Effect of operating parameters on TrpA productivity and performance of a recombinant fermentation process in a two-stage continuous fermentation

Abstract

A two‐stage continuous system in combination with a temperature‐sensitive expression system were used as model systems to maximize the productivity of a cloned gene and minimize the problem associated with the plasmid instability for a high‐expression recombinant. In order to optimize the two‐stage fermentation process, the effects of such operational variables as temperature and dilution rate on productivity of cloned gene were studied using the model systems and a recombinant, Escherichia coli K12 ΔH1 Δtrp/pPLc23trp A1. When the expression of cloned gene is induced by raising the operating temperature above 38°C, a significant decrease in the colony‐forming‐units (CFU) of the plasmid‐harboring cell was observed, and the decrease was related to the product concentration. In order to describe this phenomenon, a new kinetic parameter related to the metabolic stress (metabolic stress factor) was introduced. It is defined as the ratio of the rate of change of pheno‐type from colony‐forming to non‐colony‐forming cells to the product accumulation per unit cell mass. At a fixed temperature of 40°C, the varying dilution rate D in the range of 0.35‐0.90 h−1 did not affect the metabolic stress factor significantly. At a fixed dilution rate of D = 0.35 h−1, this factor remained practically constant up to 41°C but increased rapidly beyond 41°C. The effects of temperature and dilution rate in the second stage on the specific production rate were also studied while maintaining the apparent specific growth rate (μ 2app) of the second stage constant at or near μ 2app = 0.26 h−1. Under a constant dilution rate, D2 = 0.35 h−1, the maximum specific production rate obtained was about qp, max = 38 units TrpA/mg cell/h at 41°C. At a constant temperature, T2 = 40°C, specific production rate increased with decreasing dilution rate with in the dilution rate range of D2 = 0.35‐0.90 h−1. Based on the results of our study, the optimal operating conditions found were dilution rate D2 = 0.35 h−1 and operating temperature T2 = 41°C at the apparent specific growth rate of 0.26 h−1. Under the optimal operating conditions, about threefold increase in productivity was achieved compared to the best batch culture result. In addition, the fermentation period could be extended for more than 100 h.