Combining optical reflectometry and atomic force microscopy (AFM), we have studied the effects of the surfactant counterion on the adsorption isotherms, kinetics, and layer structure for cationic hexadecyltrimethylammonium (C16TA + ) surfactants on negatively charged silica surfaces. The adsorption kinetics suggest that the adsorption mechanism changes at the critical micelle concentration (cmc). A change in mechanism is also suggested by differences observed in the state of interfacial self-assembly on either side of the cmc. Above the cmc, increasing the binding affinity of the counterion (from chloride to bromide) increased the surface excess concentration by approximately 60% and changed the structure of the adsorbed surfactant layer from aggregates with circular projections to wormlike micelles. The addition of 10 mM KCl or KBr increased the surfactant surface excess concentration for both counterions. Below the cmc, the counterion has only a small effect on the structure of the adsorbed layer, and the isotherms are similar, provided the surfactant concentration is scaled by the appropriate cmc. By quantitatively analyzing the AFM images and comparing this to the surface excess concentration measured by reflectometry, we determined that surfactants pack differently in adsorbed aggregates than they do in aggregates formed by self-assembly in solution. Finally, we show that an impurity present in poly(vinyl chloride) tubing explains anomalous adsorption behavior previously reported for C16TAB on silica.

Probing nanoscale interactions between an atomic force microscope tip and a bacterium may provide insight into the molecular level origins of bacterial adhesion. Distinguishing between the relevant surface interaction forces, such as steric and electrostatic interactions, is complicated by the elastic deformation of the bacterium. To probe the possible role of lipopolysaccharides (LPS) in bacterial adhesion and cell elasticity, atomic force microscopy (AFM) force images were obtained between a bare silicon nitride AFM tip and three different Escherichia coli K12 strains, each having a different length of LPS on their surface. Cell elasticity was varied with glutaraldehyde fixation. Bacterial force curves consisted of a nonlinear region 20−30 nm above the cell surface and a constant compliance (linear region) with a slope that was significantly less than that of a hard surface. AFM force curves obtained on the top of the cell were identical (linear and nonlinear regions) for all three strains, indicating a lack of a steric contribution of LPS to the force curve. Force images obtained off the center of the cell produced apparent long-range forces, but these were considered to be imaging artifacts produced by tip−surface geometries at the cell edges. Glutaraldehyde strongly affected the elasticity of the cell but did not affect the nonlinear portion of the force curve. The effective spring constant of the bacterium, calculated from the constant compliance region of the force curve, was found to increase 4-fold with the addition of 2.5% glutaraldehyde and was independent of the spring constant of the cantilever. The nonlinear portion of the curve is not consistent with electrostatic forces, because interaction distances were not a function of solution ionic strength. These results suggested that nonlinear forces were due to deformation of the bacterial surface layer.

The influence of the lipopolysaccharide (LPS) chain length on bacterial adhesion was investigated by measuring the collision efficiencies of three Escherichia coli K12 strains, each having a different length LPS, to silica glass beads in column tests (macroscale tests). Nanoscale interactions between the bacteria and a silicon nitride tip were probed utilizing atomic force microscopy (AFM). Adhesion results based on column tests indicated that collision efficiencies of the three bacteria were not consistently correlated to LPS length. Under conditions of low ionic strength (1 mM NaCl), collision efficiencies increased with LPS length for the three strains of E. coli. However, if cells were fixed with glutaraldehyde (2.5%), the strain with the shortest LPS chain had the greatest adhesion, while the bacterium with the mid-length LPS had the least adhesion to glass beads. Collision efficiencies increased when the solution ionic strength was increased from 1 to 100 mM as expected, and in most cases glutaraldehyde treatment also increased adhesion. AFM force curves failed to distinguish the adhesion characteristics of these bacteria measured in column tests, as all AFM force curves on the bacteria were identical. Changes in adhesion were also not predictable by more conventional measurements of bacterial properties based on ζ potential or contact angle. These results suggest that the LPS molecule length is not the sole determinant of adhesion of the three E. coli strains in porous media and that AFM force curve analysis, zeta potential, or contact angle data cannot yet be used to fully predict adhesion of these three strains to glass beads.

Atomic force microscopy (AFM) has become an important tool for investigating various biological materials, and it is now being applied more routinely for imaging bacteria. By imaging bacteria in water, AFM can provide in-situ images of viable cells and be used to measure interaction forces between the AFM tip (or a colloid probe) and the cell surface. However, the relatively large height and compliance of the bacterium can also lead to imaging artifacts. AFM images of Escherichia coli K12 were consistently found to contain image shadows that were oriented in parallel lines 27°from the direction of the cantilever tilt, regardless of the scan direction. Similar image shadows were also observed for 1 µm diameter polystyrene latex microspheres. Using a simple geometric model for the interaction of the tip and the bacterium, it is demonstrated here that these lines observed for bacteria are image artifacts produced by the pyramidal shape of the tip, the 10°tilt of the cantilever, and the height of the bacterium relative to the size of the tip. Such image artifacts disappear when we image dehydrated bacteria that are lower in height, or bacteria that become damaged and deflated during imaging in water. The interaction of the edge of the tip with the bacterium is also shown to result in inconsistent shapes of force curves unless the force curve is centered on the crest of the rounded bacterial surface.

A cationic protein isolated from the seeds of the Moringa oleifera tree has been extensively studied for use in water treatment in developing countries and has been proposed for use in antimicrobial and therapeutic applications. However, the molecular basis for the antimicrobial action of this peptide, Moringa oleifera cationic protein (MOCP), has not been previously elucidated. We demonstrate here that a dominant mechanism of MOCP antimicrobial activity is membrane fusion. We used a combination of cryogenic electron microscopy (cryo-EM) and fluorescence assays to observe and study the kinetics of fusion of membranes in liposomes representing model microbial cells. We also conducted cryo-EM experiments on E. coli cells where MOCP was seen to fuse the inner and outer membranes. Coarse-grained molecular dynamics simulations of membrane vesicles with MOCP molecules were used to elucidate steps in peptide adsorption, stalk formation, and fusion between membranes.

Coadsorbing poly-L-lysine hydrobromide inhibits hexadecyltrimethylammonium bromide adsorption to silica surfaces. Using optical reflectometry to measure surface excess concentrations, we find that this inhibition depends on the concentration of added 1:1 electrolyte, both quantitatively and qualitatively. In the absence of added salt, the polyelectrolyte causes a rather uniform decrease in the extent of surfactant adsorption, and the shape of the surfactant coadsorption isotherm is qualitatively similar to the isotherm for adsorption in the absence of polyelectrolyte. In particular, the surfactant concentration marking the onset of cooperative adsorption, where admicelles are formed at bulk concentrations below the critical micelle concentration, is unchanged by the presence of the polyelectrolyte. In contrast, when the surfactant coadsorbs in the presence of 10 mM KBr, the polyelectrolyte eliminates this adsorption regime altogether. Admicelles do not form until the bulk surfactant concentration exceeds the critical micelle concentration. Upon addition of the 1:1 electrolyte, the inhibition mechanism changes from a simple competition for available surface area to a more profound disruption of surfactant interfacial self-assembly. By comparing a low molecular weight oligolysine with three higher molecular weight polylysine samples, we find that this change in inhibition mechanism can be traced to the effect of salt on the relative adsorption energies of the surfactant and the polyelectrolyte, but kinetically trapped or frustrated states exert a large influence on the composition of the mixed adsorbed layer in the case of higher molecular weight polyelectrolytes.

Moringa oleifera (Moringa) seeds contain a natural cationic protein (MOCP) that can be used as an antimicrobial flocculant for water clarification. Currently, the main barrier to using Moringa seeds for producing potable water is that the seeds release other water-soluble proteins and organic matter, which increase the concentration of dissolved organic matter (DOM) in the water. The presence of this DOM supports the regrowth of pathogens in treated water, preventing its storage and later use. A new strategy has been established for retaining the MOCP protein and its ability to clarify and disinfect water while removing the excess organic matter. The MOCP is first adsorbed and immobilized onto sand granules, followed by a rinsing step wherein the excess organic matter is removed, thereby preventing later growth of bacteria in the purified water. Our hypotheses are that the protein remains adsorbed onto the sand after the functionalization treatment, and that the ability of the antimicrobial functionalized sand (f-sand) to clarify turbidity and kill bacteria, as MOCP does in bulk solution, is maintained. The data support these hypotheses, indicating that the f-sand removes silica microspheres and pathogens from water, renders adhered Escherichia coli bacteria nonviable, and reduces turbidity of a kaolin suspension. The antimicrobial properties of f-sand were assessed using fluorescent (live-dead) staining of bacteria on the surface of the f-sand. The DOM that can contribute to bacterial regrowth was shown to be significantly reduced in solution, by measuring biochemical oxygen demand (BOD). Overall, these results open the possibility that immobilization of the MOCP protein onto sand can provide a simple, locally sustainable process for producing storable drinking water.

3820Langmuir 2004, 20,