Identification of residues in the Staphylococcus aureus fibrinogen-binding MSCRAMM clumping factor A (ClfA) that are important for ligand binding

Comparative proteomic analysis of vancomycin-sensitive and vancomycin-intermediate resistant Staphylococcus aureus

PURPOSE

The extensive use of vancomycin has led to the development of Staphylococcus aureus strains with varying degrees of resistance to vancomycin. The present study aimed to explore the molecular causes of vancomycin resistance by conducting a proteomics analysis of subcellular fractions isolated from vancomycin-intermediate resistant S. aureus (VISA) and vancomycin-sensitive S. aureus (VSSA) strains.

METHODS

We conducted proteomics analysis of subcellular fractions isolated from 2 isogenic S. aureus strains: strain 11 (VSSA) and strain 11Y (VISA). We used an integrated quantitative proteomics approach assisted by bioinformatics analysis, and comprehensively investigated the proteome profile. Intensive bioinformatics analysis, including protein annotation, functional classification, functional enrichment, and functional enrichment-based cluster analysis, was used to annotate quantifiable targets.

RESULTS

We identified 128 upregulated proteins and 21 downregulated proteins in strain 11Y as compared to strain 11. The largest group of differentially expressed proteins was composed of enzymatic proteins associated with metabolic and catalytic activity, which accounted for 32.1% and 50% of the total proteins, respectively. Some proteins were indispensable parts of the regulatory networks of S. aureus that were altered with vancomycin treatment, and these proteins were related to cell wall metabolism, cell adhesion, proteolysis, and pressure response.

CONCLUSION

Our proteomics study revealed regulatory proteins associated with vancomycin resistance in S. aureus. Some of these proteins were involved in the regulation of cell metabolism and function, which provides potential targets for the development of strategies to manage vancomycin resistance in S. aureus.

Management of Biofilm with Breast Implant Surgery

LEARNING OBJECTIVES

After studying this article, the participant should be able to: 1. Understand how bacteria negatively impact aesthetic and reconstructive breast implants. 2. Understand how bacteria infect breast implants. 3. Understand the evidence associated with common implant infection-prevention strategies, and their limitations. 4. Understand why implementation of bacteria-mitigation strategies such as antibiotic administration or "no-touch" techniques may not indefinitely prevent breast implant infection.

SUMMARY

Bacterial infection of aesthetic and reconstructive breast implants is a common and expensive problem. Subacute infections or chronic capsular contractures leading to device explantation are the most commonly documented sequelae. Although bench and translational research underscores the complexities of implant-associated infection, high-quality studies with adequate power, control groups, and duration of follow-up are lacking. Common strategies to minimize infections use antibiotics-administered systemically, in the breast implant pocket, or by directly bathing the implant before insertion-to limit bacterial contamination. Limiting contact between the implant and skin or breast parenchyma represents an additional common strategy. The clinical prevention of breast implant infection is challenged by the clean-contaminated nature of breast parenchyma, and the variable behavior of not only specific bacterial species but also their strains. These factors impact bacterial virulence and antibiotic resistance.

Effect of savirin in the prevention of biofilm-related Staphylococcus aureus prosthetic joint infection

Background: Most of the arthroplasty surgery failure due to prosthetic joint infections (PJI) is caused by biofilm-associated Staphylococcus aureus. In a recent experimental study, savirin has been used to prevent and treat S. aureus skin infections in animal models. We explored the application of savirin in a PJI mouse model to determine its utility as an adjunct therapy to prevent PJI.

Materials and methods: The in-vitro antibacterial and antibiofilm activity of savirin, with or without antibiotics (cefazolin, rifampicin, and vancomycin), against S. aureus were investigated using broth microdilution and crystal violet staining method, respectively. The effect of savirin treatment on the expression of the key biofilm-related genes (icaA, icaD, eno, fib, ebps, and agr) in S. aureus was studied using quantitative reverse transcriptase polymerase chain reaction (qRTPCR). The in-vivo efficacy of savirin alone and with cefazolin to prevent S. aureus PJI was determined using a clinically relevant PJI mouse model. Mice were randomized into five groups (n = 8/group): 1) infected K-wire savirin treated group, 2) infected K-wire cefazolin treated group, 3) infected K-wire savirin plus cefazolin treated group, 4) infected K-wire PBS treated group, 5) sterile K-wire group. Savirin was administered subcutaneously immediately post-surgery and intravenous cefazolin was given on day seven.

Results: Savirin inhibited planktonic and biofilm in-vitro growth of S. aureus, showed enhanced inhibitory activity when combined with antibiotics, and down-regulated the expression of key S. aureus biofilm-related genes (icaA, icaD, eno, fib, ebps, and agr). Savirin significantly reduced bacterial counts on joint implants in comparison with the PBS treated control, while savirin plus cefazolin reduced bacterial counts on both implants and peri-prosthetic tissues.

Conclusion: Savirin adjuvant therapy may prevent biofilm formation and S. aureus PJI. This study gives baseline data for using savirin for the prevention as well as treatment of S. aureus PJI in future animal studies.

Non-Antimicrobial Adjuvant Therapy Using Ticagrelor Reduced Biofilm-Related Staphylococcus aureus Prosthetic Joint Infection

Background: Prosthetic joint infection (PJI), frequently caused by Staphylococcus aureus, leads to a significant arthroplasty failure rate. Biofilm is a crucial virulence factor of S. aureus that is intrinsic to the pathogenesis of PJI. Biofilm-related infections are recalcitrant to antibiotic treatment. Surgical and antibiotic therapy could be combined with non-antibacterial adjuvants to improve overall treatment success. Ticagrelor, a P2Y12 receptor inhibitor antiplatelet drug, is known to have anti-staphylococcal antibacterial and antibiofilm activity. However, the molecular mechanism for ticagrelor’s antibiofilm activity and its efficacy in the treatment of S. aureus PJI are unknown.

Methods: To study the in vitro antibacterial and antibiofilm activity of ticagrelor, broth microdilution and crystal violet staining method were used. Ticagrelor’s effect on the expression of S. aureus biofilm genes (icaA, icaD, ebps, fib, eno, and agr) was studied using the relative quantification method. To test ticagrelor’s in vivo efficacy to treat S. aureus PJI, mice were randomized into five groups (n = 8/group): infected femoral implants treated with ticagrelor alone; infected implants treated with cefazolin alone; infected implants treated with ticagrelor and cefazolin; infected implants treated with phosphate buffer solution (PBS)-positive controls, and sterile implants-negative controls. Ticagrelor was administered orally from day 4 to day 7 post-surgery, while cefazolin was injected intravenously on day 7.

Results: Ticagrelor, alone and with selected antibiotics, showed in vitro antibacterial and antibiofilm activity against S. aureus. Strain-specific downregulation of biofilm-related genes, fib, icaD, ebps, and eno, was shown. In an animal model of biofilm-related S. aureus PJI, ticagrelor alone and combined with cefazolin significantly reduced bacterial concentrations on the implants compared with the positive control group. Ticagrelor significantly reduced bacterial dissemination to periprosthetic tissue compared with the positive controls.

Conclusion: Ticagrelor adjuvant therapy reduced S. aureus PJI in an animal model. However, this study is very preliminary to make a conclusion on the clinical implication of the findings. Based on the current results, more studies are recommended to better understand its implication.

Allantodapsone is a Pan-Inhibitor of Staphylococcus aureus Adhesion to Fibrinogen, Loricrin, and Cytokeratin 10

Staphylococcus aureus infections have become a major challenge in health care due to increasing antibiotic resistance. We aimed to design small molecule inhibitors of S. aureus surface proteins to be developed as colonization inhibitors. We identified allantodapsone in an initial screen searching for inhibitors of clumping factors A and B (ClfA and ClfB). We used microbial adhesion assays to investigate the effect of allantodapsone on extracellular matrix protein interactions. Allantodapsone inhibited S. aureus Newman adhesion to fibrinogen with an IC50 of 21.3 μM (95% CI 4.5-102 μM), minimum adhesion inhibitory concentration (MAIC) of 100 μM (40.2 μg/mL). Additionally, allantodapsone inhibited adhesion of Lactococcus lactis strains exogenously expressing the clumping factors to fibrinogen (L. lactis ClfA, IC50 of 3.8 μM [95% CI 1.0-14.3 μM], MAIC 10 μM, 4.0 μg/mL; and L. lactis ClfB, IC50 of 11.0 μM [95% CI 0.9-13.6 μM], MAIC 33 μM, 13.3 μg/mL), indicating specific inhibition. Furthermore, the dapsone and alloxan fragments of allantodapsone did not have any inhibitory effect. Adhesion of S. aureus Newman to L2v loricrin is dependent on the expression of ClfB. Allantodapsone caused a dose dependent inhibition of S. aureus adhesion to the L2v loricrin fragment, with full inhibition at 40 μM (OD600 0.11 ± 0.01). Furthermore, recombinant ClfB protein binding to L2v loricrin was inhibited by allantodapsone (P < 0.0001). Allantodapsone also demonstrated dose dependent inhibition of S. aureus Newman adhesion to cytokeratin 10 (CK10). Allantodapsone is the first small molecule inhibitor of the S. aureus clumping factors with potential for development as a colonization inhibitor. IMPORTANCE S. aureus colonization of the nares and the skin provide a reservoir of bacteria that can be transferred to wounds that can ultimately result in systemic infections. Antibiotic resistance can make these infections difficult to treat with significant associated morbidity and mortality. We have identified and characterized a first-in-class small molecule inhibitor of the S. aureus clumping factors A and B, which has the potential to be developed further as a colonization inhibitor.

The prevalence of virulence determinants in methicillin-resistant Staphylococcus aureus isolated from different infections in hospitalized patients in Poland

Methicillin-resistant Staphylococcus aureus (MRSA) is responsible for hard-to-treat infections. The presence of 19 virulence genes in 120 MRSA isolates obtained from hospitalized patients and genetic relationships of these isolates were investigated. The eno (100%) and ebps (93.3%) genes encoding laminin- and elastin binding proteins, respectively, were ubiquitous. Other adhesion genes: fib (77.5%), fnbB (41.6%), bbp (40.8%), cna (30.8%) encoding proteins binding fibrinogen, fibronectin, bone sialoprotein and collagen, respectively, and map/eap (62.5%), encoding Eap, were also frequent. The etB and etD genes, encoding exfoliative toxins, were present in 15.6% and 12.5% isolates, respectively. The splA, splE and sspA, encoding serine protease were detected in 100%, 70.8% and 94.2% isolates, respectively. The tst gene, encoding toxic shock syndrome toxin-1 was found in 75% isolates. The cna, map/eap and tst genes were the most common in wound isolates and much less common in blood isolates. We identified 45 different spa types, t003 (21.7%) and t008 (18.8%) being the most common. The t003 was the most frequent among isolates from the respiratory tract (35.5%), while t008 in blood isolates (40%). Identification of virulence factors of MRSA is important for evaluation of pathogen transmission rate and disease development.

Structure-function characterization of Streptococcus intermedius surface antigen Pas

Streptococcus intermedius, an oral commensal bacterium, is found at various sites including subgingival dental plaque, purulent infections, and in cystic fibrosis lungs. Oral streptococci utilize proteins on their surface to adhere to tissues and/or surfaces localizing the bacteria, which subsequently leads to the development of biofilms, colonization and infection. Among the 19 genomically annotated cell-wall attached surface proteins on S. intermedius, Pas is an adhesin that belongs to the Antigen I/II (AgI/II) family. Here we have structurally and functionally characterized Pas, particularly focusing on its microbial-host as well as microbial-microbial interactions. The crystal structures of V^Pas and C₁₂₃^Pas show high similarity with AgI/II of S. mutans. V^Pas hosts a conserved metal binding site, and likewise the C₁₂₃^Pas structure retains its conserved metal binding sites and isopeptide bonds within its three DEv-IgG domains. Pas interacts with nanomolar affinity to lung alveolar glycoprotein 340 (Gp340), its scavenger receptor cysteine rich domains (SRCRs) and with fibrinogen. Both Candida albicans and Pseudomonas aeruginosa, the opportunistic pathogens that cohabitate with S. intermedius in the lungs of CFTR patients were studied in dual-species biofilm studies. The Pas deficient mutant (Δpas) displayed significant reduction in dual biofilm formation with C. albicans. In similar studies with P. aeruginosa, Pas did not mediate the biofilm formation with either the acute isolate (PAO1), or the chronic isolate (FRD1). However, the Sortase A deficient mutant (ΔsrtA) displayed reduced biofilm formation with both C. albicans and P. aeruginosa FRD1. Taken together, our findings highlight the role of Pas in both microbial-host and interkingdom interactions and expose its potential role in disease outcomes. Importance Streptococcus intermedius, an oral commensal bacterium, has been clinically observed in subgingival dental plaque, purulent infections, and in cystic fibrosis lungs. In this study, we have (a) determined the crystal structure of the V- and C-regions of Pas; (b) shown that its surface protein Pas adheres to fibrinogen, which could potentially ferry the microbe through the blood stream from the oral cavity; (c) characterized Pas's high affinity adherence to lung alveolar protein Gp340 that could fixate the microbe on lung epithelial cells; and (d) most importantly shown that these surface proteins on the oral commensal S. intermedius enhances biofilms of known pathogens Candida albicans and Pseudomonas aeruginosa.

Host fibrinogen drives antimicrobial function in Staphylococcus aureus peritonitis through bacterial-mediated prothrombin activation

The blood-clotting protein fibrinogen has been implicated in host defense following Staphylococcus aureus infection, but precise mechanisms of host protection and pathogen clearance remain undefined. Peritonitis caused by staphylococci species is a complication for patients with cirrhosis, indwelling catheters, or undergoing peritoneal dialysis. Here, we sought to characterize possible mechanisms of fibrin(ogen)-mediated antimicrobial responses. Wild-type (WT) (Fib+) mice rapidly cleared S. aureus following intraperitoneal infection with elimination of ∼99% of an initial inoculum within 15 min. In contrast, fibrinogen-deficient (Fib-) mice failed to clear the microbe. The genotype-dependent disparity in early clearance resulted in a significant difference in host mortality whereby Fib+ mice uniformly survived whereas Fib- mice exhibited high mortality rates within 24 h. Fibrin(ogen)-mediated bacterial clearance was dependent on (pro)thrombin procoagulant function, supporting a suspected role for fibrin polymerization in this mechanism. Unexpectedly, the primary host initiator of coagulation, tissue factor, was found to be dispensable for this antimicrobial activity. Rather, the bacteria-derived prothrombin activator vWbp was identified as the source of the thrombin-generating potential underlying fibrin(ogen)-dependent bacterial clearance. Mice failed to eliminate S. aureus deficient in vWbp, but clearance of these same microbes in WT mice was restored if active thrombin was administered to the peritoneal cavity. These studies establish that the thrombin/fibrinogen axis is fundamental to host antimicrobial defense, offer a possible explanation for the clinical observation that coagulase-negative staphylococci are a highly prominent infectious agent in peritonitis, and suggest caution against anticoagulants in individuals susceptible to peritoneal infections.

Effect of manuka honey on biofilm-associated genes expression during methicillin-resistant Staphylococcus aureus biofilm formation

Methicillin-resistant Staphylococcus aureus (MRSA) are among the most important biofilm-forming pathogens responsible for hard-to-treat infections. Looking for alternatives to antibiotics that prevent biofilm formation, we investigated the effects of manuka honey on the transcriptional profile of genes essential for staphylococcal biofilm formation using qRT-PCR. mRNA from two hospital MRSA strains (strong and weak biofilm producer) were isolated after 4, 8, 12 and 24 h from cells grown in biofilm. Manuka honey at 1/2 minimum biofilm inhibition concentration (MBIC) significantly reduced MRSA cell viability in biofilm. Manuka honey downregulated the genes encoding laminin- (eno), elastin- (ebps) and fibrinogen binding protein (fib), and icaA and icaD involved in biosynthesis of polysaccharide intercellular adhesin in both weakly and strongly adhering strain compared to the control (untreated biofilm). Expression levels of cna (collagen binding protein) and map/eap (extracellular adherence protein—Eap) were reduced in weakly adhering strain. The lowest expression of investigated genes was observed after 12 h of manuka honey treatment at 1/2 MBIC. This study showed that the previously unknown mechanism of manuka honey action involved inhibition of S. aureus adhesion due to reduction in expression of crucial genes associated with staphylococcal biofilm.

Deposition of Host Matrix Proteins on Breast Implant Surfaces Facilitates Staphylococcus Epidermidis Biofilm Formation: In Vitro Analysis

BACKGROUND

Staphylococcus epidermidis is a primary cause of breast implant-associated infection. S epidermidis possesses several virulence factors that enable it to bind both abiotic surfaces and host factors to form a biofilm. In addition S epidermidis colocalizes with matrix proteins coating explanted human breast implants.

OBJECTIVES

The authors sought to identify matrix proteins that S epidermidis may exploit to infect various breast implant surfaces in vitro.

METHODS

A combination of in vitro assays was used to characterize S epidermidis strains isolated from human breast implants to gain a better understanding of how these bacteria colonize breast implant surfaces. These included determining the (1) minimum inhibitory and bactericidal concentrations for irrigation solutions commonly used to prevent breast implant contamination; (2) expression and carriage of polysaccharide intercellular adhesin and serine-aspartate repeat proteins, which bind fibrinogen (SdrG) and collagen (SdrF), respectively; and (3) biofilm formation on varying implant surface characteristics, in different growth media, and supplemented with fibrinogen and Types I and III collagen. Scanning electron microscopy and immunofluorescence staining analyses were performed to corroborate findings from these assays.

RESULTS

Textured breast implant surfaces support greater bacterial biofilm formation at baseline, and the addition of collagen significantly increases biomass on all surfaces tested. We found that S epidermidis isolated from breast implants all encoded SdrF. Consistent with this finding, these strains had a clear affinity for Type I collagen, forming dense, highly structured biofilms in its presence.

CONCLUSIONS

The authors found that S epidermidis may utilize SdrF to interact with Type I collagen to form biofilm on breast implant surfaces.

LEVEL OF EVIDENCE: 5

Effect of trans-Cinnamaldehyde on Methicillin-Resistant Staphylococcus aureus Biofilm Formation: Metabolic Activity Assessment and Analysis of the Biofilm-Associated Genes Expression

The effects of trans-cinnamaldehyde (TC) on transcriptional profiles of biofilm-associated genes and the metabolic activity of two methicillin-resistant Staphylococcus aureus (MRSA) strains showing a different degree of adherence to polystyrene, were evaluated. Metabolic activity of S. aureus in biofilm was significantly decreased in the presence of TC at 1/2 minimum biofilm inhibition concentration (MBIC). Expression levels of the genes encoding laminin binding protein (eno), elastin binding protein (ebps) and fibrinogen binding protein (fib) in the presence of TC at 1/2 MBIC were lower than in untreated biofilm in both the weakly and strongly adhering strain. The highest decrease of expression level was observed in case of fib in the strongly adhering strain, in which the amount of fib transcript was 10-fold lower compared to biofilm without TC. In the presence of TC at 1/2 MBIC after 3, 6, 8 and 12 h, the expression level of icaA and icaD, that are involved in the biosynthesis of polysaccharide intercellular adhesin, was above half lower in the weakly adhering strain compared to biofilm without TC. In the strongly adhering strain the highest decrease in expression of these genes was observed after 3 and 6 h. This study showed that TC is a promising anti-biofilm agent for use in MRSA biofilm-related infections.

Targeted interplay between bacterial pathogens and host autophagy

Due to the critical role played by autophagy in pathogen clearance, pathogens have developed diverse strategies to subvert it. Despite previous key findings of bacteria-autophagy interplay, asystems-level insight into selective targeting by the host and autophagy modulation by the pathogens is lacking. We predicted potential interactions between human autophagy proteins and effector proteins from 56 pathogenic bacterial species by identifying bacterial proteins predicted to have recognition motifs for selective autophagy receptors SQSTM1/p62, CALCOCO2/NDP52 and MAP1LC3/LC3. Using structure-based interaction prediction, we identified bacterial proteins capable to modify core autophagy components. Our analysis revealed that autophagy receptors in general potentially target mostly genus-specific proteins, and not those present in multiple genera. The complementarity between the predicted SQSTM1/p62 and CALCOCO2/NDP52 targets, which has been shown for Salmonella, Listeria and Shigella, could be observed across other pathogens. This complementarity potentially leaves the host more susceptible to chronic infections upon the mutation of autophagy receptors. Proteins derived from enterotoxigenic and non-toxigenic Bacillus outer membrane vesicles indicated that autophagy targets pathogenic proteins rather than non-pathogenic ones. We also observed apathogen-specific pattern as to which autophagy phase could be modulated by specific genera. We found intriguing examples of bacterial proteins that could modulate autophagy, and in turn being targeted by autophagy as ahost defense mechanism. We confirmed experimentally an interplay between a Salmonella protease, YhjJ and autophagy. Our comparative meta-analysis points out key commonalities and differences in how pathogens could affect autophagy and how autophagy potentially recognizes these pathogenic effectors. Abbreviations: ATG5: autophagy related 5; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; GST: glutathione S-transferase; LIR: MAP1LC3/LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3 alpha; OMV: outer membrane vesicles; SQSTM1/p62: sequestosome 1; SCV: Salmonella containing vesicle; TECPR1: tectonin beta-propeller repeat containing 1; YhjJ: hypothetical zinc-protease.

Expression of the Biofilm-Associated Genes in Methicillin-Resistant Staphylococcus aureus in Biofilm and Planktonic Conditions

The role of genes that are essential for development of Staphylococcus aureus biofilm during infection is not fully known. mRNA from two methicillin-resistant S. aureus strains that formed weak and strong biofilm on polystyrene plates were isolated at five time points from cells grown in biofilm and planktonic culture. Quantitative real-time PCR analysis showed that the expression levels of investigated genes under biofilm conditions were significantly higher than under planktonic conditions. The expression levels of the gene encoding elastin binding protein (ebps) and laminin binding protein (eno) were significantly increased in biofilm at 3 h, both in strongly and weakly adhering strain. The peak expression of fib gene encoding fibrinogen binding protein was found at 6 and 8 h in the case of strongly and weakly adhering strain, respectively. The expression of icaA and icaD genes in both strains was significantly higher under biofilm conditions when comparing to planktonic cells during 12 h. The expression level of the genes encoding binding proteins and the glucosamine polymer polysaccharide intercellular adhesin (PIA) slowly decreased after 24 h. Finally, we found that the expression levels of genes encoding binding factors in weakly adhering strain were significantly lower than in strongly adhering strain.

Transcriptional regulation of virulence factors Spa and ClfB by the SpoVG-Rot cascade in Staphylococcus aureus

Staphylococcus aureus can produce numerous surface proteins involved in the adhesion and internalization of host cells, immune evasion, and inflammation initiation. Among these surface proteins, the microbial surface components recognizing adhesive matrix molecules contain many crucial cell wall-anchored virulence factors. The Sar-family regulatory protein Rot has been reported to regulate many important extracellular virulence factors at the transcriptional level, including Spa and clumping factor B. SpoVG, a global regulator in S. aureus, is known to control the expression of numerous genes. Here, we demonstrate that SpoVG can positively regulate the transcription of rot by directly binding to its promoter. SpoVG can also positively regulate the transcription of spa and clfB through direct-binding to their promoters and in a Rot-mediated manner. Furthermore, SpoVG can positively modulate the human fibrinogen-binding ability of S. aureus. In addition, phosphorylation of SpoVG by the serine/threonine kinase, Stk1, can positively regulate its binding to the promoters of rot, spa, and clfB. The human cell infection assay showed that the adhesion and internalization abilities were reduced in the spoVG mutant strain in comparison to those in the wild-type strain. Collectively, our data reveal a SpoVG-Rot regulatory cascade and novel molecular mechanisms in the virulence control in S. aureus.

Human Fibrinogen Inhibits Amyloid Assembly of Biofilm-Forming CsgA

Curli is a biofilm-forming amyloid that is expressed on the surface of Gram-negative enteric bacteria such as Escherichia coli and Salmonella spp. Curli is primarily composed of the major structural subunit, CsgA, and interacts with a wide range of human proteins that contribute to bacterial virulence. The adsorption of curli onto the contact-phase proteins and fibrinogen results in a hypocoagulatory state. Using an array of biochemical and biophysical tools, we elucidated the molecular mechanism of interaction between human fibrinogen and CsgA. Our results revealed that a substoichiometric concentration of fibrinogen delays the onset of CsgA aggregation by inhibiting the early events of CsgA assembly. The presence of fibrinogen prevents the maturation of CsgA into fibrils and maintains the soluble state of CsgA. We also demonstrate that fibrinogen interacts more effectively with the disordered conformational state of CsgA than with the ordered β-rich state. Our study suggested that fibrinogen is an anti-curli protein and that the interplay of CsgA and fibrinogen might be a host defense mechanism against curli biogenesis, biofilm formation, bacterial colonization, and infection.

Staphylococcus aureus clumping factor A is a force-sensitive molecular switch that activates bacterial adhesion

The Staphylococcus aureus surface protein clumping factor A (ClfA) binds to the blood plasma protein fibrinogen (Fg) via molecular interactions that are poorly understood. Here, we unravel the forces guiding the interaction between ClfA and immobilized Fg, showing that it is dramatically enhanced by tensile loading. Our findings favor a model whereby ClfA interacts with Fg via two distinct binding sites, the adhesive function of which is tightly regulated by mechanical force. Reminiscent of a catch bond mechanism, this force-enhanced adhesion explains the ability of ClfA to promote S. aureus colonization of host tissues and biomedical devices under physical stress.

Clumping factor A (ClfA), a cell-wall–anchored protein from Staphylococcus aureus, is a virulence factor in various infections and facilitates the colonization of protein-coated biomaterials. ClfA promotes bacterial adhesion to the blood plasma protein fibrinogen (Fg) via molecular forces that have not been studied so far. A unique, yet poorly understood, feature of ClfA is its ability to favor adhesion to Fg at high shear stress. Unraveling the strength and dynamics of the ClfA–Fg interaction would help us better understand how S. aureus colonizes implanted devices and withstands physiological shear stress. By means of single-molecule experiments, we show that ClfA behaves as a force-sensitive molecular switch that potentiates staphylococcal adhesion under mechanical stress. The bond between ClfA and immobilized Fg is weak (∼0.1 nN) at low tensile force, but is dramatically enhanced (∼1.5 nN) by mechanical tension, as observed with catch bonds. Strong bonds, but not weak ones, are inhibited by a peptide mimicking the C-terminal segment of the Fg γ-chain. These results point to a model whereby ClfA interacts with Fg via two distinct binding sites, the adhesive function of which is regulated by mechanical tension. This force-activated mechanism is of biological significance because it explains at the molecular level the ability of ClfA to promote bacterial attachment under high physiological shear stress.

Staphylococcus Biofilm Components as Targets for Vaccines and Drugs

Staphylococci have become the most common cause of nosocomial infections, especially in patients with predisposing factors such as indwelling or implanted foreign polymer bodies. The pathogenesis of foreign-body associated infections with S. aureus and S. epidermidis is mainly related to the ability of these bacteria to form thick, adherent multilayered biofilms. In a biofilm, staphylococci are protected against antibiotic treatment and attack from the immune system, thus making eradication of the infections problematic. This necessitates the discovery of novel prophylactic and therapeutic strategies to treat these infections. In this review, we provide an overview of staphylococcal biofilm components and discuss new possible approaches to controlling these persistent biofilm-dwelling bacteria.

Inhibition of bacterial adhesion and biofilm formation by dual functional textured and nitric oxide releasing surfaces

In separate prior studies, physical topographic surface modification or nitric oxide (NO) release has been demonstrated to each be an effective approach to inhibit and control bacterial adhesion and biofilm formation on polymeric surfaces. Such approaches can prevent biomaterial-associated infection without causing the antibiotic resistance of the strain. In this work, both techniques were successfully integrated and applied to a polyurethane (PU) biomaterial surface that bears ordered pillar topographies (400/400nm and 500/500nm patterns) at the top surface and a S-nitroso-N-acetylpenicillamine (SNAP, NO donor) doped sub-layer in the middle, via a soft lithography two-stage replication process. Upon placing the SNAP textured PU films into PBS at 37°C, the decomposition of SNAP within polymer film initiates NO release with a lifetime of up to 10days at flux levels >0.5×10^-10molmin^-1cm^-2 for a textured polyurethane layer containing 15wt% SNAP. The textured surface reduces the accessible surface area and the opportunity of bacteria-surface interaction, while the NO release from the same surface further inhibits bacterial growth and biofilm formation. Such dual functionality surfaces are shown to provide a synergistic effect on inhibition of Staphylococcus epidermidis bacterial adhesion that is significantly greater than the inhibition of bacterial adhesion achieved by either single treatment approach alone. Longer term experiments to observe biofilm formation demonstrate that the SNAP doped-textured PU surface can inhibit the biofilm formation for >28d and provide a practical approach to improve the biocompatibility of current biomimetic biomaterials and thereby reduce the risk of pathogenic infection.

STATEMENT OF SIGNIFICANCE

Microbial infection remains a significant barrier to development and implementation of advanced blood-contacting medical devices. Clearly, determining how to design and control material properties that can reduce microbial infection is a central question to biomaterial researchers. In separate prior studies, physical topographic surface modification or nitric oxide (NO) release has been demonstrated to each be an effective approach to inhibit and control bacterial adhesion and biofilm formation on polymeric surfaces. Such approaches can prevent biomaterial-associated infection without causing antibiotic resistance of the bacterial strain. However, efficiency of antimicrobial properties of each approach is still limited and far from sufficient for widespread clinical use. This work successfully integrates both techniques and applies them to a polyurethane (PU) biomaterial surface that bears dual functions, surface topographic modification and NO release. The former reduces the surface contact area and changes surface wettability, resulting in reduction of bacterial adhesion, and NO release further inhibits bacteria growth. Such dual functionalized surfaces provide a synergistic effect on inhibition of Staphylococcus epidermidis bacterial adhesion that is significantly greater than the inhibition of bacterial adhesion achieved by either single treatment approach alone. Furthermore, longer-term experiments demonstrate that the dual functionalized surfaces can inhibit biofilm formation for >28days. The success of this work provides a practical approach to improve the biocompatibility of current biomaterials and thereby reduce the risk of pathogenic infection.

Fibrinogen Is at the Interface of Host Defense and Pathogen Virulence in Staphylococcus aureus Infection

Fibrinogen not only plays a pivotal role in hemostasis but also serves key roles in antimicrobial host defense. As a rapidly assembled provisional matrix protein, fibrin(ogen) can function as an early line of host protection by limiting bacterial growth, suppressing dissemination of microbes to distant sites, and mediating host bacterial killing. Fibrinogen-mediated host antimicrobial activity occurs predominantly through two general mechanisms, namely, fibrin matrices functioning as a protective barrier and fibrin(ogen) directly or indirectly driving host protective immune function. The potential of fibrin to limit bacterial infection and disease has been countered by numerous bacterial species evolving and maintaining virulence factors that engage hemostatic system components within vertebrate hosts. Bacterial factors have been isolated that simply bind fibrinogen or fibrin, promote fibrin polymer formation, or promote fibrin dissolution. Staphylococcus aureus is an opportunistic gram-positive bacterium, the causative agent of a wide range of human infectious diseases, and a prime example of a pathogen exquisitely sensitive to host fibrinogen. Indeed, current data suggest fibrinogen serves as a context-dependent determinant of host defense or pathogen virulence in Staphylococcus infection whose ultimate contribution is dictated by the expression of S. aureus virulence factors, the path of infection, and the tissue microenvironment.

Protection of mice against Staphylococcus aureus infection by a recombinant protein ClfA-IsdB-Hlg as a vaccine candidate

Staphylococcus aureus is one of the most important causes of nosocomial infections. An effective vaccine to prevent S. aureus infections is urgently required due to the dramatic increase in the number of antibiotic-resistant strains. In this report, we evaluated a newly recombinant protein composed of selected antigenic regions of clumping factor A (ClfA), iron surface determinant B (IsdB) and gamma hemolysin B (HlgB) of S. aureus and sequence coding for hydrophobic linkers between three domains. The recombinant gene was constructed in pET-28a (+) and expressed in Escherichia coli BL21. In addition, sequence coding for a His(6)-tag was added followed by a hybrid procedure of nickel chelate protein purification. Immunization of BALB/c mice with the recombinant protein ClfA-IsdB-Hlg evoked antigen-specific antibodies that could opsonize S. aureus cells, enhancing in vitro phagocytosis by macrophages. Vaccination with the recombinant protein also reduced the bacterial load recovered from mice spleen samples and increased survival following the intraperitoneal challenge with pathogenic S. aureus compared to the control mice. Our results showed that the recombinant protein ClfA-IsdB-Hlg is a promising vaccine candidate for the prevention of S. aureus bacteremia infections.

Giving structure to the biofilm matrix: an overview of individual strategies and emerging common themes

Biofilms are communities of microbial cells that underpin diverse processes including sewage bioremediation, plant growth promotion, chronic infections and industrial biofouling. The cells resident in the biofilm are encased within a self-produced exopolymeric matrix that commonly comprises lipids, proteins that frequently exhibit amyloid-like properties, eDNA and exopolysaccharides. This matrix fulfils a variety of functions for the community, from providing structural rigidity and protection from the external environment to controlling gene regulation and nutrient adsorption. Critical to the development of novel strategies to control biofilm infections, or the capability to capitalize on the power of biofilm formation for industrial and biotechnological uses, is an in-depth knowledge of the biofilm matrix. This is with respect to the structure of the individual components, the nature of the interactions between the molecules and the three-dimensional spatial organization. We highlight recent advances in the understanding of the structural and functional role that carbohydrates and proteins play within the biofilm matrix to provide three-dimensional architectural integrity and functionality to the biofilm community. We highlight, where relevant, experimental techniques that are allowing the boundaries of our understanding of the biofilm matrix to be extended using Escherichia coli, Staphylococcus aureus, Vibrio cholerae, and Bacillus subtilis as exemplars.

Examining the structure and function of the biofilm extracellular matrix.

In silico analysis for identifying potential vaccine candidates against Staphylococcus aureus

Purpose

Staphylococcus aureus is one of the most important causes of nosocomial and community-acquired infections. The increasing incidence of multiple antibiotic-resistant S. aureus strains and the emergence of vancomycin resistant S. aureus strains have placed renewed interest on alternative means of prevention and control of infection. S. aureus produces a variety of virulence factors, so a multi-subunit vaccine will be more successful for preventing S. aureus infections than a mono-subunit vaccine.

Materials and Methods

We selected three important virulence factors of S. aureus, clumping factor A (ClfA), iron-regulated surface determinant (IsdB), and gamma hemolysin (Hlg) that are potential candidates for vaccine development. We designed synthetic genes encoding the clfA, isdB, and hlg and used bioinformatics tools to predict structure of the synthetic construct and its stabilities. VaxiJen analysis of the protein showed a high antigenicity. Linear and conformational B-cell epitopes were identified.

Results

The proteins encoded by these genes were useful as vaccine candidates against S. aureus infections.

Conclusion

In silico tools are highly suited to study, design, and evaluate vaccine strategies.

Characterization of fibrinogen binding by glycoproteins Srr1 and Srr2 of Streptococcus agalactiae

The serine-rich repeat glycoproteins of Gram-positive bacteria comprise a large family of cell wall proteins. Streptococcus agalactiae (group B streptococcus, GBS) expresses either Srr1 or Srr2 on its surface, depending on the strain. Srr1 has recently been shown to bind fibrinogen, and this interaction contributes to the pathogenesis of GBS meningitis. Although strains expressing Srr2 appear to be hypervirulent, no ligand for this adhesin has been described. We now demonstrate that Srr2 also binds human fibrinogen and that this interaction promotes GBS attachment to endothelial cells. Recombinant Srr1 and Srr2 bound fibrinogen in vitro, with affinities of K_D = 2.1 × 10⁻⁵ and 3.7 × 10⁻⁶m, respectively, as measured by surface plasmon resonance spectroscopy. The binding site for Srr1 and Srr2 was localized to tandem repeats 6–8 of the fibrinogen Aα chain. The structures of both the Srr1 and Srr2 binding regions were determined and, in combination with mutagenesis studies, suggest that both Srr1 and Srr2 interact with a segment of these repeats via a “dock, lock, and latch” mechanism. Moreover, properties of the latch region may account for the increased affinity between Srr2 and fibrinogen. Together, these studies identify how greater affinity of Srr2 for fibrinogen may contribute to the increased virulence associated with Srr2-expressing strains.

Structures of SdrD from Staphylococcus aureus reveal the molecular mechanism of how the cell surface receptors recognize their ligands

Staphylococcus aureus is the most important Gram-positive colonizer of human skin and nasal passage, causing high morbidity and mortality. SD-repeat containing protein D (SdrD), an MSCRAMM (Microbial Surface Components Recognizing Adhesive Matrix Molecules) family surface protein, plays an important role in S. aureus adhesion and pathogenesis, while its binding target and molecular mechanism remain largely unknown. Here we solved the crystal structures of SdrD N2-N3 domain and N2-N3-B1 domain. Through structural analysis and comparisons, we characterized the ligand binding site of SdrD, and proposed a featured sequence motif of its potential ligands. In addition, the structures revealed for the first time the interactions between B1 domain and N2-N3 domain among B domain-containing MSCRAMMs. Our results may help in understanding the roles SdrD plays in S. aureus adhesion and shed light on the development of novel antibiotics.

Novel inhibitors of Staphylococcus aureus virulence gene expression and biofilm formation

Staphylococcus aureus is a major human pathogen and one of the more prominent pathogens causing biofilm related infections in clinic. Antibiotic resistance in S. aureus such as methicillin resistance is approaching an epidemic level. Antibiotic resistance is widespread among major human pathogens and poses a serious problem for public health. Conventional antibiotics are either bacteriostatic or bacteriocidal, leading to strong selection for antibiotic resistant pathogens. An alternative approach of inhibiting pathogen virulence without inhibiting bacterial growth may minimize the selection pressure for resistance. In previous studies, we identified a chemical series of low molecular weight compounds capable of inhibiting group A streptococcus virulence following this alternative anti-microbial approach. In the current study, we demonstrated that two analogs of this class of novel anti-virulence compounds also inhibited virulence gene expression of S. aureus and exhibited an inhibitory effect on S. aureus biofilm formation. This class of anti-virulence compounds could be a starting point for development of novel anti-microbial agents against S. aureus.

Binding of glycoprotein Srr1 of Streptococcus agalactiae to fibrinogen promotes attachment to brain endothelium and the development of meningitis

The serine-rich repeat glycoprotein Srr1 of Streptococcus agalactiae (GBS) is thought to be an important adhesin for the pathogenesis of meningitis. Although expression of Srr1 is associated with increased binding to human brain microvascular endothelial cells (hBMEC), the molecular basis for this interaction is not well defined. We now demonstrate that Srr1 contributes to GBS attachment to hBMEC via the direct interaction of its binding region (BR) with human fibrinogen. When assessed by Far Western blotting, Srr1 was the only protein in GBS extracts that bound fibrinogen. Studies using recombinant Srr1-BR and purified fibrinogen in vitro confirmed a direct protein-protein interaction. Srr1-BR binding was localized to amino acids 283–410 of the fibrinogen Aα chain. Structural predictions indicated that the conformation of Srr1-BR is likely to resemble that of SdrG and other related staphylococcal proteins that bind to fibrinogen through a “dock, lock, and latch” mechanism (DLL). Deletion of the predicted latch domain of Srr1-BR abolished the interaction of the BR with fibrinogen. In addition, a mutant GBS strain lacking the latch domain exhibited reduced binding to hBMEC, and was significantly attenuated in an in vivo model of meningitis. These results indicate that Srr1 can bind fibrinogen directly likely through a DLL mechanism, which has not been described for other streptococcal adhesins. This interaction was important for the pathogenesis of GBS central nervous system invasion and subsequent disease progression.

Streptococcus agalactiae (Group B streptococcus, GBS) is a leading cause of meningitis in newborns and infants. This life-threatening infection of the brain and surrounding tissues continues to result in a high incidence of morbidity and mortality, despite antibiotic therapy. A key factor in disease production is the ability of this organism to invade the central nervous system, via the bloodstream. We now report that a GBS surface protein called Srr1 binds fibrinogen, a major protein in human blood. This interaction enhances the attachment of GBS to brain vascular endothelial cells, and contributes to the development of meningitis. A mutation in Srr1 that specifically disrupted binding to fibrinogen significantly reduced GBS attachment to brain endothelium, and markedly reduced virulence in an in vivo model of GBS disease. These studies have identified a new mechanism by which Srr1 contributes to GBS invasion of the central nervous system and may provide a basis for novel therapies targeting Srr1 binding.

Leptospiral outer membrane protein microarray, a novel approach to identification of host ligand-binding proteins

Leptospirosis is a zoonosis with worldwide distribution caused by pathogenic spirochetes belonging to the genus Leptospira. The leptospiral life cycle involves transmission via freshwater and colonization of the renal tubules of their reservoir hosts. Infection requires adherence to cell surfaces and extracellular matrix components of host tissues. These host-pathogen interactions involve outer membrane proteins (OMPs) expressed on the bacterial surface. In this study, we developed an Leptospira interrogans serovar Copenhageni strain Fiocruz L1-130 OMP microarray containing all predicted lipoproteins and transmembrane OMPs. A total of 401 leptospiral genes or their fragments were transcribed and translated in vitro and printed on nitrocellulose-coated glass slides. We investigated the potential of this protein microarray to screen for interactions between leptospiral OMPs and fibronectin (Fn). This approach resulted in the identification of the recently described fibronectin-binding protein, LIC10258 (MFn8, Lsa66), and 14 novel Fn-binding proteins, denoted Microarray Fn-binding proteins (MFns). We confirmed Fn binding of purified recombinant LIC11612 (MFn1), LIC10714 (MFn2), LIC11051 (MFn6), LIC11436 (MFn7), LIC10258 (MFn8, Lsa66), and LIC10537 (MFn9) by far-Western blot assays. Moreover, we obtained specific antibodies to MFn1, MFn7, MFn8 (Lsa66), and MFn9 and demonstrated that MFn1, MFn7, and MFn9 are expressed and surface exposed under in vitro growth conditions. Further, we demonstrated that MFn1, MFn4 (LIC12631, Sph2), and MFn7 enable leptospires to bind fibronectin when expressed in the saprophyte, Leptospira biflexa. Protein microarrays are valuable tools for high-throughput identification of novel host ligand-binding proteins that have the potential to play key roles in the virulence mechanisms of pathogens.

A recombinant clumping factor A-containing vaccine induces functional antibodies to Staphylococcus aureus that are not observed after natural exposure

Staphylococcus aureus is a Gram-positive pathogen that causes devastating disease and whose pathogenesis is dependent on interactions with host cell factors. Staphylococcal clumping factor A (ClfA) is a highly conserved fibrinogen (Fg)-binding protein and virulence factor that contributes to host tissue adhesion and initiation of infection. ClfA is being investigated as a possible component of a staphylococcal vaccine. We report the development of an Fg-binding assay that is specific for ClfA-mediated binding. Using the assay, we show that despite the presence of anti-ClfA antibodies, human sera from unvaccinated subjects are unable to prevent the binding of S. aureus to an Fg-coated surface. In contrast, antibodies elicited by a recombinant ClfA-containing vaccine were capable of blocking the ClfA-dependent binding of a diverse and clinically relevant collection of staphylococcal strains to Fg. These functional antibodies were also able to displace S. aureus already bound to Fg, suggesting that the ligand-binding activity of ClfA can be effectively neutralized through vaccination.

Crystal structures reveal the multi-ligand binding mechanism of Staphylococcus aureus ClfB

Staphylococcus aureus (S. aureus) pathogenesis is a complex process involving a diverse array of extracellular and cell wall components. ClfB, an MSCRAMM (Microbial Surface Components Recognizing Adhesive Matrix Molecules) family surface protein, described as a fibrinogen-binding clumping factor, is a key determinant of S. aureus nasal colonization, but the molecular basis for ClfB-ligand recognition remains unknown. In this study, we solved the crystal structures of apo-ClfB and its complexes with fibrinogen α (Fg α) and cytokeratin 10 (CK10) peptides. Structural comparison revealed a conserved glycine-serine-rich (GSR) ClfB binding motif (GSSGXGXXG) within the ligands, which was also found in other human proteins such as Engrailed protein, TCF20 and Dermokine proteins. Interaction between Dermokine and ClfB was confirmed by subsequent binding assays. The crystal structure of ClfB complexed with a 15-residue peptide derived from Dermokine revealed the same peptide binding mode of ClfB as identified in the crystal structures of ClfB-Fg α and ClfB-CK10. The results presented here highlight the multi-ligand binding property of ClfB, which is very distinct from other characterized MSCRAMMs to-date. The adherence of multiple peptides carrying the GSR motif into the same pocket in ClfB is reminiscent of MHC molecules. Our results provide a template for the identification of other molecules targeted by S. aureus during its colonization and infection. We propose that other MSCRAMMs like ClfA and SdrG also possess multi-ligand binding properties.

Submicron-textured biomaterial surface reduces staphylococcal bacterial adhesion and biofilm formation

Staphylococci are among the most important pathogens causing bloodstream infections associated with implanted medical devices. Control of bacterial adhesion to material surfaces is important for prevention of biofilm formation and biomaterial-associated infections. In this study, we hypothesized that submicron (staphylococcal bacterial dimension) surface textures may reduce the bacterial adhesion via a decrease in surface area that bacteria can contact, and subsequently inhibit biofilm formation. Poly(urethane urea) films were textured with two different sizes of submicron pillars via a two-stage replication process. Adhesion of two bacterial strains (Staphylococcus epidermidis RP62A and S. aureus Newman) was assessed over a shear stress range of 0-13.2 dyn cm(-2) using a rotating disk system in physiological buffer solutions. Significant decreases in bacterial adhesion were observed on textured surfaces for both strains compared with smooth controls. Biofilm formation was further tested on surfaces incubated in solution for either 2 or 5 days and it was found that biofilm formation was dramatically inhibited on textured surfaces. The results of the approaches used in this work demonstrate that patterned surface texturing of biomaterials provides an effective means to reduce staphylococcal adhesion and biofilm formation on biomaterial surfaces, and thus to prevent biomaterial-associated infections.

Characterization of the fibrinogen binding domain of bacteriophage lysin from Streptococcus mitis

The binding of bacteria to human platelets is a likely central mechanism in the pathogenesis of infective endocarditis. Platelet binding by Streptococcus mitis SF100 is mediated in part by a lysin encoded by the lysogenic bacteriophage SM1. In addition to its role in the phage life cycle, lysin mediates the binding of S. mitis to human platelets via its interaction with fibrinogen on the platelet surface. To better define the region of lysin mediating fibrinogen binding, we tested a series of purified lysin truncation variants for their abilities to bind this protein. These studies revealed that the fibrinogen binding domain of lysin is contained within the region spanned by amino acid residues 102 to 198 (lysin(102-198)). This region has no sequence homology to other known fibrinogen binding proteins. Lysin(102-198) bound fibrinogen comparably to full-length lysin and with the same selectivity for the fibrinogen Aα and Bβ chains. Lysin(102-198) also inhibited the binding in vitro of S. mitis to human fibrinogen and platelets. When assessed by platelet aggregometry, the disruption of the lysin gene in SF100 resulted in a significantly longer time to the onset of aggregation of human platelets than that of the parent strain. The preincubation of platelets with purified lysin(102-198) also delayed the onset of aggregation by SF100. These results indicate that the binding of lysin to fibrinogen is mediated by a specific domain of the phage protein and that this interaction is important for both platelet binding and aggregation by S. mitis.

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.

Effect of filaggrin breakdown products on growth of and protein expression by Staphylococcus aureus

Background

Colonization of the skin by Staphylococcus aureus in individuals with atopic dermatitis exacerbates inflammation. Atopic dermatitis is associated with loss-of-function mutations in the filaggrin (FLG) gene, accompanied by reduced levels of filaggrin breakdown products on the skin.

Objective

To assess the affect of growth in the presence of the filaggrin breakdown products urocanic acid (UCA) and pyrrolidone carboxylic acid (PCA) on fitness of and protein expression by S aureus.

Methods

S aureus was grown for 24 hours in the presence of UCA and PCA, and the density of the cultures was monitored by recording OD₆₀₀ values. Cell wall extracts and secreted proteins of S aureus were isolated and analyzed by SDS-PAGE. Cell wall–associated proteins known to be involved in colonization and immune evasion including clumping factor B, fibronectin binding proteins, protein A, iron-regulated surface determinant A, and the serine-aspartate repeat proteins were examined by Western immunoblotting.

Results

Acidification of growth media caused by the presence of UCA and PCA resulted in reduced growth rates and reduced final cell density of S aureus. At the lower pH, reduced expression of secreted and cell wall–associated proteins, including proteins involved in colonization (clumping factor B, fibronectin binding protein A) and immune evasion (protein A), was observed. Decreased expression of iron-regulated surface determinant A due to growth with filaggrin breakdown products appeared to be independent of the decreased pH.

Conclusion

S aureus grown under mildly acidic conditions such as those observed on healthy skin expresses reduced levels of proteins that are known to be involved in immune evasion.

Evaluation of clumping factor A binding region A in a subunit vaccine against Staphylococcus aureus-induced mastitis in mice

The present study evaluated the potential of recombinant binding region A of clumping factor A (rClfA-A) to be an effective component of a vaccine against mastitis induced by Staphylococcus aureus in the mouse. rClfA-A and inactivated S. aureus were each emulsified in Freund's adjuvant, mineral oil adjuvant, and Seppic adjuvant; phosphate-buffered saline was used as a control. Seven groups of 12 mice each were immunized intraperitoneally three times at 2-week intervals. The titers of IgG and subtypes thereof (IgG1 and IgG2a) in the rClfA-A-immunized group were more than 1,000-fold higher than those in the killed-bacteria-immunized group (P < 0.01). Of the three adjuvants used, mineral oil adjuvant induced the highest antibody levels for both antigens (P < 0.001). Furthermore, the anti-rClfA-A antibody capacities for bacterial adhesion and opsonizing phagocytosis were significantly greater in the rClfA-A-immunized group than in the killed-bacteria-immunized group (P < 0.05). Lactating mice immunized with either rClfA-A or inactivated vaccine were challenged with S. aureus via the intramammary route. The numbers of bacteria recovered from the murine mammary glands 24 h after inoculation were significantly lower in the rClfA-A group than in the killed-bacteria-immunized group (P < 0.001). Histologic examination of the mammary glands showed that rClfA-A immunization effectively preserved tissue integrity. Thus, rClfA-A emulsified in an oil adjuvant provides strong immune protection against S. aureus-induced mastitis in the mouse.

Characterization, crystallization and preliminary X-ray analysis of the adhesive domain of SdrE from Staphylococcus aureus

The adhesive domain of SdrE from Staphylococcus aureus was recombinantly expressed in Escherichia coli. The purified protein was identified by SDS-PAGE and MALDI-TOF MS. The protein was crystallized using the vapour-diffusion method in hanging-drop mode with PEG 8000 as the primary precipitating agent. X-ray diffraction data were collected to 1.8 A resolution from a single crystal of the protein. Preliminary X-ray analysis indicated that the crystal belonged to space group P1, with unit-cell parameters a = 40.714, b = 66.355, c = 80.827 A, alpha = 111.19, beta = 93.99, gamma = 104.39 degrees.

Direct interaction of iron-regulated surface determinant IsdB of Staphylococcus aureus with the GPIIb/IIIa receptor on platelets

The interaction of bacteria with platelets is implicated in the pathogenesis of endovascular infections, including infective endocarditis, of which Staphylococcus aureus is the leading cause. Several S. aureus surface proteins mediate aggregation of platelets by fibrinogen- or fibronectin-dependent processes, which also requires specific antibodies. In this study S. aureus was grown in iron-limited medium to mimic in vivo conditions in which iron is unavailable to pathogens. Under such conditions, a S. aureus mutant lacking the known platelet-activating surface proteins adhered directly to platelets in the absence of plasma proteins and triggered aggregation. Platelet adhesion and aggregation was prevented by inhibiting expression of iron-regulated surface determinant (Isd) proteins. Mutants defective in IsdB, but not IsdA or IsdH, were unable to adhere to or aggregate platelets. Antibodies to the platelet integrin GPIIb/IIIa inhibited platelet adhesion by IsdB-expressing strains, as did antagonists of GPIIb/IIIa. Surface plasmon resonance demonstrated that recombinant IsdB interacts directly with GPIIb/IIIa.

beta-Neurexin is a ligand for the Staphylococcus aureus MSCRAMM SdrC

Gram-positive bacteria contain a family of surface proteins that are covalently anchored to the cell wall of the organism. These cell-wall anchored (CWA) proteins appear to play key roles in the interactions between pathogenic organisms and the host. A subfamily of the CWA has a common structural organization with multiple domains adopting characteristic IgG-like folds. The identified microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) belong to this subfamily, as does SdrC from S. aureus. However, an interactive host ligand for the putative MSCRAMM SdrC was not previously identified. We have screened a phage display peptide library and identified a peptide sequence found in beta-neurexin that binds SdrC. A synthetic peptide corresponding to the identified sequence as well as a recombinant form of the beta-neurexin 1 exodomain binds SdrC with high affinity and specificity. Furthermore, expression of SdrC on bacteria greatly enhances microbial adherence to cultured mammalian cells expressing beta-neurexin on their surface. Taken together, our experimental results demonstrate that beta-neurexin is a ligand for SdrC. This interaction involves a specific sequence located in the N-terminal region of the mammalian protein and the N(2)N(3) domain of the MSCRAMM. The fact that these two proteins interact when expressed on the appropriate cells demonstrates the functionality of the interaction. Possible implications of this interaction are discussed.

Molecular characterization of the interaction of staphylococcal microbial surface components recognizing adhesive matrix molecules (MSCRAMM) ClfA and Fbl with fibrinogen

The ligand-binding domain of Fbl (the fibrinogen binding protein from Staphylococcus lugdunensis) shares 60% sequence identity with ClfA (clumping factor A) of Staphylococcus aureus. Recombinant Fbl corresponding to the minimum fibrinogen-binding region (subdomains N2N3) was compared with ClfA for binding to fibrinogen. Fbl and ClfA had very similar affinities for fibrinogen by surface plasmon resonance. The binding site for Fbl in fibrinogen was localized to the extreme C terminus of the fibrinogen gamma-chain at the same site recognized by ClfA. Isothermal titration calorimetry showed that Fbl and ClfA had very similar affinities for a peptide mimicking the C-terminal segment of the fibrinogen gamma-chain. The peptide also inhibited binding of Fbl and ClfA to fibrinogen. A series of substituted gamma-chain variant peptides behaved very similarly when used to inhibit ClfA and Fbl binding to immobilized fibrinogen. Both ClfA and Fbl bound to bovine fibrinogen with a lower affinity compared with human fibrinogen and did not bind detectably to ovine fibrinogen. The structure of the N2N3 subdomains of Fbl in complex with the fibrinogen gamma-chain peptide was modeled based on the crystal structure of the N2N3 subdomains of the ClfA-gamma-chain peptide complex. Residues in the putative binding trench likely to be involved in fibrinogen binding were identified. Fbl variant proteins with alanine substitutions in key residues had reduced affinities for fibrinogen. Thus Fbl and ClfA bind the same site in fibrinogen by similar mechanisms.

Purification, characterization, and crystallization of the adhesive domain of SdrD from Staphylococcus aureus

The adhesive domain of SdrD from Staphylococcus aureus was solubly expressed in Escherichia coli in high yield. After a series of purification steps, the purified protein was >95% pure, which was SdrD from S. aureus identified by SDS-PAGE and MALDI-TOF MS. Crystals were grown at 18 degrees C using 25% polyethylene glycol 3350 as precipitant. Diffraction by the crystal extends to 1.65A resolution, and the crystal belongs to the space group C2, with the unit cell parameters a=133.3, b=58.3, c=112.3A, alpha=90.00, beta=111.14, gamma=90.00.

The Panton-Valentine leukocidin vaccine protects mice against lung and skin infections caused by Staphylococcus aureus USA300

Methicillin-resistant Staphylococcus aureus is increasingly responsible for staphylococcal infections in the community. A large percentage of the community-acquired methicillin-resistant (CA-MRSA) strains in the USA produce Panton-Valentine leukocidin (PVL), which is associated with severe infections. The virulence of the clinical CA-MRSA strain USA300 was compared to that of its isogenic pvl-deleted mutant, and it was shown that PVL contributes to lung and muscle tissue destruction, respectively, in murine necrotizing pneumonia and skin infection models. Mice infected with the USA300 strain developed a dominant anti-PVL response. The PVL subunits were therefore tested as vaccinogens against this isolate, and their vaccine efficacy correlated with both the route of vaccination and infection. These data suggest that PVL is a virulence factor in murine CA-MRSA infections.

A structural model of the Staphylococcus aureus ClfA-fibrinogen interaction opens new avenues for the design of anti-staphylococcal therapeutics

The fibrinogen (Fg) binding MSCRAMM Clumping factor A (ClfA) from Staphylococcus aureus interacts with the C-terminal region of the fibrinogen (Fg) γ-chain. ClfA is the major virulence factor responsible for the observed clumping of S. aureus in blood plasma and has been implicated as a virulence factor in a mouse model of septic arthritis and in rabbit and rat models of infective endocarditis. We report here a high-resolution crystal structure of the ClfA ligand binding segment in complex with a synthetic peptide mimicking the binding site in Fg. The residues in Fg required for binding to ClfA are identified from this structure and from complementing biochemical studies. Furthermore, the platelet integrin α_IIbβ₃ and ClfA bind to the same segment in the Fg γ-chain but the two cellular binding proteins recognize different residues in the common targeted Fg segment. Based on these differences, we have identified peptides that selectively antagonize the ClfA-Fg interaction. The ClfA-Fg binding mechanism is a variant of the “Dock, Lock and Latch” mechanism previously described for the Staphylococcus epidermidis SdrG–Fg interaction. The structural insights gained from analyzing the ClfANFg peptide complex and identifications of peptides that selectively recognize ClfA but not α_IIbβ₃ may allow the design of novel anti-staphylococcal agents. Our results also suggest that different MSCRAMMs with similar structural organization may have originated from a common ancestor but have evolved to accommodate specific ligand structures.

Staphylococcus aureus (S. aureus) is a common pathogen that can cause a range of diseases from mild skin infections to life-threatening sepsis in humans. Some surface proteins on S. aureus play important roles in the S. aureus disease process. One of these bacterial surface proteins is clumping factor A (ClfA) that binds to the C-terminal region of one of the three chains of fibrinogen (Fg), a blood protein that plays a key role in coagulation. We carried out biochemical and structural studies to understand the binding mechanism of ClfA to Fg and to define the residues in Fg that interact with ClfA. Interestingly, the platelet integrin, which is important for platelet aggregation and thrombi formation, also binds to the same region of Fg as ClfA. Despite the fact that the two proteins bind at the same region, the mode of recognition is significantly different. Exploiting this difference in recognition, we have demonstrated that agents could be designed that inhibit the ClfA–Fg interaction but do not interfere with the interaction of Fg with the platelet integrin. This opens the field for the design of a novel class of anti-staph therapeutics.

Staphylococcus aureus adhesion via Spa, ClfA, and SdrCDE to immobilized platelets demonstrates shear-dependent behavior

OBJECTIVE

The objective of this study is to delineate the molecular mechanisms responsible for Staphylococcus aureus-platelet adhesion as a function of physiologically relevant wall shear stresses.

METHODS AND RESULTS

A parallel plate flow chamber was used to quantify adhesion of wild-type, Spa-, ClfA- and SdrCDE- strains to immobilized platelet layers. In the absence of plasma, adhesion increases with increasing wall shear rate from 100 to 5000 seconds(-1). The presence of plasma significantly enhances adhesion at all shear levels. Addition of exogenous fibrinogen yields adhesion levels similar to plasma in the lower shear regimes, but has a diminishing effect on potentiating adhesion at higher shear rates. Alternatively, as shear rate increases von Willebrand factor (VWF) plays an increasingly significant role in mediating binding.

CONCLUSIONS

Addition of plasma proteins potentiates S aureus-platelet interactions at all shear rates examined. Whereas fibrinogen plays a significant role in all shear regimes, VWF mediation becomes increasingly important as wall shear rate increases. Fibrinogen binding is dependent on bacterial adhesins ClfA and SdrCDE whereas Spa is the dominant receptor for VWF.

Surface Adhesins of Staphylococcus aureus

An important facet in the interaction between Staphylococcus aureus and its host is the ability of the bacterium to adhere to human extracellular matrix components and serum proteins. In order to colonise the host and disseminate, it uses a wide range of strategies, the molecular and genetic basis of which are multifactorial, with extensive functional overlap between adhesins. Here, we describe the current knowledge of the molecular features of the adhesive components of S. aureus, mechanisms of adhesion and the impact that these have on host-pathogen interaction.

Characterization of the fibrinogen-binding surface protein Fbl of Staphylococcus lugdunensis

The fbl gene of Staphylococcus lugdunensis encodes a protein Fbl that is 58 % identical to the clumping factor A (ClfA) of Staphylococcus aureus. The fbl gene was present in eight clinical isolates of S. lugdunensis. When Fbl was expressed on the surface of Lactococcus lactis it promoted adherence to immobilized fibrinogen and cell clumping in a fibrinogen solution. Purified recombinant Fbl region A bound to immobilized fibrinogen in a dose-dependent manner and inhibited the adherence of both Fbl-expressing and ClfA-expressing strains of L. lactis to fibrinogen. Adherence of S. lugdunensis and L. lactis Fbl(+) to immobilized fibrinogen was also inhibited by rabbit anti-Fbl region A antibodies and rabbit anti-ClfA region A antibodies, as well as by human immunoglobulin with a high level of anti-ClfA antibodies. Alignment of the A domains of CflA and Fbl revealed that all of the ClfA residues implicated in binding to the gamma-chain of fibrinogen are conserved in Fbl. Nevertheless Fbl had a tenfold lower affinity for fibrinogen, suggesting that sequence differences that occur elsewhere in the protein, possibly in beta-strand E of domain N2, affect ligand binding.

Structure and function comparison of Micropechis ikaheka snake venom phospholipase A2 isoenzymes

Comparison of the crystal structures of three Micropechis ikaheka phospholipase A2 isoenzymes (MiPLA2, MiPLA3 and MiPLA4, which exhibit different levels of pharmacological effects) shows that their C-terminus (residues 110-124) is the most variable. M-Type receptor binding affinity of the isoenzymes has also been investigated and MiPLA4 binds to the rabbit M-type receptor with high affinity. Examination of surface charges of the isoenzymes reveals a trend of increase in positive charges with potency. The isoenzymes are shown to oligomerize in a concentration-dependent manner in a semi-denaturing gel. The C-termini of the medium (MiPLA4) and highly potent (MiPLA2) isoenzyme molecules cluster together, forming a highly exposed area. A BLAST search using the sequence of the most potent MiPLA2 results in high similarity to Staphylococcus aureus clotting factor A and cadherin 11. This might explain the myotoxicity, anticoagulant and hemoglobinuria effects of MiPLA2s.