Engineering Lactococcus Lactis for D-Lactic Acid Production from Starch

Bioprocess development is a current requirement to enhance the global production of D-lactic acid. Herein, we report a new bioprocess for D-lactic acid production directly from starch using engineered Lactococcus lactis NZ9000. To modify L. lactis as a D-lactic acid producer, its major endogenous L-lactate dehydrogenase (L-Ldh) gene was replaced with a heterologous D-Ldh gene from Lactobacillus delbrueckii subsp. lactis JCM 1107. The resulting strain AH1 showed a somewhat slower growth rate but similar lactic acid production compared to those of the intact strain when cultivated with glucose as a carbon source. The chemical purity of D-lactic acid produced by L. lactis AH1 was 93.8%, and the enzymatic activities of D- and L-Ldh in AH1 were 1.54 U/mL and 0.05 U/mL, respectively. Next, a heterologous α-amylase gene from Streptococcus bovis NRIC 1535 cloned into an expression vector pNZ8048 was introduced into AH1. The resulting strain AH2 showed an amylolytic activity of 0.26 U/mL in the culture supernatant. Direct production of D-lactic acid from starch as the carbon source was demonstrated using L. lactis AH2, resulting in D-lactic acid production at a concentration of 15.0 g/L after 24 h cultivation. To our knowledge, this is the first report on D-lactic acid production in engineered L. lactis.