Ethanol Production by Modified Caldicellulosiruptor bescii at High Temperatures

The bacteria Caldicellulosiruptor bescii does not produce ethanol. However, the recent introduction of an NADH-dependent AdhE from C. thermocellum into a mutant of C. bescii resulted in production of ethanol from un-pretreated switchgrass.

However, C. thermocellum AdhE becomes inactive in high temperatures, which is the optimum growth temperature of C. bescii. This means ethanol can not be produced above 65°C. Researchers from the University of Georgia and Oak Ridge National Laboratory expressed the adhB and adhE genes from Thermoanaerobacter pseudethanolicus 39E, a thermophilic bacteria capable of producing ethanol, in a mutant of C. bescii.

The modified strains produced ethanol at 75°C. The AdhB expressing strain produced ethanol and acetate from switchgrass. On the other hand, AdhE expressing strain produced more ethanol but reduced levels of acetate. The use of thermostable enzymes will be critical in achieving high temperature production in bacteria.