We have developed an integrated approach to studying cell surface chemistry that includes simplified chemical systems of pure compound biopolymers as models of the cell wall1. The specific polymer selected to construct the model cell surface is based on the known physiology of key classes of bacteria. Our work has focussed on specific strains of the genus Rhodococcus. This biological model is selected due to the surfactant properties of mycolic acids. These are a key chemical constituent of the outer cell wall where some are covalently bonded to trehalose that is further bonded to the arabinogalactan layer. The mycolic acids are proposed to play a key role in the adaptation of the organism to effectively anchor itself to solid substrata with a range of surface properties such as degree of hydrophobicity and charge. Mycolic acid is a high molecular weight α-alkyl-β-hydroxy fatty acid with 50-60 carbon units. In order to study the nanometre-scale interactions of mycolic acid with solid surfaces, we first construct pure compound monolayers of either hydrophobic or anionic character, by binding these molecules to surfaces using respectively either the Langmuir-Blodgett or Langmuir-Schäffer technique. These monolayers thus represent specific manifestations of cell wall architecture that may encounter mineral surfaces during the initiation of cell attachment as incipient biofilm formation.
Interaction of this mycolic acid layer with solid surfaces is subsequently studied using quantitative colloidal force microscopy where the monolayer is interrogated with an AFM tip holding a bead of either silica (hydrophilic, negatively charged) or polystyrene (hydrophobic). Cantilever force interaction curves are determined for the approach and retraction of the AFM tip into and away from the mycolic acid monolayer interface. Parameterisation of mathematical models of colloidal force interactions, using the Extended DVLO theory, allows a quantitative interpretation of the contribution of ionic charge and solvent interactions to mycolic acid binding at the mineral surface. This work is being followed by studies that mount a Rhodococcus cell on the AFM tip in order to probe silica and polystyrene surfaces, and directly compare to what degree the mycolic acid model captures the key aspects of whole cell-mineral interactions.
Our biological models include additional bacterial strains that will be used to guide new studies of macromolecular binding to minerals. These strains include Sphingomonas that demonstrates a dominant role for cell surface proteins, and strains of Pseudomonas that exhibit a determining role for extracellular nucleic acids, in cell-mineral binding. We thus propose to extend our work on the polymer binding of mycolic acid to mineral surfaces, to study much longer-chain proteins, and eventually polysaccharides and nucleic acids, as key classes of biological macromolecules. We will study these interactions on two synthetic mineral surfaces; silica as a representative negatively charged surface and subsequently hematite (Fe2O3(s)) as a representative positively charged surface.
We propose to tackle the following 3 activities as key research challenges that advance progressively from pure compound interactions with aqueous electrolyte solutions, to whole cell interactions with the mineral-aqueous solution interface.