Effects of oxygen diffusion on recombinant E. coli B (pTG201) plasmid stability,growth rate,biomass production,and enzyme activity in immobilized and free bacteria during continuous culture

The influence of oxygen diffusion on kinetic properties, plasmid instability, biomass production and enzyme production of recombinant Escherichia coli B containing plasmid pTG201 was investigated. The experiments were carried out using continuous cultures and two types of medium, with immobilized bacteria in carrageenan gel and free bacteria. The specific growth rate of immobilized bacteria was a monotonic function of dissolved oxygen (DO) level from 0% DO to 100% DO, and it increased 63% in L-Broth medium and 70% in minimal medium with respect to 0% DO, but the growth rate was quickly reduced above 75% DO (L-Broth medium) or 50% DO (minimal medium) in free bacteria. The specific growth rate was generally higher for free bacteria than for immobilized bacteria. The plasmid was substantially more stable at 0% than at 50% or 100% DO in both immobilized and free bacteria. Biomass production was proportional to the growth rate, and this in turn was greatly influenced by the DO concentration. Measurements of enzyme production showed that the enzyme activity was diminished in both immobilized and free bacteria when oxygen was eliminated. The observed increase in plasmid stability may have resulted from the disturbance of low growth rate and decreased expression of the enzyme.