Analysis of the Interfacial Properties of Fibrillated and Nonfibrillated Oral Streptococcal Strains from Electrophoretic Mobility and Titration Measurements:  Evidence for the Shortcomings of the ‘Classical Soft-Particle Approach

Chemical and structural intricacies of bacterial cells complicate the quantitative evaluation of the physicochemical properties pertaining to the cell surface. The presence of various types of cell surface appendages has a large impact on those properties and therefore on various interfacial phenomena, such as aggregation and adhesion. In this paper, an advanced analysis of the electrophoretic mobilities of fibrillated and nonfibrillated strains (Streptococcus salivarius HB and Streptococcus salivarius HB-C12, respectively) is performed over a wide range of pH and ionic strength conditions on the basis of a recent electrokinetic theory for soft particles. The latter extends the approximate formalism originally developed by Ohshima by solving rigorously the fundamental electrokinetic equations without restrictions on the bacterial size, charge, and double layer thickness. It further allows (i) a straightforward implementation of the dissociation characteristics, as evaluated from titration experiments, of the ionogenic charged groups distributed throughout the bacterial cell wall and/or the surrounding exopolymer layer and (ii) the inclusion of possible specific interactions between the charged groups and ions from the background electrolyte other than charge-determining ions. The theory also enables an estimation of possible swelling/shrinking processes operating on the outer polymeric layer of the bacterium. Application of the electrokinetic model to HB and HB-C12 clearly shows a significant discrepancy between the amount of surface charges probed by electrophoresis and by protolytic titration. This is ascribed to the specific adsorption of cations onto pristine charged sites in the cell wall. Physicochemical parameters pertaining to the hydrodynamics (softness degree) and electrostatics of the bacterial cell wall (HB-C12) and soft polymeric layer (HB) are quantitatively derived.