Abstract

The stability of recombinant cells in continuous culture fermentations of the system B. subtilis DN1885(pCH7) and the effects on recombinant enzyme synthesis were studied experimentally and theoretically. A mathematical model that studies the effect of plasmid content distribution in the population of plasmid-containing cells and plasmid replication kinetics on the culture stability and hence on recombinant enzyme synthesis was developed. Results obtained in fermentations at different dilution rates (0.03-0.7 hr-1) were used to find the parameters and validate the model. It was observed that the stability of the culture was higher at higher dilution rates, but the recombinant enzyme synthesis was lower at higher dilution rates. From the model simulations it was found that the average plasmid content (plasmids per cell) is only one of the variables needed to predict the stability of recombinant cultures; as important are the standard deviation from the average and the plasmid replication kinetics. The plasmid replication kinetics and the plasmid distribution content both have a very important effect on the stability of the recombinant culture and on enzyme synthesis and titers in the bioreactor.