Cell cycle parameters of slowly growing Escherichia coli B/r studied by flow cytometry

The cell cycle kinetics of Escherichia coli B/r A and B/r K cells were studied by flow cytometry. Three-dimensional histograms of cell cultures show the number of cells as a function of cellular DNA and protein contents and give detailed pictures of the cell cycle distribution with regard to these parameters. Histograms of slowly growing chemostat cultures showed that cell cycle periods B and C + D increase with a decreasing growth rate and that the B period occupies an increasing fraction of the cycle. The DNA replication patterns of B/r A and K were found to be quite similar. At extremely low growth rates (doubling time [T] = 17 h), B/r A cells had a B period of 0.8 T, a C period of 0.1 T, and a D period of 0.1 T, and B/r K cells (T = 16 h) had a B period of 0.6 T, a C period of 0.15 T, and a D period of 0.25 T. Mass increase, i.e., essentially protein synthesis, was seen in all three periods of the cell cycle. For B/r A cells, the average rate of mass increase was 11 times greater in the D period than in the B period, whereas for B/r K cells the rate of mass increase was twice as great in the D period as in the B period. The DNA and cell size distributions of batch cultures in exponential growth were found to vary with time, indicating that such cultures are not suitable for studies of cell cycle kinetics.