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Physicochemical impacts of soluble metals on bacterial lipid chemistry and function

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Title: Physicochemical impacts of soluble metals on bacterial lipid chemistry and function
Author: French, Shawn
Department: Department of Land Resource Science
Program: Biophysics
Advisor: Glasauer, Susan
Abstract: Bacterial membranes are dynamic structures, and contain lipid components that are individually simple, but complex as a whole system. The presence of charged functional groups makes them capable of interaction with ubiquitous environmental metals. Physiological responses of bacteria to metals, in preservation of membrane functions and integrity, are unclear. In this study, membrane lipid profiles were characterized for Shewanella putrefaciens CN32. Both fatty acid chemistry and hydrophilic headgroup chemistry were assayed, after growing the cells in a chemically defined medium spiked with Mn, V, or U. Cultures were grown in both aerobic and anaerobic conditions, to examine the effects of O2 and CO2 gases, as well as the combined effects of these gases with metals. The results were compared to scanning transmission X-ray microscopy (STXM) elemental maps and near-edge X-ray absorption fine structure (NEXAFS) spectra of isolated and purified S. putrefaciens CN32 envelopes at V, Mn, Ca, C, N, and O edges. It was found that there were strong correlations between membrane fluidity and fatty acid composition of strain CN32 membranes. The acyl chain chemistry was minimally affected by metal presence in the growth medium, however these subtle changes correlated with significant alterations in the fluid states of the membranes. Uranium seemed to fall outside this relationship, strongly stabilizing cell membranes. Metals in all treatments adsorbed to cell membranes, determined using either NEXAFS or electron microscopy, with the exception of V in aerobic conditions. Permeability effects of metal exposure to Ca(II), Cu(II), Mn(II), U(VI), V(IV), and Zn(II) were also assessed. Bacterial strains for these studies included S. putrefaciens CN32, Escherichia coli AB264 (wildtype K-12), Pseudomonas aeruginosa PAO1 wildtype, and Bacillus subtilis 168, in order to compare published data from the membrane chemistry of those organisms to S. putrefaciens CN32 membranes. Each metal had the same overall impact on each bacterial strain, regardless of variations in cell membrane and surface sugar chemistry, however the strengths of these effects were different for each organism. All metals with the exception of U permeabilized cell walls, while U rendered the membrane much less permeable. These impacts on permeability were concentration dependent from 0.001 mM to 1 mM concentrations. The research demonstrated that growth environment has a significant impact on the physicochemical state of bacterial membranes. Metals in those environments have varying complexation chemistry according to pH and redox conditions, and impact membrane attributes and dynamics depending on cell wall chemical composition.
Date: 2011-08
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