ABSTRACT A macroscopic-level examination of bacterial adhesion in natural and engineered aquatic systems has so far been insufficient to understand the factors that control the initial events in microbial attachment to mineral and natural surfaces.  A central reason is a lack of a molecular-level understanding of the chemical and physical interactions of colloidal-sized bacteria with surfaces. Historically, the DLVO theory (and its various extensions) has been used to explain colloid-colloid and colloid-surface interactions based on mean-field models and surface properties.  While the DLVO model has some experimental support for very ideal systems like homogeneous, molecularly-smooth mica surfaces, the theory often fails to describe colloid-surface forces, even in “ideal” cases.  This failure arises in part because DLVO theory was not meant to account for all physical and chemical interfacial forces and the inherent complexity of bacterial adhesion.   We hypothesize that in environmental systems bacterial adhesion is controlled by the interaction of cell-surface biopolymers (exopolymers, or EPS), natural organic matter (e.g. humic acids, which are ubiquitous in natural waters) and mineral surfaces. These molecular-level polymer factors are absent in DLVO-type models because they do not incorporate the effects of solution chemistry on the polymers themselves. The emergence of molecular-level surface chemistry techniques and new molecular level computational models make it possible for the first time to directly interrogate the behavior of bacterial polymers and their interactions with mineral surfaces in the presence of natural organic matter. We will use atomic force microscopy (AFM), FTIR spectroscopic techniques, molecular modeling, and several electro-hydrodynamic techniques to probe bacterial polymer and natural organic matter polymer size, strength, and conformation at surfaces as a function of solution chemistry, surface charge and surface composition. AFM is a powerful tool for examining polymers and bacterial surfaces, but surface elasticity must be included in electrosteric models to translate raw AFM data into absolute values of polymer distances and interaction forces. Differential electrophoresis and AFM measurements will be used to measure the non-homogeneous distribution of surface charge on bacteria.  Macromolecule-surface interactions will be probed using attenuated total reflectance (ATR) FTIR spectroscopy and ab initio molecular models of polymer behavior at surfaces. The chemical-hydrodynamic coupling of particle motion will then be used to relate these molecular scale events to the initial attachment and/or release of bacteria at surfaces and in packed columns of porous media.  Our research into the molecular-level description of bacterial adhesion will enable breakthroughs in the control of bacterial adhesion and the use of surface chemical techniques to study microorganisms, biopolymers, and colloidal systems.  Although our focus is on microbial adhesion, our results will impact the understanding of forces between many types of surfaces with polymer layers and surface heterogeneity, and this will have significant implications for many environmental and industrial applications where colloids are important.