Abstract
The potential contribution of chemical bonds formed between bacterial cells and metal surfaces during biofilm initiation has received little attention. Previous work has suggested that bacterial siderophores may play a role in bacterial adhesion to metals. It has now been shown using in situ ATR-IR spectroscopy that enterobactin, a catecholate siderophore secreted by Escherichia coli, forms covalent bonds with particle films of titanium dioxide, boehmite (AIOOH), and chromium oxide-hydroxide which model the surfaces of metals of significance in medical and industrial settings. Adsorption of enterobactin to the metal oxides occurred through the 2,3-dihydroxybenzoyl moieties, with the trilactone macrocycle having little involvement Vibrational modes of the 2,3-dihydroxybenzoyl moiety of enterobactin, adsorbed to TiO2, were assigned by comparing the observed IR spectra with those calculated by the density functional method. Comparison of the observed adsorbate IR spectrum with the calculated spectra of catecholate-type [H2NCOC6H3O2Ti(OH)(4)](2) and salicylate-type [H2NCOC6H3O2HTi(OH)(4)](2-) surface complexes indicated that the catecholate type is dominant. Analysis of the spectra for enterobactin in solution and that adsorbed to TiO2 revealed that the amide of the 2,3-dihydroxybenzoylserine group reorientates during coordination to surface Ti(IV) ions. Investigation into the pH dependence of enterobactin adsorption to TiO2 surfaces showed that all 2,3-dihydroxybenzoyl groups are involved. Infrared absorption bands attributed to adsorbed enterobactin were also strongly evident for E. coli cells attached to TiO2 particle films. These studies give evidence of enterobadin-metal bond formation and further suggest the generality of siderophore involvement in bacterial biofilm initiation on metal surfaces.