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. How ever, this equation has been derived under several more or less restri ctive conditions, which are easily violated by complex colloidal syste ms, such as bacterial cell suspensions. In recent theories as derived by Dukhin, O'Brien, and Fixman, the effect of double layer polarizatio n on the electrophoretic mobility of colloidal particles is accounted for. These theories predict that, at high surface charge densities, th e electrophoretic mobility may be strongly retarded compared to the He lmholtz-Smoluchowski equation. In this paper the effect of the mobile charge in the bacterial wall on the electrophoretic mobility is consid ered. For this purpose a comprehensive equation for the electrophoreti c mobility has been derived, which also includes surface conduction wi thin the hydrodynamically stagnant layer. To that end, Fixman's theory , valid for large kappa alpha, has been modified. It is shown that cel l wall conduction can have a considerable effect on the electrophoreti c mobility of bacterial cells, especially at low salt concentrations. In 1 and 10 mM electrolyte solution, the classical Helmholtz-Smoluchow ski equation underestimates the zeta-potential by approximately a fact or of 2 and 1.3, respectively. Obviously a full description of the com position of the electrical double layer of bacterial cell surfaces can not be based on electrophoretic mobility measurements only but should be obtained from a combination of experimental techniques, including t itration and conductivity measurements.