Lateral Mobility and Surface Density of Lipopolysaccharide in the Outer Membrane of Salmonella typhimurium

Salmonella typhimurium 1195 mutant bacteria were allowed to produce clusters of lipopolysaccharide with new, wild‐type specificity over the export sites. Ferritin‐labeling experiments showed that the lipopolysaccharide diffuses away from the export sites at 37 °C and is found dispersed on the cell surface, whereas at 0 °C it remains immobile. The density of lipopolysaccharide molecules on the bacterial surface, measured as number of lipopolysaccharide molecules per μm 2 , or as percentage of the bacterial surface covered by lipopolysaccharide was determined in two wild‐type S. typhimurium strains and one mutant strain, which was grown under conditions either permissive or nonpermissive for the formation of wild‐type lipopolysaccharide. There are 0.7—1.0 × 10 5 lipopolysaccharide molecules per μm 2 surface area. About 15—25% of the bacterial surface was found to be covered by lipopolysaccharide. The mean intermolecular distance at hypothetical uniform distribution of lipopolysaccharide was calculated to be in the range of 3—4 nm. It was of no consequence to the lipopolysaccharide surface density whether the mutant strain produced wildtype or mutant lipopolysaccharide. An estimate of the lateral diffusion constant D ( D ∼ 3 × 10 −13 cm 2 /s) of lipopolysaccharide in the outer membrane was made from earlier published data 2. Possible reasons for the low order of magnitude of D , based on the observed surface density, are discussed.