Persistence of the pBR 322 plasmid in Escherichia coli K 12 grown in chemostat cultures.

Populations of a Escherichia coli K 12 strain, containing the vector plasmid pBR 322, were grown in chemostat culture under glucose- and phosphate-limited conditions. Resistance to tetracycline and ampicillin were lost after prolonged cultivation, resulting in the production of apparent plasmid-free populations which were more competitive than the original population. This competitiveness between plasmid-free and plasmid-containing populations was greatest in environments where the nutrient restriction was severe. Also during sequential subcultivation in batch cultures loss of plasmid was observed.