Cloning of d-lactate dehydrogenase genes of Lactobacillus delbrueckii subsp. bulgaricus and their roles in d-lactic acid production

Lactobacillus delbrueckii subsp. bulgaricus is a heterogenous lactic acid bacterium that converts pyruvate mainly to d-lactic acid using d-lactate dehydrogenases (d-LDHs), whose functional properties remain poorly characterized. Here, the d-LDHs genes (ldb0101, ldb0813, ldb1010, ldb1147 and ldb2021) were cloned and overexpressed in Escherichia coli JM109 from an inducible pUC18 vector, respectively, and the resulting strains were compared in terms of d-lactic acid production. The strain expressing ldb0101 and ldb1010 gene individually produced more d-lactate than other three strains. Further study revealed that Ldb0101 activity was down-regulated by the oxygen and, therefore, achieved a highest titer of d-lactate (1.94 g/L) under anaerobic condition, and introduction of ldb1010 gene enhanced d-lactate formation (0.94 and 0.85 g/L, respectively) both in aerobic and anaerobic conditions due to a relatively stable qd-lactate. Our results suggested that the enzyme Ldb0101 and Ldb1010 played a role of more importance in d-lactate formation. To the best of our knowledge, we demonstrate for the first time the roles of different d-LDH homologs from L. bulgaricus in d-lactic acid production.