Bacterial metabolic engineering for the production of second-generation (2 G) bioethanol and biobutanol; a review

The second generation (2 G) biofuels were introduced to solve the issues associated with first-generation biofuel (dependency on food materials) and fossil fuels, such as reservoirs diminution, high demand, price fluctuation, and lethal greenhouse gases emission. Butanol and ethanol are the main 2 G biofuels. They are used as a disinfectant, antiseptic, and chemical solvent in the pharmaceutical, plastic, textiles, cosmetics, and fuel industries. Currently, their bacterial biological production from lignocellulosic material at the industrial level with primitive microorganisms is under development and not economical and qualitative compatible as compared to that of fossil origin, due to the slow growth rate, low titer, recalcitrant nature of lignocellulose, strain intolerance to a higher amount of butanol and ethanol, and strain inability to tolerate inhibitors accumulated during pretreatment of lignocellulosic materials. Therefore, metabolic engineering strategies such as redirection of carbon flux, knocking out competing pathways, enhancing strain robustness and wide range of substrate utilization ability, and overexpression of enzymes involved in their biological synthesis have been applied to bacteria for enhancing their ability for 2 G ethanol and butanol production in a highly cost-effective amount from lignocellulosic materials. Herein, we summarized and reviewed the progress in metabolic engineering of bacterial species such as Clostridium spp, Escherichia coli, and Zymomonas mobilis for the synthesis of 2 G butanol and ethanol, especially from lignocellulosic materials.