Insight into the molecular basis of pathogen abundance: group A Streptococcus inhibitor of complement inhibits bacterial adherence and internalization into human cells

Interplay between group A Streptococcus and host innate immune responses

SUMMARYGroup A Streptococcus (GAS), also known as Streptococcus pyogenes, is a clinically well-adapted human pathogen that harbors rich virulence determinants contributing to a broad spectrum of diseases. GAS is capable of invading epithelial, endothelial, and professional phagocytic cells while evading host innate immune responses, including phagocytosis, selective autophagy, light chain 3-associated phagocytosis, and inflammation. However, without a more complete understanding of the different ways invasive GAS infections develop, it is difficult to appreciate how GAS survives and multiplies in host cells that have interactive immune networks. This review article attempts to provide an overview of the behaviors and mechanisms that allow pathogenic GAS to invade cells, along with the strategies that host cells practice to constrain GAS infection. We highlight the counteractions taken by GAS to apply virulence factors such as streptolysin O, nicotinamide-adenine dinucleotidase, and streptococcal pyrogenic exotoxin B as a hindrance to host innate immune responses.

BifA Triggers Phosphorylation of Ezrin to Benefit Streptococcus equi subsp. zooepidemicus Survival from Neutrophils Killing

Streptococcus equi subsp. zooepidemicus (SEZ) ATCC35246 can invade the brain and cause severe neutrophils infiltration in brain tissue. This microorganism can survive and reproduce to an extremely high CFU burden (10⁸–10⁹/organ) under stressful neutrophils infiltration circumstances. The aim of this research is to explore the mechanism of the SEZ hypervirulent strain with its specific bifA gene which avoids being eliminated by neutrophils in the brain. We isolated the primary mouse neutrophils to treat SEZ WT and bifA gene defective (ΔBif) strains. The ΔBif strain had a weakened function of defending against neutrophils killing in vitro. The interaction between BifA and ezrin proteins in neutrophils were identified by co-IP and immunoblot. In neutrophils, the BifA interacts with ezrin and triggers the phosphorylation of ezrin at its Thr567 site in a PKC-dependent manner, then the excessive elevation of phosphorylated-ezrin recruits Dbl and activates Rac1. Since the Rac1 is closely relevant to several critical cellular functions, its abnormal activation will lead to neutrophils dysfunction and benefit to SEZ survival from neutrophils killing. Our findings reveal a novel consequence of BifA and ERM family protein (for ezrin, radixin, moesin) interaction, which happens between BifA and ezrin in neutrophils and contributes to SEZ survival in the brain. BifA should be considered as a potential target for drug development to prevent SEZ infection.

Understanding nanoparticle endocytosis to improve targeting strategies in nanomedicine

Nanoparticles (NPs) have attracted considerable attention in various fields, such as cosmetics, the food industry, material design, and nanomedicine. In particular, the fast-moving field of nanomedicine takes advantage of features of NPs for the detection and treatment of different types of cancer, fibrosis, inflammation, arthritis as well as neurodegenerative and gastrointestinal diseases. To this end, a detailed understanding of the NP uptake mechanisms by cells and intracellular localization is essential for safe and efficient therapeutic applications. In the first part of this review, we describe the several endocytic pathways involved in the internalization of NPs and we discuss the impact of the physicochemical properties of NPs on this process. In addition, the potential challenges of using various inhibitors, endocytic markers and genetic approaches to study endocytosis are addressed along with the principal (semi) quantification methods of NP uptake. The second part focuses on synthetic and bio-inspired substances, which can stimulate or decrease the cellular uptake of NPs. This approach could be interesting in nanomedicine where a high accumulation of drugs in the target cells is desirable and clearance by immune cells is to be avoided. This review contributes to an improved understanding of NP endocytic pathways and reveals potential substances, which can be used in nanomedicine to improve NP delivery.

Streptococcal protein SIC activates monocytes and induces inflammation

Streptococcus pyogenes is a major bacterial pathogen in the human population and isolates of the clinically important M1 serotype secrete protein Streptococcal inhibitor of complement (SIC) known to interfere with human innate immunity. Here we find that SIC from M1 bacteria interacts with TLR2 and CD14 on monocytes leading to the activation of the NF-κB and p38 MAPK pathways and the release of several pro-inflammatory cytokines (e.g. TNFα and INFγ). In human plasma, SIC binds clusterin and histidine-rich glycoprotein, and whole plasma, and these two purified plasma proteins enhanced the activation of monocytes by SIC. Isolates of the M55 serotype secrete an SIC homolog, but this protein did not activate monocytes. M1 isolates are common in cases of invasive S. pyogenes infections characterized by massive inflammation, and the results of this study indicate that the pro-inflammatory property of SIC contributes to the pathology of these severe clinical conditions.

Vaccine-induced, but not natural immunity, against the Streptococcal inhibitor of complement protects against invasive disease

Highly pathogenic emm1 Streptococcus pyogenes strains secrete the multidomain Streptococcal inhibitor of complement (SIC) that binds and inactivates components of the innate immune response. We aimed to determine if naturally occurring or vaccine-induced antibodies to SIC are protective against invasive S. pyogenes infection. Immunisation with full-length SIC protected mice against systemic bacterial dissemination following intranasal or intramuscular infection with emm1 S. pyogenes. Vaccine-induced rabbit anti-SIC antibodies, but not naturally occurring human anti-SIC antibodies, enhanced bacterial clearance in an ex vivo whole-blood assay. SIC vaccination of both mice and rabbits resulted in antibody recognition of all domains of SIC, whereas naturally occurring human anti-SIC antibodies recognised the proline-rich region of SIC only. We, therefore, propose a model whereby natural infection with S. pyogenes generates non-protective antibodies against the proline-rich region of SIC, while vaccination with full-length SIC permits the development of protective antibodies against all SIC domains.

Group A Streptococcus-Mediated Host Cell Signaling

In the past decade, the field of the cellular microbiology of group A Streptococcus (S. pyogenes) infection has made tremendous advances and touched upon several important aspects of pathogenesis, including receptor biology, invasive and evasive phenomena, inflammasome activation, strain-specific autophagic bacterial killing, and virulence factor-mediated programmed cell death. The noteworthy aspect of S. pyogenes-mediated cell signaling is the recognition of the role of M protein in a variety of signaling events, starting with the targeting of specific receptors on the cell surface and on through the induction and evasion of NETosis, inflammasome, and autophagy/xenophagy to pyroptosis and apoptosis. Variations in reports on S. pyogenes-mediated signaling events highlight the complex mechanism of pathogenesis and underscore the importance of the host cell and S. pyogenes strain specificity, as well as in vitro/in vivo experimental parameters. The severity of S. pyogenes infection is, therefore, dependent on the virulence gene expression repertoire in the host environment and on host-specific dynamic signaling events in response to infection. Commonly known as an extracellular pathogen, S. pyogenes finds host macrophages as safe havens wherein it survives and even multiplies. The fact that endothelial cells are inherently deficient in autophagic machinery compared to epithelial cells and macrophages underscores the invasive nature of S. pyogenes and its ability to cause severe systemic diseases. S. pyogenes is still one of the top 10 causes of infectious mortality. Understanding the orchestration of dynamic host signaling networks will provide a better understanding of the increasingly complex mechanism of S. pyogenes diseases and novel ways of therapeutically intervening to thwart severe and often fatal infections.

Streptococcal inhibitor of complement (SIC) modulates fibrinolysis and enhances bacterial survival within fibrin clots

Some strains of the bacterial pathogen Streptococcus pyogenes secrete protein SIC (streptococcal inhibitor of complement), including strains of the clinically relevant M1 serotype. SIC neutralizes the effect of a number of antimicrobial proteins/peptides and interferes with the function of the host complement system. Previous studies have shown that some S. pyogenes proteins bind and modulate coagulation and fibrinolysis factors, raising the possibility that SIC also may interfere with the activity of these factors. Here we show that SIC interacts with both human thrombin and plasminogen, key components of coagulation and fibrinolysis. We found that during clot formation, SIC binds fibrin through its central region and that SIC inhibits fibrinolysis by interacting with plasminogen. Flow cytometry results indicated that SIC and plasminogen bind simultaneously to S. pyogenes bacteria, and fluorescence microscopy revealed co-localization of the two proteins at the bacterial surface. As a consequence, SIC-expressing bacteria entrapped in clots inhibit fibrinolysis, leading to delayed bacterial escape from the clots as compared with mutant bacteria lacking SIC. Moreover, within the clots SIC-expressing bacteria were protected against killing. In an animal model of subcutaneous infection, SIC-expressing bacteria exhibited a delayed systemic spread. These results demonstrate that the bacterial protein SIC interferes with coagulation and fibrinolysis and thereby enhances bacterial survival, a finding that has significant implications for S. pyogenes virulence.

Paediatric Autoimmune Neuropsychiatric Disorder Associated with Group A Beta-Haemolytic Streptococcal Infection: An Indication for Tonsillectomy? A Review of the Literature

Background. Paediatric Autoimmune Neuropsychiatric Disorder Associated with Streptococcal Infection (PANDAS) is the acute onset of neuropsychiatric symptoms following group A beta-haemolytic streptococcal infection. The aetiology remains elusive. However, with group A streptococcus being the most common bacterial cause of tonsillitis, surgical intervention in the form of tonsillectomy has often been considered as a potential therapy. Methods. A MEDLINE® search was undertaken using keywords “PANDAS” or “paediatric autoimmune neuropsychiatric disorders associated with streptococcus” combined with “tonsillectomy”. Results. Six case reports and 3 case series met the inclusion criteria. Demesh et al. (case series) reported a dramatic reduction in neuropsychiatric symptom severity in the patient cohort undergoing tonsillectomy. Two case series suggest that there is no association between tonsillectomy and resolution of PANDAS. Conclusion. Due to the lack of uniform data and sporadic reports, tonsillectomy should be carefully adopted for the treatment of this disorder. In particular, tonsillectomies/adenoidectomies to alleviate neuropsychiatric symptoms should be avoided until more definitive evidence is at our disposal. This review highlights the importance of a potential collaborative prospective study.

Streptococcus anginosus (milleri) Group Strains Isolated in Poland (1996-2012) and their Antibiotic Resistance Patterns

Streptococcus anginosus, Streptococcus intermedius and Streptococcus constellatus form a group of related streptococcal species, namely the Streptococcus Anginosus Group (SAG). The group, previously called "milleri" had been rarely described until 1980/1990 as source of infections. Nowadays SAG bacteria are often described as pathogens causing predominantly purulent infections. The number of infections is highly underestimated, as SAG strains are often classified in the microbiology laboratory as less virulent "viridans streptococci" Epidemiological situation regarding SAG infections in Poland has been unrecognized, therefore we performed a retrospective analysis of strains isolated between 1996 and 2012. Strains suspected of belonging to SAG were re-identified using an automated biochemical approach (Vitek2) and MALDI-TOF MS. We performed first analysis of antibiotic resistance among SAG strains isolated in Poland using automated methods (Vitek2), disk diffusion tests and E-Tests. We also performed PCR detection of resistance determinants in antibiotic resistant strains. Clonal structure of analyzed strains was evaluated with PFGE and MLVF methods. All three species are difficult to distinguish using automated diagnostic methods and the same is true for automated MIC evaluation. Our analysis revealed SAG strains are rarely isolated in Poland, predominantly from purulent infections. All isolates are very diverse on the genomic level as estimated by PFGE and MLVF analyses. All analyzed strains are sensitive to penicillin, a substantial group of strains is resistant to macrolides and the majority of strains are resistant to tetracycline.

Mechanisms of group A Streptococcus resistance to reactive oxygen species

Streptococcus pyogenes, also known as group A Streptococcus (GAS), is an exclusively human Gram-positive bacterial pathogen ranked among the ‘top 10’ causes of infection-related deaths worldwide. GAS commonly causes benign and self-limiting epithelial infections (pharyngitis and impetigo), and less frequent severe invasive diseases (bacteremia, toxic shock syndrome and necrotizing fasciitis). Annually, GAS causes 700 million infections, including 1.8 million invasive infections with a mortality rate of 25%. In order to establish an infection, GAS must counteract the oxidative stress conditions generated by the release of reactive oxygen species (ROS) at the infection site by host immune cells such as neutrophils and monocytes. ROS are the highly reactive and toxic byproducts of oxygen metabolism, including hydrogen peroxide (H₂O₂), superoxide anion (O₂•⁻), hydroxyl radicals (OH•) and singlet oxygen (O₂*), which can damage bacterial nucleic acids, proteins and cell membranes. This review summarizes the enzymatic and regulatory mechanisms utilized by GAS to thwart ROS and survive under conditions of oxidative stress.

This review discusses the mechanisms utilized by the bacterial pathogen group A Streptococcus to detoxify reactive oxygen species and survive in the human host under conditions of oxidative stress.

Responses of innate immune cells to group A Streptococcus

Group A Streptococcus (GAS), also called Streptococcus pyogenes, is a Gram-positive beta-hemolytic human pathogen which causes a wide range of mostly self-limiting but also several life-threatening diseases. Innate immune responses are fundamental for defense against GAS, yet their activation by pattern recognition receptors (PRRs) and GAS-derived pathogen-associated molecular patterns (PAMPs) is incompletely understood. In recent years, the use of animal models together with the powerful tools of human molecular genetics began shedding light onto the molecular mechanisms of innate immune defense against GAS. The signaling adaptor MyD88 was found to play a key role in launching the immune response against GAS in both humans and mice, suggesting that PRRs of the Toll-like receptor (TLR) family are involved in sensing this pathogen. The specific TLRs and their ligands have yet to be identified. Following GAS recognition, induction of cytokines such as TNF and type I interferons (IFNs), leukocyte recruitment, phagocytosis, and the formation of neutrophil extracellular traps (NETs) have been recognized as key events in host defense. A comprehensive knowledge of these mechanisms is needed in order to understand their frequent failure against GAS immune evasion strategies.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

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

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

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

Interaction between M-like protein and macrophage thioredoxin facilitates antiphagocytosis for Streptococcus equi ssp. zooepidemicus

Streptococcus equi ssp. zooepidemicus (S. zooepidemicus, S.z) is one of the common pathogens that can cause septicemia, meningitis, and mammitis in domesticated species. M-like protein (SzP) is an important virulence factor of S. zooepidemicus and contributes to bacterial infection and antiphagocytosis. The interaction between SzP of S. zooepidemicus and porcine thioredoxin (TRX) was identified by the yeast two-hybrid and further confirmed by co-immunoprecipitation. SzP interacted with both reduced and the oxidized forms of TRX without inhibiting TRX activity. Membrane anchored SzP was able to recruit TRX to the surface, which would facilitate the antiphagocytosis of the bacteria. Further experiments revealed that TRX regulated the alternative complement pathway by inhibiting C3 convertase activity and associating with factor H (FH). TRX alone inhibited C3 cleavage and C3a production, and the inhibitory effect was additive when FH was also present. TRX inhibited C3 deposition on the bacterial surface when it was recruited by SzP. These new findings indicated that S. zooepidemicus used SzP to recruit TRX and regulated the alternative complement pathways to evade the host immune phagocytosis.

Pathogen-mimetic stealth nanocarriers for drug delivery: a future possibility

The Mononuclear Phagocyte System (MPS) is a major constraint to nanocarrier-based drug-delivery systems (DDS) by exerting a negative impact on blood circulation times and biodistribution. Current approaches rely on the protein- and cell-repelling properties of inert hydrophilic polymers, to enable escape from the MPS. Poly(ethylene glycol) (PEG) has been particularly useful in this regard, and it also exerts positive effects in other blood compatibility parameters, being correlated with decreased hemolysis, thrombogenicity, complement activation and protein adsorption, due to its uncharged and hydrophilic nature. However, PEGylated nanocarriers are commonly found in the liver and spleen, the major MPS organs. In fact, a hydrophilic and cell-repelling delivery system is not always beneficial, as it might decrease the interaction with the target cell and hinder drug release. Here, a full scope of the immunological and biochemical barriers is presented along with some selected examples of alternatives to PEGylation. We present a novel conceptual approach that includes virulence factors for the engineering of bioactive, immune system-evasive stealth nanocarriers.

FROM THE CLINICAL EDITOR

The efficacy of nanocarrier-based drug-delivery systems is often dampened by the Mononuclear Phagocyte System (MPS). Current approaches to circumvent MPS rely on protein- and cell-repelling properties of inert hydrophilic polymers, including PEG. This paper discusses the full scope of the immunological and biochemical barriers along with selected examples of alternatives to PEGylation.

Streptococcus pyogenes Ser/Thr kinase-regulated cell wall hydrolase is a cell division plane-recognizing and chain-forming virulence factor

Cell division and cell wall synthesis are closely linked complex phenomena and play a crucial role in the maintenance and regulation of bacterial virulence. Eukaryotic-type Ser/Thr kinases reported in prokaryotes, including that in group A Streptococcus (GAS) (Streptococcus pyogenes Ser/Thr kinase (SP-STK)), regulate cell division, growth, and virulence. The mechanism of this regulation is, however, unknown. In this study, we demonstrated that SP-STK-controlled cell division is mediated under the positive regulation of secretory protein that possesses a cysteine and histidine-dependent aminohydrolases/peptidases (CHAP) domain with functionally active cell wall hydrolase activity (henceforth named as CdhA (CHAP-domain-containing and chain-forming cell wall hydrolase). Deletion of the CdhA-encoding gene resulted in severe cell division and growth defects in GAS mutants. The mutant expressing the truncated CdhA (devoid of the CHAP domain), although displayed no such defects, it became attenuated for virulence in mice and highly susceptible to cell wall-acting antibiotics, as observed for the mutant lacking CdhA. When CdhA was overexpressed in the wild-type GAS as well as in heterologous strains, Escherichia coli and Staphylococcus aureus, we observed a distinct increase in bacterial chain length. Our data reveal that CdhA is a multifunctional protein with a major function of the N-terminal region as a cell division plane-recognizing domain and that of the C-terminal CHAP domain as a virulence-regulating domain. CdhA is thus an important therapeutic target.

Streptococcal inhibitor of complement promotes innate immune resistance phenotypes of invasive M1T1 group A Streptococcus

Streptococcal inhibitor of complement (SIC) is a highly polymorphic extracellular protein and putative virulence factor secreted by M1 and M57 strains of group A Streptococcus (GAS). The sic gene is highly upregulated in invasive M1T1 GAS isolates following selection of mutations in the covR/S regulatory locus in vivo. Previous work has shown that SIC (allelic form 1.01) binds to and inactivates complement C5b67 and human cathelicidin LL-37. We examined the contribution of SIC to innate immune resistance phenotypes of GAS in the intact organism, using (1) targeted deletion of sic in wild-type and animal-passaged (covS mutant) M1T1 GAS harboring the sic 1.84 allele and (2) heterologous expression of sic in M49 GAS, which does not possess the sic genein its genome. We find that M1T1 SIC production is strongly upregulated upon covS mutation but that the sic gene is not required for generation and selection of covS mutants in vivo. SIC 1.84 bound both human and murine cathelicidins and was necessary and sufficient to promote covS mutant M1T1 GAS resistance to LL-37, growth in human whole blood and virulence in a murine model of systemic infection. Finally, the sic knockout mutant M1T1 GAS strain was deficient in growth in human serum and intracellular macrophage survival. We conclude that SIC contributes to M1T1 GAS immune resistance and virulence phenotypes.

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.

The streptococcal inhibitor of complement (SIC) protects Streptococcus pyogenes from bacteriocin-like inhibitory substance (BLIS) from Streptococcus salivarius

Streptococcus salivarius inhibits the growth of Streptococcus pyogenes in vitro. Streptococcus pyogenes has various virulence factors, including the streptococcus inhibitor of complement (SIC). Although SIC inhibits the activity of the peptides LL-37 and NAP1, the relationship between SIC and the bacteriocin-like inhibitory substance (BLIS) has not been elucidated. Here, we evaluated whether S. salivarius BLIS affects S. pyogenes SIC. We created three deltasic mutant strains from three S. pyogenes strains and performed deferred antagonism assays. The test strains were BLIS-positive S. salivarius JCM5707 and BLIS-negative S. salivarius NCU12. Deferred antagonism assays with JCM5707 showed that the inhibitory zones in the three deltasic mutant strains were wider than those in the three wild-type strains. Streptococcus pyogenes was cultured in BLIS-containing broth and the change in SIC in the supernatant was assessed by two-dimensional gel electrophoresis (2-DE). The 2-DE analysis of S. pyogenes exoproteins with the JCM5707 supernatant showed reduced SIC compared with those without the JCM5707 supernatant. Changes in sic mRNA levels affected by S. salivarius BLIS were evaluated by a reverse transcriptase-PCR. The sic mRNA level was affected more by the BLIS-positive S. salivarius than by the BLIS-negative strain. Our result indicates that SIC plays a role in the inhibition of S. salivarius BLIS.

Inactivation of DltA modulates virulence factor expression in Streptococcus pyogenes

BACKGROUND

D-alanylated lipoteichoic acid is a virtually ubiquitous component of gram-positive cell walls. Mutations in the dltABCD operon of numerous species exhibit pleiotropic effects, including reduced virulence, which has been attributed to increased binding of cationic antimicrobial peptides to the more negatively charged cell surface. In this study, we have further investigated the effects that mutating dltA has on virulence factor expression in Streptococcus pyogenes.

METHODOLOGY/PRINCIPAL FINDINGS

Isogenic Delta dltA mutants had previously been created in two distinct M1T1 isolates of S. pyogenes. Immunoblots, flow cytometry, and immunofluorescence were used to quantitate M protein levels in these strains, as well as to assess their ability to bind complement. Bacteria were tested for their ability to interact with human PMN and to grow in whole human blood. Message levels for emm, sic, and various regulatory elements were assessed by quantitative RT-PCR. Cell walls of Delta dltA mutants contained much less M protein than cell walls of parent strains and this correlated with reduced levels of emm transcripts, increased deposition of complement, increased association of bacteria with polymorphonuclear leukocytes, and reduced bacterial growth in whole human blood. Transcription of at least one other gene of the mga regulon, sic, which encodes a protein that inactivates antimicrobial peptides, was also dramatically reduced in Delta dltA mutants. Concomitantly, ccpA and rofA were unaffected, while rgg and arcA were up-regulated.

CONCLUSIONS/SIGNIFICANCE

This study has identified a novel mechanism for the reduced virulence of dltA mutants of Streptococcus pyogenes in which gene regulatory networks somehow sense and respond to the loss of DltA and lack of D-alanine esterification of lipoteichoic acid. The mechanism remains to be determined, but the data indicate that the status of D-alanine-lipoteichoic acid can significantly influence the expression of at least some streptococcal virulence factors and provide further impetus to targeting the dlt operon of gram-positive pathogens in the search for novel antimicrobial compounds.

Species-specific interaction of Streptococcus pneumoniae with human complement factor H

Streptococcus pneumoniae naturally colonizes the nasopharynx as a commensal organism and sometimes causes infections in remote tissue sites. This bacterium is highly capable of resisting host innate immunity during nasopharyngeal colonization and disseminating infections. The ability to recruit complement factor H (FH) by S. pneumoniae has been implicated as a bacterial immune evasion mechanism against complement-mediated bacterial clearance because FH is a complement alternative pathway inhibitor. S. pneumoniae recruits FH through a previously defined FH binding domain of choline-binding protein A (CbpA), a major surface protein of S. pneumoniae. In this study, we show that CbpA binds to human FH, but not to the FH proteins of mouse and other animal species tested to date. Accordingly, deleting the FH binding domain of CbpA in strain D39 did not result in obvious change in the levels of pneumococcal bacteremia or virulence in a bacteremia mouse model. Furthermore, this species-specific pneumococcal interaction with FH was shown to occur in multiple pneumococcal isolates from the blood and cerebrospinal fluid. Finally, our phagocytosis experiments with human and mouse phagocytes and complement systems provide additional evidence to support our hypothesis that CbpA acts as a bacterial determinant for pneumococcal resistance to complement-mediated host defense in humans.

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.

How group A Streptococcus circumvents host phagocyte defenses

Group A Streptococcus (GAS) is a Gram-positive bacterium associated with a variety of mucosal and invasive human infections. GAS systemic disease reflects the diverse abilities of this pathogen to avoid eradication by phagocytic defenses of the innate immune system. Here we review how GAS can avoid phagocyte engagement, inhibit complement and antibody functions required for opsonization, impair phagocytotic uptake mechanisms, promote phagocyte lysis or apoptosis, and resist specific effectors of phagocyte killing such as antimicrobial peptides and reactive oxygen species. Understanding the molecular basis of GAS phagocyte resistance may reveal novel therapeutic targets for treatment and prevention of invasive human infections.

Dynamics in prophage content of invasive and noninvasive M1 and M28 Streptococcus pyogenes isolates in The Netherlands from 1959 to 1996

Invasive group A streptococcal (GAS) disease re-emerged in The Netherlands in the late 1980s. To seek an explanation for this resurgence, the genetic compositions of 22 M1 and 19 M28 GAS strains isolated in The Netherlands between 1960s and the mid-1990s were analyzed by using a mixed-genome DNA microarray. During this four-decade period, M1 and especially M28 strains acquired prophages on at least eight occasions. All prophages carried a superantigen (speA2, speC, speK) or a streptodornase (sdaD2, sdn), both associated with invasive GAS disease. Invasive and noninvasive GAS strains did not differ in prophage acquisition, suggesting that there was an overall increase in the pathogenicity of M1 and M28 strains over the last four decades rather than emergence of hypervirulent subclones. The increased overall pathogenic potential may have contributed to the reemergence of invasive GAS disease in The Netherlands.

Determination of the relationship between group A streptococcal genome content, M type, and toxic shock syndrome by a mixed genome microarray

Group A streptococci (GAS), or Streptococcus pyogenes, are associated with a remarkable variety of diseases, ranging from superficial infections to life-threatening diseases such as toxic-shock-like syndrome (TSS). GAS strains belonging to M types M1 and M3 are associated with TSS. This study aims to obtain insight into the gene profiles underlying different M types and disease manifestations. Genomic differences between 76 clinically well characterized GAS strains collected in The Netherlands were examined using a mixed-genome microarray. Inter-M-type genomic differences clearly outweighed intra-M-type genome variation. Phages were major contributors to observed genome diversification. We identified four novel genes, including two genes encoding fibronectin-binding-like proteins, which are highly specific to a subset of M types and thus may contribute to M-type-associated disease manifestations. All M12 strains were characterized by the unique absence of the citrate lyase complex and reduced growth under hypoxic, nutrient-deprived conditions. Furthermore, six virulence factors, including genes encoding a complement-inhibiting protein (sic), an exotoxin (speA), iron(III) binding factor, collagen binding factor (cpa), and fibrinogen binding factor (prt2-like), were unique to M1 and/or M3 strains. These virulence factors may contribute to the potential of these strains to cause TSS. Finally, in contrast to M-type-specific virulence profiles, we did not identify a common virulence profile among strains associated with TSS irrespective of their M type.

Dynamics of the immune response against extracellular products of group A streptococci during infection

The immune response against the infecting group A streptococcus (GAS) extracellular products (EP) was determined in acute- and convalescent-phase sera from 75 patients with different clinical manifestations of GAS infection. All EP elicited a high proliferative response in human peripheral blood mononuclear cells. In patients with bacteremia, low neutralization in acute-phase sera was associated with development of streptococcal toxic shock syndrome. Lack of neutralization in acute-phase sera was more common in patients infected with the T1emm1 serotype. The majority of patients did not develop the ability to neutralize the mitogenic activity of their infecting isolate despite a significant increase in enzyme-linked immunosorbent assay titer in early convalescent-phase sera. In patients with the ability to neutralize GAS EP, the immune response remained high over at least 3 years. In contrast, the neutralization capacity conferred by intravenous immunoglobulin and/or plasma treatment disappeared within 3 months.

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

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

Inactivation of the group A Streptococcus regulator srv results in chromosome wide reduction of transcript levels, and changes in extracellular levels of Sic and SpeB

Group A Streptococcus is characterized by the ability to cause a diverse number of human infections including pharyngitis, necrotizing fasciitis, toxic shock syndrome, and acute rheumatic fever, yet the regulation of streptococcal genes involved in disease processes and survival in the host is not completely understood. Genome scale analysis has revealed a complex regulatory network including 13 two-component regulatory systems and more than 100 additional putative regulators, the majority of which remain uncharacterized. Among these is the streptococcal regulator of virulence, Srv, the first Group A Streptococcus member of the Crp/Fnr family of transcriptional regulators. Previous work demonstrated that the loss of srv resulted in a significant decrease in Group A Streptococcus virulence. To begin to define the gene products influenced by Srv, we combined microarray and two-dimensional gel electrophoresis analysis. Loss of srv results in a chromosome wide reduction of gene transcription and changes in the production of the extracellular virulence factors Sic (streptococcal inhibitor of complement) and SpeB (cysteine proteinase). Sic levels are reduced in the srv mutant, whereas the extracellular concentration and activity of SpeB is increased. These data link Srv to the increasingly complex GAS regulatory network.

Bacterial complement evasion

The human complement system is elemental to recognize bacteria, opsonize them for handling by phagocytes, or kill them by direct lysis. However, successful bacterial pathogens have in turn evolved ingenious strategies to overcome this part of the immune system. In this review we discuss the different stages of complement activation sequentially and illustrate the immune evasion strategies that various bacteria have developed to evade each subsequent step. The focus is on bacterial proteins, either surface-bound or excreted, that block complement activation. The underlying molecular mechanism of action and the possible role in pathophysiology of bacterial infections are discussed.

Roles of the alternative complement pathway and C1q during innate immunity to Streptococcus pyogenes

Complement is important for innate immunity to the common bacterial pathogen Streptococcus pyogenes, but the relative importance of the alternative and classical pathways has not been investigated. Using mice and human serum deficient in either C1q, the first component of the classical pathway, or factor B, an important component of the alternative pathway, we have investigated the role of both pathways for innate immunity to S. pyogenes. C3b deposition on four different strains of S. pyogenes was mainly dependent on factor B. As a consequence opsonophagocytosis of S. pyogenes was reduced in serum from factor B-deficient mice, and these mice were very susceptible to S. pyogenes infection. In contrast, C3b deposition was not dependent on C1q for two of the strains investigated, H372 and H305, yet opsonophagocytosis of all four S. pyogenes strains was impaired in serum deficient in C1q. Furthermore, infection in C1q-deficient mice with strain H372 resulted in a rapidly progressive disease associated with large numbers of bacteria in target organs. These results demonstrate the important role of the alternative pathway and C1q for innate immunity to S. pyogenes and suggest that C1q-mediated innate immunity to at least some strains of S. pyogenes may involve mechanisms that are independent of C3b on the bacteria.

Inhibition of antimicrobial peptides by group A streptococci: SIC and DRS

SIC (streptococcal inhibitor of complement) is a 31 kDa protein secreted by a few highly virulent strains of GAS (group A streptococci), predominantly by the M1 strain. Initially described as an inhibitor of the membrane attack complex of complement, it has turned out to be a polyfunctional inhibitor of the innate mucosal immune response. The SIC protein sequence contains three domains: an N-terminal SRR (short repeat region), followed by three longer tandem repeats [LRR (long repeat region)] and a C-terminal PRR (proline-rich region). SIC inhibits the antibacterial activity of a wide range of antimicrobial peptides and proteins: i.e. lysozyme, SLPI (secretory leucocyte proteinase inhibitor), LL-37, hNP-1 (human neutrophil peptide-1) and the human β-defensins 1, 2 and 3. Analysis of the functional properties of recombinant domains of SIC shows that binding and inhibition of lysozyme and human β-defensin-3 require the SRR+LRR, as does binding to SLPI. Complement inhibition is confined to the SRR. M12 GAS secrete a protein ‘distantly related to SIC’ (DRS). DRS contains a C-terminal PRR which is significantly similar to that of SIC, but it has no central LRR and the N-terminal SRR is very different. DRS inhibits human β-defensin-3, but has no effect on lysozyme, SLPI or complement.

Genome-wide analysis of group a streptococci reveals a mutation that modulates global phenotype and disease specificity

Many human pathogens produce phenotypic variants as a means to circumvent the host immune system and enhance survival and, as a potential consequence, exhibit increased virulence. For example, it has been known for almost 90 y that clinical isolates of the human bacterial pathogen group A streptococci (GAS) have extensive phenotypic heterogeneity linked to variation in virulence. However, the complete underlying molecular mechanism(s) have not been defined. Expression microarray analysis of nine clinical isolates identified two fundamentally different transcriptomes, designated pharyngeal transcriptome profile (PTP) and invasive transcriptome profile (ITP). PTP and ITP GAS differed in approximately 10% of the transcriptome, including at least 23 proven or putative virulence factor genes. ITP organisms were recovered from skin lesions of mice infected subcutaneously with PTP GAS and were significantly more able to survive phagocytosis and killing by human polymorphonuclear leukocytes. Complete genome resequencing of a mouse-derived ITP GAS revealed that the organism differed from its precursor by only a 7-bp frameshift mutation in the gene (covS) encoding the sensor kinase component of a two-component signal transduction system implicated in virulence. Genetic complementation, and sequence analysis of covR/S in 42 GAS isolates confirmed the central role of covR/S in transcriptome, exoproteome, and virulence modulation. Genome-wide analysis provides a heretofore unattained understanding of phenotypic variation and disease specificity in microbial pathogens, resulting in new avenues for vaccine and therapeutics research.

Phenotypic heterogeneity within an infecting population is a strategy commonly used by bacterial pathogens to evade the host immune system and enhance survival. Such phenotypic variation has been observed for the human pathogen group A streptococci (GAS), which can cause a wide range of diseases with differing severity. However, the underlying mechanisms that control this variation, and the survival- and virulence-associated effects of this variation, have not been fully elucidated.

By assaying total gene expression the authors found that clinical GAS isolates from invasive and pharyngeal diseases had distinct gene expression patterns during growth in standard laboratory media. These two gene expression patterns conferred distinct virulence-associated attributes on the expressing GAS strain, as assessed using bacteremia and soft-tissue infection models of disease. Likewise, the ability to survive the bactericidal activity of human neutrophils was significantly different between GAS strains with the two distinct expression patterns. Transition from one gene expression pattern to the other required the mutation of the two-component signal transduction system CovRS (control of virulence R/S). The authors conclude that the ability of GAS to remodel its transcriptome plays a major contribution in its ability to colonize distinct niches of the human body and cause disease.

Growth characteristics of and virulence factor production by group A Streptococcus during cultivation in human saliva

Group A Streptococcus (GAS) commonly infects the human oropharynx, but the initial molecular events governing this process are poorly understood. Saliva is a major component of the innate and acquired immune defense in this anatomic site. Although landmark studies were done more than 60 years ago, investigation of GAS-saliva interaction has not been addressed extensively in recent years. Serotype M1 GAS strain MGAS5005 cultured in human saliva grew to approximately 10(7) CFU/ml and, remarkably, maintained this density for up to 28 days. Strains of several other M-protein serotypes had similar initial growth patterns but did not maintain as high a CFU count during prolonged culture. As revealed by analysis of the growth of isogenic mutant strains, the ability of GAS to maintain high numbers of CFU/ml during the prolonged stationary phase in saliva was dependent on production of streptococcal inhibitor of complement (Sic) and streptococcal pyrogenic exotoxin B (SpeB). During cultivation in human saliva, GAS had growth-phase-dependent production of multiple proven and putative extracellular virulence factors, including Sic, SpeB, streptococcal pyrogenic exotoxin A, Mac protein, and streptococcal phospholipase A(2). Our results clearly show that GAS responds in a complex fashion to growth in human saliva, suggesting that the molecular processes that enhance colonization and survival in the upper respiratory tract of humans are well under way before the organism reaches the epithelial cell surface.

Attribution of the Various Inhibitory Actions of the Streptococcal Inhibitor of Complement (SIC) to Regions within the Molecule

Some strains of Streptococcus pyogenes secrete a virulence factor called the streptococcal inhibitor of complement (SIC) function. SIC is a polyfunctional protein that interacts with a number of host proteins and peptides, especially with those that are involved in host defense systems. In addition to inhibiting the complement-mediated lysis of cells, SIC inhibits lysozyme, secretory leukocyte proteinase inhibitor, and beta-defensins. SIC also binds to proteins associated with the cytoskeleton and thereby may cause cytoskeletal derangement. The SIC molecule has three distinct structural domains constituting the N-proximal short repeat region (SRR), the central long repeat region (LRR), and the C-proximal proline-rich region (PRR). To map various functions to the structural domains, we have analyzed recombinant subclones expressing various parts of SIC and elastase-generated discrete fragments of SIC for binding to various ligands and for determining their biological properties. The results demonstrate the following. (a) SRR alone was sufficient to confer inhibition of complement function. (b) Anti-defensin and anti-lysozyme activities were mapped to the SRR plus LRR. (c) The LRR plus PRR harbored ezrin binding activity.

Impact of the SpeB protease on binding of the complement regulatory proteins factor H and factor H-like protein 1 by Streptococcus pyogenes

Microbial pathogens often exploit human complement regulatory proteins such as factor H (FH) and factor H-like protein 1 (FHL-1) for immune evasion. Fba is an FH and FHL-1 binding protein expressed on the surface of the human pathogenic bacterium Streptococcus pyogenes, a common agent of pharyngeal, skin, and soft-tissue infections. Fba has been shown to contribute to phagocytosis resistance, intracellular invasion, and virulence in mice. Here, we look at the role of Fba in recruitment of FH and FHL-1 by five serotype M1 isolates of streptococci. Inactivation of fba greatly inhibited binding of FH and FHL-1 by all isolates, indicating that Fba is a major FH and FHL-1 binding factor of serotype M1 streptococci. For three isolates, FH binding was significantly reduced in stationary-phase cultures and correlated with high levels of protease activity and SpeB (an extracellular cysteine protease) protein in culture supernatants. Analysis of a speB mutant confirmed that SpeB accounts for the loss of Fba from the cell surface, suggesting that the protease may modulate FH and FHL-1 recruitment during infection. Comparisons of fba DNA sequences revealed that the FH and FHL-1 binding site in Fba is conserved among the M1 isolates. Although the ligand binding site is not strictly conserved in Fba from a serotype M49 isolate, the M49 Fba protein was found to bind both FH and FHL-1. Collectively, these data indicate that binding of FH and FHL-1 is a conserved function of Fba while modulation of Fba function by SpeB is variable.

Presence of fibronectin-binding protein gene prtF2 in invasive group A streptococci in tropical Australia is associated with increased internalisation efficiency

The fibronectin-binding proteins (FnBPs) PrtF1 and PrtF2 are considered to be major group A streptococcal virulence factors, mediating adherence to and internalisation of host cells. The present study investigated an association between the presence of prtF1 and prtF2 genes and internalisation efficiency in group A streptococci (GAS) isolated from patients with invasive disease. Of the 80 isolates tested, 58 (73%) had prtF1 and 71 (89%) possessed prtF2. Three isolates (4%) had neither gene, seven (9%) had prtF1 only, 19 (24%) had prtF2 only and 51 isolates (64%) had both prtF1 and prtF2. prtF2-positive isolates internalised up to three times more efficiently than isolates that had prtF1 alone (P<0.001), and 1.5-fold better than isolates that had neither gene. No significant association was found between internalisation efficiency and presence of the prtF1 gene. Analysis of the fibronectin-binding repeat domain (FBRD) of prtF2 revealed that this gene can contain 2, 3, 4 or 5 repeat regions and that five repeat regions conferred very high internalisation efficiency in invasive GAS isolates.

Effect of antibiotics on group A Streptococcus exoprotein production analyzed by two-dimensional gel electrophoresis

High-dose clindamycin (CLDM) and benzylpenicillin (PCG) are the recommended chemotherapeutic remedies for toxic shock-like syndrome caused by group A streptococci. One reason for this is that it has been shown that CLDM suppresses the expression of some exoproteins, e.g., SpeB, SpeA, and streptolysin O (Slo). We analyzed the effects of antibiotics on the production of whole exoproteins by two-dimensional gel electrophoresis. Unexpectedly, we found that the levels of several exoproteins, Slo, NAD(+)-glycohydrolase (Nga), M protein, and Sic, were increased by CLDM treatment, although we also confirmed previous findings that the levels of various exoproteins, including SpeB, were decreased. The increases in exoprotein levels were also detected by using other protein synthesis inhibitor antibiotics: erythromycin, kanamycin, tetracycline, chloramphenicol, and linezolid. Peptidoglycan synthesis inhibitors (such as PCG, cefazolin, and imipenem), DNA replication inhibitors (such as gatifloxacin), and an RNA polymerase inhibitor (rifampin) did not have significant effects on exoprotein production. The combination of CLDM and PCG had no advantageous effects with regard to exoprotein production compared to the effect achieved with CLDM alone. We also analyzed the transcriptional levels of slo and nga by reverse transcription-PCR and found that this change was also detected at the transcriptional level. Furthermore, the phenomenon was seen not only in strains of the M1 serotype but also in strains of the other M serotypes. Our study suggests that the clinical effectiveness of CLDM might be due to the inhibition of the production of a limited number of exoproteins.

Streptococcus pyogenes and human neutrophils: a paradigm for evasion of innate host defense by bacterial pathogens

Human polymorphonuclear leukocytes (PMNs) are the first line of defense against invading microorganisms. Although most invading bacteria are eliminated by PMNs, some have evolved complex strategies to prevent normal PMN function. This review focuses on the interaction of human PMNs with Streptococcus pyogenes as a paradigm for successful pathogen evasion mechanisms.

Engagement of the pathogen survival response used by group A Streptococcus to avert destruction by innate host defense

Neutrophils are a critical component of human innate host defense and efficiently kill the vast majority of invading microorganisms. However, bacterial pathogens such as group A Streptococcus (GAS) successfully avert destruction by neutrophils to cause human infections. Relatively little is known about how pathogens detect components of the innate immune system to respond and survive within the host. In this study, we show that inactivation of a two-component gene regulatory system designated Ihk-Irr significantly attenuates streptococcal virulence in mouse models of soft tissue infection and bacteremia. Microarray analysis of wild-type and irr-negative mutant (irr mutant) GAS strains revealed that Ihk-Irr influenced expression of 20% of all transcripts in the pathogen genome. Notably, at least 11 genes involved in cell wall synthesis, turnover, and/or modification were down-regulated in the irr mutant strain. Compared with the wild-type strain, significantly more of the irr mutant strain was killed by human neutrophil components that destroy bacteria by targeting the cell envelope (cell wall and/or membrane). Unexpectedly, expression of ihk and irr was dramatically increased in the wild-type strain exposed to these same neutrophil products under conditions that favored cell envelope damage. We report a GAS mechanism for detection of innate host defense that initiates the pathogen survival response, in which cell wall synthesis is critical. Importantly, our studies identify specific genes in the pathogen survival response as potential targets to control human infections.

Characterization of a complement-binding protein, DRS, from strains of Streptococcus pyogenes containing the emm12 and emm55 genes

An extracellular protein of Streptococcus pyogenes, streptococcal inhibitor of complement (SIC), and its variant, called DRS (distantly related to SIC), are expressed by some S. pyogenes strains. SIC from type 1 (M1) isolates of S. pyogenes interferes with complement-mediated cell lysis, reportedly via its interaction with complement proteins. In this study we demonstrate that S. pyogenes strains carrying emm12 and emm55 (the genes for the M12 and M55 proteins, respectively) express and secrete DRS. This protein, like SIC, binds to the C6 and C7 complement proteins, and competition enzyme-linked immunosorbent assay experiments demonstrate that DRS competes with SIC for C6 and C7 binding. Similarly, SIC competes with DRS for binding to the complement proteins. Despite this, the recombinant DRS preparation showed no significant effect on complement function, as determined by lysis of sensitized sheep erythrocytes. Furthermore, the presence of DRS is not inhibitory to SIC activity.

The low-density lipoprotein receptor is regulated by estrogen and forms a functional complex with the estrogen-regulated protein ezrin in pituitary GH3 somatolactotropes

Estrogen regulates the function, growth, and proliferation of lactotropes in the pituitary. We report here that low-density lipoprotein (LDL) receptor (LDLR) gene expression and LDL uptake are strongly up-regulated by estrogen in pituitary somatolactotropic GH(3) cells. The uptake of LDL was significantly inhibited by the F-actin-severing drug, swinholide A, indicating that LDL uptake is dependent on the integrity of the cortical actin cytoskeleton in GH(3) cells. We examined whether the estrogen-inducible cytoskeletal linker protein, ezrin, interacts with the LDLR. The LDLR coimmunoprecipitated with ezrin, and fluorescently labeled LDL bound to regions of the cell membrane that colocalized with the active, phosphorylated form of ezrin (phosphoezrin). Evidence for a functional interaction between ezrin and the LDLR was obtained by transient transfection experiments using ezrin-green fluorescent protein (GFP) expression constructs. We observed that transient transfection of GH(3) cells with an ezrin N terminus-GFP dominant-negative construct prevented the uptake of LDL particles, whereas expression of GFP alone or an ezrin C terminus-GFP construct had no effect on LDL uptake. Transfection with the ezrin N terminus dominant- negative construct had no effect on the endocytosis of transferrin. Thus, estrogen stimulates the expression of ezrin and the LDLR in GH(3) cells, which interact physically and functionally to facilitate the endocytosis of LDL. We propose that the up-regulation and interaction of ezrin and the LDLR serves to augment the delivery of cholesterol and other lipids in support of the hypertrophic and proliferative response of cells to estrogen.

Polysaccharide intercellular adhesin (PIA) protects Staphylococcus epidermidis against major components of the human innate immune system

The skin commensal and opportunistic pathogen Staphylococcus epidermidis is the leading cause of nosocomial and biofilm-associated infections. Little is known about the mechanisms by which S. epidermidis protects itself against the innate human immune system during colonization and infection. We used scanning electron microscopy to demonstrate that the exopolysaccharide intercellular adhesin (PIA) resides in fibrous strands on the bacterial cell surface, and that lack of PIA production results in complete loss of the extracellular matrix material that has been suggested to mediate immune evasion. Phagocytosis and killing by human polymorphonuclear leucocytes was significantly increased in a mutant strain lacking PIA production compared with the wild-type strain. The mutant strain was also significantly more susceptible to killing by major antibacterial peptides of human skin, cationic human beta-defensin 3 and LL-37, and anionic dermcidin. PIA represents the first defined factor of the staphylococcal biofilm matrix that protects against major components of human innate host defence.

The interaction of streptococcal inhibitor of complement (SIC) and its proteolytic fragments with the human beta defensins

Streptococcal inhibitor of complement (SIC) is a 31 kDa extracellular protein produced by a few highly virulent strains of Streptococcus pyogenes (in particular the M1 strain). It has been shown additionally to inhibit four further components of the mucosal innate response-lysozyme, secretory leucocyte proteinase inhibitor, human alpha-defensin 1 and the cathelicidin LL-37 which are all bactericidal against Group A Streptococci (GAS). We now show that SIC also inhibits variably the antibacterial action of hBD-1, -2 and -3. By enzyme-linked immunosorbent assay (ELISA), SIC binds strongly to hBD-2 and hBD-3, but not at all to hBD-1. Investigation of the antimicrobial action of beta-defensins hBD-1, -2 and -3 against GAS in two different buffer systems shows that both the killing efficiencies of all three defensins, and the binding of SIC to them, occurs more efficiently in 10 mm Tris buffer than in 10 mm phosphate. The lower ionic strength of the Tris buffer may underlie this effect. hBD-1 kills the M1 strain of GAS only in 10 mm Tris, but is able to kill an M6 (SIC negative) strain in 10 mm phosphate. The inhibition of hBD-3 by SIC is clearly of physiological relevance, that of hBD-2 is likely to be so, but the inhibition of hBD-1 occurs only at lower ionic strength than is likely to be encountered in vivo. Elastase digestion of SIC yields three major fragments of MW 3.843 kDa comprising residues 1-33 (fragment A); 10.369 kDa comprising residues 34-126 (fragment B); and MW 16.487 kDa, comprising residues 127-273 (fragment C). By ELISA, only fragment B binds to hBD-2 and hBD-3 and this may indicate the inhibitory portion of the SIC molecule.

Stable isotope labeling of a Group A Streptococcus virulence factor using a chemically defined growth medium

A secreted, hypervariable virulence factor called the streptococcal inhibitor of complement (Sic) has been linked to the reemergence of epidemics due to the human pathogenic bacterium Group A Streptococcus. This paper describes a method for expressing and purifying Sic from an attenuated GAS strain using a chemically defined growth medium. This method was used to label specific amino acid residue types in Sic with forms containing the magnetically active isotope (15)N, at the amide nitrogen. The (15)N-labeling of Sic permits a detailed investigation of the structure and dynamics of the protein using nuclear magnetic resonance spectroscopy. The level of stable isotope incorporation was established using mass spectrometry and nuclear magnetic resonance spectroscopy.

The Mig protein ofStreptococcus dysgalactiaeinhibits bacterial internalization into bovine mammary gland epithelial cells

The role of the Mig protein of Streptococcus dysgalactiae in bacterial adhesion and internalization of bovine mammary gland epithelial cells (MAC-T) was investigated with the wild-type and isogenic mig mutant strains. While there was no difference in adhesion between the strains, the wild-type strain exhibited a significantly lower level of invasion than the mutants. The lower level of internalization of the Mig(+) strain is likely due to Mig-mediated interference with uptake of the microorganisms rather than the host protein binding properties of Mig. Avoidance of intimate interactions with the host cells might be an alternative strategy for S. dysgalactiae to survive and persist in the bovine mammary glands.

Genomic location and variation of the gene for CRS, a complement binding protein in the M57 strains of Streptococcus pyogenes

All isolates of serotype M1 of group A streptococci possess a gene for streptococcal inhibitor of complement (SIC) in the mga regulon, which harbors genes for other virulence factors, such as M and M-like proteins, C5a peptidase, and a regulator. In serotype M57 the gene for a protein that is closely related to SIC (crs57) is located outside the mga regulon. We mapped the location of the crs57 gene in six strains of emm57 (gene encoding the M57 protein) sequence types to an intergenic region between the ABC transporter gene (SPy0778) and the gene for a small ribosomal protein (rpsU). The noncoding sequences on both sides of crs57 exhibited high degrees of identity to the corresponding regions of sic from M1 strains. This included one of the inverted repeat sequences of IS1562 but not the insertion element itself. These observations suggest that crs57 was recently acquired by serotype M57 or its progenitor via horizontal acquisition from serotype M1. The six emm57 sequence type isolates analyzed in this study belong to two distinct molecular types (vir types VT8 and VT101). Although the crs57 sequences from VT8 strains had very few substitution mutations, the VT101 crs57 sequence had a large number of such mutations. The CRS57 proteins from these strains are secretory products and have the ability to bind to complement proteins. All these proteins contain several tryptophan-rich repeats designated DWS motifs and internal repeat sequences. In all of these structural and biochemical characteristics CRS57 resembles SIC from M1 strains. Hence, CRS57 has a functional role similar to that of SIC in an M1 strain.

Characterization of an extracellular virulence factor made by group A Streptococcus with homology to the Listeria monocytogenes internalin family of proteins

Leucine-rich repeats (LRR) characterize a diverse array of proteins and function to provide a versatile framework for protein-protein interactions. Importantly, each of the bacterial LRR proteins that have been well described, including those of Listeria monocytogenes, Yersinia pestis, and Shigella flexneri, have been implicated in virulence. Here we describe an 87.4-kDa group A Streptococcus (GAS) protein (designated Slr, for streptococcal leucine-rich) containing 10 1/2 sequential units of a 22-amino-acid C-terminal LRR homologous to the LRR of the L. monocytogenes internalin family of proteins. In addition to the LRR domain, slr encodes a gram-positive signal secretion sequence characteristic of a lipoprotein and a putative N-terminal domain with a repeated histidine triad motif (HxxHxH). Real-time reverse transcriptase PCR assays indicated that slr is transcribed abundantly in vitro in the exponential phase of growth. Flow cytometry confirmed that Slr was attached to the GAS cell surface. Western immunoblot analysis of sera obtained from 80 patients with invasive infections, noninvasive soft tissue infections, pharyngitis, and rheumatic fever indicated that Slr is produced in vivo. An isogenic mutant strain lacking slr was significantly less virulent in an intraperitoneal mouse model of GAS infection and was significantly more susceptible to phagocytosis by human polymorphonuclear leukocytes. These studies characterize the first GAS LRR protein as an extracellular virulence factor that contributes to pathogenesis and may participate in evasion of the innate host defense.

Regulation of the neutrophil-mediated inflammatory response to infection

Human polymorphonuclear leukocytes (PMNs) are the first line of defense against invading microorganisms and contribute significantly to inflammation. Recent evidence suggests that resolution of neutrophil-mediated inflammation is facilitated by an apoptosis differentiation program, a final stage of transcriptionally regulated PMN maturation that is accelerated significantly by phagocytosis.

drs (Distantly related sic) gene polymorphisms among emm12-type Streptococcus pyogenes isolates

Twenty-eight emm12-type Streptococcus pyogenes isolates from patients with invasive and noninvasive infections or from asymptomatic carriers were genetically typed. Sequencing of drs (distantly related sic [streptococcal inhibitor of complement]) genes identified two novel alleles and revealed a polymorphism for drs similar to that of sic. No association was observed between the five different drs alleles and the five restriction patterns of the vir regulon for the isolates studied. These data suggest that drs sequencing may be useful for further differentiation of S. pyogenes isolates with emm12 and identical vir regulon restriction patterns.