Scientists just
discovered plastic-eating bacteria that can break down PET
Bon appétit!
This article was written by Mark Lorch from
the University of
Hull, and was originally published by The Conversation.
We manufacture over 300 million
tonnes of plastics each year for use in everything from
packaging to clothing. Their resilience is great when you want a product to
last. But once discarded, plastics linger in the environment, littering
streets, fields and oceans alike.
Every corner of our planet has been
blighted by our addiction to plastic. But now we may have some
help to clean up the mess in the form of bacteria that have been found slowly
munching away on discarded bottles in the sludge of a recycling centre.
Plastics are
polymers, long thin molecules made of repeating (monomer) building
blocks. These are cross-linked to one another to build a durable, malleable
mesh. Most plastics are made from carbon-based monomers, so in theory they are
a good source of food for microorganisms.
But unlike natural polymers (such as cellulose
in plants) plastics aren’t
generally biodegradable. Bacteria and fungi co-evolved with natural
materials, all the while coming up with new biochemical methods to harness the
resources from dead matter.
But plastics have only been around for about
70 years. So microorganisms simply haven’t had much time to evolve the
necessary biochemical tool kit to latch onto the plastic fibres, break them up
into the constituent parts and then utilise the resulting chemicals as a source
of energy and carbon that they need to grow.
Enzyme innovation
Now a team at Kyoto University has, by
rummaging around in piles of waste, found a plastic munching
microbe. After five years of searching through 250 samples, they
isolated a bacteria that could live on poly(ethylene terephthalate) (PET),
a common plastic used in bottles and clothing. They named the new species of
bacteria Ideonella sakaiensis.
You may think this is the rerun of an old
story, as plastic-eating microbes have already been touted as saviours of the
planet. But there are several important differences here.
First, previous reports were of tricky-to-cultivate
fungi, where in this case the microbe is easily grown. The researchers more or
less left the PET in a warm jar with the bacterial culture and some other
nutrients, and a few weeks later all the plastic was gone.
Bottle breakdown. Illustration: P. Huey. Reprinted with
permission from U.T. Bornscheuer, Science 351:1154 (2016)
Second - and the real innovation - is that the
team has identified the enzymes that Ideonella sakaiensis uses
to breakdown the PET. All living things contain enzymes that
they use to speed up necessary chemical reactions. Some enzymes help digest our
food, dismantling it into useful building blocks. Without the necessary enzymes
the body can’t access certain sources of food.
For example, people who are lactose intolerant
don’t have the enzyme that breaks down the lactose sugar found in dairy
produce. And no human can digest cellulose, while some microbes can. Ideonella
sakaiensis seems to have evolved an efficient enzyme that the bacteria
produces when it is in an environment that is rich in PET.
The Kyoto researchers identified the gene in
the bacteria’s DNA that is responsible for the PET-digesting enzyme. They then
were able to manufacture more of the enzyme and then demonstrate that PET could
be broken down with the enzyme alone.
First real recycling
This opens a whole new approach to plastic
recycling and decontamination. At present, most plastic bottles are not truly
recycled. Instead they are melted and reformed into other hard plastic
products. Packaging companies typically preferfreshly
made 'virgin' plastics that are created from chemical starting materials that
are usually derived from oil.
The PET-digesting enzymes offer a way to truly
recycle plastic. They could be added to vats of waste, breaking all the bottles
or other plastic items down into into easy-to-handle chemicals. These could
then be used to make fresh plastics, producing a true recycling system.
Manufactured enzymes are already used to great effect in a wide
range of everyday items. Biological
washing powders contain enzymes that digest fatty stains. The
enzymes known as rennet that are used to harden cheese once came from calfs’
intestines but are now manufactured using genetically
engineered bacteria. Maybe we can now use a similar manufacturing
method to clean up our mess