Plastic-Eating Bacteria: Nature's Recyclers

The Plastic Problem

Think about all the plastic you see everyday. You drink water out of plastic bottles and you eat food that's wrapped in plastic. The pen in your hand is probably plastic, and the device you’re reading this on is partly made of plastic. Where does all this plastic end up? We hope that it gets recycled into new plastic. But most of it does not get recycled. Instead, it ends up in landfills and in our waterways.

Did you know?

Only 9% of the plastic ever made has been recycled.

The amount of plastic in the environment has become a big problem. And it gets worse every year. Over 12 million metric tonnes (Mt) of plastic enter the ocean every year. This is on top of the 362 million Mt that are already there. 4.1 million Mt of that comes from Canada. 

All this plastic is a serious problem for marine life. Plastic waste is 80% of all marine debris. It gets eaten by animals like whales, fish, and seabirds. These animals then develop major health issues.

Other kinds of plastic debris can entangle or strangle sea life.

Microplastics are another problem. Microplastics break off of larger plastic items and enter the food chain.

Image - Text Version

Shown is a colour photograph of a turtle swimming underwater, while tangled in a plastic bag. The camera is just under the surface of the dark blue water. Bubbles and sunlight are visible along the top of the image. The turtle is extending both of its flippers to swim. The handle of a translucent plastic bag is looped around its neck. The bag is almost as large as the turtle.

There are many advantages to using plastic. The big disadvantage, though, is that most of it doesn’t biodegrade. An empty jug of laundry detergent that enters the ocean will still be in the ocean hundreds of years from now. Even the plastic engineered to be biodegradable takes a very long time to break down. Dealing with this needs some innovative thinking.

What if something ate it? 

In March 2016, scientists in Japan made a startling discovery. They found that some of the bottles at a recycling plant were being broken down by bacteria. The newly discovered bacteria was named Ideonella sakaiensis.

Did you know?

This bacteria was named after the recycling plant in Sakai, Japan.

Ideonella sakaiensis is a gram-negative, rod-shaped bacteria. Scientists believe that it has naturally evolved to use a certain type of plastic for food. For some time, scientists have thought about using microorganisms, like bacteria, to break down plastic. They have mostly tried using Escherichia coli (E. coli) bacteria. But E. coli doesn’t break down plastic very well. It prefers sugars over everything else. Ideonella sakaiensis, on the other hand, prefers a type of plastic called polyethylene terephthalate, or PET. PET is a type of plastic commonly used to make water bottles and food packages. 

How to Eat a Plastic Bottle

What do we mean when we say that bacteria can “break down” plastic?

First, it’s important to know that PET plastic is made up of repeated units of C₁₀H₈O₄. This type of molecule is called a monomer. Monomers can chemically react with other monomers to form long chains called polymers. Different types of plastic use different monomers to make polymers.

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Shown is a black and white diagram of the structure of a PET molecule. Most of the structure is inside a pair of brackets labelled n. In the centre is a hexagon with extra lines representing double bonds on three sides. Single horizontal lines extend to the right and left of this. Each one branches into one double and one single diagonal line. Each of these two single and two double bonds leads to a letter O. A single horizontal line leads from the O on the lower left, outside the bracket, to a letter H. On the right, another horizontal line leads from the top O. This turns diagonally upward, then horizontal again as it crosses the right n bracket. At the end of this line are the letters OH.

The bonds between the monomers are very strong. This makes plastic tough and durable. You can experience this yourself. Try to rip apart a plastic water bottle with your bare hands. It’s impossible! The strength of polymers is what allows plastics to remain in the environment for such a long time. Natural processes can normally only break plastic into smaller pieces. They cannot break polymer chains apart.

Ideonella sakaiensis is different because it can break down the bonds between monomers. It does this using enzymes. Living creatures use enzymes for many different life processes. They speed up the chemical reactions that take place in cells.

One of the common roles of enzymes is in digestion. Digestive enzymes in bacteria work to break down larger molecules into smaller ones. These smaller molecules can then be absorbed by the bacteria. The bacteria takes what it needs and gets rid of (excretes) the rest.

Ideonella sakaiensis produces an enzyme called hydrolyzing PET (or PETase). This breaks down the bonds in the polymer to form monomers. Then it absorbs the monomers to use for energy. This is similar to the way humans break down food.

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Shown is a colour diagram of a tubular purple bacteria with its flagellum touching parts of a water bottle. On the left, a curving purple tube shaped creature. This is labelled “Bacteria.” Long, thin, squiggly structures extend from its right end. These are labelled “Flagellum.” The flagellum reach out toward the blue water bottle on the right, where they touch small orange circles.The bottle is labelled “Plastic,” and the orange dots are labelled “hydrolyzing PET.”

Hurry Up!

On the surface, using bacteria seems like a great way to get rid of all that plastic waste clogging up the environment. We could grow a bunch of Ideonella sakaiensis, add it to the plastic waste, and let the bacteria feast! Unfortunately, there are a few drawbacks to this idea.

The first problem is that there is an awful lot of plastic waste. In the history of plastic, we have produced 8.3 billion Mt of the stuff. 79% of that has gone into landfills or into the environment. It will remain there until something finally comes along to break it down. It would be hard to make enough bacteria to eat the plastic we send into the environment every year, let alone the plastic that is already there. The second problem is that bacteria digest the plastic very slowly.

Image - Text Version

Shown is a colour photograph of a vast pile of plastic drink bottles. The pile is so big it looks like a hilly landscape that fills the whole photograph. Close to the camera, individual bottles are visible. Some are clear plastic and some are transparent green. Many are very dirty and most still have labels. The pile stretches off into the distance.

So, scientists wondered if there was a way to speed up the process. In 2018, researchers at the University of Portsmouth figured out the 3D structure of PETase. This allowed them to engineer PETase to break down bottles in days, rather than months. This is still too slow to overcome all the waste we’ve created, but it’s a good start. In 2020, researchers at the same university combined PETase with another, similar enzyme. This produced a ‘super-enzyme’ that does the job six times faster.

Is it hot in here? 

A second problem is that the enzymes need to be in an environment where the temperature is above 30℃. These environments are only found in certain parts of the world. The rest of the world is too cold. This means that the cost of heating up the bacteria to get them to digest plastic, both in terms of money and environmental impact, is too expensive.

Luckily, Swiss scientists found bacteria that can digest some types of plastic in colder temperatures. They saw that these bacteria made enzymes that broke down plastic at 15℃. They believe the enzymes could work in temperatures as low as 4℃.

Everything Must Go!

The downside of the cold-temperature bacteria is that they can only break down a few specific types of plastic. This is a problem with plastic-eating bacteria in general. For example, only bacteria that can make PETase can break down PET plastic. And the cold-temperature bacteria can only break down biodegradable plastics like polyester-polyurethane (PUR).

Other common items are made from a different type of plastic called polyethylene (PE). PE is the plastic that is used in shampoo and laundry detergent bottles. It is also used in things like plastic buckets and children’s toys. It is not biodegradable, and so far none of the bacteria can digest it.

But there is still hope. In February 2023, scientists suggested the answer might be in waxworms. Waxworms are the larvae of wax moths. The scientists found that bacteria in the gut of the Lesser Waxworm (Achroia grisella) were able to digest low-density PE. This kind of plastic is used in things like shopping bags and bubble wrap. While there is still a lot of research to do, it is a promising start.

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Shown is a colour photograph of five pale grey and beige coloured worms on a table. The worms are short, plump, curved and segmented. They are mostly pale beige with darker areas about three quarters of the way along their bodies. A few specks of mud stick to their sides. The table below is light brown woodgrain.

What Goes In Must Come Out 

The last thing to note about plastic-eating bacteria is that, much like when you eat food, they produce waste. This waste is in the form of the small monomers that the plastic was made from in the first place. It looks like goopy liquid. Unlike some other waste, this has some potential uses. It can be recycled into other plastic objects. For example, a recycling plant in France is using bacteria to break down PET plastic. And the waste is being used to make new PET objects that researchers say are good as regular PET plastic.

The future of using bacteria to break down plastic looks bright, and maybe a little slimy.