Continuous succinic acid fermentation by Escherichia coli KJ122 with cell recycle

• Productivity of 3 g L−1 h−1 and yield of 0.77 g g−1 achieved at D = 0.15 h−1.
• Increased pyruvate dehydrogenase activity at higher D contributes to yield increase.
• Minimal oxidative TCA/glyoxylate flux detected from analysis.
• Batch results inferior in productivity but superior in yield and titre.

High cell densities were obtained by separating the cells with an external hollow fibre filter. Extreme product inhibition at high succinate titres resulted in cell death and subsequent lysis. Accordingly, the highest succinate titre obtained during continuous fermentation was 25 g L−1 at a dilution rate of 0.05 h−1. The highest volumetric productivity of 3 g L−1 h−1 and the highest succinate yield (0.77 g g−1) were obtained at the highest dilution rate (0.15 h−1). The improved yield was caused by increased pyruvate dehydrogenase activity, leading to a decrease in pyruvate and formate excretion and an increase in the reductive flux towards succinate as additional reduction power was produced. The oxidative tricarboxylic acid cycle flux was determined to be minimal, with most of the acetyl coenzyme A (acetyl-CoA) culminating as acetate. Although comparative batch fermentations exhibited a fivefold lower volumetric productivity than the maximum obtained in the cell recycle runs, higher succinate titres (56 g L−1) and yields (0.85 g L−1) were obtained. The higher batch yields were attributed to pyruvate and formate consumption after the termination of cell growth.

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