Researchers from the Universidad Nacional Autonoma de Mexico have genetically engineered the bacterium Bacillus subtilis to ferment glucose sugar directly to ethanol with a high (86%) yield. This is the first step in their quest to develop bacteria that can breakdown and ferment cellulose biomass directly to ethanol.
In a presentation at last week’s 106th General meeting of the American Society for Microbiology, the team members described their work in engineering the bacterium to produce ethanol as a single fermentation product from the sugars resulting from the breakdown of plant cellulose into simple carbohydrates.
With its very efficient secretion systems, B. subtilis is widely used in the production of extracellular hydrolytic enzymes, but it does not produce ethanol. The researchers engineered in the ethanol production capability by using the pyruvate decarboxylase (PDC) and alcohol dehydrogenase B (ADHII) from Zymomonas mobilis, a bacterium that does produce ethanol.
Initially, the team found that the integration of the ethanol synthetic pathways allowed ethanol production with a 50% yield, but with a large amount of butanediol still produced. Further tweaking avoided the butanediol formation, and bumped the ethanol yield up to 86%.
They found, however, that while the yield was high, the rate of production from the recombinant strains was low. Further work is necessary to increase the rate.
Beyond that, the next step is to engineer the bacterium to produce the cellulase enzymes that can also initially break down the stems and leaves into the simple carbohydrates required for fermentation.