title: Enhancement of D-lactic acid production from a mixed glucose and xylose substrate by the Escherichia coli strain JH15 devoid of the glucose effect.
abstract: BACKGROUND
  A thermal tolerant stereo-complex poly-lactic acid (SC-PLA) can be made by mixing Poly-D-lactic acid (PDLA) and poly-L-lactic acid (PLLA) at a defined ratio. This environmentally friendly biodegradable polymer could replace traditional recalcitrant petroleum-based plastics. To achieve this goal, however, it is imperative to produce optically pure lactic acid isomers using a cost-effective substrate such as cellulosic biomass. The roadblock of this process is that: 1) xylose derived from cellulosic biomass is un-fermentable by most lactic acid bacteria; 2) the glucose effect results in delayed and incomplete xylose fermentation. An alternative strain devoid of the glucose effect is needed to co-utilize both glucose and xylose for improved D-lactic acid production using a cellulosic biomass substrate.
  
  
  RESULTS
  A previously engineered L-lactic acid Escherichia coli strain, WL204 (ΔfrdBC ΔldhA ΔackA ΔpflB ΔpdhR ::pflBp6-acEF-lpd ΔmgsA ΔadhE, ΔldhA::ldhL), was reengineered for production of D-lactic acid, by replacing the recombinant L-lactate dehydrogenase gene (ldhL) with a D-lactate dehydrogenase gene (ldhA). The glucose effect (catabolite repression) of the resulting strain, JH13, was eliminated by deletion of the ptsG gene which encodes for IIBC(glc) (a PTS enzyme for glucose transport). The derived strain, JH14, was metabolically evolved through serial transfers in screw-cap tubes containing glucose. The evolved strain, JH15, regained improved anaerobic cell growth using glucose. In fermentations using a mixture of glucose (50 g L(-1)) and xylose (50 g L(-1)), JH15 co-utilized both glucose and xylose, achieving an average sugar consumption rate of 1.04 g L(-1)h(-1), a D-lactic acid titer of 83 g L(-1), and a productivity of 0.86 g L(-1) h(-1). This result represents a 46 % improved sugar consumption rate, a 26 % increased D-lactic acid titer, and a 48 % enhanced productivity, compared to that achieved by JH13.
  
  
  CONCLUSIONS
  These results demonstrated that JH15 has the potential for fermentative production of D-lactic acid using cellulosic biomass derived substrates, which contain a mixture of C6 and C5 sugars.
authors.affiliation: Hubei Provincial Cooperative Innovation Center of Industrial Fermentation, Key Laboratory of Fermentation Engineering (Ministry of Education), College of Bioengineering, Hubei University of Technology, Wuhan, 430068, P. R. China. 183459815@qq.com.
authors.forename: Hongying
authors.initials: H
authors.lastname: Lu
authors.affiliation: Hubei Provincial Cooperative Innovation Center of Industrial Fermentation, Key Laboratory of Fermentation Engineering (Ministry of Education), College of Bioengineering, Hubei University of Technology, Wuhan, 430068, P. R. China. elaine82_82@hotmail.com.
authors.forename: Xiao
authors.initials: X
authors.lastname: Zhao
authors.affiliation: Hubei Provincial Cooperative Innovation Center of Industrial Fermentation, Key Laboratory of Fermentation Engineering (Ministry of Education), College of Bioengineering, Hubei University of Technology, Wuhan, 430068, P. R. China. wyzzju@126.com.
authors.forename: Yongze
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Hubei Provincial Cooperative Innovation Center of Industrial Fermentation, Key Laboratory of Fermentation Engineering (Ministry of Education), College of Bioengineering, Hubei University of Technology, Wuhan, 430068, P. R. China. 858709213@qq.com.
authors.forename: Xiaoren
authors.initials: X
authors.lastname: Ding
authors.affiliation: Hubei Provincial Cooperative Innovation Center of Industrial Fermentation, Key Laboratory of Fermentation Engineering (Ministry of Education), College of Bioengineering, Hubei University of Technology, Wuhan, 430068, P. R. China. wangjinhua@mail.hbut.edu.cn.
authors.forename: Jinhua
authors.initials: J
authors.lastname: Wang
authors.affiliation: Department of Biological Sciences, Northern Illinois University, DeKalb, IL, 60115, USA. egarza@niu.edu.
authors.forename: Erin
authors.initials: E
authors.lastname: Garza
authors.affiliation: Department of Biological Sciences, Northern Illinois University, DeKalb, IL, 60115, USA. usa300@live.com.
authors.forename: Ryan
authors.initials: R
authors.lastname: Manow
authors.affiliation: Department of Biological Sciences, Northern Illinois University, DeKalb, IL, 60115, USA. aiverson@harpercollege.edu.|William Rainey Harper College, Palatine, IL, 60142, USA. aiverson@harpercollege.edu.
authors.forename: Andrew
authors.initials: A
authors.lastname: Iverson
authors.affiliation: Hubei Provincial Cooperative Innovation Center of Industrial Fermentation, Key Laboratory of Fermentation Engineering (Ministry of Education), College of Bioengineering, Hubei University of Technology, Wuhan, 430068, P. R. China. szhou@niu.edu.|Department of Biological Sciences, Northern Illinois University, DeKalb, IL, 60115, USA. szhou@niu.edu.
authors.forename: Shengde
authors.initials: S
authors.lastname: Zhou
mesh_terms: D003470:Culture Media; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D005285:Fermentation; D005947:Glucose / Q000378:metabolism*; D019344:Lactic Acid / Q000032:analysis / Q000378:metabolism*; D060847:Metabolic Engineering; D014994:Xylose / Q000378:metabolism*
chemical_list: D003470:Culture Media; D019344:Lactic Acid; D014994:Xylose; D005947:Glucose