Microbial Adhesion to Medical Implant Materials: An Atomic Force Microscopy Study

Emerson, Ray Jenkins

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Microbial Adhesion to Medical Implant Materials: An Atomic Force Microscopy Study

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Microbial infections of medical implants occur in more than 2 million surgical cases each year in the United States alone. These increase patient morbidity and mortality, as well as patient cost and recovery time. Many treatments are available, but none are guaranteed to remove the infection. The purpose of this work is to examine the initial events in microbial adhesion by simulating the approach and contact between a planktonic cell, immobilized on an Atomic Force Microscope (AFM) cantilever, and a biomaterial or biofilm substrate. Distinct adhesive interactions exist between Candida parapsilosis and both unmodified silicone rubber and Pseudomonas aeruginosa biofilms. Using C. parapsilosis cells immobilized on AFM cantilevers with a silicone substrate, we have measured attractive interactions with magnitude of 2.3 Â± 0.5 nN (SD) in the approach portion of the force cycle. On P. aeruginosa biofilms, the magnitude of the attractive force increases to 3.5 Â± 0.75 nN (SD), and is preceded by a 2.5 nN repulsion at approximately 175 nm from the cell surface. This repulsion may be attributed to steric and electrostatic interactions between the two microbial polymer brushes. Young's moduli for microbes and biofilms were calculated using Hertzian contact models. These produced values of 0.21 Â± 0.003 MPa (SD) for the C. parapsilosis-silicone rubber system, and 0.84 Â± 0.015 MPa (SD) for the C. parapsilosis-biofilm system. This technique may be extended to calculate the work per unit contact area involved in the attractions in experimental data. For example, the work of adhesion using a spore probe is an order of magnitude greater for unmodified silicone rubber than for a P. aeruginosa biofilm. This indicates a high affinity for silicone rubber, and suggests that this material is vulnerable to infection by C. parapsilosis in vivo. We have also demonstrated that AFM force curve analysis using established qualitative and quantitative models fails to accurately represent the physical interactions taking place between the probe and sample for the case where a polymer brush exists on the substrate, the probe, or both. As such, an approximate method defining the sample surface as the actual surface plus some vertical dimension associated with the maximum compressible thickness of the polymer brush is discussed. Characterization of cell-biomaterial and cell-cell interactions allows for a quantitative evaluation of the materials used for medical implantation. It also provides a link between the physicochemical and physicomechanical properties of these materials and the nanoscale interactions leading to microbial colonization and infection. The goal of this research is to study this link and determine how best to exploit it to prevent microbial infections of medical implant materials.

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