FERMENTATION OF HEXOSE AND PENTOSE SUGAR MIXTURES TO LACTIC ACID BY RECOMBINANT BACTERIA

Authors: Dien, Bruce; Nichols, Nancy; Cotta, Michael

Submitted to: American Institute of Chemical Engineers Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 11/21/2003
Publication Date: 11/21/2003
Citation: Dien, B.S., Nichols, N.N., Cotta, M.A. 2003. Fermentation of hexose and pentose sugar mixtures to lactic acid by recombinant bacteria [abstract]. American Institute Of Chemical Engineers. Paper No. 161D.

Interpretive Summary:

Technical Abstract: Conversion of lignocellulose to lactic acid requires strains capable of fermenting mixtures of hexose and pentose sugars. We have developed recombinant Escherichia coli strains engineered to selectively produce L-lactic acid from such sugar mixtures. The best strain (FBR11) was able to ferment 10% w/v glucose within 40 hr, and the maximum lactic acid concentration (7.32% w/v) was 93% of the maximum theoretically possible. When used to ferment xylose, FBR11 produced 6.33% w/v lactic acid in 100 hr, though the yield was only 72% of theoretical because of unfermented xylose (1% w/v). However, fermentations with mixtures of glucose and xylose stopped once the glucose was exhausted from the culture. Because glucose represses xylose use in E. coli, the failure to completely utilize xylose in these fermentations was likely due to regulation of substrate utilization. Subsequently, a catabolite repression mutation (ptsG**-) was introduced into the strains that conferred the ability to simultaneously ferment glucose and other sugars. The best results were obtained for ptsG**- strain FBR19. FBR19 had a yield of 0.77 (g lactic acid/g added sugar) and consumed all of the glucose and 75% of the xylose. Further work is currently being directed at understanding the mechanism of continued xylose repression subsequent to completion of glucose.