Influence of cold, surfactant or carbon dioxide adaptation on the sensitivity of Listeria monocytogenes to nisin: A mechanistic study of the membrane composition and physical properties

Listeria monocytogenes is a foodborne pathogen of great concern. The cell membrane of the bacteria plays a critical function in its stress responses. As an effective antimicrobial agent against L. monocytogenes , nisin interacts with target membranes in four sequential steps: binding, insertion, aggregation, and poration. Alterations in membrane lipid composition could affect any of these steps and therefore the efficiency of nisin. We hypothesized that adaptations to cold (10°C) and surfactant (0.1% Tween 20) or CO₂ would influence the membrane lipid composition, membrane fluidity and nisin sensitivity of both wildtype (Scott A) and nisin-resistant (NR10) L. monocytogenes.; Compared to non-adapted L. monocytogenes, membranes of cold-adapted L. monocytogenes had increased amounts of shorter, branched-chain fatty acids, increased CL in the expense of PG as the polar head groups of membrane phospholipids, increased lipid membrane fluidity (as measured by fluorescence anisotropy) and increased nisin sensitivity. Surfactant-adaptation did not affect lipid membrane fluidity of L. monocytogenes Scott A, but increased nisin sensitivity. Further study on the binding isotherms of 14C-labeled nisin with surfactant-adapted Scott A membranes indicated surfactant-adaptation increased the binding capacity of the membrane and therefore nisin sensitivity of the cells. CO₂-adapted L. monocytogenes Scott A had increased straight chain fatty acids and membrane fluidity compared with cold-adapted Scott A. However, CO ₂-adapted Scott A appeared less sensitive to nisin. In parallel, the same set of experiments was carried out on L. monocytogenes NR10.; In this research, we demonstrated that stress-adaptations would induce membrane lipid compositional changes to L. monocytogenes, which further regulated lipid membrane fluidity. We also found a correlation of increased lipid membrane fluidity with increased cell sensitivity to nisin in most of the cases. CO₂-adaptation is an exception and needs further investigations. Since nisin sensitivity of the lipid vesicles from different stress-adapted cells were the same in spite of the difference in membrane fluidity, we speculate that other membrane components such as membrane proteins and their interactions with membrane lipids may be part of the nisin sensitivity regulation in addition to the membrane fluidity as determined from pure lipid vesicles. (Abstract shortened by UMI.)...