Recombinant Escherichia coli engineered for production of L-lactic acid from hexose and pentose sugars

Recombinant Escherichia coli have been constructed for the conversion of gl ucose as well as pentose sugars into L-lactic acid. The strains carry the l actate dehydrogenase gene from Streptococcus bovis on a low copy number pla smid for production of L-lactate. Three E. coli strains were transformed wi th the plasmid for producing L-lactic acid. Strains FBR9 and FBR11 were ser ially transferred 10 times in anaerobic cultures in sugar-limited medium co ntaining 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-con trolled batch fermentations with medium containing 10% w/v glucose. Ferment ation results were superior for FBR11, an E. coli B strain, compared to tho se 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 theoretic al maximum. The other side-products detected were cell mass and succinic ac id (0.5 g/l).