title: Simultaneous achievement of high ethanol yield and titer in Clostridium thermocellum.
abstract: BACKGROUND
  Biofuel production from plant cell walls offers the potential for sustainable and economically attractive alternatives to petroleum-based products. Fuels from cellulosic biomass are particularly promising, but would benefit from lower processing costs. Clostridium thermocellum can rapidly solubilize and ferment cellulosic biomass, making it a promising candidate microorganism for consolidated bioprocessing for biofuel production, but increases in product yield and titer are still needed.
  
  
  RESULTS
  Here, we started with an engineered C. thermocellum strain where the central metabolic pathways to products other than ethanol had been deleted. After two stages of adaptive evolution, an evolved strain was selected with improved yield and titer. On chemically defined medium with crystalline cellulose as substrate, the evolved strain produced 22.4 ± 1.4 g/L ethanol from 60 g/L cellulose. The resulting yield was about 0.39 gETOH/gGluc eq, which is 75 % of the maximum theoretical yield. Genome resequencing, proteomics, and biochemical analysis were used to examine differences between the original and evolved strains.
  
  
  CONCLUSIONS
  A two step selection method successfully improved the ethanol yield and the titer. This evolved strain has the highest ethanol yield and titer reported to date for C. thermocellum, and is an important step in the development of this microbe for industrial applications.
authors.affiliation: Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Liang
authors.initials: L
authors.lastname: Tian
authors.affiliation: Bredesen Center, University of Tennessee, Knoxville, TN 37996 USA.|Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Beth
authors.initials: B
authors.lastname: Papanek
authors.affiliation: Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Daniel G
authors.initials: DG
authors.lastname: Olson
authors.affiliation: Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Thomas
authors.initials: T
authors.lastname: Rydzak
authors.affiliation: Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Evert K
authors.initials: EK
authors.lastname: Holwerda
authors.affiliation: Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Tianyong
authors.initials: T
authors.lastname: Zheng
authors.affiliation: Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Jilai
authors.initials: J
authors.lastname: Zhou
authors.affiliation: Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Marybeth
authors.initials: M
authors.lastname: Maloney
authors.affiliation: Bredesen Center, University of Tennessee, Knoxville, TN 37996 USA.|Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Nannan
authors.initials: N
authors.lastname: Jiang
authors.affiliation: Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.|Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Giannone
authors.affiliation: Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.|Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Robert L
authors.initials: RL
authors.lastname: Hettich
authors.affiliation: Bredesen Center, University of Tennessee, Knoxville, TN 37996 USA.|Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Adam M
authors.initials: AM
authors.lastname: Guss
authors.affiliation: Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755 USA.|Bioenergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.
authors.forename: Lee R
authors.initials: LR
authors.lastname: Lynd
keywords: Adaptive evolution; Cellulosic ethanol; Clostridium thermocellum; Consolidated bioprocessing