Switching Clostridium acetobutylicum to an ethanol producer by disruption of the butyrate/butanol fermentative pathway

Solventogenic clostridia are well-known since almost a century due to their unique capability to biosynthesize the solvents acetone and butanol. Based on recently developed genetic engineering tools, a targeted 3-hydroxybutyryl-CoA dehydrogenase (Hbd)-negative mutant of Clostridium acetobutylicum was generated. Interestingly, the entire butyrate/butanol (C₄) metabolic pathway of C. acetobutylicum could be inactivated without a severe growth limitation and indicated the general feasibility to manipulate the central fermentative metabolism for product pattern alteration. Cell extracts of the mutant C. acetobutylicum hbd::int(69) revealed clearly reduced thiolase, Hbd and crotonase but increased NADH-dependent alcohol dehydrogenase enzyme activities as compared to the wildtype strain. Neither butyrate nor butanol were detected in cultures of C. acetobutylicum hbd::int(69), and the formation of molecular hydrogen was significantly reduced. Instead up to 16 and 20 g/l ethanol were produced in glucose and xylose batch cultures, respectively. Further sugar addition in glucose fed-batch fermentations increased the ethanol production to a final titer of 33 g/l, resulting in an ethanol to glucose yield of 0.38 g/g.