Fba, a novel fibronectin-binding protein from Streptococcus pyogenes, promotes bacterial entry into epithelial cells, and the fba gene is positively transcribed under the Mga regulator

Complement-mediated opsonization of invasive group A Streptococcus pyogenes strain AP53 is regulated by the bacterial two-component cluster of virulence responder/sensor (CovRS) system

Group A Streptococcus pyogenes (GAS) strain AP53 is a primary isolate from a patient with necrotizing fasciitis. These AP53 cells contain an inactivating mutation in the sensor component of the cluster of virulence (cov) responder (R)/sensor (S) two-component gene regulatory system (covRS), which enhances the virulence of the primary strain, AP53/covR(+)S(-). However, specific mechanisms by which the covRS system regulates the survival of GAS in humans are incomplete. Here, we show a key role for covRS in the regulation of opsonophagocytosis of AP53 by human neutrophils. AP53/covR(+)S(-) cells displayed potent binding of host complement inhibitors of C3 convertase, viz. Factor H (FH) and C4-binding protein (C4BP), which concomitantly led to minimal C3b deposition on AP53 cells, further showing that these plasma protein inhibitors are active on GAS cells. This resulted in weak killing of the bacteria by human neutrophils and a corresponding high death rate of mice after injection of these cells. After targeted allelic alteration of covS(-) to wild-type covS (covS(+)), a dramatic loss of FH and C4BP binding to the AP53/covR(+)S(+) cells was observed. This resulted in elevated C3b deposition on AP53/covR(+)S(+) cells, a high level of opsonophagocytosis by human neutrophils, and a very low death rate of mice infected with AP53/covR(+)S(+). We show that covRS is a critical transcriptional regulator of genes directing AP53 killing by neutrophils and regulates the levels of the receptors for FH and C4BP, which we identify as the products of the fba and enn genes, respectively.

Factor H binds to the hypervariable region of many Streptococcus pyogenes M proteins but does not promote phagocytosis resistance or acute virulence

Many pathogens express a surface protein that binds the human complement regulator factor H (FH), as first described for Streptococcus pyogenes and the antiphagocytic M6 protein. It is commonly assumed that FH recruited to an M protein enhances virulence by protecting the bacteria against complement deposition and phagocytosis, but the role of FH-binding in S. pyogenes pathogenesis has remained unclear and controversial. Here, we studied seven purified M proteins for ability to bind FH and found that FH binds to the M5, M6 and M18 proteins but not the M1, M3, M4 and M22 proteins. Extensive immunochemical analysis indicated that FH binds solely to the hypervariable region (HVR) of an M protein, suggesting that selection has favored the ability of certain HVRs to bind FH. These FH-binding HVRs could be studied as isolated polypeptides that retain ability to bind FH, implying that an FH-binding HVR represents a distinct ligand-binding domain. The isolated HVRs specifically interacted with FH among all human serum proteins, interacted with the same region in FH and showed species specificity, but exhibited little or no antigenic cross-reactivity. Although these findings suggested that FH recruited to an M protein promotes virulence, studies in transgenic mice did not demonstrate a role for bound FH during acute infection. Moreover, phagocytosis tests indicated that ability to bind FH is neither sufficient nor necessary for S. pyogenes to resist killing in whole human blood. While these data shed new light on the HVR of M proteins, they suggest that FH-binding may affect S. pyogenes virulence by mechanisms not assessed in currently used model systems.

The human complement system may be rapidly activated upon infection and thereby plays a key role in innate immunity. However, activation must be tightly controlled, to avoid attack on self tissues. A key component of this control system is the plasma protein factor H (FH). Many pathogens bind FH, as first described for Streptococcus pyogenes, and it has been proposed that the surface-localized M protein of this bacterium “hijacks” FH to escape phagocytosis. However, it remains unclear whether FH-binding to M protein indeed protects S. pyogenes against phagocytosis and promotes bacterial growth in vivo. Here, we demonstrate that FH binds to some but not all M proteins and solely binds to the hypervariable region (HVR), a part of M protein important for virulence. Nevertheless, several lines of evidence, including studies with transgenic mice, indicated that FH-binding ability did not contribute to acute virulence or phagocytosis resistance. These data shed new light on the HVR of M proteins but underline the difficulty in determining the in vivo role of a ligand-binding region. Binding of FH may contribute to S. pyogenes virulence by mechanisms not assessed in currently used models.

Cysteine proteinase from Streptococcus pyogenes enables evasion of innate immunity via degradation of complement factors

Streptococcus pyogenes is an important human pathogen that causes invasive diseases such as necrotizing fasciitis, sepsis, and streptococcal toxic shock syndrome. We investigated the function of a major cysteine protease from S. pyogenes that affects the amount of C1-esterase inhibitor (C1-INH) and other complement factors and aimed to elucidate the mechanism involved in occurrence of streptococcal toxic shock syndrome from the aspect of the complement system. First, we revealed that culture supernatant of a given S. pyogenes strain and recombinant SpeB degraded the C1-INH. Then, we determined the N-terminal sequence of the C1-INH fragment degraded by recombinant SpeB. Interestingly, the region containing one of the identified cleavage sites is not present in patients with C1-INH deficiency. Scanning electron microscopy of the speB mutant incubated in human serum showed the abnormal superficial architecture and irregular oval structure. Furthermore, unlike the wild-type strain, that mutant strain showed lower survival capacity than normal as compared with heat-inactivated serum, whereas it had a significantly higher survival rate in serum without the C1-INH than in normal serum. Also, SpeB degraded multiple complement factors and the membrane attack complex. Flow cytometric analyses revealed deposition of C9, one of the components of membrane the attack complex, in greater amounts on the surface of the speB mutant, whereas lower amounts of C9 were bound to the wild-type strain surface. These results suggest that SpeB can interrupt the human complement system via degrading the C1-INH, thus enabling S. pyogenes to evade eradication in a hostile environment.

Group A streptococcal cysteine protease cleaves epithelial junctions and contributes to bacterial translocation

BACKGROUND

Group A Streptococcus (GAS) translocates across the host epithelial barrier.

RESULTS

Streptococcal pyrogenic exotoxin B (SpeB) directly cleaves junctional proteins.

CONCLUSION

The proteolytic efficacy of SpeB allows GAS to translocate across the epithelial barrier.

SIGNIFICANCE

SpeB-mediated dysfunction of the epithelial barrier may have important implications for not only bacterial invasion but also dissemination of other virulence factors throughout intercellular spaces. Group A Streptococcus (GAS) is an important human pathogen that possesses an ability to translocate across the epithelial barrier. In this study, culture supernatants of tested GAS strains showed proteolytic activity against human occludin and E-cadherin. Utilizing various types of protease inhibitors and amino acid sequence analysis, we identified SpeB (streptococcal pyrogenic exotoxin B) as the proteolytic factor that cleaves E-cadherin in the region neighboring the calcium-binding sites within the extracellular domain. The cleaving activities of culture supernatants from several GAS isolates were correlated with the amount of active SpeB, whereas culture supernatants from an speB mutant showed no such activities. Of note, the wild type strain efficiently translocated across the epithelial monolayer along with cleavage of occludin and E-cadherin, whereas deletion of the speB gene compromised those activities. Moreover, destabilization of the junctional proteins was apparently relieved in cells infected with the speB mutant, as compared with those infected with the wild type. Taken together, our findings indicate that the proteolytic efficacy of SpeB in junctional degradation allows GAS to invade deeper into tissues.

Contribution of fibronectin-binding protein to pathogenesis of Streptococcus equi ssp. zooepidemicus

Streptococcus equi ssp. zooepidemicus (S. zooepidemicus) is responsible for a wide variety of infections in many species. Fibronectin-binding protein is a bacterial cell surface protein, which specifically binds fibronectin (FN). Considering the specific role of FN-binding protein in host-pathogen interactions, we investigated the function of a novel FN-binding domain in the FN-binding protein (FNZ) of S. zooepidemicus. Five recombinant FNZ gene fragments [N1 (amino acids, 38-197), N2 (amino acids, 38-603), N3 (amino acids, 41-315), N4 (amino acids, 192-370), and N5 (amino acids, 38-225)] were expressed in Escherichia coli, and their FN-binding activities were tested. The results showed that amino acids 192-225 in the NH2 -terminal region of FNZ could be responsible for binding fibronectin. The FNZ knockout mutant was constructed in S. zooepidemicus, which results in the reduced capacity to adhere to HEp-2 cell, defective virulence in vivo, decreased biofilm formation, and decreased colonization capacity in blood, liver, lung, and spleen tissues of mice as compared to the wild type. These results suggest that FNZ participates in biofilm formation, FN binding, cell adhesion, and pathogenesis of S. zooepidemicus. Furthermore, this work offers a novel FN-binding domain within FNZ, which will help in further characterization of S. zooepidemicus FN-binding properties.

The group A streptococcal collagen-like protein-1, Scl1, mediates biofilm formation by targeting the extra domain A-containing variant of cellular fibronectin expressed in wounded tissue

Wounds are known to serve as portals of entry for group A Streptococcus (GAS). Subsequent tissue colonization is mediated by interactions between GAS surface proteins and host extracellular matrix components. We recently reported that the streptococcal collagen-like protein-1, Scl1, selectively binds the cellular form of fibronectin (cFn) and also contributes to GAS biofilm formation on abiotic surfaces. One structural feature of cFn, which is predominantly expressed in response to tissue injury, is the presence of a spliced variant containing extra domain A (EDA/EIIIA). We now report that GAS biofilm formation is mediated by the Scl1 interaction with EDA-containing cFn. Recombinant Scl1 proteins that bound cFn also bound recombinant EDA within the C-C' loop region recognized by the α(9)β(1) integrin. The extracellular 2-D matrix derived from human dermal fibroblasts supports GAS adherence and biofilm formation. Altogether, this work identifies and characterizes a novel molecular mechanism by which GAS utilizes Scl1 to specifically target an extracellular matrix component that is predominantly expressed at the site of injury in order to secure host tissue colonization.

Pleiotropic virulence factor - Streptococcus pyogenes fibronectin-binding proteins

Streptococcus pyogenes causes a broad spectrum of infectious diseases, including pharyngitis, skin infections and invasive necrotizing fasciitis. The initial phase of infection involves colonization, followed by intimate contact with the host cells, thus promoting bacterial uptake by them. S. pyogenes recognizes fibronectin (Fn) through its own Fn-binding proteins to obtain access to epithelial and endothelial cells in host tissue. Fn-binding proteins bind to Fn to form a bridge to α5 β1 -integrins, which leads to rearrangement of cytoskeletal actin in host cells and uptake of invading S. pyogenes. Recently, several structural analyses of the invasion mechanism showed molecular interactions by which Fn converts from a compact plasma protein to a fibrillar component of the extracellular matrix. After colonization, S. pyogenes must evade the host innate immune system to spread into blood vessels and deeper organs. Some Fn-binding proteins contribute to evasion of host innate immunity, such as the complement system and phagocytosis. In addition, Fn-binding proteins have received focus as non-M protein vaccine candidates, because of their localization and conservation among different M serotypes.Here, we review the roles of Fn-binding proteins in the pathogenesis and speculate regarding possible vaccine antigen candidates.

Variability in the distribution of genes encoding virulence factors and putative extracellular proteins of Streptococcus pyogenes in India, a region with high streptococcal disease burden, and implication for development of a regional multisubunit vaccine

Streptococcus pyogenes causes a wide variety of human diseases and is a significant cause of morbidity and mortality. Attempts to develop a vaccine were hampered by the genetic diversity of S. pyogenes across different regions of the world. This study sought to identify streptococcal antigens suitable for a region-specific vaccine in India. We used a two-step approach, first performing epidemiological analysis to identify the conserved antigens among Indian isolates. The second step consisted of validating the identified antigens by serological analysis. The 201 streptococcal clinical isolates from India used in this study represented 69 different emm types, with emm12 being the most prevalent. Virulence profiling of the North and South Indian S. pyogenes isolates with a custom-designed streptococcal virulence microarray identified seven conserved putative vaccine candidates. Collagen-like surface protein (SCI), putative secreted 5'-nucleotidase (PSNT), and C5a peptidase were found in 100% of the isolates, while R28, a putative surface antigen (PSA), and a hypothetical protein (HYP) were found in 90% of the isolates. A fibronectin binding protein, SfbI, was present in only 78% of the isolates. In order to validate the identified potential vaccine candidates, 185 serum samples obtained from patients with different clinical manifestations were tested for antibodies. Irrespective of clinical manifestations, serum samples showed high antibody titers to all proteins except for SCI and R28. Thus, the data indicate that PSNT, C5a peptidase, PSA, HYP, and SfbI are promising candidates for a region-specific streptococcal vaccine for the different parts of India.

Group A streptococcus adheres to pharyngeal epithelial cells with salivary proline-rich proteins via GrpE chaperone protein

Group A Streptococcus pyogenes (GAS) is an important human pathogen that frequently causes pharyngitis. GAS organisms can adhere to and invade pharyngeal epithelial cells, which are overlaid by salivary components. However, the role of salivary components in GAS adhesion to pharyngeal cells has not been reported precisely. We collected human saliva and purified various salivary components, including proline-rich protein (PRP), statherin, and amylase, and performed invasion assays. The GAS-HEp-2 association ratio (invasion/adhesion ratio) and invasion ratio of GAS were increased significantly with whole human saliva and PRP, while the anti-PRP antibody inhibited the latter. GAS strain NY-5, which lacks M and F proteins on the cell surface, was promoted to cohere with HEp-2 cells by whole human saliva and PRP. The 28-kDa protein of GAS bound to PRP and was identified as GrpE, a chaperone protein, whereas the N-terminal of GrpE was found to bind to PRP. A GrpE-deficient mutant of GAS strain B514Sm, TR-45, exhibited a reduced ability to adhere to and invade HEp-2 cells. Microscopic observations showed the GrpE was mainly expressed on the surface of the cell division site of GAS. Furthermore, GrpE-deficient mutants of GAS and Streptococcus pneumoniae showed an elongated morphology as compared with the wild type. Taken together, this is the first study to show an interaction between salivary PRP and GAS GrpE, which plays an important role in GAS infection on the pharynx, whereas the expression of GrpE on the surface of GAS helps to maintain morphology.

Vru (Sub0144) controls expression of proven and putative virulence determinants and alters the ability of Streptococcus uberis to cause disease in dairy cattle

The regulation and control of gene expression in response to differing environmental stimuli is crucial for successful pathogen adaptation and persistence. The regulatory gene vru of Streptococcus uberis encodes a stand-alone response regulator with similarity to the Mga of group A Streptococcus. Mga controls expression of a number of important virulence determinants. Experimental intramammary challenge of dairy cattle with a mutant of S. uberis carrying an inactivating lesion in vru showed reduced ability to colonize the mammary gland and an inability to induce clinical signs of mastitis compared with the wild-type strain. Analysis of transcriptional differences of gene expression in the mutant, determined by microarray analysis, identified a number of coding sequences with altered expression in the absence of Vru. These consisted of known and putative virulence determinants, including Lbp (Sub0145), SclB (Sub1095), PauA (Sub1785) and hasA (Sub1696).

Manuka honey inhibits the development of Streptococcus pyogenes biofilms and causes reduced expression of two fibronectin binding proteins

Streptococcus pyogenes (group A Streptococcus; GAS) is always of clinical significance in wounds where it can initiate infection, destroy skin grafts and persist as a biofilm. Manuka honey has broad spectrum antimicrobial activity and its use in the clinical setting is beginning to gain acceptance with the continuing emergence of antibiotic resistance and the inadequacy of established systemic therapies; novel inhibitors may affect clinical practice. In this study, the effect of manuka honey on S. pyogenes (M28) was investigated in vitro with planktonic and biofilm cultures using MIC, MBC, microscopy and aggregation efficiency. Bactericidal effects were found in both planktonic cultures and biofilms, although higher concentrations of manuka honey were needed to inhibit biofilms. Abrogation of adherence and intercellular aggregation was observed. Manuka honey permeated 24 h established biofilms of S. pyogenes, resulting in significant cell death and dissociation of cells from the biofilm. Sublethal concentrations of manuka honey effectively prevented the binding of S. pyogenes to the human tissue protein fibronectin, but did not inhibit binding to fibrinogen. The observed inhibition of fibronectin binding was confirmed by a reduction in the expression of genes encoding two major fibronectin-binding streptococcal surface proteins, Sof and SfbI. These findings indicate that manuka honey has potential in the topical treatment of wounds containing S. pyogenes.

Adherence and invasion of streptococci to eukaryotic cells and their role in disease pathogenesis

Streptococcal adhesion, invasion, intracellular trafficking, dissemination, and persistence in eukaryotic cells have a variety of implications in the infection pathogenesis. While cell adhesion establishes the initial host contact, adhering bacteria exploit the host cell for their own benefit. Internalization into the host cell is an essential step for bacterial survival and subsequent dissemination and persistence, thus playing a key role in the course of infection. This chapter summarizes the current knowledge about the diverse mechanisms of streptococcal adhesion to and invasion into different eukaryotic cells and the impact on dissemination and persistence which is reflected by consequences for the pathogenesis of streptococcal infections.

Group a streptococcal vaccine candidates: potential for the development of a human vaccine

Currently there is no commercial Group A Streptococcus (GAS; S. pyogenes) vaccine available. The development of safe GAS vaccines is challenging, researchers are confronted with obstacles such as the occurrence of many unique serotypes (there are greater than 150 M types), antigenic variation within the same serotype, large variations in the geographical distribution of serotypes, and the production of antibodies cross-reactive with human tissue which can lead to host auto-immune disease. Cell wall anchored, cell membrane associated, secreted and anchorless proteins have all been targeted as GAS vaccine candidates. As GAS is an exclusively human pathogen, the quest for an efficacious vaccine is further complicated by the lack of an animal model which mimics human disease and can be consistently and reproducibly colonized by multiple GAS strains.

A novel monoclonal antibody against FbaA can inhibit the binding of the complement regulatory protein factor H to group A streptococcus

Some microbial pathogens utilize human complement regulatory proteins, such as factor H (FH) and factor H-like protein 1 (FHL-1), for immune evasion. FbaA is an FHL-1 and FH binding protein expressed on the surface of group A streptococcus (GAS), a common agent of pharyngeal, skin, and soft tissue infections. In this study, we prepared monoclonal antibodies (MAbs) against FbaA, assayed them for specificity, and located their binding domains in FbaA. We found an MAb called FbaA MAb2, which demonstrated the highest affinity to GAS among all of the MAbs. Based on the binding with component peptides, the detected epitope, which was specific for FbaA MAb2, was the amino acid residues 95 to 118 of FbaA; on the other hand, it did not bind with the truncated protein of the internally deleted residues of the segment from 95 to 118 of FbaA. Furthermore, the predominant amino acids specific for FbaA MAb2 screened by phage display epitope library were I, T, P, D, and L, corresponding to the amino acid residues 101, 103, 105, 106, and 110 of FbaA, respectively. The binding location of FbaA with FH and FHL-1 was a 16-amino-acid region corresponding to amino acid residues 97 to 112 of FbaA, which overlapped the FbaA MAb2 binding domain, as confirmed by competitive inhibition enzyme-linked immunosorbent assay and immunofluorescence microscopy. Based on the results of the invasion assay, FbaA MAb2 can inhibit the binding of FH to GAS.

Role of fibronectin-binding protein A in Clostridium difficile intestinal colonization

Clostridium difficile is a frequent cause of severe, recurrent, post-antibiotic diarrhoea and pseudomembranous colitis. Its pathogenicity is mediated mainly by two toxins, TcdA and TcdB. However, different adhesins have also been described as important colonization factors which are implicated in the first step of the intestinal infection. In this study, we focused our interest on one of these adhesins, fibronectin-binding protein A (FbpA), and on its role in the intestinal colonization process. A mutant of FbpA (CDΔFbpA) was constructed in C. difficile strain 630Δerm by using ClosTron technology. This mutant was characterized in vitro and in vivo and compared to the isogenic wild-type strain. Adhesion of the CDΔFbpA mutant to the human colonic epithelial cell line Caco-2 and to mucus-secreting HT29-MTX cells was examined. Surprisingly, the CDΔFbpA mutant adhered more than the wild-type parental strain. The CDΔFbpA mutant was also analysed in three different mouse models by following the intestinal implantation kinetics (faecal shedding) and caecal colonization (7 days post-challenge). We showed that in monoxenic mice, CDΔFbpA shed C. difficile in faeces at the same rate as that of the isogenic wild-type strain but its colonization of the caecal wall was significantly reduced. In dixenic mice, the shedding rate was slower for the CDΔFbpA mutant than for the isogenic wild-type strain during the first days of infection, but no significant difference was observed in caecal colonization. Similar rates of intestinal implantation and caecal colonization were observed for both strains in assays performed in human microbiota-associated mice. Taken together, our data suggest that FbpA plays a role in intestinal colonization by C. difficile.

Streptolysin S contributes to group A streptococcal translocation across an epithelial barrier

Group A Streptococcus pyogenes (GAS) is a human pathogen that causes local suppurative infections and severe invasive diseases. Systemic dissemination of GAS is initiated by bacterial penetration of the epithelial barrier of the pharynx or damaged skin. To gain insight into the mechanism by which GAS penetrates the epithelial barrier, we sought to identify both bacterial and host factors involved in the process. Screening of a transposon mutant library of a clinical GAS isolate recovered from an invasive episode allowed identification of streptolysin S (SLS) as a novel factor that facilitates the translocation of GAS. Of note, the wild type strain efficiently translocated across the epithelial monolayer, accompanied by a decrease in transepithelial electrical resistance and cleavage of transmembrane junctional proteins, including occludin and E-cadherin. Loss of integrity of intercellular junctions was inhibited after infection with a deletion mutant of the sagA gene encoding SLS, as compared with those infected with the wild type strain. Interestingly, following GAS infection, calpain was recruited to the plasma membrane along with E-cadherin. Moreover, bacterial translocation and destabilization of the junctions were partially inhibited by a pharmacological calpain inhibitor or genetic interference with calpain. Our data indicate a potential function of SLS that facilitates GAS invasion into deeper tissues via degradation of epithelial intercellular junctions in concert with the host cysteine protease calpain.

Fibronectin-binding proteins of Clostridium perfringens recognize the III1-C fragment of fibronectin

The Clostridium perfringens strain 13 genome contains two genes (fbpA, fbpB) that encode putative Fbp. Both rFbpA and rFbpB were purified and their reactivity with human serum Fn was analyzed. To determine the region of the Fn molecule recognized by rFbp, a plate binding assay using N-terminal 70-kDa peptide, III1-C peptide, and 110-kDa peptide containing III2-10 of Fn was performed. Both rFbp bound to the III1-C peptide of Fn but not to the other peptides. However, the III1-C fragment of Fn is known to be cryptic in serum Fn. Then, rFbp-BP from Fn were purified by rFbp-affinity chromatography. The yield of purified proteins was approximately 1% of the applied Fn on a protein basis. Western blotting analysis of the rFbp-BP, using four different anti-Fn monoclonal antibodies, revealed that the rFbp-BP carried partial Fn antigenicity. Bindings of rFbp to rFbp-BP were inhibited by the presence of the III1-C peptide, suggesting that rFbp-BP express the III1-C fragment. The binding of Fn to III1-C was inhibited by the presence of either rFbpA or rFbpB. This result that suggests C. perfringens Fbps may inhibit the formation of Fn-matrix in vivo.

Specific behavior of intracellular Streptococcus pyogenes that has undergone autophagic degradation is associated with bacterial streptolysin O and host small G proteins Rab5 and Rab7

Streptococcus pyogenes (group A streptococcus (GAS)) is a pathogen that invades non-phagocytic host cells, and causes a variety of acute infections such as pharyngitis. Our group previously reported that intracellular GAS is effectively degraded by the host-cell autophagic machinery, and that a cholesterol-dependent cytolysin, streptolysin O (SLO), is associated with bacterial escape from endosomes in epithelial cells. However, the details of both the intracellular behavior of GAS and the process leading to its autophagic degradation remain unknown. In this study, we found that two host small G proteins, Rab5 and Rab7, were associated with the pathway of autophagosome formation and the fate of intracellular GAS. Rab5 was involved in bacterial invasion and endosome fusion. Rab7 was clearly multifunctional, with roles in bacterial invasion, endosome maturation, and autophagosome formation. In addition, this study showed that the bacterial cytolysin SLO supported the escape of GAS into the cytoplasm from endosomes, and surprisingly, a SLO-deficient mutant of GAS was viable longer than the wild-type strain although it failed to escape the endosomes. This intracellular behavior of GAS is unique and distinct from that of other types of bacterial invaders. Our results provide a new picture of GAS infection and host-cell responses in epithelial cells.

Characterization of MabA, a modulator of Lactobacillus rhamnosus GG adhesion and biofilm formation

Abstract The probiotic Lactobacillus rhamnosus GG, first isolated from healthy human gut microbiota, has been reported to adhere very well to components of the intestinal mucosa, thereby enabling transient colonization of the gastrointestinal tract (GIT). In a search for the genes responsible for the good adherence capacity of this strain, a genomic region encoding a protein with homology to putative adhesion proteins (LGG_01865) and its putative regulator (LGG_01866) was identified. The sequence of the L. rhamnosus GG LGG_01865 encodes a polypeptide of 2419 amino acid residues containing 26 repetitive DUF1542 domains and a C-terminal LPxTG cell wall-anchoring motif. Phenotypic analyses of a dedicated LGG_01865 knockout mutant revealed a reduced biofilm formation capacity on abiotic surfaces and decreased adhesion to intestinal epithelial cells and tissues of the murine GIT. This suggests a modulating role for LGG_01865 in L. rhamnosus GG-host interactions. Therefore, we propose a new name for LGG_01865, i.e. MabA, modulator of adhesion and biofilm. Expression analysis indicated that LGG_01866 plays a conditional role in the regulation of LGG_01865 expression, i.e. when cells are grown under conditions of sugar starvation.

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.

Contribution of AmyA, an extracellular alpha-glucan degrading enzyme, to group A streptococcal host-pathogen interaction

alpha-Glucans such as starch and glycogen are abundant in the human oropharynx, the main site of group A Streptococcus (GAS) infection. However, the role in pathogenesis of GAS extracellular alpha-glucan binding and degrading enzymes is unknown. The serotype M1 GAS genome encodes two extracellular proteins putatively involved in alpha-glucan binding and degradation; pulA encodes a cell wall anchored pullulanase and amyA encodes a freely secreted putative cyclomaltodextrin alpha-glucanotransferase. Genetic inactivation of amyA, but not pulA, abolished GAS alpha-glucan degradation. The DeltaamyA strain had a slower rate of translocation across human pharyngeal epithelial cells. Consistent with this finding, the DeltaamyA strain was less virulent following mouse mucosal challenge. Recombinant AmyA degraded alpha-glucans into beta-cyclomaltodextrins that reduced pharyngeal cell transepithelial resistance, providing a physiologic explanation for the observed transepithelial migration phenotype. Higher amyA transcript levels were present in serotype M1 GAS strains causing invasive infection compared with strains causing pharyngitis. GAS proliferation in a defined alpha-glucan-containing medium was dependent on the presence of human salivary alpha-amylase. These data delineate the molecular mechanisms by which alpha-glucan degradation contributes to GAS host-pathogen interaction, including how GAS uses human salivary alpha-amylase for its own metabolic benefit.

Adhesion determinants of the Streptococcus species

Streptococci are clinically important Gram-positive bacteria that are capable to cause a wide variety of diseases in humans and animals. Phylogenetic analyses based on 16S rRNA sequences of the streptococcal species reveal a clustering pattern, reflecting, with a few exceptions, their pathogenic potential and ecological preferences. Microbial adhesion to host tissues is the initial critical event in the pathogenesis of most infections. Streptococci use multiple adhesins to attach to the epithelium, and their expression is regulated in response to environmental and growth conditions. Bacterial adhesins recognize and bind cell surface molecules and extracellular matrix components through specific domains that for certain adhesin families have been well defined and found conserved across the streptococcal species. In this review, we present the different streptococcal adhesin families categorized on the basis of their adhesive properties and structural characteristics, and, when available, we focus the attention on conserved functional domains.

Complement evasion strategies of pathogens-acquisition of inhibitors and beyond

Activation of the complement system and resulting opsonisation with C3b are key events of the innate immune defense against infections. However, a wide variety of bacterial pathogens subvert complement attack by binding host complement inhibitors such as C4b-binding protein, factor H and vitronectin, which results in diminished opsonophagocytosis and killing of bacteria by lysis. Another widely used strategy is production of proteases, which can effectively degrade crucial complement components. Furthermore, bacterial pathogens such as Moraxella catarrhalis and Staphylococcus aureus capture and incapacitate the key complement component C3. The current review describes examples of these three strategies. Targeting binding sites for complement inhibitors on bacterial surfaces and complement-degrading proteases with vaccine-induced antibodies may be used to enhance a common vaccine design strategy that depends on the generation of complement-dependent bactericidal and opsonophagocytic antibody activities.

Capturing host-pathogen interactions by protein microarrays: identification of novel streptococcal proteins binding to human fibronectin, fibrinogen, and C4BP

Microbial pathogen entry and survival in the host is mediated by a network of molecular interactions between the two partners, which has been the subject of many research efforts. A complex picture is emerging in which host-pathogen crosstalk involves a high number of proteins, often with redundant functions. In the present study, we investigated the potential of protein microarrays to simultaneously scan interactions between surface proteins from two main human streptococcal pathogens, Streptococcus pyogenes and Streptococcus agalactiae, and three human ligands, fibronectin, fibrinogen, and C4 binding protein, known to play an important role in streptococcal pathogenesis. By using this technology, we confirmed interactions described in the literature and detected a novel set of streptococcal proteins with binding capacities for the human ligands. The observations were validated by Western blot and ELISA techniques. Three of the newly identified proteins were isoforms of a group B streptococcus-secreted component named Fib and displayed differential binding capacities for fibronectin, fibrinogen, and C4BP. The protein regions involved in the interaction with each ligand were identified by constructing fragments of one of the Fib variants. The approach proved valuable for the acquisition of novel insights into the complex network of protein-protein interactions occurring during microbial infection.

Risk factors for severe impetiginized atopic dermatitis in Japan and assessment of its microbiological features

Patients with atopic dermatitis (AD) are susceptible to cutaneous bacterial infection. When such patients develop infection, some have extensive impetiginized dermatitis with high fever. To clarify the risk factors for severe impetiginized AD and its microbiological features, we reviewed clinical and microbiological data of 14 patients with impetiginized AD who were admitted to our hospital between the years 1999 and 2006. All patients had poorly controlled AD with eczematous lesions on the extensive body surface. The mean age was 28.2 years (range 18-35). Cultures of the lesional skin yielded both Streptococcus pyogenes and Staphylococcus aureus in 12 patients. S. pyogenes alone was isolated in two cases. These observations suggest that poorly controlled AD in adults is a risk factor for severe impetiginized AD and that S. pyogenes might play an important role in the development of severe clinical symptoms.

Rapid screen for epithelial internalization of Tn917-mutagenized Streptococcus pyogenes

Group A streptococci (GAS) cause a number of human diseases ranging from pharyngitis to necrotizing fasciitis. GAS are hypothesized to escape killing by either the immune system or beta lactam antibiotics by internalization into epithelial cells. A Tn917 library of transposon mutants was screened for capacity to invade and survive in human epithelial cells using a novel blood agar overlay method. Although the screen revealed that a majority of Tn917 insertions occurred within a 10 kb region of the genome, GAS genes identified as essential for internalization into epithelial cells included ABC transporters, and DNA maintenance proteins, and citrate metabolism enzymes, underlining the importance of adaptation to the intracellular environment.

Population biology of the human restricted pathogen, Streptococcus pyogenes

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

Positive correlation between low adhesion of group A Streptococcus to mammalian cells and virulence in a mouse model

We previously reported that a mouse model reflected, at least in part, the pathogenic mechanism of bacteremia observed during streptococcal toxic shock syndrome caused by group A Streptococcus (GAS). We have extended this study by assaying the in vitro adhesion of these same isolates to mammalian cells. Unexpectedly, we found that high-virulence GAS isolates in the mouse model showed low adhesion to the host cells. Similarly, the rate of recovery from the peritoneal cavity and cardiac blood of mice after intraperitonial injection was higher for high- than for low-virulence strains. Levels of expression of molecules that affect the adhesion of GAS to host cells were not significantly correlated with GAS virulence. Taken together, these results indicate that the invasiveness of GAS, reflected as higher virulence, is correlated directly with lower adhesion to host cells.

PfbA, a novel plasmin- and fibronectin-binding protein of Streptococcus pneumoniae, contributes to fibronectin-dependent adhesion and antiphagocytosis

Streptococcus pneumoniae is a major causative agent of mortality throughout the world. The initial event in invasive pneumococcal disease is the attachment of pneumococci to epithelial cells in the upper respiratory tract. Several bacterial proteins can bind to host extracellular matrix proteins and function as adhesins and invasins. To identify adhesins or invasins on the pneumococcal cell surface, we searched for several proteins with an LPXTG anchoring motif in the whole-genome sequence of Streptococcus pneumoniae and identified one, which we called PfbA (plasmin- and fibronectin-binding protein A), that bound to human serum proteins. Immunofluorescence microscopy and fluorescence-activated cell sorter analysis revealed that PfbA was expressed on the pneumococcal cell surface. A DeltapfbA mutant strain was only half as competent as the wild-type strain at adhering to and invading lung and laryngeal epithelial cells. In addition, epithelial cells infected with DeltapfbA showed morphological changes, including cell flattening and a loss of microvilli, that did not occur in cells infected with the wild-type strain. The mutant strain also exhibited a weaker antiphagocytotic activity than wild type in human peripheral blood. Moreover, the growth of wild-type bacteria in human whole blood containing anti-PfbA antibodies was reduced by approximately 50% after 3 h compared with its growth without the antibody. These results suggest that PfbA is an important factor in the development of pneumococcal infections.

Complement factor H allotype 402H is associated with increased C3b opsonization and phagocytosis of Streptococcus pyogenes

The main virulence factor of group A streptococcus (GAS), M protein, binds plasma complement regulators factor H (FH) and FH-like protein 1 (FHL-1) leading to decreased opsonization. The M protein binding site on FH is within domain 7 in which also the age-related macular degeneration (AMD)-associated polymorphism Y402H is located. We studied if FH allotypes 402H and 402Y have different binding affinities to GAS. Plasma-derived FH allotype 402H and its recombinant fragment FH5-7(402H) showed decreased binding to several GAS strains. Growth of GAS in human blood taken from FH(402H) homozygous individuals was decreased when compared with blood taken from FH(402Y) homozygous individuals. The effect of the allotype 402H can be explained by combining the previous M protein mutagenesis data and the recently published crystal structure of FH6-8. In conclusion the data indicate that the AMD-associated allotype 402H leads to diminished binding of FH to GAS and increased opsonophagocytosis of the bacteria in blood. These results suggest that the homozygous presence of the allele 402H could be associated with decreased risk for severe GAS infections offering an explanation for the high frequency of the allele despite its association with visual impairment.

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.

Inactivation of a gene for a fibronectin-binding protein of the oral bacterium Streptococcus mutans partially impairs its adherence to fibronectin

A sequence of 1647 base pairs in length of Streptococcus mutans DNA that encodes for a 63-kDa protein with significant amino acid similarity with fibronectin-binding proteins of Streptococcus pyogenes and Streptococcus gordonii was cloned. The putative recombinant fibronectin-binding protein of S. mutans was purified using affinity chromatography and the cloned protein was used to prepare polyclonal antibodies against the recombinant protein. In immunoblot assays, antibodies against the S. pyogenes fibronectin-binding protein, FBP54, were cross-reactive with the S. mutans protein that was designated SmFnB. Additionally, antibodies to the S. mutans SmFnB protein reacted with the S. pyogenes FBP54 protein. The S. mutans SmFnB protein was found to bind to immobilized fibronectin in a concentration dependant manner. A mutant strain of S. mutans M51 that was constructed by allelic exchange did not express the SmFnB protein. This mutant strain, S. mutans DeltaSmFnB, was determined in an ELISA to bind to immobilized fibronectin 30% less when compared to the parental strain S. mutans M51. The results are consistent with the conclusion that the 63-kDa SmFnB protein of S. mutans is a fibronectin-binding protein that may contribute to the interaction of S. mutans with damaged heart tissue during pathogenesis of infective endocarditis. Also, the study suggests that multiple molecules may mediate the interaction of S. mutans with fibronectin.

Molecular and biological characterization of histidine triad protein in group A streptococci

Four Streptococcus pneumoniae genes, phtA, phtB, phtD, and phtE, as well as the slr gene of group A streptococci (GAS), encode proteins with a histidine triad motif (HxxHxH). Pht proteins function as protective antigens against S. pneumoniae infection. A search of the GAS genome database identified a novel protein, HtpA, possessing five histidine triad motifs. The htpA gene was shown to encode a 92.5-kDa protein located downstream of the fbaA and lbp genes, while Western blot analyses revealed that HtpA protein was expressed on the cell surfaces of all group A, B, C, and G streptococcal isolates tested. Immunization of mice with rHtpA induced antigen-specific antibody production and was effective after a single immunization, with antibody titers remaining constant for at least 84days. In addition, HtpA-immunized mice survived after challenge with GAS strains isolated from patients with streptococcal toxic shock syndrome for significantly longer periods than sham-immunized mice. In that experiment, the HtpA-specific antibody was effectively induced by a single immunization and the specific antibody titer remained constant for at least 84days. These results indicate that the novel histidine triad protein HtpA is a candidate vaccine for GAS infection.

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

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