Enhanced succinic acid production under acidic conditions by introduction of glutamate decarboxylase system in E. coli AFP111

Biological synthesis of succinic acid at low pH values was favored since it not only decreased investment cost but also simplified downstream purification process. In this study, the feasibility of using glutamate decarboxylase system to improve succinic acid production of Escherichia coli AFP111, a succinate-producing candidate with mutations in pfl, ldhA, and ptsG, under acidic conditions was investigated. By overexpressing gadBC operon in AFP111, a recombinant named as BA201 (AFP111/pMD19T-gadBC) was constructed. Fermentation at pH 5.6 showed that 30 g L⁻¹ glucose was consumed and 26.58 g L⁻¹ succinic acid was produced by BA201, which was 1.22- and 1.32-fold higher than that by the control BA200 (AFP111/pMD19T) containing the empty vector. Analysis of intracellular enzymes activities and ATP concentrations revealed that the activities of key enzymes involved in glucose uptake and products synthesis and intracellular ATP levels were all increased after overexpression of gadBC which were benefit for cell metabolism under weak acidic conditions. To further improve the succinic acid titer by recombinant BA201 at pH 5.6, the extracellular glutamate concentration was optimized and the final succinic acid titer increased 20.4% to 32.01 g L⁻¹. Besides, the fermentation time was prolonged by repetitive fermentation and additional 15.78 g L⁻¹ succinic acid was produced by recovering cells into fresh medium. The results here demonstrated a potential strategy of overexpressing gadBC for increased succinic acid production of E. coli AFP111 under weak acidic conditions.