Fractionation of Escherichia coli cell populations at different stages during growth transition to stationary phase.

Cultures of Escherichia coli could be separated into more than 15 cell populations, each forming a discrete band after Percoll gradient centrifugation. The cell separation was found to result from the difference in buoyant density but not the size difference. The cell density increases upon transition from exponential growth to stationary phase. Exponential phase cultures formed at least five discrete bands with lower densities, whereas stationary phase cultures formed more than 10 bands with higher densities. Two molecular markers characterizing each cell population were identified: the functioning promoter species, as identified by measuring the expression of green fluorescent protein under the control of test promoters; and the expressed protein species, as monitored by quantitative immunoblotting. These findings together suggest that the growth phase-coupled transition of E. coli phenotype is discontinuous.