Antilisterial activity of peptide AS-48 and study of changes induced in the cell envelope properties of an AS-48-adapted strain of Listeria monocytogenes

The peptide AS-48 is highly active on all Lister ia species. It has a bacte ricidal and bacteriolytic mode of action on Listeria monocytogenes CECT 403 2, causing depletion of the membrane electrical potential and pH gradient. The producer strain Enterococcus faecalis A-48-32, releases sufficient amou nts of AS-48 into the growth medium to suppress L, monocytogenes in cocultu res at enterococcus-to-listeria ratios above 1 at 37 degrees C or above 10 at 15 degrees C. As the temperature decreases, the bactericidal effects of AS-48 are less pronounced, but at 2.5 mu g/ml it still can inhibit the grow th of listeria at 6 degrees C. AS-48 is highly active on liquid cultures, a lthough concentrations above 0.2 mu g/ml are required to avoid adaptation o f listeria. AS-48-adapted cells can be selected at low (but still inhibitor y) concentrations, and they can be inhibited completely by AS-48 at 0.5 mu g/ml. The adaptation is lost gradually upon repeated subcultivation. AS48(a d) cells are cross-resistant to nisin and show an increased resistance to m uramidases, Their fatty acid composition is modified: they show a much high er proportion of branched fatty acids as well as a higher C-15:0 (An)-to-C- 17:0 An ratio. Resistance to AS-48 is also maintained by protoplasts from A S48(ad) cells. Electron microscopy observations show that the cell wall of AS48(ad) cells is thicker and less dense. The structure of wild-type cells is severely modified after AS-48 treatment: the cell wall and the cytoplasm ic membrane are disorganized, and the cytoplasmic content is lost, Intracyt oplasmic membrane vesicles are also observed when the wild-type strain is t reated with high AS-48 concentrations.