Effect of variations in lipopolysaccharide on the fluidity of the outer membrane of Escherichia coli.

The lipid hydrocarbon chains in the outer membrane of gram-negative bacteria appear from previous experiments to be less mobile than in the cytoplasmic membrane. To determine whether lipopolysaccharide, a unique outer membrane component, is a cause of this restricted mobility, outer membranes differing in the amount of lipopolysaccharide, and the length of the polysaccharide side chain, were prepared from Escherichia coli J5. Cytoplasmic membranes were prepared for comparison. The probes, 5- and 12-doxylstearate, were introduced into these membranes, electron spin resonance spectra were analyzed, and the order parameter (S) and empirical motion parameter (tau0) were calculated. Outer membrane preparations containing long chain lipopolysaccharide were much less fluid by these criteria than were preparations containing short chain lipopolysaccharide. Removing about 40% of the lipopolysaccharide from the former preparations greatly increased their fluidity. The lipid in the cytoplasmic membrane preparations was more fluid than in the outer membrane and cytoplasmic membranes were similar to each other regardless of the composition of the outer membrane. These results indicate that lipopolysaccharide, and especially the polysaccharide portion, directly or indirectly causes the restricted mobility of the lipid hydrocarbon chains observed in the outer membrane.