The direct conversion of xylan to lactic acid by Lactobacillus brevis transformed with a xylanase gene.

A xylanase gene (xynR8), obtained from the DNA of a pool of uncultured rumen microbes, was introduced via a plasmid into Lactobacillus brevis. The recombinant xylanase, with an estimated molecular weight of 32 KDa, was expressed in the transformants and showed obvious xylanase activity (0.412 U ml−1) against oat-spelt xylan in broth when compared to the wild-type Lactobacillus brevisATCC367. The transformants all shared a similar ability to utilize and metabolize xylooligosaccharides. When a selected transformant was inoculated into modified MRS medium containing xylan as the main carbon source, the cell density reached 2.20 × 109 CFU ml−1 on day 4, while the wild-type strain without the plasmid containing the recombinant xylanase did not grow at all under the same conditions. After fermentation, 1.70 g l−1 of lactic acid and 0.44 g l−1 of ethanol were present in the culture supernatant of the strain containing the recombinant xylanase. These results indicate that Lactobacillus brevis containing the xylanase gene is capable of directly saccharifying and fermenting xylan to produce lactic acid in one step. This strain will enable the development of a feasible and economical approach to the production of lactic acid directly from xylan.