Influence of DNA supercoiling on the loss of culturability of Escherichia coli cells incubated in seawater

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Abstract
The relationship between the loss of culturability of Escherichia coli cells in seawater and the DNA supercoiling level of a reporter plasmid (pUC8) have been studied under different experimental conditions. Transfer to seawater of cells grown at low osmolarity decreased their ability to grow without apparent modification of the plasmid supercoiling. We found that E. coli cells could be protected against seawater-induced loss of culturability by increasing their DNA-negative supercoiling in response to environmental factors: either a growth at high osmolarity before the transfer to seawater, or addition of organic matter (50-mg/l peptone) in seawater. We further found conditions where a DNAinduced relaxation was accompanied by an increase in seawater sensitivity. Indeed, inactivation of either one of the subunits A and B of DNA gyrase, which leads to important DNA relaxation, was accompanied in both cases by an increased loss of culturability of conditional mutants after transfer to seawater which could not be explained uniquely by the increase in the temperature required to inactivate the gyrase. Similarly, a strain harbouring a mutation in topoisomerase I, compensated by another mutation in subunit B of the gyrase, was more sensitive to seawater than the isogenic wild-type cell and this greater sensitivity was correlated to a relaxation of plasmid DNA. Again, in these different cases, a previous growth at high osmolarity protected against this seawater sensitivity. We thus propose that the ability of E. coli cells to survive in seawater and maintain their ability to grow on culture media could be linked, at least in part, to the topological state of their DNA. This latter state might govern expression of genes whose products could allow restoration of cellular homeostasis in this medium.