Submitted to: Journal of Industrial Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 22, 2002
Publication Date: November 1, 2002
Citation: DIEN, B.S., NICHOLS, N.N., BOTHAST, R.J. FERMENTATION OF SUGAR MIXTURES USING ESCHERICHIA COLI CATABOLITE REPRESSION MUTANTS ENGINEERED FOR PRODUCTION OF L-LACTIC ACID. JOURNAL OF INDUSTRIAL MICROBIOLOGY AND BIOTECHNOLOGY. 2002. V. 29. P. 221-227. Interpretive Summary: Global lactate production is approximately 130,000 metric tons per year, most of which is used by the food industry. Demand for lactic acid is expected to grow because companies are beginning to use it for making biodegradable plastics. Currently, lactate is produced from glucose, but the cost of producing the plastic is too expensive for many applications. One way to lower costs is to use a more inexpensive sugar source. Biomass fiber, such as found in agricultural crop residues, offers a less expensive source of sugars. Use of biomass will also benefit farmers by adding value to material that has little or no net value. Biomass is comprised of several different kinds of sugars, some of which are not fermented efficiently by common lactate producing microorganisms. We have developed novel recombinant microorganisms that produce lactic acid from all of the major sugars found in biomass. The lactate produced is suitable for making gplastics. The best performing strain yield was 74-100% lactic acid from a sugar mixture similar to that produced from biomass.
Technical Abstract: Conversion of lignocellulose to lactic acid requires strains capable of fermenting sugar mixtures of glucose and xylose. Recombinant E. coli strains engineered to selectively produce L-lactic acid were used to ferment sugar mixtures. Three of these strains were catabolite repression mutants (ptsG**-) that have the ability to simultaneously ferment glucose and xylose. The best results were obtained for ptsG**- strain FBR19. FBR19 had a yield of 0.77 (g lactic acid/g added sugar) and consumed 75% of the xylose. In comparison, the ptsG**+ strains had yields of 0.47-0.48 g/g and consumed 18-22% of the xylose. FBR19 was subsequently screened on a variety of glucose (0-40 g/l) and xylose (40 g/l) mixtures. The lactic acid yields ranged from 0.74-1.00 g/g. Further experiments were conducted to discover the mechanism leading to the poor yields for ptsG**+ strains. Xylose isomerase (XI) activity, a convenient marker for xylose induction of fxylose metabolism, was monitored for FBR19 and a ptsG**+ control during fermentations of a sugar mixture. Crude protein extracts prepared from FBR19 had 10-12 times the specific XI activity of comparable samples from ptsG**+ strains. Therefore, it was concluded higher levels of xylose induction in the ptsG**- strain led to superior conversion of xylose to product compared to the ptsG**+ fermentations.