Contributions of Bacterial Surface Polymers, Electrostatics, and Cell Elasticity to the Shape of AFM Force Curves

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.