A leucine auxotroph strain of Saccharomyces cerevisiae was used to study plasmid stability and expression using a recombinant plasmid, which contained a foreign gene for firefly luciferase (luc). This recombinant yeast was tested in a series of continuous cultures in semi-defined media with varying concentrations of yeast extract in order to study its effect on stability. While the biomass concentration and luciferase activity increased with increasing concentrations of yeast extract, the plasmid stability declined. An analysis of the growth rates showed that the recombinants enjoyed a growth rate advantage over the plasmid-free cells at critically low yeast extract concentrations, possibly due to leucine starvation in the media. A two-stage cultivation strategy was designed in order to create a yeast extract limited environment so that plasmid-free cells could not grow and overtake the recombinant cells. The cells were cultivated in selective media in the first stage, and then transferred continuously to the second stage where the media was enriched by feeding yeast extract. The feed rate was kept low in order to ensure yeast extract and hence leucine starvation, thereby selecting against the plasmid-free cells. This strategy resulted in a stable existence of recombinant cells, which stabilized around 60% at steady state during the tested period of cultivation. The complex nitrogen feed helped in increasing the cell density and Volumetric activity by similar to9 and 18-fold respectively with respect to that achieved in minimal medium. The experimental data was used to formulate a mathematical model to predict cell growth and plasmid stability in two-stage cultivation, which correctly explained the experimental data. (C) 2001 Elsevier Science Inc. All rights reserved.