Nucleotide sequence of the gene for a fibronectin-binding protein from Staphylococcus aureus: use of this peptide sequence in the synthesis of biologically active peptides

99mTc-DTPA-Collagen Radiotracer for the Noninvasive Detection of Infective Endocarditis

Infective endocarditis (IE) represents a significant concern among hospital-acquired infections, frequently caused by the Gram-positive bacterium Staphylococcus aureus. Nuclear imaging is emerging as a noninvasive and precise diagnostic tool. However, the gold standard radiotracer [18F]-FDG cannot distinguish between infection and inflammation, resulting in false positives. Based on the presence of collagen-binding proteins in the cell wall of S. aureus, we propose the radiolabeling of collagen for its evaluation in IE animal models by single-photon emission computed tomography (SPECT) imaging. We radiolabeled rat tail collagen I using DTPA chelator and [99mTc]NaTcO4. Selectivity was evaluated in vitro using 3 Gram-positive bacteria, 1 Gram-negative bacteria and 1 yeast. In vivo SPECT/computed tomography (CT) imaging was conducted on 8 SD rat models of IE and 8 sterile sham model as controls. Ex vivo biodistribution and autoradiography were performed following imaging. Diagnosis of IE was confirmed through microbiological studies and H&E histopathology. [99mTc]-DTPA-Collagen was synthesized successfully with a yield of 42.86 ± 6.35%, a purity of 95.84 ± 1.85% and a stability higher than 90% after 50 h postincubation. In vitro uptake demonstrated the selectivity for Gram-positive bacteria (63.85 ± 15.15%). Ex vivo analysis confirmed hepato-splenic excretion. In vivo SPECT/CT imaging revealed highly localized uptake within the aortic valve with a sensitivity of 62.5% and specificity of 87.5%. We successfully synthesized and characterized a new SPECT radiotracer based on [99mTc]Tc-radiolabeled collagen. In vitro studies demonstrated the selectivity of the radiotracer for Gram-positive bacteria. In vivo SPECT/CT-based assessment in an IE model confirmed the potential of this approach to detect active IE.

HfaE Is a Component of the Holdfast Anchor Complex That Tethers the Holdfast Adhesin to the Cell Envelope

Bacteria use adhesins to colonize different surfaces and form biofilms. The species of the Caulobacterales order use a polar adhesin called holdfast, composed of polysaccharides, proteins, and DNA, to irreversibly adhere to surfaces. In Caulobacter crescentus, a freshwater Caulobacterales species, the holdfast is anchored at the cell pole via the holdfast anchor (Hfa) proteins HfaA, HfaB, and HfaD. HfaA and HfaD colocalize with holdfast and are thought to form amyloid-like fibers that anchor holdfast to the cell envelope. HfaB, a lipoprotein, is required for the translocation of HfaA and HfaD to the cell surface. Deletion of the anchor proteins leads to a severe defect in adherence resulting from holdfast not being properly attached to the cell and shed into the medium. This phenotype is greater in a ΔhfaB mutant than in a ΔhfaA ΔhfaD double mutant, suggesting that HfaB has other functions besides the translocation of HfaA and HfaD. Here, we identify an additional HfaB-dependent holdfast anchoring protein, HfaE, which is predicted to be a secreted protein. HfaE is highly conserved among Caulobacterales species, with no predicted function. In planktonic culture, hfaE mutants produce holdfasts and rosettes similar to those produced by the wild type. However, holdfasts from hfaE mutants bind to the surface but are unable to anchor cells, similarly to other anchor mutants. We showed that fluorescently tagged HfaE colocalizes with holdfast and that HfaE forms an SDS-resistant high-molecular-weight species consistent with amyloid fiber formation. We propose that HfaE is a novel holdfast anchor protein and that HfaE functions to link holdfast material to the cell envelope. IMPORTANCE For surface attachment and biofilm formation, bacteria produce adhesins that are composed of polysaccharides, proteins, and DNA. Species of the Caulobacterales produce a specialized polar adhesin, holdfast, which is required for permanent attachment to surfaces. In this study, we evaluate the role of a newly identified holdfast anchor protein, HfaE, in holdfast anchoring to the cell surface in two different members of the Caulobacterales with drastically different environments. We show that HfaE plays an important role in adhesion and biofilm formation in the Caulobacterales. Our results provide insights into bacterial adhesins and how they interact with the cell envelope and surfaces.

Antimicrobial properties of nanostructured surfaces - demonstrating the need for a standard testing methodology

Bioinspired nanostructured materials that exhibit antimicrobial properties are being synthesized and tested at increasing rates for use in healthcare, manufacturing processes, and diagnostics. Although progress has been made in improving and understanding their bactericidal activity, arguably, the biggest problem currently in the field is the lack of a standard testing methodology that allows for optimal characterization and better comparison of emerging nanostructures. Here, we examine two forms of nanostructured silicon that vary in their ability to kill certain bacterial species due to different physical mechanisms and derive guidelines for the comparative testing. We perform a comprehensive evaluation of methodologies used extensively in the field (e.g., colony counting and live dead analysis) and the novel application of high-throughput flow cytometry. The data reveal how the techniques are complementary but not always directly equivalent or correlative. Therefore, comparison of results obtained using different methodologies on different materials can be grossly misleading. We report significant variations in bactericidal efficiencies depending on experimental environments (medium type, etc.) and methodologies employed. In addition, we demonstrate how cytometry is yet another powerful complementary tool that can aid the mechanistic understanding of antimicrobial activities of rough surfaces. Besides standardization for comparison, ultimately, evaluation methods need to consider anticipated applications. Then and only then can the true potential (or limitation) of a novel material be determined for its suitability for advancement in a particular field of use.

A Comprehensive Landscape for Fibril Association Behaviors Encoded Synergistically by Saccharides and Peptides

Nature provides us a panorama of fibrils with tremendous structural polymorphism from molecular building blocks to hierarchical association behaviors. Despite recent achievements in creating artificial systems with individual building blocks through self-assembly, molecularly encoding the relationship from model building blocks to fibril association, resulting in controlled macroscopic properties, has remained an elusive goal. In this paper, by employing a designed set of glycopeptide building blocks and combining experimental and computational tools, we report a library of controlled fibril polymorphism with elucidation from molecular packing to fibril association and the related macroscopic properties. The growth of the fibril either axially or radially with right- or left-handed twisting is determined by the subtle trade-off of oligosaccharide and oligopeptide components. Meanwhile, visible evidence for the association process of double-strand fibrils has been experimentally and theoretically proposed. Finally the fibril polymorphs demonstrated significant different macroscopic properties on hydrogel formation and cellular migration control.

The Multivalent Role of Fibronectin-Binding Proteins A and B (FnBPA and FnBPB) of Staphylococcus aureus in Host Infections

Staphylococcus aureus, one of the most important human pathogens, is the causative agent of several infectious diseases including sepsis, pneumonia, osteomyelitis, endocarditis and soft tissue infections. This pathogenicity is due to a multitude of virulence factors including several cell wall-anchored proteins (CWA). CWA proteins have modular structures with distinct domains binding different ligands. The majority of S. aureus strains express two CWA fibronectin (Fn)-binding adhesins FnBPA and FnBPB (Fn-binding proteins A and B), which are encoded by closely related genes. The N-terminus of FnBPA and FnBPB comprises an A domain which binds ligands such as fibrinogen, elastin and plasminogen. The A domain of FnBPB also interacts with histones and this binding results in the neutralization of the antimicrobial activity of these molecules. The C-terminal moiety of these adhesins comprises a long, intrinsically disordered domain composed of 11/10 fibronectin-binding repeats. These repetitive motifs of FnBPs promote invasion of cells that are not usually phagocytic via a mechanism by which they interact with integrin α₅β₁ through a Fn mediated-bridge. The FnBPA and FnBPB A domains engage in homophilic cell-cell interactions and promote biofilm formation and enhance platelet aggregation. In this review we update the current understanding of the structure and functional properties of FnBPs and emphasize the role they may have in the staphylococcal infections.

Adaptation of the group A Streptococcus adhesin Scl1 to bind fibronectin type III repeats within wound-associated extracellular matrix: implications for cancer therapy

The human‐adapted pathogen group A Streptococcus (GAS) utilizes wounds as portals of entry into host tissue, wherein surface adhesins interact with the extracellular matrix, enabling bacterial colonization. The streptococcal collagen‐like protein 1 (Scl1) is a major adhesin of GAS that selectively binds to two fibronectin type III (FnIII) repeats within cellular fibronectin, specifically the alternatively spliced extra domains A and B, and the FnIII repeats within tenascin‐C. Binding to FnIII repeats was mediated through conserved structural determinants present within the Scl1 globular domain and facilitated GAS adherence and biofilm formation. Isoforms of cellular fibronectin that contain extra domains A and B, as well as tenascin‐C, are present for several days in the wound extracellular matrix. Scl1‐FnIII binding is therefore an example of GAS adaptation to the host's wound environment. Similarly, cellular fibronectin isoforms and tenascin‐C are present in the tumor microenvironment. Consistent with this, FnIII repeats mediate GAS attachment to and enhancement of biofilm formation on matrices deposited by cancer‐associated fibroblasts and osteosarcoma cells. These data collectively support the premise for utilization of the Scl1‐FnIII interaction as a novel method of anti‐neoplastic targeting in the tumor microenvironment.

Antibiotic resistant zoonotic bacteria in Irrawaddy squirrel (Callosciurus pygerythrus)

Irrawaddy squirrel (Callosciurus pygerythrus) may play an important role in the transmission of zoonotic bacteria, but little is known about the carriage of zoonotic bacteria in this common frugivorous rodent in Bangladesh. We aimed to investigate the presence of common zoonotic bacterial pathogens in Irrawaddy squirrel in the southeast part of Bangladesh. A total of 27 rectal and 27 oro‐nasal swabs were collected from 27 healthy wild Irrawaddy squirrels. Four common zoonotic bacteria were isolated following routine laboratory procedures, and were identified based on colony morphology, and biochemical and staining properties. The pathogenic potential of the identified bacteria was confirmed by detection of virulence genes by PCR. All isolates were subjected to antimicrobial susceptibility test against seven antibiotics from six generic groups which are commonly used in human and veterinary medicine in Bangladesh. The prevalence of Escherichia coli, Salmonella spp., Yersinia spp. and Staphylococcus spp. was 44.4% (95% CI, 32.0–57.6), 13% (95% CI, 6.1–24.7), 44.4% (95% CI, 32.0–57.6), and 72.2% (95% CI, 59.0–82.5), respectively. We identified potential zoonotic virulence genes in all of these four bacterial species. Antimicrobial susceptibility testing revealed the presence of several multidrug resistant bacterial strains in squirrels. To the best of our knowledge, this is the first report in Bangladesh of the detection of antibiotic resistant zoonotic bacteria in Irrawaddy squirrels. The findings underpin the role of Irrawaddy squirrel as a source of pathogenic antibiotic resistant bacteria, consequently, fruit rejected because of squirrel consumption and squirrel‐bites deserve more concern than previously.

In vitro methods for the evaluation of antimicrobial surface designs

Bacterial adhesion and subsequent biofilm formation on biomedical implants and devices are a major cause of their failure. As systemic antibiotic treatment is often ineffective, there is an urgent need for antimicrobial biomaterials and coatings. The term "antimicrobial" can encompass different mechanisms of action (here termed "antimicrobial surface designs"), such as antimicrobial-releasing, contact-killing or non-adhesivity. Biomaterials equipped with antimicrobial surface designs based on different mechanisms of action require different in vitro evaluation methods. Available industrial standard evaluation tests do not address the specific mechanisms of different antimicrobial surface designs and have therefore been modified over the past years, adding to the myriad of methods available in the literature to evaluate antimicrobial surface designs. The aim of this review is to categorize fourteen presently available methods including industrial standard tests for the in vitro evaluation of antimicrobial surface designs according to their suitability with respect to their antimicrobial mechanism of action. There is no single method or industrial test that allows to distinguish antimicrobial designs according to all three mechanisms identified here. However, critical consideration of each method clearly relates the different methods to a specific mechanism of antimicrobial action. It is anticipated that use of the provided table with the fourteen methods will avoid the use of wrong methods for evaluating new antimicrobial designs and therewith facilitate translation of novel antimicrobial biomaterials and coatings to clinical use. The need for more and better updated industrial standard tests is emphasized.

STATEMENT OF SIGNIFICANCE

European COST-action TD1305, IPROMEDAI aims to provide better understanding of mechanisms of antimicrobial surface designs of biomaterial implants and devices. Current industrial evaluation standard tests do not sufficiently account for different, advanced antimicrobial surface designs, yet are urgently needed to obtain convincing in vitro data for approval of animal experiments and clinical trials. This review aims to provide an innovative and clear guide to choose appropriate evaluation methods for three distinctly different mechanisms of antimicrobial design: (1) antimicrobial-releasing, (2) contact-killing and (3) non-adhesivity. Use of antimicrobial evaluation methods and definition of industrial standard tests, tailored toward the antimicrobial mechanism of the design, as identified here, fulfill a missing link in the translation of novel antimicrobial surface designs to clinical use.

Human Immunoglobulin G Cannot Inhibit Fibrinogen Binding by the Genetically Diverse A Domain of Staphylococcus aureus Fibronectin-Binding Protein A

The fibronectin-binding protein A (FnBPA) is a cell surface-associated protein of Staphylococcus aureus which mediates adherence to the host extracellular matrix and is important for bacterial virulence. Previously, substantial sequence diversity was found among strains in the fibrinogen-binding A domain of this protein, and 7 different isotypes were described. The effect of this sequence diversity on the human antibody response, in terms of both antibody production and antibody function, remains unclear. In this study, we identify five different FnBPA A domain isotypes based on the sequence results of 22 clinical S. aureus isolates, obtained from the same number of patients suffering from bacteremia. Using a bead-based Luminex technique, we measure the patients' total immunoglobulin G (IgG) against the 7 FnBPA isotypes at the onset and during the time course of bacteremia (median of 10 serum samples per patient over a median of 35 days). A significant increase in IgG against the FnBPA A domain, including the isotype carried by the infecting strain, is observed in only three out of 22 patients (14%) after the onset of bacteremia. Using a Luminex-based FnBPA-fibrinogen-binding assay, we find that preincubation of recombinant FnBPA isotypes with IgG from diverse patients does not interfere with binding to fibrinogen. This observation is confirmed using an alternative Luminex-based assay and enzyme-linked immunosorbent assay (ELISA). IMPORTANCE Despite the many in vitro and murine in vivo studies involving FnBPA, the actual presence of this virulence factor during human infection is less well established. Furthermore, it is currently unknown to what extent sequence variation in such a virulence factor affects the human antibody response and the ability of antibodies to interfere with FnBPA function. This study sheds new light on these issues. First, the uniform presence of a patient's IgG against FnBPA indicates the presence and importance of this virulence factor during S. aureus pathogenesis. Second, the absence of an increase in antibody production in most patients following bacteremia indicates the complexity of S. aureus-host interactions, possibly involving immune evasion or lack of expression of FnBPA during invasive infection. Finally, we provide new insights into the inability of human antibodies to interfere with FnBPA-fibrinogen binding. These observations should be taken into account during the development of novel vaccination approaches.

Perspectives on DNA Vaccines. Targeting Staphylococcal Adhesins to Prevent Implant Infections

DNA vaccines consist of a plasmid DNA genetically engineered to produce one or more proteins able to elicit protective immune responses against virulence factors of infectious pathogens. Once introduced into the cells of the host, a DNA vaccine induces a high production of antigens by the endogenous presence of the peptide codifying gene; improves antigen processing and presentation; may be able to simultaneously co-express multiple antigenic molecules; and, lastly, switches on both humoral and cellular immune responses. In this mini-review, we underscore the advantageous characteristics of DNA vaccines compared with traditional ones and provide summaries of some of the more recent studies on them, mainly focusing the possibility of their use in targeting the staphylococcal adhesins that play a key role in the first adhesive phase of implant infections.

Methicillin-Resistant Staphylococcus aureus Sequence Type (ST) 5 Isolates from Health Care and Agricultural Sources Adhere Equivalently to Human Keratinocytes

Staphylococcus aureus is part of the nasal microbiome of many humans and has become a significant public health burden due to infections with antibiotic-resistant strains, including methicillin-resistant S. aureus (MRSA) strains. Several lineages of S. aureus, including MRSA, are found in livestock species and can be acquired by humans through contact with animals. These livestock-associated MRSA (LA-MRSA) isolates raise public health concerns because of the potential for livestock to act as reservoirs for MRSA outside the hospital setting. In the United States, swine harbor a mixed population of LA-MRSA isolates, with the sequence type 398 (ST398), ST9, and ST5 lineages being detected. LA-MRSA ST5 isolates are particularly concerning to the public health community because, unlike the isolates in the ST398 and ST9 lineages, isolates in the ST5 lineage are a significant cause of human disease in both the hospital and community settings globally. The ability of swine-associated LA-MRSA ST5 isolates to adhere to human keratinocytes in vitro was investigated, and the adherence genes harbored by these isolates were evaluated and compared to those in clinical MRSA ST5 isolates from humans with no swine contact. The two subsets of isolates adhered equivalently to human keratinocytes in vitro and contained an indistinguishable complement of adherence genes that possessed a high degree of sequence identity. Collectively, our data indicate that, unlike LA-MRSA ST398 isolates, LA-MRSA ST5 isolates do not exhibit a reduced genotypic or phenotypic capacity to adhere to human keratinocytes.IMPORTANCE Our data indicate that swine-associated livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) ST5 isolates are as capable of adhering to human skin and have the same genetic potential to adhere as clinical MRSA ST5 isolates from humans. This suggests that humans in contact with livestock have the potential to become colonized with LA-MRSA ST5 isolates; however, the genes that contribute to the persistence of S. aureus on human skin were absent in LA-MRSA ST5 isolates. The data presented here are important evidence in evaluating the potential risks that LA-MRSA ST5 isolates pose to humans who come into contact with livestock.

Variation and association of fibronectin-binding protein genes fnbA and fnbB in Staphylococcus aureus Japanese isolates

Fibronectin-binding proteins A and B (FnBPA and FnBPB) mediate adhesion of Staphylococcus aureus to fibrinogen, elastin and fibronectin. FnBPA and FnBPB are encoded by two closely linked genes, fnbA and fnbB, respectively. With the exception of the N-terminal regions, the amino acid sequences of FnBPA and FnBPB are highly conserved. To investigate the genetics and evolution of fnbA and fnbB, the most variable regions, which code for the 67th amino acids of the A through B regions (A67-B) of fnbA and fnbB, were focused upon. Eighty isolates of S. aureus in Japan were sequenced and 19 and 18 types in fnbA and fnbB, respectively, identified. Although the phylogeny of fnbA and fnbB were found to be quite different, each fnbA type connected with a specific fnbB type, indicating that fnbA and fnbB mutate independently, whereas the combination of both genes after recombination is stable. Hence those fnbA-fnbB combinations were defined as FnBP sequence types (FnSTs). Representative isolates of each FnST were assigned distinct STs by multilocus sequence typing, suggesting correspondence of FnST with genome lineage. Linkage disequilibrium (LD) analysis of the A67-B region revealed that subdomains N2, N3 and FnBR1 form a LD block in fnbA, whereas N2 and N3 form two independent LD blocks in fnbB. N2-N3 three-dimensional structural models indicated that not only the variable amino acid residues, but also well-conserved amino acid residues between FnBPA and FnBPB, are located on the surface of the protein. These results highlight a molecular process of the FnBP that has evolved by mingled mutation and recombination with retention of functions.

The remarkably multifunctional fibronectin binding proteins of Staphylococcus aureus

Staphylococcus aureus expresses two distinct but closely related multifunctional cell wall-anchored (CWA) proteins that bind to the host glycoprotein fibronectin. The fibronectin binding proteins FnBPA and FnBPB comprise two distinct domains. The C-terminal domain comprises a tandem array of repeats that bind to the N-terminal type I modules of fibronectin by the tandem β-zipper mechanism. This causes allosteric activation of a cryptic integrin binding domain, allowing fibronectin to act as a bridge between bacterial cells and the α5β1 integrin on host cells, triggering bacterial uptake by endocytosis. Variants of FnBPA with polymorphisms in fibronectin binding repeats (FnBRs) that increase affinity for the ligand are associated with strains that infect cardiac devices and cause endocarditis, suggesting that binding affinity is particularly important in intravascular infections. The N-terminal A domains of FnBPA and FnBPB have diverged into seven antigenically distinct isoforms. Each binds fibrinogen by the 'dock, lock and latch' mechanism characteristic of clumping factor A. However, FnBPs can also bind to elastin, which is probably important in adhesion to connective tissue in vivo. In addition, they can capture plasminogen from plasma, which can be activated to plasmin by host and bacterial plasminogen activators. The bacterial cells become armed with a host protease which destroys opsonins, contributing to immune evasion and promotes spreading during skin infection. Finally, some methicillin-resistant S. aureus (MRSA) strains form biofilm that depends on the elaboration of FnBPs rather than polysaccharide. The A domains of the FnBPs can interact homophilically, allowing cells to bind together as the biofilm accumulates.

Antimicrobial susceptibility, virulence genes, and randomly amplified polymorphic DNA analysis of Staphylococcus aureus recovered from bovine mastitis in Ningxia, China

Staphylococcus aureusis the leading pathogen involved inbovine mastitis, but knowledgeabout antimicrobial resistance, virulence factors, and genotypes of Staphylococcus aureus resulting in bovine mastitis in Ningxia, China, is limited. Therefore, antimicrobial susceptibility, virulence gene, and randomly amplified polymorphic DNA (RAPD) analyses of Staph. aureus were carried out. A total of 327 milk samples from cows with clinical and subclinical mastitis in 4 regions of Ningxia were used for the isolation and identification of pathogens according to phenotypic and molecular characteristics. Antimicrobial susceptibility against 22 antimicrobial agents was determined by disk diffusion. The presence of 8 virulence genes in Staph. aureus isolates was tested by PCR. Genotypes of isolates were investigated based on RAPD. Results showed that 35 isolates obtained from mastitis milk samples were identified as Staph. aureus. The isolates were resistant to sulfamethoxazole (100%), penicillin G (94.3%), ampicillin (94.3%), erythromycin (68.6%), azithromycin (68.6%), clindamycin (25.7%), amoxicillin (11.4%), and tetracycline (5.7%). All of the isolates contained one or more virulence genes with average (standard deviation) of 6.6±1.6. The most prevalent virulence genes were hlb (97.1%), followed by fnbpA, hla, coa (94.3% each), nuc (85.7%), fnbpB (80%), clfA (77.1%), and tsst-1 (40%). Nine different gene patterns were found and 3 of them were the dominant gene combinations (77.1%). Staphylococcus aureus isolates (n=35) were divided into 6 genotypes by RAPD tying, the genotypes III and VI were the most prevalent genotypes. There was greatvariation in genotypes of Staph. aureus isolates, not only among different farms, but also within the same herd in Ningxia province. The study showed a high incidence of Staph. aureus with genomic variation of resistance genes, which is matter of great concern in public and animal health in Ningxia province of China.

Stuck in the Middle: Fibronectin-Binding Proteins in Gram-Positive Bacteria

Fibronectin is a multidomain glycoprotein found ubiquitously in human body fluids and extracellular matrices of a variety of cell types from all human tissues and organs, including intestinal epithelial cells. Fibronectin plays a major role in the regulation of cell migration, tissue repair, and cell adhesion. Importantly, fibronectin also serves as a common target for bacterial adhesins in the gastrointestinal tract. Fibronectin-binding proteins (FnBPs) have been identified and characterized in a wide variety of host-associated bacteria. Single bacterial species can contain multiple, diverse FnBPs. In pathogens, some FnBPs contribute to virulence via host cell attachment, invasion, and interference with signaling pathways. Although FnBPs in commensal and probiotic strains are not sufficient to confer virulence, they are essential for attachment to their ecological niches. Here we describe the interaction between human fibronectin and bacterial adhesins by highlighting the FnBPs of Gram-positive pathogens and commensals. We provide an overview of the occurrence and diversity of FnBPs with a focus on the model pathogenic organisms in which FnBPs are most characterized. Continued investigation of FnBPs is needed to fully understand their divergence and specificity in both pathogens and commensals.

Staphylococcus aureus Aggregation and Coagulation Mechanisms, and Their Function in Host-Pathogen Interactions

The human commensal bacterium Staphylococcus aureus can cause a wide range of infections ranging from skin and soft tissue infections to invasive diseases like septicemia, endocarditis, and pneumonia. Muticellular organization almost certainly contributes to S. aureus pathogenesis mechanisms. While there has been considerable focus on biofilm formation and its role in colonizing prosthetic joints and indwelling devices, less attention has been paid to nonsurface-attached group behavior like aggregation and clumping. S. aureus is unique in its ability to coagulate blood, and it also produces multiple fibrinogen-binding proteins that facilitate clumping. Formation of clumps, which are large, tightly packed groups of cells held together by fibrin(ogen), has been demonstrated to be important for S. aureus virulence and immune evasion. Clumps of cells are able to avoid detection by the host's immune system due to a fibrin(ogen) coat that acts as a shield, and the size of the clumps facilitates evasion of phagocytosis. In addition, clumping could be an important early step in establishing infections that involve tight clusters of cells embedded in host matrix proteins, such as soft tissue abscesses and endocarditis. In this review, we discuss clumping mechanisms and regulation, as well as what is known about how clumping contributes to immune evasion.

Staphylococcus aureus Sortase A-Mediated Incorporation of Peptides: Effect of Peptide Modification on Incorporation

The endogenous Staphylococcus aureus sortase A (SrtA) transpeptidase covalently anchors cell wall-anchored (CWA) proteins equipped with a specific recognition motif (LPXTG) into the peptidoglycan layer of the staphylococcal cell wall. Previous in situ experiments have shown that SrtA is also able to incorporate exogenous, fluorescently labelled, synthetic substrates equipped with the LPXTG motif (K(FITC)LPETG-amide) into the bacterial cell wall, albeit at high concentrations of 500 μM to 1 mM. In the present study, we have evaluated the effect of substrate modification on the incorporation efficiency. This revealed that (i) by elongation of LPETG-amide with a sequence of positively charged amino acids, derived from the C-terminal domain of physiological SrtA substrates, the incorporation efficiency was increased by 20-fold at 10 μM, 100 μM and 250 μM; (ii) Substituting aspartic acid (E) for methionine increased the incorporation of the resulting K(FITC)LPMTG-amide approximately three times at all concentrations tested; (iii) conjugation of the lipid II binding antibiotic vancomycin to K(FITC)LPMTG-amide resulted in the same incorporation levels as K(FITC)LPETG-amide, but much more efficient at an impressive 500-fold lower substrate concentration. These newly developed synthetic substrates can potentially find broad applications in for example the in situ imaging of bacteria; the incorporation of antibody recruiting moieties; the targeted delivery and covalent incorporation of antimicrobial compounds into the bacterial cell wall.

Contribution of Adsorbed Protein Films to Nanoscopic Vibrations Exhibited by Bacteria Adhering through Ligand-Receptor Bonds

Bacteria adhering to surfaces exhibit nanoscopic vibrations that depend on the viscoelasticity of the bond. The quantification of the nanoscopic vibrations of bacteria adhering to surfaces provides new opportunities to better understand the properties of the bond through which bacteria adhere and the mechanisms by which they resist detachment. Often, however, bacteria do not adhere to bare surfaces but to adsorbed protein films, on which adhesion involves highly specific ligand-receptor binding next to nonspecific DLVO interaction forces. Here we determine the contribution of adsorbed salivary protein and fibronectin films to vibrations exhibited by adhering streptococci and staphylococci, respectively. The streptococcal strain used has the ability to adhere to adsorbed salivary proteins films through antigen I/II ligand-receptor binding, while the staphylococcal strain used adheres to adsorbed fibronectin films through a proteinaceous ligand-receptor bond. In the absence of ligand-receptor binding, electrostatic interactions had a large impact on vibration amplitudes of adhering bacteria on glass. On an adsorbed salivary protein film, vibration amplitudes of adhering streptococci depended on the film softness as determined by QCM-D and were reduced after film fixation using glutaraldehyde. On a relatively stiff fibronectin film, cross-linking the film in glutaraldehyde hardly reduced its softness, and accordingly fibronectin film softness did not contribute to vibration amplitudes of adhering staphylococci. However, fixation of the staphylococcus-fibronectin bond further decreased vibration amplitudes, while fixation of the streptococcus bond hardly impacted vibration amplitudes. Summarizing, this study shows that both the softness of adsorbed protein films and the properties of the bond between an adhering bacterium and an adsorbed protein film play an important role in bacterial vibration amplitudes. These nanoscopic vibrations reflect the viscoelasticity of the bacterial bond with a substratum and play important roles in bacterial adhesion, detachment and susceptibility to antimicrobials.

The fibronectin-binding protein Fnm contributes to adherence to extracellular matrix components and virulence of Enterococcus faecium

The interaction between bacteria and fibronectin is believed to play an important role in the pathogenicity of clinically important Gram-positive cocci. In the present study, we identified a gene encoding a predicted fibronectin-binding protein of Enterococcus faecium (fnm), a homologue of Streptococcus pneumoniae pavA, in the genomes of E. faecium strain TX82 and all other sequenced E. faecium isolates. Full-length recombinant Fnm from strain TX82 bound to immobilized fibronectin in a concentration-dependent manner and also appeared to bind collagen type V and laminin, but not other proteins, such as transferrin, heparin, bovine serum albumin, mucin, or collagen IV. We demonstrated that the N-terminal fragment of Fnm is required for full fibronectin binding, since truncation of this region caused a 2.4-fold decrease (P < 0.05) in the adhesion of E. faecium TX82 to fibronectin. Deletion of fnm resulted in a significant reduction (P < 0.001) in the ability of the mutant, TX6128, to bind fibronectin relative to that of the wild-type strain; in situ reconstitution of fnm in the deletion mutant strain restored adherence. In addition, the Δfnm mutant was highly attenuated relative to TX82 (P ≤ 0.0001) in a mixed-inoculum rat endocarditis model. Taken together, these results demonstrate that Fnm affects the adherence of E. faecium to fibronectin and is important in the pathogenesis of experimental endocarditis.

The expanding field of platelet-bacterial interconnections

Given their small size, platelets are emerging as being one of the most important entities in the bloodstream. Not only do they play a key role in maintaining thrombosis and haemostasis, platelets also play a critical role in orchestrating the immune response. Being the first cell at the site of injury, they are perfectly placed to assess the extent of the damage and recruit immune cells as is necessary. As a first line of defence, platelets can act as primitive immune cells themselves by interacting with invading pathogens. A number of platelet receptors have been shown to interact with bacteria either directly or indirectly, involving a plasma protein bridge. This review will discuss the molecular mechanisms that exist between platelets and bacteria and the functional response to the interaction. We will also discuss the importance of considering animal models of disease and the use of physiological shear when studying platelet-bacterial interactions.

Host-Bacteria Interactions in Foreign Body Infections

Persistent staphylococcal infections are a major medical problem, especially when they occur on implanted materials or intravascular catheters. This review describes some of the recently discovered molecular mechanisms ofStaphylococcus aureusattachment to host proteins coating biomedical implants. These interactions involve specific surface proteins, called bacterial adhesins, that recognize specific domains of host proteins deposited on indwelling devices, such as fibronectin, fibrinogen, or fibrin. Elucidation of molecular mechanisms ofS aureusadhesion to the different host proteins may lead to the development of specific inhibitors blocking attachment ofS aureus, which may decrease the risk of bacterial colonization of indwelling devices.

Amino acid alterations in fibronectin binding protein A (FnBPA) and bacterial genotype are associated with cardiac device related infection in Staphylococcus aureus bacteraemia

OBJECTIVES

Staphylococcus aureus initiates cardiac device-related infection (CDI) by binding of fibronectin binding protein A (FnBPA) to the device's surface. In FnBPA, specific "binding enhancing" amino acid alterations are associated with CDI. However, no study has investigated whether these mutations also occur in geographically different regions and whether they arise during infection or are inherent properties of the infecting isolate.

METHODS

We analysed bacterial isolates from 34 patients with S. aureus bacteraemia and implanted cardiac devices for association with CDI, FnBPA sequence, classification into a clonal complex (CC), and binding to fibronectin (Fn).

RESULTS

We confirmed that amino acid alterations at positions 652, 782, and 786 in FnBPA were associated with CDI (p = 0.005). Furthermore, CC15 and CC45 isolates were associated with CDI (p = 0.004). All isolates within a CC exhibited a characteristic mutation pattern, with major changes occurring in CC45 including a duplication of D1 and an altered immunogenic epitope in the D3 repeat. Isolates harbouring the "binding enhancing" mutations showed a slightly increased Fn binding capability, whereas Fn binding was decreased in CC45 isolates, according to a microtiter plate assay.

CONCLUSIONS

FnBPA sequence variations are lineage specific and display inherent properties of the infecting isolate. Sequence analysis of FnBPA, as well as the bacterial genotype, may be used to predict the risk for device-related infection.

The role of hemostasis in infective endocarditis

Infective endocarditis (IE) is a thromboinflammatory disease of the endocardium, with pathophysiology mostly the result of the interplay between microorganisms and modifiers of the hemostasis system. In this setting, the evidence gathered so far warrants a more systematic appraisal. In this review article, experimental and clinical data on the role of hemostasis in IE are summarized. Starting from the current pathogenetic model of IE, we discuss the dual role of platelets in this condition, the microbial interaction with the hemostasis system, also describing nonspecific hemostasis changes during sepsis. We finally propose our hypothesis of thrombophilia as a possible trigger of IE, highlighting the challenges that the study of hemostasis in IE presents. The role of hemostasis in IE appears to be an exciting field of research. The activity of the hemostasis system is highly relevant in terms of susceptibility, progression, and treatment of IE. Pharmacologic modulation of hemostasis before and after IE onset is possible and represents still a largely unexplored area of study.

Surface adhesins and exopolymers of selected foodborne pathogens

The ability of bacteria to bind different compounds and to adhere to biotic and abiotic surfaces provides them with a range of advantages, such as colonization of various tissues, internalization, avoidance of an immune response, and survival and persistence in the environment. A variety of bacterial surface structures are involved in this process and these promote bacterial adhesion in a more or less specific manner. In this review, we will focus on those surface adhesins and exopolymers in selected foodborne pathogens that are involved mainly in primary adhesion. Their role in biofilm development will also be considered when appropriate. Both the clinical impact and the implications for food safety of such adhesion will be discussed.

Gingipains from Porphyromonas gingivalis - Complex domain structures confer diverse functions

Gingipains, a group of arginine or lysine specific cysteine proteinases (also known as RgpA, RgpB and Kgp), have been recognized as major virulence factors in Porphyromonas gingivalis. This bacterium is one of a handful of pathogens that cause chronic periodontitis. Gingipains are involved in adherence to and colonization of epithelial cells, haemagglutination and haemolysis of erythrocytes, disruption and manipulation of the inflammatory response, and the degradation of host proteins and tissues. RgpA and Kgp are multi-domain proteins composed of catalytic domains and haemagglutinin/adhesin (HA) regions. The structure of the HA regions have previously been defined by a gingipain domain structure hypothesis which is a set of putative domain boundaries derived from the sequences of fragments of these proteins extracted from the cell surface. However, multiple sequence alignments and hidden Markov models predict an alternative domain architecture for the HA regions of gingipains. In this alternate model, two or three repeats of the so-called "cleaved adhesin" domains (and one other undefined domain in some strains) are the modules which constitute the substructure of the HA regions. Recombinant forms of these putative cleaved adhesin domains are indeed stable folded protein modules and recently determined crystal structures support the hypothesis of a modular organisation of the HA region. Based on the observed K2 and K3 structures as well as multiple sequence alignments, it is proposed that all the cleaved adhesin domains in gingipains will share the same β-sandwich jelly roll fold. The new domain model of the structure for gingipains and the haemagglutinin (HagA) proteins of P. gingivalis will guide future functional studies of these virulence factors.

Structural determinants of the interaction between the Haemophilus influenzae Hap autotransporter and fibronectin

Haemophilus influenzae is a Gram-negative cocco-bacillus that initiates infection by colonizing the upper respiratory tract. Hap is an H. influenzae serine protease autotransporter protein that mediates adherence, invasion and microcolony formation in assays with human epithelial cells and is presumed to facilitate the process of colonization. Additionally, Hap mediates adherence to fibronectin, laminin and collagen IV, extracellular matrix (ECM) proteins that are present in the respiratory tract and are probably important targets for H. influenzae colonization. The region of Hap responsible for adherence to ECM proteins has been localized to the C-terminal 511 aa of the Hap passenger domain (HapS). In this study, we characterized the structural determinants of the interaction between HapS and fibronectin. Using defined fibronectin fragments, we established that Hap interacts with the fibronectin repeat fragment called FNIII(1-2). Using site-directed mutagenesis, we found a series of motifs in the C-terminal region of HapS that contribute to the interaction with fibronectin. Most of these motifs are located on the F1 and F3 faces of the HapS structure, suggesting that the F1 and F3 faces may be responsible for the HapS-fibronectin interaction.

Identifying functional anti-Staphylococcus aureus antibodies by sequencing antibody repertoires of patient plasmablasts

Infection by Staphylococcus aureus is on the rise, and there is a need for a better understanding of host immune responses that combat S. aureus. Here we use DNA barcoding to enable deep sequencing of the paired heavy- and light-chain immunoglobulin genes expressed by individual plasmablasts derived from S. aureus-infected humans. Bioinformatic analysis of the antibody repertoires revealed clonal families of heavy-chain sequences and enabled rational selection of antibodies for recombinant expression. Of the ten recombinant antibodies produced, seven bound to S. aureus, of which four promoted opsonophagocytosis of S. aureus. Five of the antibodies bound to known S. aureus cell-surface antigens, including fibronectin-binding protein A. Fibronectin-binding protein A-specific antibodies were isolated from two independent S. aureus-infected patients and mediated neutrophil killing of S. aureus in in vitro assays. Thus, our DNA barcoding approach enabled efficient identification of antibodies involved in protective host antibody responses against S. aureus.

Release of protein A from the cell wall of Staphylococcus aureus

Staphylococcal protein A (SpA) is anchored to the cell wall envelope of Staphylococcus aureus by sortase A, which links the threonyl (T) of its C-terminal LPXTG motif to peptidoglycan cross-bridges (i.e., Gly5). SpA binds the Fcγ domains of IgG and protects staphylococci from opsonophagocytic clearance. Moreover, SpA cross-links B-cell receptors to modify host adaptive immune responses. The mechanisms whereby SpA is released from the bacterial surface to access the host's immune system are not known. Here we demonstrate that SpA is released with murein tetrapeptide-tetraglycyl [L-Ala-D-iGln-(SpA-Gly5)L-Lys-D-Ala-Gly4] linked to its C-terminal threonyl. LytN, a cross-wall murein hydrolase, contributes to the release of SpA by removing amino sugars [i.e., N-acetylmuramic acid-N-acetylglucosamine (MurNAc-GlcNAc)] from attached peptidoglycan, whereas LytM, a pentaglycyl-endopeptidase, triggers polypeptide release from the bacterial envelope. A model is proposed whereby murein hydrolases cleave the anchor structure of released SpA to modify host immune responses.

Emergence of the epidemic methicillin-resistant Staphylococcus aureus strain USA300 coincides with horizontal transfer of the arginine catabolic mobile element and speG-mediated adaptations for survival on skin

The arginine catabolic mobile element (ACME) is the largest genomic region distinguishing epidemic USA300 strains of methicillin-resistant Staphylococcus aureus (MRSA) from other S. aureus strains. However, the functional relevance of ACME to infection and disease has remained unclear. Using phylogenetic analysis, we have shown that the modular segments of ACME were assembled into a single genetic locus in Staphylococcus epidermidis and then horizontally transferred to the common ancestor of USA300 strains in an extremely recent event. Acquisition of one ACME gene, speG, allowed USA300 strains to withstand levels of polyamines (e.g., spermidine) produced in skin that are toxic to other closely related S. aureus strains. speG-mediated polyamine tolerance also enhanced biofilm formation, adherence to fibrinogen/fibronectin, and resistance to antibiotic and keratinocyte-mediated killing. We suggest that these properties gave USA300 a major selective advantage during skin infection and colonization, contributing to the extraordinary evolutionary success of this clone.

IMPORTANCE

Over the past 15 years, methicillin-resistant Staphylococcus aureus (MRSA) has become a major public health problem. It is likely that adaptations in specific MRSA lineages (e.g., USA300) drove the spread of MRSA across the United States and allowed it to replace other, less-virulent S. aureus strains. We suggest that one major factor in the evolutionary success of MRSA may have been the acquisition of a gene (speG) that allows S. aureus to evade the toxicity of polyamines (e.g., spermidine and spermine) that are produced in human skin. Polyamine tolerance likely gave MRSA multiple fitness advantages, including the formation of more-robust biofilms, increased adherence to host tissues, and resistance to antibiotics and killing by human skin cells.

Expression and Distribution of Very Late Antigen-5 in Mouse Peritoneal Macrophages upon Ingestion of Fibronectin-BoundStaphylococcus aureus

Many pathogens colonize host tissues by binding to the extracellular matrix via their cell surface adhesion molecules, which are called MSCRAMMs (microbial surface components recognizing adhesive matrix molecules). Staphylococcus aureus expresses several of these adhesion molecules, some of which bind to fibronectin. Of these adhesion molecules, fibronectin-binding proteins play a role in the pathogenicity of S. aureus, although it is not yet clear whether they enhance its virulence. We have previously shown that fibronectin-bound S. aureus is efficiently phagocytosed by thioglycolate-induced mouse peritoneal macrophages. Bacterial ingestion is mediated by Very Late Antigen-5 (VLA-5; alpha5beta1 integrin) and is accompanied by the formation of adhesion complexes. Here we show that the expression of VLA-5 is restricted to thioglycolate-induced inflammatory macrophages and is not found in the resident macrophages. When cells were in suspension, alpha5 integrin was not expressed on the surface of either resident or inflammatory macrophages, whereas in adherent cells, this integrin was distributed on the surface of inflammatory but not resident macrophages. A high level of this integrin was present in the cytoplasmic region only in inflammatory macrophages. In agreement with this, fibronectin-mediated phagocytosis of S. aureus was observed only in the inflammatory macrophages. In inflammatory macrophages ingesting fibronectin-bound S. aureus, alpha5 integrin was concentrated close to the phagocytosed bacteria. This change in distribution was not found in macrophages ingesting untreated bacteria. Together with our previous work, these results indicate that, upon ingestion of fibronectin-bound S. aureus, VLA-5 accumulates in the area of phagocytosis in inflammatory macrophages, where it forms adhesion complexes.

Interactions in bacterial biofilm development: a structural perspective

A community-based life style is the normal mode of growth and survival for many bacterial species. These cellular accretions or biofilms are initiated upon recognition of solid phases by cell surface exposed adhesive moieties. Further cell-cell interactions, cell signalling and bacterial replication leads to the establishment of dense populations encapsulated in a mainly self-produced extracellular matrix; this comprises a complex mixture of macromolecules. These fascinating architectures protect the inhabitants from radiation damage, dehydration, pH fluctuations and antimicrobial compounds. As such they can cause bacterial persistence in disease and problems in industrial applications. In this review we discuss the current understandings of these initial biofilm-forming processes based on structural data. We also briefly describe latter biofilm maturation and dispersal events, which although lack high-resolution insights, are the present focus for many structural biologists working in this field. Finally we give an overview of modern techniques aimed at preventing and disrupting problem biofilms.

Mapping the ligand-binding region of Borrelia hermsii fibronectin-binding protein

Many pathogenic microorganisms express fibronectin-binding molecules that facilitate their adherence to the extracellular matrix and/or entry into mammalian cells. We have previously described a Borrelia recurrentis gene, cihC that encodes a 40-kDa surface receptor for both, fibronectin and the complement inhibitors C4bp and C1-Inh. We now provide evidence for the expression of a group of highly homologues surface proteins, termed FbpA, in three B. hermsii isolates and two tick-borne relapsing fever spirochetes, B. parkeri and B. turicatae. When expressed in Escherichia coli or B. burgdorferi, four out of five proteins were shown to selectively bind fibronectin, whereas none of five proteins were able to bind the human complement regulators, C4bp and C1-Inh. By applying deletion mutants of the B. hermsii fibronectin-binding proteins a putative high-affinity binding site for fibronectin was mapped to its central region. In addition, the fibronectin-binding proteins of B. hermsii were found to share sequence homology with BBK32 of the Lyme disease spirochete B. burgdorferi with similar function suggesting its involvement in persistence and/or virulence of relapsing fever spirochetes.

Subdomains N2N3 of fibronectin binding protein A mediate Staphylococcus aureus biofilm formation and adherence to fibrinogen using distinct mechanisms

Health care-associated methicillin-resistant Staphylococcus aureus (HA-MRSA) forms biofilm in vitro that is dependent on the surface-located fibronectin binding proteins A and B (FnBPA, FnBPB). Here we provide new insights into the requirements for FnBP-dependent biofilm formation by MRSA. We show that expression of FnBPs is sustained at high levels throughout the growth cycle in the HA-MRSA strain BH1CC in contrast to laboratory strain SH1000, where expression could be detected only in exponential phase. We found that FnBP-mediated biofilm accumulation required Zn(2+), while the removal of Zn(2+) had no effect on the ability of FnBPA to mediate bacterial adherence to fibrinogen. We also investigated the role of FnBPA expressed on the surface of S. aureus in promoting biofilm formation and bacterial adhesion to fibrinogen. The minimum part of FnBPA required for ligand binding has so far been defined only with recombinant proteins. Here we found that the N1 subdomain was not required for biofilm formation or for FnBPA to promote bacterial adherence to fibrinogen. Residues at the C terminus of subdomain N3 required for FnBPA to bind to ligands using the "dock, lock, and latch" mechanism were necessary for FnBPA to promote bacterial adherence to fibrinogen. However, these residues were not necessary to form biofilm, allowing us to localize the region of FnBPA required for biofilm accumulation to residues 166 to 498. Thus, FnBPA mediates biofilm formation and bacterial adhesion to fibrinogen using two distinct mechanisms. Finally, we identified a hitherto-unrecognized thrombin cleavage site close to the boundary between subdomains N1 and N2 of FnBPA.

Protective immunization against Staphylococcus aureus infection in a novel experimental wound model in mice

A novel murine experimental wound infection model was used to assess the efficacy of multi-component immunization against Staphylococcus aureus infection. Necrotic lesions were induced in mice with venom from Bothrops asper and infected with a low inoculum, 1 × 10(2) CFU. The wound infection model therefore more resembles a clinical case of S. aureus infection compared with conventional infection models where far more bacteria are required. Before infection, mice were immunized with four recombinant S.aureus proteins expressed from Escherichia coli: (i) domains 1-3 of Extracellular adherence protein (Eap), (ii) Efb - D (fusion protein combining Extracellular fibrinogen binding protein (Efb) and a fibronectin binding domain (D) of the fibronectin binding protein (FnBP) and (iii) clumping factor A (ClfA). In the immunized group, lower bacterial colonization, undisturbed crust formation and significantly faster wound healing were found compared with the unimmunized control group. Efb and Eap have previously been found to impair wound healing and neutralization of these proteins by antibodies restores a more natural wound healing process. This effect is further also enhanced by the proposed opsonic activity of antibodies against ClfA and FnBP.

Detection of virulence-associated genes in Staphylococcus aureus isolated from bovine clinical mastitis milk samples in Guangxi

Staphylococcus aureus is recognized worldwide as a pathogen causing many serious diseases in humans and animals and is one of the most common etiological agents of clinical and subclinical bovine mastitis. The purpose of this study was to determine the presence of genes encoding clfA, fnbA, fnbB, cap5, cap8, hla, hlb, nuc, sea, and tst of S. aureus strains (n = 39) isolated from bovine clinical mastitis in Guangxi by polymerase chain reaction amplification. The results of the present study indicated that all isolates were found to contain one or more virulence-associated genes. The most frequently encountered genes were fnbA (97 %) and nuc (90 %), followed by hla (85 %) and hlb (82 %), respectively. None of the investigated S. aureus strains harbored fnbB and sea genes. The data in the present study showed a relatively wide distribution of the genes fnbA and nuc among the investigated isolates, indicating that they play an important role on bovine mastitis pathogenesis. The study provides a valuable insight into the virulence-associated genes of this important pathogen.

Polymorphisms in fibronectin binding protein A of Staphylococcus aureus are associated with infection of cardiovascular devices

Medical implants, like cardiovascular devices, improve the quality of life for countless individuals but may become infected with bacteria like Staphylococcus aureus. Such infections take the form of a biofilm, a structured community of bacterial cells adherent to the surface of a solid substrate. Every biofilm begins with an attractive force or bond between bacterium and substratum. We used atomic force microscopy to probe experimentally forces between a fibronectin-coated surface (i.e., proxy for an implanted cardiac device) and fibronectin-binding receptors on the surface of individual living bacteria from each of 80 clinical isolates of S. aureus. These isolates originated from humans with infected cardiac devices (CDI; n = 26), uninfected cardiac devices (n = 20), and the anterior nares of asymptomatic subjects (n = 34). CDI isolates exhibited a distinct binding-force signature and had specific single amino acid polymorphisms in fibronectin-binding protein A corresponding to E652D, H782Q, and K786N. In silico molecular dynamics simulations demonstrate that residues D652, Q782, and N786 in fibronectin-binding protein A form extra hydrogen bonds with fibronectin, complementing the higher binding force and energy measured by atomic force microscopy for the CDI isolates. This study is significant, because it links pathogenic bacteria biofilms from the length scale of bonds acting across a nanometer-scale space to the clinical presentation of disease at the human dimension.

Rsp inhibits attachment and biofilm formation by repressing fnbA in Staphylococcus aureus MW2

Biofilms contribute to virulence of Staphylococcus aureus. Formation of biofilms is multifactorial, involving polysaccharide, protein, and DNA components, which are controlled by various regulators. Here we report that deletion of the rsp gene resulted in an increase in biofilm formation in strain MW2, suggesting that Rsp is a repressor of biofilm formation. Using SDS-PAGE, we found that Rsp profoundly affected cell surface and secreted proteins. The rsp gene was transcribed monocistronically, and the transcripts were most abundant at the exponential growth phase. Microarray analyses revealed that Rsp represses 75 genes, including 9 genes encoding cell wall-anchored proteins, and activates 22 genes, including 5 genes encoding secreted proteases. Among these genes, fnbA, fnbB, sasG, and spa (which encode cell wall-anchored proteins) and splABCD (which encode secreted proteases) have been implicated in biofilm formation. To deconvolute Rsp's contribution to biofilm formation, we analyzed deletion mutants of these genes either in the wild-type or in the rsp mutant background. We found that fnbA deletion in the rsp mutant restored biofilm formation to the wild-type level, indicating that FnbA plays a major role in Rsp regulation of biofilm formation. Further studies revealed that Rsp inhibited biofilm formation at the stage of primary attachment through repressing fnbA. Rsp belongs to the AraC/XylS family of regulatory proteins. We expressed the putative Rsp DNA binding domain (RspDBD) in Escherichia coli and showed that RspDBD was able to specifically bind to a short DNA fragment containing the fnbA promoter, suggesting that Rsp represses fnbA expression by direct DNA binding.

The A domain of fibronectin-binding protein B of Staphylococcus aureus contains a novel fibronectin binding site

The fibronectin-binding proteins FnBPA and FnBPB are multifunctional adhesins than can also bind to fibrinogen and elastin. In this study, the N2N3 subdomains of region A of FnBPB were shown to bind fibrinogen with a similar affinity to those of FnBPA (2 μM). The binding site for FnBPB in fibrinogen was localized to the C-terminus of the γ-chain. Like clumping factor A, region A of FnBPB bound to the γ-chain of fibrinogen in a Ca(2+)-inhibitable manner. The deletion of 17 residues from the C-terminus of domain N3 and the substitution of two residues in equivalent positions for crucial residues for fibrinogen binding in clumping factor A and FnBPA eliminated fibrinogen binding by FnBPB. This indicates that FnBPB binds fibrinogen by the dock-lock-latch mechanism. In contrast, the A domain of FnBPB bound fibronectin with K(D) = 2.5 μM despite lacking any of the known fibronectin-binding tandem repeats. A truncate lacking the C-terminal 17 residues (latching peptide) bound fibronectin with the same affinity, suggesting that the FnBPB A domain binds fibronectin by a novel mechanism. The substitution of the two residues required for fibrinogen binding also resulted in a loss of fibronectin binding. This, combined with the observation that purified subdomain N3 bound fibronectin with a measurable, but reduced, K(D) of 20 μM, indicates that the type I modules of fibronectin bind to both the N2 and N3 subdomains. The fibronectin-binding ability of the FnBPB A domain was also functional when the protein was expressed on and anchored to the surface of staphylococcal cells, showing that it is not an artifact of recombinant protein expression.

Adhesive polypeptides of Staphylococcus aureus identified using a novel secretion library technique in Escherichia coli

BACKGROUND

Bacterial adhesive proteins, called adhesins, are frequently the decisive factor in initiation of a bacterial infection. Characterization of such molecules is crucial for the understanding of bacterial pathogenesis, design of vaccines and development of antibacterial drugs. Because adhesins are frequently difficult to express, their characterization has often been hampered. Alternative expression methods developed for the analysis of adhesins, e.g. surface display techniques, suffer from various drawbacks and reports on high-level extracellular secretion of heterologous proteins in Gram-negative bacteria are scarce. These expression techniques are currently a field of active research. The purpose of the current study was to construct a convenient, new technique for identification of unknown bacterial adhesive polypeptides directly from the growth medium of the Escherichia coli host and to identify novel proteinaceous adhesins of the model organism Staphylococcus aureus.

RESULTS

Randomly fragmented chromosomal DNA of S. aureus was cloned into a unique restriction site of our expression vector, which facilitates secretion of foreign FLAG-tagged polypeptides into the growth medium of E. coli ΔfliCΔfliD, to generate a library of 1663 clones expressing FLAG-tagged polypeptides. Sequence and bioinformatics analyses showed that in our example, the library covered approximately 32% of the S. aureus proteome. Polypeptides from the growth medium of the library clones were screened for binding to a selection of S. aureus target molecules and adhesive fragments of known staphylococcal adhesins (e.g coagulase and fibronectin-binding protein A) as well as polypeptides of novel function (e.g. a universal stress protein and phosphoribosylamino-imidazole carboxylase ATPase subunit) were detected. The results were further validated using purified His-tagged recombinant proteins of the corresponding fragments in enzyme-linked immunoassay and surface plasmon resonance analysis.

CONCLUSIONS

A new technique for identification of unknown bacterial adhesive polypeptides was constructed. Application of the method on S. aureus allowed us to identify three known adhesins and in addition, five new polypeptides binding to human plasma and extracellular matrix proteins. The method, here used on S. aureus, is convenient due to the use of soluble proteins from the growth medium and can in principle be applied to any bacterial species of interest.

Adhesion mechanisms of staphylococci

Staphylococcal adherence to an either biotic or abiotic surface is the critical first event in the establishment of an infection with these serious pathogens. Especially Staphylococcus aureus harbours a variety of proteinaceous and non-proteinaceous adhesins that mediate attachment to a multitude of host factors, such as extracellular matrix and plasma proteins and human host cells, or intercellular adhesion, which is essential for biofilm accumulation. Proteinaceous adhesins may be classified in covalently surface-anchored proteins of the MSCRAMM (microbial surface components recognizing adhesive matrix molecules) family or in proteins that are surface-associated by different means, such as ionic or hydrophobic interactions. Non-covalently surface-associated proteins include the autolysin/adhesins, proteins of the SERAM (secretable expanded repertoire adhesive molecules) family, or membrane-spanning proteins. Non-proteinaceous adhesins comprise the polysaccharide PIA (polysaccharide intercellular adhesin) and wall teichoic and lipoteichoic acids. The features and functions of surface and surface-associated protein adhesins as well as of non-proteinaceous adhesins are discussed.

SpA, ClfA, and FnbA genetic variations lead to Staphaurex test-negative phenotypes in bovine mastitis Staphylococcus aureus isolates

Staphylococcus aureus encodes many proteins that act as virulence factors, leading to a variety of diseases, including mastitis in cows. Among these virulence factors, SpA, ClfA, ClfB, FnbA, and FnbB are important for the ability of S. aureus to adhere to and invade host cells as well as to evade host immune responses. The interaction between these S. aureus surface proteins and human immunoglobulin G and fibrinogen that are coupled to latex particles is utilized to induce latex agglutination reactions, which are used widely in diagnostic kits for confirmation of presumptive S. aureus isolates. In this study, the Staphaurex latex agglutination test was performed on a collection of confirmed bovine mastitis S. aureus isolates. Notably, 54% (43/79 isolates) of these isolates exhibited latex agglutination-negative phenotypes (Staphaurex-negative result). To gain insights into the reasons for the high frequency of Staphaurex-negative bovine mastitis S. aureus isolates, the spa, clfA, clfB, fnbA, and fnbB genes were examined. Specific genetic changes in spa, clfA, and fnbA, as well as a loss of fnbB, which may impair SpA, ClfA, FnbA, and FnbB functions in latex agglutination reactions, were detected in Staphaurex-negative S. aureus isolates. The genetic changes included a premature stop codon in the spa gene, leading to a truncated SpA protein that is unable to participate in S. aureus cell-mediated agglutination of latex particles. In addition, clfA and fnbA genetic polymorphisms were detected that were linked to ClfA and FnbA amino acid changes that may significantly reduce fibrinogen-binding activity. The genetic variations in these S. aureus isolates might also have implications for their bovine mastitis virulence capacity.

The streptococcal binding site in the gelatin-binding domain of fibronectin is consistent with a non-linear arrangement of modules

Fibronectin-binding proteins (FnBPs) of Staphylococcus aureus and Streptococcus pyogenes mediate invasion of human endothelial and epithelial cells in a process likely to aid the persistence and/or dissemination of infection. In addition to binding sites for the N-terminal domain (NTD) of fibronectin (Fn), a number of streptococcal FnBPs also contain an upstream region (UR) that is closely associated with an NTD-binding region; UR binds to the adjacent gelatin-binding domain (GBD) of Fn. Previously, UR was shown to be required for efficient streptococcal invasion of epithelial cells. Here we show, using a Streptococcus zooepidemicus FnBP, that the UR-binding site in GBD resides largely in the (8)F1(9)F1 module pair. We also show that UR inhibits binding of a peptide from the α1 chain of type I collagen to (8)F1(9)F1 and that UR binding to (8)F1 is likely to occur through anti-parallel β-zipper formation. Thus, we propose that streptococcal proteins that contain adjacent NTD- and GBD-binding sites form a highly unusual extended tandem β-zipper that spans the two domains and mediates high affinity binding to Fn through a large intermolecular interface. The proximity of the UR- and NTD-binding sequences in streptococcal FnBPs is consistent with a non-linear arrangement of modules in the tertiary structure of the GBD of Fn.

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

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

Bonds between fibronectin and fibronectin-binding proteins on Staphylococcus aureus and Lactococcus lactis

Bacterial cell-wall-associated fibronectin binding proteins A and B (FnBPA and FnBPB) form bonds with host fibronectin. This binding reaction is often the initial step in prosthetic device infections. Atomic force microscopy was used to evaluate binding interactions between a fibronectin-coated probe and laboratory-derived Staphylococcus aureus that are (i) defective in both FnBPA and FnBPB (fnbA fnbB double mutant, DU5883), (ii) capable of expressing only FnBPA (fnbA fnbB double mutant complemented with pFNBA4), or (iii) capable of expressing only FnBPB (fnbA fnbB double mutant complemented with pFNBB4). These experiments were repeated using Lactococcus lactis constructs expressing fnbA and fnbB genes from S. aureus. A distinct force signature was observed for those bacteria that expressed FnBPA or FnBPB. Analysis of this force signature with the biomechanical wormlike chain model suggests that parallel bonds form between fibronectin and FnBPs on a bacterium. The strength and covalence of bonds were evaluated via nonlinear regression of force profiles. Binding events were more frequent (p < 0.01) for S. aureus expressing FnBPA or FnBPB than for the S. aureus double mutant. The binding force, frequency, and profile were similar between the FnBPA and FnBPB expressing strains of S. aureus. The absence of both FnBPs from the surface of S. aureus removed its ability to form a detectable bond with fibronectin. By contrast, ectopic expression of FnBPA or FnBPB on the surface of L. lactis conferred fibronectin binding characteristics similar to those of S. aureus. These measurements demonstrate that fibronectin-binding adhesins FnBPA and FnBPB are necessary and sufficient for the binding of S. aureus to prosthetic devices that are coated with host fibronectin.