Having an understanding of the reactive nature of the bacterial surface is enhanced when its reactivity is considered in a thermodynamic framework. Towards this end, isothermal titration calorimetry was used to measure heats of proton adsorption onto Pseudomonas putida, a common Gram negative soil bacterium. Proton adsorption generated large exothermic heats and proton uptake continued down to pH 2.5. Applying a surface complexation model to the calorimetric data allowed for the derivation of site-specific enthalpies and entropies of proton adsorption. The 4-site non-electrostatic model of Borrok et al. [D.M. Borrok. J.B. Fein, J. Colloid Interface Sci. 286 (2005)110] was chosen to describe proton adsorption and enabled derivation of site-specific enthalpies of -2.4 +/- 0.3, -3.7 +/- 0.2. -9.0 +/- 0.6, and -36.0 +/-1.2 kJ/mol for Sites 1-4, respectively. Entropies of proton adsorption were calculated to be 51 +/- 3, 75 +/- 1, 91 +/- 2, and 55 +/- 4 J/mol K, for Sites 1-4, respectively. Enthalpies and entropies of Sites 1 and 3 are consistent with that of multifunctional organophosphonic acids, Site 2 is consistent with multifunctional carboxylic acids, and Site 4 is consistent with an amine. Temperature dependence of the acidity constants for Sites 1-3 is predicted to be minimal; however. Site 4 is predicted to more substantially affected by temperature. (C) 2009 Elsevier Inc. All rights reserved.