Plastics are everywhere. Once they get into the environment as trash, they stay there for years, decades, or even centuries. That's because most plastic is chemically inert and immune to the enzymatic processes involved in biodegradation. We've tried to curtail plastic pollution through recycling and by creating plastics that are biodegradable or compostable. But what about all the plastic litter that's already out there and could persist long after our grandchildren are gone?

Life may be coming to our aid. A team of scientists in Japan, led by Shosuke Yoshida of Kyoto University, has recently discovered a species of bacteria that can degrade a plastic called PET.

Identifying microbes that degrade PET

PET stands for polyethylene terephthalate, a plastic with good mechanical, barrier, and optical properties. Bottles for water and soft drinks are just a couple of PET's many, many uses. PET is a polyester compound with a high aromatic content, which makes it chemically inert. As a result, it is typically considered resistant to microbial degradation, although certain fungi grow on a mineral medium containing PET. Roughly 56 million tons of PET are produced each year, and a lot of that ends up in the environment.

To see whether organisms other than a few fungi can manage to digest this plastic, Dr. Yoshida and his team screened 250 PET debris-contaminated environmental samples. These samples originated in everything from sediment to wastewater. The scientists looked for microorganisms that could use low-crystallinity (1.9 percent) PET film as a major carbon source for growth.

The team identified a distinct microbial consortium that, once cultured, was able to grow on PET. The PET film surface degraded at a rate of 0.13mg per square centimeter each day at 30°C. Under similar conditions, the organisms turned 75 percent of the carbon it obtained from the PET into CO 2 .

In order to identify the particular organism that was using PET as a carbon source, the scientists diluted the microbial consortium before growing it on PET. The team isolated a novel bacterial species of the genus Ideonella, which they gave the catchy name Ideonella sakaiensis 201-F6. When they discovered a subconsortium that lost the ability to degrade PET, further analysis revealed it lacked I. sakaiensis.

Using the newly identified bacteria, the team almost completely degraded a PET film in just six weeks.

Identifying the enzymes that break down PET

Currently, there are few known enzymes capable of breaking down PET through a chemical process known as hydrolysis. In order to determine what enzymes I. sakaiensis uses, the scientists sequenced its genome. They identified one gene, ISF6_4831 that encodes a protein that shares half of its amino acids with another enzyme that hydrolyzes PET. The area of similarity includes the parts of the enzyme that are used for catalytic activity.

The scientists purified the recombinant protein from I. sakaiensis and incubated it with a PET film at 30°C for 18 hours. The incubation resulted in pitting at the film surface, which is a good indication of degradation. They also performed chemical analysis of the surface, which revealed the presence of certain chemicals including mono(2-hydroxyethyl) terephthalic acid, which is an intermediate for PET hydrolysis.

Using the DNA sequence of the enzyme ISF6_4831, the team built a phylogenetic tree based on enzymes that are known to degrade PET. Using this tree, they identified three other enzymes that they hypothesized could catalyze PET hydrolysis. They then tested their ability to hydrolyze several polymers.

Compared to the three other enzymes, ISF6_4831 had a high preference for PET vs. other polymers with aliphatic esters. As a result, the team called this enzyme “PETase.” They assessed the ability of the PETase to break down the PET found in a typical soda bottle, which has higher crystallinity than their original PET samples. Again, it was more active than other enzymes.

While the PETase could break open the PET polymer, it didn't take the polymer all the way to its original starting components, so the team wanted to know whether another enzyme was responsible. Through further gene analysis, the scientists discovered the enzyme ISF6_0224, which has a protein sequence matching those of another enzyme family that is known to hydrolyze the ester linkage of aromatic compounds.

The team purified recombinant ISF6_0224 and found that it was able to efficiently hydrolyze mono(2-hydroxyethyl) terephthalic acid. But when tested against PET, ISF6_0224 did not show any hydrolytic activity. The results strongly suggest that ISF6_0224 protein is responsible for the conversion of mono(2-hydroxyethyl) terephthalic acid (MEHT) into PET’s two environmentally benign monomers, terephthalic acid and ethylene glycol. As such, the team decided that ISF6_0224 should be termed a MEHT hydrolase abbreviated to MEHTase.

This investigation has opened the possibility for a viable remediation strategy for PET. Through further research and development, we could have bacteria that efficiently clean up PET waste.

Science, 2016. DOI: 10.1126/science.aad6359 (About DOIs).