Effect of methicillin on the fatty acid composition of phospholipids in methicillin sensitive Staphylococcus aureus.

The effect of two concentrations of methicillin on the distribution of fatty acid (FA) in the individual phospholipids of the middle-log phase cultures of Staphylococcus aureus 5814S sensitive to methicillin was studied during a period of 2 h. In the presence of 1/2 X minimum inhibitory concentration (MIC) of methicillin (= 1 microgram/ml) the ratio of unsaturated fatty acid (UFA) + branched fatty acid (BFA) to saturated fatty acid (SFA) in the phosphatidic acid (PA) and in the C55-isoprenylphosphate (C55-IP) became significantly higher due mainly to the accumulation of n-C12 : 3, ai-C17 : 0, and ai-C19 : 0, and to the fall of n-C14 : 0, n-C16 : 0, and n-C20 : 0 in the PA, and to the 3-fold increase in n-C12 : 3 and a 2-fold decrease in the SFAs of the C55-IP. In contrast, the ratio of UFA + BFA to SFA was smaller in the phosphatidylglycerol (PG) and lysylphosphatidylglycerol (LPG) than that of the control. The reduction was attributed to a loss of ai-C15 : 0 and a gain of n-C20 : 0 in the PG, and to the decrease in ai-C17 : 0 and to the increase in n-C20 : 0 in the LPG. On the whole, the fluidity of the membrane phospholipids decreased. In the presence of 1 X MIC of methicillin (= 2 micrograms/ml) the ratio of UFA + BFA to SFA became higher in the PA due mainly to the prevention of the release of n-C18 : 1. In all the other phospholipids the ratio of UFA + BFA to SFA was smaller than in the control of the same age. The decrease in the ratios ranged in the order of C55-IP, PG, diphosphatidylglycerol (DPG) and LPG. The changes were attributed to the decrease in the proportion of n-C12 : 3 and ai-C15 : 0 or ai-C17, and to the increase in n-C20 : 0, except DPG. Thus the membranes in methicillin treated cocci were significantly less fluid than in the controls.