Credit: Shosuke Yoshida et al.

Bacteria can break down plastic

Monday 30 Sep 19
|
by Morten Andersen

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Anne S. Meyer
Professor
DTU Bioengineering
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How PET is degraded

The bacterium Ideonella sakaiensis degrades PET by producing two enzymes performing each their own task. The first enzyme, PETase, cuts the polyester polymer of which PET is constructed into smaller pieces. During this process, PET is converted into mono-(2-hydroxyethyl) terephthalate acid (MHET), terephthalate (TPA), and bis(2-hydroxyethyl) TPA (BHET). Then the second enzyme produced by Ideonella sakaiensis, MHETase, comes into play. This enzyme converts the MHET into ethylene glycol and TPA. Thus, the bacteria enzymes can effectively solve the environmental problem caused by the PET bottle, since the two substances that are left can easily be broken down by other microorganisms, with CO2 and water as the final result. 

Mountains of plastic waste

Around 480 billion plastic bottles are produced worldwide each year (2016 figures). Less than half of these are recycled. Even though Denmark and other countries have a deposit scheme where bottles are recycled, these bottles also eventually end up being discarded. Furthermore, only 30 per cent of all the PET in the world is used in bottles. The other 60 per cent are used for synthetic clothing fibres—polyester. Today, a negligible fraction of PET waste is used to produce new products. The rest is burned, buried, or ends up in the oceans, where degradation can take hundreds of years.

SOURCE: THE GUARDIAN/EUROMONITOR 

In 2016, Japanese scientists discovered a bacterium that can break down PET plastic, which is primarily known from plastic bottles and clothing fibres. Now, Danish researchers will speed up the degradation process so all the mountains of plastic waste can be put to use. 

Nature has yet again outperformed humans. In just 80 years, evolution has succeeded in producing an organism that can defeat a man-made material—plastic—that did not previously exist in nature. This organism is the bacterium known as Ideonella sakaiensis 201-F6, which is capable of breaking down polyethylene terephthalate (PET), a type of plastic used in soda bottles, among other things.

“The fact that Ideonella sakaiensis can break down PET is interesting in itself. Just a few years ago, we regarded PET as being imperishable in nature. Furthermore, the degradation appears to happen via just two enzymes,” says Professor Anne Meyer from DTU Bioengineering, who is spearheading a research group which, in addition to solving the environmental issue, will also transform PET waste into new products that can be used sustainably.

PET is made from ethylene glycol and dimethyltryptamin (DMT)—both of which are predominantly made from crude oil. Ideonella sakaiensis is able to break down PET into the same two compounds. The Japanese scientists discovered the bacterium’s abilities as they saw it grow on large quantities of PET bottles at a recycling station.

Need more speed
The DTU researchers are more interested in the two enzymes than in the bacterium itself.

“Ideonella sakaiensis breaks down PET, but the process is very slow. In addition, there are a number of practical problems with using microorganisms for industrial purposes. It takes a certain amount of time before the bacteria have established themselves and reach a volume that really makes a difference. Furthermore, you often run into unforeseen issues where the microorganisms are not thriving. Therefore, it may be more interesting to skip the bacteria itself and instead focus on imitating the enzyme’s process,” says Anne Meyer.

“We want to understand how we can make the enzymes work faster. We need to accelerate the process by a factor of 100 or maybe even 1,000 before we have a solid business case. This may sound drastic, but we now have extensive experience with protein engineering, where we take natural processes and speed them up. I am sure that we can reach an improvement of a factor of 10 pretty soon, and I think that a factor of 100 is also realistic,” says Anne Meyer.

“Usually, when we examine new biotechnological processes, we have to figure in the cost of the substrate on which the microorganisms will feed as an expense. But in this case, we have a substrate that exists in vast amounts, and which society actually needs to get rid of. In addition, we have a great advantage in that many countries—including Denmark—have already established systems for collecting plastic waste. Currently, 335 million tonnes of PET waste is collected globally. If we can make use of just a few percent of that waste, that will quickly make a good business case.”

Possible uses include antifreeze and carrier bags
Once the PET has been broken down into ethylene glycol and DMT, you can either use the two substances directly or use them as a starting point to produce other products.

“For example, ethylene glycol can be used directly in antifreeze. The demand for antifreeze is large, and the liquid is currently made from non-renewable materials. Another example is new materials that can replace the plastic carrier bags that are on the way to being banned in many countries. Paper bags are not as good quality, but you can produce a hybrid material from paper and recycled plastic components that have the right properties and can be recycled or degraded biologically,” says Anne Meyer.

Unique approach
In other words, the researchers are dealing with huge amounts of raw materials and big societal challenges. Consequently, DTU Bioengineering is not the only player on the pitch.

"Working with enzymatic degradation and recycling of plastic is incredibly challenging. And that is exactly why we are so drawn to the challenge."
PROFESSOR ANNE MEYER, DTU BIOENGINEERING

“Since the Japanese discovery in 2016, research groups from China, the United States, and South Korea, as well as several other countries, have devoted themselves to this field. But we feel quite confident. We have a solid tradition for working with enzymes at both the molecular and kinetic levels. The interesting thing is not just the reactions themselves, but also how to make them more efficient and fast. We are unique in that we don’t only have a biological approach. We learn from biology, but we also make calculations based on the physics and chemistry of it and improve the enzymes through protein engineering. That’s ‘the DTU way’. I’m convinced that this field will be big for us,” says Anne Meyer and emphasizes that her research group does not only focus on applications but also on new scientific knowledge:

“It is astounding that nature was able to adapt itself to a new situation in just 80 years. It gives me hope that we can learn a lot by studying the Japanese bacterium. At the same time, nature has provided us with a template for how we can design enzymes that can solve very large tasks ourselves. I’m not saying that it will be easy—on the contrary. Working with enzymatic degradation and recycling of plastic is incredibly challenging. And that is exactly why we are so drawn to the challenge.”

Other enzymes for other plastics
Another interesting question is whether the new perspectives also apply to other types of plastic. In fact, PET is only the fourth most popular type of plastic, after polyethylene, polypropylene, and PVC. PET only makes up a few percent of the world’s total amount of plastic waste.

“The two enzymes produced by Ideonella sakaiensis only breaks down PET. However, there is reason to believe that we can find other enzymes that can break down other plastics. Among other things, there have recently been reports of findings of bacteria that can grow on polyethylene, and others that can grow on polypropylene. This data needs to be verified, but we also have methods for directly looking for new enzymes by going through the ever-increasing amounts of genome sequences for bacteria and fungi deposited in databases.”

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