title: Lignocellulosic saccharification by a newly isolated bacterium, Ruminiclostridium thermocellum M3 and cellular cellulase activities for high ratio of glucose to cellobiose.
abstract: BACKGROUND
  Lignocellulosic biomass is one of earth's most abundant resources, and it has great potential for biofuel production because it is renewable and has carbon-neutral characteristics. Lignocellulose is mainly composed of carbohydrate polymers (cellulose and hemicellulose), which contain approximately 75 % fermentable sugars for biofuel fermentation. However, saccharification by cellulases is always the main bottleneck for commercialization. Compared with the enzyme systems of fungi, bacteria have evolved distinct systems to directly degrade lignocellulose. However, most reported bacterial saccharification is not efficient enough without help from additional β-glucosidases. Thus, to enhance the economic feasibility of using lignocellulosic biomass for biofuel production, it will be extremely important to develop a novel bacterial saccharification system that does not require the addition of β-glucosidases.
  
  
  RESULTS
  In this study, a new thermophilic bacterium named Ruminiclostridium thermocellum M3, which could directly saccharify lignocellulosic biomass, was isolated from horse manure. The results showed that R. thermocellum M3 can grow at 60 °C on a variety of carbon polymers, including microcrystalline cellulose, filter paper, and xylan. Upon utilization of these substrates, R. thermocellum M3 achieved an oligosaccharide yield of 481.5 ± 16.0 mg/g Avicel, and a cellular β-glucosidase activity of up to 0.38 U/mL, which is accompanied by a high proportion (approximately 97 %) of glucose during the saccharification. R. thermocellum M3 also showed potential in degrading natural lignocellulosic biomass, without additional pretreatment, to oligosaccharides, and the oligosaccharide yields using poplar sawdust, corn cobs, rice straw, and cornstalks were 52.7 ± 2.77, 77.8 ± 5.9, 89.4 ± 9.3, and 107.8 ± 5.88 mg/g, respectively.
  
  
  CONCLUSIONS
  The newly isolated strain R. thermocellum M3 degraded lignocellulose and accumulated oligosaccharides. R. thermocellum M3 saccharified lignocellulosic feedstock without the need to add β-glucosidases or control the pH, and the high proportion of glucose production distinguishes it from all other known monocultures of cellulolytic bacteria. R. thermocellum M3 is a potential candidate for lignocellulose saccharification, and it is a valuable choice for the refinement of bioproducts.
authors.affiliation: State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150090 China.
authors.forename: Tao
authors.initials: T
authors.lastname: Sheng
authors.affiliation: State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150090 China ; Advanced Water Management Centre, Faculty of Engineering, Architecture and Information Technology, The University of Queensland, Brisbane, QLD 4072 Australia.
authors.forename: Lei
authors.initials: L
authors.lastname: Zhao
authors.affiliation: CAS Key Laboratory of Environmental Biotechnology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China.
authors.forename: Ling-Fang
authors.initials: LF
authors.lastname: Gao
authors.affiliation: CAS Key Laboratory of Environmental Biotechnology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China.
authors.forename: Wen-Zong
authors.initials: WZ
authors.lastname: Liu
authors.affiliation: State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150090 China.
authors.forename: Min-Hua
authors.initials: MH
authors.lastname: Cui
authors.affiliation: State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150090 China.
authors.forename: Ze-Chong
authors.initials: ZC
authors.lastname: Guo
authors.affiliation: State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150090 China.
authors.forename: Xiao-Dan
authors.initials: XD
authors.lastname: Ma
authors.affiliation: State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150090 China.
authors.forename: Shih-Hsin
authors.initials: SH
authors.lastname: Ho
authors.affiliation: State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin, 150090 China ; CAS Key Laboratory of Environmental Biotechnology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China.
authors.forename: Ai-Jie
authors.initials: AJ
authors.lastname: Wang
keywords: Cellulase; Glucose; Lignocellulose; Oligosaccharides; Ruminiclostridium thermocellum; Saccharification