Mesenterocin 52A (Mes 52A) is a class IIa bacteriocin produced by Leuconostoc mesenteroides ssp. mesenteroides FR52. The interaction of Mes 52A with bacterial membranes of sensitive, resistant and insensitive Leuconostoc strains has been investigated. The degree of insertion of Mes 52A on the phospholipid bilayer was studied by fluorescence anisotropy measurements using two probes, 1-(4-trimethylammonium)-6-phenyl-1,3,5-hexatriene (TMA-DPH) and DPH, located at different positions in the membrane, and the consequence for K(+) efflux and proton motive force was analyzed. Mes 52A caused an increase in the fluorescence of TMA-DPH and DPH in the membrane of the sensitive strain L. mesenteroides ssp. mesenteroides LMA 7, indicating that Mes 52A inserts into the cytoplasmic membrane of this sensitive strain. This insertion leads to K(+) efflux, without perturbation of DeltapH and a weak modification of DeltaPsi, and is consistent with pore formation. With the high-level resistant strain L. mesenteroides ssp. mesenteroides LMA 7AR, or with the insensitive strain Leuconostoc citreum CIP 103405, no modification of TMA-DPH or DPH anisotropy occurred, even in the presence of high Mes 52A levels. The membrane potential was not modified and no K(+) efflux was detected. There is a clear correlation between the physico-chemical characteristics of the membrane, the degree of Mes 52A penetration, the mechanism of action and the resistance or insensitivity characteristic of the target strains.