The structure of the cell wall of the Gram-negative bacterium Proteus vulgaris: III. A lipopolysaccharide “unit membrane”

Abstract

Oriented specimens of lipopolysaccharide extracted from the Gram-negative bacterium Proteus vulgaris were examined by both low- and high-angle X-ray diffraction and by electron microscopy of thin sections. X-ray patterns were also recorded from heated specimens and specimens fixed and embedded for electron microscopy. The X-ray patterns from untreated and fixed material show the presence of 96 A thick centrosymmetrical sheets. After embedding, the sheet thickness is unchanged although the disorder is greatly increased. Electron micrographs show disordered arrays of sheets of minimum thickness ∼ 94 A, and each sheet exhibits the distinctive dense-light-dense triple-layered appearance of a unit membrane. The X-ray patterns show that the lipid hydrocarbon chains are oriented in a direction normal to the plane of the sheet. The contraction of the structure on heating is similar to that found for other lipid bilayers. It is concluded that although the relationship of the present structure to the structure of the lipopolysaccharide layer in the cell wall is not known, the extracted lipopolysaccharide is a lipid bilayer with superficial polysaccharide.