Initial Adhesion of Microorganisms to Polymeric Membranes

Biofilms form in nearly every environment that provides a surface, nutrients, and water. They can be found, for example, in almost all natural aquatic environments; on teeth, bone interstices, and oral epithelia of animals and humans; on hulls of marine installations and ship bottoms; on prosthetic devices and medical implants; on water conduits and in filters; and even ion the “sterile” surfaces of the computer chip manufacturing industry. The generally accepted stages in the development of microbial biofilms are transport to the surface, initial attachment, “more permanent” adhesion, proliferation, and biofilm formation [1]. In some of the systems described above transport is dominated by microorganism mobility (combination of thermal diffusion and motility), in others convective drag forces influence the transport of microbes toward the surface of interest. Once in close proximity to a surface, microorganisms may initially attach via any combination of intermolecular (van der Waals), electrostatic, or hydrophobic forces like simple colloids, and later become more permanently adhered by exuding extra-cellular polymeric substances (EPS). In this investigation, we are interested in understanding the fundamental mechanisms governing the transport and initial attachment of microorganisms to polymeric membrane surfaces. Biofouling of membranes is defined as an unacceptable loss of performance due to biofilm formation on a membrane, and has been described as the “Achilles heel” of membrane processes [2]. In crossflow membrane filtration, solvent permeation through the membrane creates a convective drag force normal to the surface. Even when conditions are chemically “unfavorable” for deposition, microbes may be held near the membrane surface under force of permeation drag (i.e., in “secondary minima”) long enough to enable more permanent adhesion to the membrane surface via EPS-membrane interactions. The specific aim of this investigation is to elucidate the role of physico-chemical surface properties governing the attachment of microorganisms to membrane surfaces in crossflow membrane filtration; however, the work is fundamental in nature and may have application to a host of other engineered and natural systems. Commercially available polymeric ultrafiltration membranes were used as model surfaces; yeast cells (Saccharomyces cerevisiae) and latex micro-spheres were used as model biocolloids (microorganisms). An optically clear crossflow membrane filter mounted on a microscope stage coupled with a CCD camera allowed direct visual observation of initial microbial attachment during filtration. Post-filtration image analysis via a commercial software enabled quantification of the net particle deposition rate at various solution chemistries and operating conditions. Relevant physico-chemical properties of membranes and model biocolloids were experimentally determined and used to model biocolloid-membrane (van der Waals and electrostatic) interactions. A relatively simple force balance model accounting for permeation drag, inertial lift (due to tangential convection), and colloid-membrane interactions has provided valuable insight into the relative rates of initial microbial adhesion. Ongoing work will estimate the colloidmembrane interaction forces via experiment-based means (atomic force microscopy) and more fundamental modeling techniques to include the role of nano-scale surface roughness.