S123: Metabolic Engineering of thermotolerant, acidophilic Bacillus coagulans for production of D(-)-lactic acid

Optically pure lactic acid is an attractive chemical for production of bio-based, renewable, polylactide-derived plastics and this is produced by microbial fermentation of sugars at temperatures below 40°C. Fermentations at 50-55 °C is expected to enhance the use of non-food carbohydrates for production of optically pure lactic acid while also reducing potential contamination that could lower the optical purity. Bacillus coagulans that grows optimally at 50-55 °C and produces (L+)-lactic acid as the primary fermentation product was engineered to produce D(-)-lactic acid by deleting the genes in the competing pathways: ldh (L-lactate dehydrogenase) and alsS (acetolactate synthase) genes. Neither a single (ldh), double (ldh, alsS) (strain QZ5) or a triple mutant (ldh, alsS, pflB) produced D(-)-lactic acid although a native ldhA encoding D-LDH is present in the chromosome. Upon metabolic evolution of strain QZ5 for anaerobic growth at pH 5.0 and D(-)-lactate production, a derivative, strain QZ19, was selected. Strain QZ19 produced about 90 g/L of optically pure D(-)-lactic acid in less than 48 hours in batch fermentations at 50°C.