[Ultrastructural study of lipopolysaccharide and of polymyxin B-induced changes of the outer membrane of Serratia marcescens (author's transl)].

Electron micrographs of lipopolysaccharide (LPS) from Serratia marcescens which have been extracted with phenol/water, suspended in dist. water and subsequently negatively stained reveale round to ovoid particles besides singular ribbon-like structures. These structures are interpreted as collapsed LPS-strands of the outer membrane (OM). Fine structure investigations were carried out on strand-like structures which had been obtained by light alcalization of the particle suspension. Partial denaturation of LPS in ethylene-diaminotetracetic acid with polymyxin B (PB) gave rise to broad bends with periodic 180 degrees-torsions, indicating a helical structure. Chemically fixed LPS in phosphate buffer which were only partial transformed into LPS-strands, additionally revealed that a given LPS-strand consists of two electron-microscopic identical sub-strands which form a double helix. After short times of exposure to PB, negatively stained cells of Serratia marcescens show strand-like cell wall components on the cell surface consisting of longitudinal fibrils. In a further stage of denaturation, the strand-like structures form "projections" of the OM or are completely loosened. Based on a helical arrangement in the negative staining preparations as well as in the thin sections, they are identified as LPS-strands. Presumably, the LPS in the OM exists as contiguous strains. The development of the "double track"-aspect of the LPS in thin sections may be explained as a result of the projections of the helical longitudinal fibrils into the image plane.