Isolation and characterisation of a vitronectin-binding surface protein from Staphylococcus aureus

Prolonged Heat Stress of Lactobacillus paracasei GCRL163 Improves Binding to Human Colorectal Adenocarcinoma HT-29 Cells and Modulates the Relative Abundance of Secreted and Cell Surface-Located Proteins

Lactobacillus casei group bacteria improve cheese ripening and may interact with host intestinal cells as probiotics, where surface proteins play a key role. Three complementary methods [trypsin shaving (TS), LiCl-sucrose (LS) extraction, and extracellular culture fluid precipitation] were used to analyze cell surface proteins of Lactobacillus paracasei GCRL163 by label-free quantitative proteomics after culture to the mid-exponential phase in bioreactors at pH 6.5 and temperatures of 30-45 °C. A total of 416 proteins, including 300 with transmembrane, cell wall anchoring, and secretory motifs and 116 cytoplasmic proteins, were quantified as surface proteins. Although LS caused significantly greater cell lysis as growth temperature increased, higher numbers of extracytoplasmic proteins were exclusively obtained by LS treatment. Together with the increased positive surface charge of cells cultured at supra-optimal temperatures, proteins including cell wall hydrolases Msp1/p75 and Msp2/p40, α-fucosidase AlfB, SecA, and a PspC-domain putative adhesin were upregulated in surface or secreted protein fractions, suggesting that cell adhesion may be altered. Prolonged heat stress (PHS) increased binding of L. paracasei GCRL163 to human colorectal adenocarcinoma HT-29 cells, relative to acid-stressed cells. This study demonstrates that PHS influences cell adhesion and relative abundance of proteins located on the surface, which may impact probiotic functionality, and the detected novel surface proteins likely linked to the cell cycle and envelope stress.

The Streptococcus agalactiae cell wall-anchored protein PbsP mediates adhesion to and invasion of epithelial cells by exploiting the host vitronectin/αv integrin axis

Binding of microbial pathogens to host vitronectin (Vtn) is a common theme in the pathogenesis of invasive infections. In this study, we characterized the role of Vtn in the invasion of mucosal epithelial cells by Streptococcus agalactiae (i.e. group B streptococcus or GBS), a frequent human pathogen. Moreover, we identified PbsP, a previously described plasminogen-binding protein of GBS, as a dual adhesin that can also interact with human Vtn through its streptococcal surface repeat (SSURE) domains. Deletion of the pbsP gene decreases both bacterial adhesion to Vtn-coated inert surfaces and the ability of GBS to interact with epithelial cells. Bacterial adherence to and invasion of epithelial cells were either inhibited or enhanced by cell pretreatment with, respectively, anti-Vtn antibodies or Vtn, confirming the role of Vtn as a GBS ligand on host cells. Finally, antibodies directed against the integrin α_v subunit inhibited Vtn-dependent cell invasion by GBS. Collectively, these results indicate that Vtn acts as a bridge between the SSURE domains of PbsP on the GBS surface and host integrins to promote bacterial invasion of epithelial cells. Therefore, inhibition of interactions between PbsP and extracellular matrix components could represent a viable strategy to prevent colonization and invasive disease by GBS.

Conserved Patterns of Microbial Immune Escape: Pathogenic Microbes of Diverse Origin Target the Human Terminal Complement Inhibitor Vitronectin via a Single Common Motif

Pathogenicity of many microbes relies on their capacity to resist innate immunity, and to survive and persist in an immunocompetent human host microbes have developed highly efficient and sophisticated complement evasion strategies. Here we show that different human pathogens including Gram-negative and Gram-positive bacteria, as well as the fungal pathogen Candida albicans, acquire the human terminal complement regulator vitronectin to their surface. By using truncated vitronectin fragments we found that all analyzed microbial pathogens (n = 13) bound human vitronectin via the same C-terminal heparin-binding domain (amino acids 352-374). This specific interaction leaves the terminal complement complex (TCC) regulatory region of vitronectin accessible, allowing inhibition of C5b-7 membrane insertion and C9 polymerization. Vitronectin complexed with the various microbes and corresponding proteins was thus functionally active and inhibited complement-mediated C5b-9 deposition. Taken together, diverse microbial pathogens expressing different structurally unrelated vitronectin-binding molecules interact with host vitronectin via the same conserved region to allow versatile control of the host innate immune response.

Ultra-Sensitive, High-Resolution Liquid Chromatography Methods for the High-Throughput Quantitative Analysis of Bacterial Cell Wall Chemistry and Structure

High-performance liquid chromatography (HPLC) analysis has been critical for determining the structural and chemical complexity of the cell wall. However this method is very time consuming in terms of sample preparation and chromatographic separation. Here we describe (1) optimized methods for peptidoglycan isolation from both Gram-negative and Gram-positive bacteria that dramatically reduce the sample preparation time, and (2) the application of the fast and highly efficient ultra-performance liquid chromatography (UPLC) technology to muropeptide separation and quantification. The advances in both analytical instrumentation and stationary-phase chemistry have allowed for evolved protocols which cut run time from hours (2-3 h) to minutes (10-20 min), and sample demands by at least one order of magnitude. Furthermore, development of methods based on organic solvents permits in-line mass spectrometry (MS) of the UPLC-resolved muropeptides. Application of these technologies to high-throughput analysis will expedite the better understanding of the cell wall biology.

Pseudomonas aeruginosa Uses Dihydrolipoamide Dehydrogenase (Lpd) to Bind to the Human Terminal Pathway Regulators Vitronectin and Clusterin to Inhibit Terminal Pathway Complement Attack

The opportunistic human pathogen Pseudomonas aeruginosa controls host innate immune and complement attack. Here we identify Dihydrolipoamide dehydrogenase (Lpd), a 57 kDa moonlighting protein, as the first P. aeruginosa protein that binds the two human terminal pathway inhibitors vitronectin and clusterin. Both human regulators when bound to the bacterium inhibited effector function of the terminal complement, blocked C5b-9 deposition and protected the bacterium from complement damage. P. aeruginosa when challenged with complement active human serum depleted from vitronectin was severely damaged and bacterial survival was reduced by over 50%. Similarly, when in human serum clusterin was blocked by a mAb, bacterial survival was reduced by 44%. Thus, demonstrating that Pseudomonas benefits from attachment of each human regulator and controls complement attack. The Lpd binding site in vitronectin was localized to the C-terminal region, i.e. to residues 354-363. Thus, Lpd of P. aeruginosa is a surface exposed moonlighting protein that binds two human terminal pathway inhibitors, vitronectin and clusterin and each human inhibitor when attached protected the bacterial pathogen from the action of the terminal complement pathway. Our results showed insights into the important function of Lpd as a complement regulator binding protein that might play an important role in virulence of P. aeruginosa.

Comparison of five methods for direct extraction of surface proteins from Listeria monocytogenes for proteomic analysis by orbitrap mass spectrometry

Extracts of surface proteins, with minimal artifacts from contaminating cytosolic components, are highly desirable for investigating surface factors involved in the attachment and formation of biofilms by bacteria that are problematic in commercial food processing facilities. In this study, we compared the protein profiles of the food pathogen, Listeria monocytogenes, recovered after applying different surface protein extraction methods compiled from the literature: trypsin-enzymatic shaving with BICAM/sucrose or Tris/sucrose buffers (Tryp B+S, Tryp T+S), Tris-buffered urea (UB), lithium chloride (LiCl) and Tris-buffered urea applied with hypotonic-stressed cells (UB-Ghost), and subjected them to liquid chromatography tandem mass spectrometry and protein identification. The data indicate that the UB-Ghost extraction method provides a cleaner extract of surface proteins including the predicted (this study and the literature) or validated members (literature) from L. monocytogenes. This was determined by an accumulative lower unique peptide number exhibited by mass spectrometry for total cytoplasmic proteins among different surface extracts, with a majority of proteins demonstrating hydrophilic properties. The extracted proteins were from different functional categories and have associations with the cell surface, intermediary metabolism, information pathways, or functionally unknown proteins as suggested by in silico analyses performed by other groups (Leger and ListiList). The utilization of an optimized method for surface protein extraction should greatly facilitate identification by LC-MS/MS that could be useful to anyone working on molecular proteomics of bacterial surfaces.

Genetic variation in Staphylococcus aureus surface and immune evasion genes is lineage associated: implications for vaccine design and host-pathogen interactions

Background

S. aureus is a coloniser and pathogen of humans and mammals. Whole genome sequences of 58 strains of S. aureus in the public domain and data from multi-strain microarrays were compared to assess variation in the sequence of proteins known or putatively interacting with host.

Results

These included 24 surface proteins implicated in adhesion (ClfA, ClfB, Cna, Eap, Ebh, EbpS, FnBPA, FnBPB, IsaB, IsdA, IsdB, IsdH, SasB, SasC, SasD, SasF, SasG, SasH, SasK, SdrC, SdrD, SdrE, Spa and SraP) and 13 secreted proteins implicated in immune response evasion (Coa, Ecb, Efb, Emp, EsaC, EsxA, EssC, FLIPr, FLIPr like, Sbi, SCIN-B, SCIN-C, VWbp) located on the stable core genome. Many surface protein genes were missing or truncated, unlike immune evasion genes, and several distinct variants were identified. Domain variants were lineage specific. Unrelated lineages often possess the same sequence variant domains proving that horizontal transfer and recombination has contributed to their evolution. Surprisingly, sequenced strains from four animal S. aureus strains had surface and immune evasion proteins remarkably similar to those found in human strains, yet putative targets of these proteins vary substantially between different hosts. This suggests these proteins are not essential for virulence. However, the most variant protein domains were the putative functional regions and there is biological evidence that variants can be functional, arguing they do play a role.

Conclusion

Surface and immune evasion genes are candidates for S. aureus vaccines, and their distribution and functionality is key. Vaccines should contain cocktails of antigens representing all variants or they will not protect against naturally occurring S. aureus populations.

Neisseria meningitidis Opc invasin binds to the sulphated tyrosines of activated vitronectin to attach to and invade human brain endothelial cells

The host vasculature is believed to constitute the principal route of dissemination of Neisseria meningitidis (Nm) throughout the body, resulting in septicaemia and meningitis in susceptible humans. In vitro, the Nm outer membrane protein Opc can enhance cellular entry and exit, utilising serum factors to anchor to endothelial integrins; but the mechanisms of binding to serum factors are poorly characterised. This study demonstrates that Nm Opc expressed in acapsulate as well as capsulate bacteria can increase human brain endothelial cell line (HBMEC) adhesion and entry by first binding to serum vitronectin and, to a lesser extent, fibronectin. This study also demonstrates that Opc binds preferentially to the activated form of human vitronectin, but not to native vitronectin unless the latter is treated to relax its closed conformation. The direct binding of vitronectin occurs at its Connecting Region (CR) requiring sulphated tyrosines Y(56) and Y(59). Accordingly, Opc/vitronectin interaction could be inhibited with a conformation-dependent monoclonal antibody 8E6 that targets the sulphotyrosines, and with synthetic sulphated (but not phosphorylated or unmodified) peptides spanning the vitronectin residues 43-68. Most importantly, the 26-mer sulphated peptide bearing the cell-binding domain (45)RGD(47) was sufficient for efficient meningococcal invasion of HBMECs. To our knowledge, this is the first study describing the binding of a bacterial adhesin to sulphated tyrosines of the host receptor. Our data also show that a single region of Opc is likely to interact with the sulphated regions of both vitronectin and of heparin. As such, in the absence of heparin, Opc-expressing Nm interact directly at the CR but when precoated with heparin, they bind via heparin to the heparin-binding domain of the activated vitronectin, although with a lower affinity than at the CR. Such redundancy suggests the importance of Opc/vitronectin interaction in meningococcal pathogenesis and may enable the bacterium to harness the benefits of the physiological processes in which the host effector molecule participates.

Integrin-linked kinase is required for vitronectin-mediated internalization of Streptococcus pneumoniae by host cells

By interacting with components of the human host, including extracellular matrix (ECM) proteins, Streptococcus pneumoniae has evolved various strategies for colonization. Here, we characterized the interaction of pneumococci with the adhesive glycoprotein vitronectin and the contribution of this protein to pneumococcal uptake by host cells in an integrin-dependent manner. Specific interaction of S. pneumoniae with the heparin-binding sites of purified multimeric vitronectin was demonstrated by flow cytometry analysis. Host-cell-bound vitronectin promoted pneumococcal adherence to and invasion into human epithelial and endothelial cells. Pneumococci were trapped by microspike-like structures, which were induced upon contact of pneumococci with host-cell-bound vitronectin. Alphavbeta3 integrin was identified as the major cellular receptor for vitronectin-mediated adherence and uptake of pneumococci. Ingestion of pneumococci by host cells via vitronectin required a dynamic actin cytoskeleton and was dependent on integrin-linked kinase (ILK), phosphatidylinositol 3-kinase (PI3K), and protein kinase B (Akt), as demonstrated by gene silencing or in inhibition experiments. In conclusion, pneumococci exploit the vitronectin-alphavbeta3-integrin complex as a cellular receptor for invasion and this integrin-mediated internalization requires the cooperation between the host signalling molecules ILK, PI3K and Akt.

SdrI, a serine-aspartate repeat protein identified in Staphylococcus saprophyticus strain 7108, is a collagen-binding protein

A gene encoding a serine-aspartate repeat protein of Staphylococcus saprophyticus, an important cause of urinary tract infections in young women, has been cloned and sequenced. In contrast to other SD repeat proteins, SdrI carries 21 additional N-terminal repeats with a consensus sequence of (P/A)ATKE(K/E)A(A/V)(T/I)(A/T/S)EE and has the longest SD(AD)(1-5) repetitive region (854 amino acids) described so far. This highly repetitive sequence contains only the amino acids serine, asparagine, and a distinctly greater amount of alanine (37%) than all other known SD repeat proteins (2.3 to 4.4%). In addition, it is a collagen-binding protein of S. saprophyticus and the second example in this organism of a surface protein carrying the LPXTG motif. We constructed an isogenic sdrI knockout mutant that showed decreased binding to immobilized collagen compared with wild-type S. saprophyticus strain 7108. Binding could be reconstituted by complementation. Collagen binding is specifically caused by SdrI, and the recently described UafA protein, the only LPXTG-containing protein in the genome sequence of the type strain, is not involved in this trait. Our experiments suggest that, as in other staphylococci, the presence of different LPXTG-anchored cell wall proteins is common in S. saprophyticus and support the notion that the presence of matrix-binding surface proteins is common in staphylococci.

Heparan sulfate proteoglycans mediate Staphylococcus aureus interactions with intestinal epithelium

Staphylococcus aureus can be internalized by non-professional phagocytes, and may colonize the intestine in normal and antibiotic-treated individuals. Intestinal colonization may depend on the interactions of S. aureus with the intestinal epithelium. The best described mechanism of S. aureus binding to eukaryotic cells involves S. aureus fibronectin binding proteins (FnBPs), using fibronectin as a bridging molecule to beta1 integrins on the eukaryotic cell surface. Because S. aureus can be internalized by enterocytes, and because S. aureus is known to bind heparan sulfate (HS), we hypothesized that heparan sulfate proteoglycans (HSPGs) widely expressed on epithelia may mediate S. aureus interactions with intestinal epithelial cells. Internalization of S. aureus RN6390 by cultured intestinal epithelial cells was inhibited in a dose-dependent fashion by the HS mimic heparin, and by HS itself. Internalization of S. aureus DU5883, which lacks expression of staphylococcal FnBPs, was also inhibited by heparin. S. aureus adherence to ARH-77 cells, transfected to express the HSPG syndecan-1, was greatly increased when compared to adherence to plasmid control ARH-77 cells which have little detergent extractable HS. In addition, compared to wild-type HS-expressing Chinese hamster ovary (CHO) cells, internalization of S. aureus was decreased using mutant CHO cells with decreased HS expression. These findings are consistent with a model wherein S. aureus internalization by intestinal epithelial cells (and perhaps other epithelia) is mediated by S. aureus binding to the HS moiety of cell-surface HSPGs, and this interaction appears independent of fibronectin binding.

Proteome analysis of membrane and cell wall associated proteins from Staphylococcus aureus

Pathogenesis of Staphylococcus aureus, an opportunistic human pathogen, is complex and involves many virulence factors including an array of surface proteins (adhesins) that promote bacterial interactions with extracellular matrix components. A better understanding of these interactions can be achieved by studying the expression of membrane and cell wall associated proteins using a proteome analysis approach. To accomplish this, our goal here was to construct a reference map of membrane and cell wall associated proteins for S. aureus. Various lytic and solubilization methods have been tested to identify a suitable methodology for detection of these proteins in two-dimensional electrophoresis (2DE). Results demonstrate that cell lysis with lysostaphin, which lyses staphylococcal peptidoglycan, followed by solubilization with urea, thiourea, amidosulfobetaine 14 (ASB 14) and dithiothreitol (DTT) is an effective method, yielding a sample comprising proteins of wide molecular ranges and isoelectric points with minimum contamination from cytosolic proteins. Mass spectrometric analysis was employed to identify the membrane and cell surface proteins present in the sample and consequently an initial proteomic map of membrane and cell wall associated proteins for S. aureus is presented.

Interaction of vitronectin with Haemophilus influenzae

Eight strains of Haemophilus influenzae were tested for binding to human vitronectin. All strains adhered to vitronectin-coated glass slides but no binding was detected using soluble vitronectin, suggesting that surface association of vitronectin is a prerequisite. Vitronectin binding was not likely to be mediated by fimbriae as non-fimbriated and fimbriated isogenic strains adhered equally. Adhesion could be blocked by heparin, which is also known to block vitronectin binding to Staphylococcus aureus. However, no blocking was achieved with sialic acid-rich glycoproteins such as fetuin and mucin contrasting with Helicobacter pylori for which sialic acid seems to play an important role. With Streptococcus pneumoniae binding was detected both with soluble and surface-associated vitronectin and could not be blocked by heparin. Our results suggest that H. influenzae, Streptococcus pneumoniae and Helicobacter pylori all use distinct modes to interact with vitronectin.

Identification and characterization of a novel 38.5-kilodalton cell surface protein of Staphylococcus aureus with extended-spectrum binding activity for extracellular matrix and plasma proteins

The ability to attach to host ligands is a well-established pathogenic factor in invasive Staphylococcus aureus disease. In addition to the family of adhesive proteins bound to the cell wall via the sortase A (srtA) mechanism, secreted proteins such as the fibrinogen-binding protein Efb, the extracellular adhesion protein Eap, or coagulase have been found to interact with various extracellular host molecules. Here we describe a novel protein, the extracellular matrix protein-binding protein (Emp) initially identified in Western ligand blots as a 40-kDa protein due to its broad-spectrum recognition of fibronectin, fibrinogen, collagen, and vitronectin. Emp is expressed in the stationary growth phase and is closely associated with the cell surface and yet is extractable by sodium dodecyl sulfate. The conferring gene emp (1,023 nucleotides) encodes a signal peptide of 26 amino acids and a mature protein of a calculated molecular mass of 35.5 kDa. Using PCR, emp was demonstrated in all 240 S. aureus isolates of a defined clinical strain collection as well as in 6 S. aureus laboratory strains, whereas it is lacking in all 10 S. epidermidis strains tested. Construction of an allelic replacement mutant (mEmp50) revealed the absence of Emp in mEmp50, a significantly decreased adhesion of mEmp50 to immobilized fibronectin and fibrinogen, and restoration of these characteristics upon complementation of mEmp50. Emp expression was also demonstrable upon heterologous complementation of S. carnosus. rEmp expressed in Escherichia coli interacted with fibronectin, fibrinogen, and vitronectin in surface plasmon resonance experiments at a K(d) of 21 nM, 91 nM, and 122 pM, respectively. In conclusion, the biologic characterization of Emp suggests that it is a member of the group of secreted S. aureus molecules that interact with an extended spectrum of host ligands and thereby contribute to S. aureus pathogenicity.

Binding of human clusterin by Staphylococcus epidermidis

To hypothesise that Staphylococcus epidermidis may possess clusterin receptor(s), bacterial binding of human clusterin was determined. In a fluid phase, the binding was markedly influenced by culture medium and three out of 12 S. epidermidis strains grown on ISO-sensitest agar expressed clusterin-binding ability. S. epidermidis J9P, selected for further study, showed saturable binding of iodine-labelled clusterin, and the binding was only inhibited by unlabelled clusterin. The binding was sensitive to protease treatment. Scatchard plot indicated one single class of binding sites (K(d)=104.2 nM). None of the S. epidermidis strains bound immobilised clusterin. These data imply that ligand-receptor interaction exists between S. epidermidis and clusterin in fluid phase, but the domain(s) recognised by bacteria may have been occluded when clusterin was adsorbed on a surface.

Vitronectin interaction with glycosaminoglycans. Kinetics, structural determinants, and role in binding to endothelial cells

Vitronectin (VN) is a high affinity heparin-binding protein. The physiological role of this binding has hitherto received little attention, and its molecular determinants are subject to controversy. In this study, we characterized vitronectin interaction with heparin, heparin analogues, bacterial extracts, and cell surface glycosaminoglycans. As assessed by (i) fluorescence assays, (ii) precipitation with heparin-Sepharose beads, or (iii) Western blotting with antibodies against VN(347-361) (the heparin-binding site), we demonstrate an exposure of the VN heparin-binding site in multimeric but not monomeric vitronectin. Through its heparin-binding site, vitronectin also bound other glycosaminoglycans and Staphylococcus aureus extracts. The kinetics of heparin binding to vitronectin were complex. After a fast association phase (tau = 0.3 s), a slow conversion of an unstable to a stable heparin-vitronectin complex (tau = 180 s) occurred. Heparin binding kinetics and transition to a stable complex were mimicked by VN(347-361), demonstrating that this area is the fully functional heparin-binding site of vitronectin. Multimeric vitronectin bound to endothelial cells. This binding was blocked by soluble heparin and was not observed when endothelial cells were pretreated with glycosaminoglycan-removing enzymes. Glycosaminoglycan-dependent interaction of endothelial cells with multimeric vitronectin might be a relevant mechanism for removal of multimeric vitronectin from plasma. Conversion of an unstable to a stable glycosaminoglycan-vitronectin complex is likely to be relevant for association with endothelial cells under flow conditions.

A lithium chloride-extracted, broad-spectrum-adhesive 42-kilodalton protein of Staphylococcus epidermidis is ornithine carbamoyltransferase

To identify novel putative staphylococcal adhesins, lithium chloride extraction (an established method for selective surface molecule solubilization) was employed. N-terminal sequencing and functional assays identified a 42-kDa fibronectin-binding protein from Staphylococcus epidermidis as ornithine carbamoyltransferase (OCTase). However, OCTase was not recognizable extracellularly, and this fact together with the fact that LiCl induced DNA release and a decrease in viability suggests that LiCl extraction may not be the method of choice for selective surface molecule extraction from staphylococci.

Matrix-binding proteins of Staphylococcus aureus: functional analysis of mutant and hybrid molecules

The fibrinogen-binding protein ClfA and the collagen-binding protein Cna are surface-associated adhesins of Staphylococcus aureus. ClfA has a dipeptide repeat region R composed mainly of serine and aspartate residues, more than 40 of which are required along with the 28-residue region W, the LPXTG motif and region M to display the ligand-binding region A on the cell surface in a functional form. Cna has a 61-residue region W and at least one 187-residue region B linking the collagen-binding region A to peptidoglycan. A cna mutant of S. aureus lacking region B was shown to bind collagen at the same level as wild-type Cna+ cells, indicating that region B is not necessary for ligand binding. Furthermore, altering the number of B repeats did not influence the level of collagen binding. In order to study the ability of C-terminal domains of Cna and ClfA to support functional ligand-binding activity of different adhesins, chimeric proteins were constructed and expressed in S. aureus. Surprisingly, the presence of a single Cna B domain and a nonapeptide linker located between ClfA region A and Cna region WM failed to support fibrinogen binding by S. aureus cells, despite the fact that ClfA region A was detected on the bacterial surface by immunoblotting. In contrast, the ClfA region A-Cna region B hybrid expressed as a recombinant protein in Escherichia coli did bind fibrinogen in Western ligand blots and in an ELISA-type assay. It is concluded that Cna region B cannot support functional display of ClfA region A on the bacterial cell surface. However, the ClfA dipeptide repeat region R and region WM did promote functional surface expression of the Cna collagen-binding domain in a hybrid Cna-ClfA protein.

Transcription of Staphylococcus aureus fibronectin binding protein genes is negatively regulated by agr and an agr-independent mechanism

The production of cell surface proteins in Staphylococcus aureus is generally down-regulated in the postexponential growth phase by the global regulator agr. The effector of this regulation is the RNAIII molecule, which is encoded within the agr locus. RNAIII seems to regulate most target genes at the level of transcription, but it also has an effect on the translation of some genes. To study the role of agr on the expression of fibronectin binding proteins (FnBPs), we investigated the transcription and translation of fnb genes in agr mutant strain WA250 and its parent strain, 8325-4. The results show that fnb genes are negatively regulated by agr and also by an agr-independent mechanism that restricts fnb mRNA synthesis to the early exponential phase of growth. Transcription and Western blot analysis of cell-associated FnBPs demonstrated that synthesis of both FnBPA and FnBPB in the wild-type and agr mutant strains took place preferentially during the first hour of growth and rapidly decreased after the second hour. We also confirmed previous results showing that the agr mutant strain has an increased capacity to bind fibronectin compared to its parent agr+ strain. However, while the concentrations of fnb mRNAs and proteins differed by a factor of 16 between the strains, the difference in fibronectin binding was only twofold, indicating that the binding of fibronectin to the bacteria is not proportional to the amount of FnBPs on their surface.

Clusterin, a putative complement regulator, binds to the cell surface of Staphylococcus aureus clinical isolates

The ability of Staphylococcus aureus Cowan I strain and a number of S. aureus clinical isolates to bind to the human blood glycoprotein clusterin was investigated. Binding of clusterin to these strains was tested by both enzyme-linked immunosorbent assay and flow cytometry. All of the S. aureus strains examined appeared to bind clusterin to some extent, while nonpathogenic control strains Bacillus subtilis BR151 and Escherichia coli JM109 did not. Three S. aureus isolates were selected for more detailed study; binding of labeled clusterin was saturable, inhibited in the presence of excess unlabeled clusterin, and prevented by pretreatment of bacteria with proteases. From the saturation binding studies, estimates of the affinity constants for the binding of clusterin to the bacteria ranged from 31 to 57 nM. Addition of clusterin to S. aureus cultures was also found to result in aggregation of the bacterial cells; aggregation was not detected when clusterin was added to B. subtilis BR151 or E. coli JM109 cultures. These results suggest that at least some S. aureus strains possess specific proteinaceous receptors for clusterin. Such receptors may be an important new bacterial virulence determinant for S. aureus, as clusterin has been proposed to have a role in the regulation of complement activity.