Inhibition by heparin and derivatized dextrans of Staphylococcus epidermidis adhesion to in vitro fibronectin-coated or explanted polymer surfaces

The ability of Staphylococcus to recognize several extracellular matrix or plasma proteins (e.g., fibrinogen, fibronectin, and collagen) promotes bacterial attachment to artificial surfaces. Whereas most S. aureus clinical isolates elaborate a wide repertoire of bacterial surface 'receptors' called adhesins, exhibiting specific binding of individual host proteins, S. epidermidis is lacking most of such protein adhesins. To document the interactions between S. epidermidis and various surface-adsorbed proteins, we first compared promotion of bacterial attachment by seven purified human proteins immobilized onto poly(methyl methacrylate) (PMMA) coverslips. Only two of them, namely fibronectin and fibrinogen, exhibited adhesion-promoting activities. In the presence of native heparin or two functionalized dextrans (CMDBS for Carboxy Methyl, Benzylamide sulfonate/sulfate), a dose-dependent inhibition of S. epidermidis adhesion to fibronectin-coated, but not to fibrinogen-coated surfaces was observed. The inhibitory effects of each CMDBS were much stronger than that of native heparin. In contrast, a control highly negatively charged dextran exclusively substituted with carboxy methyl groups exerted no inhibition on S. epidermidis adhesion. To evaluate how CMDBS could interfere with S. epidermidis attachment to coverslips coated in vivo with extracellular matrix components, we also tested PMMA surfaces retrieved from tissue cages Subcutaneously implanted in guinea pigs. Each CMDBS, but not heparin, strongly inhibited S. epidermidis adhesion to explanted coverslips, even in the presence of tissue cage fluid. In conclusion, fibronectin plays an important role in promoting S. epidermidis attachment to implanted biomaterials. Furthermore, S. epidermidis adhesion to fibronectin-coated or implanted biomaterials can be efficiently blocked in vitro by CMDBS.     

Inhibition by heparin and derivatized dextrans of Staphylococcus epidermidis adhesion to in vitro fibronectin-coated or explanted polymer surfaces

The ability of Staphylococcus aureus to recognize several extracellular matrix or plasma proteins (e.g., fibrinogen, fibronectin, and collagen) promotes bacterial attachment to artificial surfaces. Whereas most S. aureus clinical isolates elaborate a wide repertoire of bacterial surface receptors' called adhesins, exhibiting specific binding of individual host proteins, S. epidermidis is lacking most of such protein adhesins. To document the interactions between S. epidermidis and various surface-adsorbed proteins, we first compared promotion of bacterial attachment by seven purified human proteins immobilized onto poly(methyl methacrylate) (PMMA) coverslips. Only two of them, namely fibronectin and fibrinogen, exhibited adhesion-promoting activities. In the presence of native heparin or two functionalized dextrans (CMDBS for Carboxy Methyl, Benzylamide sulfonate/sulfate), a dose-dependent inhibition of S. epidermidis adhesion to fibronectin-coated, but not to fibrinogen-coated surfaces was observed. The inhibitory effects of each CMDBS were much stronger than that of native heparin. In contrast, a control highly negatively charged, dextran exclusively substituted with carboxy methyl groups exerted no inhibition on S. epidermidis adhesion. To evaluate how CMDBS could interfere with S. epidermidis attachment to coverslips coated in vivo with extracellular matrix components, we also tested PMMA surfaces retrieved from tissue cages subcutaneously implanted in guinea pigs. Each CMDBS, but not heparin, strongly inhibited S. epidermidis adhesion to explanted coverslips, even in the presence of tissue cage fluid. In conclusion, fibronectin plays an important role in promoting S. epidermidis attachment to implanted biomaterials. Furthermore, S. epidermidis adhesion to fibronectin-coated or implanted biomaterials can be efficiently blocked in vitro by CMDBS.     

Effects on antibiotic resistance of Staphylococcus epidermidis following adhesion to polymethylmethacrylate and to silicone surfaces

A number of studies appears to give emphasis to the role of prosthetic materials in determining microbial adherence and resistance to host defence and drug therapy. Aim of this study was to explore whether the direct contact with biomaterial substrata of different chemical nature could influence bacterial behaviour, determining possible changes in the bacteria population as far as antibiotic resistance is concerned. To this end, susceptibility to penicillin, erythromycin, clindamycin, cefamandole, imipenem, vancomycin, ciprofloxacin. ampicillin, cefazolin, trimethoprim-sulfamethoxazole, chloramphenicol, amikacin and netilmicin was evaluated in a methicillin-, gentamicin- and tobramycin-resistant Staphylococcus epidermidis strain, after in vitro adhesion to polymethylmethacrylate (PM MA) and to silicone elastomer. The susceptibility to antibiotics of both adherent bacteria and bacteria which, although exposed to the materials, had not undergone adhesion was measured as bacterial growth inhibition area onto a plate antibiogram. according to Kirby-Bauer and using an image analyser system. The results obtained suggest that the two test materials considered in this study were capable to condition bacterial behaviour. In particular. the adhesion onto PMMA surfaces induced a marked and significant decrease in susceptibility to the following beta-lactam antibiotics: cefamandole (32%), cefazolin (23%), imipenem (27%), ampicillin (31%). Moreover, PMMA caused a lower but significant reduction in resistance to vancomycin (15%), chloramphenicol (16%), amikacin (13%). netilmicin (13%), erythromycin (11%) trimethoprim-sulfamethoxazole (13%). In contrast, the adhesion onto silicone elastomer appeared to influence bacterial changes to a lesser extent and elicited a significant decrease in susceptibility only to cefazolin (10%) and amikacin (11). Further studies are required to thoroughly investigate the mechanisms of these variations, even though, also according to other authors, one of the best conceivable conclusions is that some material substrata can lead to selection of variant adhesive bacteria with increased antibiotic resistance.     

Characterization of poly(ethylene oxide) brushes on glass surfaces and adhesion of Staphylococcus epidermidis

Poly(ethylene oxide) brushes have been covalently bound to glass surfaces and their presence was demonstrated by an increase in water contact angles from fully wettable on glass to advancing contact angles of 54 degrees, with a hysteresis of 32 degrees. In addition, electrophoretic mobilities of glass and brush-coated glass were determined using streaming potential measurements. The dependence of the electrophoretic mobilities on the ionic strength was analyzed in terms of a softlayer model, yielding an electrophoretic softness and fixed charge density of the layer. Brush-coated glass could be distinguished from glass by a 2-3-fold decrease in fixed charge density, while both surfaces were about equally soft. Adhesion of Staphylococcus epidermidis HBH276 to glass in a parallel plate flow chamber was extremely high and after 4 h, 19.0 x 10(6) bacteria were adhering per cm2. In contrast, the organisms did not adhere to brush-coated glass, with numbers below the detection limit, i.e. 0.1 x 10(6) per cm2. These results attest to the great potential of polymer brushes in preventing bacterial adhesion to surfaces.     

Quantitative analysis of adhesion and biofilm formation on hydrophilic and hydrophobic surfaces of clinical isolates of Staphylococcus epidermidis

Staphylococcus epidermidis is now well established as a major nosocomial pathogen associated with infections of indwelling medical devices. The major virulence factor of these organisms is their ability to adhere to devices and form biofilms. However, it has not been established that adherence and biofilm formation are closely linked phenotypes for clinical isolates. In this study, the initial adhesion to different materials (acrylic and glass) of 9 clinical isolates of S. epidermidis, along with biofilm-positive and biofilm-negative control strains, was assayed using physico-chemical interactions to analyze the basis for bacterial adherence to the substratum. X-ray photo electron spectroscopy (XPS) analysis of the cell surface elemental composition was also performed in an attempt to find a relationship between chemical composition and adhesion capabilities. Biofilm formation on the two surfaces was evaluated by dry weight measurements. Human erythrocytes were used to evaluate the ability of S. epidermidis strains to cause hemagglutination, an indicator of the production of a poly-N-acetyl glucosamine cell surface polysaccharide also involved in biofilm formation. The clinical isolates exhibited different cell wall physico-chemical properties, resulting in differing abilities to adhere to surfaces. Adhesion to hydrophobic substrata for all strains occurred to a greater extent than that to hydrophilic surfaces. Bacterial cell hydrophobicity seemed to have little or no influence on adhesion. X-ray photoelectron spectroscopy analysis showed a high ratio of oxygen/carbon for all strains, which is a common characteristic of S. epidermidis species. No relevant relationship was found between XPS data and adhesion values. All strains forming biofilms were able to agglutinate erythrocytes. However, no direct relationship was found between the amount of biofilm formed and the initial adhesion extent. These results indicate that high levels of initial adherence do not necessarily lead to thick biofilm formation. These two aspects of the pathogenesis of medical device related-infection may need to be evaluated independently to ascertain the contribution of each to the virulence of S. epidermidis causing device-related infections.     

Staphylococcus epidermidis adhesion on modified urea/urethane elastomers

Block urea/urethane co-polymer films present elastomeric properties with the possible tuning of their surface properties within a wide range and are therefore considered relevant surfaces for possible medical applications. In particular, thin free standing films of urea/urethane elastomers with two soft segments, polypropylene oxide and more hydrophobic polybutadiene, develop multistable states with surface topography features with remarkable regularity. Moreover, complex surface structures may be obtained by UV radiation treatment followed by suitable mechanical action and also by extraction of the elastomer with a suitable solvent. In the present work, different modified elastomer samples were assayed for Staphylococcus epidermidis adhesion during 2 h and the extent of bacterial adhesion was evaluated by automatic cell enumeration. Bacterial adhesion assays demonstrate that the typical trend relating the increase in the number of adhered bacteria with the increase of the surface roughness does not hold for all materials. Results may be interpreted taking into account both the surface topography and the different types of micro-phase segregation of hydrophobic and hydrophilic parts of the elastomer.     

Staphylococcus epidermidis adhesion on modified urea/urethane elastomers

Block urea/urethane co-polymer films present elastomeric properties with the possible tuning of their surface properties within a wide range and are therefore considered relevant surfaces for possible medical applications. In particular, thin free standing films of urea/urethane elastomers with two soft segments, polypropylene oxide and more hydrophobic polybutadiene, develop multi-stable states with surface topography features with remarkable regularity. Moreover, complex surface structures may be obtained by UV radiation treatment followed by suitable mechanical action and also by extraction of the elastomer with a suitable solvent. In the present work, different modified elastomer samples were assayed for Staphylococcus epidermidis adhesion during 2 h and the extent of bacterial adhesion was evaluated by automatic cell enumeration. Bacterial adhesion assays demonstrate that the typical trend relating the increase in the number of adhered bacteria with the increase of the surface roughness does not hold for all materials. Results may be interpreted taking into account both the surface topography and the different types of micro-phase segregation of hydrophobic and hydrophilic parts of the elastomer.     

Neutrophil chemotactic activity of peptidoglycan. A comparison between Staphylococcus aureus and Staphylococcus epidermidis

The ability of peptidoglycan from Staphylococcus epidermidis and Staphylococcus aureus to generate in human serum chemoattractant for peripheral blood neutrophils was studied. It was shown that PG from the two bacteria was able to induce chemotactic activity in normal human serum. Sonication of PG was required to generate this activity. Very little or no activity was generated in heat-treated or C5-deficient human serum by PG, indicating that PG treatment of serum resulted in generation of chemoattractants by activation of complement. Kinetics studies employing C2-deficient or MgEGTA-chelated serum revealed that S. epidermidis induced chemotactic activity by activating the alternative complement pathway. The alternative complement activation induced by S. epidermidis occurred rapidly and was completed after 15 min, whereas S. aureus activated the alternative pathway much more slowly, with activation reaching a maximum at 60 min. The rapid activation of the alternative complement pathway by S. epidermidis PG may partly explain why this bacterium does not normally cause infections in healthy individuals.     

Effects of controlled fibronectin surface orientation on subsequent Staphylococcus epidermidis adhesion

Several bacterial species, including Staphylococcus aureus and Staphylococcus epidermidis (SE) are known to express cell receptors that bind specifically to surface immobilized or extracellular matrix ligands, such as the protein fibronectin (FN). Yet, few existing studies have examined the effect of protein surface orientation on bacterial adhesion. We report here a substratum modification protocol that allows for the specific orientation of FN molecules on a surface at known levels of surface coverage. Monoclonal antibodies (Mabs), specific to either the COOH-terminus or NH3-terminus of FN, are conjugated to biotin, then immobilized to streptavidin-coated glass substrata. Specific orientation of the bound FN molecules is verified using the same Mabs in an ELISA. Bacterial adhesion of Staphylococcus epidermidis (SE) to FN bound by either its C-terminus or its NH3-terminus was quantified in batch static adhesion assays. Results indicate an increase in SE adhesion to FN-coated surfaces when the FN is bound by its C-terminus (NH3-terminus free), indicating SE receptor-specific adhesion to the FN NH3-terminus. These studies demonstrate that antifibronectin monoclonal antibodies can be used to specifically bind and orient fibronectin on a surface. In addition, adhesion of SE to these model substrata can be controlled by the orientation of the protein.     

Effect of surface modification of siliconeon Staphylococcus epidermidis adhesion and colonization

Cerebrospinal fluid (CSF) shunts for the treatment of hydrocephalus are generally made of silicone rubber. The growth of bacterial colonies on the silicone surface leads to frequent CSF shunt complications. A systematic study of the effect of the surface modification of silicone on Staphylococcus epidermidis adhesion and colonization was performed for different incubation times by means of colony counting and scanning electron microscopy (SEM). Silicone was modified with different biopolymers and silanes, including heparin, hyaluronan, octadecyltrichlorosilane (OTS), and fluoroalkylsilane (FAS) to provide a stable and biocompatible surface with different surface functional groups and degrees of hydrophobicity. The modified silicone surfaces were studied by using contact angle measurements, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). After 4 and 8 h of incubation, the FAS- and OTS-coated silicone and the hyaluronan coated OTS/silicone surfaces showed significantly reduced bacterial adhesion and colonization compared to blank silicone by both quantification methods. However, the heparin coated OTS/silicone showed significantly increased bacterial adhesion. These results indicate that the nature of the surface functional group and surface roughness determine the extent of bacterial adhesion and colonization. However, the degree of hydrophobicity of the surface did not appear to play a determining role in bacterial adhesion and colonization.     

Expression of leukocyte adhesion molecules by endothelial cells seeded on various polymer surfaces

Although endothelial cell seeding in small-diameter vascular prostheses significantly improves graft survival, the detachment of adherent endothelial cells after the restoration of circulation remains one of the major obstacles. Because in vivo experiments indicate that leukocyte infiltration is involved in endothelial cell loss, we hypothesize that seeded endothelial cells become activated and express leukocyte adhesion molecules and cytokines because of an interaction with the underlying polymer surface. The aim of this study was to investigate the expression of the leukocyte adhesion molecules ICAM-1, VCAM-1, PECAM-1, and E-selectin by cultured human umbilical vein endothelial cells (HUVECs) and human adipose microvascular endothelial cells (HAMVECs). The cells were seeded on tissue culture poly(styrene) and the vascular graft materials Dacron and Teflon. The results of this study indicate that the expression of leukocyte adhesion molecules by cultured endothelial cells is mainly affected by the endothelial cell origin, that is, umbilical vein or adipose tissue. Expressions of both ICAM-1 and E-selectin by HUVECs and HAMVECs are characterized by the presence of two cell populations with distinct levels of expression. With respect to endothelial cell seeding in vascular prostheses, the increased expression of E-selectin by microvascular endothelial cells deserves further attention.     

Influence of silicone surface roughness and hydrophobicity on adhesion and colonization of Staphylococcus epidermidis

Bacterial adhesion and colonization are complicated processes that depend on many factors, including surface chemistry, hydrophobicity, and surface roughness. The contribution of each of these factors has not been fully elucidated because most previous studies used different polymeric surfaces to achieve differences in properties. The objective of this study was to modify hydrophobicity and roughness on one polymeric surface, eliminating the confounding contribution of surface chemistry. Mechanically assembled monolayer (MAM) preparation methods (both one- and two-dimensional) were used to impart different degrees of hydrophobicity on fluoroalkylsilane (FAS)-coated silicone. Surface roughness was varied by casting the silicone to templates prepared with different abrasives. Surface hydrophobicity was determined by contact angle measurement, whereas surface roughness was determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Bacterial adhesion and colonization were analyzed using a direct colony-counting method and SEM images. Hydrophobicity increased as a function of stretched length or width (Deltax or Deltay); it reached a maximum at Deltax = 60% with one-dimensional MAM and decreased as Deltax further increased to 80 and 100%. The same trend was observed for the two-dimensional MAM. After 12-h incubation, all the FAS/silicone surfaces had significantly reduced adherence of Staphylococcus epidermidis by 42-89%, compared to untreated silicone, and the degree of which is inversely related to surface hydrophobicity. On the other hand, surface roughness had a significant effect on bacterial adhesion and colonization only when the root-mean-square roughness was higher than 200 nm.     

Human erythrocyte adhesion and spreading on protein-coated polymer surfaces

Protein adsorption is the first event which occurs when polymer surfaces are exposed to blood. The adsorption of proteins modifies the surface properties of the substrates and therefore influences subsequent cell-surface interactions. In an attempt to elucidate the fundamental mechanisms governing cell-proteinated-surface interactions, the extent of fresh human erythrocyte adhesion and spreading on protein-coated surfaces was examined. Five human serum proteins (albumin, fibrinogen, immunoglobulin G, fibronectin, and transferrin) were used at bulk concentrations ranging from 0.01 mg/mL to 50 mg/mL. Polymer substrates covering a wide range of wettability were employed. Protein adsorption significantly reduces erythrocyte adhesion and spreading on all test surfaces with minimum adhesion observed on fibrinogen: IgG greater than albumin greater than fibronectin greater than transferrin greater than fibrinogen. The extent of these effects is dependent on the nature of the adsorbed protein, the protein bulk concentration, and the surface properties of the underlying polymer substrates.     

Initial adhesion and surface growth of Staphylococcus epidermidis and Pseudomonas aeruginosa on biomedical polymers

The infection risk of biomaterials implants varies between different materials and is determined by an interplay of adhesion and surface growth of the infecting organisms. In this study, we compared initial adhesion and surface growth of Staphylococcus epidermidis HBH(2) 102 and Pseudomonas aeruginosa AK1 on poly(dimethylsiloxane), Teflon, polyethylene, polypropylene, polyurethane, poly(ethylene terephthalate), poly(methyl methacrylate), and glass. Initial adhesion was measured in situ in a parallel plate flow chamber with microorganisms suspended in phosphate-buffered saline, while subsequent surface growth was followed in full and in 20 times diluted growth medium. Initial adhesion of both bacterial strains was similar to all biomaterials. In full growth medium, generation times of surface growing S. epidermidis ranged from 17 to 38 min with no relation to wettability, while in diluted growth medium generation times increased from 44 to 98 min with increasing surface wettability. For P. aeruginosa no influence of surface wettability on generation times was observed, but generation times increased with decreasing desorption rates, maximal generation times being 47 min and minimal values down to 30 min. Generally, generation times of adhering bacteria were shorter than of planktonic bacteria. In conclusion, surface growth of initially adhering bacteria is influenced by biomaterials surface properties to a greater extent than initial adhesion.     

Involvement of adherence and adhesion Staphylococcus epidermidis genes in pacemaker lead-associated infections

We explored three genes of attachment (fbe and atlE) and adhesion (ica) in 27 and 10 Staphylococcus epidermidis strains involved in pacemaker-related infections (PMI) and intravascular-catheter-related infections (IVCI), respectively, and in 25 saprophytic strains. The detection rates of fbe and atlE were identical in PMI and IVCI strains, but ica detection rates were identical in PMI and saprophytic strains.     

血糖对树鼩感染表皮葡萄球菌清除能力及植入性材料细菌黏附性的影响

背景：高浓度葡萄糖培养条件下,细菌在生物材料表面有较强的生物膜形成能力。 目的：观察血糖升高对表皮葡萄球菌在动物体内清除能力及植入性生物材料上细菌生物膜形成的影响。 方法：注射链脲佐菌素建立高血糖树鼩模型(血糖≥11.1 mmol/L),采用生物膜形成阳性与阴性表皮葡萄球菌感染高血糖与正常对照树鼩,并同时在动物股静脉内植入PVC导管。 结果与结论：感染生物膜形成阳性的表皮葡萄球菌株后,血糖≥11.1 mmol/L树鼩血液、心脏、肝脏、肾脏、胰腺的细菌感染率及菌落计数较正常对照组高(P < 0.05);扫描电镜观察血糖≥11.1 mmol/L组植入生物材料上有明显的生物膜形成。感染生物膜形成阴性表皮葡萄球菌株后无论血糖高低,均未观察到生物膜形成。表明血糖升高不仅使植入生物材料树鼩的细菌清除能力下降,同时可诱导细菌在植入生物材料表面形成明显的生物膜。     

Adherence of slime-producing strains of Staphylococcus epidermidis to smooth surfaces.

Slime production is not a generally recognized feature of Staphylococcus epidermidis. In a recent outbreak of S. epidermidis intravascular catheter-associated sepsis, we noted that 63% of clinically implicated strains grew as a slimy film coating the culture tube walls when propagated in tryptic soy broth. Only 37% of randomly collected blood culture contaminants and skin isolates demonstrated a similar phenomenon (p less than 0.05). Transmission electron micrographs of these coating bacteria showed them to be encased in an extracellular matrix that stained with alcian blue. Slime production was most evident in autoclaved media containing Casamino Acids and glucose supplementation (0.25% wt/vol). There were strain and media preparation variability of slime production in the presence of other carbohydrates. Some strains were not able to produce slime under any of the tested conditions. The production or nonproduction of slime did not influence growth rate. When grown in vitro, slime producers accumulated on the surface of intravascular catheters as macrocolonies, whereas non-slime, producers did not. Transmission and scanning electron micrographs showed slime producers to be encased in an adhesive layer on the catheter surface, whereas nonproducers were not encased. These results suggest that slime-mediated adherence may be a critical factor in the pathogenesis of S. epidermidis infections of medical devices.     

Teichoic acid enhances adhesion of Staphylococcus epidermidis to immobilized fibronectin.

Adhesion is a prerequisite for coagulase-negative staphylococci to cause invasive disease and may be mediated by adhesive host molecules adsorbed on implanted polymers. In this study, we can confirm previous observations demonstrating binding of Staphylococcus epidermidis to fibronectin (FN) adsorbed polymer surfaces. So far, the nature of FN-recognizing adhesin(s) in S. epidermidis remains elusive. Since teichoic acids (TA) have been shown to exert binding functions for extracellular matrix molecules in several Gram-positive species, we have purified wall TA of S. epidermidis laboratory strains KH11 and RP62A, as well as clinical isolate AB9. Using a polymethylmethacrylate (PMMA) coverslip adhesion assay, a microtitre plate assay and a particle agglutination assay, we found that purified TA significantly enhanced adhesion of S. epidermidis KH11 and RP62A to FN coated surfaces. Enhanced adhesion was dose-dependent and saturable. Preincubation, either of microorganisms or of FN coated surfaces, with TA promoted adhesion, while adhesion to TA-adsorbed PMMA was comparably low. This observation may suggest a potential role of cell wall carbohydrates as bridging molecules between microorganisms and immobilized FN in early steps of S. epidermidis pathogenesis.     

Kinetics of cell adhesion to polymer surfaces

Results of the kinetics of adhesion of granulocytes as well as fresh and glutaraldehyde-fixed erythrocytes, suspended in Hanks Balanced Salt Solution (HBSS; pH 7.2, ionic strength of 0.15) to various polymeric substrates are presented. Cell adhesion increases rapidly initially and reaches a plateau value after approximately 30 minutes. There is no evidence for a lag-time in the onset of cell adhesion, suggesting that electrostatic double-layer forces are negligible under these experimental conditions. For the experiments in which the cells are suspended in HBSS, which has a surface tension larger than that of the cells, the level of cell adhesion increases with decreasing surface tension of the polymeric substrates. An additional experiment with fresh human granulocytes suspended in HBSS containing 10% dimethyl sulfoxide was also performed. The surface tension of the resulting liquid medium is below that of the cells and the pattern of adhesion is reversed, in agreement with the predictions of a thermodynamic model for cell adhesion. The slightly different behavior of siliconized glass as a substrate is discussed in terms of "screening."     

The rate of adhesion of melanoma cells onto nonionic polymer surfaces.

The rates of adhesion of melanoma cells (carcinogenic) onto nonionic polymer surfaces were studied by using radioactively labeled cells and measuring the fraction of cells which adhered to the surface in a given time. Glow discharge (plasma) polymerization of 1,1,3,3-tetramethyldisiloxane and of nitrogen-acetylene-water (mole ratio 0.4:1.0:0.2) was used to modify the surface energy of the substrate. The cell adhesion rate was found to be given by Y = 1 - exp [-k0(gammas - gamma0)t], where Y is the fraction of cells adhered, -k0 is a characteristic rate constant, gammas is the total surface energy of the substrate, gamma0 is the threshold surface energy of cell adhesion, and t is time.