Plasminogen is a critical host pathogenicity factor for group A streptococcal infection

Comparative genomic analysis of the Streptococcus dysgalactiae species group: gene content, molecular adaptation, and promoter evolution

Comparative genomics of closely related bacterial species with different pathogenesis and host preference can provide a means of identifying the specifics of adaptive differences. Streptococcus dysgalactiae (SD) is comprised of two subspecies: S. dysgalactiae subsp. equisimilis is both a human commensal organism and a human pathogen, and S. dysgalactiae subsp. dysgalactiae is strictly an animal pathogen. Here, we present complete genome sequences for both taxa, with analyses involving other species of Streptococcus but focusing on adaptation in the SD species group. We found little evidence for enrichment in biochemical categories of genes carried by each SD strain, however, differences in the virulence gene repertoire were apparent. Some of the differences could be ascribed to prophage and integrative conjugative elements. We identified approximately 9% of the nonrecombinant core genome to be under positive selection, some of which involved known virulence factors in other bacteria. Analyses of proteomes by pooling data across genes, by biochemical category, clade, or branch, provided evidence for increased rates of evolution in several gene categories, as well as external branches of the tree. Promoters were primarily evolving under purifying selection but with certain categories of genes evolving faster. Many of these fast-evolving categories were the same as those associated with rapid evolution in proteins. Overall, these results suggest that adaptation to changing environments and new hosts in the SD species group has involved the acquisition of key virulence genes along with selection of orthologous protein-coding loci and operon promoters.

The nonideal coiled coil of M protein and its multifarious functions in pathogenesis

The M protein is a major virulence factor of Streptococcus pyogenes (group A Streptococcus, GAS). This gram-positive bacterial pathogen is responsible for mild infections, such as pharyngitis, and severe invasive disease, like streptococcal toxic shock syndrome. M protein contributes to GAS virulence in multifarious ways, including blocking deposition of antibodies and complement, helping formation of microcolonies, neutralizing antimicrobial peptides, and triggering a proinflammatory and procoagulatory state. These functions are specified by interactions between M protein and many host components, especially C4BP and fibrinogen. The former interaction is conserved among many antigenically variant M protein types but occurs in a strikingly sequence-independent manner, and the latter is associated in the M1 protein type with severe invasive disease. Remarkably for a protein of such diverse interactions, the M protein has a relatively simple but nonideal α-helical coiled coil sequence. This sequence nonideality is a crucial feature of M protein. Nonideal residues give rise to specific irregularities in its coiled-coil structure, which are essential for interactions with fibrinogen and establishment of a proinflammatory state. In addition, these structural irregularities are reminiscent of those in myosin and tropomyosin, which are targets for crossreactive antibodies in patients suffering from autoimmune sequelae of GAS infection.

Contribution of plasminogen activation towards the pathogenic potential of oral streptococci

Oral streptococci are a heterogeneous group of human commensals, with a potential to cause serious infections. Activation of plasminogen has been shown to increase the virulence of typical human pathogenic streptococci such as S. pneumoniae. One important factor for plasminogen activation is the streptococcal α-enolase. Here we report that plasminogen activation is also common in oral streptococci species involved in clinical infection and that it depends on the action of human plasminogen activators. The ability to activate plasminogen did not require full conservation of the internal plasminogen binding sequence motif FYDKERKVY of α-enolase that was previously described as crucial for increased plasminogen binding, activation and virulence. Instead, experiments with recombinant α-enolase variants indicate that the naturally occurring variations do not impair plasminogen binding. In spite of these variations in the internal plasminogen binding motif oral streptococci showed similar activation of plasminogen. We conclude that the pathomechanism of plasminogen activation is conserved in oral streptococci that cause infections in human. This may contribute to their opportunistic pathogenic character that is unfurled in certain niches.

Small RNAs promote mRNA stability to activate the synthesis of virulence factors

Although most bacterial small RNAs act to repress target mRNAs, some also activate messengers. The predominant mode of activation has been seen in 'anti-antisense' regulation whereby a small RNA prevents the formation of an inhibitory 5' mRNA structure that otherwise impairs translational initiation and protein synthesis. The translational activation might also stabilize the target yet this was considered a secondary effect in the examples known thus far. Two recent papers in Molecular Microbiology investigate post-transcriptional activation of collagenase mRNA by Clostridium VR-RNA, and streptokinase mRNA by Streptococcus FasX RNA, to suggest that small RNAs exert positive regulation of virulence genes primarily at the level of mRNA stabilization.

The group A Streptococcus small regulatory RNA FasX enhances streptokinase activity by increasing the stability of the ska mRNA transcript

Small RNA molecules play key regulatory roles in many bacterial species. However, little mechanistic data exists for the action of small regulatory RNAs in the human pathogen group A Streptococcus (GAS). Here, we analysed the relationship between a putative GAS sRNA and production of the secreted virulence factor streptokinase (SKA). SKA promotes GAS dissemination by activating conversion of host plasminogen into the fibrin-degrading protease plasmin. Homologues of the putative sRNA-encoding gene fibronectin/fibrinogen-binding/haemolytic-activity/streptokinase-regulator-X (fasX) were identified in four different pyogenic streptococcal species. However, despite 79% fasX nucleotide identity, a fasX allele from the animal pathogen Streptococcus zooepidemicus failed to complement a GAS fasX mutant. Using a series of precisely constructed fasX alleles we discovered that FasX is a bona-fide sRNA that post-transcriptionally regulates SKA production in GAS. By base-pairing to the 5' end of ska mRNA, FasX enhances ska transcript stability, resulting in a ∼10-fold increase in SKA activity. Our data provide new insights into the mechanisms used by small regulatory RNAs to activate target mRNAs, and enhances our understanding of the regulation of a key GAS virulence factor.

Histidine-rich glycoprotein promotes bacterial entrapment in clots and decreases mortality in a mouse model of sepsis

Streptococcus pyogenes is a significant bacterial pathogen in humans. In this study, histidine-rich glycoprotein (HRG), an abundant plasma protein, was found to kill S pyogenes. Furthermore, S pyogenes grew more efficiently in HRG-deficient plasma, and clots formed in this plasma were significantly less effective at bacterial entrapment and killing. HRG-deficient mice were strikingly more susceptible to S pyogenes infection. These animals failed to control the infection at the local subcutaneous site, and abscess formation and inflammation were diminished compared with control animals. As a result, bacterial dissemination occurred more rapidly in HRG-deficient mice, and they died earlier and with a significantly higher mortality rate than control animals. HRG-deficient mice supplemented with purified HRG gave the same phenotype as control animals, demonstrating that the lack of HRG was responsible for the increased susceptibility. The results demonstrate a previously unappreciated role for HRG as a regulator of inflammation and in the defense at the local site of bacterial infection.

Corruption of innate immunity by bacterial proteases

The innate immune system of the human body has developed numerous mechanisms to control endogenous and exogenous bacteria and thus prevent infections by these microorganisms. These mechanisms range from physical barriers such as the skin or mucosal epithelium to a sophisticated array of molecules and cells that function to suppress or prevent bacterial infection. Many bacteria express a variety of proteases, ranging from non-specific and powerful enzymes that degrade many proteins involved in innate immunity to proteases that are extremely precise and specific in their mode of action. Here we have assembled a comprehensive picture of how bacterial proteases affect the host's innate immune system to gain advantage and cause infection. This picture is far from being complete since the numbers of mechanisms utilized are as astonishing as they are diverse, ranging from degradation of molecules vital to innate immune mechanisms to subversion of the mechanisms to allow the bacterium to hide from the system or take advantage of it. It is vital that such mechanisms are elucidated to allow strategies to be developed to aid the innate immune system in controlling bacterial infections.

Parameters governing invasive disease propensity of non-M1 serotype group A streptococci

Group A Streptococcus (GAS) causes rare but life-threatening syndromes of necrotizing fasciitis and toxic shock-like syndrome in humans. The GAS serotype M1T1 clone has globally disseminated, and mutations in the control of virulence regulatory sensor kinase (covRS) operon correlate with severe invasive disease. Here, a cohort of non-M1 GAS was screened to determine whether mutation in covRS triggers systemic dissemination in divergent M serotypes. A GAS disease model defining parameters governing invasive propensity of differing M types is proposed. The vast majority of GAS infection is benign. Nonetheless, many divergent M types possess limited capacity to cause invasive infection. M1T1 GAS readily switch to a covRS mutant form that is neutrophil resistant and frequently associated with systemic infection. Whilst non-M1 GAS are shown in this study to less frequently accumulate covRS mutations in vivo, such mutants are isolated from invasive infections and exhibit neutrophil resistance and enhanced virulence. The reduced capacity of non-M1 GAS to switch to the hypervirulent covRS mutant form provides an explanation for the comparatively less frequent isolation of non-M1 serotypes from invasive human infections.

Reciprocal coupling of coagulation and innate immunity via neutrophil serine proteases

Blood neutrophils provide the first line of defense against pathogens but have also been implicated in thrombotic processes. This dual function of neutrophils could reflect an evolutionarily conserved association between blood coagulation and antimicrobial defense, although the molecular determinants and in vivo significance of this association remain unclear. Here we show that major microbicidal effectors of neutrophils, the serine proteases neutrophil elastase and cathepsin G, together with externalized nucleosomes, promote coagulation and intravascular thrombus growth in vivo. The serine proteases and extracellular nucleosomes enhance tissue factor- and factor XII-dependent coagulation in a process involving local proteolysis of the coagulation suppressor tissue factor pathway inhibitor. During systemic infection, activation of coagulation fosters compartmentalization of bacteria in liver microvessels and reduces bacterial invasion into tissue. In the absence of a pathogen challenge, neutrophil-derived serine proteases and nucleosomes can contribute to large-vessel thrombosis, the main trigger of myocardial infarction and stroke. The ability of coagulation to suppress pathogen dissemination indicates that microvessel thrombosis represents a physiological tool of host defense.

Plasma lipoproteins are important components of the immune system

Plasma lipoproteins (VLDL, LDL, Lp[a] and HDL) function primarily in lipid transport among tissues and organs. However, cumulative evidence suggests that lipoproteins may also prevent bacterial, viral and parasitic infections and are therefore a component of innate immunity. Lipoproteins can also detoxify lipopolysaccharide and lipoteichoic acid. Infections can induce oxidation of LDL, and oxLDL in turn plays important anti-infective roles and protects against endotoxin-induced tissue damage. There is also evidence that apo(a) is protective against pathogens. Taken together, the evidence suggests that it might be valuable to introduce the concept that plasma lipoproteins belong in the realm of host immune response.

Plasma lipoproteins are important components of the immune system

Plasma lipoproteins (VLDL, LDL, Lp[a] and HDL) function primarily in lipid transport among tissues and organs. However, cumulative evidence suggests that lipoproteins may also prevent bacterial, viral and parasitic infections and are therefore a component of innate immunity. Lipoproteins can also detoxify lipopolysaccharide and lipoteichoic acid. Infections can induce oxidation of LDL, and oxLDL in turn plays important anti-infective roles and protects against endotoxin-induced tissue damage. There is also evidence that apo(a) is protective against pathogens. Taken together, the evidence suggests that it might be valuable to introduce the concept that plasma lipoproteins belong in the realm of host immune response.

NMR backbone dynamics of VEK-30 bound to the human plasminogen kringle 2 domain

To gain insights into the mechanisms for the tight and highly specific interaction of the kringle 2 domain of human plasminogen (K2(Pg)) with a 30-residue internal peptide (VEK-30) from a group A streptococcal M-like protein, the dynamic properties of free and bound K2(Pg) and VEK-30 were investigated using backbone amide (15)N-NMR relaxation measurements. Dynamic parameters, namely the generalized order parameter, S(2), the local correlation time, tau(e), and the conformational exchange contribution, R(ex), were obtained for this complex by Lipari-Szabo model-free analysis. The results show that VEK-30 displays distinctly different dynamic behavior as a consequence of binding to K2(Pg), manifest by decreased backbone flexibility, particularly at the binding region of the peptide. In contrast, the backbone dynamics parameters of K2(Pg) displayed similar patterns in the free and bound forms, but, nonetheless, showed interesting differences. Based on our previous structure-function studies of this interaction, we also made comparisons of the VEK-30/K2(Pg) dynamics results from different kringle modules complexed with small lysine analogs. The differences in dynamics observed for kringles with different ligands provide what we believe to be new insights into the interactions responsible for protein-ligand recognition and a better understanding of the differences in binding affinity and binding specificity of kringle domains with various ligands.

Tranexamic acid, an inhibitor of plasminogen activation, aggravates staphylococcal septic arthritis and sepsis

Haemostatic balance shifts towards pro-coagulation during infection. Plasminogen, a key molecule of fibrinolysis, may play an important role in the pathogenesis of staphylococcal infections. In the present study, we assessed the impact of inhibition of plasminogen activation by tranexamic acid on the course of staphylococcal sepsis and septic arthritis in mice. We found significantly down-regulated plasmin activity and increased D-dimer levels in the blood from the mice with staphylococcal sepsis. Treatment with tranexamic acid significantly increased the severity and mortality of staphylococcal infection. In addition, tranexamic acid reduced the survival rate in a murine model for staphylococcal enterotoxin A-induced death. The aggravation of diseases by tranexamic acid was due neither to the pro-inflammatory cytokine network, nor to impairment of bacterial clearance. Modulation of fibrinolysis, either by supplement of fibrinolytic molecules (tissue plasminogen activator or plasmin) or by fibrinogen depletion, did not reduce the mortality of staphylococcal sepsis. In conclusion, we report that treatment with tranexamic acid led to distinct aggravation of staphylococcal septic arthritis and sepsis in mice, suggesting the clinical importance of fibrinolytic balance in staphylococcal infection.

Effects on human plasminogen conformation and activation rate caused by interaction with VEK-30, a peptide derived from the group A streptococcal M-like protein (PAM)

In vertebrates, fibrinolysis is primarily carried out by the serine protease plasmin (Pm), which is derived from activation of the zymogen precursor, plasminogen (Pg). One of the most distinctive features of Pg/Pm is the presence of five homologous kringle (K) domains. These structural elements possess conserved Lys-binding sites (LBS) that facilitate interactions with substrates, activators, inhibitors and receptors. In human Pg (hPg), K2 displays weak Lys affinity, however the LBS of this domain has been implicated in an atypical interaction with the N-terminal region of a bacterial surface protein known as PAM (Pg-binding group A streptococcal M-like protein). A direct correlation has been established between invasiveness of group A streptococci and their ability to bind Pg. It has been previously demonstrated that a 30-residue internal peptide (VEK-30) from the N-terminal region of PAM competitively inhibits binding of the full-length parent protein to Pg. We have attempted to determine the effects of this ligand-protein interaction on the regulation of Pg zymogen activation and conformation. Our results show minimal effects on the sedimentation velocity coefficients (S degrees (20,w)) of Pg when associated to VEK-30 and a direct relationship between the concentration of VEK-30 or PAM and the activation rate of Pg. These results are in contrast with the major conformational changes elicited by small-molecule activators of Pg, and point towards a novel mechanism of Pg activation that may underlie group A streptococcal (GAS) virulence.

Streptococcal M proteins and their role as virulence determinants

Group A streptococci (GAS, Streptococcus pyogenes) are exclusive human pathogens that have been extensively studied for many decades. The spectrum of diseases caused by these bacteria ranges from uncomplicated and superficial to severe and invasive infections. In order to give rise to these complications, GAS have evolved a number of surface-bound and secreted virulence factors, of which the M proteins are probably the best characterized. Evidence has emerged that M proteins are multifunctional pathogenic determinants, and over the years many interactions between M proteins and the human host have been reported. The present review article aims to present a state-of-the-art overview of the most important virulence mechanisms employed by M proteins to trigger disease.

Skizzle is a novel plasminogen- and plasmin-binding protein from Streptococcus agalactiae that targets proteins of human fibrinolysis to promote plasmin generation

Skizzle (SkzL), secreted by Streptococcus agalactiae, has moderate sequence identity to streptokinase and staphylokinase, bacterial activators of human plasminogen (Pg). SkzL binds [Glu]Pg with low affinity (K(D) 3-16 mum) and [Lys]Pg and plasmin (Pm) with indistinguishable high affinity (K(D) 80 and 50 nm, respectively). Binding of SkzL to Pg and Pm is completely lysine-binding site-dependent, as shown by the effect of the lysine analog, 6-aminohexanoic acid. Deletion of the COOH-terminal SkzL Lys(415) residue reduces affinity for [Lys]Pg and active site-blocked Pm 30-fold, implicating Lys(415) in a lysine-binding site interaction with a Pg/Pm kringle. SkzL binding to active site fluorescein-labeled Pg/Pm analogs demonstrates distinct high and low affinity interactions. High affinity binding is mediated by Lys(415), whereas the source of low affinity binding is unknown. SkzL enhances the activation of [Glu]Pg by urokinase (uPA) approximately 20-fold, to a maximum rate indistinguishable from that for [Lys]Pg and [Glu]Pg activation in the presence of 6-aminohexanoic acid. SkzL binds preferentially to the partially extended beta-conformation of [Glu]Pg, which is in unfavorable equilibrium with the compact alpha-conformation, thereby converting [Glu]Pg to the fully extended gamma-conformation and accelerating the rate of its activation by uPA. SkzL enhances [Lys]Pg and [Glu]Pg activation by single-chain tissue-type Pg activator, approximately 42- and approximately 650-fold, respectively. SkzL increases the rate of plasma clot lysis by uPA and single-chain tissue-type Pg activator approximately 2-fold, confirming its cofactor activity in a physiological model system. The results suggest a role for SkzL in S. agalactiae pathogenesis through fibrinolytic enhancement.

Highly frequent mutations in negative regulators of multiple virulence genes in group A streptococcal toxic shock syndrome isolates

Streptococcal toxic shock syndrome (STSS) is a severe invasive infection characterized by the sudden onset of shock and multiorgan failure; it has a high mortality rate. Although a number of studies have attempted to determine the crucial factors behind the onset of STSS, the responsible genes in group A Streptococcus have not been clarified. We previously reported that mutations of csrS/csrR genes, a two-component negative regulator system for multiple virulence genes of Streptococcus pyogenes, are found among the isolates from STSS patients. In the present study, mutations of another negative regulator, rgg, were also found in clinical isolates of STSS patients. The rgg mutants from STSS clinical isolates enhanced lethality and impaired various organs in the mouse models, similar to the csrS mutants, and precluded their being killed by human neutrophils, mainly due to an overproduction of SLO. When we assessed the mutation frequency of csrS, csrR, and rgg genes among S. pyogenes isolates from STSS (164 isolates) and non-invasive infections (59 isolates), 57.3% of the STSS isolates had mutations of one or more genes among three genes, while isolates from patients with non-invasive disease had significantly fewer mutations in these genes (1.7%). The results of the present study suggest that mutations in the negative regulators csrS/csrR and rgg of S. pyogenes are crucial factors in the pathogenesis of STSS, as they lead to the overproduction of multiple virulence factors.

Group A streptococcus (GAS) causes life-threatening severe invasive diseases, including necrotizing fasciitis and streptococcal toxic shock-like syndrome. Although many studies have attempted to determine factors that are crucial for the onset of streptococcal toxic shock syndrome (STSS), bacterial factors responsible for it have not been clarified. By comparing genome sequences of clinical GAS isolates from STSS with those of non-invasive infections, we showed that mutations of negative regulator genes (csrS, csrR, rgg) were detected at a high frequency of more than 50% in STSS isolates, but at a low frequency of less than 2% in non-invasive isolates. These mutations of negative regulators were found in various emm-genotyped STSS isolates but not in a particular emm genotype. These mutants enhanced virulence in mouse models. Such results indicated that mutations of bacterial negative regulators are crucial for the pathogenesis of STSS due to the overproduction of multiple virulence factors under the de-repressed conditions.

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.

The streptococcal M protein: a highly versatile molecule

Interaction of the M-protein of group A Streptococcus (GAS) with its numerous host binding partners might assist the bacteria in evading host immune responses. Although the extensive diversity of this protein has been highlighted by different GAS typing schemes, most of the structural and functional information has been obtained from a limited number of types. Increasing numbers of epidemiological, clinical and biological reports suggest that the structure and function of the M protein is less conserved than previously thought. This review focuses on the known interactions between M proteins and host ligand proteins, emphasizing that our understanding of this well-studied molecule is fragmented.

Molecular pathogenesis of necrotizing fasciitis

Necrotizing fasciitis, also known as the flesh-eating disease, is a severe invasive infection associated with very high rates of human morbidity and mortality. It is most commonly caused by group A Streptococcus(GAS), a versatile human pathogen that causes diseases ranging in severity from uncomplicated pharyngitis (or strep throat) to life-threatening infections such as necrotizing fasciitis. Herein, we review recent discoveries bearing on the molecular pathogenesis of GAS necrotizing fasciitis. Importantly, the integration of new technologies and the development of human-relevant animal models have markedly expanded our understanding of the key pathogen-host interactions underlying GAS necrotizing fasciitis. For example, we now know that GAS organisms secrete a variety of proteases that disrupt host tissue and that these proteolytic enzymes are regulated by multiple transcriptional and posttranslational processes. This pathogenesis knowledge will be crucial to supporting downstream efforts that seek to develop novel vaccines and therapeutic agents for this serious human infection.

Binding and activation of host plasminogen on the surface of Francisella tularensis

Background

Francisella tularensis (FT) is a gram-negative facultative intracellular coccobacillus and is the causal agent of a life-threatening zoonotic disease known as tularemia. Although FT preferentially infects phagocytic cells of the host, recent evidence suggests that a significant number of bacteria can be found extracellularly in the plasma fraction of the blood during active infection. This observation suggests that the interaction between FT and host plasma components may play an important role in survival and dissemination of the bacterium during the course of infection. Plasminogen (PLG) is a protein zymogen that is found in abundance in the blood of mammalian hosts. A number of both gram-positive and gram-negative bacterial pathogens have the ability to bind to PLG, giving them a survival advantage by increasing their ability to penetrate extracellular matrices and cross tissue barriers.

Results

We show that PLG binds to the surface of FT and that surface-bound PLG can be activated to plasmin in the presence of tissue PLG activator in vitro. In addition, using Far-Western blotting assays coupled with proteomic analyses of FT outer membrane preparations, we have identified several putative PLG-binding proteins of FT.

Conclusions

The ability of FT to acquire surface bound PLG that can be activated on its surface may be an important virulence mechanism that results in an increase in initial infectivity, survival, and/or dissemination of this bacterium in vivo.

Pathogen entrapment by transglutaminase--a conserved early innate immune mechanism

Clotting systems are required in almost all animals to prevent loss of body fluids after injury. Here, we show that despite the risks associated with its systemic activation, clotting is a hitherto little appreciated branch of the immune system. We compared clotting of human blood and insect hemolymph to study the best-conserved component of clotting systems, namely the Drosophila enzyme transglutaminase and its vertebrate homologue Factor XIIIa. Using labelled artificial substrates we observe that transglutaminase activity from both Drosophila hemolymph and human blood accumulates on microbial surfaces, leading to their sequestration into the clot. Using both a human and a natural insect pathogen we provide functional proof for an immune function for transglutaminase (TG). Drosophila larvae with reduced TG levels show increased mortality after septic injury. The same larvae are also more susceptible to a natural infection involving entomopathogenic nematodes and their symbiotic bacteria while neither phagocytosis, phenoloxidase or—as previously shown—the Toll or imd pathway contribute to immunity. These results firmly establish the hemolymph/blood clot as an important effector of early innate immunity, which helps to prevent septic infections. These findings will help to guide further strategies to reduce the damaging effects of clotting and enhance its beneficial contribution to immune reactions.

One of the main functions of immune systems is to prevent the dissemination of microbes and the resulting sepsis. Blood clotting during sepsis has until now been primarily regarded as harmful, leading to the formation of widespread clots in blood vessels and as a result to organ failure. Here we show that clotting also has a protective function to limit and prevent infections. This is achieved by capturing bacteria in the clot. Our infection studies were performed in the insect model Drosophila melanogaster where, due to the presence of an open circulatory system, the negative effects of clotting are less pronounced. We show that clotting of hemolymph—the insect blood equivalent—is essential in Drosophila to prevent septic death arising from injection of bacteria or infection with a natural pathogen. We also show that both Drosophila transglutaminase and its human homologue clotting factor XIII are key enzymes for sequestration of bacteria in the clot matrix, indicating the conserved nature of the clot's function in immunity. Our data are expected to lead to a much stronger appreciation of the role of blood clotting in innate immunity, and will guide future therapies which target this process.

Oxidative modification of von Willebrand factor by neutrophil oxidants inhibits its cleavage by ADAMTS13

Elevated plasma von Willebrand factor (VWF) and low ADAMTS13 activity have been reported in several inflammatory states, including sepsis and acute respiratory distress syndrome. One hallmark of inflammation is neutrophil activation and production of reactive oxygen species, including superoxide radical, hydrogen peroxide, and hypochlorous acid (HOCl). HOCl is produced from hydrogen peroxide and chloride ions through the action of myeloperoxidase. HOCl can oxidize methionine to methionine sulfoxide and tyrosine to chlorotyrosine. This is of interest because the ADAMTS13 cleavage site in VWF, the Tyr(1605)-Met(1606) peptide bond, contains both oxidation-prone residues. We hypothesized that HOCl would oxidize either or both of these residues and possibly inhibit ADAMTS13-mediated cleavage. We therefore treated ADAMTS13 substrates with HOCl and examined their oxidative modification by mass spectrometry. Met(1606) was oxidized to the sulfoxide in a concentration-dependent manner, with complete oxidation at 75muM HOCl, whereas only a miniscule percentage of Tyr(1605) was converted to chlorotyrosine. The oxidized substrates were cleaved much more slowly by ADAMTS13 than the nonoxidized substrates. A similar result was obtained with multimeric VWF. Taken together, these findings indicate that reactive oxygen species released by activated neutrophils have a prothrombotic effect, mediated in part by inhibition of VWF cleavage by ADAMTS13.

The secreted esterase of group a streptococcus is important for invasive skin infection and dissemination in mice

Virulence factors regulated by the CovRS/CsrRS two-component gene regulatory system contribute to the invasive diseases caused by group A Streptococcus (GAS). To determine whether the streptococcal secreted esterase (Sse), an antigen that protects against subcutaneous GAS infection, is one of these virulence factors, we investigated the phenotype of a nonpolar sse deletion mutant strain (Deltasse). In addition, we examined the effects of covS mutation on sse expression. As assessed using a mouse model of subcutaneous infection, the virulence of the Deltasse strain is attenuated and the overall pathology is reduced. Furthermore, GAS was detected in the blood and spleens from mice subcutaneously infected with the parental strain, whereas mice subcutaneously infected with the Deltasse strain had no GAS present in their blood and spleens. The ability of the mutant to survive in the subcutis of mice appeared to be compromised. The growth of the Deltasse strain in rich and chemically defined media and nonimmune human blood and sera was slower than that of the wild-type strain. Complementation restored the phenotype of the Deltasse strain to that of the wild-type strain. The wild-type, Deltasse, and complement strains had no detectable SpeB activity. Expression of Sse is negatively controlled by CovRS. These findings suggest that Sse is a CovRS-regulated virulence factor that is important for the virulence of GAS in subcutaneous infection and plays an important role in severe soft tissue infections and systemic dissemination of GAS from the skin.

Clindamycin-induced CovS-mediated regulation of the production of virulent exoproteins streptolysin O, NAD glycohydrolase, and streptokinase in Streptococcus pyogenes

The administration of high-dose clindamycin (CLI) along with penicillin is recommended for the treatment of streptococcal toxic shock syndrome. However, the prevalence of CLI-resistant Streptococcus pyogenes strains is increasing worldwide, and the effect of CLI on CLI-resistant S. pyogenes strains remains unknown. We aimed to evaluate the effect of CLI on the in vitro production of three major virulent exoproteins, namely, streptolysin O (Slo), NAD glycohydrolase (Nga), and streptokinase (Ska), by CLI-resistant S. pyogenes strains. After the incubation of M1 serotype CLI-resistant S. pyogenes D2TY in the presence of 1 microg/ml CLI, the amounts of Slo, Nga, and Ska and the levels of slo, nga, and ska mRNA in the supernatant were analyzed by Northern blotting and Western blotting, respectively. The results of both assays showed that the production of Slo, Nga, and Ska was higher with CLI treatment than without CLI treatment. We evaluated the role of the sensor kinase CovS, which is involved in the two-component system of S. pyogenes, in the CLI-induced production of these three exoproteins. Northern blotting analysis revealed that CLI induced the expression of covS mRNA in wild-type strain D2TY. Furthermore, both Northern blotting and Western blotting analyses showed that CLI decreased the levels of expression of Slo, Nga, and Ska in isogenic covS mutant D2TYcovS. These results suggest that CLI increases the production of three virulent exoproteins in CLI-resistant S. pyogenes strains via the action of CovS.

Functional dissection of Streptococcus pyogenes M5 protein: the hypervariable region is essential for virulence

The surface-localized M protein of Streptococcus pyogenes is a major virulence factor that inhibits phagocytosis, as determined ex vivo. Because little is known about the role of M protein in vivo we analyzed the contribution of different M protein regions to virulence, using the fibrinogen (Fg)-binding M5 protein and a mouse model of acute invasive infection. This model was suitable, because M5 is required for mouse virulence and binds mouse and human Fg equally well, as shown here. Mixed infection experiments with wild type bacteria demonstrated that mutants lacking the N-terminal hypervariable region (HVR) or the Fg-binding B-repeat region were strongly attenuated, while a mutant lacking the conserved C-repeats was only slightly attenuated. Because the HVR of M5 is not required for phagocytosis resistance, our data imply that this HVR plays a major but unknown role during acute infection. The B-repeat region is required for phagocytosis resistance and specifically binds Fg, suggesting that it promotes virulence by binding Fg. However, B-repeat mutants were attenuated even in Fg-deficient mice, implying that the B-repeats may have a second function, in addition to Fg-binding. These data demonstrate that two distinct M5 regions, including the HVR, are essential to virulence during the early stages of an infection. In particular, our data provide the first in vivo evidence that the HVR of an M protein plays a major role in virulence, focusing interest on the molecular role of this region.

Plasminogen substrate recognition by the streptokinase-plasminogen catalytic complex is facilitated by Arg253, Lys256, and Lys257 in the streptokinase beta-domain and kringle 5 of the substrate

Streptokinase (SK) conformationally activates the central zymogen of the fibrinolytic system, plasminogen (Pg). The SK.Pg* catalytic complex binds Pg as a specific substrate and cleaves it into plasmin (Pm), which binds SK to form the SK.Pm complex that propagates Pm generation. Catalytic complex formation is dependent on lysine-binding site (LBS) interactions between a Pg/Pm kringle and the SK COOH-terminal Lys(414). Pg substrate recognition is also LBS-dependent, but the kringle and SK structural element(s) responsible have not been identified. SK mutants lacking Lys(414) with Ala substitutions of charged residues in the SK beta-domain 250-loop were evaluated in kinetic studies that resolved conformational and proteolytic Pg activation. Activation of [Lys]Pg and mini-Pg (containing only kringle 5 of Pg) by SK with Ala substitutions of Arg(253), Lys(256), and Lys(257) showed decreases in the bimolecular rate constant for Pm generation, with nearly total inhibition for the SK Lys(256)/Lys(257) double mutant. Binding of bovine Pg (BPg) to the SK.Pm complex containing fluorescently labeled Pm demonstrated LBS-dependent assembly of a SK.labeled Pm.BPg ternary complex, whereas BPg did not bind to the complex containing the SK Lys(256)/Lys(257) mutant. BPg was activated by SK.Pm with a K(m) indistinguishable from the K(D) for BPg binding to form the ternary complex, whereas the SK Lys(256)/Lys(257) mutant did not support BPg activation. We conclude that SK residues Arg(253), Lys(256), and Lys(257) mediate Pg substrate recognition through kringle 5 of the [Lys]Pg and mini-Pg substrates. A molecular model of the SK.kringle 5 complex identifies the putative interactions involved in LBS-dependent Pg substrate recognition.

Reduction of canine plasminogen leads to an expanded molecule which precipitates

Canine plasminogen is made up of seven domains. In each domain there are several cysteines that are linked by disulfide bonds. Reduction of a limited number of the cystines destabilizes the protein such that it precipitates. The bond or bonds that are broken provide about 14 kcal of stabilization energy. Circular dichroism and dynamic light scattering indicate that there is probably an intermediate that is formed prior to precipitation and that the intermediate is somewhat larger than the compact form of plasminogen.

Critical roles for thrombin in acute and chronic inflammation

Thrombin can amplify inflammation induced by other stimuli, either through ischemia (consequent upon thrombosis), indirectly through generation of downstream mediators such as activated protein C, or directly via signals through protease activated receptors (PAR). This paper will summarize recent data from our laboratory indicating that thrombin is required to initiate CCR2-dependent leukocyte recruitment and that it is the principal determinant of the outcome after vascular injury, via PAR-1 activation of a distinct subset of smooth muscle cell progenitors. In both, tissue factor (TF) initiates thrombin generation and the thrombin acts locally, exemplifying that the initiation phase can generate autocrine or paracrine signalling molecules. Thrombin is an important constituent of innate immunity, able to amplify and modify responses to invading pathogens or tissue damage. With novel anti-thrombin therapeutics and agents to target PAR, a new understanding of the importance of thrombin may allow the development of innovative anti-inflammatory strategies.

Defining the structural basis of human plasminogen binding by streptococcal surface enolase

The flesh-eating bacterium group A Streptococcus (GAS) binds and activates human plasminogen, promoting invasive disease. Streptococcal surface enolase (SEN), a glycolytic pathway enzyme, is an identified plasminogen receptor of GAS. Here we used mass spectrometry (MS) to confirm that GAS SEN is octameric, thereby validating in silico modeling based on the crystal structure of Streptococcus pneumoniae alpha-enolase. Site-directed mutagenesis of surface-located lysine residues (SEN(K252 + 255A), SEN(K304A), SEN(K334A), SEN(K344E), SEN(K435L), and SEN(Delta434-435)) was used to examine their roles in maintaining structural integrity, enzymatic function, and plasminogen binding. Structural integrity of the GAS SEN octamer was retained for all mutants except SEN(K344E), as determined by circular dichroism spectroscopy and MS. However, ion mobility MS revealed distinct differences in the stability of several mutant octamers in comparison with wild type. Enzymatic analysis indicated that SEN(K344E) had lost alpha-enolase activity, which was also reduced in SEN(K334A) and SEN(Delta434-435). Surface plasmon resonance demonstrated that the capacity to bind human plasminogen was abolished in SEN(K252 + 255A), SEN(K435L), and SEN(Delta434-435). The lysine residues at positions 252, 255, 434, and 435 therefore play a concerted role in plasminogen acquisition. This study demonstrates the ability of combining in silico structural modeling with ion mobility-MS validation for undertaking functional studies on complex protein structures.

Regulation of streptokinase expression by CovR/S in Streptococcus pyogenes: CovR acts through a single high-affinity binding site

The important human pathogen Streptococcus pyogenes (the group A streptococcus or GAS) produces many virulence factors that are regulated by the two-component signal transduction system CovRS (CsrRS). Dissemination of GAS infection originating at the skin has been shown to require production of streptokinase, whose transcription is repressed by CovR. In this work we have studied the interaction of CovR and phosphorylated CovR (CovR-P) with the promoter for streptokinase, Pska. We found that, in contrast to the other CovR-repressed promoters, Pska regulation by CovR occurs through binding at a single ATTARA consensus binding sequence (CB) that overlaps the -10 region of the promoter. Binding of CovR to other nearby consensus sequences occurs upon phosphorylation of the protein, but these other CBs do not contribute to the regulation of Pska by CovR. Thus, binding at a specific site does not necessarily indicate that the site is involved in regulation by CovR. In addition, at Pska, CovR binding to the different sites does not appear to involve cooperative interactions, which simplifies the analysis of CovR binding and gives us insight into the modes of interaction that occur between CovR and its specific DNA-binding sites. Finally, the observation that regulation of transcription from Pska occurs at a very low concentration of phosphorylated CovR may have important implications for the regulation of virulence gene expression during GAS infection.

SufA - a bacterial enzyme that cleaves fibrinogen and blocks fibrin network formation

Finegoldia magna is a member of the normal human bacterial flora on the skin and other non-sterile body surfaces, but this anaerobic coccus is also an important opportunistic pathogen. SufA was the first F. magna proteinase to be isolated and characterized. Many bacterial pathogens interfere with different steps of blood coagulation, and here we describe how purified SufA efficiently and specifically cleaves fibrinogen in human plasma. SufA is both secreted by F. magna and associated with the bacterial surface. Successful gene targeting has previously not been performed in anaerobic cocci, but in order to study the role of the SufA that is present at the bacterial surface, we constructed an F. magna mutant that expresses a truncated SufA lacking proteolytic activity. In contrast to wild-type bacteria that delayed the coagulation of human plasma, mutant bacteria had no such effect. Wild-type and mutant bacteria adhered to keratinocytes equally well, but in a plasma environment only wild-type bacteria blocked the formation of fibrin networks surrounding adherent bacteria. The effective cleavage of fibrinogen by SufA suggests that the interference with fibrin network formation represents an adaptive mechanism of F. magna with potential implications also for pathogenicity.

Spatial localization of bacteria controls coagulation of human blood by 'quorum acting'

Blood coagulation often accompanies bacterial infections and sepsis and is generally accepted as a consequence of immune responses. Though many bacterial species can directly activate individual coagulation factors, they have not been shown to directly initiate the coagulation cascade that precedes clot formation. Here we demonstrated, using microfluidics and surface patterning, that the spatial localization of bacteria substantially affects coagulation of human and mouse blood and plasma. Bacillus cereus and Bacillus anthracis, the anthrax-causing pathogen, directly initiated coagulation of blood in minutes when bacterial cells were clustered. Coagulation of human blood by B. anthracis required secreted zinc metalloprotease InhA1, which activated prothrombin and factor X directly (not via factor XII or tissue factor pathways). We refer to this mechanism as 'quorum acting' to distinguish it from quorum sensing--it does not require a change in gene expression, it can be rapid and it can be independent of bacterium-to-bacterium communication.

Reduced thrombin generation increases host susceptibility to group A streptococcal infection

Bacterial plasminogen activators are commonplace among microbial pathogens, implying a central role of host plasmin in supporting bacterial virulence. Group A streptococci (GAS) secrete streptokinase, a specific activator of human plasminogen (PLG). The critical contribution of the streptokinase-PLG interaction to GAS pathogenicity was recently demonstrated using mice expressing human PLG. To examine the importance of thrombin generation in antimicrobial host defense, we challenged mice with deficiency of factor V (FV) in either the plasma or platelet compartment. Reduction of FV in either pool resulted in markedly increased mortality after GAS infection, with comparison to heterozygous F5-deficient mice suggesting a previously unappreciated role for the platelet FV pool in host defense. Mice with complete deficiency of fibrinogen also demonstrated markedly increased mortality to GAS infection relative to controls. Although FV Leiden may be protective in the setting of severe sepsis in humans, no significant survival advantage was observed in GAS-infected mice carrying the FV Leiden mutation. Taken together, our data support the hypothesis that local thrombosis/fibrin deposition limits the survival and dissemination of at least a subset of microbial pathogens and suggest that common variation in hemostatic factors among humans could affect host susceptibility to a variety of infectious diseases.

Plasminogen-dependent proteolytic activity in Bifidobacterium lactis

Bifidobacteria represent one of the most important health-promoting bacterial groups of the intestinal microbiota. The binding of plasminogen to species of Bifidobacterium has been recently reported. To further explore the interaction between bifidobacteria and plasminogen, we investigated the role of Bifidobacterium lactis BI07 plasminogen-dependent proteolytic activity in the degradation of host-specific substrates. Our experimental data demonstrate that the recruitment of plasminogen on the bacterial cell surface and its subsequent conversion into plasmin by host-derived plasminogen activators provide B. lactis BI07 with a surface-associated plasmin activity effective in degradation of physiological substrates such as extracellular matrix, fibronectin and fibrinogen. The ability of bifidobacteria to intervene in the host plasminogen/plasmin system may contribute to facilitating colonization of the host gastrointestinal tract.

Antibodies against a surface protein of Streptococcus pyogenes promote a pathological inflammatory response

Streptococcal toxic shock syndrome (STSS) caused by Streptococcus pyogenes is a clinical condition with a high mortality rate despite modern intensive care. A key feature of STSS is excessive plasma leakage leading to hypovolemic hypotension, disturbed microcirculation and multiorgan failure. Previous work has identified a virulence mechanism in STSS where M1 protein of S. pyogenes forms complexes with fibrinogen that activate neutrophils to release heparin-binding protein (HBP), an inducer of vascular leakage. Here, we report a marked inter-individual difference in the response to M1 protein-induced HBP release, a difference found to be related to IgG antibodies directed against the central region of the M1 protein. To elicit massive HBP release, such antibodies need to be part of the M1 protein-fibrinogen complexes. The data add a novel aspect to bacterial pathogenesis where antibodies contribute to the severity of disease by promoting a pathologic inflammatory response.

Extensive adaptive changes occur in the transcriptome of Streptococcus agalactiae (group B streptococcus) in response to incubation with human blood

To enhance understanding of how Streptococcus agalactiae (group B streptococcus, GBS) adapts during invasive infection, we performed a whole-genome transcriptome analysis after incubation with whole human blood. Global changes occurred in the GBS transcriptome rapidly in response to blood contact following shift from growth in a rich laboratory medium. Most (83%) of the significantly altered transcripts were down-regulated after 30 minutes of incubation in blood, and all functional categories of genes were abundantly represented. We observed complex dynamic changes in the expression of transcriptional regulators and stress response genes that allow GBS to rapidly adapt to blood. The transcripts of relatively few proven virulence genes were up-regulated during the first 90 minutes. However, a key discovery was that genes encoding proteins involved in interaction with the host coagulation/fibrinolysis system and bacterial-host interactions were rapidly up-regulated. Extensive transcript changes also occurred for genes involved in carbohydrate metabolism, including multi-functional proteins and regulators putatively involved in pathogenesis. Finally, we discovered that an incubation temperature closer to that occurring in patients with severe infection and high fever (40°C) induced additional differences in the GBS transcriptome relative to normal body temperature (37°C). Taken together, the data provide extensive new information about transcriptional adaptation of GBS exposed to human blood, a crucial step during GBS pathogenesis in invasive diseases, and identify many new leads for molecular pathogenesis research.

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.

A leaky blood-brain barrier, fibrinogen infiltration and microglial reactivity in inflamed Alzheimer's disease brain

This study has used immunohistochemical examination of tissue obtained from Alzheimer’s disease (AD) brains and rat hippocampus injected with Aβ_1-42 peptide to determine effects of induced inflammatory reactivity on integrity of blood–brain barrier (BBB) and viability of neurons. Tissue from AD, but not non-demented, brains exhibited a diffuse pattern of staining for fibrinogen and immunoglobulin (IgG) indicative of BBB leakiness with considerable fibrinogen immunoreactivity (ir) appearing in association with Aβ deposits. Immunostaining for the endothelial cell specific glycoprotein, von Willebrand factor, showed morphological evidence for altered blood vessels in AD tissue. AD brains also demonstrated extensive areas of fibrinogen ir in association with microglial reactivity. In vivo, intra-hippocampal injection of Aβ_1-42 caused time-dependent (1–7 days after injection) increases in double staining of fibrinogen with areas of microgliosis. Two independent pharmacological strategies were employed to examine how Aβ_1-42 stimulation (7 days injection) may be linked to neurodegeneration. The defibrinogenating compound, ancrod, reduced inflammatory reactivity, levels of parenchymal fibrinogen and IgG, and was neuroprotective. These results prompted use of Aβ_1-42 plus fibrinogen as a novel in vivo inflammatory stimulus and this combination significantly enhanced inflammatory reactivity, vascular perturbations and neuronal damage compared to Aβ_1-42 alone. A second approach, using anti-Mac-1 (antibody for antigen CD11b) to block activation of microglia, was highly effective in attenuating effects of Aβ_1-42 plus fibrinogen amplification of inflammatory and vascular responses and conferred significant neuroprotection. The overall findings from study of AD tissue and in vivo in Aβ_1-42 and Aβ_1-42 plus fibrinogen stimulated rat hippocampus suggest microglial responses to promote increased extravasation of blood protein as a critical component in amplifying inflammatory reactivity and causing neuronal damage in inflamed AD brain.

Protease-activated receptor 1 activation is necessary for monocyte chemoattractant protein 1-dependent leukocyte recruitment in vivo

Thrombin, acting through a family of protease-activated receptors (PARs), is known to amplify inflammatory responses, but the in vivo importance of PARs in inflammation is not fully appreciated. In a mouse heart-to-rat transplant model, where it is possible to distinguish graft (mouse) from systemic (rat) chemokines, we show that donor PAR-1 is required to generate the local monocyte chemoattractant protein (MCP)-1 needed to recruit rat natural killer cells and macrophages into the hearts. We have confirmed the importance of this mechanism in a second model of thioglycollate-induced peritonitis and also show that PAR-1 is important for the production of MCP-3 and MCP-5. Despite the presence of multiple other mediators capable of stimulating chemokine production in these models, these data provide the first evidence that thrombin and PAR activation are required in vivo to initiate inflammatory cell recruitment.

Characteristics of plasminogen binding to Trypanosoma cruzi epimastigotes

The binding constants of the interaction between plasminogen and Trypanosoma cruzi epimastigotes were determined. An indirect method in which the bound plasminogen is detached from the cell by epsilon-aminocaproic acid and a direct method through biotinylated plasminogen were used. The analyses revealed a dissociation constant (Kd) from 0.4 to 1.2microM, these values being compatible with recognition in vivo. Moreover, epimastigotes from the gut of Rhodnius prolixus were able to bind plasminogen from the blood meal. Fragments derived from elastase digestion of plasminogen were tested for their ability to bind T. cruzi cells. The fragment with highest ability to interact with the parasite was miniplasminogen that bound in a concentration-dependent and saturable manner with a Kd similar to that for plasminogen. This binding was inhibited by epsilon-aminocaproic acid indicating that the lysine-binding site of kringle 5 may be responsible for the interaction of plasminogen with T. cruzi.

Plasminogen alleles influence susceptibility to invasive aspergillosis

Invasive aspergillosis (IA) is a common and life-threatening infection in immunocompromised individuals. A number of environmental and epidemiologic risk factors for developing IA have been identified. However, genetic factors that affect risk for developing IA have not been clearly identified. We report that host genetic differences influence outcome following establishment of pulmonary aspergillosis in an exogenously immune suppressed mouse model. Computational haplotype-based genetic analysis indicated that genetic variation within the biologically plausible positional candidate gene plasminogen (Plg; Gene ID 18855) correlated with murine outcome. There was a single nonsynonymous coding change (Gly110Ser) where the minor allele was found in all of the susceptible strains, but not in the resistant strains. A nonsynonymous single nucleotide polymorphism (Asp472Asn) was also identified in the human homolog (PLG; Gene ID 5340). An association study within a cohort of 236 allogeneic hematopoietic stem cell transplant (HSCT) recipients revealed that alleles at this SNP significantly affected the risk of developing IA after HSCT. Furthermore, we demonstrated that plasminogen directly binds to Aspergillus fumigatus. We propose that genetic variation within the plasminogen pathway influences the pathogenesis of this invasive fungal infection.

M protein-mediated plasminogen binding is essential for the virulence of an invasive Streptococcus pyogenes isolate

The human protease plasmin plays a crucial role in the capacity of the group A streptococcus (GAS; Streptococcus pyogenes) to initiate invasive disease. The GAS strain NS88.2 was isolated from a case of bacteremia from the Northern Territory of Australia, a region with high rates of GAS invasive disease. Mutagenesis of the NS88.2 plasminogen binding M protein Prp was undertaken to examine the contribution of plasminogen binding and cell surface plasmin acquisition to virulence. The isogenic mutant NS88.2prp was engineered whereby four amino acid residues critical for plasminogen binding were converted to alanine codons in the GAS genome sequence. The mutated residues were reverse complemented to the wild-type sequence to construct GAS strain NS88.2prpRC. In comparison to NS88.2 and NS88.2prpRC, the NS88.2prp mutant exhibited significantly reduced ability to bind human plasminogen and accumulate cell surface plasmin activity during growth in human plasma. Utilizing a humanized plasminogen mouse model of invasive infection, we demonstrate that the capacity to bind plasminogen and accumulate surface plasmin activity plays an essential role in GAS virulence.

Plasminogen binding by oral streptococci from dental plaque and inflammatory lesions

Plasminogen binding by bacteria is a virulence factor important for the entry and dissemination of bacteria in the body. A wide variety of bacteria bind plasminogen, including both organisms causing disease and components of the normal oral flora. The purpose of this study was to examine the characteristics of plasminogen binding by six clinical isolates of oral streptococci from both dental plaque and inflammatory lesions. All the strains bound plasminogen with approximately the same affinity, and binding was specific and lysine-dependent as evidenced by its inhibition by epsilon-aminocaproic acid. All of the test strains were capable of activating bound plasminogen to plasmin without the addition of a plasminogen activator, and subsequent analysis revealed the presence of streptokinase in all strains. However, the streptococci exhibited fibrinolytic activity only in the presence of plasminogen and this could be inhibited by the addition of epsilon-aminocaproic acid. SDS-PAGE and 2D gel electrophoresis coupled with plasminogen ligand blotting showed that only a subset of the total proteins (2-15) were involved in the binding of plasminogen. Partial identification of the binding proteins revealed that four glycolytic enzymes, enolase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate mutase, were predominant in binding plasminogen. The binding of plasminogen by bacteria from pus did not differ from that of the strains from supragingival plaque. The findings illustrate how apparently innocuous commensal bacteria are capable of utilizing a mechanism that is generally regarded as being of importance to pathogenicity and suggest an additional role of plasminogen binding.

Skin microbiota: a source of disease or defence?

Microbes found on the skin are usually regarded as pathogens, potential pathogens or innocuous symbiotic organisms. Advances in microbiology and immunology are revising our understanding of the molecular mechanisms of microbial virulence and the specific events involved in the host-microbe interaction. Current data contradict some historical classifications of cutaneous microbiota and suggest that these organisms may protect the host, defining them not as simple symbiotic microbes but rather as mutualistic. This review will summarize current information on bacterial skin flora including Staphylococcus, Corynebacterium, Propionibacterium, Streptococcus and Pseudomonas. Specifically, the review will discuss our current understanding of the cutaneous microbiota as well as shifting paradigms in the interpretation of the roles microbes play in skin health and disease.

The lack of binding of VEK-30, an internal peptide from the group A streptococcal M-like protein, PAM, to murine plasminogen is due to two amino acid replacements in the plasminogen kringle-2 domain

VEK-30, a 30-amino acid internal peptide present within a streptococcal M-like plasminogen (Pg)-binding protein (PAM) from Gram-positive group-A streptococci (GAS), represents an epitope within PAM that shows high affinity for the lysine binding site (LBS) of the kringle-2 (K2) domain of human (h)Pg. VEK-30 does not interact with this same region of mouse (m)Pg, despite the high conservation of the mK2- and hK2-LBS. To identify the molecular basis for the species specificity of this interaction, hPg and mPg variants were generated, including an hPg chimera with the mK2 sequence and an mPg chimera containing the hK2 sequence. The binding of synthetic VEK-30 to these variants was studied by surface plasmon resonance. The data revealed that, in otherwise intact Pg, the species specificity of VEK-30 binding in these two cases is entirely dictated by two K2 residues that are different between hPg and mPg, namely, Arg-220 of hPg, which is a Gly in mPg, and Leu-222 of hPg, which is a Pro in mPg, neither of which are members of the canonical K2-LBS. Neither the activation of hPg, nor the enzymatic activity of its activated product, plasmin (hPm), are compromised by replacing these two amino acids by their murine counterparts. It is also demonstrated that hPg is more susceptible to activation to hPm after complexation with VEK-30 and that this property is greatly reduced as a result of the R220G and L222P replacements in hPg. These mechanisms for accumulation of protease activity on GAS likely contribute to the virulence of PAM(+)-GAS strains and identify targets for new therapeutic interventions.