Summary: The effects of changing the composition of the growth medium, the dilution rate and the source of the bacterial host on maintenance of the plasmid pAT153 in HB101 have been studied. In a medium supplemented with Casamino acids, the plasmid was maintained longer during phosphate-limited growth at a dilution rate of 0·3 h than at 0·15 h. In contrast, phosphate-limited growth was not achieved when the Casamino acids were replaced by proline, leucine and thiamin to satisfy the auxotrophic requirements of the host. Although 100% of the bacteria were still ampicillin resistant after 72 generations of growth at a dilution rate of 0·15 h, the original plasmid had almost totally been replaced by a structurally modified plasmid which lacked a functional gene. Further experiments confirmed that neither the host nor the plasmid was retained unchanged in the minimal medium. The changes were highly reproducible and reflected periodic selection of sub-populations which were either plasmid-free or carried a structurally modified plasmid, which had reverted to Leu or Pro, or had acquired other chromosomal mutations which gave them a selective advantage. We conclude that in complex media the plasmid is maintained longer by HB101 at a high than at a low growth rate and that different results reported from different laboratories are largely due to differences in analytical techniques and the growth medium rather than to differences in the bacterial host or the plasmid used. A fermenter-adapted strain was isolated which reproducibly maintained the plasmid longer during phosphate-limited continuous growth than the original strain which had been cultured on laboratory media.


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