Escherichia coli DNA distributions measured by flow cytometry and compared with theoretical computer simulations

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Abstract
A computer simulation routine was made to calculate the DNA distributions of exponentially growing cultures of E. coli. Calculations were based on a previously published model. Simulated distributions were compared with experimental DNA distributions (histograms) recorded by flow cytometry. Cell cycle parameters were determined by varying the parameters to find the best fit of theoretical to experimental histograms. A culture of E. coli B/r A with a doubling time of 27 min was found to have a DNA replication period (C) of 43 min and an average postreplication period .hivin.D of 22-23 min. Similar cell cycle parameters were found for a 60-min B/r A culture. Initiations of DNA replication at multiple origins in one and the same cell were shown to be essentially synchronous. A slowly growing B/r A culture (doubling time, 5.5 h) had an average prereplication period .hivin.B of 2.3 h; C = 2.4 h and .hivin.D = 0.8 h. Thus, the C period has a constant duration of 43 min (at 37.degree. C) at fast growth rates (doubling times, < 1 h) but increases at slow growth rates. These results obtained with unperturbed exponential cultures in steady state support the model of Cooper and Helmstetter which was based on data obtained with synchronized cells.