In situ potentiometric method to evaluate bacterial outer membrane-permeabilizing ability of drugs: example using antiprotozoal diamidines.

We introduced a new assay system, combining tyrocidine A and a K(+)-selective electrode, to evaluate the bacterial outer membrane-permeabilizing ability of drugs. Tyrocidine A, in the presence of an outer membrane permeabilizer, increased the permeability to K(+) of the cytoplasmic membrane of Escherichia coli, because this antibiotic could markedly increase the permeability of phospholipid layers constituting the cytoplasmic membrane, while it acted weakly on the outer membrane. Hence, the novel function of agents increasing the permeability of the outer membrane could be examined directly by monitoring the tyrocidine A-induced leakage of K(+) from the bacterial cytoplasm using a K(+)-selective electrode. We found that antiprotozoal diamidines, such as diminazene, pentamidine, and 4',6-diamidino-2-phenylindole (DAPI), can increase the permeability of the bacterial outer membrane and appropriate lipophilicity is important for diamidines to permeabilize the outer membrane.