Direct Conversion of Plant Biomass to Ethanol by Engineered Caldicellulosiruptor bescii

Ethanol is the most widely used renewable transportation biofuel in the US with the production of 13.3 billion gallons in 2012. First-generation biofuels produced from food crops, such as corn, are limited by cost and competition with food supply and generating fuel from abundant lignocellulosic plant biomass is an essential requirement for enabling a viable biofuels industry. Members of the genus Caldicellulosiruptor are the most thermophilic cellulolytic bacterium so far described, growing optimally at ~80 ̊C with the ability to utilize a wide range of substrates such as cellulose, hemicellulose, and lignocellulosic plant biomass without pretreatment. It efficiently ferments both C₅ and C₆ sugars derived from plant biomass and the recent development of genetic methods for manipulation of these bacteria allows metabolic engineering for the direct conversion of plant biomass to liquid fuels such as ethanol. We have engineered Caldicellulosiruptor bescii for the direct production of ethanol from plant biomass without conventional pretreatment. This was accomplished by the heterologous expression of a bi-functional acetaldehyde/alcohol dehydrogenase gene (adhE) from Clostridium thermocellum. The engineered strain produced ethanol directly from switchgrass – a real world substrate, produced no lactate, and decreased the production of acetate compared to wild type. Direct conversion of lignocellulosic biomass to ethanol represents a new paradigm for consolidated bioprocessing (CBP) and offers the potential for carbon neutral, cost effective, sustainable fuel production.