Filamentation promotes F'lac loss in Escherichia coli K12

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Abstract
The stability of plasmid F'lac in Escherichia coli strain SP45 (a temperature conditional mutant which grows as spherical cells at 42 °C and as a rod at 30 °C) was studied. F'lac elimination was demonstrated when bacteria exposed to subinhibitory concentrations of various chemicals were induced to form filaments. No plasmid loss was found when spherical cells were subjected to the same treatments. Plasmid loss was also observed in dnaA46 and lexA41 mutants when cell filamentation was induced at 42 °C, but not when they were cultured at 30 °C. Nalidixic acid promoted F'lac elimination at 0.25 μg ml-1in a recA13 mutant and at 1.5 μg ml-1in the recA+counterpart. A marked difference was found in the rate of F'lac elimination from thermosensitive DNA gyrase mutants [gyrA43(Ts) and gyrB41(Ts)] between rods and their spherical (rodA51) derivatives growing at semipermissive temperature (36.5 °C). Plasmids carrying the ccd segment of F in DNA gyrase mutants were lost after 2.5 generations from rods and after 6 generation from spherical cells. Plasmid segregation into non-viable minicell-like elements was found after induction of filaments. These data suggest that plasmid stability is correlated with cell shape and that curing is more easily achieved when bacteria can elongate normally.

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