Escherichia coli Membrane Fluidity as Detected by Excimerization of Dipyrenylpropane: Sensitivity to the Bacterial Fatty Acid Profile

A coordinated study of membrane fluidity and fatty acid composition has been carried out in Escherichia coli W3110. The lipid acyl chain profile of the bacteria, altered by growing cells in steady state at 30, 37, 42, or 45°C, was determined by gas chromatography of the fatty acid methyl esters. In parallel experiments, total membranes obtained from cells of the above-mentioned cultures were labeled with dipyrenylpropane and their relative fluidity was measured on the basis of the excimer to monomer fluorescence intensity ratio of the fluorophore. It has been found that, at constant assay temperature, fluidity determined with dipyrenylpropane decreases gradually with the growth temperature increment, from 30 to 45°C. Interestingly, when fatty acid composition is taken into account, fluidity increases linearly in the range under study, with the proportion of unsaturated fatty acyl chains, both variables being highly correlated (0.924 ≤ r2 ≤ 0.996). Our results show that dipyrenylpropane is a reliable and quantitative indicator of changes in membrane fluidity, driven by modifications in the acyl chain composition of bacterial lipids.