Effect of feeding strategy on l-tryptophan production by recombinant Escherichia coli

Abstract

Acetic acid is a primary inhibitory metabolite in Escherichia coli cultivation, and is detrimental to bacterial growth and formation of desired products. The effects of acetic acid on l-tryptophan production by recombinant E. coli were evaluated in this study. Results indicated that growth of an l-tryptophan-producing bacterium and l-tryptophan formation were inhibited significantly by acetic acid concentrations above 2 g/L. Different feeding strategies, including exponential feeding, pseudo-exponential feeding, and feeding based on DO-stat and glucose-stat were conducted and evaluated for l-tryptophan fermentation in 30-L jar fermenters. It was found that the maximum specific growth rate should be controlled below 0.25 h−1 in a variable specific growth rate fed-batch process, and glucose concentration should be controlled at a low level in l-tryptophan production, which suggested a combined feeding strategy of pseudo-exponential feeding and glucose-stat feeding for optimal l-tryptophan production. Applying this combined feeding strategy resulted in high cell density (54.5 g/L) and l-tryptophan production (38.8 g/L). In addition, a high glucose conversion rate (19.9%) with suitable ranges for specific growth rate and glucose concentration were obtained due to the low acetic acid concentration (0.9 g/L).