Enhanced production of glutamate decarboxylase by batch, fed-batch, and repeated batch cultivations of Escherichia coli

In order to obtain high yields of glutamate decarboxylase (GAD) to satisfy the growing market for gamma-aminobutyric acid (GABA), production of GAD by Escherichia coli was examined in batch, fed-batch, and repeated batch cultures. In fed-batch mode, the yield of dry cell mass was enhanced, which is beneficial for GAD production, and corresponded to 155.57% improvement of GAD production as compared to batch cultivation. However, extending the culture time to more than 52.5 h resulted in loss of cell mass and decreased GAD activity. Repeated batch culture demonstrated increases in average cell mass and GAD productivity (around 0.27 g L^-1 h^-1 and 1.8 U mL^-1 h ^-1) in five cycles. An overall faster fermentation with a steady increase in yield (50.6% to 80.6%) occurred in the first three cycles by repeatedly replacing a portion of culture with fresh medium. Nevertheless, long-term operation (after the fifth batch) is not beneficial for cell growth or GAD production. Growthassociated characteristics of GAD production (GAD yield increased or decreased as the amount of cell mass altered) were observed in different cultivation modes of E. coli. The importance of this work was to first investigate various cultivation modes for GAD yield enhancement, which will serve as important research references for industrial purposes and fill in the research gap in comprehensively elucidating the performance of different feeding strategies with respect to GAD yield, productivity, by-product accumulation, process stability, and incubation cycle.