Microbial cell factory serves as a robust platform for the production of diverse commodity chemicals and biofuels from renewable resources. Renewable or cheap waste resources constitute complex sugars which require to be hydrolyzed into their monomeric forms. The constituent sugars can be a single sugar or different sugar moieties. It is inherently challenging for a single microbe to utilize all the monomeric sugars simultaneously and achieve high product titre, rate and yield (TRY). Microbial co-culture or consortium is a promising alternative for concomitant utilization of multiple substrates to enhance the TRY criteria of different platform chemicals.

The current work successfully established a co-culture system of Lactobacillus delbrueckii and engineered Lactococcus lactis for improved production of D-LA from whey lactose. This is the first study to my knowledge which has demonstrated the production of D-LA in L. lactis which is actually a native producer of L-LA. The yield of D-LA was increased from 0.37 g/g of lactose in mono-culture of L. delbrueckii to 0.88 g/g of lactose in co-culture.

With the assistance of a mathematical model, fed-batch co-culture process was developed. A continuous fed batch co-culture with feed rate of 2.25 g/h achieved ~128 g/L of D-LA. But the D-LA yield and productivity were lower as compared to batch co-culture. The D-LA yield (0.84 g/g of whey lactose) and productivity (1.07 g/L/h) was improved in repeated fed batch co-culture with a comparable D-LA titre (~122 g/L) in continuous fed batch co-culture. This study has achieved one of the highest reported D-LA titre (~128 g/L) using whey based substrate. The co-culture strategy also successfully improved the D-LA yield and productivity as compared to mono-culture of L. delbrueckii.

The successful development of a co-culture process involving multiple substrates in this work demonstrates that such a strategy can be designed and employed for a range of fermentation processes with mixed sugars as substrates.

Publications:

1] Sahoo, T.K. & Jayaraman, G. (2019) Co-culture of Lactobacillus delbrueckii and engineered Lactococcus lactis enhances the stoichiometric yield of D-lactic acid from whey permeate. Appl Microbiol Biotechnol. https://doi.org/10.1007/s00253-019-09819-7.

2] *Vishnu Prasad, J., *Sahoo, T.K., Naveen, S., Jayaraman, G. (2020) Evolutionary engineering of Lactobacillus bulgaricus reduces enzyme usage and enhances conversion of lignocellulosics to D-lactic acid by simultaneous saccharification and fermentation. Biotechnol Biofuels 13, 171. https://doi.org/10.1186/s13068-020- 01812-x. (*Joint first authors)