See Inside April 2010

Biofuel from Bacteria

E. coli turned into cellulose-chomping biodiesel refineries

Photo Researchers, Inc.

The bacterium that causes the most cases of food poisoning in the U.S. could someday be responsible for much of the country’s transportation fuel. Researchers have used the tools of synthetic biology to manipulate the genes of Escherichia coli, a common gut bacterium, so that it can chew up vegetation to produce diesel and other hydrocarbons.

E. coli is popular in genetic engineering because it is deeply studied and quite hardy, able to tolerate genetic changes well, says chemical engineer Jay Keasling of the University of California, Berkeley. Researchers have already modified E. coli to make medicines and chemicals, and now Keasling and his colleagues have turned the organisms into biodiesel factories.

The scientists first genetically modified E. coli to consume sugar and secrete engine-grade biodiesel, which can float to the top of a fermentation vat—no need for distilling, purifying or breaking cells open to get the oil out, as is the case for making biodiesel from algae.

To minimize any impacts on food supplies, the investigators then sought out genes from other bacterial species that can break down cellulose, the tough material that makes up the bulk of plants but is not fit for human consumption. The team added those enzyme genes plus an extra bit of genetic code that instructs the altered E. coli cells to secrete the enzyme. The enzyme then breaks down plant cellulose and thereby turns it into sugar that the altered E. coli can consume to make diesel.

The process, described in the January 28 Nature (Scientific American is part of Nature Publishing Group), is perfect for creating hydrocarbons with at least 12 carbon atoms in them—besides diesel, the group includes jet fuel (kerosene). But it cannot yet make shorter-chain hydrocarbons such as gasoline, a deficiency Keasling is working on. After all, the U.S. alone burns some 530 billion liters of gasoline a year, compared with around 170 billion liters of diesel (and just 7.5 billion liters of biodiesel). 

Keasling is not alone in his regard for E. coli as a fuel maker; several companies are pursuing commercial production from the hardy microbe. It “grows fast, three times faster than yeast,” explains geneticist and technology developer George Church of Harvard Medical School, and “100 times faster than most agricultural microbes.”

Still, Keasling’s E. coli needs more work to boost its efficiency in making fuel. “We are at about 10 percent of the theoretical maximum yield from sugar,” Keasling notes. “We would like to be at 80 to 90 percent to make this commercially viable. Furthermore, we would need a large-scale production process.” In creating a novel organism, they will also remove key metabolic pathways so that it could never survive in the wild.

The challenges are not trivial, but the promise is great. Keasling has estimated that in the past such engineered microbes would need to chew up only 40.5 million hectares—roughly one quarter of the current amount of land devoted to raising crops in the U.S.—of a common tall Asian grass to produce enough fuel to meet all of the U.S.’s transportation needs.

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