High-efficiency conversion of glycerol to D-lactic acid with metabolically engineered Escherichia coli

Escherichia coli strain CICIM B0013 was genetically engineered to efficiently produce optically pure D-lactate (higher than 99.9%) from glycerol with a minimum of by-products. When E. coli B0013-070 (DackA, Dpta, Dpps, DpflB, Ddld, DpoxB, DadhE,DfrdA) was cultivated aerobically for 9 h followed by 27 h under microaerobic fermentation, it produced 98.5 g l-1 of D-lactate with no more than 2 g l-1 total by-products from glycerol. During the microaerobic phase, the average D-lactate productivity and yield were 3.45 g l-1 h-1 and 64 g/100 g glycerol, respectively. Elevated expression of the lactate dehydrogenase gene (ldhA) in strain B0013-070 improved conversion of glycerol to D-lactate resulting in a yield and productivity of 78 g/100 g glycerol and 3.65 g l-1 h-1, respectively. The metabolically engineered E. colistrain B0013-070 (pTH-ldhA) efficiently converted glycerol to D-lactate with a 2.1-foldhigher D-lactate productivity than previously reported. It is concluded that overexpression of ldhA at an appropriate level is important for the balance between cell growth and D-lactate synthesis. Furthermore, biodiesel-based glycerol can be an appropriate substrate for industrial scale D-lactate production.   Key words: D-Lactate, glycerol, Escherichia coli, metabolic engineering, overexpression of ldhA.