Interaction forces measured using AFM between colloids and surfaces coated with both dextran and protein

Both proteins and polysaccharides are biopolymers present on a bacterial surface that can simultaneously affect bacterial adhesion. To better understand how the combined presence of proteins and polysaccharides might influence bacterial attachment, adhesion forces were examined using atomic force microscopy (AFM) between colloids (COOH- or protein-coated) and polymer-coated surfaces (BSA, lysozyme, dextran, BSA+dextran and lysozyme+dextran) as a function of residence time and ionic strength. Protein and dextran were competitively covalently bonded onto glass surfaces, forming a coating that was 22-33% protein and 68-77% dextran. Topographic and phase images of polymer-coated surfaces obtained with tapping mode AFM indicated that proteins at short residence times (<1 s) were shielded by dextran. Adhesion forces measured between colloid and polymer-coated surfaces at short residence times increased in the order protein+dextran ≤ protein < dextran. However, the adhesion forces for protein+dextran-coated surface substantially increased with longer residence times, producing the largest adhesion forces between polymer coated surfaces and the colloid over the longest residence times (50-100 s). It was speculated that with longer interaction times the proteins extended out from beneath the dextran and interacted with the colloid, leading to a molecular rearrangement that increased the overall adhesion force. These results show the importance of examining the effect of the combined adhesion force with two different types of biopolymers present and how the time of interaction affects the magnitude of the force obtained with two-polymer-coated surfaces.