Metabolic Engineering of Lactobacillus plantarum for Direct l‐Lactic Acid Production From Raw Corn Starch

Fermentative production of optically pure lactic acid (LA) has attracted great interest because of the increased demand for plant‐based plastics. For cost‐effective LA production, an engineered Lactobacillus plantarum NCIMB 8826 strain, which enables the production of optically pure l‐LA from raw starch, is constructed. The wild‐type strain produces a racemic mixture of d‐ and l‐LA from pyruvate by the action of the respective lactate dehydrogenases (LDHs). Therefore, the gene encoding D‐LDH (ldhD) is deleted. Although no decrease in d‐LA formation is observed in the ΔldhD mutant, additional disruption of the operon encoding lactate racemase (larA‐E), which catalyzes the interconversion between d‐ and l‐LA, completely abolished d‐LA production. From 100 g L−1 glucose, the ΔldhD ΔlarA‐E mutant produces 87.0 g L−1 of l‐LA with an optical purity of 99.4%. Subsequently, a plasmid is introduced into the ΔldhD ΔlarA‐E mutant for the secretion of α‐amylase from Streptococcus bovis 148. The resulting strain could produce 50.3 g L−1 of l‐LA from raw corn starch with a yield of 0.91 (g per g of consumed sugar) and an optical purity of 98.6%. The engineered L. plantarum strain would be useful in the production of l‐LA from starchy materials. Fermentative production of optically pure lactic acid (LA) has attracted great interest because of the increased demand for plant‐based plastics. In this study, production of optically pure l‐LA is achieved by double‐knockout of the gene encoding D‐lactate dehydrogenase (ldhD) and the operon encoding lactate racemase (larA‐E) in Lactobacillus plantarum NCIMB 8826 strain. In addition, direct l‐LA production is achieved by introducing the plasmid for secretion of α‐amylase from Streptococcus bovis 148 to the ΔldhD ΔlarA‐E strain. The engineered L. pantarum strain would be applicable for l‐LA production from starchy materials.