Production of xylitol from d-xylose by recombinant Lactococcus lactis

Abstract

The d-xylose reductase from Pichia stipitis CBS 5773 and the xylose transporter from Lactobacillus brevis ATCC 8287 were expressed in active form in Lactococcus lactis NZ9800. Xylitol production was investigated using non-growing recombinant cells in high cell-density under microaerobic conditions in the presence of xylose and glucose. Besides xylose, the recombinant strain with xylose reductase activity reduced l-arabinose and d-ribose in significant extent to the corresponding pentitols. The ratio of xylitol produced per glucose consumed was almost 10-fold higher under glucose limitation than the ratio in the presence of excess initial glucose. The co-expression of the xylose transporter with the xylose reductase did not increase the efficiency of xylitol production appreciably when compared to the strain in which only the xylose reductase gene was expressed. A fed-batch experiment with high initial xylose concentration (160 g l −1) under glucose limitation was carried out using the strain co-expressing xylose reductase and xylose transporter genes. The xylitol yield from xylose was 1.0 mol mol −1 and the ratio of xylitol produced per glucose consumed was 2.5 mol mol −1. The volumetric productivity was 2.72 g l −1 h −1 at 20 h. Of the xylose initially present, 34% was consumed. Analysis of the fermentation metabolites revealed a shift from homolactic to mixed acid fermentation at early stages of the experiment.