Abstract
Mechanistic information about the bacteriocin nisin was obtained by examining the efflux of 5(6)-carboxy-fluorescein from Listeria monocytogenes-derived liposomes. The initial leakage rate (percentage of efflux per minute) of the entrapped dye was dependent on both nisin and lipid concentrations. At all nisin concentrations tested, 5(6)-carboxyfluorescein efflux plateaued before all of the 5(6)-carboxyfluorescein was released (suggesting that pore formation was transient), but efflux resumed when more nisin was added. Isotherms for the binding of nisin to liposomes constructed on the basis of the Langmuir isotherm gave an apparent binding constant of 6.2 x 10(5)M(-1) at pH 6.0. The critical number of nisin molecules required to induce efflux from liposomes at pH 6.0 was approximately 7,000 molecules per liposome. The pH affected the 5(6)-carboxyfluorescein leakage rates, with higher pH values resulting in higher leakage rates. The increased leakage rate observed at higher pH values was not due to an increase in the binding affinity of the nisin molecules towards the liposomal membrane. Rather, the critical number of nisin molecules required to induce activity was decreased (approximately 1,000 nisin molecules per liposome at pH 7.0). These data are consistent with a poration mechanism in which the ionization state of histidine residues in nisin plays an important role in membrane permeabilization.