Shear stress prevents fibronectin binding protein-mediated Staphylococcus aureus adhesion to resting endothelial cells

Vasculature-on-a-chip technologies as platforms for advanced studies of bacterial infections

Bacterial infections frequently occur within or near the vascular network as the vascular network connects organ systems and is essential in delivering and removing blood, essential nutrients, and waste products to and from organs. In turn, the vasculature plays a key role in the host immune response to bacterial infections. Technological advancements in microfluidic device design and development have yielded increasingly sophisticated and physiologically relevant models of the vasculature including vasculature-on-a-chip and organ-on-a-chip models. This review aims to highlight advancements in microfluidic device development that have enabled studies of the vascular response to bacteria and bacterial-derived molecules at or near the vascular interface. In the first section of this review, we discuss the use of parallel plate flow chambers and flow cells in studies of bacterial adhesion to the vasculature. We then highlight microfluidic models of the vasculature that have been utilized to study bacteria and bacterial-derived molecules at or near the vascular interface. Next, we review organ-on-a-chip models inclusive of the vasculature and pathogenic bacteria or bacterial-derived molecules that stimulate an inflammatory response within the model system. Finally, we provide recommendations for future research in advancing the understanding of host-bacteria interactions and responses during infections as well as in developing innovative antimicrobials for preventing and treating bacterial infections that capitalize on technological advancements in microfluidic device design and development.

Effect of Gravity on Bacterial Adhesion to Heterogeneous Surfaces

Bacterial adhesion is the first step in the formation of surface biofilms. The number of bacteria that bind to a surface from the solution depends on how many bacteria can reach the surface (bacterial transport) and the strength of interactions between bacterial adhesins and surface receptors (adhesivity). By using microfluidic channels and video microscopy as well as computational simulations, we investigated how the interplay between bacterial transport and adhesivity affects the number of the common human pathogen Escherichia coli that bind to heterogeneous surfaces with different receptor densities. We determined that gravitational sedimentation causes bacteria to concentrate at the lower surface over time as fluid moves over a non-adhesive region, so bacteria preferentially adhere to adhesive regions on the lower, inflow-proximal areas that are downstream of non-adhesive regions within the entered compartments. Also, initial bacterial attachment to an adhesive region of a heterogeneous lower surface may be inhibited by shear due to mass transport effects alone rather than shear forces per se, because higher shear washes out the sedimented bacteria. We also provide a conceptual framework and theory that predict the impact of sedimentation on adhesion between and within adhesive regions in flow, where bacteria would likely bind both in vitro and in vivo, and how to normalize the bacterial binding level under experimental set-ups based on the flow compartment configuration.

Staphylococcus aureus adhesion in endovascular infections is controlled by the ArlRS-MgrA signaling cascade

Staphylococcus aureus is a leading cause of endovascular infections. This bacterial pathogen uses a diverse array of surface adhesins to clump in blood and adhere to vessel walls, leading to endothelial damage, development of intravascular vegetations and secondary infectious foci, and overall disease progression. In this work, we describe a novel strategy used by S. aureus to control adhesion and clumping through activity of the ArlRS two-component regulatory system, and its downstream effector MgrA. Utilizing a combination of in vitro cellular assays, and single-cell atomic force microscopy, we demonstrated that inactivation of this ArlRS—MgrA cascade inhibits S. aureus adhesion to a vast array of relevant host molecules (fibrinogen, fibronectin, von Willebrand factor, collagen), its clumping with fibrinogen, and its attachment to human endothelial cells and vascular structures. This impact on S. aureus adhesion was apparent in low shear environments, and in physiological levels of shear stress, as well as in vivo in mouse models. These effects were likely mediated by the de-repression of giant surface proteins Ebh, SraP, and SasG, caused by inactivation of the ArlRS—MgrA cascade. In our in vitro assays, these giant proteins collectively shielded the function of other surface adhesins and impaired their binding to cognate ligands. Finally, we demonstrated that the ArlRS—MgrA regulatory cascade is a druggable target through the identification of a small-molecule inhibitor of ArlRS signaling. Our findings suggest a novel approach for the pharmacological treatment and prevention of S. aureus endovascular infections through targeting the ArlRS—MgrA regulatory system.

Adhesion is central to the success of Staphylococcus aureus as a bacterial pathogen. We describe a novel mechanism through which S. aureus alters adhesion to ligands by regulating expression of giant inhibitory surface proteins. These giant proteins shield normal surface adhesins, preventing binding to ligands commonly found in the bloodstream and vessel walls. Using this unique regulatory scheme, S. aureus can bypass the need for individualized regulation of numerous adhesins to control overall adhesive properties. Our study establishes the importance of these giant proteins for S. aureus pathogenesis and demonstrates that a single regulatory cascade can be targeted for treating infections.

In Vitro Attachment of Staphylococcus Epidermidis to Surgical Sutures with and without Ag-Containing Bioactive Glass Coating

The ability of a silver-doped bioactive glass (AgBG) coating to prevent bacterial colonization on surgical sutures was investigated in vitro. Bioactive glass powders, in the form of 45S5 Bioglass® and AgBG, were used to coat Mersilk® sutures using an optimized ‘in house’ slurry-dipping process. In vitro experiments were carried out using Staphylococcus epidermidis under both batch and flow conditions. While the traditional batch culture testing was used to determine the number of viable cells adhered to the surface, the flow-cell was used to visualize attachment and detachment over time. Under batch conditions of up to 180 min, statistically significant differences were observed in the colony forming units (CFU) per suture for both the coated and uncoated Mersilk® sutures. The results showed that the AgBG coating had the greatest effect on limiting bacterial attachment (8 102 CFU) when compared to the 45S5 Bioglass® coating (3.2 103 CFU) and the uncoated Mersilk® (1.2 104 CFU). Also under flow conditions differences were seen between the coated and uncoated sutures. Therefore, this preliminary study has demonstrated the quantification and visualization of bacterial attachment onto sutures in order to compare the antibacterial properties of Ag-containing bioactive glass coatings. The bactericidal properties imparted by Ag-containing glass open new opportunities for use of the composite sutures in wound healing and body wall repair.

Multi-disciplinary antimicrobial strategies for improving orthopaedic implants to prevent prosthetic joint infections in hip and knee

Like any foreign object, orthopaedic implants are susceptible to infection when introduced into the human body. Without additional preventative measures, the absolute number of annual prosthetic joint infections will continue to rise, and may exceed the capacity of health care systems in the near future. Bacteria are difficult to eradicate from synovial joints due to their exceptionally diverse taxonomy, complex mechanistic attachment capabilities, and tendency to evolve antibiotic resistance. When a primary orthopaedic implant fails from prosthetic joint infection, surgeons are generally challenged by limited options for intervention. In this review, we highlight the etiology and taxonomic groupings of bacteria known to cause prosthetic joint infections, and examine their key mechanisms of attachment. We propose that antimicrobial strategies should focus on the most harmful bacteria taxa within the context of occurrence, taxonomic diversity, adhesion mechanisms, and implant design. Patient-specific identification of organisms that cause prosthetic joint infections will permit assessment of their biological vulnerabilities. The latter can be targeted using a range of antimicrobial techniques that exploit different colonization mechanisms including implant surface attachment, biofilm formation, and/or hematogenous recruitment. We anticipate that customized strategies for each patient, joint, and prosthetic component will be most effective at reducing prosthetic joint infections, including those caused by antibiotic-resistant and polymicrobial bacteria.

Shear-Resistant Binding to von Willebrand Factor Allows Staphylococcus lugdunensis to Adhere to the Cardiac Valves and Initiate Endocarditis

BACKGROUND

Staphylococcus lugdunensis is an emerging cause of endocarditis. To cause endovascular infections, S. lugdunensis requires mechanisms to overcome shear stress. We investigated whether platelets and von Willebrand factor (VWF) mediate bacterial adhesion to the vessel wall and the cardiac valves under flow.

METHODS

S. lugdunensis binding to VWF, collagen, and endothelial cells was studied in a parallel flow chamber in the absence and presence of platelets. In vivo adhesion of S. lugdunensis was evaluated in a mouse microvasculature perfusion model and a new mouse model of endocarditis.

RESULTS

Contrary to other coagulase-negative staphylococci, S. lugdunensis bound to VWF under flow, thus enabling its adhesion to endothelial cells and to the subendothelial matrix. In inflamed vessels of the mesenteric circulation, VWF recruited S. lugdunensis to the vessel wall. In a novel endocarditis mouse model, local inflammation and the resulting release of VWF enabled S. lugdunensis to bind and colonize the heart valves.

CONCLUSIONS

S. lugdunensis binds directly to VWF, which proved to be vital for withstanding shear forces and for its adhesion to the vessel wall and cardiac valves. This mechanism explains why S. lugdunensis causes more-aggressive infections, including endocarditis, compared with other coagulase-negative staphylococci.

Proteomic analysis of Staphylococcus aureus biofilm cells grown under physiologically relevant fluid shear stress conditions

BACKGROUND

The biofilm forming bacterium Staphylococcus aureus is responsible for maladies ranging from severe skin infection to major diseases such as bacteremia, endocarditis and osteomyelitis. A flow displacement system was used to grow S. aureus biofilms in four physiologically relevant fluid shear rates (50, 100, 500 and 1000 s(-1)) to identify proteins that are associated with biofilm.

RESULTS

Global protein expressions from the membrane and cytosolic fractions of S. aureus biofilm cells grown under the above shear rate conditions are reported. Sixteen proteins in the membrane-enriched fraction and eight proteins in the cytosolic fraction showed significantly altered expression (p < 0.05) under increasing fluid shear. These 24 proteins were identified using nano-LC-ESI-MS/MS. They were found to be associated with various metabolic functions such as glycolysis / TCA pathways, protein synthesis and stress tolerance. Increased fluid shear stress did not influence the expression of two important surface binding proteins: fibronectin-binding and collagen-binding proteins.

CONCLUSIONS

The reported data suggest that while the general metabolic function of the sessile bacteria is minimal under high fluid shear stress conditions, they seem to retain the binding capacity to initiate new infections.

Mechanical stretch and shear flow induced reorganization and recruitment of fibronectin in fibroblasts

It was our objective to study the role of mechanical stimulation on fibronectin (FN) reorganization and recruitment by exposing fibroblasts to shear fluid flow and equibiaxial stretch. Mechanical stimulation was also combined with a Rho inhibitor to probe their coupled effects on FN. Mechanically stimulated cells revealed a localization of FN around the cell periphery as well as an increase in FN fibril formation. Mechanical stimulation coupled with chemical stimulation also revealed an increase in FN fibrils around the cell periphery. Complimentary to this, fibroblasts exposed to fluid shear stress structurally rearranged pre-coated surface FN, but unstimulated and stretched cells did not. These results show that mechanical stimulation directly affected FN reorganization and recruitment, despite perturbation by chemical stimulation. Our findings will help elucidate the mechanisms of FN biosynthesis and organization by furthering the link of the role of mechanics with FN.

Methicillin resistant Staphylococcus aureus adhesion to human umbilical vein endothelial cells demonstrates wall shear stress dependent behaviour

Background

Methicillin-resistant Staphylococcus aureus (MRSA) is an increasingly prevalent pathogen capable of causing severe vascular infections. The goal of this work was to investigate the role of shear stress in early adhesion events.

Methods

Human umbilical vein endothelial cells (HUVEC) were exposed to MRSA for 15-60 minutes and shear stresses of 0-1.2 Pa in a parallel plate flow chamber system. Confocal microscopy stacks were captured and analyzed to assess the number of MRSA. Flow chamber parameters were validated using micro-particle image velocimetry (PIV) and computational fluid dynamics modelling (CFD).

Results

Under static conditions, MRSA adhered to, and were internalized by, more than 80% of HUVEC at 15 minutes, and almost 100% of the cells at 1 hour. At 30 minutes, there was no change in the percent HUVEC infected between static and low flow (0.24 Pa), but a 15% decrease was seen at 1.2 Pa. The average number of MRSA per HUVEC decreased 22% between static and 0.24 Pa, and 37% between 0.24 Pa and 1.2 Pa. However, when corrected for changes in bacterial concentration near the surface due to flow, bacteria per area was shown to increase at 0.24 Pa compared to static, with a subsequent decline at 1.2 Pa.

Conclusions

This study demonstrates that MRSA adhesion to endothelial cells is strongly influenced by flow conditions and time, and that MSRA adhere in greater numbers to regions of low shear stress. These areas are common in arterial bifurcations, locations also susceptible to generation of atherosclerosis.

Staphylococcus aureus SigB activity promotes a strong fibronectin-bacterium interaction which may sustain host tissue colonization by small-colony variants isolated from cystic fibrosis patients

Genes encoding cell-surface proteins regulated by SigB are stably expressed in Staphylococcus aureus small-colony variants (SCVs) isolated from cystic fibrosis (CF) patients. Our hypothesis is that CF-isolated SCVs are locked into a colonization state by sustaining the expression of adhesins such as fibronectin-binding proteins (FnBPs) throughout growth. Force spectroscopy was used to study the fibronectin-FnBPs interaction among strains varying for their SigB activity. The fibronectin-FnBPs interaction was described by a strength of 1000+/-400 pN (pulling rate of 2 microm s(-1)), an energetic barrier width of 0.6+/-0.1 A and an off-rate below 2 x 10(-4) s(-1). A CF-isolated SCV highly expressed fnbA throughout growth and showed a sustained capacity to bind fibronectin, whereas a prototypic strain showed a reduced frequency of fibronectin-binding during the stationary growth phase when its fnbA gene was down-regulated. Reduced expression of fnbA was observed in sigB mutants, which was associated with an overall decrease adhesion to fibronectin. These results suggest that the fibronectin-FnBPs interaction plays a role in the formation of a mechanically resistant adhesion of S. aureus to host tissues and supports the hypothesis that CF-isolated SCVs are locked into a colonization state as a result of a sustained SigB activity.

Microfluidic devices for studying growth and detachment of Staphylococcus epidermidis biofilms

Microfluidic devices were used to study the influences of hydrodynamics of local microenvironments on Staphylococcus epidermidis (S. epidermidis) biofilm formation and the effects of a poly(beta-1,6-N-acetyl glucosamine)-hydrolyzing enzyme (dispersin B) and/or an antibiotic (rifampicin) on the detachment of the biofilm. Elongated, monolayered biofilm morphologies were observed at high flow velocity and fluid shear locations whereas large clump-like, multilayered biofilm structures were produced at low flow velocity and fluid shear locations. Upon dispersin B treatment, most of the biofilm was detached from the microchannel surface. However, a trace amount of bacterial cells could not be removed from corner locations most likely due to the insufficient wall shear stress of the fluid at these locations. Dispersin B or rifampicin treatment was effective in delaying the dispersal behavior of bacterial cells, but could not completely remove the biofilm. Combined dynamic delivery of dispersin B and rifampicin was found to be effective for complete removal of the S. epidermidis biofilm.

Molecular basis for Staphylococcus aureus-mediated platelet aggregate formation under arterial shear in vitro

OBJECTIVE

Staphylococcus aureus is the most frequent causative organism of infective endocarditis (IE) and is characterized by thrombus formation on a cardiac valve that can embolize to a distant site. Previously, we showed that S. aureus clumping factor A (ClfA) and fibronectin-binding protein A (FnBPA) can stimulate rapid platelet aggregation.

METHODS AND RESULTS

In this study we investigate their relative roles in mediating aggregate formation under physiological shear conditions. Platelets failed to interact with immobilized wild-type S. aureus (Newman) at shear rates <500 s(-1) but rapidly formed an aggregate at shear rates >800 s(-1). Inactivation of the ClfA gene eliminated aggregate formation at any shear rate. Using surrogate hosts that do not interact with platelets bacteria overexpressing ClfA supported rapid aggregate formation under high shear with a similar profile to Newman whereas bacteria overexpressing FnBPA did not. Fibrinogen binding to ClfA was found to be essential for aggregate formation although fibrinogen-coated surfaces only allowed single-platelets to adhere under all shear conditions. Blockade of the platelet immunoglobulin receptor Fc gammaRIIa inhibited aggregate formation.

CONCLUSIONS

Thus, fibrinogen and IgG binding to ClfA is essential for aggregate formation under arterial shear conditions and may explain why S. aureus is the major cause of IE.

Cellular coating of the left ventricular assist device textured polyurethane membrane reduces adhesion of Staphylococcus aureus

OBJECTIVE

Infections are among the most common and serious complications of ventricular assist device implantation. These infections generally occur within the first 2 months after surgery. The basis for this high incidence of infection is not well established, so a murine intravascular infection model was developed with aortic implantation of the textured polyurethane patch material currently used in HeartMate ventricular assist devices (Thoratec Corporation Pleasanton, Calif).

METHODS

Polyurethane patch material was placed in the wall of the mouse descending aorta. Mice were then infected with Staphylococcus aureus 1 or 14 days after implantation. In vitro adhesion studies were conducted with polyurethane membranes coated with endothelial cells and membranes coated with fibrinogen.

RESULTS

Mice were susceptible to infection in both dose- and time-dependent fashions. The patch material was significantly more susceptible to infection at day 1 than day 14. Immunohistologic and morphologic studies demonstrated that the CD31+ cells deposited on the membrane surface phenotypically appeared to be endothelial cells. In vitro adhesion studies of polyurethane membranes coated with endothelial cells showed them to be less susceptible to S. aureus binding than were membranes coated with fibrinogen.

CONCLUSION

Textured polyurethane membranes are less susceptible to infection as cellular deposition occurs. The time frame within which these membranes become populated with cellular material is consistent with the time-dependent clinical incidence of infection. Cellular coating of polyurethane may provide a strategy for reducing the risk of infection.

Measuring cell adhesion on RGD-modified, self-assembled PEG monolayers using the quartz crystal microbalance technique

In this study, the suitability of a flow-through quartz crystal microbalance system for the detection of the adhesion of rMSCs and 3T3-L1 fibroblasts on different surfaces is demonstrated. Frequency shifts for rMSCs of -6.7 mHz x cell(-1) and -2.0 mHz x cell(-1) for 3T3-L1 cells could be detected on non-modified gold sensors, revealing that the frequency shift per cell is comparable to that of a static setup. Modifying the sensor surface with SAMs of thioalkylated omega-amine-terminated PEG derivatives led to cell-adhesion-resistant surfaces. Total frequency shifts of only -20 +/- 7 Hz showed that protein adsorption was also significantly reduced. Attaching 35 pmol x mm(-2) of the GRGDS cell adhesion motif to the SAMs induced specific cell adhesion due to RGD-integrin interactions; the resonance frequency dropped by 3.4 mHz x cell(-1). Furthermore, the kinetics of cell detachment could be determined. The corresponding processes were completed after 10 min for trypsin, and not before 90 min with GRGDS. Moreover, the detectability of cell adhesion was shown to increase after the addition of manganese cations. The total decrease in the resonance frequency was almost 80 Hz in the presence of Mn(2+) (6.4 mHz x cell(-1)). [image: see text] Staining the cytoskeleton of the rMSCs shows that the GRGDS-modified surfaces are almost completely covered with well-spread cells.

Mycobacterium tuberculosis binding to human surfactant proteins A and D, fibronectin, and small airway epithelial cells under shear conditions

A crucial step in infection is the initial attachment of a pathogen to host cells or tissue. Mycobacterium tuberculosis has evolved multiple strategies for establishing an infection within the host. The pulmonary microenvironment contains a complex milieu of pattern recognition molecules of the innate immune system that play a role in the primary response to inhaled pathogens. Encounters of M. tuberculosis with these recognition molecules likely influence the outcome of the bacillus-host interaction. Here we use a novel fluid shear assay to investigate the binding of M. tuberculosis to innate immune molecules that are produced by pulmonary epithelial cells and are thought to play a role in the lung innate immune response. Virulent and attenuated M. tuberculosis strains bound best to immobilized human fibronectin (FN) and surfactant protein A (SP-A) under this condition. Binding under fluid shear conditions was more consistent and significant compared to binding under static conditions. Soluble FN significantly increased the adherence of both virulent and attenuated M. tuberculosis strains to human primary small airway epithelial cells (SAEC) under fluid shear conditions. In contrast, SP-A and SP-D effects on bacterial adherence to SAEC differed between the two strains. The use of a fluid shear model to simulate physiological conditions within the lung and select for high-affinity binding interactions should prove useful for studies that investigate interactions between M. tuberculosis and host innate immune determinants.

The pathogenesis and control of Staphylococcus aureus-induced mastitis: study models in the mouse

The intramammary colonisation by Staphylococcus aureus provokes mastitis in the cow. Once established, the infection is difficult to eradicate with available therapies and may become chronic. The present article focuses on the use of the experimental mouse model of S. aureus-induced mastitis as a practical approach for the study of bovine mastitis. Results obtained regarding the pathogenesis of S. aureus and the development of new therapeutic approaches are discussed.

Fluid Shear Regulates the Kinetics and Receptor Specificity ofStaphylococcus aureusBinding to Activated Platelets

The interaction between surface components on the invading pathogen and host cells such as platelets plays a key role in the regulation of endovascular infections. However, the mechanisms mediating Staphylococcus aureus binding to platelets under shear remain largely unknown. This study was designed to investigate the kinetics and molecular requirements of platelet-S. aureus interactions in bulk suspensions subjected to a uniform shear field. Hydrodynamic shear-induced collisions augment platelet-S. aureus binding, which is further potentiated by platelet activation with stromal derived factor-1beta. Peak adhesion efficiency occurs at low shear (100 s(-1)) and decreases with increasing shear. The molecular interaction of platelet alpha(IIb)beta(3) with bacterial clumping factor A through fibrinogen bridging is necessary for stable bacterial binding to activated platelets under shear. Although this pathway is sufficient at low shear (</=400 s(-1)), the involvement of platelet gpIb and staphylococcal protein A through von Willebrand factor bridging is essential for optimal recruitment of S. aureus cells by platelets in the high shear regime. IgG plays an inhibitory role in the adhesion process, presumably by interfering with the binding of von Willebrand factor to staphylococcal protein A. This study demonstrates that platelet activation and a fluid-mechanical environment representative of the vasculature affect platelet-S. aureus cell-adhesive interactions pertinent to the process of S. aureus-induced bloodstream infections.

Dynamics of the interaction between a fibronectin molecule and a living bacterium under mechanical force

Fibronectin (Fn) is an important mediator of bacterial invasions and of persistent infections like that of Staphylococcus epidermis. Similar to many other types of cell-protein adhesion, the binding between Fn and S. epidermidis takes place under physiological shear rates. We investigated the dynamics of the interaction between individual living S. epidermidis cells and single Fn molecules under mechanical force by using the scanning force microscope. The mechanical strength of this interaction and the binding site in the Fn molecule were determined. The energy landscape of the binding/unbinding process was mapped, and the force spectrum and the association and dissociation rate constants of the binding pair were measured. The interaction between S. epidermidis cells and Fn molecules is compared with those of two other protein/ligand pairs known to mediate different dynamic states of adhesion of cells under a hydrodynamic flow: the firm adhesion mediated by biotin/avidin interactions, and the rolling adhesion, mediated by L-selectin/P-selectin glycoprotein ligand-1 interactions. The inner barrier in the energy landscape of the Fn case characterizes a high-energy binding mode that can sustain larger deformations and for significantly longer times than the correspondent high-strength L-selectin/P-selectin glycoprotein ligand-1 binding mode. The association kinetics of the former interaction is much slower to settle than the latter. On this basis, the observations made at the macroscopic scale by other authors of a strong lability of the bacterial adhesions mediated by Fn under high turbulent flow are rationalized at the molecular level.

In vivo and in vitro demonstration that Staphylococcus aureus is an intracellular pathogen in the presence or absence of fibronectin-binding proteins

Staphylococcus aureus is the most significant bacterial pathogen associated with bovine mastitis. However, the relevance of intracellular infection to mastitis pathogenesis is poorly understood. We used in vitro assays and a mouse model of mastitis to demonstrate the intracellular component of the infection and to identify the importance of fibronectin-binding proteins in the processes of colonization and internalization. In vitro, a mutant strain, lacking fibronectin-binding protein (FnBPs(-)), had a reduced ability to bind fibronectin and to infect epithelial cells when compared to its parental wild type strain. After 2 h of infection, the internalization of the mutant bacteria into epithelial cell cultures was reduced by 60% compared with the wild type. After in vivo infection, microscopic examination using the FnBPs(-) strain revealed that production of a high density of live bacteria within the mammary gland epithelial cells was delayed. Both parental and mutant strains were identified within neutrophils, macrophages and epithelial cells suggesting a close similarity between the mouse mastitis model and bovine mastitis. These results demonstrate that S. aureus was able to cause an intracellular infection in the mouse model of mastitis and that the elimination of one adhesion protein delayed, but did not prevent, infection.

The fibronectin-binding proteins of Staphylococcus aureus may promote mammary gland colonization in a lactating mouse model of mastitis

The fibronectin-binding proteins (FnBPs) of Staphylococcus aureus are believed to be implicated in the pathogen's adherence to and colonization of bovine mammary glands, thus leading to infectious mastitis. In vitro studies have shown that FnBPs help the adhesion of the pathogen to bovine mammary epithelial cells. However, the importance of FnBPs for the infection of mammary glands has never been directly established in vivo. In this study with a mouse model of mastitis, the presence of FnBPs on the surface of S. aureus increased the capacity of the bacterium to colonize mammary glands under suckling pressure compared to that of a mutant lacking FnBPs.

Complement activation influences Staphylococcus aureus adherence to endothelial cells

The ability of Staphylococcus aureus to adhere to endothelial cells (EC) is a critical step in the development of metastatic infection. The role of complement in S. aureus binding to EC remains uninvestigated. Log-phase S. aureus, expressing minimal capsule, was incubated with serum under various conditions, washed, and then incubated at 37 degrees C for 30 min with cultured human umbilical vein EC (ATCC CRL-1730). Adherence was scored visually after staining with acridine orange. Incubation in 10% heat-inactivated human serum increased adherence to endothelial cells by 488% compared to organisms incubated in buffer. Incubating S. aureus in complement-active normal human serum (NHS) decreased binding to EC by 58% compared to organisms incubated in heat-inactivated serum. The importance of active complement was confirmed by experiments using serum with added EDTA or cobra venom factor, a protein that depletes C3. The expression of capsule by S. aureus strongly interfered with adherence. It has been shown that an important protein for S. aureus adhesion to EC is fibronectin. S. aureus adherence to purified fibronectin increased by 511% after incubation in heat-inactivated serum, compared to that of organisms incubated in buffer. This decreased by 56% in complement-active serum, suggesting that inhibition of S. aureus adherence to EC is due, in part, to complement-mediated diminished binding to fibronectin. Interestingly, when EC were exposed to S. aureus-activated serum and then washed, binding by S. aureus was 234% higher than that of EC exposed to NHS. Thus, complement-activated EC have increased S. aureus binding, while complement on the bacterial surface markedly reduces adherence.

Venous shear stress enhances platelet mediated staphylococcal adhesion to artificial and damaged biological surfaces

We investigated the role of blood components in the adhesion of staphylococci to biological and artificial surfaces under well-defined flow conditions by using the Cone and Plate(let) Analyzer. An enzyme-linked immunosorbent assay-like binding assay with biotinylated bacteria determined the extent of bacterial adhesion to subendothelial extracellular matrix (ECM), polystyrene (PS) and adult bovine aortic endothelial (ABAE) cell monolayer. The presence of adsorbed plasma proteins on PS and ECM did not increase and in some cases reduced staphylococcal adhesion under flow conditions (200s(-1)). However, their presence on ABAE cells increased bacterial adhesion but to a level still lower than the adhesion to PS and ECM. In contrast, adhered platelets significantly increased staphylococcal adhesion to both PS and ECM, but did not affect the adhesion to ABAE cells. Furthermore, bacterial adhesion to the platelets coated ECM and PS under flow conditions (200s(-1)) was increased by 1.4 to 2.6-fold compare to static conditions. The platelet-enhanced bacterial adhesion was markedly inhibited by blockade of the platelet GPIb receptor. In conclusion, staphylococcal extensive adhesion to ECM and PS surfaces is increased by venous flow and mediated by surface adhered activated platelets via a GPIb dependent mechanism. On the other hand, ABAE cells demonstrated limited bacterial adhesion that is mediated by adsorbed plasma proteins. Our results suggest that under physiological venous flow conditions the intact vessel wall is less prone for bacterial adhesion than damaged vessel wall.

Host-parasite interactions in Staphylococcus aureus keratitis

Ulcerative keratitis is among the leading ocular bacterial infections, and Streptococcus aureus accounts for approximately 25% of cases in some surveys. Although S. aureus expresses numerous virulence factors, many of which are under the control of staphylococcal global regulatory genes, their pathophysiologic roles in keratitis are largely unknown. Similarly, the nature of the host response during S. aureus keratitis is unclear. Following a review of previously published research on the pathophysiology of S. aureus ocular infection, we present the results of a study designed to assess the host-parasite relationship between S. aureus and human corneal epithelial cells (HCECs) in vitro. In this model system, a wild-type S. aureus strain and its isogenic mutants harboring mutations in agr and sar global regulatory genes or fibronectin-binding proteins A and B (fnbAB) were tested for their ability to bind and invade confluent HCEC monolayers. The contribution of host cell factors was assessed by preincubating HCECs with various inhibitory agents. These studies demonstrated that S. aureus not only adhered to the surface of HCECs but was also internalized, as has been previously observed in other nonocular cell lines. Adherence and invasion of HCECs was saturable at 1 h of incubation in the presence of approximately 10(7) CFU per HCEC monolayer (multiplicity of infection approximately 10). A mutant defective in both agr and sar global regulators was not significantly different in invasive capacity compared to its isogenic wild-type parent strain. In contrast, mutations in fibronectin-binding proteins A and B (fnbAB) reduced the invasiveness of S. aureus by 99% compared to the wild-type strain. Pretreatment of HCECs with colchicine had little effect on S. aureus invasion. In sharp contrast, cytochalasin D and genistein were each capable of inhibiting invasion by >99%. In summary, the results of this study point to fibronectin-binding protein as a key S. aureus surface adhesin facilitating invasion of HCECs in vitro. Furthermore, these results suggest an active mechanism for S. aureus internalization by HCECs, likely involving actin polymerization and tyrosine kinase activity. Additional studies are warranted to determine the applicability of these findings in vivo, and to facilitate the rational design of therapeutic agents aimed at blocking the establishment and progression of S. aureus keratitis.

Functional variation of the antigen I/II surface protein in Streptococcus mutans and Streptococcus intermedius

Although Streptococcus intermedius and Streptococcus mutans are regarded as members of the commensal microflora of the body, S. intermedius is often associated with deep-seated purulent infections, whereas S. mutans is frequently associated with dental caries. In this study, we investigated the roles of the S. mutans and S. intermedius antigen I/II proteins in adhesion and modulation of cell surface characteristics. By using isogenic mutants, we show that the antigen I/II in S. mutans, but not in S. intermedius, was involved in adhesion to a salivary film under flowing conditions, as well as in binding to rat collagen type I. Binding to human fibronectin was a common function associated with the S. mutans and S. intermedius antigen I/II. Adhesion of S. mutans or S. intermedius to human collagen types I or IV was negligible. Hydrophobicity, as measured by water contact angles, and zeta potentials were unaltered in the S. intermedius mutant. The S. mutans isogenic mutants, on the other hand, exhibited more positive zeta potentials at physiological pH values than did the wild type. The results indicate common and species-specific roles for the antigen I/II in mediating the attachment of S. mutans and S. intermedius to host components and in determining cell surface properties.