The production of extracellular polymeric substances (EPS) by bacteria significantly alters the physical structure of the cell wall-solution interface, potentially affecting cell surface reactivity and colloidal (transport) properties of the cells. This study investigated the effect of the EPS layer on the electrostatic properties of the cell surface by conducting electrophoretic mobility (EM) analysis and potentiometric titrations at ionic strengths of 0.001 M, 0.01 M and 0.1 M, using both native cells, with the EPS-layer intact, and EPS-free cells of the gram-positive thermophile, Bacillus licheniformis S-86. Electrophoretic mobility measurements indicated that the isoelectric point for both cell types was below pH 2. At low ionic strength (0.001 M), expansion of the EPS layer causes the EM of the native cells to have a smaller magnitude than that of the EPS-free cells. As ionic strength increases, this effect diminishes and at 0.1 M the native cells have a higher EM than the EPS-free cells, which indicates that there is a relatively high charge density in the EPS layer. Higher total site concentrations obtained using potentiometric titration data modelling for the native cells are in agreement with the electrophoretic mobility data, and indicate a higher proton-active site density in the EPS layer compared to the EPS-free cell wall. The results of data modelling for titrations conducted at all three ionic strength values showed that there are no systematic changes in deprotonation constants or site concentrations as a function of ionic strength, indicating that the use of a non-electrostatic model is justified for this EPS-producing bacterial strain. (C) 2010 Elsevier Inc. All rights reserved.