Nonspecific and Specific Adhesion of Bacterial Cells to Membranes

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This proposal is concerned with studying the adhesion of bacterial cells to membranes which are used in a variety of industrial scale separations technologies such as reverse osmosis and desalination. The adhesion of a cell to a membrane surface is a two step process. First, cells in the vicinity of the membrane surface are immobilized by both long- range colloidal forces such as van der Waals attraction, electrostatic attraction, and short range interactions such as hydrogen bonding. Once immobilized, the cells begin to project structures and synthesize extracellular polymeric molecules which bridge the gap between the cells and the membrane, and firmly adhere the cell to the surface. Experiments will be undertaken in a parallel plate flow chamber which is mounted onto the stage of a phase contrast microscope equipped for videomicroscopy and sensitive light intensity measurement via a photomultiplier tube. The bottom plate of the chamber is a glass slide onto which thin polymeric membranes are cast. The membranes will be made of various polymers such as cellulose acetate and polymethyl methacrylate (PMMA), and these will have different surface charges (characterized by zeta potential measurements) and hydrophobicity (characterized by contact angle measurements). Two experiments will be undertaken using the cell. In the first, a dilute suspension of cells wills be slowly perfused through the chamber, and the cells will be allowed to absorb at low density onto the polymeric membrane support. The separation distance between the cell and the membrane will be measured by total internal reflection microscopy. In this technique a laser beam, impinging against the glass slide at a small angle of incidence, is totally reflected off of the glass surface and produces an evanescent wave which propagates normal to the surface and into the chamber. The scattering of the wave by the cell is detected by the photomultiplier tube, and the distance of separation of the cell from the membrane can be obtained from this measurement. The cell separation distance will be measured as a function of both the polymeric membrane surface charge and hydrophobicity, and the charge and hydrophobicity of the bacterial cell. The latter will be correlated with the growth cycle phase. In the second set of experiments, the detachment to attached cells will be measured as a function of applied shear rate by observing the fate of attached cells by video microscopy as the flow rate through the parallel plate chamber is varied. The detachment characteristics will then be correlated with the distance of separation, and ultimately to the hydrophobicity and surface charge characteristics of the membrane and cell.