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Direct Evaluation of DLVO Theory for Predicting Long-Range Forces between a Yeast Cell and a Surface Jeffrey M. Sharp† and Richard B. Dickinson*,†,‡ Departments of Chemical Engineering and Biomedical Engineering, University of Florida, Gainesville, Florida 32611 Received December 29, 2004. In Final Form: May 3, 2005 Using a combined gradient optical trap and evanescent wave light-scattering force-measurement technique, long-range colloidal forces were measured between a single Candida albicans yeast cell and a flat, bare glass surface in electrolyte concentrations ranging from 0.1 to 100 mM NaCl. The DerjaguinLandau-Verwey-Overbeek (DLVO) theory was compared to experimentally measured equilibrium force curves and found to provide a close approximation to the decay length of the measured forces for electrolyte concentrations up to about 0.23 mM NaCl. At higher electrolyte concentrations (g0.5 mM NaCl), decay lengths of force curves in experimental measurements were consistently longer than Debye lengths calculated from the electrolyte concentrations. In electrolyte concentrations of 10 and 100 mM NaCl, most cells attached rapidly, which prevented measurements of long-range forces. The small fraction of cells remaining unattached in these higher electrolyte concentrations displayed purely repulsive forces. These results show that the DLVO theory accurately describes cell-surface interactions when the Debye length is in the range of 20-30 nm but underpredicts the decay length of the interactions at higher electrolyte concentrations.
(1) Xu, L. C.; Chan, K. Y.; Fang, H. H. P. Mater. Charact. 2002, 5476,
in solution and van der Waals forces caused by the specific alignment and coupling interactions of molecular dipoles. While useful in many situations where surfaces may be assumed uniform and relatively ideal, the DLVO theory does not account for several types of interaction forces thought to be relevant to microbial attachment, including hydrophobic,11 hydration,12 and steric13,14 (electrosteric) forces, as well as the effects of surface roughness15,16 or receptor-ligand.17,18 In many cases, discrepancies between measured interaction forces and the DLVO theory have been attributed to the influence of either hydrophobic forces or bridging between cell-surface macromolecules and the substratum.19,20 Additionally, polysaccharide chains and macromolecules attached to the cell surface can influence both the surface-charge distribution and the steric interactions between a cell and a surface. Together, these “electrosteric” interactions14 can significantly affect the forces that a cell experiences in the vicinity of a solid surface. In recent years, the extended DLVO theory has been implemented to account for these types of complexities such as steric2 and hydrophobic forces.5 Improvement of theories for colloidal adhesion requires accurate methods to measure weaker long-range forces. Recently, we demonstrated that piconewton-level longrange interaction forces between a yeast cell and a flat surface could be accurately measured by a combined gradient optical trap and evanescent wave light-scattering
(2) Jucker, B. A.; Zehnder, A. J. B.; Harms, H. Quantification of polymer interactions in bacterial adhesion. Environ. Sci. Technol. 1998, 32, 2909. (3) Dillow, A. K.; Tirrell, M. Curr. Opin. Solid State Mater. Sci. 1998, 3, 252. (4) Fleming, H.-C. Water Res. 1987, 21, 745. (5) Ong, Y.-L.; Razatos, A.; Georgiou, G.; Sharma, M. M. Langmuir 1999, 15, 2719. (6) Grutsch, J. F. Environ. Sci. Technol. 1978, 12, 1022. (7) Dickinson, R. B. In Encyclopedia of Colloid Science; Hubbard, A., Ed.; Marcel-Dekker: New York, 2002; p 4972. (8) Derjaguin, B. V.; Landau, L. D. Acta Physicochim. USSR 1941, 14, 633. (9) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, The Netherlands, 1948. (10) Ma, H.; Dickinson, R. B. J. Theor. Biol. 2004, 226, 237.
(11) van Oss, C. J. Colloids Surf., B 1995, 5, 91. (12) Chang, Y.-I.; Chang, P.-K. Colloids Surf., A 2002, 211, 67. (13) Rijnaarts, H. M.; Norde, W.; Lyklema, J.; Zehnder, A. J. B. Colloids Surf., B 1999, 14, 179. (14) Camesano T. A.; Logan, B. E. Environ. Sci. Technol. 2000, 34, 3354. (15) Suresh, L.; Walz, J. Y. J. Colloid Interface Sci. 1997, 196, 177. (16) Walz, J. Y. Adv. Colloid Interface Sci. 1998, 74, 119. (17) Bouchara, J. P.; Tronchin, G.; Annaix, V.; Robert, R.; Senet, J.-M. Infect. Immun. 1990, 58, 48. (18) Dickinson, R. B. J. Colloid Interface Sci. 1997, 190, 142. (19) van Loosdrecht, M. C. M.; Norde, W.; Lyklema, J.; Zehnder, A. J. B. Aquatic Sci. 1990, 52, 103. (20) Rijnaarts, H. H. M.; Norde, W.; Bouwer, E. J.; Lyklema, J.; Zehnder, A. J. B. Appl. Environ. Microbiol. 1993, 59, 3255.
Introduction Prediction of surface forces that affect the interaction and adhesion of microbial cells with surfaces is relevant to industrial,1 environmental,2 and biomedical3 applications. Accurate understanding and prediction of interaction forces can lead to methods for control and prevention of the microbial colonization or fouling of, for example, process equipment4 and biomedical device implants.3,5 In other cases, the prediction of cell adhesion or aggregation based on solution and surface properties is necessary for removal processes, such as in flocculation of cells for removal in wastewater decontamination.6 Microbial adhesion depends on both nonspecific and specific interaction forces.7 Several models exist that quantitatively predict particle-surface interaction energies (or forces) that lead to cell-surface adhesion, including the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory,8,9 and more detailed models that account for other interactions such as macromolecular binding.10 The DLVO theory for colloidsurface forces accounts for the long-range electrostatic interactions that arise from the existence of overlapping diffuse double layers of counterions near charged surfaces * To whom correspondence should be addressed. Telephone: 352392-0898. Fax: 352-392-9513. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Biomedical Engineering. 1.
10.1021/la046765s CCC: $30.25 © 2005 American Chemical Society Published on Web 08/05/2005
Direct Evaluation of DLVO Theory
(EWLS) technique.21 This technique uses the axial component of the laser trap to position a particle or cell against a surface to probe its interaction forces. The restoring trapping force on a colloidal particle is linear for displacements from the trap center up to around the distance of one particle radius.22,23 In the present study, we measured the forces between Candida albicans and glass surfaces in varying electrolyte concentrations and compared these measurements to predictions from the DLVO theory. Experimental results indicated close agreement with the theory for lower electrolyte concentrations but a departure from the theory at moderate to high electrolyte concentrations. Experimental Procedures Electrolyte Solutions. Experimental solutions were prepared using a Barnstead Nanopure (Barnstead-Thermolyne, Dubuque, IA) filtration system to obtain water with a resistivity of 18.2 MΩ cm. The pH of the solutions was measured to be in the range of 7.4 ( 0.4. The appropriate amounts of sodium chloride were added to the solutions to obtain concentrations of 0.1, 0.23, 0.5, 1.0, 2.0, 10, and 100 mM. Subsequent conductivity measurements confirmed the electrolyte content of each solution within 2.2%. Yeast preparation Modified Sabouraud dextrose agar (Becton Dickinson, Sparks, MD) plates (pH 7.0 ( 0.2 at 25 °C) were prepared according to instructions of the supplier. Plates were then streaked from a lyophilized C. albicans (ATCC 10231) pellet, which was rehydrated in deionized microfiltered water and allowed to grow for 48 h (to stationary phase) at 25 °C. Prior to force-measurement experiments, C. albicans cells were taken from single plate colony and grown in modified Sabouraud dextrose broth (pH 5.6 ( 0.2 at 25 °C) by inoculating test tubes containing 5 mL of broth with the rehydrated pellet and incubating at 25 °C for 48 h. Following growth of C. albicans, test tubes were shaken, centrifuged at 1900 rpm for 15 min, decanted, and resuspended in electrolyte solution. After centrifugation for a second 15-min period, the remaining yeast cells were resuspended in 10 mL of the electrolyte solution of desired concentration for optical trapping experiments and sonicated for 15 min. Prior experimental work with adhesion of C. albicans to glass surfaces24 demonstrated that the cell surface equilibrates with the electrolyte medium over a period of minutes, which is necessary to achieve consistent adhesion behavior. Cells placed in solutions used in our experiments were allowed to reside for at least 1 h before measurement. The final concentration of cells within the suspension was approximately 1 × 105 cells/mL. Finally, 25 µL of 15 µm polystyrene beads (producing a concentration similar to that of the yeast cells) was added to the solution to maintain a constant fluid gap within the sample. Solutions were prepared on the same day of the measurements. ζ Potential Measurements. A Brookhaven Zetaplus (Brookhaven Instruments Corporation, Holtsville, NY) was used to measure yeast-cell electrophoretic mobilities, from which ζ potentials were calculated using the Smoluchowski equation.25 A concentration of approximately 105 cells/mL was used for these measurements. Results of these measurements are shown in Figure 1 and agreed closely with results from Jones et al.,26 who concluded that the electrophoretic behavior of C. albicans in solution approximates that of a simple colloidal suspension. Slide Preparation. Precleaned borosilicate glass slides (Fisher Scientific, Pittsburgh, PA) were immersed in acetone (new slides) or chromerge (for previously used slides containing (21) Sharp, J. M.; Clapp, A. R.; Dickinson, R. B. Colloids Surf., B 2003, 27, 355-364. (22) Clapp, A. R.; Ruta, A. G.; Dickinson, R. B. Rev. Sci. Inst. 1999, 70, 2627. (23) Clapp, A. R.; Dickinson, R. B. Langmuir 2001, 17, 2182. (24) Jones, L.; O’Shea, P. Exp. Mycol. 1994, 18, 111. (25) Smoluchowski, M. Handbook of Electricity and Magnetism; Barth: Leipzig, Germany, 1921; Vol. 2, p 366. (26) Klein, J. D.; Dickinson, R. B. J. Colloid Interface Sci. 2003, 261, 379.
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Figure 1. ζ potential of yeast phase C. albicans measured in solutions of various electrolyte concentrations. A minimum of three measurements were taken for each data point. a well) for at least 24 h and rinsed thoroughly with ethanol and ultrafiltered water to remove any residual contaminants from the sample surfaces before sample preparation. For high electrolyte experiments (100 mM NaCl), a well of 5 mm diameter and 0.5 mm depth was drilled in the center of each slide to hold cells for delaying attachment to the surface prior to force measurements, as described in Klein et al.27 Calculation of DLVO Force Profiles. Theoretical curves for electrostatic double-layer forces, FR, as a function of the separation distance, h, were calculated by differentiating the expression derived by Hogg, Healy, and Fuerstenau28 for interaction potential between two spheres
FR(h) )
a1a2κ -2κh
(1 - e
[ψ )(a + a ) 1
2
01ψ02e
-κh
1 ( (ψ201 + ψ202)e-2κh 2 (1)
]
where is the dielectric constant of the medium, a1 is the radius of the particle, ψ01 and ψ02 are the surface potentials, and κ is the inverse Debye length. The sign of the second term in brackets in eq 1 depends on whether constant charge (+) or constant potential (-) is assumed. Constant charge is commonly used for a biological cell interacting with a surface.29 For larger separation distances (e-κh , 1), constant-charge and constant-potential force-distance equations both converge to
F(h) ) aκψ01ψ02e-κh
(2)
to the lowest order of e-κh. The Debye length was estimated for each electrolyte solution with the formula30
κ-1[m] )
3.04 × 10-10 |z|M1/2
(3)
where z is the valence number of a symmetric electrolyte and M is the solution molarity in mol/dm3. From conductivity measurements, the electrolyte concentrations were verified for each solution and used to estimate the Debye length in eq 3 above. A surface potential of the glass slide of -22.7 mV was assumed for calculation of DLVO profiles at all electrolyte concentrations examined, which was measured through streaming potential by Gu and Li3231-32 for glass in contact with 1 mM NaCl aqueous solution. Additionally, these authors demonstrated that ζ potential of glass in an aqueous solution of monovalent electrolyte became much less sensitive to additional electrolyte as concentrations exceeded 1 mM. Calculations of the van der Waals force were performed using the pairwise additivity approach of (27) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday Soc. 1966, 62, 1638. (28) Poortinga, A. T.; Bos, R.; Norde, W.; Busscher, H. J. Surf. Sci. Rep. 2002, 47, 1. (29) van der Waal, A.; Minor, M.; Norde, W.; Zehnder, A. J. B.; Lyklema, J. J. Colloid Interface Sci. 1997, 186, 71. (30) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1991. (31) Shepherd, M. G. Cell envelope of Candida albicans. CRC Crit. Rev. Microbiol. 1987, 15, 7. (32) Gu, Y.; Li, D. J. Colloid Interface Sci. 2000, 226, 328.
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Hamaker given by Suresh and Walz33 for the interaction between a sphere and a flat surface, accounting for retardation of forces that typically occurs at separation distances greater than about 5 nm
[
]
U -2.45λ 2.17λ2 0.59λ3 ) 2πA + R 120π2h2 720π3h3 3360π4h4
(4)
R is the particle radius, λ is the London wavelength, and h is the absolute sphere-surface separation distance. λ was assumed to be equal to 100 nm,34 which is commonly applied to systems of biological particles and surfaces for estimation of this parameter. A value of 6 × 10-21 J was assumed for the Hamaker constant A, which has been used to reasonably predict the van der Waals interaction for a system of a bacteria cell interacting with a glass plate through water.35 Force Measurements. An optical trap/evanescent-wave light-scattering force-measurement technique was used to measure equilibrium forces between individual yeast cells and the glass surface. A detailed description of this force-measurement methodology, the apparatus, and the data analysis, which has been successfully applied previously to microbial cells,21,26 is described elsewhere.23 Briefly, a 100-mW 830-nm diode laser (Cell Robotics, Albuquerque, NM) is used to trap the particle and probe it against the surface, and the scatter from the exponentially decaying evanescent wave, which is created by total internal reflection of 17-mW helium-neon laser (Melles Griot, Carlsbad, CA), determines relative distances of particle positions from the surface (see Figure 2). The difference between any two positions z1 and z2 of the particle is related to the scattered intensity of the evanescent wave by
z2 - z1 ) - β-1 ln
I2 I1
(5)
where β-1 ) 184.4 nm is the decay length of the evanescent wave. At each trap position, a time series of the intensity is generated and analyzed to determine the most probable particle position zp and the autocovariance function of the position fluctuations Gz(τ), which, for small linear fluctuations, has the form, Gz(τ) ) σ2z e -Dτ/σ2z , where σ2z is the variance and D is the diffusivity at zp. The analysis, described in detail elsewhere,23 estimates zp, σ2z , and D for each trap position, while correcting for the background intensity and shot noise in the intensity fluctuations. The force-distance relationship for the particle displacement from the trap center is
Ftrap(z) ) - γz(z - z0)
(6)
where γz is the axial trap stiffness. For trap positions outside the range of surface forces (Figure 2A), Brownian fluctuations in z obey a stationary Gaussian distribution with mean z0 ) zp and variance σ2z ) kT/γz, which allows the stiffness to be estimated. Within the range of surface forces (Figure 2B), the cell interactions with the glass surface cause zp to deviate from z0, allowing the surface force to be determined at zp from eq 6. After cells were first trapped and positioned just outside the evanescent wave, an automated LABView (National Instruments, Austin, TX) program stepped the objective at 20-nm increments to move the trap position toward the surface while collecting a time series of intensity (32 768 samples at 5 kHz) for each trap position. The trap stiffness γz was estimated from the average variance over the range of trap positions clearly outside the range of forces, identified by the exponential dependence of the intensity with the trap position. The value of σ2z was nearly constant over the calibration range, confirming no effect of surface forces, which would narrow (if repulsive) or broaden (if attractive) the range of position fluctuations. The value of γz varied little between (33) Suresh, L.; Walz, J. Y. J. Colloid Interface Sci. 1996, 183, 199. (34) Atteia, O. Colloids Surf., A 1998, 139, 171. (35) Rijnaarts, H. M.; Norde, W.; Bouwer, E. J.; Lyklema, J.; Zehnder, A. J. B. Colloids Surf., B 1995, 4, 5.
Figure 2. Illustration of the force-measurement method for an optically trapped yeast cell probed against a glass surface. (A) Trap calibration is performed to obtain the stiffness parameter γz in the region where scatter of the evanescent wave is significant but surface forces are negligible, such that the most probable position zp is equal to the position of the cell in the trap center z0. (B) At closer distances, significant surface forces deflect the zp from z0, allowing the force at zp to be measured. runs from its average value of 3.4 × 10-3 pN/nm, which compares closely to values that we previously obtained with yeast cells.21 Because the evanescent wave only determines relative positions and not the absolute distances from the glass surface, the absolute distance scale needed to be estimated separately. However, for biological cells, which have a three-dimensional surface architecture, the “absolute separation distance” cannot clearly be defined on the nanometer length scale. As described in Clapp and Dickinson,23 we chose to define here the absolute separation distance as the “hydrodynamic distance” based on fitting the viscous drag coefficient δ ) kT/D to the hydrodynamic theory for hydrodynamic interactions between a sphere and a flat plate,36 with the zero-point distance as the fitted parameter. The fitted data were taken from the range of trap positions outside the range of strong interaction forces, where good agreement between the measured drag coefficient and theory was observed. Using an approach described previously,23 the beam center was shifted slightly off axis from the center of the objective to minimize surface-reflection effects, which, when present, result in periodic deviations from a linear particle-position versus trapposition profile in the calibration region. This approach has previously been successfully applied to eliminate such deviations even with larger particles (up to 5 µm in diameter),21 which were initially found to be more susceptible.23 For all force-distance profiles reported here, the particle-position versus trap-position (36) Brenner, H. Chem. Eng. Sci. 1961, 16, 242.
Direct Evaluation of DLVO Theory
Figure 3. Equilibrium forces measured between a single C. albicans cell and a borosilicate glass surface in various concentrations of NaCl. DLVO curves are plotted with each data set on a linear scale (A) and a log scale (B) to show the extent of agreement between data and theory. Good agreement is achieved for low electrolyte cases (e.g., 5 pN) adhesion force between the particle and surface. In 100 mM NaCl solution, consistent attractive particle-surface forces were obtained for a silica particle probed against the glass surface in 100 mM solution, which were accompanied by large jump distances of the particle to the surface and followed by irreversible adhesion (inset of Figure 5B). When attractive forces become significant upon the close approach of the particle to the surface, the effective trap stiffness is reduced slightly because of the potential created by the combined effects of the trap and surface.23 Although attractive forces are measurable at the data points leading up to adhesion, the points become spaced farther apart as the particle diffuses into locations of increasingly attractive potential energy. As a result, fewer data points of the equilibrium force are obtained at smaller separation distances in these runs and the points nearest the surface indicate a jump of the particle into the region of attractive surface forces.
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Figure 5. (A) Equilibrium forces measured between a single C. albicans cell and a borosilicate glass surface in 10 mM NaCl (main plot), which are consistently long-range and repulsive. The inset contains a plot of equilibrium force versus distance measured between a 5.0-µm silica sphere and a glass surface in 10 mM NaCl solution for two different experimental runs. 0 of the inset demonstrates that in some runs no particlesurface attachment occurs, where the decay length of the repulsive force approximates the Debye length (see the text). In other measurements (b), the particle attaches after a small jump (∼50 nm) to the surface. (B) Equilibrium forces measured between a single C. albicans cell and glass in 100 mM NaCl (main plot), which are purely repulsive with cells available for measurement. Inset presents the equilibrium force versus distance measured between a 5.0-µm silica sphere and a glass surface in 100 mM NaCl solution, with corresponding DLVO theoretical curve. All measurements for silica in these solution conditions concluded with irreversible adhesion of the particle to the surface (adhesion force > 5 pN).
As shown in the main plots of parts A and B of Figure 5, the measured force profiles for the few nonattaching cells in higher electrolyte concentrations exhibited monotonically increasing repulsive forces despite the significant attractive forces predicted by the DLVO theory. Although the nonattaching cells are likely not representative of the cell population, of which most became attached before they could be trapped and measured, the shape of the force-distance profiles nevertheless informs about the range and decay of the repulsive surface forces. For force measurements in 10 mM NaCl (Figure 5A), experimental force profiles displayed a similar decay length (21.5 ( 6.5 nm) to those obtained at lower electrolyte concentrations (1.0 and 2.0 mM), whereas the theory predicts the existence of a secondary minimum of about 1.8 pN (or energy of 9 kT). At 100 mM NaCl (Figure 5B), the DLVO theory predicts no energy barrier and a primary energy minimum with appreciable attractive interactions (>5 kT) at distances of less than 30 nm from the surface. However, only repulsive forces were measured between the cell (chosen from those that remained unattached after sample preparation) and the surface with an average force decay length of 19.5 ( 5.6 nm. No differences in forces were observed when cells were grown at 37 °C. Discussion Measured force-distance profiles generally show good agreement with the DLVO theory at low electrolyte concentrations with increasing deviation at higher concentrations. One explanation is that at very low electrolyte
Sharp and Dickinson
concentrations (e.g., 0.1 and 0.23 mM NaCl) cells remain far enough from the surface to experience negligible effects of any interactions other than repulsion caused by diffuse double layers, even when pressed against the surface with the trap. At slightly higher electrolyte concentrations, however, the trapped cell may have approached the surface close enough to be influenced by additional repulsive forces (e.g., steric) with a different characteristic decay length, rendering the DLVO theory inadequate for accurate quantitative predictions. Supporting this explanation is the observation that force decay lengths extracted from fitting the DLVO theory were always greater than the Debye length calculated from the electrolyte concentration. It is likely that the surface polymers, which can vary from 100 to 200 nm in length,37 undergo rapid compression and expansion as the cell diffuses near the surface. Jones et al. have noted that a net attraction between a C. albicans cell and glass may result even in the presence of a significant repulsive electrostatic force,24 although this behavior was not observed in our measurements. Force measurements between C. albicans and glass at 10 and 100 mM NaCl generally did not result in adhesion despite the DLVO theory predicting a lack of repulsive energy barrier under these conditions. However, it should be noted that most cells in the 10 and 100 mM solutions did settle and attach from suspension within 10 min of preparing the sample. The cells selected for measurement were among the fewer cells that remained unattached, which presented a selection bias toward cells with a lesser tendency to attachment. Possible explanations for the variability in the population include variable surface coverage of stabilizing surface polymers or variable surface potential28 because of the inherent heterogeneous nature of microbial surfaces. We do not anticipate nonspherical geometries to have played a role in deviation from the DLVO theory or variability among cells. All cells chosen for experimental force measurements appeared spherical when viewed with the 1000× magnification achieved with our microscope, and budding cells or those with oblate geometry were avoided. As discussed in Klein et al.,26 elongated objects tend to orient vertically in the trap but then reorient parallel to the surface during the measurement, an event that is clearly identifiable by a sharp increase in the intensity of scattered light. Runs where this occurred could not be easily interpreted as a force measurement and were discarded. Because of the cell diameter of C. albicans, which can be as large as 6 µm, the gap thickness chosen for force measurements was 15 µm. The effect of spherical aberrations can be a concern for trap positions at large distances into the optically less dense aqueous medium on the objective lens side of the light path.38 However, no dependence of trap stiffness on trap position was observed within the calibration region, and the close agreement with the monotonic exponential force-distance profiles is consistent with linearity of the optical trap over the range of the force measurements. Moreover, for C. albicans and for 5.0 µm silica particles, no appreciable difference in trap stiffnesses was found with gap thicknesses of 10 and 15 µm, suggesting the trapping properties at 15 µm were not seriously affected by spherical aberrations. In conclusion, the force-measurement technique employed here successfully measured repulsive electrostatic forces up to 4 pN with decay lengths that agreed closely (37) Jabra-Rizk, M. A.; Falkler, W. A., Jr.; Merz, W. G.; Kelley, J. I.; Baqui, A. A. M. A.; Meiler, T. F. Rev. Iberoam. Micol. 1999, 16, 187. (38) Rohrbach, A.; Stelzer, E. H. K. Appl. Opt. 2002, 41, 2494.
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with DLVO force profiles at electrolyte concentrations of e0.23 mM NaCl. Additionally, the influence of attractive van der Waals forces was notably absent from the measured interactions at higher electrolyte concentrations (10 and 100 mM) where, at sufficiently small absolute separation distances, they would be expected from the DLVO theory to dominate the overall interaction. Interactions between cell-surface polymers and the glass surface are likely responsible for an additional repulsion that occurs at smaller separation distances, and the observation that the decay length of experimentally measured forces exceeded that predicted by the DLVO theory suggests the presence of these additional nonDLVO repulsive forces. Studies of microorganisms such as C. albicans have shown that surface structures often enhance the attractive interactions with both nonbiological and biological surfaces.23,39 Therefore, while the present work focused mainly on the effects of solution electrolyte on forces that control cell adhesion, results from other literature40 suggest that additional work to study yeast cells grown in different media or in a different phase of (39) Cotter, G.; Kavanagh, K. Brit. J. Biomed. Sci. 2000, 57, 241. (40) Douglas, L. J. In The Yeasts, 2nd ed.; Rose, A. H., Harrison, J. S., Eds.; Academic Press: San Diego, CA, 1986; Vol. 2, p 239.
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the life cycle (e.g., exponential phase growth or death phase) could provide further information on the attachment behavior because of altered cell-surface composition of adhesion molecules. In present and future work, this technique will be utilized to isolate contributions to the interaction profile of molecular layers (e.g., proteins and lipids) adsorbed to the substrate surface. With the present characterization of the DLVO-type forces that occur in basic systems of colloidal and microbial particles, systems of biological colloids will be studied more effectively to isolate contributions to the overall profiles by non-DLVO or specific interaction forces. Acknowledgment. Financial support for this work was provided in part by the National Science Foundation (BES-9704236) and the Engineering Research Center for Particle Science and Technology (EEC-94-0289). The instrumentation development was supported by a Major Research Instrumentation grant from the National Science Foundation (CTS-9977459). The authors would also like to thank Chris Hughes for assistance in measurement of ζ potentials of yeast cells. LA046765S
