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
1999
Mendoza, F. | Maqueda, M. | Galvez, A. | Martinez-Bueno, M. | Valdivia, E.
The peptide AS-48 is highly active on all Listeria species. It has a bactericidal and bacteriolytic mode of action on Listeria monocytogenes CECT 4032, causing depletion of the membrane electrical potential and pH gradient. The producer strain Enterococcus faecalis A-48-32, releases sufficient amounts of AS-48 into the growth medium to suppress L. monocytogenes in cocultures 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 microgram/ml it still can inhibit the growth of listeria at 6 degrees C. AS-48 is highly active on liquid cultures, although concentrations above 0.2 micrograms/ml are required to avoid adaptation of listeria. AS-48-adapted cells can be selected at low (but still inhibitory) concentrations, and they can he inhibited completely by AS-48 at 0.5 microgram/ml. The adaptation is lost gradually upon repeated subcultivation. AS48(ad) cells are cross-resistant to nisin and show an increased resistance to muramidases. Their fatty acid composition is modified: they show a much higher proportion of branched fatty acids as well as a higher C15:0 (An)-to-C17:0 (An) ratio. Resistance to As-48 is also maintained by protoplasts from AS48(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 cytoplasmic membrane are disorganized, and the cytoplasmic content is lost. Intracytoplasmic membrane vesicles are also observed when the wild-type strain is treated with high AS-48 concentrations.
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