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'

Abstract

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.

Theory of electrostatics and electrokinetics of soft particles

“…The diffuse character of the polyelectrolyte layer is defined by a gradual distribution of the density of polymer segments in the interspatial region separating the core from the bulk electrolyte solution. Successful application of that model has been reported for humic acids [46], bacteria [47] and red blood cells [48]. For numerical calculations of the electrophoretic mobility based on more rigorous theories, the readers are referred to the studies of Hill et al [49][50][51][52] and Lopez-Garcia et al [53][54][55] as well as that of Duval and Ohshima [45].…”

Electrohydrodynamic Properties of Succinoglycan as Probed by Fluorescence Correlation Spectroscopy, Potentiometric Titration and Capillary Electrophoresis

“…12 The charge measured by potentiometric titration, denoted as Q o , is defined by the spatial integration of all local charges over the macromolecular volume, i.e.…”

“…Using the measured protolytic properties and sizes of the succinoglycan, the measured electrophoretic mobility µ could be analyzed as a function of ionic strength at pH ) 10.7 ( Figure 3B) 12,14,30 with the hydrodynamic permeability 1/λ o as the unique unknown variable. Very good agreement between theory and experiment was observed over the whole range of ionic strength 5 mM-400 mM with 1/λ o ≈ 0.70 ( 0.02 nm.…”

“…These methods, however, are known to be extremely sensitive to variations of the density of the electric charge carried by the macromolecule, 9 the molecular geometry, structural arrangement, hydrodynamic permeability, and size. [10][11][12][13] This paper was designed to investigate the electrostatic and hydrodynamic character of succinoglycan on the basis of electrophoretic mobility measurements performed by capillary electrophoresis (CE) over a large range of pH and ionic strengths in aqueous medium. These features are intrinsically related to the conformational characteristics of the macromolecule.…”

Alteration of bacterial surface electrostatic potential and pH upon adhesion to a solid surface and impacts to cellular bioenergetics

“…While more complex models may be applicable, such as the soft-particle approach (Duval et al, 2005;Duval and Ohshima, 2006;Ohshima, 1995), the model developed here is sufficient to demonstrate the impact of a charge-regulated surface on surface pH and electrostatic potential (Hong and Brown, 2008). The basis of these models is the Poisson-Boltzmann equation, which describes the electrostatic potential in an electrolyte solution as a function of distance from a charged surface.…”