN0901, the absorption of EtBr dyes was significantly reduced (P < 0.01) in ΔsfpB, implying an altered cell membrane permeability. In addition, the ΔsfpB strain had a significantly lower survival rate in macrophages (P < 0.01) and a 61.85 % lower adhesion ability to the EPC cells (P < 0.01) compared to THN0901. In the in vivo colonization experiment using tilapia as a model, 210 fish were selected and injected with different bacterial strains at a concentration of 3 × 106 CFU/tail. At 6, 12, 24, 48, 72 and 96 h post-injection, three fish were randomly selected from each group and their brain, liver, spleen, and kidney tissues were isolated. Subsequently, it was demonstrated that the ΔsfpB strain exhibited a markedly diminished capacity for colonization in tilapia. Additionally, the cumulative mortality of ΔsfpB in fish after intraperitoneal injection was reduced by 19.92-23.85 %. In conclusion, the findings in this study have demonstrated that the SfpB plays a significant role in S. agalactiae cell membrane stability and immune evasion. The immune evasion is fundamental for the development and transmission of invasive diseases, the serine protease SfpB may be a promising candidate for the development of antimicrobial agents to reduce the transmission of S. agalactiae.

Endogenous Type I-C CRISPR-Cas system of Streptococcus equi subsp. zooepidemicus promotes biofilm formation and pathogenicity

Streptococcus equi subsp. zooepidemicus (SEZ) is a significant zoonotic pathogen that causes septicemia, meningitis, and mastitis in domestic animals. Recent reports have highlighted high-mortality outbreaks among swine in the United States. Traditionally recognized for its adaptive immune functions, the CRISPR-Cas system has also been implicated in gene regulation, bacterial pathophysiology, virulence, and evolution. The Type I-C CRISPR-Cas system, which is prevalent in SEZ isolates, appears to play a pivotal role in regulating the pathogenicity of SEZ. By constructing a Cas3 mutant strain (ΔCas3) and a CRISPR-deficient strain (ΔCRISPR), we demonstrated that this system significantly promotes biofilm formation and cell adhesion. However, the deficiency in the CRISPR-Cas system did not affect bacterial morphology or capsule production. In vitro studies showed that the CRISPR-Cas system enhances pro-inflammatory responses in RAW264.7 cells. The ΔCas3 and ΔCRISPR mutant strains exhibited reduced mortality rates in mice, accompanied by a decreased bacterial load in specific organs. RNA-seq analysis revealed distinct expression patterns in both mutant strains, with ΔCas3 displaying a broader range of differentially expressed genes, which accounted for over 70% of the differential genes observed in ΔCRISPR. These genes were predominantly linked to lipid metabolism, the ABC transport system, signal transduction, and quorum sensing. These findings enhance our understanding of the complex role of the CRISPR-Cas system in SEZ pathogenesis and provide valuable insights for developing innovative therapeutic strategies to combat infections.

Identification of ClpB, a molecular chaperone involved in the stress tolerance and virulence of Streptococcus agalactiae

Bacterial ClpB is an ATP-dependent disaggregate that belongs to the Hsp100/Clp family and facilitates bacterial survival under hostile environmental conditions. Streptococcus agalactiae, which is regarded as the major bacterial pathogen of farmed Nile tilapia (Oreochromis niloticus), is known to cause high mortality and large economic losses. Here, we report a ClpB homologue of S. agalactiae and explore its functionality. S. agalactiae with a clpB deletion mutant (∆clpB) exhibited defective tolerance against heat and acidic stress, without affecting growth or morphology under optimal conditions. Moreover, the ΔclpB mutant exhibited reduced intracellular survival in RAW264.7 cells, diminished adherence to the brain cells of tilapia, increased sensitivity to leukocytes from the head kidney of tilapia and whole blood killing, and reduced mortality and bacterial loads in a tilapia infection assay. Furthermore, the reduced virulence of the ∆clpB mutant was investigated by transcriptome analysis, which revealed that deletion of clpB altered the expression levels of multiple genes that contribute to the stress response as well as certain metabolic pathways. Collectively, our findings demonstrated that ClpB, a molecular chaperone, plays critical roles in heat and acid stress resistance and virulence in S. agalactiae. This finding provides an enhanced understanding of the functionality of this ClpB homologue in gram-positive bacteria and the survival strategy of S. agalactiae against immune clearance during infection.

Tackling Antibiotic Resistance: Exploring 5-Fluorouracil as a Promising Antimicrobial Strategy for the Treatment of Streptococcus suis Infection

Streptococcus suis (S. suis) is a zoonotic pathogen with a global distribution, which causes serious diseases in both humans and animals and economic losses in the swine industry. As antibiotic resistance increases, there is an urgent imperative to explore novel antibacterial alternatives. In the present study, we selected the anticancer drug 5-fluorouracil (5-FU) approved by the Food and Drug Administration (FDA) as a candidate drug to treat S. suis infections. The results showed that various pathogens, especially S. suis, are more sensitive to 5-FU. Moreover, the cytotoxicity of 5-FU is relatively low. Extensive in vitro assays demonstrated the pronounced bacteriostatic and bactericidal efficacy of 5-FU against susceptible and multidrug-resistant S. suis strains. Its mechanisms of action include damage to the bacterial cell walls and membranes, resulting in the leakage of intracellular components, and the inhibition of thymidylate synthase (TS), leading to a depletion of deoxythymidine triphosphate (dTTP) pools, ultimately causing thymine-less death and lethal DNA damage in bacteria. Gene-knockout experiments further showed that 5-FU played a role by inhibiting the thyA gene-encoding thymidine synthase. Finally, we determined that S. suis infections can be alleviated by 5-FU in the mouse infection model. This study emphasizes the antibacterial potential of 5-FU against S. suis and provides evidence for its targeting of bacterial membrane damage and DNA damage. In summary, 5-FU can control S. suis infection and is expected to become a new alternative to antibiotics.

Lipoteichoic acids influence cell shape and bacterial division of Streptococcus suis serotype 2, but play a limited role in the pathogenesis of the infection

Streptococcus suis serotype 2 is a major swine pathogen and a zoonotic agent, causing meningitis in both swine and humans, responsible for substantial economic losses to the swine industry worldwide. The pathogenesis of infection and the role of bacterial cell wall components in virulence have not been fully elucidated. Lipoproteins, peptidoglycan, as well as lipoteichoic acids (LTA) have all been proposed to contribute to virulence. In the present study, the role of the LTA in the pathogenesis of the infection was evaluated through the characterisation of a mutant of the S. suis serotype 2 strain P1/7 lacking the LtaS enzyme, which mediates the polymerization of the LTA poly-glycerolphosphate chain. The ltaS mutant was confirmed to completely lack LTA and displayed significant morphological defects. Although the bacterial growth of this mutant was not affected, further results showed that LTA is involved in maintaining S. suis bacterial fitness. However, its role in the pathogenesis of the infection appears limited. Indeed, LTA presence reduces self-agglutination, biofilm formation and even dendritic cell activation, which are important aspects of the pathogenesis of the infection caused by S. suis. In addition, it does not seem to play a critical role in virulence using a systemic mouse model of 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.

Characterization of AI-2/LuxS quorum sensing system in biofilm formation, pathogenesis of Streptococcus equi subsp. zooepidemicus

Streptococcus equi subsp. zooepidemicus (SEZ) is an opportunistic pathogen of both humans and animals. Quorum sensing (QS) plays an important role in the regulation of bacterial group behaviors. The aim of this study was to characterize the LuxS in SEZ and evaluate its impact on biofilm formation, pathogenesis and gene expression. The wild-type SEZ and its LuxS mutant (ΔluxS) were examined for growth, biofilm formation, virulence factors, and transcriptomic profiles. Our results showed that LuxS deficiency did not affect SEZ hemolytic activity, adhesion or capsule production. For biofilm assay demonstrated that mutation in the luxS gene significantly enhances biofilm formation, produced a denser biofilm and attached to a glass surface. RAW264.7 cell infection indicated that ΔluxS promoted macrophage apoptosis and pro-inflammatory responses. In mice infection, there was no significant difference in mortality between SEZ and ΔluxS. However, the bacterial load in the spleen of mice infected with ΔluxS was significantly higher than in those infected with SEZ. And the pathological analysis further indicated that spleen damage was more severe in the ΔluxS group. Moreover, transcriptomics analysis revealed significant alterations in carbon metabolism, RNA binding and stress response genes in ΔluxS. In summary, this study provides the first evidence of AI-2/LuxS QS system in SEZ and reveals its regulatory effects on biofilm formation, pathogenicity and gene expression.

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.

Contribution of nadR to the cell growth and virulence of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2) has been reported to be a highly invasive pathogen in swine and a zoonotic agent for humans. Although many bacterial virulence factors have been identified, our an insightful understanding of SS2 pathogenicity is lacking. The gene nadR, encoding nicotinamide-nucleotide adenylyltransferase, was first reported as a regulator and transporter of the intracellular NAD synthesis pathway in Salmonella typhimurium. In this study, we constructed a mutant strain of nadR (ΔnadR) to test the phenotypic and virulence variations between the deletion mutant and the wild-type strain ZY05719. The phenotypic experimental results showed that ΔnadR obviously inhibited cell growth and exhibited shorter chains than WT. The growth defect of ΔnadR was caused by the loss of the function of nadR for transporting the substrates nicotinamide mononucleotide and nicotinamide riboside in the intracellular NAD synthesis pathway. In the process of interaction with the host, ΔnadR participated in adherence and invasion to the host cells, and it was more easily cleared by RAW264.7 macrophages. More importantly, both zebrafish and BALB/c mice in vivo virulence experimental results showed that ΔnadR dramatically attenuated the virulence of SS2, and the ability of ΔnadR to colonize tissues was notably reduced in comparison with that of WT in the BALB/c mice infection model. To the best of our knowledge, this is the first time to demonstrate that nadR not only plays an important role in bacterial growth, but also in connection with the virulence of SS2 as a global transcriptional regulator.

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.

Analysis of FctB3 crystal structure and insight into its structural stabilization and pilin linkage mechanisms

Streptococcus pyogenes harboring an FCT type 3 genomic region display pili composed of three types of pilins. In this study, the structure of the base pilin FctB from a serotype M3 strain (FctB3) was determined at 2.8 Å resolution. In accordance with the previously reported structure of FctB from a serotype T9 strain (FctB9), FctB3 was found to consist of an immunoglobulin-like domain and proline-rich tail region. Data obtained from structure comparison revealed main differences in the omega (Ω) loop structure and the proline-rich tail direction. In the Ω loop structure, a differential hydrogen bond network was observed, while the lysine residue responsible for linkage to growing pili was located at the same position in both structures, which indicated that switching of the hydrogen bond network in the Ω loop without changing the lysine position is advantageous for linkage to the backbone pilin FctA. The difference in direction of the proline-rich tail is potentially caused by a single residue located at the root of the proline-rich tail. Also, the FctB3 structure was found to be stabilized by intramolecular large hydrophobic interactions instead of an isopeptide bond. Comparisons of the FctB3 and FctA structures indicated that the FctA structure is more favorable for linkage to FctA. In addition, the heterodimer formation of FctB with Cpa or FctA was shown to be mediated by the putative chaperone SipA. Together, these findings provide an alternative FctB structure as well as insight into the interactions between pilin proteins.

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.

Group A Streptococcus cation diffusion facilitator proteins contribute to immune evasion by regulating intracellular metal concentrations

Cation diffusion facilitators (CDFs) are a large family of divalent metal transporters with broad specificities that contribute to intracellular metal homeostasis and toxicity in bacterial pathogens. Streptococcus pyogenes (Group A Streptococcus [GAS]) expresses two homologous CDF efflux transporters, MntE and CzcD, which selectively transport Mn and Zn, respectively. We discovered that the MntE- and CzcD-deficient strains exhibited a marked decrease in the viability of macrophage-differentiated THP-1 cells and neutrophils. In addition, the viability of mice infected with both deficient strains markedly increased. Consistent with a previous study, our results suggest that MntE regulates the PerR-dependent oxidative stress response by maintaining intracellular Mn levels and contributing to the growth of GAS. The maturation and proteolytic activity of streptococcal cysteine protease (SpeB), an important virulence factor in GAS, has been reported to be abrogated by zinc and copper. Zn inhibited the maturation and proteolytic activity of SpeB in the culture supernatant of the CzcD-deficient strain. Furthermore, Mn inhibited SpeB maturation and proteolytic activity in a MntE-deficient strain. Since the host pathogenicity of the SpeB-deficient strain was significantly reduced, maintenance of intracellular manganese and zinc levels in the GAS via MntE and CzcD may not only confer metal resistance to the bacterium, but may also play an essential role in its virulence. These findings provide new insights into the molecular mechanisms of pathogenicity, which allow pathogens to survive under stressful conditions associated with elevated metal ion concentrations during host infection.

Impact of mutagenesis and lateral gene transfer processes in bacterial susceptibility to phage in food biocontrol and phage therapy

Introduction

The emergence of resistance and interference mechanisms to phage infection can hinder the success of bacteriophage-based applications, but the significance of these mechanisms in phage therapy has not been determined. This work studies the emergence of Salmonella isolates with reduced susceptibility to a cocktail of three phages under three scenarios: i) Salmonella cultures (LAB), ii) biocontrol of cooked ham slices as a model of food safety (FOOD), and iii) oral phage therapy in broilers (PT).

Methods

S. Typhimurium ATCC 14028 RifR variants with reduced phage susceptibility were isolated from the three scenarios and conventional and molecular microbiology techniques were applied to study them.

Results and discussion

In LAB, 92% of Salmonella isolates lost susceptibility to all three phages 24 h after phage infection. This percentage was lower in FOOD, with 4.3% of isolates not susceptible to at least two of the three phages after seven days at 4°C following phage treatment. In PT, 9.7% and 3.3 % of isolates from untreated and treated broilers, respectively, displayed some mechanism of interference with the life cycle of some of the phages. In LAB and FOOD scenarios, resistant variants carrying mutations in rfc and rfaJ genes involved in lipopolysaccharide synthesis (phage receptor) were identified. However, in PT, the significant decrease of EOP, ECOI, and burst size observed in isolates was prompted by lateral gene transfer of large IncI1 plasmids, which may encode phage defense mechanisms. These data indicate that the acquisition of specific conjugative plasmids has a stronger impact than mutagenesis on the emergence of reduced phage-susceptibility bacteria in certain environments. In spite of this, neither mechanism seems to significantly impair the success of Salmonella biocontrol and oral phage therapy.

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.

Phosphorylation of GntR reduces Streptococcus suis oxidative stress resistance and virulence by inhibiting NADH oxidase transcription

GntR transcription factor of Streptococcus suis serotype 2 (SS2) is a potential substrate protein of STK, but the regulation mechanisms of GntR phosphorylation are still unclear. This study confirmed that STK phosphorylated GntR in vivo, and in vitro phosphorylation experiments showed that STK phosphorylated GntR at Ser-41. The phosphomimetic strain (GntR-S41E) had significantly reduced lethality in mice and reduced bacterial load in the blood, lung, liver, spleen, and brain of infected mice compared to wild-type (WT) SS2. Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) experiments demonstrated that the promoter of nox was bound by GntR. The phosphomimetic protein GntR-S41E cannot bind to the promoter of nox, and the nox transcription levels were significantly reduced in the GntR-S41E mutant compared to WT SS2. The virulence in mice and the ability to resist oxidative stress of the GntR-S41E strain were restored by complementing transcript levels of nox. NOX is an NADH oxidase that catalyzes the oxidation of NADH to NAD+ with the reduction of oxygen to water. We found that NADH is likely accumulated under oxidative stress in the GntR-S41E strain, and higher NADH levels resulted in increased amplified ROS killing. In total, we report GntR phosphorylation could inhibit the transcription of nox, which impaired the ability of SS2 to resist oxidative stress and virulence.

Mutations in troABCD against Copper Overload in a copA Mutant of Streptococcus suis

Streptococcus suis is a major swine pathogen that is increasingly recognized as a porcine zoonotic pathogen that threatens the health of both pigs and humans. Metal homeostasis plays a critical role during the process of bacterial infection. In this study, RNA sequencing was used to identify potential candidate genes involved in the maintenance of intracellular copper homeostasis. CopA was identified as the primary copper exporter in S. suis. The copA deletion mutant strain was found to be more sensitive to copper and accumulated more intracellular copper than the wild-type (WT) parent strain. In addition, adding manganese increased the ability of S. suis to resist copper, and the manganese transporter, TroABCD, was involved in tolerance to copper. The copA deletion mutant strain accumulated less copper when supplemented with manganese. Furthermore, when cultured with copper, the double deletion mutant (ΔcopAΔtroA) exhibited improved growth compared to the copA deletion mutant strain. In addition, the double deletion mutant (ΔcopAΔtroA) accumulated less copper than the copA deletion mutant strain. These data were consistent with a model wherein defective TroABCD resulted in decreased cellular copper accumulation and protected the strain against copper poisoning. IMPORTANCE Metal homeostasis plays a critical role during the process of bacterial infection. We identified three important potential candidate genes involved in maintenance of intracellular copper homeostasis. CopA was demonstrated to be the main copper exporter in Streptococcus suis, and manganese increased the tolerance of S. suis to copper. The double deletion mutant (ΔcopAΔtroA) improved growth ability over the copA deletion mutant strain in the presence of high concentrations of copper and accumulated less copper. These findings are consistent with a model wherein defective TroABCD resulted in decreased cellular accumulation of copper and protected the strain against copper poisoning.

Characterization and virulence of Streptococcus agalactiae deficient in SaeRS of the two-component system

There are a variety of regulatory systems in bacteria, among which the two-component system (TCS) can sense external environmental changes and make a series of physiological and biochemical reactions, which is crucial for the life activities of bacteria. As a member of TCS, SaeRS is considered to be an important virulence factor in Staphylococcus aureus, but its function in tilapia (Oreochromis niloticus)-derived Streptococcus agalactiae remains unknown. To explore the role of SaeRS in regulating virulence in the two-component system (TCS) of S. agalactiae from tilapia, ΔSaeRS mutant strain and CΔSaeRS complementary strain were constructed by homologous recombination. The results showed that the abilities of growth and biofilm formation of ΔSaeRS strain were significantly decreased when cultured in a brain heart infusion (BHI) medium (P < 0.01). Also, the survival rate of the ΔSaeRS strain in blood was decreased when compared with the wild strain S. agalactiae THN0901. Under the higher infection dose, the accumulative mortality of tilapia caused by the ΔSaeRS strain was significantly decreased (23.3%), of which THN0901 and CΔSaeRS strains were 73.3%. The results of competition experiments in tilapia showed that the invasion and colonization abilities of the ΔSaeRS strain were also dramatically lower than those of the wild strain (P < 0.01). Compared with the THN0901, the mRNA expression levels of virulence factors (fbsB, sip, cylE, bca, etc.) in the ΔSaeRS strain were significantly down-regulated (P < 0.01). SaeRS is one of the virulence factors of S. agalactiae. It plays a role in promoting host colonization and achieving immune evasion during the infection of tilapia, which provides a basis for exploring the pathogenic mechanism of S. agalactiae infected with tilapia.

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.

The aminotransferase Aat initiates 3-phenyllactic acid biosynthesis in Pediococcus acidilactici

The function of the aminotransferase Aat (GenBank Protein WP_159211138) from Pediococcus acidilactici FAM 18098 was studied in vivo. For this purpose, the gene was replaced with an erythromycin resistance gene using the temperature-sensitive Escherichia coli-Pediococcus shuttle plasmid pSET4T_Δaat. The knockout was verified by PCR and genome sequencing. Subsequently, the differences between the metabolism of the knockout and of the wild-type strain were investigated by determining the free amino acids and organic acids in culture supernatants. It was found that the knockout mutant no longer synthesized 3-phenyllactic acid (PLA) and 4-hydroxyphenyllactic acid (HPLA). Additionally, the mutant strain no longer catabolized phenylalanine. Metabolic pathway analysis using the KEGG database indicate that P. acidilactici cannot synthesize α-ketoglutarate that is a predominant amino-group acceptor in many transamination reactions. To study the transfer of the amino group of phenylalanine, the wild-type strain was incubated with [¹⁵N] phenylalanine. Mass spectrometry showed that during fermentation, [¹⁵N] alanine was formed, indicating that pyruvic acid is an amino group acceptor in P. acidilactici. The present study shows that Aat plays a crucial role in PLA/HPLA biosynthesis and pyruvic acid is an amino acceptor in transamination reactions in P. acidilactici.

Comparative transcriptomic analysis reveal genes involved in the pathogenicity increase of Streptococcus suis epidemic strains

Streptococcus suis epidemic strains were responsible for two outbreaks in China and possessed increased pathogenicity which was featured prominently by inducing an excessive inflammatory response at the early phase of infection. To discover the critical genes responsible for the pathogenicity increase of S. suis epidemic strains, the genome-wide transcriptional profiles of epidemic strain SC84 were investigated at the early phase of interaction with BV2 cells. The overall low expression levels of 89K pathogenicity island (PAI) and 129 known virulence genes in the SC84 interaction groups indicated that its pathogenicity increase should be attributed to novel mechanisms. Using highly pathogenic strain P1/7 and intermediately pathogenic strain 89–1591 as controls, 11 pathogenicity increase crucial genes (PICGs) and 38 pathogenicity increase-related genes (PIRGs) were identified in the SC84 incubation groups. The PICGs encoded proteins related to the methionine biosynthesis/uptake pathway and played critical roles in the pathogenicity increase of epidemic strains. A high proportion of PIRGs encoded surface proteins related to host cell adherence and immune escape, which may be conducive to the pathogenicity increase of epidemic strains by rapidly initiating infection. The fact that none of PICGs and PIRGs belonged to epidemic strain-specific gene indicated that the pathogenicity increase of epidemic strain may be determined by the expression level of genes, rather than the presence of them. Our results deepened the understanding on the mechanism of the pathogenicity increase of S. suis epidemic strains and provided novel approaches to control the life-threatening infections of S. suis epidemic strains.

Exogenous glycogen utilization effects the transcriptome and pathogenicity of Streptococcus suis serotype 2

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen that causes severe infections in humans and the swine industry. Acquisition and utilization of available carbon sources from challenging host environments is necessary for bacterial pathogens to ensure growth and proliferation. Glycogen is abundant in mammalian body and may support the growth of SS2 during infection in hosts. However, limited information is known about the mechanism between the glycogen utilization and host adaptation of SS2. Here, the pleiotropic effects of exogenous glycogen on SS2 were investigated through transcriptome sequencing. Analysis of transcriptome data showed that the main basic metabolic pathways, especially the core carbon metabolism pathways and virulence-associated factors, of SS2 responded actively to glycogen induction. Glycogen induction led to the perturbation of the glycolysis pathway and citrate cycle, but promoted the pentose phosphate pathway and carbohydrate transport systems. Extracellular glycogen utilization also promoted the mixed-acid fermentation in SS2 rather than homolactic fermentation. Subsequently, apuA, a gene encoding the unique bifunctional amylopullulanase for glycogen degradation, was deleted from the wild type and generated the mutant strain ΔapuA. The pathogenicity details of the wild type and ΔapuA cultured in glucose and glycogen were investigated and compared. Results revealed that the capsule synthesis or bacterial morphology were not affected by glycogen incubation or apuA deletion. However, extracellular glycogen utilization significantly enhanced the hemolytic activity, adhesion and invasion ability, and lethality of SS2. The deletion of apuA also impaired the pathogenicity of bacteria cultured in glucose, indicating that ApuA is indeed an important virulence factor. Our results revealed that exogenous glycogen utilization extensively influenced the expression profile of the S. suis genome. Based on the transcriptome response, exogenous glycogen utilization promoted the carbon adaption and pathogenicity of SS2.

Improved genetic transformation by disarmament of type II Restriction-Modification system in Streptococcus zooepidemicus

Streptococcus zooepidemicus, group C Streptococci, is currently used for the industrial production of hyaluronic acid (HA). However, genetic manipulation of S. zooepidemicus is severely limited by its low transformation efficiency, which might be in part due to the Restriction-Modification (R-M) systems. The complete genome sequence of S. zooepidemicus ATCC39920 revealed the presence of two putative R-M systems, type I and type II. The putative type I R-M system is encoded by three closely linked genes: hsdR (SeseC_01315), hsdS, hsdM (SeseC_01318), and the putative type II R-M system consists of two closely linked genes: SeseC_02360 and yhdJ (SeseC_02362). Inactivation of hsdR, encoding the restriction endonuclease (REase) of the type I R-M system, showed no apparent effects on transformation efficiency, implying that disarmament of the type I R-M system alone is not sufficient for increasing transformation efficiency. However, inactivation of SeseC_02360, encoding the REase of the type II R-M system, improved transformation efficiency by 4.97 folds, indicating that type II R-M system is the major barrier that restricts genetic transformation in S. zooepidemicus. Furthermore, S. zooepidemicus strains lacking either of the two R-M systems are phenotypically indistinguishable from the wild-type in terms of cell growth and HA production. In summary, our study revealed that the type II R-M system is the main barrier to genetic transformation in S. zooepidemicus ATCC39920, and that the deletion of the type II R-M system renders S. zooepidemicus more transformable, thus facilitating metabolic engineering of this industrially important microorganism.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03227-x.

Mn uptake system affects the virulence of Streptococcus suis by mediating oxidative stress

Manganese (Mn) is an important micronutrient that is not readily available to pathogens during infection. Hosts resist the invasion of pathogens through nutritional immunity and oxidative stress. To overcome this nutrient restriction, bacteria utilize high affinity transporters to compete with nutrient-binding proteins (e.g., calprotectin). Little is known about the role of Mn in the pathophysiology of Streptococcus suis. Here, we revealed that the tolerance of S. suis to calprotectin and oxidative stress was associated with Mn. Inactivation of Mn uptake system, TroABCD, in S. suis decreased the tolerance to calprotectin and oxidative stress. Furthermore, Mn uptake system mutant strains reduced capacity for bacterial cellular survival, and attenuated virulence in a mouse model. To explore the regulatory mechanism, we determined the transcriptional start site of troABCD using capping rapid amplification of cDNA ends. Furthermore, we revealed that TroR was a transcriptional regulatory repressor of troABCD. In the absence of troR, transcription levels of troA, troB, troC, and troD were not inhibited by low or high Mn levels, and intracellular Mn contents of mutant strains were higher than that of the wild-type strain. Finally, we used electrophoretic mobility shift assay to demonstrate that TroR bound the promoter region of troABCD. Collectively, this study revealed that Mn acquisition was essential for pathogenesis of S. suis and Mn uptake systems should be targets for the development of new antimicrobials.

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.

Streptococcal autolysin promotes dysfunction of swine tracheal epithelium by interacting with vimentin

Streptococcus suis serotype 2 (SS2) is a major zoonotic pathogen resulting in manifestations as pneumonia and septic shock. The upper respiratory tract is typically thought to be the main colonization and entry site of SS2 in pigs, but the mechanism through which it penetrates the respiratory barrier is still unclear. In this study, a mutant with low invasive potential to swine tracheal epithelial cells (STECs) was screened from the TnYLB-1 transposon insertion mutant library of SS2, and the interrupted gene was identified as autolysin (atl). Compared to wild-type (WT) SS2, Δatl mutant exhibited lower ability to penetrate the tracheal epithelial barrier in a mouse model. Purified Atl also enhanced SS2 translocation across STEC monolayers in Transwell inserts. Furthermore, Atl redistributed the tight junctions (TJs) in STECs through myosin light chain kinase (MLCK) signaling, which led to increased barrier permeability. Using mass spectrometry, co-immunoprecipitation (co-IP), pull-down, bacterial two-hybrid and saturation binding experiments, we showed that Atl binds directly to vimentin. CRISPR/Cas9-targeted deletion of vimentin in STECs (VIM KO STECs) abrogated the capacity of SS2 to translocate across the monolayers, SS2-induced phosphorylation of myosin II regulatory light chain (MLC) and MLCK transcription, indicating that vimentin is indispensable for MLCK activation. Consistently, vimentin null mice were protected from SS2 infection and exhibited reduced tracheal and lung injury. Thus, MLCK-mediated epithelial barrier opening caused by the Atl-vimentin interaction is found to be likely the key mechanism by which SS2 penetrates the tracheal epithelium.

Streptococcus suis serotype 2 (SS2), an emerging zoonotic agent, can breach the respiratory barrier and cause invasive disease in pigs. Here, we identified the novel role of autolysin Atl in penetration of the respiratory barrier by SS2 and its systemic dissemination and identified its binding partner, vimentin, a type III intermediate filament protein. Atl contributed to the MLCK-triggered redistribution of tight junctions to open the tracheal epithelial barrier. Knockout of vimentin abolished the ability of SS2 to penetrate the monolayer barrier and the activation of MLCK. Furthermore, vimentin null mice were protected from infection by intranasally administered SS2. This study is the first to demonstrate that the interaction between the GBS Bsp-like domain of Atl and vimentin promotes MLCK-mediated dysfunction of the epithelial barrier, which may provide theoretical information for prophylactic and/or therapeutic treatments against diseases caused by similar respiratory pathogens.

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.

Natural mutation in the regulatory gene (srrG) influences virulence-associated genes and enhances invasiveness in Streptococcus dysgalactiae subsp. equisimilis strains isolated from cases of streptococcal toxic shock syndrome

Background

Streptococcus dysgalactiae subspecies equisimilis (SDSE) has emerged as an important cause of severe invasive infections including streptococcal toxic shock syndrome (STSS). The present study aimed to identify genes involved in differences in invasiveness between STSS and non-invasive SDSE isolates.

Methods

STSS and non-invasive SDSE isolates were analysed to identify csrS/csrR mutations, followed by a comparative analysis of genomic sequences to identify mutations in other genes. Mutant strains were generated to examine changes in gene expression profiles and altered pathogenicity in mice.

Findings

Of the 79 STSS-SDSE clinical isolates, 15 (19.0%) harboured csrS/csrR mutations, while none were found in the non-invasive SDSE isolates. We identified a small RNA (sRNA) that comprised three direct repeats along with an inverted repeat and was transcribed in the same direction as the sagA gene. The sRNA was referred to as srrG (streptolysin S regulatory RNA in GGS). srrG mutations were identified in the STSS-SDSE strains and were found to be associated with elevated expression of the streptolysin S (SLS) gene cluster and enhanced pathogenicity in mice.

Interpretation

The csrS/csrR and srrG mutations that increased virulence gene expression in STSS-SDSE isolates were identified, and strains carrying these mutations caused increased lethality in mice. A significantly higher frequency of mutations was observed in STSS-SDSE isolates, thereby highlighting their importance in STSS.

Funding

Japan Agency for Medical Research and Development, the Japan Society for the Promotion of Science (JSPS), and the Ministry of Health, Labor, and Welfare of Japan.

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.

Advances of genetic engineering in Streptococci and Enterococci

In the post-genome era, reverse genetic engineering is an indispensable methodology for experimental molecular biology to provide a deeper understanding of the principal relationship between genomic features and biological phenotypes. Technically, genetic engineering is carried out through allele replacement of a target genomic locus with a designed nucleotide sequence, so called site-directed mutagenesis. To artificially manipulate allele replacement through homologous recombination, researchers have improved various methodologies that are optimized to the bacterial species of interest. Here, we review widely used genetic engineering technologies, particularly for streptococci and enterococci, and recent advances that enable more effective and flexible manipulation. The development of genetic engineering has been promoted by synthetic biology approaches based on basic biology knowledge of horizontal gene transfer systems, such as natural conjugative transfer, natural transformation, and the CRISPR/Cas system. Therefore, this review also describes basic insights into molecular biology that underlie improvements in genetic engineering technology. This article is protected by copyright. All rights reserved.

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.

PerR-Regulated Manganese Import Contributes to Oxidative Stress Defense in Streptococcus suis

Streptococcus suis has been increasingly recognized as a porcine zoonotic pathogen that threatens the health of both pigs and humans. Metal homeostasis plays a critical role in the antioxidative capability of bacteria, thus facilitating the escape of pathogenic species from the innate immunity systems of hosts. Here, we revealed that manganese increased the ability of S. suis to resist oxidative stress. RNA sequencing was used to identify potential candidate genes involved in the maintenance of intracellular manganese homeostasis. Four genes, termed troABCD, were identified by NCBI BLASTp analysis. The troA, troB, troC, and troD deletion mutant strains exhibited decreased intracellular manganese content and tolerance to H₂O₂ compared to the wild-type strain. Thus, troABCD were determined to be involved in manganese uptake and played an important role in H₂O₂ tolerance in S. suis. Furthermore, the inactivation of perR increased the survival of H₂O₂-pulsed S. suis 2.18-fold and elevated the intracellular manganese content. H₂O₂-pulsed S. suis and perR deletion mutants upregulated troABCD. This finding suggested that H₂O₂ released the suppression of troABCD by perR. In addition, an electrophoretic mobility shift assay (EMSA) showed that PerR at 500 ng binds to the troABCD promoter, indicating that troABCD were directly regulated by PerR. In conclusion, this study revealed that manganese increases tolerance to H₂O₂ by upregulating the expression of troABCD. Moreover, PerR-regulated Mn import in S. suis and increased the tolerance of S. suis to oxidative stress by regulating troABCD. IMPORTANCE During infection, it is extremely important for bacteria to defend against oxidative stress. While manganese plays an important role in this process, its role is unclear in S. suis. Here, we demonstrated that manganese increased S. suis tolerance to oxidative stress. Four manganese ABC transporter genes, troABCD, were identified. Oxidative stress increased the content of manganese in the cell. Furthermore, PerR increased the tolerance to oxidative stress of S. suis by regulating troABCD. Manganese played an important role in bacterial defense against oxidative stress. These findings provide novel insight into the mechanism by which S. suis resists oxidative stress and approaches to inhibit bacterial infection by limiting manganese intake.

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.

d-Alanylation of Lipoteichoic Acids in Streptococcus suis Reduces Association With Leukocytes in Porcine Blood

Streptococcus suis (S. suis) is a common swine pathogen but also poses a threat to human health in causing meningitis and severe cases of streptococcal toxic shock-like syndrome (STSLS). Therefore, it is crucial to understand how S. suis interacts with the host immune system during bacteremia. As S. suis has the ability to introduce d-alanine into its lipoteichoic acids (LTAs), we investigated the working hypothesis that cell wall modification by LTA d-alanylation influences the interaction of S. suis with porcine blood immune cells. We created an isogenic mutant of S. suis strain 10 by in-frame deletion of the d-alanine d-alanyl carrier ligase (DltA). d-alanylation of LTAs was associated with reduced phagocytosis of S. suis by porcine granulocytes, reduced deposition of complement factor C3 on the bacterial surface, increased hydrophobicity of streptococci, and increased resistance to cationic antimicrobial peptides (CAMPs). At the same time, survival of S. suis was not significantly increased by LTA d-alanylation in whole blood of conventional piglets with specific IgG. However, we found a distinct cytokine pattern as IL-1β but not tumor necrosis factor (TNF)-α levels were significantly reduced in blood infected with the ΔdltA mutant. In contrast to TNF-α, activation and secretion of IL-1β are inflammasome-dependent, suggesting a possible influence of LTA d-alanylation on inflammasome regulation. Especially in the absence of specific antibodies, the association of S. suis with porcine monocytes was reduced by d-alanylation of its LTAs. This dltA-dependent phenotype was also observed with a non-encapsulated dltA double mutant indicating that it is independent of capsular polysaccharides. High antibody levels caused high levels of S. suis—monocyte—association followed by inflammatory cell death and strong production of both IL-1β and TNF-α, while the influence of LTA d-alanylation of the streptococci became less visible. In summary, the results of this study expand previous findings on d-alanylation of LTAs in S. suis and suggest that this pathogen specifically modulates association with blood leukocytes through this modification of its surface.

sRNA23, a novel small RNA, regulates to the pathogenesis of Streptococcus suis serotype 2

ABBREVIATION

sRNA: small noncoding RNA; FBA: fructose diphosphate aldolase; rplB: 50S ribosomal protein L2; RACE: rapid amplification of cDNA ends; EMSA: electrophoretic mobility shift assay; THB: Todd-Hewitt broth; FBS: fetal bovine serum; BIP: 2,2'-Bipyridine.

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

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

Comparative Phenotypic, Proteomic, and Phosphoproteomic Analysis Reveals Different Roles of Serine/Threonine Phosphatase and Kinase in the Growth, Cell Division, and Pathogenicity of Streptococcus suis

Eukaryote-like serine/threonine kinases (STKs) and cognate phosphatases (STPs) comprise an important regulatory system in many bacterial pathogens. The complexity of this regulatory system has not been fully understood due to the presence of multiple STKs/STPs in many bacteria and their multiple substrates involved in many different physiological and pathogenetic processes. Streptococci are the best materials for the study due to a single copy of the gene encoding STK and its cognate STP. Although several studies have been done to investigate the roles of STK and STP in zoonotic Streptococcus suis, respectively, few studies were performed on the coordinated regulatory roles of this system. In this study, we carried out a systemic study on STK/STP in S. suis by using a comparative phenotypic, proteomic, and phosphoproteomic analysis. Mouse infection assays revealed that STK played a much more important role in S. suis pathogenesis than STP. The ∆stk and ∆stp∆stk strains, but not ∆stp, showed severe growth retardation. Moreover, both ∆stp and ∆stk strains displayed defects in cell division, but they were abnormal in different ways. The comparative proteomics and phosphoproteomics revealed that deletion of stk or stp had a significant influence on protein expression. Interestingly, more virulence factors were found to be downregulated in ∆stk than ∆stp. In ∆stk strain, a substantial number of the proteins with a reduced phosphorylation level were involved in cell division, energy metabolism, and protein translation. However, only a few proteins showed increased phosphorylation in ∆stp, which also included some proteins related to cell division. Collectively, our results show that both STP and STK are critical regulatory proteins for S. suis and that STK seems to play more important roles in growth, cell division, and pathogenesis.

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.

CRISPR-dependent endogenous gene regulation is required for virulence in piscine Streptococcus agalactiae

The clustered regularly interspaced palindromic repeats (CRISPR)-Cas (CRISPR-associated) system is a prokaryotic defence against invading mobile genetic elements, such as bacteriophages or exogenous plasmids. Beyond this, this system has been shown to play an important role in controlling the virulence of some bacterial pathogens. Streptococcus agalactiae strain GD201008-001, a causative agent of septicemia and meningitis in tilapia, contains a single type II CRISPR-Cas system with Cas9 as a signature protein. In this study, we found that the deletion of CRISPR significantly reduced adhesion, invasion, cytotoxicity and haemolysis, and caused severely attenuated virulence in the piscine S. agalactiae strain. RNA-Seq identified 236 endogenous genes regulated by CRISPR, with 159 genes upregulated and 77 genes downregulated. The resulting change in gene transcription by CRISPR was much more pronounced than that by cas9 in this bacterium, indicating CRISPR-mediated endogenous gene regulation was mostly independently of cas9. Subsequent studies showed that CovR/S two-component system was transcriptionally upregulated due to CRISPR deletion, which repressed the expression of the cylE gene coding for a cytolytic toxin, and thus decreased the activity of β-haemolysin/cytolysin. However, upregulation of CovR/S was not the contributor to the attenuation phenotype of ΔCRISPR. Further, we demonstrated that CRISPR is capable of repressing the expression of Toll-like receptor 2 (TLR2)-activating lipoprotein Sag0671 and thus dampens the innate immune response. This study revealed that the CRISPR system of S. agalactiae exhibited extraordinary potential capability in the regulation of endogenous transcripts, which contributes to bacterial innate immune evasion and virulence.