Changes in permeability of Staphylococcus aureus and derived liposomes with varying lipid composition

The physical and barrier properties of the phospholipids phosphatidylglycerol and lysylphosphatidylglycerol were studied in membrane model systems. Packing in monolayers at the air-water interface showed a larger area per molecule for lysylphosphatidylglycerol than for phosphatidylglycerol. The non-electrolyte permeability of liposomes prepared with lysylphosphatidylglycerol was higher than of those prepared with phosphatidylglycerol. On the other hand, the permeability of 86Rb+ was higher for liposomes of phosphatidylglycerol than for those of lysylphosphatidylglycerol. Valinomycin was able to increase the permeability of this cation in the phosphatidylglycerol liposome only. Studies on the effect of the environmental pH on the lysylphosphatidylglycerol to phosphatidylglycerol ratio in intact cells of Staphylococcus aureus showed that the total amount of lysylphosphatidylglycerol, cardiolipin and neutral lipids in the membrane did not change when the pH of the medium was varied between pH 6.5 and 5.0; but the total amount of phosphatidylglycerol decreased when the pH of the medium was lowered. The permeability of the intact cells for erythritol appeared to increase with increasing lysylphosphatidylglycerol to phosphatidylglycerol ratio; whereas the valinomycin mediated exchange of 86Rb+ over the cell membrane appeared to decrease when this ratio was increased. From the correlation between the permeability properties of cells and liposomes the conclusion is drawn that in S. aureus the chemical nature of the phospholipids determines to a great extent the properties of the permeability barrier