In order to optimize the bacteria’s performance and increase ethanol yield, Mascoma researchers metabolically engineered both strains to be able to ferment xylose, without the help of added enzymes. They also cut out bacteria’s metabolic pathways that produce by-products such as lactate and acetate, so that the microbes only produce ethanol. Finally, the scientists engineered the microbe to keep breaking down cellulose in high concentrations of ethanol.
In Mascoma’s work with yeast, researchers genetically added a process not normally found in native strains. Normally, yeast is a very efficient and robust ethanol producer and can ferment sugars at high rates. It does not have any natural ability to break down cellulose, however. So Mascoma’s scientists engineered yeast to produce cellulolytic enzymes, enabling it to grow on cellulose and break it down. The researchers also inserted genes into yeast that allow it to ferment xylose, further increasing its ethanol yield. In experiments with paper sludge, the engineered yeast broke down and converted 85 percent of cellulose into sugars and produced ethanol without the help of added enzymes.
Frances Arnold, a professor of chemical engineering and biochemistry at the California Institute of Technology and a member of Mascoma’s scientific advisory board, says that the company’s work in yeast may be a near-term commercial application. “What they’re reporting, with a high-level expression of cellulase from yeast, is really impressive,” she says. It’s been difficult, Arnold says, “to get these enzymes expressed in yeast. If you look at the literature, it’s dismal–micrograms or milligrams per liter–and they’re talking about a gram per liter–many magnitudes higher than others have reported, to a point where it starts to look interesting.”
“There’s still optimization for these microbes that remain, and we want to improve their cellulolytic performance, and the rate at which they hydrolize sugars, which speeds up the overall production process,” says Jim Flatt, the Mascoma’s executive vice president of research and development. “They perform, they’re reliable, but we can improve them further, and that’s what we intend to do.”
The company has begun to test all three engineered microbes at a pilot plant in Rome, NY, and it plans to have a commercial scale-up by 2010.
Qteros, a startup based in Marlborough, MA, is also pursuing consolidated bioprocessing with a microbe that breaks down cellulose and ferments it to make ethanol. Jef Sharp, executive vice president of Qteros, says that Mascoma’s findings significantly advance the field of consolidated bioprocessing.
“Any progress is good,” says Sharp. “We think that it’s important for the industry to realize that it is likely the conversion technology that is going to have the best economics.”