Streptococcus pyogenes collagen type I-binding Cpa surface protein. Expression profile, binding characteristics, biological functions, and potential clinical impact

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.

Identification of an immunodominant region on a group A Streptococcus T-antigen reveals temperature-dependent motion in pili

Group A Streptococcus (GAS) is a globally important pathogen causing a broad range of human diseases. GAS pili are elongated proteins with a backbone comprised repeating T-antigen subunits, which extend from the cell surface and have important roles in adhesion and establishing infection. No GAS vaccines are currently available, but T-antigen-based candidates are in pre-clinical development. This study investigated antibody-T-antigen interactions to gain molecular insight into functional antibody responses to GAS pili. Large, chimeric mouse/human Fab-phage libraries generated from mice vaccinated with the complete T18.1 pilus were screened against recombinant T18.1, a representative two-domain T-antigen. Of the two Fab identified for further characterization, one (designated E3) was cross-reactive and also recognized T3.2 and T13, while the other (H3) was type-specific reacting with only T18.1/T18.2 within a T-antigen panel representative of the major GAS T-types. The epitopes for the two Fab, determined by x-ray crystallography and peptide tiling, overlapped and mapped to the N-terminal region of the T18.1 N-domain. This region is predicted to be buried in the polymerized pilus by the C-domain of the next T-antigen subunit. However, flow cytometry and opsonophagocytic assays showed that these epitopes were accessible in the polymerized pilus at 37°C, though not at lower temperature. This suggests that there is motion within the pilus at physiological temperature, with structural analysis of a covalently linked T18.1 dimer indicating "knee-joint" like bending occurs between T-antigen subunits to expose this immunodominant region. This temperature dependent, mechanistic flexing provides new insight into how antibodies interact with T-antigens during infection.

Expression of the Group A Streptococcus Fibrinogen-Binding Protein Mrp Is Negatively Regulated by the Small Regulatory RNA FasX

Small regulatory RNAs (sRNAs) represent a major class of regulatory molecule that promotes the ability of the group A Streptococcus (GAS) and other pathogens to regulate virulence factor expression. Despite FasX being the best-described sRNA in GAS, there remains much to be learned.

Streptococcus pneumoniae binds collagens and C1q via the SSURE repeats of the PfbB adhesin

The binding of Streptococcus pneumoniae to collagen is likely an important step in the pathogenesis of pneumococcal infections, but little is known of the underlying molecular mechanisms. Streptococcal surface repeats (SSURE) are highly conserved protein domains present in cell wall adhesins from different Streptococcus species. We find here that SSURE repeats of the pneumococcal adhesin plasminogen and fibronectin binding protein B (PfbB) bind to various types of collagen. Moreover, deletion of the pfbB gene resulted in a significant impairment of the ability of encapsulated or unencapsulated pneumococci to bind collagen. Notably, a PfbB SSURE domain is also bound to the complement component C1q that bears a collagen‐like domain and promotes adherence of pneumococci to host cells by acting as a bridge between bacteria and epithelial cells. Accordingly, deletion of PfbB or pre‐treatment with anti‐SSURE antibodies markedly decreased pneumococcal binding to C1q as well as C1q‐dependent adherence to epithelial and endothelial cells. Further data indicated that C1q promotes pneumococcal adherence by binding to integrin α₂β₁. In conclusion, our results indicate that the SSURE domains of the PfbB protein promote interactions of pneumococci with various types of collagen and with C1q. These repeats may be useful targets in strategies to control S. pneumoniae infections.

Multicomponent Vaccines against Group A Streptococcus Can Effectively Target Broad Disease Presentations

Group A Streptococcus (GAS) is an important global human pathogen, with a wide range of disease presentations, from mild mucosal infections like pharyngitis to invasive diseases such as toxic shock syndrome. The effect on health and mortality from GAS infections is substantial worldwide, particularly from autoimmune sequelae-like rheumatic heart disease (RHD), and there is currently no licenced vaccine. We investigated protein antigens targeting a broad range of GAS disease presentations as vaccine components in individual and combination formulations. The potency and functional immunity generated were evaluated and compared between groups. Antibodies against all components were found in pooled human IgG (IVIG) and an immune response generated following the subcutaneous immunisation of mice. A combination immunisation showed a reduction in IgG response for SpyCEP but an increase for Cpa and Mac-1 (IdeS). An opsonophagocytosis assay (OPA) showed the killing of GAS with immune sera against M protein and combination groups, with a lower killing activity observed for immune sera against other individual antigens. Specific antigen assays showed functional immunity against SpyCEP and Mac-1 from both individual and combination immunisations, with the activity correlating with antibody titres. However, efficient blocking of the binding activity of Cpa to collagen I and fibronectin could not be demonstrated with immune sera or purified IgG. Our data indicate that combination immunisations, while effective at covering a broader range of virulence factors, can also affect the immune response generated. Further, our results showed that an OPA alone is inadequate for understanding protection from vaccination, particularly when considering protection from immune evasion factors and evaluation of the colonisation leading to pharyngitis.

Streptococcus pyogenes ("Group A Streptococcus"), a Highly Adapted Human Pathogen-Potential Implications of Its Virulence Regulation for Epidemiology and Disease Management

Streptococcus pyogenes (group A streptococci; GAS) is an exclusively human pathogen. It causes a variety of suppurative and non-suppurative diseases in people of all ages worldwide. Not all can be successfully treated with antibiotics. A licensed vaccine, in spite of its global importance, is not yet available. GAS express an arsenal of virulence factors responsible for pathological immune reactions. The transcription of all these virulence factors is under the control of three types of virulence-related regulators: (i) two-component systems (TCS), (ii) stand-alone regulators, and (iii) non-coding RNAs. This review summarizes major TCS and stand-alone transcriptional regulatory systems, which are directly associated with virulence control. It is suggested that this treasure of knowledge on the genetics of virulence regulation should be better harnessed for new therapies and prevention methods for GAS infections, thereby changing its global epidemiology for the better.

Intranasal immunization with Ag85B peptide 25 displayed on Lactococcus lactis using the PilVax platform induces antigen-specific B- and T-cell responses

Mycobacterium tuberculosis (Mtb) remains a global epidemic despite the widespread use of Bacillus Calmette-Guérin (BCG). Consequently, novel vaccines are required to facilitate a reduction in Mtb morbidity and mortality. PilVax is a peptide delivery strategy for the generation of highly specific mucosal immune responses and is based on the food-grade bacterium Lactococcus lactis that is used to express selected peptides engineered within the Streptococcus pyogenes M1T1 pilus, allowing for peptide amplification, stabilization and enhanced immunogenicity. In the present study, the dominant T-cell epitope from the Mtb protein Ag85B was genetically engineered into the pilus backbone subunit and expressed on the surface of L. lactis. Western blot and flow cytometry confirmed formation of pilus containing the peptide DNA sequence. B-cell responses in intranasally vaccinated mice were analyzed by ELISA while T-cell responses were analyzed by flow cytometry. Serum titers of peptide-specific immunoglobulin (Ig) G and IgA were detected, confirming that vaccination produced antibodies against the cognate peptide. Peptide-specific IgA was also detected across several mucosal sites sampled. Peptide-specific CD4⁺ T cells were detected at levels similar to those of mice immunized with BCG. PilVax immunization resulted in an unexpected increase in the numbers of CD3⁺ CD4^- CD8^- [double negative (DN)] T cells in the lungs of vaccinated mice. Analysis of cytokine production following stimulation with the cognate peptide showed the major cytokine producing cells to be CD4⁺ T cells and DN T cells. This study provides insight into the antibody and peptide-specific cellular immune responses generated by PilVax vaccination and demonstrates the suitability of this vaccine for conducting a protection study.

Genetics, Structure, and Function of Group A Streptococcal Pili

Streptococcus pyogenes (Group A Streptococcus; GAS) is an exclusively human pathogen. This bacterial species is responsible for a large variety of infections, ranging from purulent but mostly self-limiting oropharynx/skin diseases to streptococcal sequelae, including glomerulonephritis and rheumatic fever, as well as life-threatening streptococcal toxic-shock syndrome. GAS displays a wide array of surface proteins, with antigenicity of the M protein and pili utilized for M- and T-serotyping, respectively. Since the discovery of GAS pili in 2005, their genetic features, including regulation of expression, and structural features, including assembly mechanisms and protein conformation, as well as their functional role in GAS pathogenesis have been intensively examined. Moreover, their potential as vaccine antigens has been studied in detail. Pilus biogenesis-related genes are located in a discrete section of the GAS genome encoding fibronectin and collagen binding proteins and trypsin-resistant antigens (FCT region). Based on the heterogeneity of genetic composition and DNA sequences, this region is currently classified into nine distinguishable forms. Pili and fibronectin-binding proteins encoded in the FCT region are known to be correlated with infection sites, such as the skin and throat, possibly contributing to tissue tropism. As also found for pili of other Gram-positive bacterial pathogens, GAS pilin proteins polymerize via isopeptide bonds, while intramolecular isopeptide bonds present in the pilin provide increased resistance to degradation by proteases. As supported by findings showing that the main subunit is primarily responsible for T-serotyping antigenicity, pilus functions and gene expression modes are divergent. GAS pili serve as adhesins for tonsillar tissues and keratinocyte cell lines. Of note, a minor subunit is considered to have a harpoon function by which covalent thioester bonds with host ligands are formed. Additionally, GAS pili participate in biofilm formation and evasion of the immune system in a serotype/strain-specific manner. These multiple functions highlight crucial roles of pili during the onset of GAS infection. This review summarizes the current state of the art regarding GAS pili, including a new mode of host-GAS interaction mediated by pili, along with insights into pilus expression in terms of tissue tropism.

Protein folding modulates the chemical reactivity of a Gram-positive adhesin

Gram-positive bacteria colonize mucosal tissues, withstanding large mechanical perturbations such as coughing, which generate shear forces that exceed the ability of non-covalent bonds to remain attached. To overcome these challenges, the pathogen Streptococcus pyogenes utilizes the protein Cpa, a pilus tip-end adhesin equipped with a Cys-Gln thioester bond. The reactivity of this bond towards host surface ligands enables covalent anchoring; however, colonization also requires cell migration and spreading over surfaces. The molecular mechanisms underlying these seemingly incompatible requirements remain unknown. Here we demonstrate a magnetic tweezers force spectroscopy assay that resolves the dynamics of the Cpa thioester bond under force. When folded at forces <6 pN, the Cpa thioester bond reacts reversibly with amine ligands, which are common in inflammation sites; however, mechanical unfolding and exposure to forces >6 pN block thioester reformation. We hypothesize that this folding-coupled reactivity switch (termed a smart covalent bond) could allow the adhesin to undergo binding and unbinding to surface ligands under low force and remain covalently attached under mechanical stress.

Microscale communication between bacterial pathogens and the host epithelium

Pathogenic bacteria have evolved a variety of highly selective adhesins allowing these microbes to engage specific surface determinants of their eukaryotic host cells. Receptor clustering induced by the multivalent microorganisms will not only anchor the bacteria to the tissue, but will inevitably trigger host cell signaling. It has become clear, that these bacteria-initiated signaling events can be seen as a form of localized communication with host epithelial cells. Such a microscale communication can have immediate consequences in the form of changes in host cell membrane morphology or cytoskeletal organization, but can also lead to transcriptional responses and medium- and long-term alterations in cellular physiology. In this review, we will discuss several examples of this form of microscale communication between bacterial pathogens and mammalian host cells and try to delineate their downstream ramifications in the infection process. Furthermore, we will highlight recent findings that specialized pathogenic bacteria utilize the adhesin-based interaction to diffuse the short-range messenger molecule nitric oxide into the host tissue. While anti-adhesive strategies to disrupt the initial bacterial attachment have not yet translated into medical applications, the ability to interfere with the microscale communication emanating on the host side provides an unconventional approach for preventing infectious diseases.

Deciphering Streptococcal Biofilms

Streptococci are a diverse group of bacteria, which are mostly commensals but also cause a considerable proportion of life-threatening infections. They colonize many different host niches such as the oral cavity, the respiratory, gastrointestinal, and urogenital tract. While these host compartments impose different environmental conditions, many streptococci form biofilms on mucosal membranes facilitating their prolonged survival. In response to environmental conditions or stimuli, bacteria experience profound physiologic and metabolic changes during biofilm formation. While investigating bacterial cells under planktonic and biofilm conditions, various genes have been identified that are important for the initial step of biofilm formation. Expression patterns of these genes during the transition from planktonic to biofilm growth suggest a highly regulated and complex process. Biofilms as a bacterial survival strategy allow evasion of host immunity and protection against antibiotic therapy. However, the exact mechanisms by which biofilm-associated bacteria cause disease are poorly understood. Therefore, advanced molecular techniques are employed to identify gene(s) or protein(s) as targets for the development of antibiofilm therapeutic approaches. We review our current understanding of biofilm formation in different streptococci and how biofilm production may alter virulence-associated characteristics of these species. In addition, we have summarized the role of surface proteins especially pili proteins in biofilm formation. This review will provide an overview of strategies which may be exploited for developing novel approaches against biofilm-related streptococcal infections.

Functional Analysis of the Collagen Binding Proteins of Streptococcus parasanguinis FW213

Streptococcus parasanguinis is a dominant isolate of dental plaque and an opportunistic pathogen associated with subacute endocarditis. As the expression of collagen binding proteins (CBPs) could promote the establishment of S. parasanguinis in the host, the functions of three putative CBP-encoding loci, Spaf_0420, Spaf_1570, and Spaf_1573, were analyzed using isogenic mutant strains. It was revealed that S. parasanguinis FW213 bound effectively to fibronectin and type I collagen, but the strain's affinity for laminin and type IV collagen was quite low. By using various deletion derivatives, it was found that these three loci mediated the binding of S. parasanguinis to multiple extracellular matrix molecules, with type I collagen as the common substrate. Derivative strains with a deletion in any of the three loci expressed reduced binding to trypsin-treated swine heart valves. The deletion of these loci also reduced the viable count of S. parasanguinis bacteria within macrophages, especially the loss of Spaf_0420, but only strains with deletions in Spaf_0420 and Spaf_1570 expressed reduced virulence in the Galleria mellonella larva model. The deletion of Spaf_1570 and Spaf_1573 affected mainly the structure, but not the overall mass, of biofilm cultures in a flow cell system. Thus, CBPs are likely to be more critical for the initial colonization of S. parasanguinis on host tissues during the development of endocarditis.IMPORTANCE Bacteria generally can utilize multiple adhesins to establish themselves in the host. We found that Streptococcus parasanguinis, a dominant oral commensal and an opportunistic pathogen for subacute endocarditis, possesses at least three collagen-binding proteins that enable S. parasanguinis to successfully colonize damaged heart tissues and escape innate immune clearance. The binding specificities of these three proteins for extracellular matrix molecules differ, although all three proteins participate in biofilm formation by S. parasanguinis The "multiligand for multisubstrate" feature of these adhesins may explain the high adaptability of this microbe to different tissue sites.

T4 Pili Promote Colonization and Immune Evasion Phenotypes of Nonencapsulated M4 Streptococcus pyogenes

Streptococcus pyogenes (group A Streptococcus [GAS]) is an important human pathogen causing a broad spectrum of diseases and associated with significant global morbidity and mortality. Almost all GAS isolates express a surface hyaluronic acid capsule, a virulence determinant that facilitates host colonization and impedes phagocyte killing. However, recent epidemiologic surveillance has reported a sustained increase in both mucosal and invasive infections caused by nonencapsulated GAS, which questions the indispensable role of hyaluronic acid capsule in GAS pathogenesis. In this study, we found that pilus of M4 GAS not only significantly promotes biofilm formation, adherence, and cytotoxicity to human upper respiratory tract epithelial cells and keratinocytes, but also promotes survival in human whole blood and increased virulence in murine models of invasive infection. T4 antigen, the pilus backbone protein of M4 GAS, binds haptoglobin, an abundant human acute-phase protein upregulated upon infection and inflammation, on the bacterial surface. Haptoglobin sequestration reduces the susceptibility of nonencapsulated M4 GAS to antimicrobial peptides released from activated neutrophils and platelets. Our results reveal a previously unappreciated virulence-promoting role of M4 GAS pili, in part mediated by co-opting the biology of haptoglobin to mitigate host antimicrobial defenses.IMPORTANCE Group A Streptococcus (GAS) is a strict human pathogen causing more than 700 million infections globally each year. The majority of the disease-causing GAS are encapsulated, which greatly guarantees survival and dissemination in the host. Emergence of the capsule-negative GAS, such as M4 GAS, in recent epidemiologic surveillance alarms the necessity to elucidate the virulence determinants of these pathogens. Here, we found that M4 pili play an important role in promoting M4 GAS adherence and cytotoxicity to human pharyngeal epithelial cells and keratinocytes. The same molecule also significantly enhanced M4 GAS survival and replication in human whole blood and experimental murine infection. T4 antigen, which composes the backbone of M4 pili, was able to sequester the very abundant serum protein haptoglobin to further confer M4 GAS resistance to antibacterial substances released by neutrophils and platelets.

Role for Cell-Surface Collagen of Streptococcus pyogenes in Infections

Group A Streptococcus (GAS) displays cell-surface proteins that resemble human collagen. We find that a fluorophore-labeled collagen mimetic peptide (CMP) labels GAS cells but not Escherichia coli or Bacillus subtilis cells, which lack such proteins. The CMP likely engages in a heterotrimeric helix with endogenous collagen, as the nonnatural d enantiomer of the CMP does not label GAS cells. To identify a molecular target, we used reverse genetics to "knock-in" the GAS genes that encode two proteins with collagen-like domains, Scl1 and Scl2, into B. subtilis. The fluorescent CMP labels the cells of these B. subtilis strains. Moreover, these strains bind tightly to a surface of mammalian collagen. These data are consistent with streptococcal collagen forming triple helices with damaged collagen in a wound bed and thus have implications for microbial virulence.

Extracellular Matrix Interactions with Gram-Positive Pathogens

The main strategies used by pathogenic bacteria to infect eukaryotic tissue include their adherence to cells and the extracellular matrix (ECM), the subsequent colonization and invasion as well as the evasion of immune defences. A variety of structurally and functionally characterized adhesins and binding proteins of gram-positive bacteria facilitate these processes by specifically recognizing and interacting with various components of the host ECM, including different collagens, fibronectin and other macromolecules. The ECM affects the cellular physiology of our body and is critical for adhesion, migration, proliferation, and differentiation of many host cell types, but also provides the support for infiltrating pathogens, particularly under conditions of injury and trauma. Moreover, microbial binding to a variety of adhesive components in host tissue fluids leads to structural and/or functional alterations of host proteins and to the activation of cellular mechanisms that influence tissue and cell invasion of pathogens. Since the diverse interactions of gram-positive bacteria with the ECM represent important pathogenicity mechanisms, their characterization not only allows a better understanding of microbial invasion but also provides clues for the design of novel therapeutic strategies to manage infectious diseases.

Exploring the arthritogenicity of Streptococcus dysgalactiae subspecies equisimilis

BACKGROUND

During the past decades, Streptococcus dysgalactiae subspecies equisimilis (SDSE) has been increasingly recognized as an important human pathogen. Osteoarticular infections is one of the predominant disease manifestations of SDSE, but the pathogenetic rationale for its arthritogenicity has yet to be unravelled. We aimed to explore if the rising incidence of osteoarticular infections caused by this pathogen in our region emanated from clonal expansion of strains with enhanced tropism for bone and joint tissue components or orthopaedic implants.

RESULTS

Twenty-nine SDSE-isolates associated with osteoarticular infections were retrospectively identified. Their genomic content and affinity for fibronectin, collagen and stainless steel were compared to 24 temporally and geographically matched SDSE blood culture isolates obtained from patients without bone or joint infections. Despite a thorough genetic and phenotypic dissection, neither the presence or absence of any single gene, nor the binding abilities of the SDSE isolates, were predictive of clinical entity. SNP analysis revealed a heterogenous population, and a correlation between phylogenetic relationships and disease manifestation was not evident. However, we identified a strong concordance between phenotypic binding abilities and genetic variations in the pilus-region, also denoted as the FCT-region (Fibronectin binding, Collagen binding and T-antigen). This observation could be related to the ample and varied repertoire of putative adhesins residing within this region, including proteins predicted to adhere to fibronectin and collagen, as well as fibrinogen.

CONCLUSIONS

SDSE strains associated with osteoarticular infections do not emanate from subpopulation characterized by distinct genetic or phenotypic traits. The genetic architecture of the pilus region was predictive of the adhesive properties of the SDSE-isolates, but its role in tissue tropism needs further investigation. To the best of our knowledge, this is the first comprehensive characterization of the genetic landscape of the SDSE pilus region.

PilVax - a novel peptide delivery platform for the development of mucosal vaccines

Peptide vaccines are an attractive strategy to engineer the induction of highly targeted immune responses and avoid potentially allergenic and/or reactogenic protein regions. However, peptides by themselves are often unstable and poorly immunogenic, necessitating the need for an adjuvant and a specialised delivery system. We have developed a novel peptide delivery platform (PilVax) that allows the presentation of a stabilised and highly amplified peptide as part of the group A streptococcus serotype M1 pilus structure (PilM1) on the surface of the non-pathogenic bacterium Lactococcus lactis. To show proof of concept, we have successfully inserted the model peptide Ova_324–339 into 3 different loop regions of the backbone protein Spy0128, which resulted in the assembly of the pilus containing large numbers of peptide on the surface of L. lactis. Intranasal immunisation of mice with L. lactis PilM1-Ova generated measurable Ova-specific systemic and mucosal responses (IgA and IgG). Furthermore, we show that multiple peptides can be inserted into the PilVax platform and that peptides can also be incorporated into structurally similar, but antigenically different pilus structures. PilVax may be useful as a cost-effective platform for the development of peptide vaccines against a variety of important human pathogens.

Identification and Characterization of Serotype-Specific Variation in Group A Streptococcus Pilus Expression

Isolates of a given bacterial pathogen often display phenotypic variation, and this can negatively impact public health, for example, by reducing the efficacy of preventative measures. Here, we identify that the human pathogen group A Streptococcus (GAS; Streptococcus pyogenes) expresses pili on its cell surface in a serotype-specific manner. Specifically, we show that serotype M3 GAS isolates, which are nonrandomly associated with causing particularly severe and lethal invasive infections, produce negligible amounts of pili relative to serotype M1 and M49 isolates. Performance of an interserotype transcriptome comparison (serotype M1 versus serotype M3) was instrumental in this discovery. We also identified that the transcriptional regulator Nra positively regulates pilus expression in M3 GAS isolates and that the low level of pilus expression of these isolates correlates with a low level of nra transcription. Finally, we discovered that the phenotypic consequences of low levels of pilus expression by M3 GAS isolates are a reduced ability to adhere to host cells and an increased ability to survive and proliferate in human blood. We propose that an enhanced ability to survive in human blood, in part due to reduced pilus expression, is a contributing factor in the association of serotype M3 isolates with highly invasive infections. In conclusion, our data show that GAS isolates express pili in a serotype-dependent manner and may inform vaccine development, given that pilus proteins are being discussed as possible GAS vaccine antigens.

Assessment of Cpa, Scl1 and Scl2 in clinical group A streptococcus isolates and patients from north India: an evaluation of the host pathogen interaction

Group A streptococcus (GAS) infection remains a major concern due to multiple diseases including pharyngitis, impetigo, acute rheumatic fever (ARF) and rheumatic heart disease (RHD). It uses different adhesins and virulence factors like Cpa (collagen binding protein) and Scl (collagen-like protein) in its pathogenicity. Scl having similarities with human collagen may contribute to inducing autoimmunity in the host. Here we assessed gene expression, antibody titer of Cpa, Scl1 and Scl2 in both clinical GAS isolates (n = 45) and blood (n = 45) obtained from pharyngitis, ARF (acute rheumatic fever) and RHD respectively. Skin isolates (n = 30) were obtained from impetigo patients. The study revealed a total of 27 GAS emm types. Frequency of cpa, scl1, scl2 was high in ARF isolates. The antibody titer of these proteins was high in all isolates, and also in patients with pharyngitis and ARF. All isolates showed high binding affinity toward collagen I and IV, which further indicates a potential host pathogen interaction. Our study reflects a strong association of Cpa and Scls in early and post-GAS pathogenicity. However, the increased antibody titer of Scl1 and Scl2 during ARF may be attributed to a cogent immune response in the host.

The Group A Streptococcus serotype M2 pilus plays a role in host cell adhesion and immune evasion

Group A Streptococcus (GAS), or Streptococcus pyogenes, is a human pathogen that causes diseases ranging from skin and soft tissue infections to severe invasive diseases, such as toxic shock syndrome. Each GAS strain carries a particular pilus type encoded in the variable fibronectin-binding, collagen-binding, T antigen (FCT) genomic region. Here, we describe the functional analysis of the serotype M2 pilus encoded in the FCT-6 region. We found that, in contrast to other investigated GAS pili, the ancillary pilin 1 lacks adhesive properties. Instead, the backbone pilin is important for host cell adhesion and binds several host factors, including fibronectin and fibrinogen. Using a panel of recombinant pilus proteins, GAS gene deletion mutants and Lactococcus lactis gain-of-function mutants we show that, unlike other GAS pili, the FCT-6 pilus also contributes to immune evasion. This was demonstrated by a delay in blood clotting, increased intracellular survival of the bacteria in macrophages, higher bacterial survival rates in human whole blood and greater virulence in a Galleria mellonella infection model in the presence of fully assembled FCT-6 pili.

Temporal trends of β-haemolytic streptococcal osteoarticular infections in western Norway

BACKGROUND

Beta-haemolytic streptococci are important contributors to the global burden of osteoarticular infections (OAI). Knowledge on the disease traits specific for streptococcal OAI, however, remains scarce. We wished to explore temporal trends of OAI caused by Group A Streptococci (GAS), Group B Streptococci (GBS) and Group C and G Streptococci (GCGS), and furthermore, to describe the associated host and pathogen characteristics.

METHODS

All cases of microbiologically verified β-haemolytic streptococcal OAI in Health Region Bergen, Norway, in the period 1999-2013 were retrospectively identified. Clinical data were extracted from medical records. Microbial isolates were submitted to antibiotic susceptibility testing and molecular typing.

RESULTS

A total of 24 GAS, 45 GBS and 42 GCGS acute OAI were identified. The cumulative incidence of GCGS OAI, but not GAS or GBS OAI, increased significantly from the first to the last 5-year period (IRR 5.7, p = 0.0003), with the annual incidence peaking at 1.9/100 000 in 2013. GAS OAI generally produced the most acute and severe clinical presentation, whereas GBS and GCGS predominantly affected the elderly, and were significantly associated with the presence of host risk factors of systemic and focal origin, respectively.

CONCLUSIONS

We found a significantly increasing incidence of GCGS OAI, likely related to the presence of host susceptibility factors, including prosthetic material and pre-existing joint disease. With an increasing application of therapeutic and diagnostic bone and joint procedures, the rising trend of OAI caused by GCGS is likely to continue. Sustained epidemiological attentiveness to GCGS seems warranted.

Streptococcus pyogenes adhesion and colonization

Streptococcus pyogenes (group A Streptococcus, GAS) is a human-adapted pathogen responsible for a wide spectrum of disease. GAS can cause relatively mild illnesses, such as strep throat or impetigo, and less frequent but severe life-threatening diseases such as necrotizing fasciitis and streptococcal toxic shock syndrome. GAS is an important public health problem causing significant morbidity and mortality worldwide. The main route of GAS transmission between humans is through close or direct physical contact, and particularly via respiratory droplets. The upper respiratory tract and skin are major reservoirs for GAS infections. The ability of GAS to establish an infection in the new host at these anatomical sites primarily results from two distinct physiological processes, namely bacterial adhesion and colonization. These fundamental aspects of pathogenesis rely upon a variety of GAS virulence factors, which are usually under strict transcriptional regulation. Considerable progress has been made in better understanding these initial infection steps. This review summarizes our current knowledge of the molecular mechanisms of GAS adhesion and colonization.

Collagen-binding proteins of Streptococcus mutans and related streptococci

The ability of Streptococcus mutans to interact with collagen through the expression of collagen-binding proteins (CBPs) bestows this oral pathogen with an alternative to the sucrose-dependent mechanism of colonization classically attributed to caries development. Based on the abundance and distribution of collagen throughout the human body, stringent adherence to this molecule grants S. mutans with the opportunity to establish infection at different host sites. Surface proteins, such as SpaP, WapA, Cnm and Cbm, have been shown to bind collagen in vitro, and it has been suggested that these molecules play a role in colonization of oral and extra-oral tissues. However, robust collagen binding is not achieved by all strains of S. mutans, particularly those that lack Cnm or Cbm. These observations merit careful dissection of the contribution from these different CBPs towards tissue colonization and virulence. In this review, we will discuss the current understanding of mechanisms used by S. mutans and related streptococci to colonize collagenous tissues, and the possible contribution of CBPs to infections in different sites of the host.

The Relationship of Bacterial Biofilms and Capsular Contracture in Breast Implants

Capsular contracture is a common sequelae of implant-based breast augmentation. Despite its prevalence, the etiology of capsular contracture remains controversial. Numerous studies have identified microbial biofilms on various implantable materials, including breast implants. Furthermore, biofilms have been implicated in subclinical infections associated with other surgical implants. In this review, we discuss microbial biofilms as a potential etiology of capsular contracture. The review also outlines the key diagnostic modalities available to identify the possible infectious agents found in biofilm, as well as available preventative and treatment measures.

Distribution of virulence factors and association with emm polymorphism or isolation site among beta-hemolytic group G Streptococcus dysgalactiae subspecies equisimilis

Distribution of virulence factors and association with emm polymorphism or isolation site among beta-hemolytic group G Streptococcus dysgalactiae subspecies equisimilis. Streptococcus dysgalactiae subspecies equisimilis (SDSE), the dominant human pathogenic species among group G streptococci, is the causative agent of several invasive and non-invasive diseases worldwide. However, limited information is available about the distribution of virulence factors among SDSE isolates, or their association with emm types and the isolation sites. In this study, 246 beta-hemolytic group G SDSE isolates collected in central Taiwan between February 2007 and August 2011 were under investigation. Of these, 66 isolates were obtained from normally sterile sites and 180 from non-sterile sites. emm typing revealed 32 types, with the most prevalent one being stG10.0 (39.8%), followed by stG245.0 (15.4%), stG840.0 (12.2%), stG6.1 (7.7%), and stG652.0 (4.1%). The virulence genes lmb (encoding laminin-binding protein), gapC (glyceraldehyde 3-phosphate dehydrogenase), sagA (streptolysin S), and hylB (hyaluronidase) existed in all isolates. Also, 99.2% of the isolates possessed slo (streptolysin O) and scpA (C5a peptidase) genes. In addition, 72.8%, 14.6%, 9.4%, and 2.4% of the isolates possessed the genes ska (streptokinase), cbp (putative collagen-binding protein, SDEG_1781), fbp (putative fibronectin-binding protein, SDEG_0161), and sicG (streptococcal inhibitor of complement), respectively. The only superantigen gene detected was spegg (streptococcus pyrogenic exotoxin G(dys) ), which was possessed by 74.4% of the isolates; these isolates correlated with non-sterile sites. Positive correlations were observed between the following emm types and virulence genes: stG10.0 and stG840.0 with spegg, stG6.1 and stG652.0 with ska, and stG840.0 with cbp. On the other hand, negative correlations were observed between the following: stG245.0, stG6.1, and stG652.0 types with spegg, stG10.0 with ska, and stG10.0, stG245.0, and stG6.1 types with cbp. The prevalence of emm types of SDSE in central Taiwan was investigated for the first time. Moreover, the distribution of virulence factors among beta-hemolytic group G SDSE isolates, as well as their association with emm types or isolation sites were also examined.

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.

Structural model for covalent adhesion of the Streptococcus pyogenes pilus through a thioester bond

The human pathogen Streptococcus pyogenes produces pili that are essential for adhesion to host surface receptors. Cpa, the adhesin at the pilus tip, was recently shown to have a thioester-containing domain. The thioester bond is believed to be important in adhesion, implying a mechanism of covalent attachment analogous to that used by human complement factors. Here, we have characterized a second active thioester-containing domain on Cpa, the N-terminal domain of Cpa (CpaN). Expression of CpaN in Escherichia coli gave covalently linked dimers. These were shown by x-ray crystallography and mass spectrometry to comprise two CpaN molecules cross-linked by the polyamine spermidine following reaction with the thioester bonds. This cross-linked CpaN dimer provides a model for the covalent attachment of Cpa to target receptors and thus the streptococcal pilus to host cells. Similar thioester domains were identified in cell wall proteins of other Gram-positive pathogens, suggesting that thioester domains are more widely used and provide a mechanism of adhesion by covalent bonding to target molecules on host cells that mimics that used by the human complement system to eliminate pathogens.

Mycobacterial laminin-binding histone-like protein mediates collagen-dependent cytoadherence

When grown in the presence of exogenous collagen I, Mycobacterium bovis BCG was shown to form clumps. Scanning electron microscopy examination of these clumps revealed the presence of collagen fibres cross-linking the bacilli. Since collagen is a major constituent of the eukaryotic extracellular matrices, we assayed BCG cytoadherence in the presence of exogenous collagen I. Collagen increased the interaction of the bacilli with A549 type II pneumocytes or U937 macrophages, suggesting that BCG is able to recruit collagen to facilitate its attachment to host cells. Using an affinity chromatography approach, we have isolated a BCG collagen-binding protein corresponding to the previously described mycobacterial laminin-binding histone-like protein (LBP/Hlp), a highly conserved protein associated with the mycobacterial cell wall. Moreover, Mycobacterium leprae LBP/Hlp, a well-characterized adhesin, was also able to bind collagen I. Finally, using recombinant fragments of M. leprae LBP/Hlp, we mapped the collagen-binding activity within the C-terminal domain of the adhesin. Since this protein was already shown to be involved in the recognition of laminin and heparan sulphate-containing proteoglycans, the present observations reinforce the adhesive activities of LBP/Hlp, which can be therefore considered as a multifaceted mycobacterial adhesin, playing an important role in both leprosy and tuberculosis pathogenesis.

Structure of Streptococcus agalactiae tip pilin GBS104: a model for GBS pili assembly and host interactions

The crystal structure of a 75 kDa central fragment of GBS104, a tip pilin from the 2063V/R strain of Streptococcus agalactiae (group B streptococcus; GBS), is reported. In addition, a homology model of the remaining two domains of GBS104 was built and a model of full-length GBS104 was generated by combining the homology model (the N1 and N4 domains) and the crystal structure of the 75 kDa fragment (the N2 and N3 domains). This rod-shaped GBS104 model is constructed of three IgG-like domains (the N1, N2 and N4 domains) and one vWFA-like domain (the N3 domain). The N1 and N2 domains of GBS104 are assembled with distinct and remote segments contributed by the N- and C-termini. The metal-binding site in the N3 domain of GBS104 is in the closed/low-affinity conformation. Interestingly, this domain hosts two long arms that project away from the metal-binding site. Using site-directed mutagenesis, two cysteine residues that lock the N3 domain of GBS104 into the open/high-affinity conformation were introduced. Both wild-type and disulfide-locked recombinant proteins were tested for binding to extracellular matrix proteins such as collagen, fibronectin, fibrinogen and laminin, and an increase in fibronectin binding affinity was identified for the disulfide-locked N3 domain, suggesting that induced conformational changes may play a possible role in receptor binding.

Collagen-binding microbial surface components recognizing adhesive matrix molecule (MSCRAMM) of Gram-positive bacteria inhibit complement activation via the classical pathway

Members of a family of collagen-binding microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) from Gram-positive bacteria are established virulence factors in several infectious diseases models. Here, we report that these adhesins also can bind C1q and act as inhibitors of the classical complement pathway. Molecular analyses of Cna from Staphylococcus aureus suggested that this prototype MSCRAMM bound to the collagenous domain of C1q and interfered with the interactions of C1r with C1q. As a result, C1r2C1s2 was displaced from C1q, and the C1 complex was deactivated. This novel function of the Cna-like MSCRAMMs represents a potential immune evasion strategy that could be used by numerous Gram-positive pathogens.

Identification of a novel collagen type І-binding protein from Streptococcus suis serotype 2

Streptococcus suis, a major pathogen of pigs, is an emerging zoonotic agent that causes meningitis and septic shock. cbp40 is a putative virulent gene that has been identified using suppression subtractive hybridization performed on the virulent S. suis serotype 2 strain HA9801 and the avirulent S. suis serotype 2 strain T15. Based on predicted protein features showing a shared conserved domain with the collagen-binding protein Cna of Staphylococcus aureus, Cbp40 is likely to function as a direct mediator of collagen adhesion. Here, the cbp40 gene was cloned and the recombinant protein purified. Western blotting using swine convalescent sera confirmed its role as an immunogenic protein. Collagen binding activity could be detected by western affinity blot and ELISA. Conversely, deletion of the cbp40 gene reduced bacterial adhesion to HEp-2 cells, capacity for biofilm formation, and virulence in a zebrafish infection model. The response of the bEnd.3 cell line to infection with the S. suis serotype 2 strain ZY05719 and the cbp40-knockout strain was evaluated using gene expression arrays. The differentially expressed genes were involved in inflammatory and immune responses, leukocyte adhesion and heterophilic cell adhesion. Collectively, these data suggest that Cbp40 plays an important role as an extracellular matrix adhesion protein that interacts with host cells during infection.

The small regulatory RNA FasX controls pilus expression and adherence in the human bacterial pathogen group A Streptococcus

Bacterial pathogens use cell surface-associated adhesion molecules to promote host attachment and colonization, and the ability to modulate adhesion expression is critical to pathogen success. Here, we show that the human-specific pathogen the group A Streptococcus (GAS) uses a small regulatory RNA (sRNA) to regulate the expression of adhesive pili. The fibronectin/fibrinogen-binding/haemolytic-activity/streptokinase-regulator-X (FasX) sRNA, previously shown to positively regulate expression of the secreted virulence factor streptokinase (SKA), negatively regulates the production of pili on the GAS cell surface. FasX base pairs to the extreme 5' end of mRNA from the pilus biosynthesis operon, and this RNA:RNA interaction reduces the stability of the mRNA, while also inhibiting translation of at least the first gene in the pilus biosynthesis operon (cpa, which encodes a minor pilin protein). The negative regulation of pilus expression by FasX reduces the ability of GAS to adhere to human keratinocytes. Our findings cement FasX sRNA as an important regulator of virulence factor production in GAS and identify that FasX uses at least three distinct mechanisms, positive (ska mRNA) and negative (pilus operon mRNA) regulation of mRNA stability, and negative regulation of mRNA translation (cpa mRNA), to post-transcriptionally regulate target mRNAs during infection.

Streptococcus gordonii collagen-binding domain protein CbdA may enhance bacterial survival in instrumented root canals ex vivo

INTRODUCTION

The surface-associated collagen-binding protein Ace of Enterococcus faecalis has been implicated as a virulence factor that contributes to bacterial persistence in endodontic infections. The purpose of this study was to determine if proteins with amino acid sequence similarity to Ace found in more abundant oral streptococci could play a similar role in potentially enhancing endodontic infections.

METHODS

A Streptococcus gordonii gene similar to ace was identified by genome sequence searches in silico. An isogenic derivative of strain DL1 with a disruption in the identified gene was constructed by allelic replacement. Parent and mutant strains were characterized for their ability to bind immobilized collagen type 1 in a microtiter plate-binding assay. Survival of the strains in a human tooth ex vivo-instrumented root canal model was compared by inoculating canals with parental or mutant bacteria and determining the colony-forming units (CFUs) recovered at various time points over a 12-day period.

RESULTS

The S. gordonii gene, encoding a protein with a conserved collagen-binding domain similar to that of Ace, was designated cbdA. The cbdA-deficient cells were less able to bind collagen type 1 than parental cells (P < .0001). Genetic complementation of the cbdA-deficient strain restored the collagen-binding phenotype. By day 12, significantly fewer (P = .03) cbdA-deficient than parental CFUs were recovered from instrumented canals.

CONCLUSIONS

A gene encoding a putative collagen-binding protein was identified in S. gordonii. Fewer S. gordonii cbdA-deficient cells survived ex vivo compared with parental cells, suggesting that collagen-binding proteins may contribute to the persistence of oral streptococci in instrumented root canals.

Human pathogens utilize host extracellular matrix proteins laminin and collagen for adhesion and invasion of the host

Laminin (Ln) and collagen are multifunctional glycoproteins that play an important role in cellular morphogenesis, cell signalling, tissue repair and cell migration. These proteins are ubiquitously present in tissues as a part of the basement membrane (BM), constitute a protective layer around blood capillaries and are included in the extracellular matrix (ECM). As a component of BMs, both Lns and collagen(s), thus function as major mechanical containment molecules that protect tissues from pathogens. Invasive pathogens breach the basal lamina and degrade ECM proteins of interstitial spaces and connective tissues using various ECM-degrading proteases or surface-bound plasminogen and matrix metalloproteinases recruited from the host. Most pathogens associated with the respiratory, gastrointestinal, or urogenital tracts, as well as with the central nervous system or the skin, have the capacity to bind and degrade Lns and collagen(s) in order to adhere to and invade host tissues. In this review, we focus on the adaptability of various pathogens to utilize these ECM proteins as enhancers for adhesion to host tissues or as a targets for degradation in order to breach the cellular barriers. The major pathogens discussed are Streptococcus, Staphylococcus, Pseudomonas, Salmonella, Yersinia, Treponema, Mycobacterium, Clostridium, Listeria, Porphyromonas and Haemophilus; Candida, Aspergillus, Pneumocystis, Cryptococcus and Coccidioides; Acanthamoeba, Trypanosoma and Trichomonas; retrovirus and papilloma virus.

Region specific and worldwide distribution of collagen-binding M proteins with PARF motifs among human pathogenic streptococcal isolates

Some of the variety of Streptococcus pyogenes and Streptococcus dysgalactiae ssp. equisimilis (SDSE) M proteins act as collagen-binding adhesins that facilitate acute infection. Moreover, their potential to trigger collagen autoimmunity has been implicated in the pathogenesis of acute rheumatic fever and attributed to a collagen-binding motif called PARF (peptide associated with rheumatic fever). For the first time we determine the rate of clinical isolates with collagen-binding M proteins that use a PARF motif (A/T/E)XYLXX(L/F)N in a defined geographic region, Vellore in South India. In this region both, incidence of streptococcal infections and prevalence of acute rheumatic fever are high. M proteins with PARF motif conferred collagen-binding activity to 3.9% of 153 S. pyogenes and 10.6% of 255 SDSE clinical isolates from Vellore. The PARF motif occurred in three S. pyogenes and 22 SDSE M protein types. In one of the S. pyogenes and five of the SDSE M proteins that contained the motif, collagen-binding was impaired, due to influences of other parts of the M protein molecule. The accumulated data on the collagen binding activity of certain M protein types allowed a reanalysis of published worldwide emm-typing data with the aim to estimate the rates of isolates that bind collagen via PARF. The results indicate that M proteins, which bind collagen via a PARF motif, are epidemiologically relevant in human infections, not only in Vellore. It is imperative to include the most relevant collagen-binding M types in vaccines. But when designing M protein based vaccines it should be considered that collagen binding motifs within the vaccine antigen remain potential risk factors.

Involvement of T6 pili in biofilm formation by serotype M6 Streptococcus pyogenes

The group A streptococcus (GAS) Streptococcus pyogenes is known to cause self-limiting purulent infections in humans. The role of GAS pili in host cell adhesion and biofilm formation is likely fundamental in early colonization. Pilus genes are found in the FCT (fibronectin-binding protein, collagen-binding protein, and trypsin-resistant antigen) genomic region, which has been classified into nine subtypes based on the diversity of gene content and nucleotide sequence. Several epidemiological studies have indicated that FCT type 1 strains, including serotype M6, produce large amounts of monospecies biofilm in vitro. We examined the direct involvement of pili in biofilm formation by serotype M6 clinical isolates. In the majority of tested strains, deletion of the tee6 gene encoding pilus shaft protein T6 compromised the ability to form biofilm on an abiotic surface. Deletion of the fctX and srtB genes, which encode pilus ancillary protein and class C pilus-associated sortase, respectively, also decreased biofilm formation by a representative strain. Unexpectedly, these mutant strains showed increased bacterial aggregation compared with that of the wild-type strain. When the entire FCT type 1 pilus region was ectopically expressed in serotype M1 strain SF370, biofilm formation was promoted and autoaggregation was inhibited. These findings indicate that assembled FCT type 1 pili contribute to biofilm formation and also function as attenuators of bacterial aggregation. Taken together, our results show the potential role of FCT type 1 pili in the pathogenesis of GAS infections.

Bacterial Pili exploit integrin machinery to promote immune activation and efficient blood-brain barrier penetration

Group B Streptococcus (GBS) is the leading cause of meningitis in newborn infants. Bacterial cell surface appendages, known as pili, have been recently described in streptococcal pathogens, including GBS. The pilus tip adhesin, PilA, contributes to GBS adherence to blood-brain barrier (BBB) endothelium; however, the host receptor and the contribution of PilA in central nervous system (CNS) disease pathogenesis are unknown. Here we show that PilA binds collagen, which promotes GBS interaction with the α₂β₁ integrin resulting in activation of host chemokine expression and neutrophil recruitment during infection. Mice infected with the PilA-deficient mutant exhibit delayed mortality, a decrease in neutrophil infiltration and bacterial CNS dissemination. We find that PilA-mediated virulence is dependent on neutrophil influx as neutrophil depletion results in a decrease in BBB permeability and GBS-BBB penetration. Our results suggest that the bacterial pilus, specifically the PilA adhesin, has a dual role in immune activation and bacterial entry into the CNS.

Assembly mechanism of FCT region type 1 pili in serotype M6 Streptococcus pyogenes

The human pathogen Streptococcus pyogenes produces diverse pili depending on the serotype. We investigated the assembly mechanism of FCT type 1 pili in a serotype M6 strain. The pili were found to be assembled from two precursor proteins, the backbone protein T6 and ancillary protein FctX, and anchored to the cell wall in a manner that requires both a housekeeping sortase enzyme (SrtA) and pilus-associated sortase enzyme (SrtB). SrtB is primarily required for efficient formation of the T6 and FctX complex and subsequent polymerization of T6, whereas proper anchoring of the pili to the cell wall is mainly mediated by SrtA. Because motifs essential for polymerization of pilus backbone proteins in other Gram-positive bacteria are not present in T6, we sought to identify the functional residues involved in this process. Our results showed that T6 encompasses the novel VAKS pilin motif conserved in streptococcal T6 homologues and that the lysine residue (Lys-175) within the motif and cell wall sorting signal of T6 are prerequisites for isopeptide linkage of T6 molecules. Because Lys-175 and the cell wall sorting signal of FctX are indispensable for substantial incorporation of FctX into the T6 pilus shaft, FctX is suggested to be located at the pilus tip, which was also implied by immunogold electron microscopy findings. Thus, the elaborate assembly of FCT type 1 pili is potentially organized by sortase-mediated cross-linking between sorting signals and the amino group of Lys-175 positioned in the VAKS motif of T6, thereby displaying T6 and FctX in a temporospatial manner.

The membrane bound LRR lipoprotein Slr, and the cell wall-anchored M1 protein from Streptococcus pyogenes both interact with type I collagen

Streptococcus pyogenes is an important human pathogen and surface structures allow it to adhere to, colonize and invade the human host. Proteins containing leucine rich repeats (LRR) have been indentified in mammals, viruses, archaea and several bacterial species. The LRRs are often involved in protein-protein interaction, are typically 20–30 amino acids long and the defining feature of the LRR motif is an 11-residue sequence LxxLxLxxNxL (x being any amino acid). The streptococcal leucine rich (Slr) protein is a hypothetical lipoprotein that has been shown to be involved in virulence, but at present no ligands for Slr have been identified. We could establish that Slr is a membrane attached horseshoe shaped lipoprotein by homology modeling, signal peptidase II inhibition, electron microscopy (of bacteria and purified protein) and immunoblotting. Based on our previous knowledge of LRR proteins we hypothesized that Slr could mediate binding to collagen. We could show by surface plasmon resonance that recombinant Slr and purified M1 protein bind with high affinity to collagen I. Isogenic slr mutant strain (MB1) and emm1 mutant strain (MC25) had reduced binding to collagen type I as shown by slot blot and surface plasmon resonance. Electron microscopy using gold labeled Slr showed multiple binding sites to collagen I, both to the monomeric and the fibrillar structure, and most binding occurred in the overlap region of the collagen I fibril. In conclusion, we show that Slr is an abundant membrane bound lipoprotein that is co-expressed on the surface with M1, and that both these proteins are involved in recruiting collagen type I to the bacterial surface. This underlines the importance of S. pyogenes interaction with extracellular matrix molecules, especially since both Slr and M1 have been shown to be virulence factors.

Architects at the bacterial surface - sortases and the assembly of pili with isopeptide bonds

The cell wall envelope of Gram-positive bacteria can be thought of as a surface organelle for the assembly of macromolecular structures that enable the unique lifestyle of each microorganism. Sortases - enzymes that cleave the sorting signals of secreted proteins to form isopeptide (amide) bonds between the secreted proteins and peptidoglycan or polypeptides - function as the principal architects of the bacterial surface. Acting alone or with other sortase enzymes, sortase construction leads to the anchoring of surface proteins at specific sites in the envelope or to the assembly of pili, which are fibrous structures formed from many protein subunits. The catalysis of intermolecular isopeptide bonds between pilin subunits is intertwined with the assembly of intramolecular isopeptide bonds within pilin subunits. Together, these isopeptide bonds endow these sortase products with adhesive properties and resistance to host proteases.

Relevance of pili in pathogenic streptococci pathogenesis and vaccine development

A common mechanism used by bacteria to initiate adhesion to host tissues during colonization is the expression of long filamentous structures extending from their surface. These structures, known as pili or fimbriae, were initially identified in Gram-negative bacteria, and are typically formed by noncovalent interactions between pilin subunits. Pili have only recently been described in Gram-positive bacteria. In particular, in pathogenic streptococci the proteinaceous components of pili are covalently polymerized by the action of sortase enzymes similar to those involved in the covalent attachment of Gram-positive surface proteins to the peptidoglycan cell wall. With great relevance to the development of strategies to combat Gram-positive-associated infections, pilus components from pathogenic streptococci have been shown to induce protective immunity in mouse models of streptococcal disease. In addition, recent papers have created new perspectives on the role of such organelles in streptococcal pathogenesis, from the involvement in colonization and biofilm formation to translocation of tissue barriers. All this information makes the characterization of pili a hot scientific issue that we believe will lead to important future developments in understanding bacterial dynamics that lead to successful occupation of microbial niches.

Novel conserved group A streptococcal proteins identified by the antigenome technology as vaccine candidates for a non-M protein-based vaccine

Group A streptococci (GAS) can cause a wide variety of human infections ranging from asymptomatic colonization to life-threatening invasive diseases. Although antibiotic treatment is very effective, when left untreated, Streptococcus pyogenes infections can lead to poststreptococcal sequelae and severe disease causing significant morbidity and mortality worldwide. To aid the development of a non-M protein-based prophylactic vaccine for the prevention of group A streptococcal infections, we identified novel immunogenic proteins using genomic surface display libraries and human serum antibodies from donors exposed to or infected by S. pyogenes. Vaccine candidate antigens were further selected based on animal protection in murine lethal-sepsis models with intranasal or intravenous challenge with two different M serotype strains. The nine protective antigens identified are highly conserved; eight of them show more than 97% sequence identity in 13 published genomes as well as in approximately 50 clinical isolates tested. Since the functions of the selected vaccine candidates are largely unknown, we generated deletion mutants for three of the protective antigens and observed that deletion of the gene encoding Spy1536 drastically reduced binding of GAS cells to host extracellular matrix proteins, due to reduced surface expression of GAS proteins such as Spy0269 and M protein. The protective, highly conserved antigens identified in this study are promising candidates for the development of an M-type-independent, protein-based vaccine to prevent infection by S. pyogenes.

Typing of the pilus-protein-encoding FCT region and biofilm formationas novel parameters in epidemiological investigations of Streptococcuspyogenes isolates from various infection sites

Streptococcus pyogenes is an important human pathogen for whichan association between infection site and selected epidemiological or functionalmarkers has previously been suggested. However, the studies involved oftenused strains with an insufficiently defined clinical background and laboratoryhistory. Thus, the major goal of the present study was to investigate theserelationships in 183 prospectively collected, well-defined, low-passage isolatesfrom a North-East German centre for tertiary care. For each isolate the clinicalbackground (91 respiratory, 71 skin and 21 invasive isolates) andantibiotic-resistance pattern was recorded. All isolates were classified accordingto their emm type, antibiotic-resistance and PFGE pattern (SmaI restriction analysis of genomic DNA). As novel discriminatorymethods we performed a PCR-based typing of the pilus-protein-encoding FCTregion (FCT) and biofilm-formation phenotyping in various culturemedia. Forty-one isolates were found to be resistant to at least one of thetested antibiotics. emm typing revealed emm28, emm12, emm1, emm4, emm89 and emm2 as themost frequent types in our collection. The novel FCT typing showed isolatesencoding FCT types 4 and 2 to be the most common. Overall 113 strains withunique combinations of emm and FCT types, antibiotic-resistance andPFGE patterns were identified. The majority of all isolates revealed an associationof biofilm-formation capacity with growth media. Comparing all results forpotential associations, no correlation could be established between the anatomicalsite of isolation and the emm or the FCT type. There was no relationshipbetween biofilm formation and emm type, antibiotic-resistance orPFGE patterns. However, a novel association between biofilm formation andFCT type became obvious among strains from our collection.

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.