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

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.

A novel aquaporin Aagp contributes to Streptococcus suis H2O2 efflux and virulence

Streptococcus suis is a bacterium that can cause infections in pigs and humans. Although oxidative stress is common occurrence during bacterial growth and infection, the regulation networks of S. suis under oxidative stress remain poorly understood. To address this, we utilized RNA-Seq to reveal the transcriptional landscape of S. suis in response to H2O2 stress. We identified novel genes responsible for S. suis resistance to oxidative stress, including those involved in DNA repair or protection, and essential for the biosynthesis of amino acids and nucleic acids. In addition, we found that a novel aquaporin, Aagp, belonging to atypical aquaglyceroporins and widely distributed in diverse S. suis serotypes, plays a crucial role during H2O2 stress. By performing oxidative stress assays and measuring the intracellular H2O2 concentrations of the wild-type strain and Aagp mutants during H2O2 stress, we found that Aagp facilitated H2O2 efflux. Additionally, we found that Aagp might be involved in glycerol transport, as shown by the growth inhibition and H2O2 production in the presence of glycerol. Mice infection experiments indicated that Aagp contributed to S. suis virulence. This study contributes to understanding the mechanism of S. suis oxidative stress response, S. suis pathogenesis, and the function of aquaporins in prokaryotes.

Role of Metabolic Adaptation of Streptococcus suis to Host Niches in Bacterial Fitness and Virulence

Streptococcus suis, both a common colonizer of the porcine upper respiratory tract and an invasive pig pathogen, successfully adapts to different host environments encountered during infection. Whereas the initial infection mainly occurs via the respiratory tract, in a second step, the pathogen can breach the epithelial barrier and disseminate within the whole body. Thereby, the pathogen reaches other organs such as the heart, the joints, or the brain. In this review, we focus on the role of S. suis metabolism for adaptation to these different in vivo host niches to encounter changes in nutrient availability, host defense mechanisms and competing microbiota. Furthermore, we highlight the close link between S. suis metabolism and virulence. Mutants deficient in metabolic regulators often show an attenuation in infection experiments possibly due to downregulation of virulence factors, reduced resistance to nutritive or oxidative stress and to phagocytic activity. Finally, metabolic pathways as potential targets for new therapeutic strategies are discussed. As antimicrobial resistance in S. suis isolates has increased over the last years, the development of new antibiotics is of utmost importance to successfully fight infections in the future.

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.

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.

Genome-wide association study identifies the virulence-associated marker in Streptococcus suis serotype 2

Streptococcus suis (S. suis) has been reported to be a highly invasive pathogen in swine, which causes severe infections like meningitis, arthritis and septicemia, and also a zoonotic agent for humans. Although many putative virulence factors (VFs) have been identified, the exact and wildly accepted virulence associated marker and pathogenesis mechanism of S. suis are still unclear. To establish connection of the genotypes with virulence phenotypes, we performed an "internal standard" method based on the zebrafish model to assess the virulence phenotypes of S. suis and did the genome-wide association study (GWAS) based on the genomes of 68 S. suis isolates. Through GWAS, a total number of 172 genes were identified. Among these genes, 143 of them distribute in virulent isolates. Further VFs interaction network analysis based on protein-protein interaction database found that 71 genes identified in this study could interact with known VFs and some of them even played an important role as the bridge between known VFs or formed important hub. In addition, 12 genes were found conserved in virulent isolates and 3 genes were conserved in avirulent isolates, 8 genes of the virulent conserved genes were belonging to a srtBCD pili cluster. Considering that sbp2', a member of the srtBCD pili cluster has been reported as a virulence-associated factor, we predict that sbp2' could be a fitness virulence-associated marker of virulent isolates. Taken together, our findings contribute to the insights in S. suis pathogenesis, enhance the knowledge of the genomic evolution of S. suis and provide several novel virulence-associated candidates.

mRNA-Seq reveals the quorum sensing system luxS gene contributes to the environmental fitness of Streptococcus suis type 2

Background

Streptococcus suis type 2 (SS2) is an important zoonotic pathogen. We have previously reported the structure of LuxS protein and found that the luxS gene is closely related to biofilm, virulence gene expression and drug resistance of SS2. However, the mechanism of luxS mediated SS2 stress response is unclear. Therefore, this experiment performed stress response to luxS mutant (ΔluxS) and complement strain (CΔluxS), overexpression strain (luxS+) and wild-type SS2 strain HA9801, and analyzed the differential phenotypes in combination with transcriptome data.

Results

The results indicate that the luxS gene deletion causes a wide range of phenotypic changes, including chain length. RNA sequencing identified 278 lx-regulated genes, of which 179 were up-regulated and 99 were down-regulated. Differential genes focus on bacterial growth, stress response, metabolic mechanisms and drug tolerance. Multiple mitotic genes were down-regulated; while the ABC transporter system genes, cobalamin /Fe³⁺-iron carrier ABC transporter ATPase and oxidative stress regulators were up-regulated. The inactivation of the luxS gene caused a significant reduction in the growth and survival in the acid (pH = 3.0, 4.0, 5.0) and iron (100 mM iron chelator 2,2′-dipyridyl) stress environments. However, the mutant strain ΔluxS showed increased antioxidant activity to H₂O₂ (58.8 mmol/L).

Conclusions

The luxS gene in SS2 appears to play roles in iron metabolism and protective responses to acidic and oxidative environmental conditions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02170-w.

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

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

Involvement of Various Enzymes in the Physiology and Pathogenesis of Streptococcus suis

Streptococcus suis causes severe infections in both swine and humans, making it a serious threat to the swine industry and public health. Insight into the physiology and pathogenesis of S. suis undoubtedly contributes to the control of its infection. During the infection process, a wide variety of virulence factors enable S. suis to colonize, invade, and spread in the host, thus causing localized infections and/or systemic diseases. Enzymes catalyze almost all aspects of metabolism in living organisms. Numerous enzymes have been characterized in extensive detail in S. suis, and have shown to be involved in the pathogenesis and/or physiology of this pathogen. In this review, we describe the progress in the study of some representative enzymes in S. suis, such as ATPases, immunoglobulin-degrading enzymes, and eukaryote-like serine/threonine kinase and phosphatase, and we highlight the important role of various enzymes in the physiology and pathogenesis of this pathogen. The controversies about the current understanding of certain enzymes are also discussed here. Additionally, we provide suggestions about future directions in the study of enzymes in S. suis.

Transcriptional regulator SpxA1a controls the resistance of the honey bee pathogen Melissococcus plutonius to the antimicrobial activity of royal jelly

Royal jelly (RJ), a brood food of honey bees, has strong antimicrobial activity. Melissococcus plutonius, the causative agent of European foulbrood of honey bees, exhibits resistance to this antimicrobial activity and infects larvae orally. Among three genetically distinct groups (CC3, CC12 and CC13) of M. plutonius, CC3 strains exhibit the strongest RJ resistance. In this study, to identify genes involved in RJ resistance, we generated an RJ-susceptible derivative from a highly RJ-resistant CC3 strain by UV mutagenesis. Genome sequence analysis of the derivative revealed the presence of a frameshift mutation in the putative regulator gene spxA1a. The deletion of spxA1a from a CC3 strain resulted in increased susceptibility to RJ and its antimicrobial component 10-hydroxy-2-decenoic acid. Moreover, the mutant became susceptible to low-pH and oxidative stress, which may be encountered in brood foods. Differentially expressed gene analysis using wild-type and spxA1a mutants revealed that 45 protein-coding genes were commonly upregulated in spxA1a-positive strains. Many upregulated genes were located in a prophage region, and some highly upregulated genes were annotated as universal/general stress proteins, oxidoreductase/reductase, chaperons and superoxide dismutase. These results suggest that SpxA1a is a key regulator to control the tolerance status of M. plutonius against stress in honey bee colonies.

Regulatory circuits controlling Spx levels in Streptococcus mutans

Spx is a major regulator of stress responses in Firmicutes. In Streptococcus mutans, two Spx homologues, SpxA1 and SpxA2, were identified as mediators of oxidative stress responses but the regulatory circuits controlling their levels and activity are presently unknown. Comparison of SpxA1 and SpxA2 protein sequences revealed differences at the C-terminal end, with SpxA1 containing an unusual number of acidic residues. Here, we showed that a green fluorescence protein (GFP) reporter becomes unstable when fused to the last 10 amino acids of SpxA2 but remained stable when fused to the C-terminal acidic tail of SpxA1. Inactivation of clpP or simultaneous inactivation of clpC and clpE stabilized the GFP::SpxA2_tail fusion protein. Addition of acidic amino acids to the GFP::SpxA2_tail chimera stabilized GFP, while deletion of the acidic residues destabilized GFP::SpxA1_tail . Promoter reporter fusions revealed that spxA1 transcription is co-repressed by the metalloregulators PerR and SloR while spxA2 transcription is largely dependent on the envelope stress regulator LiaFSR. In agreement with spxA2 being part of the LiaR regulon, SpxA2 was found to be critical for the growth of S. mutans under envelope stress conditions. Finally, we showed that redox sensing is essential for SpxA1-dependent activation of oxidative stress responses but dispensable for SpxA2-mediated envelope stress responses.

In vivo transcriptomes of Streptococcus suis reveal genes required for niche-specific adaptation and pathogenesis

Streptococcus suis is a Gram-positive bacterium and a zoonotic pathogen residing in the nasopharynx or the gastrointestinal tract of pigs with a potential of causing life-threatening invasive disease. It is endemic in the porcine production industry worldwide, and it is also an emerging human pathogen. After invasion, the pathogen adapts to cause bacteremia and disseminates to different organs including the brain. To gain insights in this process, we infected piglets with a highly virulent strain of S. suis, and bacterial transcriptomes were obtained from blood and different organs (brain, joints, and heart) when animals had severe clinical symptoms of infection. Microarrays were used to determine the genome-wide transcriptional profile at different infection sites and during growth in standard growth medium in vitro. We observed differential expression of around 30% of the Open Reading Frames (ORFs) and infection-site specific patterns of gene expression. Genes with major changes in expression were involved in transcriptional regulation, metabolism, nutrient acquisition, stress defenses, and virulence, amongst others, and results were confirmed for a subset of selected genes using RT-qPCR. Mutants were generated in two selected genes, and the encoded proteins, i.e., NADH oxidase and MetQ, were shown to be important virulence factors in coinfection experiments and in vitro assays. The knowledge derived from this study regarding S. suis gene expression in vivo and identification of virulence factors is important for the development of novel diagnostic and therapeutic strategies to control S. suis disease.

CopA Protects Streptococcus suis against Copper Toxicity

Streptococcus suis is a zoonotic pathogen that causes great economic losses to the swine industry and severe threats to public health. A better understanding of its physiology would contribute to the control of its infections. Although copper is an essential micronutrient for life, it is toxic to cells when present in excessive amounts. Herein, we provide evidence that CopA is required for S. suis resistance to copper toxicity. Quantitative PCR analysis showed that copA expression was specifically induced by copper. Growth curve analyses and spot dilution assays showed that the ΔcopA mutant was defective in media supplemented with elevated concentrations of copper. Spot dilution assays also revealed that CopA protected S. suis against the copper-induced bactericidal effect. Using inductively coupled plasma-optical emission spectroscopy, we demonstrated that the role of CopA in copper resistance was mediated by copper efflux. Collectively, our data indicated that CopA protects S. suis against the copper-induced bactericidal effect via copper efflux.

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.

Inactivation of cysL Inhibits Biofilm Formation by Activating the Disulfide Stress Regulator Spx in Bacillus subtilis

Bacillus subtilis forms biofilms in response to internal and external stimuli. I previously showed that the cysL deletion mutant was defective in biofilm formation, but the reason for this remains unidentified. CysL is a transcriptional activator of the cysJI operon, which encodes sulfite reductase, an enzyme involved in cysteine biosynthesis. Decreased production of sulfite reductase led to biofilm formation defects in the ΔcysL mutant. The ΔcysL mutation was suppressed by disrupting cysH operon genes, whose products function upstream of sulfite reductase in the cysteine biosynthesis pathway, indicating that defects in cysteine biosynthesis were not a direct cause for the defective biofilm formation observed in the ΔcysL mutant. The cysH gene encodes phosphoadenosine phosphosulfate reductase, which requires a reduced form of thioredoxin (TrxA) as an electron donor. High expression of trxA inhibited biofilm formation in the ΔcysL mutant but not in the wild-type strain. Northern blot analysis showed that trxA transcription was induced in the ΔcysL mutant in a disulfide stress-induced regulator Spx-dependent manner. On the basis of these results, I propose that the ΔcysL mutation causes phosphoadenosine phosphosulfate reductase to consume large amounts of reduced thioredoxin, inducing disulfide stress and activating Spx. The spx mutation restored biofilm formation to the ΔcysL mutant. The ΔcysL mutation reduced expression of the eps operon, which is required for exopolysaccharide production. Moreover, overexpression of the eps operon restored biofilm formation to the ΔcysL mutant. Taken together, these results suggest that the ΔcysL mutation activates Spx, which then inhibits biofilm formation through repression of the eps operon.IMPORTANCE Bacillus subtilis has been studied as a model organism for biofilm formation. In this study, I explored why the cysL deletion mutant was defective in biofilm formation. I demonstrated that the ΔcysL mutation activated the disulfide stress response regulator Spx, which inhibits biofilm formation by repressing biofilm matrix genes. Homologs of Spx are highly conserved among Gram-positive bacteria with low G+C contents. In some pathogens, Spx is also reported to inhibit biofilm formation by repressing biofilm matrix genes, even though these genes and their regulation are quite different from those of B. subtilis Thus, the negative regulation of biofilm formation by Spx is likely to be well conserved across species and may be an appropriate target for control of biofilm formation.

Immunogenicity and cross-protective efficacy of double-mutant Streptococcus suis ΔSspepO/ΔSspspC serotypes 2 and 7

Streptococcus suis serotype 2 (S. suis 2) is a major pathogen causing streptococcosis in swine, resulting in significant losses in swine breeding worldwide. We previously reported that the non-antibiotic-resistant double-mutant strain ΔSspepO/ΔSspspC can be used as a live vaccine, providing effective protection against S. suis 2 infections in mice. This study aimed to understand the characteristics of streptococcosis and develop vaccine candidates for immunization. Intramuscular injection of live S. suis ΔSspepO/ΔSspspC in pigs induced discernable antibody production and provided cross-protection against challenges by a type 2 strain (100% protection) and a type 7 strain (60% protection). Protection was evaluated via clinical, bacteriological, serological, and post-mortem examinations. Furthermore, vaccination induced the production of opsonizing antibodies against serotypes 2 and 7. Analysis of IgG subclasses (IgG1 and IgG2a) revealed that both Th1 and Th2 responses were induced by S. suis ΔSspepO/ΔSspspC, although the IgG2a (Th1) response predominated over the IgG1 (Th2) response. This study is the first, to our knowledge, to establish a live vaccine candidate to protect against two major S. suis serotypes. Further studies are required to assess these candidate vaccines and examine their feasibility in providing cross-protection against 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.

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.

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

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

Induction of the Spx regulon by cell wall stress reveals novel regulatory mechanisms in Bacillus subtilis

The transcription factor Spx is the master regulator of the disulfide stress response in Bacillus subtilis. Intriguingly, the activation of Spx by diamide relies entirely on posttranslational regulatory events in spite of the complex transcriptional control of the spx gene. Here, we show that cell wall stress, but not membrane stress, also results in induction of the Spx regulon. Remarkably, two major differences were found regarding the mechanism of induction of Spx under cell wall stress in comparison to disulfide stress. First, transcriptional induction of the spx gene from a σ^M -dependent promoter is required for accumulation of Spx in response to cell wall stress. Second, activation of the Spx regulon during cell wall stress is not accompanied by oxidation of the Spx disulfide switch. Finally, we demonstrate that cells lacking Spx have increased sensitivity toward antibiotics inhibiting both early and late steps in peptidoglycan synthesis, suggesting that the Spx regulon plays an important adaptive role in the cell wall stress response. This study expands the functional role of the Spx regulon and reveals novel regulatory mechanisms that result in induction of Spx in B. subtilis.

Transcriptome responses of Streptococcus mutans to peroxide stress: identification of novel antioxidant pathways regulated by Spx

The oxidative stress regulator Spx is ubiquitously found among Gram-positive bacteria. Previously, we reported identification of two Spx proteins in Streptococcus mutans - SpxA1 was the primary activator of oxidative stress genes whereas SpxA2 served a backup role. Here, we used RNA sequencing to uncover the scope of the H₂O₂ (peroxide)-stress regulon and to further explore the significance of Spx regulation in S. mutans. The transcriptome data confirmed the relationship between Spx and genes typically associated with oxidative stress, but also identified novel genes and metabolic pathways controlled by Spx during peroxide stress. While individual inactivation of newly identified peroxide stress genes had modest or no obvious consequences to bacterial survival, a phenotype enhancement screen using the ∆spxA1 strain as background for creation of double mutants revealed that four of the five genes inactivated were required for stress survival. Physiological and biochemical assays validated, at least in part, the transcriptome data indicating that SpxA1 coordinates transcriptional changes during peroxide stress that modify global metabolism and facilitate production of antioxidants. Collectively, our findings unraveled the scope of the peroxide stress regulon and expand the repertoire of oxidative stress genes in S. mutans, shedding new light on the role of Spx regulation.

Stress Physiology of Lactic Acid Bacteria

Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the "stressome" of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

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

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

Contribution of NADH oxidase to oxidative stress tolerance and virulence of Streptococcus suis serotype 2

Streptococcus suis is a major swine and zoonotic pathogen that causes severe infections. Previously, we identified 2 Spx regulators in S. suis, and demonstrated that SpxA1 affects oxidative stress tolerance and virulence. However, the mechanism behind SpxA1 function remains unclear. In this study, we targeted 4 genes that were expressed at significantly reduced levels in the spxA1 mutant, to determine their specific roles in adaptation to oxidative stress and virulence potential. The Δnox strain exhibited impaired growth under oxidative stress conditions, suggesting that NADH oxidase is involved in oxidative stress tolerance. Using murine and pig infection models, we demonstrate for the first time that NADH oxidase is required for virulence in S. suis 2. Furthermore, the enzymatic activity of NADH oxidase has a key role in oxidative stress tolerance and a secondary role in virulence. Collectively, our findings reveal that NADH oxidase plays an important part in SpxA1 function and provide a new insight into the pathogenesis of S. suis 2.

Inactivation of the spxA1 or spxA2 gene of Streptococcus mutans decreases virulence in the rat caries model

In oral biofilms, the major environmental challenges encountered by Streptococcus mutans are acid and oxidative stresses. Previously, we showed that the transcriptional regulators SpxA1 and SpxA2 are involved in general stress survival of S. mutans with SpxA1 playing a primary role in activation of antioxidant and detoxification strategies whereas SpxA2 serves as a back up activator of oxidative stress genes. We have also found that spxA1 mutant strains (∆spxA1 and ∆spxA1∆spxA2) are outcompeted by peroxigenic oral streptococci in vitro and have impaired abilities to colonize the teeth of rats fed a highly cariogenic diet. Here, we show that the Spx proteins can also exert regulatory roles in the expression of additional virulence attributes of S. mutans. Competence activation is significantly impaired in Δspx strains and the production of mutacin IV and V is virtually abolished in ΔspxA1 strains. Unexpectedly, the ∆spxA2 strain showed increased production of glucans from sucrose, without affecting the total amount of bacteria within biofilms when compared with the parent strain. By using the rat caries model, we showed that the capacity of the ΔspxA1 and ΔspxA2 strains to cause caries on smooth tooth surfaces is significantly impaired. The ∆spxA2 strain also formed fewer lesions on sulcal surfaces. This report reveals that global regulation via Spx contributes to the cariogenic potential of S. mutans and highlights that animal models are essential in the characterization of bacterial traits implicated in virulence.

Gene expression of Lactobacillus plantarum and the commensal microbiota in the ileum of healthy and early SIV-infected rhesus macaques

Chronic HIV infection results in impairment of gut-associated lymphoid tissue leading to systemic immune activation. We previously showed that in early SIV-infected rhesus macaques intestinal dysfunction is initiated with the induction of the IL-1β pathway in the small intestine and reversed by treatment with an exogenous Lactobacillus plantarum strain. Here, we provide evidence that the transcriptomes of L. plantarum and ileal microbiota are not altered shortly after SIV infection. L. plantarum adapts to the small intestine by expressing genes required for tolerating oxidative stress, modifying cell surface composition, and consumption of host glycans. The ileal microbiota of L. plantarum-containing healthy and SIV+ rhesus macaques also transcribed genes for host glycan metabolism as well as for cobalamin biosynthesis. Expression of these pathways by bacteria were proposed but not previously demonstrated in the mammalian small intestine.

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

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

Thiol-based redox switches in prokaryotes

Bacteria encounter reactive oxygen species (ROS) as a consequence of the aerobic life or as an oxidative burst of activated neutrophils during infections. In addition, bacteria are exposed to other redox-active compounds, including hypochloric acid (HOCl) and reactive electrophilic species (RES) such as quinones and aldehydes. These reactive species often target the thiol groups of cysteines in proteins and lead to thiol-disulfide switches in redox-sensing regulators to activate specific detoxification pathways and to restore the redox balance. Here, we review bacterial thiol-based redox sensors that specifically sense ROS, RES and HOCl via thiol-based mechanisms and regulate gene transcription in Gram-positive model bacteria and in human pathogens, such as Staphylococcus aureus and Mycobacterium tuberculosis. We also pay particular attention to emerging widely conserved HOCl-specific redox regulators that have been recently characterized in Escherichia coli. Different mechanisms are used to sense and respond to ROS, RES and HOCl by 1-Cys-type and 2-Cys-type thiol-based redox sensors that include versatile thiol-disulfide switches (OxyR, OhrR, HypR, YodB, NemR, RclR, Spx, RsrA/RshA) or alternative Cys phosphorylations (SarZ, MgrA, SarA), thiol-S-alkylation (QsrR), His-oxidation (PerR) and methionine oxidation (HypT). In pathogenic bacteria, these redox-sensing regulators are often important virulence regulators and required for adapation to the host immune defense.

Identification and characterization of the chromosomal yefM-yoeB toxin-antitoxin system of Streptococcus suis

Toxin-antitoxin (TA) systems are widely prevalent in the genomes of bacteria and archaea. These modules have been identified in Escherichia coli and various other bacteria. However, their presence in the genome of Streptococcus suis, an important zoonotic pathogen, has received little attention. In this study, we describe the identification and characterization of a type II TA system, comprising the chromosomal yefM-yoeB locus of S. suis. The yefM-yoeB locus is present in the genome of most serotypes of S. suis. Overproduction of S. suis YoeB toxin inhibited the growth of E. coli, and the toxicity of S. suis YoeB could be alleviated by the antitoxin YefM from S. suis and Streptococcus pneumoniae, but not by E. coli YefM. More importantly, introduction of the S. suis yefM-yoeB system into E. coli could affect cell growth. In a murine infection model, deletion of the yefM-yoeB locus had no effect on the virulence of S. suis serotype 2. Collectively, our data suggested that the yefM-yoeB locus of S. suis is an active TA system without the involvement of virulence.

Contribution of nuclease to the pathogenesis of Aeromonas hydrophila

Aeromonas hydrophila is a gram-negative bacterium that is widely distributed in aquatic environments and can cause septicemia in both fish and humans. However, the underlying mechanisms leading to severe infection are not well understood. In this study, an A. hydrophila nuclease (ahn) deletion mutant was constructed to investigate its contribution to pathogenesis. This mutant did not differ from the wild-type strain in terms of its growth or hemolytic phenotype. However, the ahn-deficient mutant was more susceptible to being killed by fish macrophages and mouse blood in vitro. Furthermore, evidence obtained using both fish and murine infection models strongly indicated that the inactivation of Ahn impaired the ability of A. hydrophila to evade innate immune clearance in vivo. More importantly, the virulence of the mutant was attenuated in both fish and mice, with reductions in dissemination capacities and mortality rates. These findings implicate Ahn in A. hydrophila virulence, with important functions in evading innate immune defenses.

Transcriptional and Phenotypic Characterization of Novel Spx-Regulated Genes in Streptococcus mutans

In oral biofilms, two of the major environmental challenges encountered by the dental pathogen Streptococcus mutans are acid and oxidative stresses. Previously, we showed that the S. mutans transcriptional regulators SpxA1 and SpxA2 (formerly SpxA and SpxB, respectively) are involved in stress survival by activating the expression of classic oxidative stress genes such as dpr, nox, sodA and tpx. We reasoned that some of the uncharacterized genes under SpxA1/A2 control are potentially involved in oxidative stress management. Therefore, the goal of this study was to use Spx-regulated genes as a tool to identify novel oxidative stress genes in S. mutans. Quantitative real-time PCR was used to evaluate the responses of ten Spx-regulated genes during H₂O₂ stress in the parent and Δspx strains. Transcription activation of the H₂O₂-induced genes (8 out of 10) was strongly dependent on SpxA1 and, to a lesser extent, SpxA2. In vitro transcription assays revealed that one or both Spx proteins directly regulate three of these genes. The gene encoding the FeoB ferrous permease was slightly repressed by H₂O₂ but constitutively induced in strains lacking SpxA1. Nine genes were selected for downstream mutational analysis but inactivation of smu127, encoding a subunit of the acetoin dehydrogenase was apparently lethal. In vitro and in vivo characterization of the viable mutants indicated that, in addition to the transcriptional activation of reducing and antioxidant pathways, Spx performs an important role in iron homeostasis by regulating the intracellular availability of free iron. In particular, inactivation of the genes encoding the Fe-S biogenesis SUF system and the previously characterized iron-binding protein Dpr resulted in impaired growth under different oxidative stress conditions, increased sensitivity to iron and lower infectivity in rats. These results serve as an entryway into the characterization of novel genes and pathways that allow S. mutans to cope with oxidative stress.

Transcription of Oxidative Stress Genes Is Directly Activated by SpxA1 and, to a Lesser Extent, by SpxA2 in Streptococcus mutans

The SpxA1 and SpxA2 (formerly SpxA and SpxB) transcriptional regulators of Streptococcus mutans are members of a highly conserved family of proteins found in Firmicutes, and they were previously shown to activate oxidative stress responses. In this study, we showed that SpxA1 exerts substantial positive regulatory influence over oxidative stress genes following exposure to H2O2, while SpxA2 appears to have a secondary regulatory role. In vitro transcription (IVT) assays using purified SpxA1 and/or SpxA2 showed that SpxA1 and, less often, SpxA2 directly activate transcription of some of the major oxidative stress genes. Addition of equimolar concentrations of SpxA1 and SpxA2 to the IVT reactions neither enhanced transcription of the tested genes nor disrupted the dominant role of SpxA1. Substitution of a conserved glycine residue (G52) present in both Spx proteins by arginine (SpxG52R) resulted in strains that phenocopied the Δspx strains. Moreover, addition of purified SpxA1G52R completely failed to activate transcription of ahpC, sodA, and tpx, further confirming that the G52 residue is critical for Spx functionality.

IMPORTANCE

Streptococcus mutans is a pathogen associated with the formation of dental caries in humans. Within the oral cavity, S. mutans routinely encounters oxidative stress. Our previous data revealed that two regulatory proteins, SpxA1 and SpxA2 (formerly SpxA and SpxB), bear high homology to the Spx regulator that has been characterized as a critical activator of oxidative stress genes in Bacillus subtilis. In this report, we prove that Spx proteins of S. mutans directly activate transcription of genes involved in the oxidative stress response, though SpxA1 appears to have a more dominant role than SpxA2. Therefore, the Spx regulators play a critical role in the ability of S. mutans to thrive within the oral cavity.