The stability of plasmid F' in strain SP45 (a temperature conditional mutant which grows as spherical cells at 42 °C and as a rod at 30 °C) was studied. F' 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 and mutants when cell filamentation was induced at 42 °C, but not when they were cultured at 30 °C. Nalidixic acid promoted F' elimination at 0.25 μg mlin a mutant and at 1.5 μg mlin the counterpart. A marked difference was found in the rate of F' elimination from thermosensitive DNA gyrase mutants [(Ts) and (Ts)] between rods and their spherical () derivatives growing at semipermissive temperature (36.5 °C). Plasmids carrying the 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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