Membrane assembly in Escherichia coli I. Segregation of preformed and newly formed membrane into daughter cells

Density labeling of the lipid phase of membranes has been used to test the possibilities that the cytoplasmic membrane of Escherichia coli is assembled at one or two fixed foci, either at the equitorial perimeter or at the poles of the organism. Synchronized cultures of E. coli were switched from light to heavy medium, and samples were removed after either one or two cell division cycles of growth in heavy medium and processed by equilibrium density gradient centrifugation. No separation of definitively light or heavy membrane bands was observed indicating that preformed and newly formed membranes are not assembled at one or two fixed foci and then segregated with conservation of structure.