From Bacterial Adhesion to Biofilm Formation: Impact of Humic Acid and Quorum Sensing
Open Access
- Author:
- Parent, Mary Elizabeth
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 14, 2006
- Committee Members:
- Darrell Velegol, Committee Chair/Co-Chair
Costas D Maranas, Committee Member
Michael Pishko, Committee Member
John Michael Regan, Committee Member
Andrew Zydney, Committee Member - Keywords:
- autoinduction
cell signaling
diffusion modeling
luminescence
population dependence - Abstract:
- Biofilms are surface-associated bacterial populations prevalent in engineered and natural settings. Biofilms cause fouling on industrial equipment, infection of medical implants, and can be utilized in bioremediation. A better understanding of biofilm formation will generate more effective biofilm control strategies. Initial bacterial adhesion (the first stage of biofilm development) was studied in the presence of humic acid. It was determined that changes in the concentration of humic acid on silica do not affect Escherichia coli adhesion. Based on this evidence and other reports that showed bacterial adhesion is not strongly affected by external surface chemistries, it was concluded that studying the events following bacterial adhesion could garner more insight into biofilm formation. For many species quorum sensing is necessary for proper biofilm formation; thus the effect of quorum sensing signal diffusion on biofilm development was examined. Set-backs in creating engineered biofilms prompted the investigation of the effect of signal diffusion on quorum sensing alone. <p> Quorum sensing is almost always regarded as a population density effect in three-dimensional bulk samples of bacteria. Two-dimensional samples of Vibrio fischeri cells adhered onto glass surfaces were created to examine the effect of local population densities on quorum sensing. The two-dimensional bacterial populations allowed for the measurement of both time and distance effects on quorum sensing, which were previously very challenging to access in typical three-dimensional bulk samples. Thus, quorum sensing was considered in terms of signal diffusion. A diffusion model guided the experiments and showed that for a given cell spacing (density) and diffusion time there exists a “true quorum”– a number of cells necessary for quorum sensing. The experiments showed that quorum sensing occurs locally in surface samples and is a function of cell density as well as diffusion time. With proof that quorum sensing occurs locally, engineered biofilm experiments were revisited. Adhesion force measurements of small isolated biofilms served as preliminary tests of the “true quorum” concept. Diffusion in quorum sensing and that there exists a “true quorum” are important to consider for biofilm formation. Even small numbers of localized surface cells can act as “biofilm seeds”.