Driven by the quest for an abundant, non-polluting alternative to fossil fuel, scientists said Tuesday they had developed a method to produce propane using sugar and the gut bacteria E. coli.

Though still far from being commercially viable, the inventors of the process said they hoped it would one day yield a renewable source of clean fuel that could be seamlessly introduced and used by existing technologies.

Propane makes up the bulk of LPG (liquid petroleum gas) used in heaters, gas burners, refrigerators and some types of cars.

It is derived as a by-product from natural gas processing and petrol refining -- both finite resources.

Adding to its attractiveness as a alternative energy source, propane is released as a gas but can be stored in an energy-dense liquid form, and is less toxic than other fuels, wrote the authors of the study in the journal Nature Communications.

However, no method existed for its manufacture from a renewable source. Until now.

"Our proof of concept study provides a method for renewable production of a fuel that previously was only accessible from fossil reserves," said study co-author Patrik Jones from Imperial College London.

"Although we have only produced tiny amounts so far, the fuel we have produced is ready to be used in an engine straight away."

- 'Phase out biofuels' -

The team grew engineered E. coli, usually harmless gut bacteria that can sometimes cause food poisoning, in sugar in the lab.

Inside the bacterial cells, some of the sugar would ordinarily be turned into fatty acid and protein molecules, which would then be turned into new cell membranes.

In the modified bacteria, however, the process of making fatty acids was interrupted to instead yield a nasty-smelling compound called butyric acid.

This was, in turn, coaxed into becoming propane through the addition of several enzymes.

"We created a system to renewably produce a product which is chemically identical to a molecule that until now only was available from fossil sources," Jones told AFP.

Productivity and yield would need to improve by a factor of 100 to 1,000 before one would be able to assess if the method is commercially viable, he added.

"Although fracking has provided a boost in the supply of liquid and gaseous fossil fuels, there is still a continued need for genuinely sustainable energy technologies over the long-term," the study authors wrote.

And they pointed to a "need to phase out current crop-based biofuels and move towards next-generation technologies that do not compete for food" -- a criticism of some biofuel crops like maize.

"I hope that over the next five to 10 years we will be able to achieve commercially viable processes that will sustainably fuel our energy demands," said Jones.