Role for a secreted cysteine proteinase in the establishment of host tissue tropism by group A streptococci

Streptococcus dysgalactiae subsp. equisimilis infection and its intersection with Streptococcus pyogenes

SUMMARYStreptococcus dysgalactiae subsp. equisimilis (SDSE) is an increasingly recognized cause of disease in humans. Disease manifestations range from non-invasive superficial skin and soft tissue infections to life-threatening streptococcal toxic shock syndrome and necrotizing fasciitis. Invasive disease is usually associated with co-morbidities, immunosuppression, and advancing age. The crude incidence of invasive disease approaches that of the closely related pathogen, Streptococcus pyogenes. Genomic epidemiology using whole-genome sequencing has revealed important insights into global SDSE population dynamics including emerging lineages and spread of anti-microbial resistance. It has also complemented observations of overlapping pathobiology between SDSE and S. pyogenes, including shared virulence factors and mobile gene content, potentially underlying shared pathogen phenotypes. This review provides an overview of the clinical and genomic epidemiology, disease manifestations, treatment, and virulence determinants of human infections with SDSE with a particular focus on its overlap with S. pyogenes. In doing so, we highlight the importance of understanding the overlap of SDSE and S. pyogenes to inform surveillance and disease control strategies.

Group A Streptococcus induces GSDMA-dependent pyroptosis in keratinocytes

Gasdermins (GSDMs) are a family of pore-forming effectors that permeabilize the cell membrane during the cell death program pyroptosis¹. GSDMs are activated by proteolytic removal of autoinhibitory carboxy-terminal domains, typically by caspase regulators^1-9. However, no activator is known for one member of this family, GSDMA. Here we show that the major human pathogen group A Streptococcus (GAS) secretes a protease virulence factor, SpeB, that induces GSDMA-dependent pyroptosis. SpeB cleavage of GSDMA releases an active amino-terminal fragment that can insert into membranes to form lytic pores. GSDMA is primarily expressed in the skin¹⁰, and keratinocytes infected with SpeB-expressing GAS die of GSDMA-dependent pyroptosis. Mice have three homologues of human GSDMA, and triple-knockout mice are more susceptible to invasive infection by a pandemic hypervirulent M1T1 clone of GAS. These results indicate that GSDMA is critical in the immune defence against invasive skin infections by GAS. Furthermore, they show that GSDMs can act independently of host regulators as direct sensors of exogenous proteases. As SpeB is essential for tissue invasion and survival within skin cells, these results suggest that GSDMA can act akin to a guard protein that directly detects concerning virulence activities of microorganisms that present a severe infectious threat.

Streptococcal pyrogenic exotoxin B cleaves GSDMA and triggers pyroptosis

Gasdermins, a family of five pore-forming proteins (GSDMA-GSDME) in humans expressed predominantly in the skin, mucosa and immune sentinel cells, are key executioners of inflammatory cell death (pyroptosis), which recruits immune cells to infection sites and promotes protective immunity1,2. Pore formation is triggered by gasdermin cleavage1,2. Although the proteases that activate GSDMB, C, D and E have been identified, how GSDMA-the dominant gasdermin in the skin-is activated, remains unknown. Streptococcus pyogenes, also known as group A Streptococcus (GAS), is a major skin pathogen that causes substantial morbidity and mortality worldwide3. Here we show that the GAS cysteine protease SpeB virulence factor triggers keratinocyte pyroptosis by cleaving GSDMA after Gln246, unleashing an active N-terminal fragment that triggers pyroptosis. Gsdma1 genetic deficiency blunts mouse immune responses to GAS, resulting in uncontrolled bacterial dissemination and death. GSDMA acts as both a sensor and substrate of GAS SpeB and as an effector to trigger pyroptosis, adding a simple one-molecule mechanism for host recognition and control of virulence of a dangerous microbial pathogen.

Unique virulence role of post-translocational chaperone PrsA in shaping Streptococcus pyogenes secretome

Streptococcus pyogenes (group A Streptococcus, GAS) is a strict human pathogen causing a broad spectrum of diseases and a variety of autoimmune sequelae. The pathogenesis of GAS infection mostly relies on the production of an extensive network of cell wall-associated and secreted virulence proteins, such as adhesins, toxins, and exoenzymes. PrsA, the only extracellular parvulin-type peptidyl-prolyl isomerase expressed ubiquitously in Gram-positive bacteria, has been suggested to assist the folding and maturation of newly exported proteins to acquire their native conformation and activity. Two PrsA proteins, PrsA1 and PrsA2, have been identified in GAS, but the respective contribution of each PrsA in GAS pathogenesis remains largely unknown. By combining comparative proteomic and phenotypic analysis approaches, we demonstrate that both PrsA isoforms are required to maintain GAS proteome homeostasis and virulence-associated traits in a unique and overlapping manner. The inactivation of both PrsA in GAS caused remarkable impairment in biofilm formation, host adherence, infection-induced cytotoxicity, and in vivo virulence in a murine soft tissue infection model. The concordance of proteomic and phenotypic data clearly features the essential role of PrsA in GAS full virulence.

Impetigo Animal Models: A Review of Their Feasibility and Clinical Utility for Therapeutic Appraisal of Investigational Drug Candidates

Impetigo (school sores), a superficial skin infection commonly seen in children, is caused by the gram-positive bacteria Staphylococcus aureus and/or Streptococcus pyogenes. Antibiotic treatments, often topical, are used as the first-line therapy for impetigo. The efficacy of potential new antimicrobial compounds is first tested in in vitro studies and, if effective, followed by in vivo studies using animal models and/or humans. Animal models are critical means for investigating potential therapeutics and characterizing their safety profile prior to human trials. Although several reviews of animal models for skin infections have been published, there is a lack of a comprehensive review of animal models simulating impetigo for the selection of therapeutic drug candidates. This review critically examines the existing animal models for impetigo and their feasibility for testing the in vivo efficacy of topical treatments for impetigo and other superficial bacterial skin infections.

The Streptococcal Protease SpeB Antagonizes the Biofilms of the Human Pathogen Staphylococcus aureus USA300 through Cleavage of the Staphylococcal SdrC Protein

Streptococcus pyogenes, or group A Streptococcus (GAS), is both a pathogen and an asymptomatic colonizer of human hosts and produces a large number of surface-expressed and secreted factors that contribute to a variety of infection outcomes. The GAS-secreted cysteine protease SpeB has been well studied for its effects on the human host; however, despite its broad proteolytic activity, studies on how this factor is utilized in polymicrobial environments are lacking. Here, we utilized various forms of SpeB protease to evaluate its antimicrobial and antibiofilm properties against the clinically important human colonizer Staphylococcus aureus, which occupies niches similar to those of GAS. For our investigation, we used a skin-tropic GAS strain, AP53CovS+, and its isogenic ΔspeB mutant to compare the production and activity of native SpeB protease. We also generated active and inactive forms of recombinant purified SpeB for functional studies. We demonstrate that SpeB exhibits potent biofilm disruption activity at multiple stages of S. aureus biofilm formation. We hypothesized that the surface-expressed adhesin SdrC in S. aureus was cleaved by SpeB, which contributed to the observed biofilm disruption. Indeed, we found that SpeB cleaved recombinant SdrC in vitro and in the context of the full S. aureus biofilm. Our results suggest an understudied role for the broadly proteolytic SpeB as an important factor for GAS colonization and competition with other microorganisms in its niche.IMPORTANCEStreptococcus pyogenes (GAS) causes a range of diseases in humans, ranging from mild to severe, and produces many virulence factors in order to be a successful pathogen. One factor produced by many GAS strains is the protease SpeB, which has been studied for its ability to cleave and degrade human proteins, an important factor in GAS pathogenesis. An understudied aspect of SpeB is the manner in which its broad proteolytic activity affects other microorganisms that co-occupy niches similar to that of GAS. The significance of the research reported herein is the demonstration that SpeB can degrade the biofilms of the human pathogen Staphylococcus aureus, which has important implications for how SpeB may be utilized by GAS to successfully compete in a polymicrobial environment.

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.

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.

Streptococcal Cysteine Protease-Mediated Cleavage of Desmogleins Is Involved in the Pathogenesis of Cutaneous Infection

Streptococcus pyogenes is responsible for a wide variety of cutaneous infections ranging from superficial impetigo to fulminant invasive necrotizing fasciitis. Dysfunction of desmosomes is associated with the pathogenesis of cutaneous diseases. We identified streptococcal pyrogenic exotoxin B (SpeB) as a proteolytic factor that cleaves the extracellular domains of desmoglein 1 and 3. In an epicutaneous infection model, lesional skin infected with an speB deletion mutant were significantly smaller as compared to those caused by the wild-type strain. Furthermore, immunohistological analysis indicated cleavage of desmogleins that developed around the invasion site of the wild-type strain. In contrast, the speB mutant was preferentially found on the epidermis surface layer. Taken together, our findings provide evidence that SpeB-mediated degradation of desmosomes has a pathogenic role in development of S. pyogenes cutaneous infection.

Incremental Contributions of FbaA and Other Impetigo-Associated Surface Proteins to Fitness and Virulence of a Classical Group A Streptococcal Skin Strain

Group A streptococci (GAS) are highly prevalent human pathogens whose primary ecological niche is the superficial epithelial layers of the throat and/or skin. Many GAS strains with a strong tendency to cause pharyngitis are distinct from strains that tend to cause impetigo; thus, genetic differences between them may confer host tissue-specific virulence. In this study, the FbaA surface protein gene was found to be present in most skin specialist strains but largely absent from a genetically related subset of pharyngitis isolates. In an ΔfbaA mutant constructed in the impetigo strain Alab49, loss of FbaA resulted in a slight but significant decrease in GAS fitness in a humanized mouse model of impetigo; the ΔfbaA mutant also exhibited decreased survival in whole human blood due to phagocytosis. In assays with highly sensitive outcome measures, Alab49ΔfbaA was compared to other isogenic mutants lacking virulence genes known to be disproportionately associated with classical skin strains. FbaA and PAM (i.e., the M53 protein) had additive effects in promoting GAS survival in whole blood. The pilus adhesin tip protein Cpa promoted Alab49 survival in whole blood and appears to fully account for the antiphagocytic effect attributable to pili. The finding that numerous skin strain-associated virulence factors make slight but significant contributions to virulence underscores the incremental contributions to fitness of individual surface protein genes and the multifactorial nature of GAS-host interactions.

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.

The detrimental impact of extracellular bacterial proteases on wound healing

In addition to clinical signs of infection (e.g. inflammation, purulence and pain), a microbial count of ≥10⁵ colony-forming units/g has historically been used to define wound infection. However, it is increasingly recognised that, rather than a high bioburden level alone being detrimental to wound healing, it is the virulence of the invading microorganism and the host's immune status that can affect clinical outcomes. Bacteria, such as Pseudomonas aeruginosa, Staphylococcus aureus and Staphylococcus epidermidis, have developed a range of virulence factors to help them overcome host defences and proliferate within the underlying soft tissue. More specifically, bacterial proteases are one such virulence factor that has been implicated in promoting the invasion and destruction of the host tissue. Because of the complexities of microorganisms, the proteases can negatively impact the wound environment, leading to delayed wound healing. The aim of the present paper is to describe various extracellular bacterial proteases; review the impact they have on the wound environment, the host immune response and biofilms; and discuss potential wound management strategies against them. The evidence discussed suggests that proteases may play a profound role in wound infections, contribute to the development of an inflammatory response and impede wound healing.

Differences in SpeB protease activity among group A streptococci associated with superficial, invasive, and autoimmune disease

The secreted cysteine proteinase SpeB is an important virulence factor of group A streptococci (GAS), whereby SpeB activity varies widely among strains. To establish the degree to which SpeB activity correlates with disease, GAS organisms were recovered from patients with pharyngitis, impetigo, invasive disease or acute rheumatic fever (ARF), and selected for analysis using rigorous sampling criteria; >300 GAS isolates were tested for SpeB activity by casein digestion assays, and each GAS isolate was scored as a SpeB-producer or non-producer. Highly significant statistical differences (p < 0.01) in SpeB production are observed between GAS recovered from patients with ARF (41.5% SpeB-non-producers) compared to pharyngitis (20.5%), invasive disease (16.7%), and impetigo (5.5%). SpeB activity differences between pharyngitis and impetigo isolates are also significant, whereas pharyngitis versus invasive isolates show no significant difference. The disproportionately greater number of SpeB-non-producers among ARF-associated isolates may indicate an altered transcriptional program for many rheumatogenic strains and/or a protective role for SpeB in GAS-triggered autoimmunity.

Preferential Acquisition and Activation of Plasminogen Glycoform II by PAM Positive Group A Streptococcal Isolates

Plasminogen (Plg) circulates in the host as two predominant glycoforms. Glycoform I Plg (GI-Plg) contains glycosylation sites at Asn289 and Thr346, whereas glycoform II Plg (GII-Plg) is exclusively glycosylated at Thr346. Surface plasmon resonance experiments demonstrated that Plg binding group A streptococcal M protein (PAM) exhibits comparative equal affinity for GI- and GII-Plg in the "closed" conformation (for GII-Plg, KD = 27.4 nM; for GI-Plg, KD = 37.0 nM). When Plg was in the "open" conformation, PAM exhibited an 11-fold increase in affinity for GII-Plg (KD = 2.8 nM) compared with that for GI-Plg (KD = 33.2 nM). The interaction of PAM with Plg is believed to be mediated by lysine binding sites within kringle (KR) 2 of Plg. PAM-GI-Plg interactions were fully inhibited with 100 mM lysine analogue ε-aminocaproic acid (εACA), whereas PAM-GII-Plg interactions were shown to be weakened but not inhibited in the presence of 400 mM εACA. In contrast, binding to the KR1-3 domains of GII-Plg (angiostatin) by PAM was completely inhibited in the presence 5 mM εACA. Along with PAM, emm pattern D GAS isolates express a phenotypically distinct SK variant (type 2b SK) that requires Plg ligands such as PAM to activate Plg. Type 2b SK was able to generate an active site and activate GII-Plg at a rate significantly higher than that of GI-Plg when bound to PAM. Taken together, these data suggest that GAS selectively recruits and activates GII-Plg. Furthermore, we propose that the interaction between PAM and Plg may be partially mediated by a secondary binding site outside of KR2, affected by glycosylation at Asn289.

Application of the C3-binding motif of streptococcal pyrogenic exotoxin B to protect mice from invasive group a streptococcal infection

Group A streptococcus (GAS) is an important human pathogen that produces several extracellular exotoxins to facilitate invasion and infection. Streptococcal pyrogenic exotoxin B (SPE B) has been demonstrated to be an important virulence factor of GAS. Our previous studies indicate that SPE B cleaves complement 3 (C3) and inhibits the activation of complement pathways. In this study, we constructed and expressed recombinant fragments of SPE B to examine the C3-binding site of SPE B. Using enzyme-linked immunosorbent assays and pull-down assays, we found that the C-terminal domain, containing amino-acid residues 345–398, of SPE B was the major binding site of human serum C3. We further identified a major, Ala³⁷⁶-Pro³⁹⁸, and a minor C3-binding motif, Gly³⁴⁶-Gly³⁶⁰, that both mediated the binding of C3 complement. Immunization with the C3-binding motifs protected mice against challenge with a lethal dose of non-invasive M49 strain GAS but not invasive M1 strains. To achieve higher efficiency against invasive M1 GAS infection, a combination of synthetic peptides derived from C-terminal epitope of streptolysin S (SLSpp) and from the major C3-binding motif of SPE B (PP6, Ala³⁷⁶-Pro³⁹⁸) was used to elicit specific immune response to those two important streptococcal exotoxins. Death rates and the severity of skin lesions decreased significantly in PP6/SLSpp-immunized mice that were infected with invasive M1 strains of GAS. These results indicate a combination of the C3-binding motif of SPE B and the protective epitope of SLS could be used as a subunit vaccine against invasive M1 strains group A streptococcal infection.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

Site-restricted plasminogen activation mediated by group A streptococcal streptokinase variants

By examining the roles of bacterial and host-derived cofactors in streptokinase-mediated plasminogen activation, we find that phenotypic streptokinase variation functionally underpins a pathogenic mechanism whereby streptokinase variants differentially focus plasminogen activation, leading to specific niche adaption within the host.

Determining antibody-binding site of streptococcal pyrogenic exotoxin B to protect mice from group a streptococcus infection

Streptococcal pyrogenic exotoxin B (SPE B), a cysteine protease, is an important virulence factor in group A streptococcal (GAS) infection. SPE B binds and cleaves antibody isotypes and further impairs the immune system by inhibiting complement activation. In this study, we examined the antibody-binding site of SPE B and used it to block SPE B actions during GAS infection. We constructed different segments of the spe B gene and induced them to express different recombinant fragments of SPE B. Using an enzyme-linked immunosorbent assay (ELISA), we found that residues 345–398 of the C-terminal domain of SPE B (rSPE B_345–398), but not the N-terminal domain, was the major binding site for antibody isotypes. Using a competitive ELISA, we also found that rSPE B_345–398 bound to the Fc portion of IgG. The in vitro functional assays indicate that rSPE B_345–398 not only interfered with cleavage of antibody isotypes but also interfered with SPE B-induced inhibition of complement activation. Immunization of BALB/c mice using rSPE B_345–398 was able to induce production of a high titer of anti-rSPE B_345–398 antibodies and efficiently protected mice from GAS-induced death. These findings suggest that SPE B uses its C-terminal domain to bind the Fc portion of IgG and that immunization of mice with this binding domain (rSPE B_345–398) could protect mice from GAS infection.

Cysteine proteinase SpeB from Streptococcus pyogenes - a potent modifier of immunologically important host and bacterial proteins

Group A streptococcus (Streptococcus pyogenes) is an exclusively human pathogen that causes a wide spectrum of diseases ranging from pharyngitis, to impetigo, to toxic shock, to necrotizing fasciitis. The diversity of these disease states necessitates that S. pyogenes possess the ability to modulate both the innate and adaptive immune responses. SpeB, a cysteine proteinase, is the predominant secreted protein from S. pyogenes. Because of its relatively indiscriminant specificity, this enzyme has been shown to degrade the extracellular matrix, cytokines, chemokines, complement components, immunoglobulins, and serum protease inhibitors, to name but a few of the known substrates. Additionally, SpeB regulates other streptococcal proteins by degrading them or releasing them from the bacterial surface. Despite the wealth of literature on putative SpeB functions, there remains much controversy about this enzyme because many of reported activities would produce contradictory physiological results. Here we review all known host and bacterial protein substrates for SpeB, their cleavage sites, and discuss the role of this enzyme in streptococcal pathogenesis based on the current literature.

Synergistic effects of streptolysin S and streptococcal pyrogenic exotoxin B on the mouse model of group A streptococcal infection

Streptococcus pyogenes is a group A streptococcus (GAS) and an important human pathogen that causes a variety of diseases. Streptococcal pyrogenic exotoxin B (SPE B) and streptolysin S (SLS) are important virulence factors involved in GAS infection, but it is not clear which one is more virulent. Using an air pouch infection model, the wild-type strain NZ131, its isogenic mutants, and complementary mutants were used to examine the effects of SPE B and SLS on GAS infection. The results of the skin lesion and mouse mortality assays showed that although SPE B and SLS had a synergistic effect on GAS infection, SPE B played a more important role in local tissue damage while SLS had a more prominent effect on mouse mortality. Surveys of the exudates from the air pouch revealed that the expression of inflammatory cytokines was significantly inhibited in the sagB/speB-double-mutant JM4-infected mice. Furthermore, in vivo and in vitro studies showed that the isogenic mutant strains were more susceptible to the immune cell killing than the wild-type strain and that the sagB/speB-double-mutant JM4 was the most sensitive among these strains. Moreover, infection with the sagB/speB-double-mutant JM4 strain caused the least amount of macrophage apoptosis compared to infection with the wild-type NZ131 and the other complementary strains, which express only SPE B or SLS or both. Taken together, these results indicate that both SPE B and SLS contributed to GAS evasion from immune cell killing, local tissue damage, and mouse mortality.

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

Streptococcal pyrogenic exotoxin B (SpeB) is an extracellular cysteine protease that is a critical virulence factor made by the major human pathogen group A Streptococcus (GAS). speB expression is dependent on the regulator of proteinase B (RopB) and is upregulated with increasing cell density and during infection. Because computer modelling suggested significant structural similarity between RopB and peptide-sensing regulatory proteins made by other Gram-positive bacteria, we hypothesized that speB expression is influenced by RopB-peptide interactions. Inactivation of the gene (vfr) encoding the virulence factor related (Vfr) protein resulted in increased speB transcript level during the exponential growth phase, whereas provision of only the amino-terminal region of Vfr comprising the secretion signal sequence in trans restored a wild-type speB expression profile. Addition of the culture supernatant from a Vfr signal peptide-expressing GAS strain restored wild-type speB transcript level to a vfr-inactivated isogenic mutant strain. A distinct peptide in the Vfr secretion signal sequence specifically bound to recombinant RopB. Finally, overexpression of the Vfr secretion signal sequence significantly decreased speB transcript level and attenuated GAS virulence in two mouse models of invasive infection. Taken together, these data delineate a previously unknown small peptide-mediated regulatory system that controls GAS virulence factor production.

Cysteine proteinase SpeB from Streptococcus pyogenes - a potent modifier of immunologically important host and bacterial proteins

Group A streptococcus (Streptococcus pyogenes) is an exclusively human pathogen that causes a wide spectrum of diseases ranging from pharyngitis, to impetigo, to toxic shock, to necrotizing fasciitis. The diversity of these disease states necessitates that S. pyogenes possess the ability to modulate both the innate and adaptive immune responses. SpeB, a cysteine proteinase, is the predominant secreted protein from S. pyogenes. Because of its relatively indiscriminant specificity, this enzyme has been shown to degrade the extracellular matrix, cytokines, chemokines, complement components, immunoglobulins, and serum protease inhibitors, to name but a few of the known substrates. Additionally, SpeB regulates other streptococcal proteins by degrading them or releasing them from the bacterial surface. Despite the wealth of literature on putative SpeB functions, there remains much controversy about this enzyme because many of reported activities would produce contradictory physiological results. Here we review all known host and bacterial protein substrates for SpeB, their cleavage sites, and discuss the role of this enzyme in streptococcal pathogenesis based on the current literature.

Molecular insight into invasive group A streptococcal disease

Streptococcus pyogenes is also known as group A Streptococcus (GAS) and is an important human pathogen that causes considerable morbidity and mortality worldwide. The GAS serotype M1T1 clone is the most frequently isolated serotype from life-threatening invasive (at a sterile site) infections, such as streptococcal toxic shock-like syndrome and necrotizing fasciitis. Here, we describe the virulence factors and newly discovered molecular events that mediate the in vivo changes from non-invasive GAS serotype M1T1 to the invasive phenotype, and review the invasive-disease trigger for non-M1 GAS. Understanding the molecular basis and mechanism of initiation for streptococcal invasive disease may expedite the discovery of novel therapeutic targets for the treatment and control of severe invasive GAS diseases.

Environmental pH changes, but not the LuxS signalling pathway, regulate SpeB expression in M1 group A streptococci

The autoinducer-2/LuxS signalling pathway participates in quorum sensing in diverse bacterial species. In group A streptococci (GAS), LuxS has been shown to be involved in regulating the expression of several important virulence factors. Streptococcal pyrogenic exotoxin B (SpeB), a cysteine protease that has important roles in GAS pathogenesis, is positively regulated by LuxS in M3 and M5 strains. In the present study, it was found that the supernatant harvested from an overnight culture stimulated M1 strains to express speB. However, mutation of the luxS gene in M1 strains or treating M1 strains with luxS mutant culture supernatant did not affect speB expression, indicating that the LuxS pathway is not involved in regulation of speB expression in M1 strains. In addition, the acid property of culture broth was found to be able to stimulate M1 strains to express speB in the same LuxS-independent manner. These results indicate that speB expression in M1 strains is induced by environmental pH changes but is not regulated by the LuxS signalling pathway.

Dispersal of Group A streptococcal biofilms by the cysteine protease SpeB leads to increased disease severity in a murine model

Group A Streptococcus (GAS) is a Gram-positive human pathogen best known for causing pharyngeal and mild skin infections. However, in the 1980's there was an increase in severe GAS infections including cellulitis and deeper tissue infections like necrotizing fasciitis. Particularly striking about this elevation in the incidence of severe disease was that those most often affected were previously healthy individuals. Several groups have shown that changes in gene content or regulation, as with proteases, may contribute to severe disease; yet strains harboring these proteases continue to cause mild disease as well. We and others have shown that group A streptococci (MGAS5005) reside within biofilms both in vitro and in vivo. That is to say that the organism colonizes a host surface and forms a 3-dimensional community encased in a protective matrix of extracellular protein, DNA and polysaccharide(s). However, the mechanism of assembly or dispersal of these structures is unclear, as is the relationship of these structures to disease outcome. Recently we reported that allelic replacement of the streptococcal regulator srv resulted in constitutive production of the streptococcal cysteine protease SpeB. We further showed that the constitutive production of SpeB significantly decreased MGAS5005Δsrv biofilm formation in vitro. Here we show that mice infected with MGAS5005Δsrv had significantly larger lesion development than wild-type infected animals. Histopathology, Gram-staining and immunofluorescence link the increased lesion development with lack of disease containment, lack of biofilm formation, and readily detectable levels of SpeB in the tissue. Treatment of MGAS5005Δsrv infected lesions with a chemical inhibitor of SpeB significantly reduced lesion formation and disease spread to wild-type levels. Furthermore, inactivation of speB in the MGAS5005Δsrv background reduced lesion formation to wild-type levels. Taken together, these data suggest a mechanism by which GAS disease may transition from mild to severe through the Srv mediated dispersal of GAS biofilms.

An insert in the covS gene distinguishes a pharyngeal and a blood isolate of Streptococcus pyogenes found in the same individual

Expression of the extensive arsenal of virulence factors by Streptococcus pyogenes is controlled by many regulators, of which CovRS is one of the best characterized and can influence ∼15 % of the genome. Animal models have established that mutants of covRS arise spontaneously in vivo resulting in highly invasive organisms. We analysed a pharyngeal and a blood isolate of S. pyogenes recovered from the same individual 13 days apart. The two isolates varied in many phenotypic properties including SpeB production, which were reflected in transcriptomic analyses. PFGE, multilocus sequence typing and partial sequencing of some key genes failed to show any differences except for an 11 bp insert in the covS gene in the blood isolate which caused a premature termination of transcription. Complementation of a fully functional covS gene into the blood isolate resulted in high expression of CovS and expression of speB. These results, showing a pharyngeal and a blood isolate from a single individual differing by a simple insertion, provide evidence for the model that regulatory gene mutations allow S. pyogenes to invade different niches in the body.

Tissue tropisms in group A streptococcal infections

Group A Streptococcus (GAS) is a human-specific pathogen that is highly prevalent throughout the world. The vast majority of GAS infections lead to a mild disease involving the epithelial surfaces of either the throat or skin. The concept of distinct sets of 'throat' and 'skin' strains of GAS has long been conceived. From an ecological standpoint, the epithelium of the throat and skin are important because it is where the organism is most successful in reproducing and transmitting to new hosts. This article examines key features of the epidemiology, population biology and molecular pathogenesis that underlie the tissue site preferences for infection exhibited by GAS, with an emphasis on work from our laboratory on skin tropisms. Recombinational replacement with orthologous gene forms, following interspecies transfer, appears to be an important genetic step leading up to the exploitation of new niches by GAS.

Decreased necrotizing fasciitis capacity caused by a single nucleotide mutation that alters a multiple gene virulence axis

Single-nucleotide changes are the most common cause of natural genetic variation among members of the same species, but there is remarkably little information bearing on how they alter bacterial virulence. We recently discovered a single-nucleotide mutation in the group A Streptococcus genome that is epidemiologically associated with decreased human necrotizing fasciitis ("flesh-eating disease"). Working from this clinical observation, we find that wild-type mtsR function is required for group A Streptococcus to cause necrotizing fasciitis in mice and nonhuman primates. Expression microarray analysis revealed that mtsR inactivation results in overexpression of PrsA, a chaperonin involved in posttranslational maturation of SpeB, an extracellular cysteine protease. Isogenic mutant strains that overexpress prsA or lack speB had decreased secreted protease activity in vivo and recapitulated the necrotizing fasciitis-negative phenotype of the DeltamtsR mutant strain in mice and monkeys. mtsR inactivation results in increased PrsA expression, which in turn causes decreased SpeB secreted protease activity and reduced necrotizing fasciitis capacity. Thus, a naturally occurring single-nucleotide mutation dramatically alters virulence by dysregulating a multiple gene virulence axis. Our discovery has broad implications for the confluence of population genomics and molecular pathogenesis research.

A decade of molecular pathogenomic analysis of group A Streptococcus

Molecular pathogenomic analysis of the human bacterial pathogen group A Streptococcus has been conducted for a decade. Much has been learned as a consequence of the confluence of low-cost DNA sequencing, microarray technology, high-throughput proteomics, and enhanced bioinformatics. These technical advances, coupled with the availability of unique bacterial strain collections, have facilitated a systems biology investigative strategy designed to enhance and accelerate our understanding of disease processes. Here, we provide examples of the progress made by exploiting an integrated genome-wide research platform to gain new insight into molecular pathogenesis. The studies have provided many new avenues for basic and translational research.

Population biology of the human restricted pathogen, Streptococcus pyogenes

Streptococcus pyogenes, also referred to as beta-hemolytic group A streptococci, are strictly human pathogens with a global distribution and high prevalence of infection. The organisms are characterized by high levels of genetic recombination, extensive strain diversity, and a narrow habitat. This review highlights many key features of the population genetics and molecular epidemiology of this biologically diverse bacterial species, with special emphasis on ecological subdivisions and tissue-specific infections, strain diversity and population dynamics in communities, selection pressures arising from the specific host immune response and antibiotic exposure, and within-host selection during the course of invasive disease.

SpeB of Streptococcus pyogenes differentially modulates antibacterial and receptor activating properties of human chemokines

Background

CXC chemokines are induced by inflammatory stimuli in epithelial cells and some, like MIG/CXCL9, IP–10/CXCL10 and I–TAC/CXCL11, are antibacterial for Streptococcus pyogenes.

Methodology/Principal Findings

SpeB from S. pyogenes degrades a wide range of chemokines (i.e. IP10/CXCL10, I-TAC/CXCL11, PF4/CXCL4, GROα/CXCL1, GROβ/CXCL2, GROγ/CXCL3, ENA78/CXCL5, GCP-2/CXCL6, NAP-2/CXCL7, SDF-1/CXCL12, BCA-1/CXCL13, BRAK/CXCL14, SRPSOX/CXCL16, MIP-3α/CCL20, Lymphotactin/XCL1, and Fractalkine/CX3CL1), has no activity on IL-8/CXCL8 and RANTES/CCL5, partly degrades SRPSOX/CXCL16 and MIP-3α/CCL20, and releases a 6 kDa CXCL9 fragment. CXCL10 and CXCL11 loose receptor activating and antibacterial activities, while the CXCL9 fragment does not activate the receptor CXCR3 but retains its antibacterial activity.

Conclusions/Significance

SpeB destroys most of the signaling and antibacterial properties of chemokines expressed by an inflamed epithelium. The exception is CXCL9 that preserves its antibacterial activity after hydrolysis, emphasizing its role as a major antimicrobial on inflamed epithelium.

A naturally occurring mutation in ropB suppresses SpeB expression and reduces M1T1 group A streptococcal systemic virulence

Epidemiological studies of group A streptococcus (GAS) have noted an inverse relationship between SpeB expression and invasive disease. However, the role of SpeB in the course of infection is still unclear. In this study we utilize a SpeB-negative M1T1 clinical isolate, 5628, with a naturally occurring mutation in the gene encoding the regulator RopB, to elucidate the role of RopB and SpeB in systemic virulence. Allelic exchange mutagenesis was used to replace the mutated ropB allele in 5628 with the intact allele from the well characterized isolate 5448. The inverse allelic exchange was also performed to replace the intact ropB in 5448 with the mutated allele from 5628. An intact ropB was found to be essential for SpeB expression. While the ropB mutation was shown to have no effect on hemolysis of RBC's, extracellular DNase activity or survival in the presence of neutrophils, strains with the mutated ropB allele were less virulent in murine systemic models of infection. An isogenic SpeB knockout strain containing an intact RopB showed similarly reduced virulence. Microarray analysis found genes of the SpeB operon to be the primary target of RopB regulation. These data show that an intact RopB and efficient SpeB production are necessary for systemic infection with GAS.

Vaccine based on a ubiquitous cysteinyl protease and streptococcal pyrogenic exotoxin A protects against Streptococcus pyogenes sepsis and toxic shock

Background

The gram-positive bacterium Streptococcus pyogenes is a common pathogen of humans that causes invasive infections, toxic-shock syndrome, rheumatic fever, necrotizing fasciitis and other diseases. Detection of antibiotic resistance in clinical isolates has renewed interest in development of new vaccine approaches for control S. pyogenes sepsis. In the study presented, a novel protein vaccine was examined. The vaccine was based on a recombinant protein fusion between streptococcal pyrogenic exotoxin B (SpeB), a cysteinyl protease expressed by all clinical isolates, and streptococcal pyrogenic exotoxin A (SpeA), a superantigen produced by a large subset of isolates.

Results

A novel protein was produced by mutating the catalytic site of SpeB and the receptor binding surface of SpeA in a fusion of the two polypeptides. Vaccination of HLA-DQ8 transgenic mice with the SpeA-SpeB fusion protein protected against a challenge with the wild-type SpeA that was lethal to naïve controls, and vaccinated mice were protected from an otherwise lethal S. pyogenes infection.

Conclusion

These results suggest that the genetically attenuated SpeA-SpeB fusion protein may be useful for controlling S. pyogenes infections. Vaccination with the SpeA-SpeB fusion protein described in this study may potentially result in protective immunity against multiple isolates of S. pyogenes due to the extensive antibody cross-reactivity previously observed among all sequence variants of SpeB and the high frequency of SpeA-producing strains.

Impact of orthologous gene replacement on the circuitry governing pilus gene transcription in streptococci

Background

The evolutionary history of several genes of the bacterial pathogen Streptococcus pyogenes strongly suggests an origin in another species, acquired via replacement of the counterpart gene (ortholog) following a recombination event. An example of orthologous gene replacement is provided by the nra/rofA locus, which encodes a key regulator of pilus gene transcription. Of biological importance is the previous finding that the presence of the nra- and rofA-lineage alleles, which are ∼35% divergent, correlates strongly with genetic markers for streptococcal infection at different tissue sites in the human host (skin, throat).

Methodology/Principal Findings

In this report, the impact of orthologous gene replacement targeting the nra/rofA locus is experimentally addressed. Replacement of the native nra-lineage allele with a rofA-lineage allele, plus their respective upstream regions, preserved the polarity of Nra effects on pilus gene transcription (i.e., activation) in the skin strain Alab49. Increased pilus gene transcription in the rofA chimera correlated with a higher rate of bacterial growth at the skin. The transcriptional regulator MsmR, which represses nra and pilus gene transcription in the Alab49 parent strain, has a slight activating effect on pilus gene expression in the rofA chimera construct.

Conclusions/Significance

Data show that exchange of orthologous forms of a regulatory gene is stable and robust, and pathogenicity is preserved. Yet, new phenotypes may also be introduced by altering the circuitry within a complex transcriptional regulatory network. It is proposed that orthologous gene replacement via interspecies exchange is an important mechanism in the evolution of highly recombining bacteria such as S. pyogenes.

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.

Molecular mechanisms underlying group A streptococcal pathogenesis

Group A Streptococcus (GAS) is a versatile human pathogen causing diseases ranging from uncomplicated mucosal infections to life-threatening invasive disease. The development of human-relevant animal models of GAS infection and introduction of new technologies have markedly accelerated the pace of discoveries related to GAS host-pathogen interactions. For example, recently investigators have identified pili on the GAS cell surface and learned that they are key components for adherence to eukaryotic cell surfaces. Similarly, the recent development of a transgenic mouse expressing human plasminogen has resulted in new understanding of the molecular processes contributing to invasive infection. Improved understanding of the molecular mechanisms underlying the pathogenesis of GAS pharyngeal, invasive and other infections holds the promise of assisting with the development of novel preventive or therapeutic agents for this prevalent human pathogen.

Heterogeneity in the polarity of Nra regulatory effects on streptococcal pilus gene transcription and virulence

Transcription of several key virulence factors of Streptococcus pyogenes is under the control of Mga and Nra/RofA. In an M serotype 49 (M49) strain, Nra is a negative regulator of pilus gene transcription; also, Nra represses mga expression, leading to downregulation of the M protein surface fibril and secreted cysteine protease SpeB. In this report, the role of Nra in the virulence of an M53 classical skin strain was investigated. In contrast to the case for the M49 strain, Nra functions as a positive regulator of pilus gene transcription in the M53 strain, and inactivation of nra leads to loss of virulence in a humanized mouse model of superficial skin infection. Furthermore, Nra has no measurable effect on mga transcription in the M53 strain; this finding is further supported by a lack of detectable Nra effects on M protein- and SpeB-dependent phenotypes. Whereas MsmR is reported to activate nra and pilus gene transcription in the M49 strain, in the M53 strain it acts as a repressor of these genes. In both strains, MsmR and Nra form a feed-forward loop network motif for pilus gene transcription, but their effects have opposite signs. The findings demonstrate key strain-specific differences in the transcriptional circuitry governing virulence gene expression in S. pyogenes and its impact on pathogenesis.

EndoS from Streptococcus pyogenes is hydrolyzed by the cysteine proteinase SpeB and requires glutamic acid 235 and tryptophans for IgG glycan-hydrolyzing activity

Background

The endoglycosidase EndoS and the cysteine proteinase SpeB from the human pathogen Streptococcus pyogenes are functionally related in that they both hydrolyze IgG leading to impairment of opsonizing antibodies and thus enhance bacterial survival in human blood. In this study, we further investigated the relationship between EndoS and SpeB by examining their in vitro temporal production and stability and activity of EndoS. Furthermore, theoretical structure modeling of EndoS combined with site-directed mutagenesis and chemical blocking of amino acids was used to identify amino acids required for the IgG glycan-hydrolyzing activity of EndoS.

Results

We could show that during growth in vitro S. pyogenes secretes the IgG glycan-hydrolyzing endoglycosidase EndoS prior to the cysteine proteinase SpeB. Upon maturation SpeB hydrolyzes EndoS that then loses its IgG glycan-hydrolyzing activity. Sequence analysis and structural homology modeling of EndoS provided a basis for further analysis of the prerequisites for IgG glycan-hydrolysis. Site-directed mutagenesis and chemical modification of amino acids revealed that glutamic acid 235 is an essential catalytic residue, and that tryptophan residues, but not the abundant lysine or the single cysteine residues, are important for EndoS activity.

Conclusion

We present novel information about the amino acid requirements for IgG glycan-hydrolyzing activity of the immunomodulating enzyme EndoS. Furthermore, we show that the cysteine proteinase SpeB processes/degrades EndoS and thus emphasize the importance of the SpeB as a degrading/processing enzyme of proteins from the bacterium itself.

New understanding of the group A Streptococcus pathogenesis cycle

Group A Streptococcus (GAS) has long been recognized as a human pathogen causing an exceptionally broad range of infections. Despite intense research, however, the molecular mechanisms of GAS disease remain unclear. Recently, many important discoveries have been made that shed light on GAS pathogenesis and open exciting avenues for future research. Advances in genome sequencing, microarray technology and proteomic analysis, in combination with the development of more suitable animal models, have markedly increased our knowledge of the mechanisms underlying GAS pathogenesis. The information gained from these studies will translate into improved diagnostics and new targets for therapeutic drugs and vaccines.

Analysis of the transcriptome of group A Streptococcus in mouse soft tissue infection

Molecular mechanisms mediating group A Streptococcus (GAS)-host interactions remain poorly understood but are crucial for diagnostic, therapeutic, and vaccine development. An optimized high-density microarray was used to analyze the transcriptome of GAS during experimental mouse soft tissue infection. The transcriptome of a wild-type serotype M1 GAS strain and an isogenic transcriptional regulator knockout mutant (covR) also were compared. Array datasets were verified by quantitative real-time reverse transcriptase-polymerase chain reaction and in situ immunohistochemistry. The results unambiguously demonstrate that coordinated expression of proven and putative GAS virulence factors is directed toward overwhelming innate host defenses leading to severe cellular damage. We also identified adaptive metabolic responses triggered by nutrient signals and hypoxic/acidic conditions in the host, likely facilitating pathogen persistence and proliferation in soft tissues. Key discoveries included that oxidative stress genes, virulence genes, genes related to amino acid and maltodextrin utilization, and several two-component transcriptional regulators were highly expressed in vivo. This study is the first global analysis of the GAS transcriptome during invasive infection. Coupled with parallel analysis of the covR mutant strain, novel insights have been made into the regulation of GAS virulence in vivo, resulting in new avenues for targeted therapeutic and vaccine research.

Population genetics and linkage analysis of loci within the FCT region of Streptococcus pyogenes

The FCT regions of Streptococcus pyogenes strains encode a variety of cell wall-anchored surface proteins that bind the extracellular matrix of the human host and/or give rise to pilus-like appendages. Strong linkage is evident between transcription-regulatory loci positioned within the FCT and emm regions and the emm pattern genotype marker for preferred infection of the throat or skin. These findings provide a basis for the hypothesis that FCT region gene products contribute to tissue-specific infection. In an initial series of steps to address this possibility, the FCT regions of 13 strains underwent comparative sequence analysis, the gene content of the FCT region was characterized for 113 strains via PCR, and genetic linkage was assessed. A history of extensive recombination within FCT regions was evident. The emm pattern D-defined skin specialist strains were highly homogenous in their FCT region gene contents, whereas the emm pattern A-C-defined throat specialist strains displayed a greater variety of forms. Most pattern A-C strains harbored prtF1 (75%) but lacked cpa (75%). In contrast, the majority of emm pattern D strains had cpa (92%) but lacked prtF1 (79%). Models based on FCT and emm region genotypes revealed the most parsimonious pathways of evolution. Using niche-determining candidate genes to infer phylogeny, emm pattern E strains--the so-called generalists, which lack a strong tissue site preference--occupied a transition zone separating most throat specialists from skin specialists. Overall, population genetic analysis supports the possibility that the FCT region gives rise to surface proteins that are largely necessary, but not always sufficient, to confer tissue site preference for infection.

Role of streptococcal T antigens in superficial skin infection

FCT region genes of Streptococcus pyogenes encode surface proteins that include fibronectin- and collagen-binding proteins and the serological markers known as T antigens, some of which give rise to pilus-like appendages. It remains to be established whether FCT region surface proteins contribute to virulence by in vivo models of infection. In this study, a highly sensitive and ecologically relevant humanized mouse model was used to measure superficial skin infection. Three genes encoding FCT region surface proteins essential for T-serotype specificity were inactivated. Both the Deltacpa and DeltaprtF2 mutants were highly attenuated for virulence when topically applied to the skin following exponential growth but were fully virulent when delivered in stationary phase. In contrast, the DeltafctA mutant was virulent at the skin, regardless of its initial growth state. Immunoblots of cell extracts revealed anti-FctA-reactive, ladder-like polymers characteristic of streptococcal pili. In addition, FctA formed a heteropolymer with the putative collagen-binding protein Cpa. The DeltafctA mutant showed a loss in anti-Cpa-reactive polymers, whereas anti-FctA-reactive polymers were reduced in the Deltacpa mutant. The findings suggest that both FctA and Cpa are required for pilus formation, but importantly, an intact pilus is not essential for Cpa-mediated virulence. Although it is an integral part of the T-antigen complex, the fibronectin-binding protein PrtF2 is not covalently linked to the FctA- and Cpa-containing heteropolymer derived from cell extracts. The data provide direct evidence that streptococcal T antigens function as virulence factors in vivo, but they also reveal that a pilus-like structure is not essential for the most common form of streptococcal skin disease.

Trigger for group A streptococcal M1T1 invasive disease

The globally disseminated Streptococcus pyogenes M1T1 clone causes a number of highly invasive human diseases. The transition from local to systemic infection occurs by an unknown mechanism; however invasive M1T1 clinical isolates are known to express significantly less cysteine protease SpeB than M1T1 isolates from local infections. Here, we show that in comparison to the M1T1 strain 5448, the isogenic mutant delta speB accumulated 75-fold more human plasmin activity on the bacterial surface following incubation in human plasma. Human plasminogen was an absolute requirement for M1T1 strain 5448 virulence following subcutaneous (s.c.) infection of humanized plasminogen transgenic mice. S. pyogenes M1T1 isolates from the blood of infected humanized plasminogen transgenic mice expressed reduced levels of SpeB in comparison with the parental 5448 used as inoculum. We propose that the human plasminogen system plays a critical role in group A streptococcal M1T1 systemic disease initiation. SpeB is required for S. pyogenes M1T1 survival at the site of local infection, however, SpeB also disrupts the interaction of S. pyogenes M1T1 with the human plasminogen activation system. Loss of SpeB activity in a subpopulation of S. pyogenes M1T1 at the site of infection results in accumulation of surface plasmin activity thus triggering systemic spread.

Regulation of SpeB in Streptococcus pyogenes by pH and NaCl: a model for in vivo gene expression

For a pathogen such as Streptococcus pyogenes, ecological success is determined by its ability to sense the environment and mount an appropriate adaptive transcriptional response. Thus, determining conditions for analyses of gene expression in vitro that are representative of the in vivo environment is critical for understanding the contributions of transcriptional response pathways to pathogenesis. In this study, we determined that the gene encoding the SpeB cysteine protease is up-regulated over the course of infection in a murine soft-tissue model. Conditions were identified, including growth phase, acidic pH, and an NaCl concentration of <0.1 M, that were required for expression of speB in vitro. Analysis of global expression profiles in response to these conditions in vitro identified a set of coregulated genes whose expression patterns showed a significant correlation with that of speB when examined during infection of murine soft tissues. This analysis revealed that a culture medium that promotes high levels of SpeB expression in vitro produced an expression profile that showed significant correlation to the profile observed in vivo. Taken together, these studies establish culture conditions that mimic in vivo expression patterns; that growth phase, pH, and NaCl may mimic relevant cues sensed by S. pyogenes during infection; and that identification of other environmental cues that alter expression of speB in vitro may provide insight into the signals that direct global patterns of gene expression in vivo.