Metabolic engineering of a Lactobacillus plantarum double ldh knockout strain for enhanced ethanol production.

Lactobacillus plantarum ferments glucose through the Embden-Meyerhof-Parnas pathway: the central metabolite pyruvate is converted into lactate via lactate dehydrogenase (LDH). By substituting LDH with pyruvate decarboxylase (PDC) activity, pyruvate may be redirected toward ethanol production instead of lactic acid fermentation. A PDC gene from the Gram-positive bacterium Sarcina ventriculi (Spdc) was introduced into an LDH-deficient strain, L. plantarum TF103, in which both the ldhL and ldhD genes were inactivated. Four different fusion genes between Spdc and either the S. ventriculi promoter or three Lactococcus lactis promoters in pTRKH2 were introduced into TF103. PDC activity was detected in all four recombinant strains. The engineered strains were examined for production of ethanol and other metabolites in flask fermentations. The recombinant strains grew slightly faster than the parent TF103 and produced 90-130 mM ethanol. Although slightly more ethanol was observed, carbon flow was not significantly improved toward ethanol, suggesting that a further understanding of this organism's metabolism is necessary.