Independence of buoyant cell density and growth rate in Escherichia coli

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Abstract
The relationship between growth rate and buoyant density was determined for cells from exponential-phase cultures of E. coli B/r NC32 by equilibrium centrifugation in Percoll gradients at growth rates ranging 0.15-2.3 doublings/h. The mean buoyant density did not change significantly with growth rate in any of 3 sets of experiments in which different gradient conditions were used. When cultures were allowed to enter the stationary phase of growth, mean cell volumes and buoyant densities usually remained unchanged for extended periods. These and earlier results support the existence of a highly regulated, discrete state of buoyant density during steady-state growth of E. coli and other cells that divide by equatorial fission.