Bacillus cereus adhesion: an investigation of the physicochemical characteristics of surface and effect of bio adhesion on the properties of silicone

The initial microorganism adhesion on substrate is an important step for biofilm formation. The surface properties of the silicone and Bacillus cereus were characterized by the sessile drop technique. Moreover, the physicochemical properties (hydrophobicity; electron donor/electron acceptor) of surface adhesion and the impact of bio adhesion on the silicone were determined at different time of contact (3, 7, and 24?h). The results showed that the strain was hydrophilic (Giwi?=?3.37?mJ/m²), whereas the silicone has hydrophobic character (Giwi?=??68.28?mJ/m²). Silicone surface presents a weak electron-donor character (γ ^??=?2.2?mJ/m²) conversely to B. cereus that presents an important electron donor-parameter (γ ^??=?31.6?mJ/m²). The adhesion of B. cereus to silicone was investigated using environmental scanning electron microscope and image analysis was assessed with the Matlab^® program. After 3?h of contact, the data analysis, confirmed the bio adhesion with an amount of 9.610⁵?cfu/cm² adhered cells. After 24?h, the percentage of silicone covered reached 93%. Furthermore, despite the difference in hydrophohbicity, the interaction between B. cereus and substrata was favoured by the thermodynamic model of adhesion (ΔG _adhesion ?<?0). The real time investigation of the effect of B. cereus adhesion on the physicochemical properties of silicone has revealed that the substrata becomes hydrophilic (θ°?=?47.3, ΔGiwi?=?23.7?mJ/m²), after 7?h of contact. This bio adhesion had also favoured the increase of electron donor/acceptor character of silicone (γ ^??=?53.1?mJ/m² and γ ⁺?=?5.3?mJ/m²).     

Physicochemical characterization of actinomycetes isolated from decayed cedar wood: contact angle measurement

Physicochemical characterization of microorganism is very important in a wide range of scientific and technological fields. In this study, we reported the isolation and the molecular identification of actinomycetes recovered from cedar wood decay. The isolates named H5 and H8 were identified by 16S rDNA sequencing and were shown to belong to the genus Nocardia and Streptomyces, respectively. Furthermore, physicochemical proprieties including hydrophobicity, electron donor/acceptor, and the Lifshitz–van der Waals (γ^LW) of these strains were evaluated using contact angle measurements. The results showed that Nocardia sp. (H5) had a hydrophobic (ΔGiwi?=??78.56?mJ/m²) and a weak electron donor/acceptor character. In contrast, results from contact angle measurements showed that the surface free energy of Streptomyces strains (H2, H3, and H8) were ΔGiwi?=?20.71?mJ/m², ΔGiwi?=?30.63?mJ/m², and ΔGiwi?=?15.35?mJ/m², respectively, classifying these microorganisms as hydrophilic bacterium. Moreover, the three strains were predominantly electron donating (γ^–?) and exhibit a weak electron-accepting (γ⁺) character.     

A Study on the Impact of the Adhesion of Penicillium expansum on the Physicochemical Surface Properties of Cedar Wood

The sessile drop technique was used to investigate the evolution of the physicochemical properties of cedar wood as a function of contact time with the Penicillium expansum spores. The most important finding showed that the impact of different contact periods (2, 4, 6, 8, 10, and 24 hr) on the wood surface were very indicative. In fact, after 2 hr of contact, the results have shown a significant impact of the bioadhesion of spores to the substrate on both the hydrophobic character (θ_W = 108.5°; ΔGiwi = ?28.25 mJ/m²), the electron donor (γ^? = 13.63 mJ/m²), and the electron acceptor (γ⁺ = 4.35 mJ/m²) parameters that were significantly reduced compared to the initial wood (θ_W = 118.5°; ΔGiwi = ?6.29 mJ/m²; γ^? = 32.1 mJ/m²; and γ⁺ = 9.1 mJ/m²). In addition, this decrease of parameters continued over time to stabilize after 10 hr of contact. Indeed, after 24 hr, the acid/base properties were almost zero and the contact angle with water decreased to 30°. Moreover, it was found that the coefficient of correlation (r²) was strong between the contact angle with water, the surface energy, and the electron acceptor character with the contact time parameter with values (r² = 0.65), (r² = 0.79), and (r² = 0.68), respectively.     

Study of marine bacteria adhesion on sea-immersed 304 and 316 stainless steels: experimental and theoretical investigations

In this study, we investigated the potential adhesion of marine bacteria isolated from seawater in the port of Chmaâla, Morocco, to sea-immersed 304 and 316 stainless steels using thermodynamic approach and the Environmental Scanning Electron Microscopy (ESEM). Furthermore, the physicochemical properties including hydrophobicity and electron donor / electron acceptor (Lewis acid-base) of bacterial isolates and both substrates were evaluated using the contact angle measurements. The molecular identification indicated that the isolated strains were Bacillus thuringiensis and Bacillus amyloliquefascience. Results also showed that both bacterial strains’s cells have a hydrophilic character with Δ_Giwi values of 29.30 and 24.12 mJ m^?2 respectively for Bacillus thuringiensis and Bacillus amyloliquefascience, and are strong electron donating (γ^?) and weakly electron accepting (γ⁺). For substrates surfaces, we found that both sea-immersed stainless steels types were hydrophilic and present strong electron-donor character (γ^? = 49 ± 0.2 mJ m^?2 for 304 and γ^? = 55.07 ± 0.02 mJ m^?2 for 316) and weak electron-acceptor character (γ⁺ = 5.4 ± 0.1 mJ m^?2 for 304 and γ⁺ = 8.3 ± 0.06 mJ m^?2 for 316). The theoretical prediction showed that both tested strains, B. thuringiensis and B. amyloliquefascience, exhibited positive values of ΔG^Total vis-à-vis the two sea-immersed stainless steels types which indicates unfavorable adhesion while the ESEM electro-micrographs show that both strains were able to adhere to both strainless steels surfaces.     

Dairy biofilm: an investigation of the impact on the surface chemistry of two materials: silicone and stainless steel

Biofilms are the most common mode of bacterial growth in nature and the formation will occur on organic or inorganic solid surfaces in contact with a liquid. The aims of this study were, by combining numeration and sessile drop technique, (i) to characterize the structural dynamics of dairy biofilm growth and the physico chemical properties on silicone and stainless steel and (ii) to evaluate the impact of bio-adhesion on chemistry of surfaces at different times of contact (2, 7, 9 and 24?h). Significantly, greater biofilm volumes were observed after 48?h on two materials. Gram-positive bacteria and fungal population exhibited a significantly higher biofilm organization than gram-negative (43–64%). Elsewhere, after 48?h, results showed a slight difference on gram-negative adhered cells on stainless steel than silicone (2.6?×?10⁷?cfu/cm² and 4.7?×?10⁵?cfu/cm², respectively). Moreover, the physico chemical properties of the surfaces showed that the silicone and stainless steel have a hydrophobic character (Giwi?=??68.28?mJ/m² and ?57.6?mJ/m², respectively). Also, both the surfaces present a weak electron donor character (γ ^??=?2.2?mJ/m² and 4.1?mJ/m², respectively). The real-time investigation of the impact of dairy biofilm on the physico chemical properties of the materials has shown a decrease of hydrophobicity degree of the silicone surface that becomes hydrophilic (ΔGiwi?=?11.47?mJ/m²) after 7?h and the increase of electron donor character (γ ^??=?75.8?mJ/m²). Elsewhere, bio-adhesion on stainless steel was accompanied with a decrease of hydrophobicity degree of the surface, which becomes hydrophilic after 7?h of contact (ΔGiwi?=?6.62?mJ/m²) and the increase of the electron donor character (γ ^??=?44.8?mJ/m²). While, after 24?h of contact, results showed a decrease of the hydrophilicity degree and surface energy components of silicone and stainless steel that become hydrophobic (ΔGiwi?=??21.2?mJ/m² and ΔGiwi?=??56.51?mJ/m², respectively) and weak electron donor (γ ^??=?14.0 and 2.3?mJ/m², respectively).     

Marine diatom Navicula jeffreyi from biochemical composition and physico-chemical surface properties to understanding the first step of benthic biofilm formation

To understand the first step of marine benthic microbial mat formation and biofouling phenomena, caused by diatoms in the marine environment, the surface properties of the epipelic diatom Navicula jeffreyi were studied and the composition of its bound Extracellular Polymeric Substances (EPS) was determined. These parameters are determining factors for the initial adhesion step of diatoms to other constituents that start marine fouling. Surface energy of a diatom cell layer was determined using the sessile drop technique and highlights that diatoms show a moderate hydrophobic character (contact angle with water >68°), no Lewis acid character (γ⁺?<1?mJ/m²), and a low Lewis basic character (γ^??=?16.1?mJ/m²). An extraction procedure using a cationic resin subtracted only the bound EPS. Biochemical assays showed that there were 2.5 times more proteins than sugars. The propensity of Navicula jeffreyi diatom to adhere to five different solid surfaces, showing a gradient in their hydrophobic and hydrophilic character, was measured. The attachment densities were high on hydrophobic surfaces such as polytetrafluoroethylene and very low on substrata with surface free energy over 40–50?mJ/m². Using a thermodynamic approach, the free energy of adhesion of the diatom to the five substrata was determined, and led to a very strong correlation with attachment densities for polytetrafluoroethylene, polyamide, polyethylene, and stainless steel.     

Impact of enzymatic treatment on wood surface free energy: contact angle analysis

The aim of this study was to investigate the effect of cellulase treatment on wood surface physicochemical characteristics. The cedar wood samples were treated by cellulase for 30 min at different concentrations: 0.2, 0.4, 0.6, 0.8, 1, and 1.2 mg/ml. Then the physicochemical properties (the wetting behavior and interfacial free energy) of the cedar samples surfaces were characterized by the sessile drop technique. The obtained results showed that the untreated cedar wood samples exhibited a hydrophobic character with a high water contact angle (θ_w = 71.9°) and a negative value of the interfacial free energy (?Giwi = ?59.3 mJ/m²). Thereafter, the wood hydrophobicity decreased continuously until it reached the hydrophilicity qualitatively (at the first concentration of cellulase (0.2 mg/ml)) and quantitatively (at 0.8 mg/ml of cellulase). The cedar wood surface treatment with cellulase also revealed a significant evolution of the acid-base parameters. Moreover, a linear relationship between the degree of hydrophobicity and the cellulase concentration has been found. This study clearly shows the impact that could have the produced enzyme by micro-organisms involved in wood biodegradation and especially their consequences on the physicochemical surface properties of the wooden materials.     

Surface tension parameters of ice obtained from contact angle data and from positive and negative particle adhesion to advancing freezing fronts

From contact angle data obtained on flat ice surfaces with a number of liquids, combined with data on particle and macromolecule adhesion or non-adhesion to advancing freezing fronts, the apolar (Lifshitz-van der Waals or LW) and polar (Lewis acid-base or AB) surface tension (γ) components and parameters have been determined. At 0°C these are γ^LW _iee = 26.9 and γ^AB _ice = 39.6 mJ/m². The latter consists of an electron-acceptor (γ^⊕) and an electron-donor (γ^?) parameter: γ^⊕ = 14 and γ^? = 28 mJ/m².     

Characterization of cellulose surface free energy

The thin-layer wicking technique was used to determine the surface free energy components and the surface character of three celluloses (Sigmaccll 101, Sigmacell 20, and Avicel 101), using an appropriate form of the Washburn equation. For this purpose, the penetration rates of probe liquids into thin porous layers of the celluloses deposited onto horizontal glass plates were measured. It was found that the wicking was a reproducible process and that the thin-layer wicking technique could be used for the determination of the celluloses' surface free energy components. The size of the cellulose particles was characterized with the Galai CIS-100 system and their crystallinity was measured by X-ray diffraction. The three celluloses have high apolar (y^LWS = 50-56 mJ/m²) and electron donor (γs = 42-45 mJ/m²) components, while the electron acceptor component (γS⁺ ) is practically zero. The free energy interactions of cellulose/water/cellulose calculated from the components are positive, regardless of the cellulose crystallinity. This would mean that the cellulose surfaces have a hydrophilic character. However, the work of spreading of water has a small negative value (3-9 mJ/m²), indicating that the surfaces are slightly hydrophobic. It is believed that the work of spreading characterizes better the hydrophobicity of the surface than the free energy of particle/water/particle interaction, because in the latter case, no electrostatic repulsion is taken into account in the calculations.     

The Anti-Adherent Activity of Plant Extracts on Penicillium Commune Spores Causing Cedar Wood Decay: An ESEM Analysis

Decay and deterioration of wood by bacteria and fungi are becoming very serious problems because of the resistance of microorganisms to antimicrobial agents. Thus, this study was aimed at investigating the anti-adhesion effect of Myrtus communis and Thymus vulgaris extracts obtained by classical and ultrasonic extraction, against of Penicillium commune spores isolated from fez Medina cedar wood. According to environmental scanning electron microscopy (ESEM) analysis, the results showed that all extracts tested have proven their ability to inhibit the adhesion of the fungal spores studied. In addition, the extract of each plant obtained by ultrasonication showed a low percentage (47% and 33%) of fungal spores adherent to a wood substrate compared to those recovered by maceration (68%). Furthermore, M. communis extracts have shown outstanding anti-adhesive activity, similar to that demonstrated by those of T. vulgaris. Finally, the treatment of wood by M. communis extracts obtained by ultrasonication showed a very important anti-adhesive activity at a concentration of 20 mg/mL compared to that at 5 mg/mL. Therefore, these extracts can be considered a potential source of bioactive metabolites acting as anti-adhesion molecules in novel formulations for the cedar wood preservation.     

Kinetics of the thermal inactivation of Bacillus subtilis α‐amylase and its application on the desizing of cotton fabrics

The thermal inactivation of Bacillus subtilis α‐amylase was studied in the presence and in the absence of Ca²⁺ at various temperatures. Inactivation rate constant (k), half‐life time (t_1/2), and activation energy (E_a) were determined to characterize the inactivation of the enzyme. Results obtained showed that the thermal inactivation of Bacillus subtilis α‐amylase followed a first‐order kinetics. The addition of Ca²⁺ had a good thermostabilizing effect on the enzyme. The stabilizing effect of Ca²⁺ is reflected by the increased values of the activation energy, which is about two times higher in the presence than that in the absence of 20 mM Ca²⁺, and the decreased values of the inactivation rate constants. The desizing of the cotton fabrics was performed through steaming at 100°C with Bacillus subtilis α‐amylase. The desizing efficiency seemed to be dependent on the concentration and pH value of the enzyme solution. It was found that through the steaming process with α‐amylase, the desizing ratio of the cotton fabrics could be beyond 98% and little damage happened to the fibers of the fabrics. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Cell surface properties of the demulsifying strain Alcaligenes sp. S-XJ-1 governing its behavior in oil–water biphasic systems

Bacterial behavior in oil–water biphasic systems plays an essential role in hydrophobic contaminant degradation, oil recovery, and emulsion breaking. Less is known about the cell surface properties that govern their behaviors in oil–water biphasic systems. In this study, biphasic partitioning and aggregation of a demulsifying strain of Alcaligenes sp. S-XJ-1 were experimentally measured and evaluated based on the cell surface properties of surface charge, surface free energy, and cell surface hydrophobicity (CSH). The S-XJ-1 was cultivated with five different carbon sources, and the results showed a highly varied partitioning, aggregation behavior, and cell surface properties. The calculated interaction energies, based on the cell surface properties, were consistent with the results of their behavior. Among the cell surface properties, the electron-donor character (γ^?, range 8.8–57.0 mJ/m²), which correlated well with CSH (ΔG_bwb), was an essential indicator of cell behavior. A low γ^? value enhanced the cell–interface and cell–cell interaction energies, which promoted cell partitioning and aggregation eventually leading to demulsification. The results and analysis provide important information for researchers concerned with cell–cell and cell–interface interactions.     

FRICTION AND ADHESION MEASUREMENTS BETWEEN A FLUOROCARBON SURFACE AND A HYDROCARBON SURFACE IN AIR

The friction and adhesion between a fluorocarbon monolayer-coated surface against a hydrocarbon monolayer-coated surface has been directly measured. The friction was found to be lower than the friction between a hydrocarbon monolayer against a hydrocarbon monolayer and a fluorocarbon monolayer against a fluorocarbon monolayer. No stick-slip sliding was observed for speeds from 0.8?µm/s to 2.6?µm/s. The fluorocarbon–hydrocarbon interface was adhesive, with the energy of interaction measured to be 14.9?mJ/m²?±?1.0?mJ/m². As predicted from theory, the magnitude of the adhesion of a fluorocarbon monolayer against a hydrocarbon monolayer is between that measured for a fluorocarbon monolayer against a fluorocarbon monolayer and a hydrocarbon monolayer against a hydrocarbon monolayer. One may note that the interfacial energy, γ, follows the general trend γ_FC/FC?<?γ_HC/FC?<?γ_HC/HC, whereas the shear stress, τ, varies according to τ_FC/HC?<?τ_HC/HC?<?τ_FC/FC.     

Seawater Ozonation of Bacillus subtilis Spores: Implications for the Use of Ozone in Ballast Water Treatment

The potential of ozone for disinfection of ships’ ballast water was investigated using Bacillus subtilis spores as an indicator. The effects of pH, presence of iron, and bacterial strain on disinfection efficacy in seawater, under simulated ballast conditions, were investigated. Ozone dosages of 9 mg/L (pH 7) and 14 mg/L (pH 8.2) and 24 h contact achieved a 4-log inactivation with the various oxidant residuals formed. Iron surface at a ratio to water of 9 m²/m³ impaired the oxidant residuals and the disinfection of spores. Different strains of B. subtilis resulted in different CT values. Ozone does not seem to be a good choice for the control of spore-forming organisms in ballast water, but may be suitable for the control of other species.     

Fermentation and oxygen transfer characteristics in serine alkaline protease production by recombinant Bacillus subtilis in molasses‐based complex medium

Serine alkaline protease (SAP) production in a complex medium based on physically pretreated molasses by recombinant Bacillus subtilis carrying pHV1431::subC gene is described. The effects of oxygen transfer were investigated in 3.5 dm³ bioreactor systems with controls for agitation rate, dissolved oxygen, pH, temperature, and foam formation under two different agitation rates, ie N = 500 and 750 min^?1, and four different air flow rates, ie Q/V_R = 0.2, 0.5, 0.7, and 1.0 vvm, at a molasses concentration equivalent to initial sucrose concentration (C_So) of 20 kg m^?3. The yield values (Y_X/S, Y_X/O, Y_S/O) and maintenance coefficient of oxygen (m_O), were calculated. m_O decreased with the increase in the air‐inlet rate. Increase in oxygen transfer rate increased the rate of growth and SAP activity, and affected the cultivation time to achieve maximum expression of SAP activity. At Q/V_R = 0.5 vvm and N = 750 min^?1, SAP activity reached 2250 U cm^?3 at t = 36 h. The oxygen transfer coefficient (K_La) and oxygen uptake rate (?r_O) were measured throughout the fermentations and their variation with the oxygen transfer conditions determined. New correlations for the calculation of K_La and ?r_O are proposed. Copyright © 2004 Society of Chemical Industry     

Biomonitoring of biosurfactant production by green fluorescent protein‐marked Bacillus subtilis W1012

BACKGROUND: Biosurfactant production was investigated using two strains of Bacillus subtilis, one being a reference strain (B. subtilis 1012) and the other a recombinant of this (B. subtilis W1012) made able to produce the green fluorescent protein (GFP). RESULTS: Batch cultivations carried out at different initial levels of glucose (G₀) in the presence of 10 g L^?1 casein demonstrated that the reference strain was able to release higher levels of biosurfactants in the medium at 5.0≤G₀≤10 g L^?1 (B_max = 104–110 mg L^?1). The recombinant strain exhibited slightly lower levels of biosurfactants (B_max = 90–104 mg L^?1) but only at higher glucose concentrations (G₀ ≥ 20 g L^?1). Under these nutritional conditions, the fluorescence intensity linked to the production of GFP was shown to be associated with the cell concentration even after achievement of the stationary phase. CONCLUSION: The ability of the genetically‐modified strain to simultaneously overproduce biosurfactant and GFP even at low biomass concentration makes it an interesting candidate for use as a biological indicator to monitor indirectly the biosurfactant production in bioremediation treatments. Copyright © 2008 Society of Chemical Industry     

Effect of woody biomass surface free energy on the mechanical properties and interface of wood/polypropylene composites

The aim of this study is to investigate the effect of surface free energy of wood flour (WF) and silanized WF on the mechanical properties and interface of wood/polypropylene (PP) composites. The contact angles of three probe liquids against unmodified and modified spruce WF were tested by capillary rise method based on the Washburn equation. Then the surface free energy and its corresponding dispersion and polar components were calculated according to the method developed by Owens–Wendt–Kaelble. The tensile strength and flexural strength of the wood/PP composite samples made with unmodified and modified WF were tested and the flexural fracture surfaces were analyzed by scanning electron microscopy (SEM). The results showed that the surface free energy of WF increased from 26.0 to 36.1?mJ/m², which was higher than that of PP (29.4?mJ/m²), and its corresponding polar component decreased from 13.1 to 4.4?mJ/m², and the dispersion component increased from 12.9 to 31.7?mJ/m² after the modification with 4 wt.% vinyltriethoxy silane, which makes it possible for spreading of PP on the surface of WF, the tensile strength and flexural strength of wood/PP composites made with modified WF were obviously improved. In addition, the improved compatibility between WF and PP was well confirmed by SEM.     

Polyether-segmented nylon hemodialysis membranes. III. Preparation and properties of new polyether-segmented nylon

In order to apply a blood-compatible polymer to hemodialysis membrane, a new polyether-segmented nylon which dissolved in common organic solvents was designed. The basic polyether-segmented nylon was synthesized by melt polycondensation from sebacic acid, m-xylenediamine, and α,ω-bisaminopropyl-poly(ethylene oxide). To improve the solubility, azelaic acid and hexamethylenediamine were copolycondensed with the basic copolymer. The solubility was correlated with the heat of fusion (ΔH_m) of the copolymer. When ΔH_m is < 30 mJ/mg, the polymer is soluble in dimethylsulfoxide and makes a stable solution. The nonthrombogenicity was investigated in the viewpoint of adhesion of platelet onto the copolymer surface. It is made clear that the surface of the block copolymer, having > 10 wt % of poly(ethylene oxide), suppresses the adhesion of platelet, and the composition of the nylon block has no effect on the adhesion of platelet. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1723–1729, 1997     

Thermodynamic work of adhesion and peel adhesion energy of dimethoxysilyl‐terminated polypropylene oxide/epoxy resin system jointed with polymeric substrates

The characterization of the interfacial surface of a dimethoxysilyl‐terminated polypropylene oxide (DMSi–PPO)/diglycidylether of bisphenol A (DGEBA) system, which has the phase structure of DGEBA particles in a DMSi–PPO matrix, was investigated by using model joints with polymeric substrates. The surface free energy (γ) of the DMSi–PPO/DGEBA system had varied with the γ of each substrate. When the system contacted to low surface free energy materials such as Teflon, polypropylene, and polyethylene, the γ of the system showed about 14.3–31.6 mJ/m²; on the other hand, when the system contacted to high surface free energy substrates such as polyethylene–telephthalate and polyimide, the γ of the system showed 50.4 and 64.6 mJ/m², respectively, because the concentration of the DGEBA as a polar component in the system changed around these interfaces. In the low surface energy substrates used, the actual peel adhesion energy value was in good agreement with the thermodynamic work of adhesion (Wa) determined independently. However, in the high surface energy materials used, the peel adhesion energies were 10³–10⁴ times larger than Wa because the energy was dissipated viscoelastically at the jointed points. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1920–1930, 2001     

Interactions Between Micron-sized Glass Particles and Poly(dimethyl siloxane) in the Absence and Presence of Applied Load

A technique using a scanning electron microscope to view a fine particle in contact with a flat substrate whilst under load and during its removal is described. The particle is attached to an atomic force microscope cantilever so that the magnitude of the load can be estimated directly from the imaged deflection. Interactions between 5 to 60 μm spherical glass particles and cross-linked poly(dimethyl siloxane) were studied in the presence and absence of load. W_A was estimated to be 74 mJ/m² from the size of the contact area in the absence of load. Using highly flexible cantilevers to apply load resulted in large shear displacements and forces, which distorted the contact area and assisted in particle removal. These shear effects were eliminated by using a more rigid cantilever to measure a normal pull-off force for which the interface toughness, G_c , exceeded 950 mJ/m². The large adhesion hysteresis indicated the presence of chemical bonding, presumed to occur between silanol and siloxane groups. The mode of particle detachment varied significantly with the choice of cantilever, showing evidence of both cohesive failure and interfacial crack propagation. The relevance of these results to the interpretation of AFM data is discussed.