Dr. Colin Charnock, Professor of Microbiology, smiles as he recounts his foray into the fascinating world of plastic-eating bacteria. Charnock’s research interests have run the gamut from drinking water quality to hunting for bacteria present in tattooing solutions.
More recently, the discovery by Japanese researchers of a new species of bacteria that metabolises polyethylene terephthalate (PET) plastic, spurred him to try his own hand at devising a better methodology for finding and culturing these types of bacteria. In the process, Charnock made a similar discovery of his own.
Kitchen chemistry leads to a better methodology
According to the data platform Statista, approximately 600 billion PET bottles are produced every year. Only a small amount is recycled. "It was outside of one of these PET recycling factories that Japanese researchers found a new species. It was able to not only break down the plastic into its components, but to also use these as a carbon and energy source," Charnock says.
The methodology used in the search was unwieldy and slow, involving suspending a piece of plastic film in samples and then looking for bacterial growth. Charnock has devised a more practical and efficient solution for finding and culturing these types of bacteria.
"I managed, using a little kitchen chemistry, to get the plastic inside an agar plate. It makes screening big samples and looking for bacteria that can break down the plastic a lot easier.” Bacteria with this ability produce zones of clearing in the incorporated plastic. The methodology has received the seal of approval of the Japanese groups who are leading the field, and Charnock is now actively collaborating with one of these.
Hunting for bacteria to monitor the environment
Before working with PET though, Charnock looked for bacteria able to break down a range of other, more biodegradable plastics included in agar. "There are a couple of factories where they rework plastic, and so basically I took samples from rivers, streams and soil close to these—as well as several other cleaner environments," Charnock explains. He tested a total of 14 different samples on seven kinds of potentially more ‘environmentally-friendly’ plastics, looking for bacteria able to biodegrade them.
"It's a convenient way to test for things in a non-toxic way," Charnock continues. He found that approximately 10% of the bacteria were able to break down at least one of the plastics, and some of them as many as four or five. By first establishing baseline levels of plastic-degrading bacteria, the bacteria can then serve as indicators to monitor changes in the environment. In addition, the bacteria could be a source of enzymes for plastic recycling in the future.
Swamp bacteria primed to tackle plastics
"If the environment starts to get contaminated with plastic you can then check again later to see if there are more plastic-degrading bacteria, or how the environment has responded," Charnock explains. Interestingly, some of the bacteria Charnock found in relatively clean environments could break down several plastics.
Speaking of some of the swamp samples, Charnock says, “You’ve got enzymes that can break down the plastic without the environment being challenged with plastics.” Charnock speculates that this is because there are natural counterparts in the environment such as cutin and other polyesters. This opens the potential for bioremediation—environmental cleanup actions run by microorganisms.
The quest for the PET plate
Charnock may not be the first to put plastics in agar plates, but he is the first to do so with PET plastic, which is no small feat. “This was more difficult because of the structure of the molecule, which is harder to work with and harder for bacteria to break down,” Charnock says. However, the challenge proved to be more interesting for Charnock, as the plastic is widely used in making bottles.
“It’s quite a fascinating story, actually. PET wasn’t widely used until the 50s or the 60s, and already by at least 2016 bacteria had developed that can not only break it down to its constituents but can also ‘eat’ or grow on it.” Charnock stresses the difference between plastic-degrading and real plastic-eating bacteria, which are less common.
There is a definite future for these bacteria in recycling PET plastics.– Colin Charnock
In the quest for finding PET-degrading bacteria, Charnock first worked with a plastic called polycaprolactone (PCL), which is chemically somewhat like PET but simpler to work with. Bacteria that grow on PCL are more likely candidates to break down PET as well. Interestingly, Charnock found that known PET-degrading thermophiles (bacteria that thrive at high temperatures) only demonstrated this property when PCL was sprinkled onto the PET agar plate. “Suddenly they come alive and start to break down the plastic, so you need to add an extra stage to find the thermophilic ones,” Charnock explains.
Finding the needle in the haystack
“Then I started to look for my own PET-degrading bacteria using the agar. I screened several hundred bacteria for PCL-degradation, and I found one of these that definitely breaks down the PET plastic.” Charnock didn’t have far to look; the bacteria in question was found at an industrial area in Ski, a short distance from Oslo. Unlike the bacteria found in Japan, his seems to be more effective at room temperature, which means it doesn’t require as high temperatures to get the job done.
More research is required to sequence his bacteria and confirm whether it also grows on the plastic. The group in Japan he is collaborating with has already performed several advanced tests confirming its ability to degrade PET.
From petri dish to recycling processes
Charnock holds out promise for plastic-eating bacteria. By breaking plastics down into their building blocks, they can then be put back together again, closing the loop. Alternatively, the components could be fed to other bacteria to make useful proteins or other compounds. “There is a definite future for these bacteria in recycling PET plastics, they are actually in some key ways more effective than the chemical methods—they require lower temperatures and don’t produce as many harmful byproducts,” Charnock says.
Charnock, Colin (2021). Norwegian Soils and Waters Contain Mesophilic, Plastic-Degrading Bacteria.