Function Of Extracellular Polymeric Substances Research Articles

Bioleaching—a result of interfacial processes caused by extracellular polymeric substances (EPS)

Extracellular polymeric substances (EPS) seem to play a pivotal role in bioleaching—the winning of precious metals as well as acid rock drainage. For a better control of both processes structure and function of extracellular polymeric substances from leaching bacteria are of crucial importance. Our research focussed on Acidithibacillus ferrooxidans, the composition of its extracellular polymeric substances and further deduction of their function. The extracellular polymeric substances of Acidithiobacillus ferrooxidans consist mainly of neutral sugars and lipids. The functions of the extracellular polymeric substances of this leaching bacterium seem to be (i) to mediate attachment to a (metal) sulfide surface, (ii) to concentrate iron(III) ions by complexation through uronic acids or other residues at the mineral surface and, thus, allowing for an oxidative attack on the sulfide. Consequently, dissolution of the metal sulfide is enhanced, which may result in an acceleration of 20- to 100-fold over chemical leaching. Experiments were performed to elucidate the importance of the iron(III) ions complexed by extracellular polymeric substances for strain-specific differences in oxidative activity for pyrite. The preliminary data indicate that strains of Acidithiobacillus ferrooxidans, with a high amount of iron(III) ions in their extracellular polymeric substances, possess a higher oxidation activity than those with less iron(III) ions. These data provide new insight into the function of extracellular polymeric substances and the consequent advantage conferred to bacteria involved in bioleaching.

Biomembrane Phospholipid–Oxide Surface Interactions: Crystal Chemical and Thermodynamic Basis

Quartz has the least favored surface among many oxides for bacterial attachment and for lipid bilayer or micelle interactions. Tetrahedrally coordinated crystalline silica polymorphs are membranolytic toward liposomes, lysosomes, erythrocytes, and macrophages. Amorphous silica, the octahedral silica polymorph, (stishovite), and oxides such as Al 2O 3, Fe 2O 3, and TiO 2 are less cytotoxic. Existing theories for membrane rupture that invoke interactions between oxide surfaces and cell membrane phospholipids (PLs) do not adequately explain these differences in membranolytic potential of the oxides. The author presents a crystal chemical, thermodynamic model for the initial interaction of oxide surfaces with the quaternary ammonium component of the PL's polar head group. The model includes solvation energy changes and electrostatic forces during adsorption, represented by the dielectric constant of the solid and the charge-to-radius ratio of the adsorbing solute. The nature of oxide–solute interactions compared with oxide–water, solute–water, and water–water interactions determines the membranolytic activity of the oxide, where the solute is TMA +, the quaternary ammonium moeity. Significant membrane rupture, as on quartz, requires unfavorable adsorption entropy (Δ S ads,TMA + <0) to maximize disruption of normal membrane structure and requires favorable Gibbs free energy of exchange between TMA + and the ambient Na + ions (Δ G exc,TMA +/Na + =Δ G ads,TMA + −Δ G ads,Na + <0) to maximize the extent of membrane affected. For amorphous silica, Δ S ads,TMA + >0, so disruption of structure is limited, even though Δ G exc,TMA +/Na + is <0. Stishovite and other oxides have Δ S ads,TMA + <0, but now Δ G exc,TMA +/Na + is >0 at the acidic to circumneutral pHs of cellular and subcellular organelle fluids. The model predicts the correct sequence of membranolytic ability: quartz ≥ amorphous SiO 2>Al 2O 3>Fe 2O 3>TiO 2. The model thus explains the relatively poor adhesion of bacterial cells to quartz and the lack of quartz as a biomineral. It is proposed that one function of extracellular polymeric substances exuded by bacteria is to render mineral surfaces more hydrophilic.

Modelling the structure and function of extracellular polymeric substances in biofilms with new numerical techniques

The objective of this study is the mathematical description of the structure and function of the extracellular polymeric substances (EPS) in biofilms. The basic assumptions of the EPS biofilm model are: the production of EPS in biofilms is coupled to the growth of micro-organisms the production of EPS is additionally coupled to the substrate conditions the EPS represent a considerable volume fraction of the matrix in biofilms and thus the density of the biofilms is strongly influenced by the EPS sorption of biocides and pollutants in biofilms occurs mainly to EPS the EPS can be used as an energy source during substrate limited phases. The mathematical model has been derived as a system of partial differential equations. The numerical solution of these complex balance equations has been done by a self-adaptive Galerkin-h-p-method. It can be shown, that on the one hand the simulation of substrate conversion and biofilm growth with the EPS-biofilm model yields similar results as the known biofilm models without consideration of the EPS fraction. On the other hand the advantage of the EPS-biofilm model is a better understanding of biofilm structure, which is mainly influenced by the EPS fraction in the biofilm. Furthermore, the sorption of pollutants, such as heavy metals and chlorinated organic substances, can be simulated in more detail.