Surface behaviour of Bacterium coli - I. The nature of the surface

Using a microelectrophoretic method the zeta potentials of Bacterium coli suspended in a series of inorganic electrolytes having either a common anion or cation have been determined. By experiment, or extrapolation to zero potential of the curve of zeta potential against log10 [electrolyte concentration], it has been possible to find the concentration (normality) of each electrolyte required to neutralize exactly the charge on the cells. For cations with common anions and vice versa, the ‘reversal of charge concentrations’ have been plotted in the form of spectra. Comparison with similar spectra given in Kruyt (1949a) for some naturally occurring substances has indicated that the exterior of the cells is polysaccharide, possibly an arabate. This conclusion is supported by the pK and isoelectric point derived from the curve of zeta potential against surface pH, and also by the relatively low zeta potentials exhibited by Bact. coli in detergents compared with the corresponding values for lipoidal substances.