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Scientists at London’s Imperial College
E. coli bacteria into making
renewable propane that could replace the petrol in your fuel tank.
Their work has caught the imagination of the nation,


, and it’s not hard to see

E. coli bacteria are commonly found in the human
gut, with some strains associated with food poisoning. It may sound
unpleasant, but if it were possible to conjure carbon-neutral gas
using clever biochemistry, what’s not to like?

Well, allow us to explain…

What the scientists did

The scientists at Imperial have genetically
modified E. coli so that they produce propane. This isn’t actually
as far fetched as it sounds. The bugs make fatty acids naturally to
build their cell walls. Chemically speaking, fatty acids are a
short hop, skip and jump away from the fat found in butter or
margarine – or alternatively, hydrocarbons like propane.

Dr Patrik Jones and his colleagues added some
genes so the bugs would turn a short fatty acid called butyric
acid, into propane. Butyric acid is what makes vomit smell like
vomit, so the researchers probably weren’t popular with their
scientific neighbours.

Bacteria are already used to ferment sugars from
wheat, corn or sugar cane into ethanol that can be blended with
petrol. But there are several reasons why ethanol isn’t actually a
very good biofuel.

It mixes with water (see: vodka, a mix of 40 per
cent ethanol in water) making it hard to separate from the
bacterial gloop. It is toxic to the bugs that make it (see: vodka).
And there is a limit to how much ethanol can be blended into fuel
for standard petrol engines.

Jones and his colleagues decided propane would
be a better choice because it is a gas, making extraction easier as
it will evaporate from the bacterial sludge on its own. It can also
easily be turned into a liquid for ease of transport, and there is
existing infrastructure associated with its use as a fuel. Many
vehicles and pumping stations already use liquid
petroleum gas
(LPG), which is mainly

In a
press release
Jones says:

“[Our work] opens up
possibilities for future sustainable production of renewable fuels
that at first could complement, and thereafter replace fossil fuels
like diesel, petrol, natural gas and jet fuel.”

LPG pump
| Shutterstock

Where’s the catch?

The concept sounds pretty cool, even if as Jones
and his team emphasise it is at the early stages of development.
Unfortunately it suffers from the same achilles heel as all
biofuels – a very low energy density.

What does that mean? At the moment the
propane-producing bugs are fed glucose, a simple sugar. The energy
in the sugar has to come from somewhere – probably sugar cane. The
sugar cane takes land to grow, energy to harvest and process and so

The researchers say the bacterial machinery that
makes the propane could be put inside cyanobacteria, also known as
blue-green algae. The algae wouldn’t need glucose to grow, just
sunlight and some basic nutrients like carbon dioxide, nitrogen and
phosphorus. That would mean you could turn sunlight into

But whether you use glucose from sugar cane or
photosynthetic algae, the energy ultimately comes from the sun, and
that is where the fundamental limitations of the idea begin to

Former chief scientific adviser to the
department of energy and climate change David MacKay lays out why
in an example from his book “Sustainable Energy –
without the hot air”

An average car driver needs 40 kilowatt hours of
energy per day to drive 50 kilometres, he explains
. Even assuming that we turn over all of the UK’s farmland to
growing biofuels, we could supply a maximum of 24 kilowatt hours of
energy per person per day by growing
– whether it be wheat, corn, oil seed rape
or whatever.

Basically, fossil fuels offer pretty amazingly
concentrated energy. Just 2.4 litres of petrol would replace the 24
kilowatt hours of energy that a single person’s share of the UK’s
farmland could produce, on a good day, if we’re lucky. That petrol
would cost about £3 – of which almost half is fuel tax.

That’s why any serious low-carbon transition for
road transport probably needs to look beyond biofuels – no matter
how cleverly they are produced.

Food poisoning-derived propane or even
ethanol derived from corn waste
may turn out
to have their uses, but they are unlikely to be a silver

P. Kallio et al. (2014) An engineered pathway for
the biosynthesis of renewable ​propane, Nature