Submitted to: American Society for Microbiology Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 5/23/2002
Publication Date: N/A
Technical Abstract: Lactic acid for manufacturing the plastic, polylactic acid (PLA), and the "green-solvent," ethyl lactate, continues to be of considerable importance for ecological and sustainability reasons. Large-scale microbial production of lactic acid is achieved primarily with Lactobacilli or Rhizopus species. However, fermentation efficiency must be improved to ensure the economic feasibility of these anticipated market expansions. Over expression of the R. oryzae lactate dehydrogenase gene (ldhA) has been very successful in enhancing lactic acid fermentation efficiency in this fungus, and we wanted to determine if comparable levels could be accomplished in other industrial organisms. The ability to produce lactic acid by Mucor strains has several appealing aspects, such as anaerobic tolerance and dimorphic growth. The ldhA gene was transformed into M. circinelloides without any modification to the promoter. Transformed strains were able to utilize all of the starting 10% (w/v) glucose, but only accumulated 3.7% lactic acid and 1.3% ethanol, in 6 days. The ldhA gene was then modified, such that expression was under control of the S. cerevisiae adh promoter and terminator. The modified gene was transformed into several S. cerevisiae strains, and fermentations were compared under different conditions. Yields of lactic acid were consistently higher in the non-aerated fermentations. Diploid strains of S. cerevisiae had higher productivity and yields. The optimal pH was 5.0, although the effect of pH was minimal between 3.5-6.0. Lactate dehydrogenase activity remained at approximately 1.6 unit/mg protein throughout the fermentation. Typical fermentations with 10% glucose resulted in 3.3% lactic acid and 2.2% ethanol in 30 hrs.