Document Type dissertation Author Name Emerson, Ray Jenkins URN etd-042706-075421 Title A Nanoscale Investigation of Pathogenic Microbial Adhesion in Biomaterial Systems Degree PhD Department Chemical Engineering Advisors Terri A. Camesano, Advisor Nancy A. Burnham, Committee Member George D. Pins, Committee Member Robert W. Thompson, Committee Member Keywords nanotechnology biomaterials atomic force microscopy pathogenic Date of Presentation/Defense 2006-04-05 Availability unrestricted
Microbial infections of medical implants occur in 10% of the more than 20 million surgical procedures carried out annually in the United States. The additional treatments required to address these infections generate more than $11 billion in additional patient costs, increase recovery time, and decrease overall patient quality of life. As the population ages, the number of necessary and voluntary surgical procedures increases; The rate of infection increases proportionately. While treatments are available, the biofilm mode of growth confers resistance to antimicrobial therapies up to 500 times greater than that of planktonic microbes. Currently, the only guaranteed method of removing an established microbial implant infection is through surgical excision of the implant and surrounding tissues. While removing the original infection, additional colonization and pathogenesis may take place.
This research explores the a priori assumption that a medical implant infection cannot occur unless a microbial cell is capable of adhering to the implant surface. From that assumption, the following sections will focus primarily on identifying the necessary and sufficient factors influencing microbial adhesion, discretizing those factors into measurable quantities, and developing methods by which those factors may be mitigated or eliminated. Following is a brief summary of each major topic treated within this research period.
Development of a Benchmark System: We have characterized the interactions between Pseudomonas aeruginosa ATCC 10145 and Candida parapsilosis ATCC 90018 using a novel method of cellular immobilization, which emphasizes minimal chemical modification of the cell surface. This research describes the very different force-separation interactions seen between C. parapsilosis and both a common medical implant material (viz., silicone rubber) and a nascent P. aeruginosa biofilm grown on the same material. This study was the first step in developing an ab initio technique which may be used to determine the relative affinity of a microbial cell for an implant material surface.
The Role of the Substrate: Microbial adhesion to a medical implant device involves two major components, being the microbe itself, and the substrate to which it adheres. Each of the two has specific and unique surface chemical and textural characteristics which, when combined, allow for microbial colonization and subsequent infection. The goal of this study was to identify correlations between the adhesive strength of Staphylococcus epidermidis to a variety of chemically and texturally distinct substrates, and common surface characterization parameters (e.g., surface roughness and water contact angle). Relationships to adhesive strength did not demonstrate statistically significant or consistent trends. To extend upon the correlation parameters, we have employed a Discrete Bonding Model, which characterizes the surface texture according to Mandelbrot fractal theory. Correlations between the adhesive strength and the observational scale show stronger relationships, indicating a significant contribution of the surface texture to a microbe's ability to colonize a surface.
Finding a Surface That Cannot Be Touched: Historically, AFM force-separation curves demonstrating only repulsive behavior on extension of the piezoactuator have been largely ignored, in terms of quantitative modeling of the interactions. In bacterial systems, such behavior describes the majority of the force profiles recorded by the instrument. As a result of the former lack of study, the latter data sets have remained unanalyzed and unanalyzable. Building on existing mathematical models, we have developed an analytical method by which the point of zero separation between a surface (viz., the microbial cell wall) coated with a polymer brush and an AFM probe may be quantitatively identified.
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