A newly discovered plastic-eating bacterium may hold the key to safely degrading millions of tonnes of polyethylene terephthalate (PET) plastics dumped each year.

Key points: PET plastic is a global pollution problem

PET plastic is a global pollution problem Scientists have discovered a new species of bacteria that produces a plastic-eating enzyme

Scientists have discovered a new species of bacteria that produces a plastic-eating enzyme When microbes were applied to PET film it broke down completely after six weeks

More than 45 million tonnes of PET plastics are produced annually. While a small percentage is recycled worldwide, much of it ends up in landfill or waterways, creating significant environmental problems.

Scientists have long searched for a biological agent that could effectively degrade PET's tough crystalline structure.

To date, they had only found a few species of fungi that could partially break down the plastic.

But a team of Japanese researchers, led by Dr Shosuke Yoshida from the Kyoto Institute of Technology, have discovered a new species of bacteria that produces a never-before-seen plastic-eating enzyme.

PET completely degraded after six weeks

A red-footed Booby on a polluted beach in Australia. Most plastics end up in landfills or waterways. ( CSIRO: Dr Denise Hardesty; file photo )

The researchers collected 250 debris samples — including soil and other sediments, wastewater and sludge — from a PET recycling plant and screened the microbial communities present in the samples to determine whether they played any role in the breakdown process.

"One sediment sample contained a distinct microbial consortium," Dr Kenji Miyamoto from Keio University, who was part of the study published today in Science, said.

"The consortium contained a mixture of bacteria, yeast-like cells, and protozoa."

When they applied the microbes to a thin PET film, prominent pits formed on the film surface as it was consumed. It completely degraded after six weeks.

"We then successfully isolated a bacterium, Ideonella sakaiensis, from the group and found that this strain produces two enzymes," researchers said.

Further analysis showed the two enzymes — PETase and MHETase — could break down both PET and another compound called MHET — (mono 92-hydroxyethyl) terephthalic acid — which forms during the breakdown process.

The environmentally benign breakdown products, ethylene glycol and terephthalic acid, are then used by the bacteria as an energy source.

New enzymes may have evolved through swapping genes

Despite promising results, scientists say solving PET pollution will not be as easy as releasing the new bacteria into a landfill. ( Flickr: WDNR; file photo )

Human-manufactured PET has only been around for around 70 years, suggesting that this trait has evolved only relatively recently.

The researchers searched genomic databases to determine whether any other organisms shared the genes responsible for the enzymes that metabolise PET.

While they did not find any other species with the complete set of genes coding for PET metabolisation, they did find 92 micro-organisms that were able to code for MHETase, with 32 of those also able to code for other enzymes used in the PET metabolisation process.

"Mutations in an [enzyme that breaks down chemical bonds] such as PET hydrolytic cutinase, may have resulted in enhanced selectivity for PET," Dr Miyamoto said.

"It is unclear whether any other similar microorganisms exist," Dr Miyamoto said.

The researchers say the volume of PET at the sample site, as well as the culturing of the microbial group during the experimental procedure, might have helped select for a bacterium that had obtained the necessary genes by swapping genes with other bacteria.

While the new discovery holds exciting possibilities for the harmless breakdown of environmentally-damaging plastics, Dr Miyamoto said that it was not as simple as unleashing armies of PET-metabolising bacteria into landfills to break down waste plastic.

"Because of its crystal structure, the rate of degradation is very slow. However, after heat-treatment at 260 degrees, the crystal structure breaks down, allowing these micro-organisms to easily decompose the plastic," he said.