Engineered Bacterium Directly Converts Biomass To Ethanol

Taking advantage of newly discovered bacterial strains, a research team led by Janet Westpheling of the University of Georgia and Oak Ridge National Laboratory has engineered the first microbe that converts raw biomass directly to ethanol without any pretreatment (Proc. Natl. Acad. Sci. USA 2014, DOI: 10.1073/pnas.1402210111). One obstacle holding up broader adoption of cellulosic biofuels has been that chemical and enzymatic biomass pretreatment adds up to 40% to the processing cost. Metabolic engineering efforts have therefore been directed toward developing single microbial strains that both break down biomass into soluble sugars and ferment the sugars into ethanol, an approach called consolidated bioprocessing. But some pretreatment is still needed and the enzyme activity remains too low for industrial-scale processing. Westpheling and coworkers genetically reengineered the biomass-digesting bacterium Caldicellulosiruptor bescii by deleting a lactate dehydrogenase gene and introducing an acetaldehyde/alcohol dehydrogenase gene from the ethanol-producing bacterium Clostridium thermocellum. Ethanol makes up more than 70% of fermentation products of the new strain, which will need additional improvement to work in a viable industrial process. The researchers believe that is doable by further tweaking the new strain and that their engineering approach could be used to produce other biobased fuel ingredients and chemicals.