Electrokinetic potential of bacterial cells

Microelectrophoresis studies are of relevance in the characterization of the electrical double layer of bacterial cell surfaces. In order to interpret the electrophoretic mobility in terms of the zeta-potential, the classical Helmholtz-Smoluchowski equation is regularly used. However, this equation has been derived under several more or less restrictive conditions, which are easily violated by complex colloidal systems, such as bacterial cell suspensions. In recent theories as derived by Dukhin, O'Brien, and Fixman, the effect of double layer polarization on the electrophoretic mobility of colloidal particles is accounted for. These theories predict that, at high surface charge densities, the electrophoretic mobility may be strongly retarded compared to the Helmholtz-Smoluchowski equation. In this paper the effect of the mobile charge in the bacterial wall on the electrophoretic mobility is considered. For this purpose a comprehensive equation for the electrophoretic mobility has been derived, which also includes surface conduction within the hydrodynamically stagnant layer. To that end, Fixman's theory, valid for large kappa alpha, has been modified. It is shown that cell wall conduction can have a considerable effect on the electrophoretic mobility of bacterial cells, especially at low salt concentrations. In 1 and 10 mM electrolyte solution, the classical Helmholtz-Smoluchowski equation underestimates the zeta-potential by approximately a factor of 2 and 1.3, respectively. Obviously a full description of the composition of the electrical double layer of bacterial cell surfaces cannot be based on electrophoretic mobility measurements only but should be obtained from a combination of experimental techniques, including titration and conductivity measurements.