An amino-terminal signal peptide of Vfr protein negatively influences RopB-dependent SpeB expression and attenuates virulence in Streptococcus pyogenes

The leaderless communication peptide (LCP) class of quorum-sensing peptides is broadly distributed among Firmicutes

The human pathogen Streptococcus pyogenes secretes a short peptide (leaderless communication peptide, LCP) that mediates intercellular communication and controls bacterial virulence through interaction with its receptor, RopB. Here, we show that LCP and RopB homologues are present in other Firmicutes. We experimentally validate that LCPs with distinct peptide communication codes act as bacterial intercellular signals and regulate gene expression in Streptococcus salivarius, Streptococcus porcinus, Enterococcus malodoratus and Limosilactobacillus reuteri. Our results indicate that LCPs are more widespread than previously thought, and their characterization may uncover new signaling mechanisms and roles in coordinating diverse bacterial traits.

RopB represses the transcription of speB in the absence of SIP in group A Streptococcus

RopB is a quorum-sensing regulator that binds to the SpeB-inducing peptide (SIP) under acidic conditions. SIP is known to be degraded by the endopeptidase PepO, whose transcription is repressed by the CovR/CovS two-component regulatory system. Both SIP-bound RopB (RopB-SIP) and SIP-free RopB (apo-RopB) can bind to the speB promoter; however, only RopB-SIP activates speB transcription. In this study, we found that the SpeB expression was higher in the ropB mutant than in the SIP-inactivated (SIP*) mutant. Furthermore, the deletion of ropB in the SIP* mutant derepressed speB expression, suggesting that apo-RopB is a transcriptional repressor of speB Up-regulation of PepO in the covS mutant degraded SIP, resulting in the down-regulation of speB We demonstrate that deleting ropB in the covS mutant derepressed the speB expression, suggesting that the speB repression in this mutant was mediated not only by PepO-dependent SIP degradation but also by apo-RopB. These findings reveal a crosstalk between the CovR/CovS and RopB-SIP systems and redefine the role of RopB in regulating speB expression in group A Streptococcus.

Signaling Peptide SpoV Is Essential for Streptococcus pyogenes Virulence, and Prophylaxis with Anti-SpoV Decreases Disease Severity

Streptococcal peptide of virulence (SpoV) is a Streptococcus pyogenes (group A streptococcus (GAS))-specific peptide that is important for GAS survival in murine blood, and the expression of the virulence factors streptolysin O (slo) and streptolysin S (sagA). We used a spoV mutant in isolate MGAS315 to assess the contribution of the SpoV peptide to virulence by using a murine model of invasive disease and an ex vivo human model (Lancefield assay). We then used antibodies to SpoV in both models to evaluate their ability to decrease morbidity and mortality. Results showed that SpoV is essential for GAS virulence, and targeting the peptide has therapeutic potential.

The Streptococcus pyogenes signaling peptide SpoV regulates streptolysin O and enhances survival in murine blood

Streptococcus pyogenes (Group A Streptococcus, GAS) is a human pathogen that causes a wide range of diseases. For successful colonization within a variety of host niches, GAS must sense and respond to environmental changes. Intercellular communication mediated by peptides is one way GAS coordinates gene expression in response to diverse environmental stressors, which enhances bacterial survival and contributes to virulence. Using peptidomics we identified SpoV (Streptococcal peptide controlling virulence) in culture supernatant fluids. SpoV is a secreted peptide encoded near the gene encoding the extracellular cholesterol-dependent cytolysin streptolysin O (slo) The addition of synthetic SpoV peptide derivatives, but not control peptides, increased slo transcript abundance in an M49 isolate but not in an M3 isolate. Deletion of spoV decreased slo transcript abundance, extracellular SLO protein levels, and SLO-specific hemolytic activity. Complementation of the spoV mutant increased slo transcript abundance. Lastly, a spoV mutant was deficient in the ability to survive in murine blood compared to the parental strain. Moreover, pre-incubation of the spoV mutant with synthetic SpoV peptide derivatives increased GAS survival. Our findings show that slo expression is regulated, in part, by the GAS-specific signaling peptide SpoV.IMPORTANCEGAS secretes signaling peptides that can alter gene expression and impact virulence. We used peptidomics to identify a signaling peptide designated SpoV. Further, we showed that SpoV altered the expression of the cholesterol-dependent cytolysin SLO. Peptide signaling plays an important regulatory role during disease progression among several bacterial pathogens, including GAS. The therapeutic potential of manipulating peptide-controlled regulatory networks is an attractive option for the development of novel therapeutic strategies that disrupt virulence gene expression.

In silico characterisation of stand-alone response regulators of Streptococcus pyogenes

Bacterial “stand-alone” response regulators (RRs) are pivotal to the control of gene transcription in response to changing cytosolic and extracellular microenvironments during infection. The genome of group A Streptococcus (GAS) encodes more than 30 stand-alone RRs that orchestrate the expression of virulence factors involved in infecting multiple tissues, so causing an array of potentially lethal human diseases. Here, we analysed the molecular epidemiology and biological associations in the coding sequences (CDSs) and upstream intergenic regions (IGRs) of 35 stand-alone RRs from a collection of global GAS genomes. Of the 944 genomes analysed, 97% encoded 32 or more of the 35 tested RRs. The length of RR CDSs ranged from 297 to 1587 nucleotides with an average nucleotide diversity (π) of 0.012, while the IGRs ranged from 51 to 666 nucleotides with average π of 0.017. We present new evidence of recombination in multiple RRs including mga, leading to mga-2 switching, emm-switching and emm-like gene chimerization, and the first instance of an isolate that encodes both mga-1 and mga-2. Recombination was also evident in rofA/nra and msmR loci with 15 emm-types represented in multiple FCT (fibronectin-binding, collagen-binding, T-antigen)-types, including novel emm-type/FCT-type pairings. Strong associations were observed between concatenated RR allele types, and emm-type, MLST-type, core genome phylogroup, and country of sampling. No strong associations were observed between individual loci and disease outcome. We propose that 11 RRs may form part of future refinement of GAS typing systems that reflect core genome evolutionary associations. This subgenomic analysis revealed allelic traits that were informative to the biological function, GAS strain definition, and regional outbreak detection.

Environmental pH and peptide signaling control virulence of Streptococcus pyogenes via a quorum-sensing pathway

Bacteria control gene expression in concert with their population density by a process called quorum sensing, which is modulated by bacterial chemical signals and environmental factors. In the human pathogen Streptococcus pyogenes, production of secreted virulence factor SpeB is controlled by a quorum-sensing pathway and environmental pH. The quorum-sensing pathway consists of a secreted leaderless peptide signal (SIP), and its cognate receptor RopB. Here, we report that the SIP quorum-sensing pathway has a pH-sensing mechanism operative through a pH-sensitive histidine switch located at the base of the SIP-binding pocket of RopB. Environmental acidification induces protonation of His144 and reorganization of hydrogen bonding networks in RopB, which facilitates SIP recognition. The convergence of two disparate signals in the SIP signaling pathway results in induction of SpeB production and increased bacterial virulence. Our findings provide a model for investigating analogous crosstalk in other microorganisms.

Turning off Bacillus cereus quorum sensing system with peptidic analogs

We explored quenching of the PlcR-PapR quorum-sensing system in Bacillus cereus. We generated PapR7-peptidic derivatives that inhibit this system and thus the production of virulence factors, reflected by a loss in hemolytic activity, without affecting bacterial growth. To our knowledge, these peptides represent the first potent synthetic inhibitors of quorum-sensing in B. cereus.

Signaling by a Conserved Quorum Sensing Pathway Contributes to Growth Ex Vivo and Oropharyngeal Colonization of Human Pathogen Group A Streptococcus

Bacterial virulence factor production is a highly coordinated process. The temporal pattern of bacterial gene expression varies in different host anatomic sites to overcome niche-specific challenges. The human pathogen group A streptococcus (GAS) produces a potent secreted protease, SpeB, that is crucial for pathogenesis. Recently, we discovered that a quorum sensing pathway comprised of a leaderless short peptide, SpeB-inducing peptide (SIP), and a cytosolic global regulator, RopB, controls speB expression in concert with bacterial population density. The SIP signaling pathway is active in vivo and contributes significantly to GAS invasive infections. In the current study, we investigated the role of the SIP signaling pathway in GAS-host interactions during oropharyngeal colonization. The SIP signaling pathway is functional during growth ex vivo in human saliva. SIP-mediated speB expression plays a crucial role in GAS colonization of the mouse oropharynx. GAS employs a distinct pattern of SpeB production during growth ex vivo in saliva that includes a transient burst of speB expression during early stages of growth coupled with sustained levels of secreted SpeB protein. SpeB production aids GAS survival by degrading LL37, an abundant human antimicrobial peptide. We found that SIP signaling occurs during growth in human blood ex vivo. Moreover, the SIP signaling pathway is critical for GAS survival in blood. SIP-dependent speB regulation is functional in strains of diverse emm types, indicating that SIP signaling is a conserved virulence regulatory mechanism. Our discoveries have implications for future translational studies.

Endopeptidase PepO Regulates the SpeB Cysteine Protease and Is Essential for the Virulence of Invasive M1T1 Streptococcus pyogenes

Streptococcus pyogenes (group A Streptococcus [GAS]) causes a wide range of human infections. The pathogenesis of GAS infections is dependent on the temporal expression of numerous secreted and surface-associated virulence factors that interact with host proteins. Streptococcal pyrogenic exotoxin B (SpeB) is one of the most extensively studied toxins produced by GAS, and the coordinate growth phase-dependent regulation of speB expression is linked to disease severity phenotypes. Here, we identified the endopeptidase PepO as a novel growth phase-dependent regulator of SpeB in the invasive GAS M1 serotype strain 5448. By using transcriptomics followed by quantitative reverse transcriptase PCR and Western blot analyses, we demonstrate through targeted mutagenesis that PepO influences growth phase-dependent induction of speB gene expression. Compared to wild-type and complemented mutant strains, we demonstrate that the 5448ΔpepO mutant strain is more susceptible to killing by human neutrophils and is attenuated in virulence in a murine model of invasive GAS infection. Our results expand the complex regulatory network that is operating in GAS to control SpeB production and suggest that PepO is a virulence requirement during GAS M1T1 strain 5448 infections.IMPORTANCE Despite the continuing susceptibility of S. pyogenes to penicillin, this bacterial pathogen remains a leading infectious cause of global morbidity and mortality. A particular subclone of the M1 serotype (M1T1) has persisted globally for decades as the most frequently isolated serotype from patients with invasive and noninvasive diseases in Western countries. One of the key GAS pathogenicity factors is the potent broad-spectrum cysteine protease SpeB. Although there has been extensive research interest on the regulatory mechanisms that control speB gene expression, its genetic regulation is not fully understood. Here, we identify the endopeptidase PepO as a new regulator of speB gene expression in the globally disseminated M1T1 clone and as being essential for virulence.

A novel chemical inducer of Streptococcus quorum sensing acts by inhibiting the pheromone-degrading endopeptidase PepO

Bacteria produce chemical signals (pheromones) to coordinate behaviors across a population in a process termed quorum sensing (QS). QS systems comprising peptide pheromones and their corresponding Rgg receptors are widespread among Firmicutes and may be useful targets for manipulating microbial behaviors, like suppressing virulence. The Rgg2/3 QS circuit of the human pathogen Streptococcus pyogenes controls genes affecting resistance to host lysozyme in response to short hydrophobic pheromones (SHPs). Considering that artificial activation of a QS pathway may be as useful in the objective of manipulating bacteria as inhibiting it, we sought to identify small-molecule inducers of the Rgg2/3 QS system. We report the identification of a small molecule, P516-0475, that specifically induced expression of Rgg2/3-regulated genes in the presence of SHP pheromones at concentrations lower than typically required for QS induction. In searching for the mode of action of P516-0475, we discovered that an S. pyogenes mutant deficient in pepO, a neprilysin-like metalloendopeptidase that degrades SHP pheromones, was unresponsive to the compound. P516-0475 directly inhibited recombinant PepO in vitro as an uncompetitive inhibitor. We conclude that this compound induces QS by stabilizing SHP pheromones in culture. Our study indicates the usefulness of cell-based screens that modulate pathway activities to identify unanticipated therapeutic targets contributing to QS signaling.

Leaderless secreted peptide signaling molecule alters global gene expression and increases virulence of a human bacterial pathogen

Successful pathogens use complex signaling mechanisms to monitor their environment and reprogram global gene expression during specific stages of infection. Group A Streptococcus (GAS) is a major human pathogen that causes significant disease burden worldwide. A secreted cysteine protease known as streptococcal pyrogenic exotoxin B (SpeB) is a key virulence factor that is produced abundantly during infection and is critical for GAS pathogenesis. Although identified nearly a century ago, the molecular basis for growth phase control of speB gene expression remains unknown. We have discovered that GAS uses a previously unknown peptide-mediated intercellular signaling system to control SpeB production, alter global gene expression, and enhance virulence. GAS produces an eight-amino acid leaderless peptide [SpeB-inducing peptide (SIP)] during high cell density and uses the secreted peptide for cell-to-cell signaling to induce population-wide speB expression. The SIP signaling pathway includes peptide secretion, reimportation into the cytosol, and interaction with the intracellular global gene regulator Regulator of Protease B (RopB), resulting in SIP-dependent modulation of DNA binding and regulatory activity of RopB. Notably, SIP signaling causes differential expression of ∼14% of GAS core genes. Several genes that encode toxins and other virulence genes that enhance pathogen dissemination and infection are significantly up-regulated. Using three mouse infection models, we show that the SIP signaling pathway is active during infection and contributes significantly to GAS pathogenesis at multiple host anatomic sites. Together, our results delineate the molecular mechanisms involved in a previously undescribed virulence regulatory pathway of an important human pathogen and suggest new therapeutic strategies.

Streptococcus pyogenes adhesion and colonization

Streptococcus pyogenes (group A Streptococcus, GAS) is a human-adapted pathogen responsible for a wide spectrum of disease. GAS can cause relatively mild illnesses, such as strep throat or impetigo, and less frequent but severe life-threatening diseases such as necrotizing fasciitis and streptococcal toxic shock syndrome. GAS is an important public health problem causing significant morbidity and mortality worldwide. The main route of GAS transmission between humans is through close or direct physical contact, and particularly via respiratory droplets. The upper respiratory tract and skin are major reservoirs for GAS infections. The ability of GAS to establish an infection in the new host at these anatomical sites primarily results from two distinct physiological processes, namely bacterial adhesion and colonization. These fundamental aspects of pathogenesis rely upon a variety of GAS virulence factors, which are usually under strict transcriptional regulation. Considerable progress has been made in better understanding these initial infection steps. This review summarizes our current knowledge of the molecular mechanisms of GAS adhesion and colonization.

Structural Mechanisms of Peptide Recognition and Allosteric Modulation of Gene Regulation by the RRNPP Family of Quorum-Sensing Regulators

The members of RRNPP family of bacterial regulators sense population density-specific secreted oligopeptides and modulate the expression of genes involved in cellular processes, such as sporulation, competence, virulence, biofilm formation, conjugative plasmid transfer and antibiotic resistance. Signaling by RRNPP regulators include several steps: generation and secretion of the signaling oligopeptides, re-internalization of the signaling molecules into the cytoplasm, signal sensing by the cytosolic RRNPP regulators, signal-specific allosteric structural changes in the regulators, and interaction of the regulators with their respective regulatory target and gene regulation. The recently determined structures of the RRNPP regulators provide insight into the mechanistic aspects for several steps in this signaling circuit. In this review, we discuss the structural principles underlying peptide specificity, regulatory target recognition, and ligand-induced allostery in RRNPP regulators and its impact on gene regulation. Despite the conserved tertiary structure of these regulators, structural analyses revealed unexpected diversity in the mechanism of activation and molecular strategies that couple the peptide-induced allostery to gene regulation. Although these structural studies provide a sophisticated understanding of gene regulation by RRNPP regulators, much needs to be learned regarding the target DNA binding by yet-to-be characterized RNPP regulators and the several aspects of signaling by Rgg regulators.

Structural and functional analysis of RopB: a major virulence regulator in Streptococcus pyogenes

Group A Streptococcus (GAS) is an exclusive human pathogen that causes significant disease burden. Global regulator RopB of GAS controls the expression of several major virulence factors including secreted protease SpeB during high cell density. However, the molecular mechanism for RopB-dependent speB expression remains unclear. To understand the mechanism of transcription activation by RopB, we determined the crystal structure of the C-terminal domain of RopB. RopB-CTD has the TPR motif, a signature motif involved in protein-peptide interactions and shares significant structural homology with the quorum sensing RRNPP family regulators. Characterization of the high cell density-specific cell-free growth medium demonstrated the presence of a low molecular weight proteinaceous secreted factor that upregulates RopB-dependent speB expression. Together, these results suggest that RopB and its cognate peptide signals constitute an intercellular signalling machinery that controls the virulence gene expression in concert with population density. Structure-guided mutational analyses of RopB dimer interface demonstrated that single alanine substitutions at this critical interface significantly altered RopB-dependent speB expression and attenuated GAS virulence. Results presented here suggested that a properly aligned RopB dimer interface is important for GAS pathogenesis and highlighted the dimerization interactions as a plausible therapeutic target for the development of novel antimicrobials.

Induction of a quorum sensing pathway by environmental signals enhances group A streptococcal resistance to lysozyme

The human-restricted pathogen Streptococcus pyogenes (Group A Streptococcus, GAS) is responsible for wide-ranging pathologies at numerous sites in the body but has the proclivity to proliferate in individuals asymptomatically. The ability to survive in diverse tissues is undoubtedly benefited by sensory pathways that recognize environmental cues corresponding to stress and nutrient availability and thereby trigger adaptive responses. We investigated the impact that environmental signals contribute to cell-to-cell chemical communication [quorum sensing (QS)] by monitoring activity of the Rgg2/Rgg3 and SHP-pheromone system in GAS. We identified metal limitation and the alternate carbon source mannose as two environmental indicators likely to be encountered by GAS in the host that significantly induced the Rgg-SHP system. Disruption of the metal regulator MtsR partially accounted for the response to metal depletion, whereas ptsABCD was primarily responsible for QS induction due to mannose, but each sensory system induced Rgg-SHP signaling apparently by different mechanisms. Significantly, we found that induction of QS, regardless of the GAS serotype tested, led to enhanced resistance to the antimicrobial agent lysozyme. These results indicate the benefits for GAS to integrate environmental signals with intercellular communication pathways in protection from host defenses.

Phosphorylation events in the multiple gene regulator of group A Streptococcus significantly influence global gene expression and virulence

Whole-genome sequencing analysis of ∼800 strains of group A Streptococcus (GAS) found that the gene encoding the multiple virulence gene regulator of GAS (mga) is highly polymorphic in serotype M59 strains but not in strains of other serotypes. To help understand the molecular mechanism of gene regulation by Mga and its contribution to GAS pathogenesis in serotype M59 GAS, we constructed an isogenic mga mutant strain. Transcriptome studies indicated a significant regulatory influence of Mga and altered metabolic capabilities conferred by Mga-regulated genes. We assessed the phosphorylation status of Mga in GAS cell lysates with Phos-tag gels. The results revealed that Mga is phosphorylated at histidines in vivo. Using phosphomimetic and nonphosphomimetic substitutions at conserved phosphoenolpyruvate:carbohydrate phosphotransferase regulation domain (PRD) histidines of Mga, we demonstrated that phosphorylation-mimicking aspartate replacements at H207 and H273 of PRD-1 and at H327 of PRD-2 are inhibitory to Mga-dependent gene expression. Conversely, non-phosphorylation-mimicking alanine substitutions at H273 and H327 relieved inhibition, and the mutant strains exhibited a wild-type phenotype. The opposing regulatory profiles observed for phosphorylation- and non-phosphorylation-mimicking substitutions at H273 extended to global gene regulation by Mga. Consistent with these observations, the H273D mutant strain attenuated GAS virulence, whereas the H273A strain exhibited a wild-type virulence phenotype in a mouse model of necrotizing fasciitis. Together, our results demonstrate phosphoregulation of Mga and its direct link to virulence in M59 GAS strains. These data also lay a foundation toward understanding how naturally occurring gain-of-function variations in mga, such as H201R, may confer an advantage to the pathogen and contribute to M59 GAS pathogenesis.

An ABC transporter is required for secretion of peptide sex pheromones in Enterococcus faecalis

Enterococci are leading causes of hospital-acquired infection in the United States and continue to develop resistances to new antibiotics. Many Enterococcus faecalis isolates harbor pheromone-responsive plasmids that mediate horizontal transfer of even large blocks of chromosomal genes, resulting in hospital-adapted strains over a quarter of whose genomes consist of mobile elements. Pheromones to which the donor cells respond derive from lipoprotein signal peptides. Using a novel bacterial killing assay dependent on the presence of sex pheromones, we screened a transposon mutant library for functions that relate to the production and/or activity of the effector pheromone. Here we describe a previously uncharacterized, but well-conserved, ABC transporter that contributes to pheromone production. Using three distinct pheromone-dependent mating systems, we show that mutants defective in expressing this transporter display a 5- to 6-order-of-magnitude reduction in conjugation efficiency. In addition, we demonstrate that the ABC transporter mutant displays an altered biofilm architecture, with a significant reduction in biofilm biomass compared to that of its isogenic parent, suggesting that pheromone activity also influences biofilm development. The conservation of this peptide transporter across the Firmicutes suggests that it may also play an important role in cell-cell communication in other species within this important phylum.

Enterococcus faecalis ranks as one of the leading causes of hospital-associated infections. Strains possessing resistance to multiple antibiotics are becoming all too common in clinical settings. Pheromone-responsive plasmids play an important role in harboring and disseminating these antibiotic resistance genes. Here we have identified a novel ABC transporter that is responsible for the secretion of peptide pheromones, which enables communication between cells to mediate plasmid transfer. We have also shown that this transporter is important for biofilm formation, providing a strong rationale for its use as a viable therapeutic target which could be targeted to curb infection, as well as the spread of existing drug resistance.

Quorum sensing in group A Streptococcus

Quorum sensing (QS) is a widespread phenomenon in the microbial world that has important implications in the coordination of population-wide responses in several bacterial pathogens. In Group A Streptococcus (GAS), many questions surrounding QS systems remain to be solved pertaining to their function and their contribution to the GAS lifestyle in the host. The QS systems of GAS described to date can be categorized into four groups: regulator gene of glucosyltransferase (Rgg), Sil, lantibiotic systems, and LuxS/AI-2. The Rgg family of proteins, a conserved group of transcription factors that modify their activity in response to signaling peptides, has been shown to regulate genes involved in virulence, biofilm formation and competence. The sil locus, whose expression is regulated by the activity of signaling peptides and a putative two-component system (TCS), has been implicated on regulating genes involved with invasive disease in GAS isolates. Lantibiotic regulatory systems are involved in the production of bacteriocins and their autoregulation, and some of these genes have been shown to target both bacterial organisms as well as processes of survival inside the infected host. Finally AI-2 (dihydroxy pentanedione, DPD), synthesized by the LuxS enzyme in several bacteria including GAS, has been proposed to be a universal bacterial communication molecule. In this review we discuss the mechanisms of these four systems, the putative functions of their targets, and pose critical questions for future studies.

Peptide conversations in Gram-positive bacteria

Within Gram-positive bacteria, the expression of target genes is controlled at the population level via signaling peptides, also known as pheromones. Pheromones control a wide range of functions, including competence, virulence, and others that remain unknown. Until now, their role in bacterial gene regulation has probably been underestimated; indeed, bacteria are able to produce, by ribosomal synthesis or surface protein degradation, an extraordinary variety of peptides which are released outside bacteria and among which, some are pheromones that mediate cell-to-cell communication. The review aims at giving an updated overview of these peptide-dependant communication pathways. More specifically, it follows the whole peptide circuit from the peptide production and secretion in the extracellular medium to its interaction with sensors at bacterial surface or re-import into the bacteria where it plays its regulation role. In recent years, as we have accumulated more knowledge about these systems, it has become apparent that they are more complex than they first appeared. For this reason, more research on peptide-dependant pathways is needed to develop new strategies for controlling functions of interest in Gram-positive bacteria. In particular, such research could lead to alternatives to the use of antibiotics against pathogenic bacteria. In perspective, the review identifies new research questions that emerge in this field and that have to be addressed.

Peptide pheromone signaling in Streptococcus and Enterococcus

Intercellular chemical signaling in bacteria, commonly referred to as quorum sensing (QS), relies on the production and detection of compounds known as pheromones to elicit coordinated responses among members of a community. Pheromones produced by Gram-positive bacteria are comprised of small peptides. Based on both peptide structure and sensory system architectures, Gram-positive bacterial signaling pathways may be classified into one of four groups with a defining hallmark: cyclical peptides of the Agr type, peptides that contain Gly-Gly processing motifs, sensory systems of the RNPP family, or the recently characterized Rgg-like regulatory family. The recent discovery that Rgg family members respond to peptide pheromones increases substantially the number of species in which QS is likely a key regulatory component. These pathways control a variety of fundamental behaviors including conjugation, natural competence for transformation, biofilm development, and virulence factor regulation. Overlapping QS pathways found in multiple species and pathways that utilize conserved peptide pheromones provide opportunities for interspecies communication. Here we review pheromone signaling identified in the genera Enterococcus and Streptococcus, providing examples of all four types of pathways.

Crystal structure of peroxide stress regulator from Streptococcus pyogenes provides functional insights into the mechanism of oxidative stress sensing

Regulation of oxidative stress responses by the peroxide stress regulator (PerR) is critical for the in vivo fitness and virulence of group A Streptococcus. To elucidate the molecular mechanism of DNA binding, peroxide sensing, and gene regulation by PerR, we performed biochemical and structural characterization of PerR. Sequence-specific DNA binding by PerR does not require regulatory metal occupancy. However, metal binding promotes higher affinity PerR-DNA interactions. PerR metallated with iron directly senses peroxide stress and dissociates from operator sequences. The crystal structure revealed that PerR exists as a homodimer with two metal-binding sites per subunit as follows: a structural zinc site and a regulatory metal site that is occupied in the crystals by nickel. The regulatory metal-binding site in PerR involves a previously unobserved HXH motif located in its unique N-terminal extension. Mutational analysis of the regulatory site showed that the PerR metal ligands are involved in regulatory metal binding, and integrity of this site is critical for group A Streptococcus virulence. Interestingly, the metal-binding HXH motif is not present in the structurally characterized members of ferric uptake regulator (Fur) family but is fully conserved among PerR from the genus Streptococcus. Thus, it is likely that the PerR orthologs from streptococci share a common mechanism of metal binding, peroxide sensing, and gene regulation that is different from that of well characterized PerR from Bacillus subtilis. Together, our findings provide key insights into the peroxide sensing and regulation of the oxidative stress-adaptive responses by the streptococcal subfamily of PerR.

Enterococcal Rgg-like regulator ElrR activates expression of the elrA operon

The Enterococcus faecalis leucine-rich protein ElrA promotes virulence by stimulating bacterial persistence in macrophages and production of the interleukin-6 (IL-6) cytokine. The ElrA protein is encoded within an operon that is poorly expressed under laboratory conditions but induced in vivo. In this study, we identify ef2687 (renamed elrR), which encodes a member of the Rgg (regulator gene for glucosyltransferase) family of putative regulatory proteins. Using quantitative reverse transcription-PCR, translational lacZ fusions, and electrophoretic mobility shift assays, we demonstrate that ElrR positively regulates expression of elrA. These results correlate with the attenuated virulence of the ΔelrR strain in a mouse peritonitis model. Virulence of simple and double elrR and elrA deletion mutants also suggests a remaining ElrR-independent expression of elrA in vivo and additional virulence-related genes controlled by ElrR.

Differential virulence gene expression of group A Streptococcus serotype M3 in response to co-culture with Moraxella catarrhalis

Streptococcus pyogenes (group A Streptococcus, GAS) and Moraxella catarrhalis are important colonizers and (opportunistic) pathogens of the human respiratory tract. However, current knowledge regarding colonization and pathogenic potential of these two pathogens is based on work involving single bacterial species, even though the interplay between respiratory bacterial species is increasingly important in niche occupation and the development of disease. Therefore, to further define and understand polymicrobial species interactions, we investigated whether gene expression (and hence virulence potential) of GAS would be affected upon co-culture with M. catarrhalis. For co-culture experiments, GAS and M. catarrhalis were cultured in Todd-Hewitt broth supplemented with 0.2% yeast extract (THY) at 37°C with 5% CO₂ aeration. Each strain was grown in triplicate so that triplicate experiments could be performed. Bacterial RNA was isolated, cDNA synthesized, and microarray transcriptome expression analysis performed. We observed significantly increased (≥4-fold) expression for genes playing a role in GAS virulence such as hyaluronan synthase (hasA), streptococcal mitogenic exotoxin Z (smeZ) and IgG endopeptidase (ideS). In contrast, significantly decreased (≥4-fold) expression was observed in genes involved in energy metabolism and in 12 conserved GAS two-component regulatory systems. This study provides the first evidence that M. catarrhalis increases GAS virulence gene expression during co-culture, and again shows the importance of polymicrobial infections in directing bacterial virulence.

Transcriptional regulation of a bacteriophage encoded extracellular DNase (Spd-3) by Rgg in Streptococcus pyogenes

The Streptococcus pyogenes transcriptional regulator Rgg controls the expression of virulence-associated genes encoded both within the core genome and within horizontally transmissible DNA such as temperate bacteriophage. Previously, we showed that Rgg binds to the non-coding DNA upstream of the bacteriophage gene encoding an extracellular DNase Spd-3. In the current study, we further characterized Rgg-mediated regulation of spd-3 expression. Two spd-3 transcripts were identified by northern blotting. The 5′ ends were 27 and 594 nucleotides upstream of the start codon as determined with primer extension analysis and 5′ RACE (rapid amplification of c-DNA ends), respectively. Results obtained with gel shift assays showed that purified Rgg bound specifically to non-coding DNA containing the promoters of both transcripts. Transcriptional fusion analyses confirmed the presence of Rgg-repressible promoters within these DNA regions. In addition, repression was associated with direct DNA binding by Rgg as determined with chromatin immunoprecipitation (ChIP) coupled with quantitative PCR (qPCR). The results show that the chromosomally encoded transcriptional regulator, Rgg, directly represses both bacteriophage promoters controlling the expression of Spd-3. The results provide new information regarding the regulation of prophage encoded virulence factors of S. pyogenes and highlight the complex evolutionary history of S. pyogenes and temperate bacteriophage.

Structural basis for the activation mechanism of the PlcR virulence regulator by the quorum-sensing signal peptide PapR

The quorum-sensing regulator PlcR is the master regulator of most known virulence factors in Bacillus cereus. It is a helix-turn-helix (HTH)-type transcription factor activated upon binding of its cognate signaling peptide PapR on a tetratricopeptide repeat-type regulatory domain. The structural and functional properties of PlcR have defined a new family of sensor regulators, called the RNPP family (for Rap, NprR, PrgX, and PlcR), in Gram-positive bacteria. To fully understand the activation mechanism of PlcR, we took a closer look at the conformation changes induced upon binding of PapR and of its target DNA, known as PlcR-box. For that purpose we have determined the structures of the apoform of PlcR (Apo PlcR) and of the ternary complex of PlcR with PapR and the PlcR-box from the plcA promoter. Comparison of the apoform of PlcR with the previously published structure of the PlcR-PapR binary complex shows how a small conformational change induced in the C-terminal region of the tetratricopeptide repeat (TPR) domain upon peptide binding propagates via the linker helix to the N-terminal HTH DNA-binding domain. Further comparison with the PlcR-PapR-DNA ternary complex shows how the activation of the PlcR dimer allows the linker helix to undergo a drastic conformational change and subsequent proper positioning of the HTH domains in the major groove of the two half sites of the pseudopalindromic PlcR-box. Together with random mutagenesis experiments and interaction measurements using peptides from distinct pherogroups, this structural analysis allows us to propose a molecular mechanism for this functional switch.

Cationic antimicrobial peptides disrupt the Streptococcus pyogenes ExPortal

Although they possess a well-characterized ability to porate the bacterial membrane, emerging research suggests that cationic antimicrobial peptides (CAPs) can influence pathogen behaviour at levels that are sublethal. In this study, we investigated the interaction of polymyxin B and human neutrophil peptide (HNP-1) with the human pathogen Streptococcus pyogenes. At sublethal concentrations, these CAPs preferentially targeted the ExPortal, a unique microdomain of the S. pyogenes membrane, specialized for protein secretion and processing. A consequence of this interaction was the disruption of ExPortal organization and a redistribution of ExPortal components into the peripheral membrane. Redistribution was associated with inhibition of secretion of certain toxins, including the SpeB cysteine protease and the streptolysin O (SLO) cytolysin, but not SIC, a protein that protects S. pyogenes from CAPs. These data suggest a novel function for CAPs in targeting the ExPortal and interfering with secretion of factors required for infection and survival. This mechanism may prove valuable for the design of new types of antimicrobial agents to combat the emergence of antibiotic-resistant pathogens.

Growth phase-dependent modulation of Rgg binding specificity in Streptococcus pyogenes

Streptococcus pyogenes Rgg is a transcriptional regulator that interacts with the cofactor LacD.1 to control growth phase-dependent expression of genes, including speB, which encodes a secreted cysteine protease. LacD.1 is thought to interact with Rgg when glycolytic intermediates are abundant in a manner that prevents Rgg-mediated activation of speB expression via binding to the promoter region. When the intermediates diminish, LacD.1 dissociates from Rgg and binds to the speB promoter to activate expression. The purpose of this study was to determine if Rgg bound to chromatin during the exponential phase of growth and, if so, to identify the binding sites. Rgg bound to 62 chromosomal sites, as determined by chromatin immunoprecipitation coupled with DNA microarrays. Thirty-eight were within noncoding DNA, including sites upstream of the genes encoding the M protein (M49), serum opacity factor (SOF), fibronectin-binding protein (SfbX49), and a prophage-encoded superantigen, SpeH. Each of these sites contained a promoter that was regulated by Rgg, as determined with transcriptional fusion assays. Purified Rgg also bound to the promoter regions of emm49, sof, and sfbX49 in vitro. Results obtained with a lacD.1 mutant showed that both LacD.1 and Rgg were necessary for the repression of emm49, sof, sfbX49, and speH expression. Overall, the results indicated that the DNA binding specificity of Rgg is responsive to environmental changes in a LacD.1-dependent manner and that Rgg and LacD.1 directly control virulence gene expression in the exponential phase of growth.

Dynamics of speB mRNA transcripts in Streptococcus pyogenes

Streptococcus pyogenes (group A streptococcus [GAS]) is a human-specific pathogen that causes a variety of diseases ranging from superficial infections to life-threatening diseases. SpeB, a potent extracellular cysteine proteinase, plays an important role in the pathogenesis of GAS infections. Previous studies show that SpeB expression and activity are controlled at the transcriptional and posttranslational levels, though it had been unclear whether speB was also regulated at the posttranscriptional level. In this study, we examined the growth phase-dependent speB mRNA level and decay using quantitative reverse transcription-PCR (qRT-PCR) and Northern blot analyses. We observed that speB mRNA accumulated rapidly during exponential growth, which occurred concomitantly with an increase in speB mRNA stability. A closer observation revealed that the increased speB mRNA stability was mainly due to progressive acidification. Inactivation of RNase Y, a recently identified endoribonuclease, revealed a role in processing and degradation of speB mRNA. We conclude that the increased speB mRNA stability contributes to the rapid accumulation of speB transcript during growth.