Effect of cell growth rate on the performance of a two-stage continuous culture system in a recombinant Escherichia coli fermentation.

As part of the process optimization of a two-stage continuous culture system, the effect of growth rate micro(2) (app) on the performance of the second stage (production stage) was studied in a recombinant Escherichia coli K12 (DeltaH1Deltatrp/pPLc23trpA1). Important parameters considered were specific gene expression rate, plasmid content, and plasmid stability, all of which were closely related to the cell growth rate and the production rate of the cloned gene product (trpalpha). When operating conditions were maintained constant (T(1) = 35 degrees C, D(1) = 0.9 h(-1), T(2) = 40 degrees C, and D(2) = 0.7 h(-1)) and micro(2) (app) was varied, plasmid content in the second stage showed its maximum at micro(2) (app) = 0.4 h(-1) and decreased thereafter. Specific gene expression rate linearly increased with increasing micro(2) (app), while plasmid stability decreased. Optimum cell growth rate giving the maximum value in overall productivity was observed at around micro(2) (app) = 0.4 h(-1). The contribution or role of the three parameters, specific gene expression rate, plasmid content, and plasmid stability in exhibiting the maximum productivity at the optimal micro(2) (app) is discussed.