Lactic acid, a widely used chemical, exists as a mixture of D and L isomers when synthesized through a chemical route . The requirement of an optically pure L isomer for applications in pharmaceutical and poly-lactic acid (PLA) bioplastic industries favors fermentative production of L-lactic acid using chiral-specific L-lactate dehydrogenase.
Glucose derived from starch biomass such as corn, is currently used for fermentative production of L-lactic acid by lactic acid bacteria like Lactobacillus. However, due to its competition with food resources, an alternative non-food substrate is needed for cost-effective production of L-lactic acid, in order to enable the environmentally friendly PLA to compete economically with petroleum based plastics .
Cellulosic biomass, the most abundant non-food resource, is a potential substrate for L-lactic acid fermentation. However, in addition to glucose, the substrate (sugar stream) derived from cellulosic biomass contains significant amounts of xylose, which is unfermentable by most lactic acid bacteria . The microorganisms that do ferment xylose to L-lactic acid, such as Lactococcus lactis IO-1  and Enterococcus mundtii, need improvements in yield, productivity, optical purity, and/or the requirement of complex nutrients.
Escherichia coli, a candidate with minimal nutrient requirements, is able to use all biomass derived hexose and pentose sugars. Derivative E. coli strains have been engineered for production of lactic acid [5–12]. However, few of these E. coli strains have demonstrated the ability to ferment xylose into L-lactic acid at high yields and/or optical purity. Furthermore, E. coli naturally produces D-lactic acid and lacks an endogenous L-lactate dehydrogenase gene. A plasmid bearing an exogenous L-lactate dehydrogenase gene from Streptococcus bovis[6, 13], Lactobacillus casei, or Clostridium thermocellum has been cloned into E. coli (pfl ldhA) to produce L-lactic acid. These plasmid bearing recombinants, however, may lack strain stability due to plasmid curing.
In this study, we report reengineering an ethanologenic E. coli strain, SZ470 (ΔfrdBC ΔldhA ΔackA ΔfocA-pflB ΔpdhR::pflBp6-pflBrbs-aceEF-lpd) , for homofermentative production of L-lactic acid from xylose. The resulting strain, WL204, contains a chromosomal integrated ldhL gene, without any antibiotic marker or plasmids.