A Spectroscopic Study of Bacterial Polymers Mediating Cell Adhesion and Mineral Transformations

Abstract

Current understanding of molecular-level interactions is inadequate to explain the initial moments of bacterial adhesion. Such information is required to develop appropriate models for bacteria-surface interactions and predictions of cell transport in subsurface environments. Bacterial adhesion is influenced by bacterial surfaces, substratum physical-chemical characteristics, and solution chemistry. Extracellular polymeric substances (EPS), surface proteins, and lipopolysaccharides (LPS) mediate cell adhesion and conditioning film formation via direct bonding to a substrate. The goal of this dissertation is to probe molecular-scale interactions of cell surface macromolecules at mineral surfaces under environmentally-relevant conditions. Four primary investigations are presented in this dissertation. The first study uses in situ attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy to reveal that prior to Mn-oxidation via Pseudomonas putida GB-1, cell adhesion to ZnSe is favorable. Subsequent Mn-oxidation results in increased extracellular proteins expression. Conversely, planktonic cell adhesion is inhibited for Mn-oxide coated cells via blocking of surface proteins. The second investigation reveals the formation of inner-sphere complexes between bacteria surface phosphoryl groups and nanohematite (α-Fe₂O₃). Spectra of bacteria (P. aeruginosa PAO1, Shewanella oneidensis MR-1, and Bacillus subtilis) on α- Fe₂O₃ contain peaks indicative of P-OFe inner-sphere bonding. Spectra collected for oxide-adsorbed model P-containing compounds give spectral signatures similar to those P-OFe bonding interactions observed for whole cell and EPS. The behavior of P. aeruginosa serotype 10 LPS in aqueous solutions was investigated in the third study. Ionic strength, pH, and electrolyte composition were varied during collection of ATR-FTIR and dynamic light scattering (DLS) data. Results reveal stable aggregate Na-LPS aggregates, whereas binding of Ca²⁺ to phosphate groups in the lipid A region leads to aggregate reorientation and increased interaction with ZnSe (hydrophobic). DLS data demonstrate decreasing hydrodynamic radius of LPS aggregates with increasing I and decreasing pH. In the fourth investigation, ATR-FTIR was used to probe the solid-solution interface of LPS on surfaces of ZnSe, Ge, α-Fe₂O₃, and α-Al₂O₃ in solutions of varying ionic composition and pH. Na-LPS aggregates remain stable and spectra are biased towards solution phase LPS. Ca-LPS aggregates are disrupted, leading to enhanced interaction with surfaces via hydrophobic (lipid A- ZnSe) and electrostatic (O-antigenhydrophilic surfaces) interactions.