Engineering the cellulolytic extreme thermophileCaldicellulosiruptorbesciito reduce carboxylic acids to alcohols using plant biomass as the energy source

Caldicellulosiruptorbesciiis the most thermophilic cellulolytic organism yet identified (T(opt)78 degrees C). It grows on untreated plant biomass and has an established genetic system thereby making it a promising microbial platform for lignocellulose conversion to bio-products. Here, we investigated the ability of engineeredC.besciito generate alcohols from carboxylic acids. Expression of aldehyde ferredoxin oxidoreductase (aorfromPyrococcusfuriosus) and alcohol dehydrogenase (adhAfromThermoanaerobactersp. X514) enabledC.besciito generate ethanol from crystalline cellulose and from biomass by reducing the acetate produced by fermentation. Deletion of lactate dehydrogenase in a strain expressing the AOR-Adh pathway increased ethanol production. Engineered strains also converted exogenously supplied organic acids (isobutyrate andn-caproate) to the corresponding alcohol (isobutanol and hexanol) using both crystalline cellulose and switchgrass as sources of reductant for alcohol production. This is the first instance of an acid to alcohol conversion pathway in a cellulolytic microbe.