ABSTRACT
In an aqueous environment, bacteria are charged, usually negatively charged. The electric characteristics of bacteria are strongly in¤uenced by their cell wall structure. By way of example, the photograph shows a “bald” and a “hairy,” brillated, variant of Streptococcus salivarius. The electric potential at the surface of the smooth variant is much higher than at the outer surface of the brillated one. The difference is due to the penetration and mobility of solvent molecules and ions in the bril coat. Knowledge of the cell wall structure, in particular, its softness with respect to solvent and ion penetration, is a necessity for the assessment of electrostatic interaction in bacterial adhesion and aggregation. (Figure courtesy of Department of Biomedical Engineering, University Medical Center Groningen and University of Groningen, Groningen, the Netherlands.)
Water has a high dielectric permittivity, and it is a good solvent for most ions (cf. Chapter 4). As a result, in an aqueous environment, most electrolytes are dissociated in their individual ions. Surfaces of (solid) materials often contain ionizable groups, which in contact with water, are electrically charged. Together with the wettability, the presence of electric charge largely determines the stability of colloidal dispersions (Chapter 16) and interfacial processes as adsorption and adhesion (Chapters 14, 15, and 20), and it plays an important role in the assembly of nanostructured objects (Chapter 11).
