The effect of extracellular polymers (EPS) on the proton adsorption characteristics of the thermophile Bacillus licheniformis S-86

This study investigated proton adsorption to an extracellular polymeric substance (EPS) producing bacterial strain, Bacillus licheniformis S-86, in order to characterise and quantify the contribution made by EPS to cell surface reactivity. Potentiometric titrations were conducted using both untreated cells and cells from which the EPS layer had been extracted. Surface-complexation modelling indicated the presence of four different functional groups in both untreated and EPS-free cells. These sites are assigned to phosphodiester, (pKa 3.3–3.4), carboxylic (pKa 5.3–5.4), phosphoryl/ (pKa 7.4–7.5) and hydroxyl/amine (pKa 9.9–10.1) type groups. The pKa values for the four groups were very similar for untreated and EPS-free cells, indicating no qualitative difference in composition, but site concentrations in the untreated cells were statistically found to be significantly higher than those in the EPS-free cells for the pKa 3.3–3.4 and pKa 9.9–10.1 sites. Infrared analysis provided supporting evidence that site 2 is carboxylic in nature but did not reveal any difference in IR absorption between the native and EPS-free cells. Dissolved organic carbon (DOC) analysis conducted during this study indicated that DOC release by cells is significant, and that the EPS layer is the major contributor.