SUMMARY: The maintenance of the plasmid vectors pTG201 and pTG206 (which both carry the E gene) and pBH3 in hosts was studied in free and immobilized continuous cultures. pTG201, containing the strong P promoter, was more quickly lost than plasmid pTG206, containing the tetracycline resistance gene promoter. The instability of pTG201 seems to be related to high expression of the cloned E gene. Fluctuations in the proportion of pTG201-containing cells were observed in the free system, suggesting the appearance of adaptive descendants (with and without plasmid) from the initial strains. The loss of plasmid vectors from cells and the fluctuations in the proportion of plasmid-containing cells could be prevented by immobilizing plasmid-containing bacteria in carrageenan gel beads.


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