The Streptococcus pyogenes stand-alone regulator RofA exhibits characteristics of a PRD-containing virulence regulator

Streptococcus pyogenes [group A streptococcus (GAS)] is a human pathogen capable of infecting diverse tissues. To successfully infect these sites, GAS must detect available nutrients and adapt accordingly. The phosphoenolpyruvate transferase system (PTS) mediates carbohydrate uptake and metabolic gene regulation to adapt to the nutritional environment. Regulation by the PTS can occur through phosphorylation of transcriptional regulators at conserved PTS-regulatory domains (PRDs). GAS has several PRD-containing stand-alone regulators with regulons encoding both metabolic genes and virulence factors [PRD-containing virulence regulators (PCVRs)]. One is RofA, which regulates the expression of virulence genes in multiple GAS serotypes. It was hypothesized that RofA is phosphorylated by the PTS in response to carbohydrate levels to coordinate virulence gene expression. In this study, the RofA regulon of M1T1 strain 5448 was determined using RNA sequencing. Two operons were consistently differentially expressed across growth in the absence of RofA; the pilus operon was downregulated, and the capsule operon was upregulated. This correlated with increased capsule production and decreased adherence to keratinocytes. Purified RofA-His was phosphorylated in vitro by PTS proteins EI and HPr, and phosphorylated RofA-FLAG was detected in vivo when GAS was grown in low-glucose C medium. Phosphorylated RofA was not observed when C medium was supplemented 10-fold with glucose. Mutations of select histidine residues within the putative PRDs contributed to the in vivo phosphorylation of RofA, although phosphorylation of RofA was still observed, suggesting other phosphorylation sites exist in the protein. Together, these findings support the hypothesis that RofA is a PCVR that may couple sugar metabolism with virulence regulation.

Loss of rpoE Encoding the δ-Factor of RNA Polymerase Impacts Pathophysiology of the Streptococcus pyogenes M1T1 Strain 5448

Streptococcus pyogenes, also known as the Group A Streptococcus (GAS), is a Gram-positive bacterial pathogen of major clinical significance. Despite remaining relatively susceptible to conventional antimicrobial therapeutics, GAS still causes millions of infections and hundreds of thousands of deaths each year worldwide. Thus, a need for prophylactic and therapeutic interventions for GAS is in great demand. In this study, we investigated the importance of the gene encoding the delta (δ) subunit of the GAS RNA polymerase, rpoE, for its impact on virulence during skin and soft-tissue infection. A defined 5448 mutant with an insertionally-inactivated rpoE gene was defective for survival in whole human blood and was attenuated for both disseminated lethality and lesion size upon mono-culture infection in mouse soft tissue. Furthermore, the mutant had reduced competitive fitness when co-infected with wild type (WT) 5448 in the mouse model. We were unable to attribute this attenuation to any observable growth defect, although colony size and the ability to grow at higher temperatures were both affected when grown with nutrient-rich THY media. RNA-seq of GAS grown in THY to late log phase found that mutation of rpoE significantly impacted (>2-fold) the expression of 429 total genes (205 upregulated, 224 downregulated), including multiple virulence and “housekeeping” genes. The arc operon encoding the arginine deiminase (ADI) pathway was the most upregulated in the rpoE mutant and this could be confirmed phenotypically. Taken together, these findings demonstrate that the delta (δ) subunit of RNA polymerase is vital in GAS gene expression and virulence.

Carbon catabolite repression on the Rgg2/3 quorum sensing system in Streptococcus pyogenes is mediated by PTSMan and Mga

Streptococcus pyogenes, also known as group A Streptococcus or GAS, is a human-restricted pathogen causing a diverse array of infections. The ability to adapt to different niches requires GAS to adjust gene expression in response to environmental cues. We previously identified the abundance of biometals and carbohydrates led to natural induction of the Rgg2/3 cell-cell communication system (quorum sensing, QS). Here we determined the mechanism by which the Rgg2/3 QS system is stimulated exclusively by mannose and repressed by glucose, a phenomenon known as carbon catabolite repression (CCR). Instead of carbon catabolite protein A, the primary mediator of CCR in Gram-positive bacteria; CCR of Rgg2/3 requires the PTS regulatory domain (PRD)-containing transcriptional regulator Mga. Deletion of Mga led to carbohydrate-independent activation of Rgg2/3 by down-regulating rgg3, the QS repressor. Through phosphoablative and phosphomimetic substitutions within Mga PRDs, we demonstrated that selective phosphorylation of PRD1 conferred repression of the Rgg2/3 system. Moreover, given the carbohydrate specificity mediating Mga-dependent governance over Rgg2/3, we tested mannose-specific PTS components and found the EIIA/B subunit ManL was required for Mga-dependent repression. These findings provide newfound connections between PTS^Man , Mga, and QS, and further demonstrate that Mga is a central regulatory nexus for integrating nutritional status and virulence.

PRD-Containing Virulence Regulators (PCVRs) in Pathogenic Bacteria

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

Protocols for Tn-seq Analyses in the Group A Streptococcus

Transposon-sequencing (Tn-seq) has revolutionized forward-genetic analyses to study genotype-phenotype associations and interrogate bacterial cell physiology. The Tn-seq approach allows the en masse monitoring of highly complex mutant libraries, leveraging massive parallel DNA sequencing as a means to characterize the composition of these mutant pools on a genome-scale with unprecedented nucleotide-level high resolution. In this chapter, we present step-by-step protocols for Tn-seq analyses in the human pathogen Streptococcus pyogenes (Group A Streptococcus or GAS) using the mariner-based Krmit transposon. We detail how to generate highly complex Krmit mutant libraries in GAS and the en masse production of Krmit insertion tags for Illumina sequencing of the transposon-genome junctions for Tn-seq analyses. Most of the protocols presented here were developed and implemented using the S. pyogenes M1T1 serotype clinical isolate 5448, but they have been successfully applied to multiple GAS serotypes as well as other pathogenic Streptococci.

Hemoglobin stimulates vigorous growth of Streptococcus pneumoniae and shapes the pathogen's global transcriptome

Streptococcus pneumoniae (Spn) must acquire iron from the host to establish infection. We examined the impact of hemoglobin, the largest iron reservoir in the body, on pneumococcal physiology. Supplementation with hemoglobin allowed Spn to resume growth in an iron-deplete medium. Pneumococcal growth with hemoglobin was unusually robust, exhibiting a prolonged logarithmic growth, higher biomass, and extended viability in both iron-deplete and standard medium. We observed the hemoglobin-dependent response in multiple serotypes, but not with other host proteins, free iron, or heme. Remarkably, hemoglobin induced a sizable transcriptome remodeling, effecting virulence and metabolism in particular genes facilitating host glycoconjugates use. Accordingly, Spn was more adapted to grow on the human α − 1 acid glycoprotein as a sugar source with hemoglobin. A mutant in the hemoglobin/heme-binding protein Spbhp-37 was impaired for growth on heme and hemoglobin iron. The mutant exhibited reduced growth and iron content when grown in THYB and hemoglobin. In summary, the data show that hemoglobin is highly beneficial for Spn cultivation in vitro and suggest that hemoglobin might drive the pathogen adaptation in vivo. The hemoglobin receptor, Spbhp-37, plays a role in mediating the positive influence of hemoglobin. These novel findings provide intriguing insights into pneumococcal interactions with its obligate human host.

The Arginine Deiminase Pathway Impacts Antibiotic Tolerance during Biofilm-Mediated Streptococcus pyogenes Infections

Bacterial biofilms are responsible for a variety of serious human infections and are notoriously difficult to treat due to their recalcitrance to antibiotics. Further work is necessary to elicit a full understanding of the mechanism of this antibiotic tolerance. The arginine deiminase (ADI) pathway is responsible for bacterial pH maintenance and is highly expressed during biofilm growth in multiple bacterial species. Using the group A Streptococcus (GAS) as a model human pathogen, the ADI pathway was demonstrated to contribute to biofilm growth. The inability of antibiotics to reduce GAS populations when in a biofilm was demonstrated by in vitro studies and a novel animal model of nasopharyngeal infection. However, disruption of the ADI pathway returned GAS biofilms to planktonic levels of antibiotic sensitivity, suggesting the ADI pathway is influential in biofilm-related antibiotic treatment failure and provides a new strategic target for the treatment of biofilm infections in GAS and potentially numerous other bacterial species.IMPORTANCE Biofilm-mediated bacterial infections are a major threat to human health because of their recalcitrance to antibiotic treatment. Through the study of Streptococcus pyogenes, a significant human pathogen that is known to form antibiotic-tolerant biofilms, we demonstrated the role that a bacterial pathway known for responding to acid stress plays in biofilm growth and antibiotic tolerance. This not only provides some insight into antibiotic treatment failure in S. pyogenes infections but also, given the widespread nature of this pathway, provides a potentially broad target for antibiofilm therapies. This discovery has the potential to impact the treatment of many different types of recalcitrant biofilm infections.

Streptococcal Lancefield polysaccharides are critical cell wall determinants for human Group IIA secreted phospholipase A2 to exert its bactericidal effects

Human Group IIA secreted phospholipase A2 (hGIIA) is an acute phase protein with bactericidal activity against Gram-positive bacteria. Infection models in hGIIA transgenic mice have suggested the importance of hGIIA as an innate defense mechanism against the human pathogens Group A Streptococcus (GAS) and Group B Streptococcus (GBS). Compared to other Gram-positive bacteria, GAS is remarkably resistant to hGIIA activity. To identify GAS resistance mechanisms, we exposed a highly saturated GAS M1 transposon library to recombinant hGIIA and compared relative mutant abundance with library input through transposon-sequencing (Tn-seq). Based on transposon prevalence in the output library, we identified nine genes, including dltA and lytR, conferring increased hGIIA susceptibility. In addition, seven genes conferred increased hGIIA resistance, which included two genes, gacH and gacI that are located within the Group A Carbohydrate (GAC) gene cluster. Using GAS 5448 wild-type and the isogenic gacI mutant and gacI-complemented strains, we demonstrate that loss of the GAC N-acetylglucosamine (GlcNAc) side chain in the ΔgacI mutant increases hGIIA resistance approximately 10-fold, a phenotype that is conserved across different GAS serotypes. Increased resistance is associated with delayed penetration of hGIIA through the cell wall. Correspondingly, loss of the Lancefield Group B Carbohydrate (GBC) rendered GBS significantly more resistant to hGIIA-mediated killing. This suggests that the streptococcal Lancefield antigens, which are critical determinants for streptococcal physiology and virulence, are required for the bactericidal enzyme hGIIA to exert its bactericidal function.

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

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

Route of Glucose Uptake in the Group a Streptococcus Impacts SLS-Mediated Hemolysis and Survival in Human Blood

The transport and metabolism of glucose has been shown to have far reaching consequences in the transcriptional profile of many bacteria. As glucose is most often the preferred carbon source for bacteria, its presence in the environment leads to the repression of many alternate carbohydrate pathways, a condition known as carbon catabolite repression (CCR). Additionally, the expression of many virulence factors is also dependent on the presence of glucose. Despite its importance, little is known about the transport routes of glucose in the human pathogen Streptococcus pyogenes. Considering that Streptococcus pyogenes is an important human pathogen responsible for over 500,000 deaths every year, we characterized the routes of glucose transport in an effort to understand its importance in GAS pathogenesis. Using a deletion of glucokinase (ΔnagC) to block utilization of glucose imported by non-PTS pathways, we determined that of the two glucose transport pathways in GAS (PTS and non-PTS), the non-PTS pathway played a more significant role in glucose transport. However, the expression of both pathways is linked by a currently unknown mechanism, as blocking the non-PTS uptake of glucose reduces ptsI (EI) expression. Similar to the effects of the deletion of the PTS pathway, lack of the non-PTS pathway also leads to the early activity of Streptolysin S. However, this early activity did not adversely or favorably affect survival of ΔnagC in whole human blood. In a subcutaneous murine infection model, ΔnagC-infected mice showed increased lesion severity at the local site of infection; although, lesion size and dissemination from the site of infection was similar to wild type. Here, we show that glucose transport in GAS is primarily via a non-PTS pathway. The route of glucose transport differentially affects the survival of GAS in whole human blood, as well as the lesion size at the local site of infection in a murine skin infection model.

Genome-wide discovery of novel M1T1 group A streptococcal determinants important for fitness and virulence during soft-tissue infection

The Group A Streptococcus remains a significant human pathogen causing a wide array of disease ranging from self-limiting to life-threatening invasive infections. Epithelium (skin or throat) colonization with progression to the subepithelial tissues is the common step in all GAS infections. Here, we used transposon-sequencing (Tn-seq) to define the GAS 5448 genetic requirements for in vivo fitness in subepithelial tissue. A near-saturation transposon library of the M1T1 GAS 5448 strain was injected subcutaneously into mice, producing suppurative inflammation at 24 h that progressed to prominent abscesses with tissue necrosis at 48 h. The library composition was monitored en masse by Tn-seq and ratios of mutant abundance comparing the output (12, 24 and 48 h) versus input (T₀) mutant pools were calculated for each gene. We identified a total of 273 subcutaneous fitness (scf) genes with 147 genes (55 of unknown function) critical for the M1T1 GAS 5448 fitness in vivo; and 126 genes (53 of unknown function) potentially linked to in vivo fitness advantage. Selected scf genes were validated in competitive subcutaneous infection with parental 5448. Two uncharacterized genes, scfA and scfB, encoding putative membrane-associated proteins and conserved among Gram-positive pathogens, were further characterized. Defined scfAB mutants in GAS were outcompeted by wild type 5448 in vivo, attenuated for lesion formation in the soft tissue infection model and dissemination to the bloodstream. We hypothesize that scfAB play an integral role in enhancing adaptation and fitness of GAS during localized skin infection, and potentially in propagation to other deeper host environments.

The WHO ranks the Group A Streptococcus (GAS) in the top 10 leading causes of morbidity and mortality from infectious diseases worldwide. GAS is a strict human pathogen causing both benign superficial infections as well as life-threatening invasive diseases. All GAS infections begin by colonization of an epithelium (throat or skin) followed by propagation into subepithelial tissues. The genetic requirements for M1T1 GAS 5448 within this niche were interrogated by in vivo transposon sequencing (Tn-seq), identifying 273 subcutaneous fitness (scf) genes with 108 of those previously of “unknown function”. Two yet uncharacterized genes, scfA and scfB, were shown to be critical during GAS 5448 soft tissue infection and dissemination into the bloodstream. Thus, this study improves the functional annotation of the GAS genome, providing new insights into GAS pathophysiology and enhancing the development of novel GAS therapeutics.

The Transcriptional Regulator CpsY Is Important for Innate Immune Evasion in Streptococcus pyogenes

As an exclusively human pathogen, Streptococcus pyogenes (the group A streptococcus [GAS]) has specifically adapted to evade host innate immunity and survive in multiple tissue niches, including blood. GAS can overcome the metabolic constraints of the blood environment and expresses various immunomodulatory factors necessary for survival and immune cell resistance. Here we present our investigation of one such factor, the predicted LysR family transcriptional regulator CpsY. The encoding gene, cpsY, was initially identified as being required for GAS survival in a transposon-site hybridization (TraSH) screen in whole human blood. CpsY is homologous with transcriptional regulators of Streptococcus mutans (MetR), Streptococcus iniae (CpsY), and Streptococcus agalactiae (MtaR) that regulate methionine transport, amino acid metabolism, resistance to neutrophil-mediated killing, and survival in vivo Our investigation indicated that CpsY is involved in GAS resistance to innate immune cells of its human host. However, GAS CpsY does not manifest the in vitro phenotypes of its homologs in other streptococcal species. GAS CpsY appears to regulate a small set of genes that is markedly different from the regulons of its homologs. The differential expression of these genes depends on the growth medium, and CpsY modestly influences their expression. The GAS CpsY regulon includes known virulence factors (mntE, speB, spd, nga [spn], prtS [SpyCEP], and sse) and cell surface-associated factors of GAS (emm1, mur1.2, sibA [cdhA], and M5005_Spy0500). Intriguingly, the loss of CpsY in GAS does not result in virulence defects in murine models of infection, suggesting that CpsY function in immune evasion is specific to the human host.

A PTS EII mutant library in Group A Streptococcus identifies a promiscuous man-family PTS transporter influencing SLS-mediated hemolysis

The Group A Streptococcus (GAS, Streptococcus pyogenes) is a Gram-positive human pathogen that must adapt to unique host environments in order to survive. Links between sugar metabolism and virulence have been demonstrated in GAS, where mutants in the phosphoenolpyruvate-dependent phosphotransferase system (PTS) exhibited Streptolysin S (SLS)-mediated hemolysis during exponential growth. This early onset hemolysis correlated with an increased lesion size and severity in a murine soft tissue infection model when compared with parental M1T1 MGAS5005. To identify the PTS components responsible for this phenotype, we insertionally inactivated the 14 annotated PTS EIIC-encoding genes in the GAS MGAS5005 genome and subjected this library to metabolic and hemolysis assays to functionally characterize each EIIC. It was found that a few EIIs had a very limited influence on PTS sugar metabolism, whereas others were fairly promiscuous. The mannose-specific EII locus, encoded by manLMN, was expressed as a mannose-inducible operon that exhibited the most influence on PTS sugar metabolism, including mannose. Importantly, components of the mannose-specific EII also acted to prevent the early onset of SLS-mediated hemolysis. Interestingly, these roles were not identical in two different M1T1 GAS strains, highlighting the possible versatility of the PTS to adapt to strain-specific needs.

GacA is essential for Group A Streptococcus and defines a new class of monomeric dTDP-4-dehydrorhamnose reductases (RmlD)

The sugar nucleotide dTDP‐L‐rhamnose is critical for the biosynthesis of the Group A Carbohydrate, the molecular signature and virulence determinant of the human pathogen Group A Streptococcus (GAS). The final step of the four‐step dTDP‐L‐rhamnose biosynthesis pathway is catalyzed by dTDP‐4‐dehydrorhamnose reductases (RmlD). RmlD from the Gram‐negative bacterium Salmonella is the only structurally characterized family member and requires metal‐dependent homo‐dimerization for enzymatic activity. Using a biochemical and structural biology approach, we demonstrate that the only RmlD homologue from GAS, previously renamed GacA, functions in a novel monomeric manner. Sequence analysis of 213 Gram‐negative and Gram‐positive RmlD homologues predicts that enzymes from all Gram‐positive species lack a dimerization motif and function as monomers. The enzymatic function of GacA was confirmed through heterologous expression of gacA in a S. mutans rmlD knockout, which restored attenuated growth and aberrant cell division. Finally, analysis of a saturated mutant GAS library using Tn‐sequencing and generation of a conditional‐expression mutant identified gacA as an essential gene for GAS. In conclusion, GacA is an essential monomeric enzyme in GAS and representative of monomeric RmlD enzymes in Gram‐positive bacteria and a subset of Gram‐negative bacteria. These results will help future screens for novel inhibitors of dTDP‐L‐rhamnose biosynthesis.

Essential Genes in the Core Genome of the Human Pathogen Streptococcus pyogenes

Streptococcus pyogenes (Group A Streptococcus, GAS) remains a major public health burden worldwide, infecting over 750 million people leading to over 500,000 deaths annually. GAS pathogenesis is complex, involving genetically distinct GAS strains and multiple infection sites. To overcome fastidious genetic manipulations and accelerate pathogenesis investigations in GAS, we developed a mariner-based system (Krmit) for en masse monitoring of complex mutant pools by transposon sequencing (Tn-seq). Highly saturated transposant libraries (Krmit insertions in ca. every 25 nucleotides) were generated in two distinct GAS clinical isolates, a serotype M1T1 invasive strain 5448 and a nephritogenic serotype M49 strain NZ131, and analyzed using a Bayesian statistical model to predict GAS essential genes, identifying sets of 227 and 241 of those genes in 5448 and NZ131, respectively. A large proportion of GAS essential genes corresponded to key cellular processes and metabolic pathways, and 177 were found conserved within the GAS core genome established from 20 available GAS genomes. Selected essential genes were validated using conditional-expression mutants. Finally, comparison to previous essentiality analyses in S. sanguinis and S. pneumoniae revealed significant overlaps, providing valuable insights for the development of new antimicrobials to treat infections by GAS and other pathogenic streptococci.

The crimson conundrum: heme toxicity and tolerance in GAS

The massive erythrocyte lysis caused by the Group A Streptococcus (GAS) suggests that the β-hemolytic pathogen is likely to encounter free heme during the course of infection. In this study, we investigated GAS mechanisms for heme sensing and tolerance. We compared the minimal inhibitory concentration of heme among several isolates and established that excess heme is bacteriostatic and exposure to sub-lethal concentrations of heme resulted in noticeable damage to membrane lipids and proteins. Pre-exposure of the bacteria to 0.1 μM heme shortened the extended lag period that is otherwise observed when naive cells are inoculated into heme-containing medium, implying that GAS is able to adapt. The global response to heme exposure was determined using microarray analysis revealing a significant transcriptome shift that included 79 up regulated and 84 down regulated genes. Among other changes, the induction of stress-related chaperones and proteases, including groEL/ES (8x), the stress regulators spxA2 (5x) and ctsR (3x), as well as redox active enzymes were prominent. The heme stimulon also encompassed a number of regulatory proteins and two-component systems that are important for virulence. A three-gene cluster that is homologous to the pefRCD system of the Group B Streptococcus was also induced by heme. PefR, a MarR-like regulator, specifically binds heme with stoichiometry of 1:2 and protoporphyrin IX (PPIX) with stoichiometry of 1:1, implicating it is one of the GAS mediators to heme response. In summary, here we provide evidence that heme induces a broad stress response in GAS, and that its success as a pathogen relies on mechanisms for heme sensing, detoxification, and repair.

An extracellular bacterial pathogen modulates host metabolism to regulate its own sensing and proliferation

Successful infection depends on the ability of the pathogen to gain nutrients from the host. The extracellular pathogenic bacterium group A Streptococcus (GAS) causes a vast array of human diseases. By using the quorum-sensing sil system as a reporter, we found that, during adherence to host cells, GAS delivers streptolysin toxins, creating endoplasmic reticulum stress. This, in turn, increases asparagine (ASN) synthetase expression and the production of ASN. The released ASN is sensed by the bacteria, altering the expression of ∼17% of GAS genes of which about one-third are dependent on the two-component system TrxSR. The expression of the streptolysin toxins is strongly upregulated, whereas genes linked to proliferation are downregulated in ASN absence. Asparaginase, a widely used chemotherapeutic agent, arrests GAS growth in human blood and blocks GAS proliferation in a mouse model of human bacteremia. These results delineate a pathogenic pathway and propose a therapeutic strategy against GAS infections.

The phosphoenolpyruvate phosphotransferase system in group A Streptococcus acts to reduce streptolysin S activity and lesion severity during soft tissue infection

Obtaining essential nutrients, such as carbohydrates, is an important process for bacterial pathogens to successfully colonize host tissues. The phosphoenolpyruvate phosphotransferase system (PTS) is the primary mechanism by which bacteria transport sugars and sense the carbon state of the cell. The group A streptococcus (GAS) is a fastidious microorganism that has adapted to a variety of niches in the human body to elicit a wide array of diseases. A ΔptsI mutant (enzyme I [EI] deficient) generated in three different strains of M1T1 GAS was unable to grow on multiple carbon sources (PTS and non-PTS). Complementation with ptsI expressed under its native promoter in single copy was able to rescue the growth defect of the mutant. In a mouse model of GAS soft tissue infection, all ΔptsI mutants exhibited a significantly larger and more severe ulcerative lesion than mice infected with the wild type. Increased transcript levels of sagA and streptolysin S (SLS) activity during exponential-phase growth was observed. We hypothesized that early onset of SLS activity would correlate with the severity of the lesions induced by the ΔptsI mutant. In fact, infection of mice with a ΔptsI sagB double mutant resulted in a lesion comparable to that of either the wild type or a sagB mutant alone. Therefore, a functional PTS is not required for subcutaneous skin infection in mice; however, it does play a role in coordinating virulence factor expression and disease progression.

Genetic manipulation of Streptococcus pyogenes (the Group A Streptococcus, GAS)

Streptococcus pyogenes (the Group A Streptococcus, GAS) is a Gram-positive bacterium responsible for a wide spectrum of diseases ranging from mild superficial infections (pharyngitis, impetigo) to severe, often life-threatening invasive diseases (necrotizing fasciitis, streptococcal toxic shock syndrome) in humans. This unit describes molecular techniques for the genetic manipulation of S. pyogenes with detailed protocols for transformation, gene disruption, allelic exchange, transposon mutagenesis, and genetic complementation.

Laboratory growth and maintenance of Streptococcus pyogenes (the Group A Streptococcus, GAS)

Streptococcus pyogenes is a Gram-positive bacterium that strictly infects humans. It is the causative agent of a broad spectrum of diseases accounting for millions of infections and at least 517,000 deaths each year worldwide. It is a nutritionally fastidious organism that ferments sugars to produce lactic acid and has strict requirements for growth. To aid in the study of this organism, this unit describes the growth and maintenance of S. pyogenes.

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

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

Genome-wide identification of genes required for fitness of group A Streptococcus in human blood

The group A streptococcus (GAS) is a strict human pathogen responsible for a wide spectrum of diseases. Although GAS genome sequences are available, functional genomic analyses have been limited. We developed a mariner-based transposon, osKaR, designed to perform Transposon-Site Hybridization (TraSH) in GAS and successfully tested its use in several invasive serotypes. A complex osKaR mutant library in M1T1 GAS strain 5448 was subjected to negative selection in human blood to identify genes important for GAS fitness in this clinically relevant environment. Mutants underrepresented after growth in blood (output pool) compared to growth in rich media (input pool) were identified using DNA microarray hybridization of transposon-specific tags en masse. Using blood from three different donors, we identified 81 genes that met our criteria for reduced fitness in blood from at least two individuals. Genes known to play a role in survival of GAS in blood were found, including those encoding the virulence regulator Mga (mga), the peroxide response regulator PerR (perR), and the RofA-like regulator Ralp-3 (ralp3). We also identified genes previously reported for their contribution to sepsis in other pathogens, such as de novo nucleotide synthesis (purD, purA, pyrB, carA, carB, guaB), sugar metabolism (scrB, fruA), zinc uptake (adcC), and transcriptional regulation (cpsY). To validate our findings, independent mutants with mutations in 10 different genes identified in our screen were confirmed to be defective for survival in blood bactericidal assays. Overall, this work represents the first use of TraSH in GAS to identify potential virulence genes.

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

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

Investigating how streptococcus responds to their environment: bringing together current research, a case study and laboratory investigation

Understanding the link between course work and unanswered authentic research questions being explored in the research lab is an important goal in undergraduate science teaching. The activity presented here focuses on current research regarding the virulence characteristics of Streptococcus pyogenes particularly targeting the control of sugar uptake regulated via catabolite repression. Students were challenged to formulate a research question and use higher-order thinking skills to analyze data, work collaboratively to solve problems, and pose and test a hypothesis in the laboratory setting. The activity employed an interrupted case study approach using both online and face-to-face settings. The case story and problems were distributed online and were followed by in-class discussions and lab work. Aspects of the activity required independent thinking, as well as collaborative work. Student learning gains were demonstrated via comparison of pre- and postscores on the Host Pathogen Interactions (HPI) concept inventory, results from an end of semester Student Perception Survey, and from analysis of students' work.

MtsR is a dual regulator that controls virulence genes and metabolic functions in addition to metal homeostasis in the group A streptococcus

MtsR is a metal-dependent regulator in the group A streptococcus (GAS) that directly represses the transcription of genes involved in haem and metal uptake. While MtsR has been implicated in GAS virulence, the DNA recognition and full regulatory scope exerted by the protein are unknown. In this study we identified the shr promoter (P(shr)) and mapped MtsR binding to a 69 bp segment in P(shr) that overlaps the core promoter elements. A global transcriptional analysis demonstrated that MtsR modulates the expression of 64 genes in GAS, 44 of which were upregulated and 20 were downregulated in the mtsR mutant. MtsR controls genes with diverse functions including metal homeostasis, nucleic acid and amino acid metabolism, and protein fate. Importantly, the MtsR regulon includes mga, emm49 and ska, which are central for GAS pathogenesis. MtsR binding to the promoter region of both negatively and positively regulated genes demonstrates that it functions as a dual regulator. MtsR footprints are large (47-130 bp) and vary between target promoters. A 16 bp motif that consists of an interrupted palindrome is implicated in the DNA recognition by the metalloregulator. In conclusion, we report here that MtsR is a global regulator in GAS that shapes the expression of vital virulence factors and genes involved in metabolic functions and metal transport, and we discuss the implications for the GAS disease process.

A model for using a concept inventory as a tool for students' assessment and faculty professional development

This essay describes how the use of a concept inventory has enhanced professional development and curriculum reform efforts of a faculty teaching community. The Host Pathogen Interactions (HPI) teaching team is composed of research and teaching faculty with expertise in HPI who share the goal of improving the learning experience of students in nine linked undergraduate microbiology courses. To support evidence-based curriculum reform, we administered our HPI Concept Inventory as a pre- and postsurvey to approximately 400 students each year since 2006. The resulting data include student scores as well as their open-ended explanations for distractor choices. The data have enabled us to address curriculum reform goals of 1) reconciling student learning with our expectations, 2) correlating student learning with background variables, 3) understanding student learning across institutions, 4) measuring the effect of teaching techniques on student learning, and 5) demonstrating how our courses collectively form a learning progression. The analysis of the concept inventory data has anchored and deepened the team's discussions of student learning. Reading and discussing students' responses revealed the gap between our understanding and the students' understanding. We provide evidence to support the concept inventory as a tool for assessing student understanding of HPI concepts and faculty development.

Stand-alone response regulators controlling global virulence networks in streptococcus pyogenes

Global regulation of virulence gene expression via transcriptional regulators plays a central role in the ability of the bacterial pathogen Streptococcus pyogenes (the group A Streptococcus, GAS) to rapidly adapt during infection. The 'stand-alone' regulators Mga, RofA-like proteins (RALPs), and RopB/Rgg control important and diverse virulence regulons in response to growth-related signals and other environmental conditions in GAS. Stand-alone regulated genes encode factors important for colonization of tissues, immune evasion, persistence, dissemination, metabolism, and the response to stressors. Although conserved 'core' regulons have been established for each, recent studies have revealed significant inter-serotype and even intra-serotype variation in the regulatory patterns presented by the stand-alone regulators. This chapter will look at each stand-alone regulatory pathway in depth and discuss how these important global networks influence virulence as well as interact with each other to produce an integrated response during GAS infection.

Assessing student understanding of host pathogen interactions using a concept inventory

As a group of faculty with expertise and research programs in the area of host-pathogen interactions (HPI), we are concentrating on students' learning of HPI concepts. As such we developed a concept inventory to measure level of understanding relative to HPI after the completion of a set of microbiology courses (presently eight courses). Concept inventories have been useful tools for assessing student learning, and our interest was to develop such a tool to measure student learning progression in our microbiology courses. Our teaching goal was to create bridges between our courses which would eliminate excessive overlap in our offerings and support a model where concepts and ideas introduced in one course would become the foundation for concept development in successive courses. We developed our HPI concept inventory in several phases. The final product was an 18-question, multiple-choice concept inventory. In fall 2006 and spring 2007 we administered the 18-question concept inventory in six of our courses. We collected pre- and postcourse surveys from 477 students. We found that students taking pretests in the advanced courses retained the level of understanding gained in the general microbiology prerequisite course. Also, in two of our courses there was significant improvement on the scores from pretest to posttest. As we move forward, we will concentrate on exploring the range of HPI concepts addressed in each course and determine and/or create effective methods for meaningful student learning of HPI aspects of microbiology.

Skim milk enhances the preservation of thawed -80 degrees C bacterial stocks

The results from bacterial strain recovery efforts following hurricanes Katrina and Rita are reported. Over 90% of strains frozen in 10% skim milk were recovered whereas various recovery rates were observed for glycerol-stored stocks (56% and 94% of Escherichia coli, depending upon the laboratory). These observations led to a viability comparison of Streptococcus pyogenes, Campylobacter jejuni, Borrelia burgdorferi, Salmonella enterica subsp. Typhimurium, Pseudomonas aeruginosa and E. coli strains stored in glycerol or skim milk. In all bacteria examined, 10% skim milk resulted in significantly longer viability after thawing than 15% glycerol solutions currently used in most laboratories.

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

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

Identification of pyruvate kinase as an antigen associated with Tourette syndrome

Immune responses to beta-hemolytic streptococcal infections are hypothesized to trigger tic disorders and early-onset obsessive-compulsive disorder (OCD) in some pediatric populations. Here we identify the M1 isoform of the glycolytic enzyme, pyruvate kinase (PK) as an autoimmune target in Tourette syndrome and associated disorders. Antibodies to PK reacted strongly with surface antigens of infectious strains of streptococcus, and antibodies to streptococcal M proteins reacted with PK. Moreover, immunoreactivity to PK in patients with exacerbated symptoms who had recently acquired a streptococcal infection was 7-fold higher compared to patients with exacerbated symptoms and no evidence of a streptococcal infection. These data suggest that PK can function as an autoimmune target and that this immunoreactivity may be associated with Tourette syndrome, OCD, and associated disorders.

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

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

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

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

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

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

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

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

Identification of residues in the Pseudomonas aeruginosa elastase propeptide required for chaperone and secretion activities

An important virulence factor of the opportunistic human pathogen Pseudomonas aeruginosa is elastase, a secreted thermolysin-like neutral zinc-metalloprotease (TNP). Elastase is synthesized as a larger precursor with an amino-terminal 18 kDa propeptide, and was the first TNP shown to require its propeptide as an intramolecular chaperone (IMC) for activity and secretion. This paper reports the analysis of the elastase propeptide to identify residues conserved among other TNP precursors that may be critical for its IMC function. The prosequences of TNP precursors from both Gram-negative (Vibrio species and Legionella species) and Gram-positive (Bacillus species) bacteria were found to show homology to the elastase propeptide. Two regions of conserved residues were observed: a hydrophilic region (ProM) found in the middle of the elastase propeptide and a more hydrophobic region (ProC) located proximal to the propeptide-processing site. To test whether such conserved motifs were important to function, single residue substitutions at eight conserved amino acids were introduced within the full-length pre-proelastase precursor by site-specific mutagenesis of lasB, the gene encoding elastase. Mutant lasB alleles were expressed from plasmids within a lasB-deleted P. aeruginosa strain, FRD740, and the effects of these propeptide alterations on elastase enzyme activity, processing, stability and accumulation inside and outside of the cell were examined. Within the ProM region, substitution at Arg74 resulted in a dramatic accumulation of proelastase in the cell, suggesting a secretion defect, and a dramatic reduction in supernatant elastolytic activity. Substitution at Asn68 adversely affected the amount of elastase in the culture supernatant, apparently as a result of the reduced stability of the mutated proelastase in the cell. Within the ProC region, mutations at Ile181 and Ala183 abolished the accumulation of a stable elastase molecule in the supernatant. Most mutations produced a phenotype consistent with a defect in protein folding and stability. Overall, the data from this preliminary study show that conserved residues within the elastase propeptide are essential for its function and begin to define the mechanisms of action of IMCs in the TNP family.

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

Streptococcus pyogenes (the group A streptococcus [GAS]) is a medically significant pathogen of humans, causing a range of diseases from pharyngitis to necrotizing fasciitis. Several important GAS virulence genes are under the control of a pleiotropic regulator called Mga, or the multiple gene regulator of GAS, including the gene encoding the streptococcal collagen-like protein, or sclA. Analysis of the genome sequence upstream of sclA revealed two potential Mga-binding sites with homology to the published Mga-binding element, which were called PsclA-I (distal) and PsclA-II (proximal) based on their location relative to a predicted start of transcription. Primer extension was used to confirm that the Mga-dependent transcriptional start site for sclA was located adjacent to the proximal PsclA-II binding site. By using overlapping PsclA promoter probes and purified Mga-His fusion protein, it was shown by electrophoretic mobility shift assays that, unlike other Mga-regulated promoters, Mga binds only to a distal DNA-binding site (PsclA-I). Binding of Mga to PsclA-I could be competed with cold probes corresponding to known Mga-regulated promoters (Pemm, PscpA, and Pmga) but not with a nonspecific probe or the proximal PsclA-II fragment. With the use of a plasmid-based green fluorescent protein transcriptional reporter system, the full-length PsclA was not sufficient to reproduce normal Mga-regulated activation. However, studies using a single-copy gusA transcriptional reporter system integrated at the native sclA chromosomal locus clearly demonstrated that the distal PsclA-I binding site is required for Mga regulation. Therefore, PsclA represents a new class of Mga-regulated promoters that requires a single distal binding site for activation.

amrA encodes a putative membrane protein necessary for maximal exponential phase expression of the Mga virulence regulon in Streptococcus pyogenes

The transcriptional regulator Mga activates a regulon of virulence genes important for colonization and immune evasion in GAS. Using transposon mutagenesis of a serotype M6 group A streptococcus (GAS) reporter strain KSM148, we have identified an open reading frame (ORF) designated amrA that is required for maximal activation of the Mga regulon during exponential phase. A deletion in amrA, but not in the downstream transcriptionally linked ORF Spy0798, was able to reproduce the phenotype seen in the transposon mutants. Northern analysis for mga and emm transcripts, as well as Western analysis of Mga, confirmed a reduction in mga expression leading to a decrease in transcription of the Mga-regulated emm in the amrA deletion and transposon mutants. Furthermore, both the amrA deletion mutant and an original transposon mutant could be complemented using amrA expressed from a nisin-inducible expression system. As amrA is strongly conserved across the sequenced streptococcal M types, and inactivation of amrA in an M3 serotype also resulted in reduction of emm transcripts, the role of amrA does not appear to be serotype specific. Although the specific function of AmrA is unknown, its putative membrane localization and homology to transporters involved in cell wall synthesis suggest a link between growth and virulence gene expression in GAS.

Role of Streptococcus pyogenes two-component response regulators in the temporal control of Mga and the Mga-regulated virulence gene emm

We examined the role of Streptococcus pyogenes two-component response regulators (SptR) in expression of Mga and the Mga-regulated gene emm. Both serotype M6 and serotype M1 mutants in 12 of the 13 identified sptR genes exhibited levels of emm transcripts and Mga protein comparable to those of the wild type during exponential and stationary phases of growth. Thus, temporal control of these virulence genes does not require Spt response regulators.

Virulence factor regulation and regulatory networks in Streptococcus pyogenes and their impact on pathogen-host interactions

Streptococcus pyogenes (group A streptococcus, GAS) is a very important human pathogen with remarkable adaptation capabilities. Survival within the harsh host surroundings requires sensing potential on the bacterial side, which leads in particular to coordinately regulated virulence factor expression. GAS 'stand-alone' response regulators (RRs) and two-component signal transduction systems (TCSs) link the signals from the host environment with adaptive responses of the bacterial cell. Numerous putative regulatory systems emerged from GAS genome sequences. Only three RRs [Mga, RofA-like protein (RALP) and Rgg/RopB] and three TCSs (CsrRS/CovRS, FasBCAX and Ihk/Irr) have been studied in some detail with respect to their growth-phase-dependent activity and their influence on GAS-host cell interaction. In particular, the Mga-, RALP- and Rgg/RopB-regulated pathways display interconnected activities that appear to influence GAS colonization, persistence and spreading mechanisms, in a growth-phase-related fashion. Here, we have summarized our current knowledge about these RRs and TCSs to highlight the questions that should be addressed in future research on GAS pathogenicity.

Two DNA-binding domains of Mga are required for virulence gene activation in the group A streptococcus

Mga is a DNA-binding protein that activates expression of several important virulence genes in the group A streptococcus (GAS), including those encoding M protein (emm), C5a peptidase (scpA) and Mga (mga). To determine the functionality of four potential helix-turn-helix DNA-binding motifs (HTH1-HTH4) identified within the amino-terminus of Mga, alanine substitutions were introduced within each domain in a MBP-Mga fusion allele and purified proteins were assayed for binding to Mga-specific promoter fragments (Pmga, PscpA and Pemm) in vitro. Although HTH-1 and HTH-2 mutations showed wild type DNA-binding activity, an altered HTH-3 domain resulted in reduced binding to the three promoters and an HTH-4 mutant was devoid of detectable binding activity. Plasmid-encoded expression of the HTH-3 and HTH-4 alleles from a constitutive promoter (Pspac) in the mga-deleted GAS strain JRS519 demonstrated that Mga-regulated emm expression correlated directly to the DNA-binding activity observed for each mutant protein in vitro. Single-copy expression of HTH-3 and HTH-4 from their native Pmga resulted in a dramatic reduction in autoregulated mga expression in both mutant strains. Thus, Mga appears to contain two DNA-binding domains (HTH-3 and HTH-4) that are required for direct activation of the Mga virulence regulon in vivo.

Restoration of Mga function to a Streptococcus pyogenes strain (M Type 50) that is virulent in mice

The Mga protein in B514Sm, a Streptococcus pyogenes strain isolated as a mouse pathogen, contains amino acid substitutions at conserved sites that render the protein defective. Replacement of mga50 with the functional homolog mga4.1 restored full expression of Mga-regulated proteins. Restoration of Mga function did not affect fibrinogen binding, nor did it affect virulence in several mouse models of group A streptococcus infection.

Attenuated expression of the mga virulence regulon in an M serotype 50 mouse-virulent group A streptococcal strain

The attenuated expression of virulence genes found in a group A streptococcal strain that is naturally pathogenic for mice was postulated to result from a defect in the strain's multigene regulator, Mga. The sequence of the mga gene reveals three amino acid changes in the gene product that might affect protein function. The defect in the mga gene was complemented by providing either the closely similar mga4 allele or a more divergent mga1 allele in trans. Complementation increased the amount of emm50 transcript and the quantity of surface-extractable M protein, restoring virulence function.

Regulation of mga transcription in the group A streptococcus: specific binding of mga within its own promoter and evidence for a negative regulator

Transcription of mga, encoding the multiple virulence gene regulator of the group A streptococcus, is positively autoregulated. This regulation requires a DNA region (Pmga) that contains both a promoter proximal to mga (P2) and a promoter located further upstream (P1). To determine if Mga has a direct role in this process, its ability to bind to specific sequences within Pmga was tested. A purified fusion of Mga to the C-terminal end of maltose-binding protein (MBP-Mga), encoded by malE-mga, was shown previously to bind to the promoter regions of Mga-regulated genes, including scpA and emm. We report here that MBP-Mga can function in vivo to regulate emm and mga. Electrophoretic mobility shift assays and DNase I footprinting were used to demonstrate specific binding of MBP-Mga to two ca. 59-bp binding sites in Pmga centered around bases -108 and -180 from the major P2 start of transcription. Mga binding sites from Pemm and PscpA were shown to compete for binding at the two Pmga sites, suggesting that the same domain of Mga interacts at all of these promoter targets. Deletion of the distal Pmga binding site (site I) in vivo resulted in loss of Mga-dependent transcription from the P2 start. However, the same lesion resulted in an increase in P1 transcription that was independent of Mga. This suggests the existence of a repressor of mga transcription with a binding site overlapping those of Mga.

A response regulator that represses transcription of several virulence operons in the group A streptococcus

A search for homologs of the Bacillus subtilis PhoP response regulator in the group A streptococcus (GAS) genome revealed three good candidates. Inactivation of one of these, recently identified as csrR (J. C. Levin and M. R. Wessels, Mol. Microbiol. 30:209-219, 1998), caused the strain to produce mucoid colonies and to increase transcription of hasA, the first gene in the operon for capsule synthesis. We report here that a nonpolar insertion in this gene also increased transcription of ska (encoding streptokinase), sagA (streptolysin S), and speMF (mitogenic factor) but did not affect transcription of slo (streptolysin O), mga (multiple gene regulator of GAS), emm (M protein), scpA (complement C5a peptidase), or speB or speC (pyrogenic exotoxins B and C). The amounts of streptokinase, streptolysin S, and capsule paralleled the levels of transcription of their genes in all cases. Because CsrR represses genes unrelated to those for capsule synthesis, and because CsrA-CsrB is a global regulatory system in Escherichia coli whose mechanism is unrelated to that of these genes in GAS, the locus has been renamed covR, for "control of virulence genes" in GAS. Transcription of the covR operon was also increased in the nonpolar insertion mutant, indicating that CovR represses its own synthesis as well. All phenotypes of the covR nonpolar insertion mutant were complemented by the covR gene on a plasmid. CovR acts on operons expressed both in exponential and in stationary phase, demonstrating that the CovR-CovS pathway is separate from growth phase-dependent regulation in GAS. Therefore, CovR is the first multiple-gene repressor of virulence factors described for this important human pathogen.

Role of mga in growth phase regulation of virulence genes of the group A streptococcus

To determine whether growth phase affects the expression of mga and other virulence-associated genes in the group A streptococcus (GAS), total RNA was isolated from the serotype M6 GAS strain JRS4 at different phases of growth and transcript levels were quantitated by hybridization with radiolabeled DNA probes. Expression of mga (which encodes a multiple gene regulator) and the Mga-regulated genes emm (which encodes M protein) and scpA (which encodes a complement C5a peptidase) was found to be maximal in exponential phase and shut off as the bacteria entered stationary phase, while the housekeeping genes recA and rpsL showed constant transcript levels over the same period of growth. Expression of mga from a foreign phage promoter in a mga-deleted GAS strain (JRS519) altered the wild-type growth phase-dependent transcription profile seen for emm and scpA, as well as for mga. Therefore, the temporal control of mga expression requires its upstream promoter region, and the subsequent growth phase regulation of emm and scpA is Mga dependent. A number of putative virulence genes in JRS4 were shown not to require Mga for their expression, although several exhibited growth phase-dependent regulation that was similar to mga, i.e., slo (which encodes streptolysin O) and plr (encoding the plasmin receptor/glyceraldehyde-3-phosphate dehydrogenase). Still others showed a markedly different pattern of expression (the genes for the superantigen toxins MF and SpeC). These results suggest the existence of complex levels of global regulation sensitive to growth phase that directly control the expression of virulence genes and mga in GAS.

Identification of isp, a locus encoding an immunogenic secreted protein conserved among group A streptococci

The protein Mga (mga), which is required for transcription of several virulence genes of group A streptococci (GAS), including the antiphagocytic M protein, was suggested to act as the response regulator element of a bacterial two-component pathway. To investigate whether a gene encoding a cognate sensor protein is located upstream of mga, 3.1 kb of DNA 5' of the mga translational start site was cloned from serotype M6 GAS strain JRS4. Sequence analysis of this region revealed two adjacent open reading frames, a previously described orf and a new locus, isp (immunogenic secreted protein), which could encode proteins of 9 and 59 kDa, respectively. Inactivation of either open reading frame had no significant effect on transcription of the gene encoding M protein (emm) under normal growth conditions, suggesting that neither isp nor orf is involved in the Mga regulatory circuit. A protein migrating at an apparent molecular weight of 65,000 was produced when isp was transcribed and translated in vitro. The predicted isp product (Isp) contains an amino-terminal signal sequence region homologous to that found in bacterial secreted proteins, and expression of isp in Escherichia coli resulted in the presence of Isp in the periplasmic fraction. Convalescent-phase serum from a patient with an active GAS infection recognized forms of Isp both from the periplasm of E. coli and the supernatant of a GAS strain. Both isp and orf are highly conserved among strains of GAS, as shown by hybridization analyses.

The elastase propeptide functions as an intramolecular chaperone required for elastase activity and secretion in Pseudomonas aeruginosa

Several proteases are secreted by Pseudomonas aeruginosa including elastase, an abundantly secreted neutral zinc-metalloprotease. Elastase (encoded by lasB) is first synthesized with a relatively large propeptide (18 kDa) domain. Here, we present evidence that this propeptide functions as an intramolecular chaperone (IMC) essential for proper maturation of elastase into a hydrolytically active enzyme. An altered elastase allele (lasB6) that encoded an elastase precursor with a precise propeptide deletion was expressed in Escherichia coli, and disrupted cells contained only inactive elastase. However, co-expression of an allele (lasB7) expressing the propeptide as an independent, non-covalently linked protein rescued about one-third of the hydrolytic activity when compared with that obtained with wild-type lasB. Thus, the propeptide was essential for elastase activity and so defined elastase as an IMC-containing protease. We examined the possibility that the propeptide of elastase also plays a role in the localization of the mature protein past the outer bacterial membrane. Expression of lasB6 in P. aeruginosa (lasB delta) in the absence of the propeptide resulted in production of inactive elastase that accumulated within the cell and was not secreted to the culture medium. When lasB7 co-expressed the non-covalently linked propeptide in the same cell with lasB6, efficient secretion was restored and active elastase was then found in the supernatant. Thus, the propeptide was needed for secretion of the mature protein as well as enzymatic activity. This chaperone-like activity of the propeptide appears to involve a direct interaction between the mature and propeptide sequences, and evidence for this was obtained by demonstrating that the non-covalently attached 18 kDa propeptide was co-precipitated with elastase using elastase antibodies. These results are consistent with a hypothesis that the propeptide domain acts as an IMC to control both enzymatic activity and competence for secretion.

Regulation of virulence by environmental signals in group A streptococci: influence of osmolarity, temperature, gas exchange, and iron limitation on emm transcription

Transcription of the gene encoding the antiphagocytic M protein (emm) of the group A streptococci has been shown to be regulated by CO2 (M. G. Caparon, R. T. Geist, J. Perez-Casal, and J. R. Scott, J. Bacteriol. 174:5693-5701, 1992). We tested the influence of additional environmental growth conditions on emm expression. Increased osmolarity, low temperature, growth with free exchange of gasses, or the restricted availability of iron resulted in decreased transcription from the emm promoter.