Cell cycle analysis of plasmid-containingEscherichia coli HB101 populations with flow cytometry

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Abstract
The relationship between cell mass and cell number dynamics for bacteria such as Escherichia coli depends on the cell cycle parameters C and D. Effects of plasmid copy number on these cell cycle parameters have been studied for Escherichia coli HB101 containing pMB1 plasmids propagated at different copy numbers ranging from 12 to 122. Determination of cell cycle and cell size parameters was accomplished using flow cytometry data on single‐cell light scattering and DNA content frequency functions in conjunction with a mathematical model of cell population statistics. Two independent methods for estimating C and D intervals based on flow cytometry were developed and applied with essentially identical results. The presence of plasmids decreases the C and D periods, mean cell sizes, and initiation masses for chromosome replication by 14, 24, 38, and 18%, respectively, relative to corresponding values for plasmid‐free host cells. Plasmid copy number has a negligible influence on these parameters, suggesting that host‐plasmid inter actions which determine these properties are centered on the single plasmid selected for replication according to the random selection model established for ColE1‐type plasmids.