Co-culture of Lactobacillus delbrueckii and engineered Lactococcus lactis enhances stoichiometric yield of d-lactic acid from whey permeate

d-Lactic acid (d-LA) is an enantiomer of lactic acid, which has a niche application in synthesis of poly-lactic acid based (PLA) polymer owing to its contribution to the thermo-stability of stereo-complex PLA polymer. Utilization of renewable substrates such as whey permeate is pivotal to economically viable production of d-LA. In present work, we have demonstrated d-LA production from whey permeate by Lactobacillus delbrueckii and engineered Lactococcus lactis. We observed that lactose fermentation by a monoculture of L. delbrueckii yields d-LA and galactose as major products. The highest yield of d-LA obtained was 0.48 g g(-1) when initial lactose concentration was 30 g L-1. Initial lactose concentration beyond 20 g L-1 resulted in accumulation of glucose and galactose, and hence, reduced the stoichiometric yield of d-LA. L. lactis naturally produces l-lactic acid (l-LA), so a mutant strain of L. lactis (L. lactis ldh ldhB ldhX) was used to prevent l-LA production and engineer it for d-LA production. Heterologous over-expression of d-lactate dehydrogenase (ldhA) in the recombinant strain L. lactis TSG1 resulted in 0.67 g g(-1) and 0.44 g g(-1) of d-LA yield from lactose and galactose, respectively. Co-expression of galactose permease (galP) and -phosphoglucomutase (pgmA) with ldhA in the recombinant strain L. lactis TSG3 achieved a d-LA yield of 0.92 g g(-1) from galactose. A co-culture batch process of L. delbrueckii and L. lactis TSG3 achieved an enhanced stoichiometric yield of 0.90 g g(-1) and 45 g L(-1)d-LA from whey permeate (lactose). This is the highest reported yield of d-LA from lactose substrate, and the titres can be improved further by a suitably designed fed-batch co-culture process.