In order to elucidate why cardiolipin increases markedly in Staphylococcus aureus cells which lack cell walls, the phase transition temperature of cardiolipin (CL) was determined and compared with that of a major phospholipid, phosphatidylglycerol (PG). CL composed of a fatty acid with a given length was synthesized from dimyristoyl PG and dipalmitoyl PG with the aid of phospholipase D prepared from cabbages and was purified by chromatography. Analysis by differential scanning calorimetry showed that the phase transition temperatures of dimyristoyl PG, tetramyristoyl CL, dipalmitoyl PG and tetrapalmitoyl CL were 25.0, 47.0, 40.5 and 62.2℃, respectively. A mixture of the two phospholipids showed a higher phase transition temperature than PG alone, but lower than CL alone. In the presence of divalent cations, especially Ca(2+), the phase transition temperature of CL increased more than that of PG. These results clearly indicate that cardiolipin can increase the membrane rigidity and suggest that S. aureus may increase cardiolipin content of the membrane to compensate for the loss of mechanical protection due to the lack of the cell wall
