Bacterial infection of biomaterial surfaces is an important problem in the biomedical and health industries. The design of materials resistant to infections necessitates an understanding of the forces driving bacterial adhesion. Escherichia coli cells were immobilized onto the tip of a standard atomic force microscope (AFM) cantilever, and force measurements were performed by approaching the modified cantilever onto mica, hydrophilic glass, hydrophobic glass, polystyrene, and Teflon. Consistent with prior qualitative observations, we show that bacterial adhesion is indeed enhanced by the surface hydrophobicity of the substrate. The forces of interaction measured with the AFM are compared to those of model predictions based on an extended-DLVO approach. In this model, short-range acid−base and steric interactions are included with the conventional van der Waals attraction and electrostatic components. The theoretical predictions agree well with experimental data for E. coli D21f2, a strain whose outer surface consists of lipopolysaccharide molecules with severely truncated carbohydrate chains. However, the adhesive behavior of E. coli strains with more complex cell surface structures was found to be more difficult to model because of the possible involvement of steric and bridging effects or specific receptor−ligand interactions that remain to be resolved.

Bacterial adhesion and the subsequent formation of biofilm are major concerns in biotechnology and medicine. The initial step in bacterial adhesion is the interaction of cells with a surface, a process governed by long-range forces, primarily van der Waals and electrostatic interactions. The precise manner in which the force of interaction is affected by cell surface components and by the physiochemical properties of materials is not well understood. Here, we show that atomic force microscopy can be used to analyze the initial events in bacterial adhesion with unprecedented resolution. Interactions between the cantilever tip and conf luent monolayers of isogenic strains of Escherichia coli mutants exhibiting subtle differences in cell surface composition were measured. It was shown that the adhesion force is affected by the length of core lipopolysaccharide molecules on the E. coli cell surface and by the production of the capsular polysaccharide, colanic acid. Furthermore, by modifying the atomic force microscope tip we developed a method for determining whether bacteria are attracted or repelled by virtually any biomaterial of interest. This information will be critical for the design of materials that are resistant to bacterial adhesion.Bacterial adhesion onto inanimate surfaces is a critical issue in processes ranging from the biofouling of industrial equipment to dental decay to infections of biomaterials for medical use. Bacterial infections associated with the formation of biofilms refractile to antibiotic therapy is one of the main reasons for the failure of devices such as catheters, vascular grafts, joint prostheses, and heart valves (1-3). The first step in bacterial adhesion is the immediate attachment of bacteria onto a substratum which is a reversible, nonspecific process (3-5). This initial interaction between bacteria and artificial surfaces is a key determinant in biofilm formation. If the approach of bacteria to a surface is unfavorable, cells must overcome an energy barrier to establish direct contact with the surface. Only when bacteria are in close proximity to the surface do shortrange interactions become significant. Thereafter, proteinligand-binding events mediated by a plethora of microbial adhesins and in some cases the production of extracellular polymers render the binding process practically irreversible (6).Initial bacterial attraction or repulsion to a particular surface can be described in terms of colloidal interactions. Consequently, the force of interaction depends on physiochemical parameters such as surface-free energy and charge density (7-11). The propensity of bacteria to adhere onto surfaces has been estimated by counting the number of bacteria that remain attached to surfaces following incubation for a specified length of time (5, 12). This approach is qualitative, time consuming, and has low sensitivity. Moreover, the resulting number of adherent bacteria is determined by multiple factors in addition to long-range attractive/repulsive interactions. A direct and ...

Failure of implanted biomaterials is commonly due to nonspecific protein adsorption, which in turn causes adverse reactions such as the formation of fibrous capsules, blood clots, or bacterial biofilm infections. Current research efforts have focused on modifying the biomaterial interface to control protein reactions. Designing biomaterial interfaces at the molecular level, however, requires an experimental technique that provides detailed, dynamic information on the forces involved in protein adhesion. The goal of this study was to develop an atomic force microscope (AFM)-based technique to evaluate protein adhesion on biomaterial surfaces. In this study, the AFM was used to evaluate (i) protein-protein, (ii) protein-substrate, and (iii) protein-dextran interactions. The AFM was first used to measure the pull-off forces between bovine serum albumin (BSA) tips/BSA surfaces and BSA tips/anti-BSA surfaces. Results from these protein-protein studies were consistent with the literature. More importantly, the successful measurement of antibody-antigen binding interactions demonstrates that both the BSA and anti-BSA proteins retain their folded conformation and remain functional following our immobilization protocol. The AFM was also used to quantify the physiochemical interactions of proteins during adhesion to various self-assembled monolayers (SAMs) and dextran-coated substrates representative of potential biomaterial interface modifications. Dextran, which renders surfaces very hydrophilic, was the only surface coating that BSA protein did not adhere to. Hydrophobic interactions were not found to play a significant role in BSA adhesion. Therefore, the dextran molecules may resist protein adhesion by repulsive steric effects or hydration pressure. Moreover, the AFM-based methodology provides dynamic, quantitative information about protein adhesion at the nanoscale level.

The atomic force microscope (AFM) was used to directly measure the forces of interaction between E. coli D21 bacteria and hydrophilic glass or hydrophobic N-octadecyltrichlorosilane (OTS)-treated glass substrates coated with the block copolymers, poly(ethylene glycol) (PEG)-lysine dendron or Pluronic F127 surfactant, respectively. Short-range repulsive interactions between bacterial cells and substrates coated with the block copolymers were detected by the AFM over distances of separation comparable to the extended length of the PEG polymer chains. In contrast, glass and OTS-treated glass devoid of PEG-lysine dendron or Pluronic F127 exerted long-range attractive forces on E. coli D21 bacteria. Thus, polymeric brush layers appear to not only block the long-range attractive forces of interaction between bacteria and substrates but also introduce repulsive steric effects.