Ultrastructure of phospholipid micelles from Staphylococcus aureus

The ultrastructure of the phospholipid micells (Phosphatidylglycerol: PG, cardiolipin: CL and lysylphosphatidylglycerol: L-PG) from S. aureus were examined by negative staining and freeze-fracture to elucidate the role of each phospholipid in staphylococcal cell membrane. The width of lipid bilayer of each phospholipid was: PG: 40A, CL: 58A and L-PG: 65A, respectively, indicating that the hydration of each lipids was different. Ca(2+) caused the fusion of cardiolipin micell followed by hexagonal-cylindrical comformation. Ca(2+) seemed to give "rigidity" to the cardiolipin bilayer. Mg(2+) modified the cardiolipin lamella causing random-ripple form. Mg(2+) gave "fluidity" to the micellar structure. PG and L-PG were less affected by Ca(2+) or Mg(2+). Conversion of PG to CL was enhanced when S. aureus was exposed to stress condition or when the cell lost the wall. Molar ratio of PG/CL was 5/2 in normal condition, while 2/5 under stress. This adaptational changes of the lipid composition in the membrane may well correlate with the characteristics of cardiolipin micellar structure