Development of a synthetic medium for continuous anaerobic growth and ethanol production with a lactate dehydrogenase mutant of Bacillus stearothermophilus

A synthetic medium was developed by the pulse and medium-shift technique for the continuous cultivation of Bacillus stearothermophilus strain LLD-15 (NCIMB 12428) under anaerobic conditions. This mutant strain lacks L-lactate dehydrogenase activity, and is a promising candidate for the production of ethanol from pentoses and hexoses, using a high-temperature two-stage process. The final medium contained four amino acids and five vitamins, and growth characteristics in this medium compared well with those in complex medium containing yeast extract and tryptone. At 70°C, the medium was capable of supporting good anaerobic and aerobic growth at 10 g input sucrose I^-1. High ethanol production indicated that pyruvate metabolism probably occurred via the combined activity of the pyruvate-formate-lyase pathway and pyruvate dehydrogenase.