Adhesive and cohesive properties of the biofilm matrix [electronic resource]

Biofilms are multicellular communities consisting of microorganisms enmeshed in an extracellular matrix of biopolymers. The matrix provides the community structure and cohesiveness and allows it to adhere to a variety of interfaces. Formation of a biofilm is advantageous to the microbial community, as it provides protection from external assaults (desiccation, oxidizing agents, predation), protection from host immune defenses, facilitates close cell-to-cell interactions for DNA exchange, and creates nutrient gradients that give rise to metabolic diversity within the community. These factors allow biofilms to persist in a variety of settings, ranging from large-scale industrial equipment to medical implants in the human host. In fact, many infections are now appreciated to be biofilm-related and are difficult to treat by traditional means such as antibiotics. To combat unwanted biofilms, a current strategy is to take a biophysical approach and interfere with the biofilm structure by disrupting the extracellular matrix. This strategy could revoke the survival advantages provided to the microorganisms by existing in the biofilm community. It also avoids the life-or-death pressure placed on microorganisms by traditional antibiotic treatment that gives rise to drug resistant mutations. However, to achieve this goal of targeting the extracellular matrix, we require an improved understanding of the underlying mechanical properties of the biofilm structure. In this work, we describe the use of modified rheological methods to quantify mechanical interactions relevant at all stages of the biofilm lifecycle, including: initial microbial adhesion to interfaces, maturation of the biofilm structure, and microbial dispersal. A Live Cell Monolayer Rheometer (LCMR) was used to study adhesion of uropathogenic Escherichia coli to bladder epithelial cells, the initial step in bladder infection. Quantitative mechanical measurements defined the contributions of bacterially produced type 1 pili, curli, and cellulose to bladder cell adhesion, and revealed an important role for cellulose in mediating these interactions. This novel use of live cell rheology can be expanded to study a variety of other relevant host-pathogen interactions. In a separate study, interfacial shear rheology was used to study the maturation of biofilms formed at the air-liquid interface by Vibrio cholerae, the causative agent of cholera. It was discovered that out of several known extracellular matrix components in the V. cholerae biofilm, a specific matrix protein called Bap1 contributed significantly to maintenance of biofilm elasticity, biofilm hydrophobicity, and development of a mature biofilm structure. Finally, mechanical measurements relevant to biofilm dispersal were performed using a custom-built device to apply large deformations to Bacillus subtilis biofilms formed at the air-liquid interface. These measurements revealed that biofilms exhibit non-uniform deformation due to inhomogeneous mechanical properties within the structure and can have both viscoelastic and viscoplastic characteristics. Together, these studies produced new tools in the field of biofilm mechanics and provided quantitative measurements of mechanical interactions relevant to all stages of the biofilm lifecycle.

Keywords: Chemical Engineering.
Advisor: Fuller Gerald G.
Cegelski Lynette advisor. Thesis advisor
Dunn Alexander Robert advisor. Thesis advisor