Plasmid inheritability and biomass production: comparison between free and immobilized cell cultures of Escherichia coli BZ18(pTG201) without selection pressure.

Maintenance of the plasmid pTG201 in Escherichia coli BZ18 was studied for both free and immobilized cells during chemostat culture, in the absence of the antibiotic against which resistance was plasmid encoded. Electron microscopic observations of immobilized proliferant cells within carrageenan gel beads showed high cell concentrations and growth into distinct cavities. The plasmid which coded for the catechol 2,3-dioxygenase activity was stably maintained during 80 generations in the case of immobilized cells. A theoretical analysis founded on the compartmentalization resulting from the immobilized growth conditions was described. However, the model still showed a plasmid stability inferior to that determined experimentally. Hypotheses dealing with physiological changes of immobilized cells were presented. In addition, the high cell concentrations obtained in the outer 50 microns of the carrageenan gel beads gave a biomass productivity within this useful volume which was 20 times higher than in free-cell cultures.