Transcriptional activation of sclA by Mga requires a distal binding site in Streptococcus pyogenes

Group A streptococcal collagen-like protein 1 restricts tumor growth in murine pancreatic adenocarcinoma and inhibits cancer-promoting neutrophil extracellular traps

Introduction

Pancreatic ductal adenocarcinoma (PDAC) is a lethal cancer associated with an immunosuppressive environment. Neutrophil extracellular traps (NETs) were initially described in the context of infection but have more recently been implicated in contributing to the tolerogenic immune response in PDAC. Thus, NETs are an attractive target for new therapeutic strategies. Group A Streptococcus (GAS) has developed defensive strategies to inhibit NETs.

Methods

In the present work, we propose utilizing intra-tumoral GAS injection to stimulate anti-tumor activity by inhibiting cancer-promoting NETs. Mice harboring Panc02 or KPC subcutaneous tumors injected with three different M-type GAS strains. Tumors and spleens were harvested at the endpoint of the experiments to assess bacterial colonization and systemic spread, while sera were analyzed for humoral responses toward the streptococcal antigens, especially the M1 and Scl1 proteins. Role of the streptococcal collagen-like protein 1 (Scl1) in anti-PDAC activity was assessed in vivo after intratumoral injection with M1 GAS wild-type, an isogenic mutant strain devoid of Scl1, or a complemented mutant strain with restored scl1 expression. In addition, recombinant Scl1 proteins were tested for NET inhibition using in vitro and ex vivo assays assessing NET production and myeloperoxidase activity.

Results

Injection of three different M-type GAS strains reduced subcutaneous pancreatic tumor volume compared to control in two different murine PDAC models. Limitation of tumor growth was dependent on Scl1, as isogenic mutant strain devoid of Scl1 did not reduce tumor size. We further show that Scl1 plays a role in localizing GAS to the tumor site, thereby limiting the systemic spread of bacteria and off-target effects. While mice did elicit a humoral immune response to GAS antigens, tested sera were weakly immunogenic toward Scl1 antigen following intra-tumoral treatment with Scl1-expressing GAS. M1 GAS inhibited NET formation when co-cultured with neutrophils while Scl1-devoid mutant strain did not. Recombinant Scl1 protein inhibited NETs ex vivo in a dose-dependent manner by suppressing myeloperoxidase activity.

Discussion

Altogether, we demonstrate that intra-tumoral GAS injections reduce PDAC growth, which is facilitated by Scl1, in part through inhibition of cancer promoting NETs. This work offers a novel strategy by which NETs can be targeted through Scl1 protein and potentiates its use as a cancer therapeutic.

MsmR1, a global transcription factor, regulates polymyxin synthesis and carbohydrate metabolism in Paenibacillus polymyxa SC2

The multiple-sugar metabolism regulator (MsmR), a transcription factor belonging to the AraC/XylS family, participates in polysaccharide metabolism and virulence. However, the transcriptional regulatory mechanisms of MsmR1 in Paenibacillus polymyxa remain unclear. In this study, knocking out msmR1 was found to reduce polymyxin synthesis by the SC2-M1 strain. Chromatin immunoprecipitation assay with sequencing (ChIP-seq) revealed that most enriched pathway was that of carbohydrate metabolism. Additionally, electromobility shift assays (EMSA) confirmed the direct interaction between MsmR1 and the promoter regions of oppC3, sucA, sdr3, pepF, yycN, PPSC2_23180, pppL, and ydfp. MsmR1 stimulates polymyxin biosynthesis by directly binding to the promoter regions of oppC3 and sdr3, while also directly regulating sucA and influencing the citrate cycle (TCA cycle). In addition, MsmR1 directly activates pepF and was beneficial for spore and biofilm formation. These results indicated that MsmR1 could regulate carbohydrate and amino acid metabolism, and indirectly affect biological processes such as polymyxin synthesis, biofilm formation, and motility. Moreover, MsmR1 could be autoregulated. Hence, this study expand the current knowledge of MsmR1 and will be beneficial for the application of P. polymyxa SC2 in the biological control against the certain pathogens in pepper.

PclR is a transcriptional activator of the gene that encodes the pneumococcal collagen-like protein PclA

The Gram-positive bacterium Streptococcus pneumoniae is a major human pathogen that shows high levels of genetic variability. The pneumococcal R6 genome harbours several gene clusters that are not present in all strains of the species. One of these clusters contains two divergent genes, pclA, which encodes a putative surface-exposed protein that contains large regions of collagen-like repeats, and spr1404 (here named pclR). PclA was shown to mediate pneumococcal adherence to host cells in vitro. In this work, we demonstrate that PclR (494 amino acids) is a transcriptional activator. It stimulates transcription of the pclA gene by binding to a specific DNA site upstream of the core promoter. In addition, we show that PclR has common features with the MgaSpn transcriptional regulator (493 amino acids), which is also encoded by the R6 genome. These proteins have high sequence similarity (60.3%), share the same organization of predicted functional domains, and generate multimeric complexes on linear double-stranded DNAs. However, on the PpclA promoter region, MgaSpn binds to a site different from the one recognized by PclR. Our results indicate that PclR and MgaSpn have similar DNA-binding properties but different DNA-binding specificities, pointing to a different regulatory role of both proteins.

Identification of a novel broad-spectrum endolysin, Ply0643, with high antibacterial activity in mouse models of streptococcal bacteriaemia and mastitis

Streptococcal infections are very common in humans and animals, and they are usually treated with antibiotics. Multidrug-resistant Streptococcus strains have continuously emerged in recent years, prompting the search for alternatives to antibiotics. The use of endolysins encoded by phages has presented a promising alternative approach to treatment. In this study, a novel prophage endolysin, Ply0643, was identified from the prophage S. a 04. At an optimal concentration (30 μg/mL), rPly0643 exhibited broad and strong lysosomal enzyme activity against 66 Streptococcus strains from different sources while also maintaining high lytic activity over a wide pH range (pH 6-10) and a broad range of temperatures (28 °C-45 °C). Two in vivo treatments of rPly0643 (total 0.8 mg/mouse) significantly protected mice (80%) from lethal bacteriaemia with Streptococcus suis, and single treatments of rPly0643 (0.1 mg/gland) significantly reduced Streptococcus agalactiae concentrations and inflammation in murine mammary glands. These findings collectively demonstrate that Ply0643 exhibits good bactericidal activity both in vitro and in vivo, and therefore represents a useful antibacterial agent for combatting streptococcal infections.

PRD-Containing Virulence Regulators (PCVRs) in Pathogenic Bacteria

Bacterial pathogens rely on a complex network of regulatory proteins to adapt to hostile and nutrient-limiting host environments. The phosphoenolpyruvate phosphotransferase system (PTS) is a conserved pathway in bacteria that couples transport of sugars with phosphorylation to monitor host carbohydrate availability. A family of structurally homologous PTS-regulatory-domain-containing virulence regulators (PCVRs) has been recognized in divergent bacterial pathogens, including Streptococcus pyogenes Mga and Bacillus anthracis AtxA. These paradigm PCVRs undergo phosphorylation, potentially via the PTS, which impacts their dimerization and their activity. Recent work with predicted PCVRs from Streptococcus pneumoniae (MgaSpn) and Enterococcus faecalis (MafR) suggest they interact with DNA like nucleoid-associating proteins. Yet, Mga binds to promoter sequences as a homo-dimeric transcription factor, suggesting a bi-modal interaction with DNA. High-resolution crystal structures of 3 PCVRs have validated the domain structure, but also raised additional questions such as how ubiquitous are PCVRs, is PTS-mediated histidine phosphorylation via potential PCVRs widespread, do specific sugars signal through PCVRs, and do PCVRs interact with DNA both as transcription factors and nucleoid-associating proteins? Here, we will review known and putative PCVRs based on key domain and functional characteristics and consider their roles as both transcription factors and possibly chromatin-structuring proteins.

Streptococcal Collagen-like Protein 1 Binds Wound Fibronectin: Implications in Pathogen Targeting

Group A Streptococcus (GAS) infections are responsible for significant morbidity and mortality worldwide. The outlook for an effective global vaccine is reduced because of significant antigenic variation among GAS strains worldwide. Other challenges in GAS therapy include the lack of common access to antibiotics in developing countries, as well as allergy to and treatment failures with penicillin and increasing erythromycin resistance in the industrialized world. At the portal of entry, GAS binds to newly deposited extracellular matrix, which is rich in cellular fibronectin isoforms with extra domain A (EDA, also termed EIIIA) via the surface adhesin, the streptococcal collagen-like protein 1 (Scl1). Recombinant Scl1 constructs, derived from diverse GAS strains, bind the EDA loop segment situated between the C and C' β-strands. Despite the sequence diversity in Scl1 proteins, multiple sequence alignments and secondary structure predictions of Scl1 variants, as well as crystallography and homology modeling studies, point to a conserved mechanism of Scl1-EDA binding. We propose that targeting this interaction may prevent the progression of infection. A synthetic cyclic peptide, derived from the EDA C-C' loop, binds to recombinant Scl1 with a micromolar dissociation constant. This review highlights the current concept of EDA binding to Scl1 and provides incentives to exploit this binding to treat GAS infections and wound colonization.

DNA methylation from a Type I restriction modification system influences gene expression and virulence in Streptococcus pyogenes

DNA methylation is pervasive across all domains of life. In bacteria, the presence of N6-methyladenosine (m6A) has been detected among diverse species, yet the contribution of m6A to the regulation of gene expression is unclear in many organisms. Here we investigated the impact of DNA methylation on gene expression and virulence within the human pathogen Streptococcus pyogenes, or Group A Streptococcus. Single Molecule Real-Time sequencing and subsequent methylation analysis identified 412 putative m6A sites throughout the 1.8 Mb genome. Deletion of the Restriction, Specificity, and Methylation gene subunits (ΔRSM strain) of a putative Type I restriction modification system lost all detectable m6A at the recognition sites and failed to prevent transformation with foreign-methylated DNA. RNA-sequencing identified 20 genes out of 1,895 predicted coding regions with significantly different gene expression. All of the differentially expressed genes were down regulated in the ΔRSM strain relative to the parent strain. Importantly, we found that the presence of m6A DNA modifications affected expression of Mga, a master transcriptional regulator for multiple virulence genes, surface adhesins, and immune-evasion factors in S. pyogenes. Using a murine subcutaneous infection model, mice infected with the ΔRSM strain exhibited an enhanced host immune response with larger skin lesions and increased levels of pro-inflammatory cytokines compared to mice infected with the parent or complemented mutant strains, suggesting alterations in m6A methylation influence virulence. Further, we found that the ΔRSM strain showed poor survival within human neutrophils and reduced adherence to human epithelial cells. These results demonstrate that, in addition to restriction of foreign DNA, gram-positive bacteria also use restriction modification systems to regulate the expression of gene networks important for virulence.

DNA methylation is common among many bacterial species, yet the contribution of DNA methylation to the regulation of gene expression is unclear outside of a limited number of gram-negative species. We characterized sites of DNA methylation throughout the genome of the gram-positive pathogen Streptococcus pyogenes or Group A Streptococcus. We determined that the gene products of a functional restriction modification system are responsible for genome-wide m6A. The mutant strain lacking DNA methylation showed altered gene expression compared to the parent strain, with several genes important for causing human disease down regulated. Furthermore, we showed that the mutant strain lacking DNA methylation exhibited altered virulence properties compared to the parent strain using various models of pathogenesis. The mutant strain was attenuated for both survival within human neutrophils and adherence to human epithelial cells, and was unable to suppress the host immune response in a murine subcutaneous infection model. Together, these results show that bacterial m6A contributes to differential gene expression and influences the ability of Group A Streptococcus to cause disease. DNA methylation is a conserved feature among bacteria and may represent a potential target for intervention in effort to interfere with the ability of bacteria to cause human disease.

Hypervirulent group A Streptococcus emergence in an acaspular background is associated with marked remodeling of the bacterial cell surface

Inactivating mutations in the control of virulence two-component regulatory system (covRS) often account for the hypervirulent phenotype in severe, invasive group A streptococcal (GAS) infections. As CovR represses production of the anti-phagocytic hyaluronic acid capsule, high level capsule production is generally considered critical to the hypervirulent phenotype induced by CovRS inactivation. There have recently been large outbreaks of GAS strains lacking capsule, but there are currently no data on the virulence of covRS-mutated, acapsular strains in vivo. We investigated the impact of CovRS inactivation in acapsular serotype M4 strains using a wild-type (M4-SC-1) and a naturally-occurring CovS-inactivated strain (M4-LC-1) that contains an 11bp covS insertion. M4-LC-1 was significantly more virulent in a mouse bacteremia model but caused smaller lesions in a subcutaneous mouse model. Over 10% of the genome showed significantly different transcript levels in M4-LC-1 vs. M4-SC-1 strain. Notably, the Mga regulon and multiple cell surface protein-encoding genes were strongly upregulated–a finding not observed for CovS-inactivated, encapsulated M1 or M3 GAS strains. Consistent with the transcriptomic data, transmission electron microscopy revealed markedly altered cell surface morphology of M4-LC-1 compared to M4-SC-1. Insertional inactivation of covS in M4-SC-1 recapitulated the transcriptome and cell surface morphology. Analysis of the cell surface following CovS-inactivation revealed that the upregulated proteins were part of the Mga regulon. Inactivation of mga in M4-LC-1 reduced transcript levels of multiple cell surface proteins and reversed the cell surface alterations consistent with the effect of CovS inactivation on cell surface composition being mediated by Mga. CovRS-inactivating mutations were detected in 20% of current invasive serotype M4 strains in the United States. Thus, we discovered that hypervirulent M4 GAS strains with covRS mutations can arise in an acapsular background and that such hypervirulence is associated with profound alteration of the cell surface.

Glucose Levels Alter the Mga Virulence Regulon in the Group A Streptococcus

Many bacterial pathogens coordinately regulate genes encoding important metabolic pathways during disease progression, including the phosphoenolpyruvate (PEP)-phosphotransferase system (PTS) for uptake of carbohydrates. The Gram-positive Group A Streptococcus (GAS) is a pathogen that infects multiple tissues in the human host. The virulence regulator Mga in GAS can be phosphorylated by the PTS, affecting Mga activity based on carbohydrate availability. Here, we explored the effects of glucose availability on the Mga regulon. RNA-seq was used to identify transcriptomic differences between the Mga regulon grown to late log phase in the presence of glucose (THY) or after glucose has been expended (C media). Our results revealed a correlation between the genes activated in C media with those known to be repressed by CcpA, indicating that C media mimics a non-preferred sugar environment. Interestingly, we found very little overlap in the Mga regulon from GAS grown in THY versus C media beyond the core virulence genes. We also observed an alteration in the phosphorylation status of Mga, indicating that the observed media differences in the Mga regulon may be directly attributed to glucose levels. Thus, these results support an in vivo link between glucose availability and virulence regulation in GAS.

The Regulatory Small RNA MarS Supports Virulence of Streptococcus pyogenes

Small regulatory RNAs (sRNAs) play a role in the control of bacterial virulence gene expression. In this study, we investigated an sRNA that was identified in Streptococcus pyogenes (group A Streptococcus, GAS) but is conserved throughout various streptococci. In a deletion strain, expression of mga, the gene encoding the multiple virulence gene regulator, was reduced. Accordingly, transcript and proteome analyses revealed decreased expression of several Mga-activated genes. Therefore, and because the sRNA was shown to interact with the 5′ UTR of the mga transcript in a gel-shift assay, we designated it MarS for mga-activating regulatory sRNA. Down-regulation of important virulence factors, including the antiphagocytic M-protein, led to increased susceptibility of the deletion strain to phagocytosis and reduced adherence to human keratinocytes. In a mouse infection model, the marS deletion mutant showed reduced dissemination to the liver, kidney, and spleen. Additionally, deletion of marS led to increased tolerance towards oxidative stress. Our in vitro and in vivo results indicate a modulating effect of MarS on virulence gene expression and on the pathogenic potential of GAS.

Collagen-like proteins of pathogenic streptococci

The collagen domain, which is defined by the presence of the Gly-X-Y triplet repeats, is amongst the most versatile and widespread known structures found in proteins from organisms representing all three domains of life. The streptococcal collagen-like (Scl) proteins are widely present in pathogenic streptococci, including Streptococcus pyogenes, S. agalactiae, S. pneumoniae, and S. equi. Experiments and bioinformatic analyses support the hypothesis that all Scl proteins are homotrimeric and cell wall-anchored. These proteins contain the rod-shaped collagenous domain proximal to cell surface, as well as a variety of outermost non-collagenous domains that generally lack predicted functions but can be grouped into one of six clusters based on sequence similarity. The well-characterized Scl1 proteins of S. pyogenes show a dichotomous switch in ligand binding between human tissue and blood environments. In tissue, Scl1 adhesin specifically recognizes the wound microenvironment, promotes adhesion and biofilm formation, decreases bacterial killing by neutrophil extracellular traps, and modulates S. pyogenes virulence. In blood, ligands include components of complement and coagulation-fibrinolytic systems, as well as plasma lipoproteins. In all, the Scl proteins signify a large family of structurally related surface proteins, which contribute to the ability of streptococci to colonize and cause diseases in humans and animals.

Unique Footprint in the scl1.3 Locus Affects Adhesion and Biofilm Formation of the Invasive M3-Type Group A Streptococcus

The streptococcal collagen-like proteins 1 and 2 (Scl1 and Scl2) are major surface adhesins that are ubiquitous among group A Streptococcus (GAS). Invasive M3-type strains, however, have evolved two unique conserved features in the scl1 locus: (i) an IS1548 element insertion in the scl1 promoter region and (ii) a nonsense mutation within the scl1 coding sequence. The scl1 transcript is drastically reduced in M3-type GAS, contrasting with a high transcription level of scl1 allele in invasive M1-type GAS. This leads to a lack of Scl1 expression in M3 strains. In contrast, while scl2 transcription and Scl2 production are elevated in M3 strains, M1 GAS lack Scl2 surface expression. M3-type strains were shown to have reduced biofilm formation on inanimate surfaces coated with cellular fibronectin and laminin, and in human skin equivalents. Repair of the nonsense mutation and restoration of Scl1 expression on M3-GAS cells, restores biofilm formation on cellular fibronectin and laminin coatings. Inactivation of scl1 in biofilm-capable M28 and M41 strains results in larger skin lesions in a mouse model, indicating that lack of Scl1 adhesin promotes bacterial spread over localized infection. These studies suggest the uniquely evolved scl1 locus in the M3-type strains, which prevents surface expression of the major Scl1 adhesin, contributed to the emergence of the invasive M3-type strains. Furthermore these studies provide insight into the molecular mechanisms mediating colonization, biofilm formation, and pathogenesis of group A streptococci.

Natural variant of collagen-like protein a in serotype M3 group a Streptococcus increases adherence and decreases invasive potential

Group A Streptococcus (GAS) predominantly exists as a colonizer of the human oropharynx that occasionally breaches epithelial barriers to cause invasive diseases. Despite the frequency of GAS carriage, few investigations into the contributory molecular mechanisms exist. To this end, we identified a naturally occurring polymorphism in the gene encoding the streptococcal collagen-like protein A (SclA) in GAS carrier strains. All previously sequenced invasive serotype M3 GAS possess a premature stop codon in the sclA gene truncating the protein. The carrier polymorphism is predicted to restore SclA function and was infrequently identified by targeted DNA sequencing in invasive strains of the same serotype. We demonstrate that a strain with the carrier sclA allele expressed a full-length SclA protein, while the strain with the invasive sclA allele expressed a truncated variant. An isoallelic mutant invasive strain with the carrier sclA allele exhibited decreased virulence in a mouse model of invasive disease and decreased multiplication in human blood. Further, the isoallelic invasive strain with the carrier sclA allele persisted in the mouse nasopharynx and had increased adherence to cultured epithelial cells. Repair of the premature stop codon in the invasive sclA allele restored the ability to bind the extracellular matrix proteins laminin and cellular fibronectin. These data demonstrate that a mutation in GAS carrier strains increases adherence and decreases virulence and suggest selection against increased adherence in GAS invasive isolates.

The pneumococcal MgaSpn virulence transcriptional regulator generates multimeric complexes on linear double-stranded DNA

The MgaSpn transcriptional regulator contributes to the virulence of Streptococcus pneumoniae. It is thought to be a member of the Mga/AtxA family of global regulators. MgaSpn was shown to activate in vivo the P1623B promoter, which is divergent from the promoter (Pmga) of its own gene. This activation required a 70-bp region (PB activation region) located between both promoters. In this work, we purified an untagged form of the MgaSpn protein, which formed dimers in solution. By gel retardation and footprinting assays, we analysed the binding of MgaSpn to linear double-stranded DNAs. MgaSpn interacted with the PB activation region when it was placed at internal position on the DNA. However, when it was positioned at one DNA end, MgaSpn recognized preferentially the Pmga promoter placed at internal position. In both cases, and on binding to the primary site, MgaSpn spread along the adjacent DNA regions generating multimeric protein–DNA complexes. When both MgaSpn-binding sites were located at internal positions on longer DNAs, electron microscopy experiments demonstrated that the PB activation region was the preferred target. DNA molecules totally or partially covered by MgaSpn were also visualized. Our results suggest that MgaSpn might recognize particular DNA conformations to achieve DNA-binding specificity.

PTS phosphorylation of Mga modulates regulon expression and virulence in the group A streptococcus

The ability of a bacterial pathogen to monitor available carbon sources in host tissues provides a clear fitness advantage. In the group A streptococcus (GAS), the virulence regulator Mga contains homology to phosphotransferase system (PTS) regulatory domains (PRDs) found in sugar operon regulators. Here we show that Mga was phosphorylated in vitro by the PTS components EI/HPr at conserved PRD histidines. A ΔptsI (EI-deficient) GAS mutant exhibited decreased Mga activity. However, PTS-mediated phosphorylation inhibited Mga-dependent transcription of emm in vitro. Using alanine (unphosphorylated) and aspartate (phosphomimetic) mutations of PRD histidines, we establish that a doubly phosphorylated PRD1 phosphomimetic (D/DMga4) is completely inactive in vivo, shutting down expression of the Mga regulon. Although D/DMga4 is still able to bind DNA in vitro, homo-multimerization of Mga is disrupted and the protein is unable to activate transcription. PTS-mediated regulation of Mga activity appears to be important for pathogenesis, as bacteria expressing either non-phosphorylated (A/A) or phosphomimetic (D/D) PRD1 Mga mutants were attenuated in a model of GAS invasive skin disease. Thus, PTS-mediated phosphorylation of Mga may allow the bacteria to modulate virulence gene expression in response to carbohydrate status. Furthermore, PRD-containing virulence regulators (PCVRs) appear to be widespread in Gram-positive pathogens.

Characterization of the Group A Streptococcus Mga virulence regulator reveals a role for the C-terminal region in oligomerization and transcriptional activation

The Group A Streptococcus (GAS) is a strict human pathogen that causes a broad spectrum of illnesses. One of the key regulators of virulence in GAS is the transcriptional activator Mga, which co-ordinates the early stages of infection. Although the targets of Mga have been well characterized, basic biochemical analyses have been limited due to difficulties in obtaining purified protein. In this study, high-level purification of soluble Mga was achieved, enabling the first detailed characterization of the protein. Fluorescence titrations coupled with filter-binding assays indicate that Mga binds cognate DNA with nanomolar affinity. Gel filtration analyses, analytical ultracentrifugation and co-immunoprecipitation experiments demonstrate that Mga forms oligomers in solution.Moreover, the ability of the protein to oligomerize in solution was found to correlate with transcriptional activation; DNA binding appears to be necessary but insufficient for full activity. Truncation analyses reveal that the uncharacterized C-terminal region of Mga, possessing similarity to phosphotransferase system EIIB proteins, plays a critical role in oligomerization and in vivo activity. Mga from a divergent serotype was found to behave similarly, suggesting that this study describes a general mechanism for Mga regulation of target virulence genes within GAS and provides insight into related regulators in other Gram-positive pathogens.

Nucleotides critical for the interaction of the Streptococcus pyogenes Mga virulence regulator with Mga-regulated promoter sequences

The Mga regulator of Streptococcus pyogenes directly activates the transcription of a core regulon that encodes virulence factors such as M protein (emm), C5a peptidase (scpA), and streptococcal inhibitor of complement (sic) by directly binding to a 45-bp binding site as determined by an electrophoretic mobility shift assay (EMSA) and DNase I protection. However, by comparing the nucleotide sequences of all established Mga binding sites, we found that they exhibit only 13.4% identity with no discernible symmetry. To determine the core nucleotides involved in functional Mga-DNA interactions, the M1T1 Pemm1 binding site was altered and screened for nucleotides important for DNA binding in vitro and for transcriptional activation using a plasmid-based luciferase reporter in vivo. Following this analysis, 34 nucleotides within the Pemm1 binding site that had an effect on Mga binding, Mga-dependent transcriptional activation, or both were identified. Of these critical nucleotides, guanines and cytosines within the major groove were disproportionately identified clustered at the 5' and 3' ends of the binding site and with runs of nonessential adenines between the critical nucleotides. On the basis of these results, a Pemm1 minimal binding site of 35 bp bound Mga at a level comparable to the level of binding of the larger 45-bp site. Comparison of Pemm with directed mutagenesis performed in the M1T1 Mga-regulated PscpA and Psic promoters, as well as methylation interference analysis of PscpA, establish that Mga binds to DNA in a promoter-specific manner.

Common regulators of virulence in streptococci

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

The streptococcal collagen-like protein-1 (Scl1) is a significant determinant for biofilm formation by group A Streptococcus

Background

Group A Streptococcus (GAS) is a human-specific pathogen responsible for a number of diseases characterized by a wide range of clinical manifestations. During host colonization GAS-cell aggregates or microcolonies are observed in tissues. GAS biofilm, which is an in vitro equivalent of tissue microcolony, has only recently been studied and little is known about the specific surface determinants that aid biofilm formation. In this study, we demonstrate that surface-associated streptococcal collagen-like protein-1 (Scl1) plays an important role in GAS biofilm formation.

Results

Biofilm formation by M1-, M3-, M28-, and M41-type GAS strains, representing an intraspecies breadth, were analyzed spectrophotometrically following crystal violet staining, and characterized using confocal and field emission scanning electron microscopy. The M41-type strain formed the most robust biofilm under static conditions, followed by M28- and M1-type strains, while the M3-type strains analyzed here did not form biofilm under the same experimental conditions. Differences in architecture and cell-surface morphology were observed in biofilms formed by the M1- and M41-wild-type strains, accompanied by varying amounts of deposited extracellular matrix and differences in cell-to-cell junctions within each biofilm. Importantly, all Scl1-negative mutants examined showed significantly decreased ability to form biofilm in vitro. Furthermore, the Scl1 protein expressed on the surface of a heterologous host, Lactococcus lactis, was sufficient to induce biofilm formation by this organism.

Conclusions

Overall, this work (i) identifies variations in biofilm formation capacity among pathogenically different GAS strains, (ii) identifies GAS surface properties that may aid in biofilm stability and, (iii) establishes that the Scl1 surface protein is an important determinant of GAS biofilm, which is sufficient to enable biofilm formation in the heterologous host Lactococcus. In summary, the GAS surface adhesin Scl1 may have an important role in biofilm-associated pathogenicity.

Functional analysis of the quorum-sensing streptococcal invasion locus (sil)

Group A streptococcus (GAS) causes a wide variety of human diseases, and at the same time, GAS can also circulate without producing symptoms, similar to its close commensal relative, group G streptococcus (GGS). We previously identified, by transposon-tagged mutagenesis, the streptococcal invasion locus (sil). sil is a quorum-sensing regulated locus which is activated by the autoinducer peptide SilCR through the two-component system SilA-SilB. Here we characterize the DNA promoter region necessary for SilA-mediated activation. This site is composed of two direct repeats of 10 bp, separated by a spacer of 11 bp. Fusion of this site to gfp allowed us to systematically introduce single-base substitutions in the repeats region and to assess the relative contribution of various positions to promoter strength. We then developed an algorithm giving different weights to these positions, and performed a chromosome-wide bioinformatics search which was validated by transcriptome analysis. We identified 13 genes, mostly bacteriocin related, that are directly under the control of SilA. Having developed the ability to quantify SilCR signaling via GFP accumulation prompted us to search for GAS and GGS strains that sense and produce SilCR. While the majority of GAS strains lost sil, all GGS strains examined still possess the locus and ∼63% are able to respond to exogenously added SilCR. By triggering the autoinduction circle using a minute concentration of synthetic SilCR, we identified GAS and GGS strains that are capable of sensing and naturally producing SilCR, and showed that SilCR can be sensed across these streptococci species. These findings suggest that sil may be involved in colonization and establishment of commensal host-bacterial relationships.

Cell-to-cell communication in bacteria is termed quorum-sensing (QS), which is triggered by signaling molecules called autoinducers. In streptococci, autoinducers are synthesized as immature peptides that are processed, secreted, and then sensed by two-component systems (TCSs). As a result, the autoinducer's own expression is upregulated (autoinduction), subsequently creating an ultrasensitive switch that turns on more genes. Group A streptococcus (GAS) is a human pathogen that causes many infections, including necrotizing fasciitis (NF). Previously, we identified in a NF GAS strain a QS locus termed streptococcal invasion locus (sil). Due to a mutation in the autoinducer peptide-SilCR, it is not produced by this strain. Here we sought to better explore sil and to examine if SilCR can be produced by other GAS strains, or strains of its close relative group G streptococcus (GGS). To this end, we characterized the DNA promoter region responsible for the TCS-mediated activation upon sensing of SilCR, and based on bioinformatics and transcriptome analyses we identified genes that are directly affected by the autoinducer peptide. By converting SilCR response to fluorescence production and turning on the autoinduction circle with minute concentrations of synthetic SilCR, we discovered naturally SilCR-producing GAS and GGS strains, and showed that SilCR can be sensed across these species. Our study describes a novel way of cell-to-cell communications among streptococci.

Role of Mga in group A streptococcal infection at the skin epithelium

Group A streptococci (GAS) primarily cause infection at epithelial tissue sites of its human host. The role of the transcriptional regulator Mga in a humanized mouse model for superficial skin infection was investigated. Inactivation of mga in a skin strain (Alab49) led to loss of virulence. The Deltamga mutant displayed >100-fold decrease in emm (pam) transcript levels, and loss of bacterial-bound plasmin activity. A slight decrease in speB transcription, accompanied by a partial decrease in cysteine protease activity but no change in PrtF2 degradation, was also observed. Mga had no effect on transcription of nra, Nra-regulated pilus genes (cpa, fctA) or other FCT-region genes (msmR, prtF2). Combined with findings on other Alab49 mutants, data show that several essential virulence genes are regulated by Mga or Nra, but not both, implying that any coordinated response during skin infection likely operates at a higher level of transcriptional control. Mga was required for bacterial autoaggregation and biofilm-like growth on an abiotic surface; however, aggregation and biofilm formation have only partial overlap with the skin virulence phenotype. Findings on numerous phenotypes for 7 mutants constructed on the same genetic background yield a detailed, integrated model for GAS pathogenesis at the skin.

RivR and the small RNA RivX: the missing links between the CovR regulatory cascade and the Mga regulon

The CovR/S two-component system regulates the transcription of many genes that are crucial for the virulence of Streptococcus pyogenes (group A Streptococcus, GAS). Previously, we demonstrated that one gene repressed directly by CovR is rivR, which encodes a member of the RofA-like family of transcriptional regulators. In this study, we deleted rivR and its downstream gene rivX in a DeltacovR background. Microarray analysis revealed that the products of the rivRX locus exert positive control over the transcription of members of the Mga regulon. Using mutational analysis, we established that rivX encodes a small regulatory RNA. We found that RivR enhances transcriptional activation by Mga in vivo and in vitro. An M1 DeltacovRDeltarivRX strain is attenuated for virulence in a murine model of invasive soft tissue infection and this attenuation is complemented by rivRX expressed from a plasmid, demonstrating the importance of the rivRX locus in pathogenesis. This study provides the first link between the CovR and Mga regulatory networks. By integrating the signals received through these two global regulators, GAS is able to select from its repertoire different combinations of specific virulence factors to express in response to a broad spectrum of environmental conditions.

The catabolite control protein CcpA binds to Pmga and influences expression of the virulence regulator Mga in the Group A streptococcus

Carbon catabolite repression (CCR) allows bacteria to alter metabolism in response to the availability of specific sugar sources, and increasing evidence suggests that CCR is involved in regulating virulence gene expression in many pathogens. A scan of the M1 SF370 group A streptococcus (GAS) genome using a Bacillus subtilis consensus identified a number of potential catabolite-responsive elements (cre) important for binding by the catabolite control protein A (CcpA), a mediator of CCR in gram-positive bacteria. Intriguingly, a putative cre was identified in the promoter region of mga upstream of its distal P1 start of transcription. Electrophoretic mobility shift assays showed that a His-CcpA fusion protein was capable of binding specifically to the cre in Pmga in vitro. Deletion analysis of Pmga using single-copy Pmga-gusA reporter strains found that Pmga P1 and its upstream cre were not required for normal autoregulated mga expression from Pmga P2 as long as Mga was produced from its native locus. In fact, the Pmga P1 region appeared to show a negative influence on Pmga P2 in these studies. However, deletion of the cre at the native Pmga resulted in a reduction of total mga transcripts as determined by real-time reverse transcription-PCR, supporting a role for CcpA in initial expression. Furthermore, normal transcriptional initiation from the Pmga P1 start site alone was dependent on the presence of the cre. Importantly, inactivation of ccpA in the M6 GAS strain JRS4 resulted in a reduction in Pmga expression and Mga protein levels in late-logarithmic-phase cell growth. These data support a role for CcpA in the early activation of the mga promoter and establish a link between CCR and Mga regulation in the GAS.

Scl1-dependent internalization of group A Streptococcus via direct interactions with the alpha2beta(1) integrin enhances pathogen survival and re-emergence

The molecular pathogenesis of infections caused by group A Streptococcus (GAS) is not fully understood. We recently reported that a recombinant protein derived from the collagen-like surface protein, Scl1, bound to the human collagen receptor, integrin alpha(2)beta(1). Here, we investigate whether the same Scl1 variant expressed by GAS cells interacts with the integrin alpha2beta(1) and affects the biological outcome of host-pathogen interactions. We demonstrate that GAS adherence and internalization involve direct interactions between surface expressed Scl1 and the alpha2beta(1) integrin, because (i) both adherence and internalization of the scl1-inactivated mutant were significantly decreased, and were restored by in-trans complementation of Scl1 expression, (ii) GAS internalization was reduced by pre-treatment of HEp-2 cells with anti-alpha2 integrin-subunit antibody and type I collagen, (iii) recombinant alpha2-I domain bound the wild-type GAS cells and (iv) internalization of wild-type cells was significantly increased in C2C12 cells expressing the alpha2beta(1) integrin as the only collagen-binding integrin. Next, we determined that internalized GAS re-emerges from epithelial cells into the extracellular environment. Taken together, our data describe a new molecular mechanism used by GAS involving the direct interaction between Scl1 and integrins, which increases the overall capability of the pathogen to survive and re-emerge.

How phosphotransferase system-related protein phosphorylation regulates carbohydrate metabolism in bacteria

The phosphoenolpyruvate(PEP):carbohydrate phosphotransferase system (PTS) is found only in bacteria, where it catalyzes the transport and phosphorylation of numerous monosaccharides, disaccharides, amino sugars, polyols, and other sugar derivatives. To carry out its catalytic function in sugar transport and phosphorylation, the PTS uses PEP as an energy source and phosphoryl donor. The phosphoryl group of PEP is usually transferred via four distinct proteins (domains) to the transported sugar bound to the respective membrane component(s) (EIIC and EIID) of the PTS. The organization of the PTS as a four-step phosphoryl transfer system, in which all P derivatives exhibit similar energy (phosphorylation occurs at histidyl or cysteyl residues), is surprising, as a single protein (or domain) coupling energy transfer and sugar phosphorylation would be sufficient for PTS function. A possible explanation for the complexity of the PTS was provided by the discovery that the PTS also carries out numerous regulatory functions. Depending on their phosphorylation state, the four proteins (domains) forming the PTS phosphorylation cascade (EI, HPr, EIIA, and EIIB) can phosphorylate or interact with numerous non-PTS proteins and thereby regulate their activity. In addition, in certain bacteria, one of the PTS components (HPr) is phosphorylated by ATP at a seryl residue, which increases the complexity of PTS-mediated regulation. In this review, we try to summarize the known protein phosphorylation-related regulatory functions of the PTS. As we shall see, the PTS regulation network not only controls carbohydrate uptake and metabolism but also interferes with the utilization of nitrogen and phosphorus and the virulence of certain pathogens.

Defining the Mga regulon: comparative transcriptome analysis reveals both direct and indirect regulation by Mga in the group A streptococcus

The regulator Mga in the group A streptococcus (GAS) is known to directly activate several virulence genes important for colonization and immune evasion. Transcriptome analysis comparing two mga-1 serotypes (M1 SF370, M6 JRS4) and one mga-2 serotype (M4 GA40634) against their isogenic mga-inactivated strains uncovered a broader Mga regulon profile containing both activated and repressed genes with predicted functions primarily related to sugar metabolism. This was reflected in the altered abilities of M1 and M4 Mga mutants to grow in chemically defined media with a single sugar source compared with their wild-type counterparts. Although the M1 and M4 Mga profiles were similar, the M6 JRS4 was clearly distinct, even from other M6 strains. Real-time RT-PCR and Northern blots confirmed that established core Mga regulon genes directly activated by Mga (emm, scpA, sof, fba) exhibited the highest activation levels across all strains tested. Spy2036 encoding a cytosolic hypothetical protein was highly activated in all three serotypes and was called gene regulated by Mga (grm). Mga bound directly to Pgrm, which overlaps the Mga-regulated Psof in OF+ strains, suggesting that grm is part of the core Mga regulon and Mga is able to activate divergently transcribed genes from a single site. Furthermore, Mga activated speB when detectable in the wild-type strain, although direct binding of Mga to PspeB could not be demonstrated. Thus, Mga is able to both directly and indirectly regulate genes shown to be important for virulence and the metabolic homeostasis of GAS.

Binding of the low-density lipoprotein by streptococcal collagen-like protein Scl1 of Streptococcus pyogenes

Several bacterial genera express proteins that contain collagen-like regions, which are associated with variable (V) non-collagenous regions. The streptococcal collagen-like proteins, Scl1 and Scl2, of group A Streptococcus (GAS) are members of this 'prokaryotic collagen' family, and they too contain an amino-terminal non-collagenous V region of unknown function. Here, we use recombinant rScl constructs, derived from several Scl1 and Scl2 variants, and affinity chromatography to identify Scl ligands present in human plasma. First, we show that Scl1, but not Scl2, proteins from different GAS serotypes bind the same ligand identified as apolipoprotein B (ApoB100), which is a major component of the low-density lipoprotein (LDL). Scl1 binding to purified ApoB100 and LDL is specific and concentration-dependent. Furthermore, the non-collagenous V region of the Scl1 protein is responsible for LDL/ApoB100 binding because only those rScls, constructed by domain swapping, which contain the V region from Scl1 proteins, were able to bind to ApoB100 and LDL ligands, and this binding was inhibited by antibodies directed against the Scl1-V region. Electron microscopy images of Scl1-LDL complexes showed that the globular V domain of Scl1 interacted with spherical particles of LDL. Importantly, live M28-type GAS cells absorbed plasma LDL on the cell surface and this binding depended on the surface expression of the Scl1.28, but not Scl2.28, protein. Phylogenetic analysis showed that the non-collagenous globular domains of Scl1 and Scl2 evolved independently to form separate lineages, which differ in amino acid sequence, and these differences may account for the variations in binding patterns of Scl1 and Scl2 proteins. Present studies provide insight into the structure-function relationship of the Scl proteins and also underline the importance of lipoprotein binding by GAS.

Mga is sufficient to activate transcription in vitro of sof-sfbX and other Mga-regulated virulence genes in the group A Streptococcus

The group A streptococcus (GAS), or Streptococcus pyogenes, is a strict human pathogen of medical significance, causing infections ranging from pharyngitis (strep throat) to necrotizing fasciitis (flesh-eating disease). Several virulence genes that encode factors important for colonization, internalization, and immune evasion are under the control of the multiple gene regulator of the GAS, or Mga. Mga functions as a DNA-binding protein that interacts with sites both proximal (Pemm and PscpA) and distal (PsclA) to the start of transcription for the genes that it regulates. The genes encoding serum opacity factor, sof, and a novel fibronectin-binding protein, sfbX, are cotranscribed and represent two uncharacterized Mga-regulated virulence genes in the GAS. Analysis of the promoter region of sof-sfbX identified a putative Mga-binding site 278 bp upstream of the regulated start of transcription as determined by primer extension. Electrophoretic mobility shift assays demonstrated that Mga is able to bind specifically to the single distal site in a fashion similar to the previously characterized PsclA. In order to better understand the events that take place at this and other Mga-regulated promoters, an in vitro transcription assay was established. Using this assay, we showed that Mga is sufficient to activate transcription in vitro for Mga-regulated promoters containing both proximal (Pemm) and distal (PsclA and Psof-sfbX) binding sites. These results indicate that additional factors are not required for Mga-specific activation at diverse promoters in vitro, although they do not rule out the potential influence of other components on the Mga virulence regulon in vivo.

Domains required for transcriptional activation show conservation in the mga family of virulence gene regulators

Mga, or the multigene regulator of the group A streptococcus (GAS) (Streptococcus pyogenes), is a transcriptional regulator of virulence genes important for colonization and immune evasion. All serotypes of the GAS possess one of two divergent mga alleles (mga-1 or mga-2), and orthologues of Mga have also been identified in other pathogenic streptococci. To date, the only functional motifs established within Mga are two amino-terminal DNA-binding domains (HTH-3 and HTH-4). To uncover novel domains, a random mutagenesis screen using an M6 Mga (mga-1) was undertaken to find mutations leading to a defect in transcriptional activation of the Mga-regulated emm gene. In addition to mutations in the established DNA-binding domains, the screen also revealed mutations in a region conserved among several Mga orthologues. Alanine scanning helped resolve the boundaries of this conserved Mga domain (CMD-1) spanning from residues 10 to 15 of the protein, with the two flanking amino acid residues likely involved in protein stability. Transcriptional reporter analyses demonstrated the importance of CMD-1 for activation of Pemm and autoactivation of Pmga in the serotype M6 Mga. Mutational analyses showed that both CMD-1 and HTH-4 are also necessary for activation of the promoter target Pmrp in a divergent serotype M4 Mga (mga-2), suggesting a conserved functionality. However, in contrast to M6, the M4 Mga mutants did not show a defect in autoregulation. Mutation of similar conserved residues in the Mga-like regulator DmgB from S. dysgalactiae subsp. dysgalactiae showed that CMD-1 and HTH-4 are critical for transcriptional activation in this orthologue, implying that a common mechanism of virulence gene activation may exist for members of the Mga family of regulators.

Anaerobic cells of Bacillus cereus F4430/73 respond to low oxidoreduction potential by metabolic readjustments and activation of enterotoxin expression

In the present study, a food-borne pathogen strain of Bacillus cereus (F4430/73) was anaerobically grown in controlled-batch conditions under low initial oxidoreduction potential (ORP=-148 mV) using hydrogen gas as reducing agent. Its physiological characteristics, including growth, glucose fermentation capacity and enterotoxin production, were compared with anaerobic conditions generated by nitrogen gas (ORP=+ 45 mV). The results showed that low ORP affected growth mainly during the early stages. Maximal specific rates of growth and glucose consumption were reduced, and drastic changes in time profiles of fermentation product concentration were observed. Production of lactate was promoted at the expense of acetate. Nevertheless, low ORP did not affect final biomass yield. Under both ORP conditions, Non-haemolytic enterotoxin (Nhe) was produced early during the exponential growth phase as a first enterotoxin and Haemolysin BL (Hbl) later during the early stationary growth phase as a second enterotoxin. The major effect of low ORP was the strong stimulation of Hbl production and, to a lesser extent, Nhe production. This control was complex, involving different levels of regulation. We discussed the regulation of enterotoxin expression and the involvement of the pleiotropic regulator PlcR.

P-Ser-HPr—a link between carbon metabolism and the virulence of some pathogenic bacteria

HPr kinase/phosphorylase phosphorylates HPr, a phosphocarrier protein of the phosphoenolpyruvate:carbohydrate phosphotransferase system, at serine-46. P-Ser-HPr is the central regulator of carbon metabolism in Gram-positive bacteria, but also plays a role in virulence development of certain pathogens. In Listeria monocytogenes, several virulence genes, which depend on the transcription activator PrfA, are repressed by glucose, fructose, etc., in a catabolite repressor (CcpA)-independent mechanism. However, the catabolite co-repressor P-Ser-HPr was found to inhibit the activity of PrfA. In an hprKV267F mutant, in which most of the HPr is transformed into P-Ser-HPr, PrfA was barely active. The ptsH1 mutation (Ser-46 of HPr replaced with an alanine) prevented the inhibitory effect of the hprKV267F mutation. Interestingly, disruption of ccpA also inhibited PrfA activity. This effect is probably also mediated via P-Ser-HPr, since ccpA disruption leads to elevated amounts of P-Ser-HPr. Indeed, a ccpA ptsH1 double mutant exhibited normal PrfA activity. In S. pyogenes, the expression of several virulence genes depends on the transcription activator Mga. Interestingly, the mga promoter is preceded by an operator site, which serves as target for the CcpA/P-Ser-HPr complex. Numerous Gram-negative pathogens also contain hprK, which is often organised in an operon with transcription regulators necessary for the development of virulence, indicating that in these organisms P-Ser-HPr also plays a role in pathogenesis. Indeed, inactivation of Neisseria meningitidis hprK strongly diminished cell adhesion of this pathogen.

Identification of residues responsible for the defective virulence gene regulator Mga produced by a natural mutant of Streptococcus pyogenes

Mga is a transcriptional regulator in the pathogen Streptococcus pyogenes that positively activates several important virulence genes involved in colonization and immune evasion in the human host. A naturally occurring mutant of Mga that is defective in its ability to activate transcription has been identified in the serotype M50 strain B514-Sm. Sequence alignment of the defective M50 Mga with the fully functional Mga from serotypes M4 and M49 revealed only three amino acid changes that might result in a defective protein. Electrophoretic mobility shift assays using purified M50 and M4 maltose binding protein-Mga found that both exhibited DNA-binding activity towards regulated promoters. Thus, the significance of each residue for the functionality of M50 Mga was explored through introduction of "gain-of-function" mutations based on M4 Mga. Transcriptional studies of the mutant alleles under both constitutive (PrpsL) and autoactivated (Pmga4) promoters illustrated that an arginine-to-methionine change at position 461 of M50 Mga protein fully restored activation of downstream genes. Western blot analyses of steady-state Mga levels suggest that the M461 residue may play a role in overall conformation and protein stability of Mga. However, despite the conservation of the M461 protein among all other Mga proteins, it does not appear to be necessary for activity in a divergent M6 Mga. These studies highlight the potential differences that exist between divergent Mga proteins in this important human pathogen.

MsmR, a specific positive regulator of the Streptococcus pyogenes FCT pathogenicity region and cytolysin-mediated translocation system genes

As a prerequisite for colonization or causing local infections, Streptococcus pyogenes (group A streptococci, GAS) need to specifically adhere to eukaryotic cell surfaces. Predominantly responsible adhesin genes are contained in a genotype-specific pattern within the FCT region of the GAS genome. In this study, MsmR, belonging to AraC/XylS type transcriptional regulators, was identified in the FCT region as a positive regulator of the major fibronectin-binding adhesin protein F2 in a serotype M49 strain. Compared with the wild-type strain, the msmR mutant showed reduced binding to immobilized fibronectin and decreased adherence to and internalization into human pharyngeal epithelial cells. These results suggested that altered levels of fibronectin-binding proteins in the mutant affect eukaryotic cell attachment and internalization. Complete transcriptome and reporter fusion assay data revealed that MsmR positively regulates FCT region genes including Nra and cytolysin-mediated translocation system genes. Consistent with the genetic data, the mutant showed attenuated streptolysin O activity and eukaryotic cell cytotoxity. Direct binding of recombinant MsmR to nga, nra/cpa and prtF2 promoter regions was confirmed by EMSA assays. As prior analysis demonstrated the Nra regulator negatively affects gene expression from the FCT region, MsmR and Nra appear to adversely control crucial virulence factor expression in GAS and thus contribute to a fine-tuned balance between local destructive process and metastatic spreading of the bacteria.