Submitted to: Journal of Industrial Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 13, 2001
Publication Date: N/A
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 increase significantly with introduction of poly-lactate, a biodegradable plastic polymer, into the marketplace. Currently, lactate is produced from fermentation of glucose. Use of sugars derived from biomass might offer an alternate less expensive feedstock. Reserves of biomass in the U.S. are estimated at 200 million tons and are continually renewed. However, 20-40% of the sugars derived from biomass is xylose, which commercial microorganisms either do not convert at all or convert inefficiently to lactic acid. We have developed recombinant E. coli strains that produce lactic acid from either glucose or xylose. Advantageously, the strains produce an optically pure L-lactic acid product. The best performing strain will convert 100 g/l of glucose to lactic acid within 30 hr with a yield that is 93% of the maximum possible. This strain also produced 63 g/l of lactic acid from xylose in 100 hr.
Technical Abstract: A new series of recombinant E. coli have been constructed for the conversion of glucose as well as pentose sugars into L-lactic acid. The strains carry the lactate dehydrogenase gene from Streptococcus bovis on a low copy number plasmid (pVALDH1) for production of L-lactate. Three E. coli strains were transformed with the plasmid to produce L-lactic acid; ;two K12 (FBR9 and FBR10) and one B (FBR11). The strains have been constructed so as to selectively maintain the plasmid when growing fermentatively. FBR9 and FBR11 were serially transferred 10 times in anaerobic cultures with sugar limited medium containing glucose or xylose, without selective antibiotic. An average of 96% of both FBR9 and FBR11 cells maintained pVALDH1 in anaerobic cultures. The fermentation performances of FBR9, FBR10, and FBR11 were compared in pH controlled batch fermentations with medium containing 10% w/v glucose. Fermentation results swere superior for FBR11, the E. coli B strain, compared to those observed for FBR9 or FBR10. FBR11 exhausted the glucose within 30 h, and the maximum lactic acid concentration (7.32% w/v) was 93% of the maximum theoretically possible. The only other side-products detected were cell mass and succinic acid (0.5 g/l). FBR9 and FBR11 were also used to ferment xylose (10% w/v). Results once again with FBR11 were superior to those for FBR9. FBR11 produced 6.33% w/v lactic acid in 100 h; however, the yield was only 72% of theoretical because of residual xylose (1% w/v). The only side-products were cell mass and succinic acid (1.7 g/l).