title: Homofermentative production of optically pure L-lactic acid from xylose by genetically engineered Escherichia coli B.
abstract: BACKGROUND
  Polylactic acid (PLA), a biodegradable polymer, has the potential to replace (at least partially) traditional petroleum-based plastics, minimizing "white pollution". However, cost-effective production of optically pure L-lactic acid is needed to achieve the full potential of PLA. Currently, starch-based glucose is used for L-lactic acid fermentation by lactic acid bacteria. Due to its competition with food resources, an alternative non-food substrate such as cellulosic biomass is needed for L-lactic acid fermentation. Nevertheless, the substrate (sugar stream) derived from cellulosic biomass contains significant amounts of xylose, which is unfermentable by most lactic acid bacteria. However, the microorganisms that do ferment xylose usually carry out heterolactic acid fermentation. As a result, an alternative strain should be developed for homofermentative production of optically pure L-lactic acid using cellulosic biomass.
  
  
  RESULTS
  In this study, an ethanologenic Escherichia coli strain, SZ470 (ΔfrdBC ΔldhA ΔackA ΔpflB ΔpdhR ::pflBp6-acEF-lpd ΔmgsA), was reengineered for homofermentative production of L-lactic acid from xylose (1.2 mole xylose = > 2 mole L-lactic acid), by deleting the alcohol dehydrogenase gene (adhE) and integrating the L-lactate dehydrogenase gene (ldhL) of Pediococcus acidilactici. The resulting strain, WL203, was metabolically evolved further through serial transfers in screw-cap tubes containing xylose, resulting in the strain WL204 with improved anaerobic cell growth. When tested in 70 g L-1 xylose fermentation (complex medium), WL204 produced 62 g L-1 L-lactic acid, with a maximum production rate of 1.631 g L-1 h-1 and a yield of 97% based on xylose metabolized. HPLC analysis using a chiral column showed that an L-lactic acid optical purity of 99.5% was achieved by WL204.
  
  
  CONCLUSIONS
  These results demonstrated that WL204 has the potential for homofermentative production of L-lactic acid using cellulosic biomass derived substrates, which contain a significant amount of xylose.
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, PR China.
authors.forename: Jinfang
authors.initials: J
authors.lastname: Zhao
authors.forename: Liyuan
authors.initials: L
authors.lastname: Xu
authors.forename: Yongze
authors.initials: Y
authors.lastname: Wang
authors.forename: Xiao
authors.initials: X
authors.lastname: Zhao
authors.forename: Jinhua
authors.initials: J
authors.lastname: Wang
authors.forename: Erin
authors.initials: E
authors.lastname: Garza
authors.forename: Ryan
authors.initials: R
authors.lastname: Manow
authors.forename: Shengde
authors.initials: S
authors.lastname: Zhou
mesh_terms: D000426:Alcohol Dehydrogenase / Q000172:deficiency / Q000235:genetics; D001426:Bacterial Proteins / Q000235:genetics; D018533:Biomass; D004926:Escherichia coli / Q000378:metabolism*; D005285:Fermentation; D005818:Genetic Engineering; D007770:L-Lactate Dehydrogenase / Q000235:genetics; D019344:Lactic Acid / Q000096:biosynthesis* / Q000138:chemical synthesis; D010376:Pediococcus / Q000201:enzymology / Q000235:genetics; D010957:Plasmids / Q000235:genetics / Q000378:metabolism; D011091:Polyesters; D011108:Polymers / Q000138:chemical synthesis; D013237:Stereoisomerism; D014994:Xylose / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D011091:Polyesters; D011108:Polymers; D019344:Lactic Acid; C033616:poly(lactide); D014994:Xylose; D000426:Alcohol Dehydrogenase; D007770:L-Lactate Dehydrogenase