M. ASTASOV-FRAUENHOFFER¹, T. WALTIMO¹, O. BRAISSANT<sup>2

1</sup>Clinic for Preventive Dentistry and Oral Microbiology, UZB - University Center for Dental Medicine, University of Basel. Hebelstrasse 3, 4056 Basel, Switzerland, ²Center of Biomechanics & Biocalorimetry, c/o Department Biomedical Engineering (DBE), University of Basel. Gewerbestrasse 14, 4123 Allschwil, Switzerland

In cariogenic biofilms, exopolysaccharides (EPS) provide an abundance of primary binding sites with different chemical functional groups such as carboxylic acids, phosphates and amines and form the core of the matrix-scaffold. Among these groups carboxylic acid and phosphates are known to bind calcium and other biologically important cations like magnesium, iron or zinc. During caries development calcium gets dissolved from enamel, however little is known whether this dissolved calcium is bound within the cariogenic biofilm. The aim of the study was to investigate and refine the role for the acid-base and calcium binding properties of the EPS of Steptococcus mutans, Lactobacillus rhamnosus, and Candida albicans. Acid-base titration of the EPS revealed that it contained mostly acidic proton binding sites. Some binding sites near neutral and alkaline pH were observed, however in much lower concentrations. Isothermal titration calorimetry (ITC) showed that EPS from bacterial strains had a binding affinity one order of magnitude higher to calcium than to lactic or citric acid. However, for EPS purified from C. albicans, titration data revealed relatively weak binding thereby not allowing estimation of an accurate binding affinity. In conclusion, the study reveals that within the biofilm matrix EPS can bind calcium with a higher affinity than organic acids thus, providing the cariogenic species with a possible mechanism to withstand higher concentrations of calcium ions which unbound could have toxic effects.