Effects of cations on the outer membrane permeability of Escherichia coli

The outer membrane permeability to beta-lactam antibiotics was determined with intact Escherichia coli cells treated with various concentrations of NaCl. It was found that the treatment of cells with moderate concentrations of NaCl caused dramatic increase in the diffusion rate of beta-lactam across the outer membrane. This increased rate of beta-lactam diffusion was not due to the high osmolarity exerted by NaCl, since the elevated osmolarity of the assay medium by sucrose had no significant effect. This increased rate of beta-lactam diffusion was restored by addition of a low concentration of MgSO4 or CaCl2 into the assay medium. These results were interpreted as that the treatment of intact E. coli cells with moderate concentration of monovalent cations squeezed out divalent cations, which presumably destabilized the outer membranes, causing electrostatic repulsion of negatively charged lipopolysaccharide molecules in site.