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

Sahoo, Tridweep K., Jayaraman, Guhan
Applied microbiology and biotechnology 2019 v.103 no.14 pp. 5653-5662
Lactobacillus delbrueckii subsp. lactis, Lactococcus lactis, batch systems, coculture, economic sustainability, enantiomers, fermentation, galactose, glucose, lactate dehydrogenase, lactic acid, lactose, mutants, phosphoglucomutase, polymers, thermal stability, whey
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 Δldh ΔldhB ΔldhX) 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 α-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⁻¹D-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.