C/N Ratio and Specific Growth Rate Plays Important Role on Enhancing Isoprene Production in Recombinant Escherichia coli.

Abstract

Effect of fermentation parameters such as C/N ratio, specific growth rate, phosphate limitation, and plasmid instability on enhancing isoprene production is the focus of the current study. Isoprene productivity in the recombinant Escherichia coli K12_MVA strain showed a bell-shaped relationship with specific growth rate in bioreactor studies with isoprene volumetric productivity peaking at 0.35/h. This behavior was depicted by a production inhibition kinetic model which envisaged a serious competition between the cellular growth, acetic acid production, and isoprene biosynthesis. The model equation derived showed a reasonable fit with the experimental values. Judicious control of the growth rates and acetate accumulation by optimizing C/N ratio, phosphate concentration, and intermittent feeding strategy resulted in maximizing the carbon flux towards isoprene. Plasmid instability caused by metabolic burden posed by the presence of dual plasmids on the bacteria was simulated using first-order degradation kinetics. The experimental plasmid loss trend was in accordance with the model simulated trend, where higher plasmid loss correlated with higher specific growth rates. Modulating the growth rate, acetate accumulation, and plasmid instability resulted in achieving maximum isoprene volumetric productivity of 1.125 g/l/h with 46.67% of carbon flux towards isoprene and a isoprene titre of 18 g/l in 16 h fermentation run.