A study of the surface charging properties of a standard strain of Escherichia coli (ATCC 11775) in aqueous solutions.

In this study, the Escherichia coli (E. coli) strain ATCC 11775 was studied to determine its surface charging properties in a range of different aqueous salt solutions, with the aim of evaluating its potential as a monitor organism for water treatment. Zeta potential measurements were carried out in various solutions containing: NaCl, CaCl2, MgSO4, ZnSO4 and C14TAB, at different pH values and concentrations. Interestingly, it was found that the zeta potential of this strain of E. coli remained fairly constant at pH values over about 6, in 1mM NaCl solutions. In order to explain the cell surface charging properties, a simple, mass action surface ionization model was developed. This model indicates that the surface charging of these E. coli cells can be modeled simply using the ionization behavior of the acid groups in the common anionic membrane lipid phosphatidylserine (PS). There appeared to be no specific, strong adsorption of either divalent anions or cations, until high salt concentrations, above about 0.1M. The results suggest that at high concentrations both Ca(2+) and SO4(2-) ions are strongly adsorbed at the cell surface. However reduction of the magnitude of the surface electrostatic potential, due to Ca(2+) ion adsorption, did not appear to cause any cellular binding. In comparison, cationic surfactant was strongly adsorbed by the cell membrane surface, even at concentrations of 0.1mM, and light scattering studies indicated that the adsorption of the surfactant appeared to lyse the cell membrane and release internal cellular materials leading to a significant reduction in cell size.