Engineered Microbe Could Ease Switch to Grass

by Kate Yandell (The Scientist) Researchers from the University of Georgia and at Tennessee’s Oak Ridge National Laboratory have engineered the thermophilic bacterium Caldicellulosiruptor bescii to directly convert switchgrass into ethanol, according to a study published on June 2 in PNAS. The new approach eliminates the need for expensive chemical and enzymatic treatments required to prepare grasses for ethanol production, potentially easing the way for use of sustainable feedstocks like switchgrass to produce biofuels.
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The researchers sought to engineer a bacterium to directly convert switchgrass into ethanol, with no need for chemical and enzymatic treatment. They selected C. bescii because it naturally produces enzymes that free glucose from lignocellulose. It is also a hyperthermophile, meaning it thrives at 80 degrees Celsius. Biomass pretreatment requires high temperatures, which help open up cell walls, making this species ideally suited for the task.
The researchers then engineered C. bescii to express a gene that the bacterium Clostridium thermocellum uses to make ethanol, a bifunctional acetaldehyde and alcohol dehydrogenase.
“We chose to take an organism that does the hard thing—digest lignocellulosic biomass— and we engineered a pathway to ethanol,” explained coauthor Janet Westpheling, a microbial geneticist at the University of Georgia.
The researchers also deleted the gene for lactate dehydrogenase, since it produces lactate, which competes with ethanol as a glucose product. They then fed the bacteria shredded switchgrass. The engineered bacteria converted 60 percent of glucose units to ethanol.
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Westpheling said that bioenergy is key to addressing climate change, as plants—particularly perennials like switchgrass—offset the emissions of burning fuel by sequestering carbon from the atmosphere in the soil. “We have to see that the long term of putting carbon dioxide in the atmosphere is bad,” she said.
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READ MORE and MORE (The Red and Black) ABSTRACT, RESEARCH PAPER (Proceedings of the National Academy of Sciences)