A new bacteria breaks down PET polymers that make up some water bottles and other common plastic containers. | Flickr/ Adam Cohn/ CC BY-NC-ND 2.0
Researchers have identified a species of bacteria that uses just two enzymes to break down a tough type of plastic polymer. The findings are published in the 11 March issue of Science.
Poly(ethylene terephthalate), or PET, is a very common type of plastic polymer; it's often used in plastic water bottles and about 56 million tons of PET were produced worldwide in 2013 alone. However, while it may be a convenient material for humans, PET is highly resistant to biodegradation, and the accumulation of PET in ecosystems around the globe, particularly in the oceans, may pollute habitats and harm wildlife. To date, very few species of fungi — and no bacteria — have been found to break down PET.
Motivated by the accumulation of PET in the environment, Shosuke Yoshida of Keio University and colleagues searched for a type of bacteria that is capable of digesting the plastic polymer. "Microbiologists know that microbes can do anything," explained Yoshida.
The researchers collected 250 environmental samples, such as soil and sludge, from the yard of a PET bottle-recycling factory and analyzed many different species of bacteria that were growing within the samples. One new bacterium, which they named Ideonella sakaiensis 201-F6, could nearly completely degrade a thin film of PET after six weeks, at a temperature of 30°C (or 86°F).
The 201-F6 strain of bacteria uses just two enzymes to "eat" PET and break it down to its simpler — and more environmentally friendly — components. The first enzyme (called a PETase) breaks down PET into a compound called MHET. The second enzyme (called a MHETase) further breaks down MET.
A scanning electron microscope image shows PET film degraded by the 201-F6 bacterium (inset is intact PET film). | Yoshida et al., (2016)
While the identification of this plastic-eating bacterium is exciting, much mystery surrounds Ideonella sakaiensis 201-F6. "We are surprised at the presence of this bacterium that degrades and assimilates PET, whose commercial production was initiated only [a little more than] 60 years ago, meaning that during such a short time the 201-F6 have evolved an efficient system to metabolize PET," said Yoshida.
Remarkably, these plastic-eating enzymes of 201-F6 share very little genetic resemblance to their closest related enzymes, suggesting that their purpose may have evolved quite recently. This study demonstrates how species can adapt very quickly to changes in their environment.
While the ability to eat plastic is helpful for 201-F6, humans may also be able to use the bacterium's enzymes to break down the masses of PET that have accumulated in nature, although much further research is needed before this can happen on commercial scales.
"Many things are left uncovered," explains Yoshida. "For example, although PETase showed higher PET-[degrading] activity, the activity level is actually too low for industrial application."
"We have to answer the fundamental questions such as why PETase is more active and specific to PET compared to other PET-[degrading] enzymes," he added, "which could lead to creating the engineered enzyme appropriate for the practical use in the future."