Bioethanol production from cellulosic substrates: Engineered bacteria and process integration challenges

Cellulosic biomass from agricultural and forestry residues, waste paper and industrial wastes could be used as an ideal and inexpensive source of sugar for sustainable fermentation into transportation fuel. As such, ethanol-producing microorganisms, mainly Zymomonas mobilis and Saccharomyces cerevisiae are potential candidates for ethanol production. However, the substrates are not cost effective, as the organisms are not able to hydrolyze complex sugars such as lignocellulose. Since last two decades, several microorganisms are manipulated for production of ethanol. Gram-negative bacteria such as Escherichia coli, Klebsiella oxytoca, Z. mobilis, Gram-positive bacteria such as Clostridium cellulolyticum, Lactobacillus casei and several yeast strains have been engineered for bioethanol production from cellulosic substrates. These engineered organisms are able to produce ethanol from a wide spectrum of sugars. This review is focused on the strategies and development of processes for ethanol production by such organisms from lignocellulosic substrates.