By David Biello
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Welding bits and pieces from various microbes and the camphor tree into the genetic code of Escherichia coli has allowed scientists to convince the stomach bug to produce hydrocarbons, rather than sickness or more E. coli. The gut microbe can now replicate the molecules, more commonly known as diesel, that burn predominantly in big trucks and other powerful moving machines.
"We wanted to make biofuels that could be used directly with existing engines to completely replace fossil fuels," explains biologist John Love of the University of Exeter in England, who led the research into fuels. "Our next step will be to try to develop a bacterium that could be deployed industrially." Love’s work was published April 22 in Proceedings of the National Academy of Sciences.
That means harnessing E. coli's already high tolerance for harsh conditions, such as the high acidity and warmth of the human digestive tract. That hardiness also seems to be helping the bacterium survive its own production of such longer-chain hydrocarbons, which could have proved toxic to the microbes, in the way brewer's yeast cells are killed off by the alcohol they ferment. The engineered E. coli used genetic code from the insect pathogen Photorhabdus luminescens and from the cyanobacterium Nostoc punctiforme as well as soil microbe Bacillus subtilis to make the fuel molecules from fatty acids, along with a gene from the camphor tree—Cinamomum camphora—to cut the resulting hydrocarbon to the right length.
The E. coli are currently fed on sugar and yeast extract, which suggests that the resulting fuel would be expensive compared with the kind refined from oil found in the ground. "We are hopeful that we could change their diet to something less valuable to humanity," Love suggests. "For example, organic wastes from agriculture or even sewage."
Exactly how the E. coli microbes expel the diesel fuel molecules is unknown at this point. The researchers have found them floating in the growth medium, suggesting the microbes are somehow secreting the hydrocarbons from their cells once produced. "We don't know how they get there yet," Love admits. But that may solve a problem posed to other would-be biofuels produced in microbes; algal oils have proved difficult to extract cheaply and effectively from inside the algae themselves, among other challenges.
Besides a better grasp of the process itself, fine-tuning the genetic engineering may one day yield other useful hydrocarbons, such as jet fuel or even gasoline (a short-chained hydrocarbon). Similar work at the University of California, Berkeley, has tinkered with E. coli genetics to allow the bacteria to digest the inedible parts of plants known as cellulose and turn them into microbial diesel that can be used in place of fossil-fuel diesel or other useful hydrocarbons. And E. coli has been harnessed in the past to make specialty oils for cosmetics; the company Amyris makes the moisturizing oil known as squalane from E. coli fed sugarcane and grown in vats in Brazil. The synthetic biologists at Amyris have also coaxed yeast to produce the antimalarial drug artemisinin, a technology that is currently being commercialized with drugmaker Sanofi.
Regardless, industrial-scale fuel production from microbes remains a much tougher proposition than making specialty oils or medicines, given the low cost and high volumes required to compete with the fuels made from fossil sources. "Fuel is actually a lot cheaper than artemisinin, so it has to be made in significantly larger quantities," Love notes. "That in itself is a challenge."
