A balanced DO-stat and its application to the control of acetic acid excretion by recombinant Escherichia coli.

During fed-batch cultivation of a recombinant Escherichia coli AT2471 harboring plasmid pSY130-14 for phenylalanine production, a large amount of acetic acid was excreted by the cells and accumulated in the culture medium. Acetic acid concentration reached 30-35 g/L at the end of a process conducted without special precautions for the reduction of this excretion. Cell growth stopped when acetic acid concentration was about 15 g/L, resulting in poor growth, 16 g/L cell concentration, and poor production - 8 g/L phenylalanine. A novel control strategy, called a balanced DO-stat. was developed to prevent acetic acid excretion. It represents a model-independent two-loop control structure, which is simple, reliable, and convenient for computer application. Using the balanced DO-stat, implemented in a computer control system, acetic acid concentration was kept at zero during the entire cultivation period. As a result, the cell concentration increased to 36 g/L and phenylalanine concentration reached 24 g/L. Aside from the phenylalanine fermentation, the proposed control approach might be applied to cultivation of other bacterial and yeast strains which have similar mechanism of the excretion of fermentative by-products.