Saccharomyces cerevisiae (baker’s yeast) has been used for industrial ethanol production from hexoses (C6 sugars) for a thousand years. However, a significant amount of pentoses (C5 sugars) derived from the hemicellulose portion of the lignocellulosic biomass is present in the hydrolysate from the pretreatment process. Modern biotechnologies enable fermenting microorganisms to use both C5 and C6 sugars available from the hydrolysate. This further increases the economic competitiveness of ethanol production and other bio-products from cellulosic biomass.

Recently, microorganisms for cellulosic ethanol production, such as Saccharomyces cerevisiae, Zymomonas mobilis and Escherichia coli, have been genetically engineered using metabolic engineering approaches. Lau et al. (2010) compared the fermentation performance of Escherichia coli KO11, Saccharomyces cerevisiae 424A(LNH-ST) and Zymomonas mobilis AX101 for cellulosic ethanol production. Three microorganisms resulted in a metabolic yield, final concentration and rate greater than 0.42 g/ g consumed sugars, 40 g/L and 0.7 g/L/h (0-48 h), respectively. They concluded that Saccharomyces cerevisiae 424A(LNH-ST) is the most promising strain for industrial production because of its ability to ferment both glucose and xylose.

Vasan et al (2011) introduced an Enterobacter cloacae cellulase gene into Zymomonas mobilis strain and 0.134 filter paper activity unit (FPU)/ml units of cellulase activity was observed with the recombinant bacterium. When using carboxymethyl cellulose and 4% NaOH pretreated bagasse as substrates, the recombinant strain produced 5.5% and 4% (V/V) ethanol respectively, which was three times higher than the amount obtained with the original E. cloacae isolate.

In 2010, Purde University scientists improved a strain of yeast that can produce more biofuel from cellulosic plant material by fermenting all five types of the plant’s sugars: galactose, manose, glucose, xylose and arabinose. Arabinose makes up about 10 percent of the sugars contained in cellulosic biomass (Casey et al., 2010).