Electron microscopic observation of crystals of Escherichia coli K-12 lipopolysaccharide

Previously we showed that Salmonella minnesota Re and Ra lipopolysaccharides (LPSs) and Escherichia coli K-12 LPS formed three-dimensional crystals, either hexagonal plates or solid columns, when they were precipitated by the addition of 2 volumes of 95% ethanol containing 375 mM MgCl2 and incubated in 70% ethanol containing 250 mM MgCl2, and stored at 4 C for 10 days. Later, Escherichia coli K-12 LPS thus treated was found to form discoid crystals as well as hexagonal plate crystals and solid column crystals. Analysis by electron diffraction of the discoid crystals from the direction perpendicular to the basal plane showed that they consisted of hexagonal lattices with the a axis of 4.62 A. This result was quite the same as that of the hexagonal plate crystals. Electron micrographs of the edges of the discoid crystals revealed stacked sheets of the hexagonal plate crystals. From these results it was concluded that formation of the discoid crystals results from irregular overlapping of the hexagonal plate crystals.