Could bacteria be the solution to our plastic problem? Maybe, but not yet

When you are done with a plastic bottle, an empty sunscreen tube or a piece of synthetic clothing, it is easy to forget about it. You toss it in the recycling bin and never see it again.

But, even if it does get recycled, that plastic is far from gone — it may take hundreds of years to decompose. One potential solution to this problem is plastic-chomping microbes. Some families of bacteria have a natural ability to break down plastics and, if we can harness that ability, it could help speed up the process of decomposition.

One such group of bacteria, the Comamonas family, is the subject of a recent Northwestern University study. Not only can Comamonas bacteria eat the chemical building blocks of plastics, the new study shows, but they can make progress at physically degrading whole pieces of plastics. 

“[This study] gave us insight into the natural fate of plastics, in addition to the fact that it’s giving us some potential avenues for solutions,” said Ludmilla Aristilde, an environmental engineering professor and lead researcher on the study.

While bacteria seem to be a natural remedy to the plastics problem, it will probably take a long time before they develop the capacity to decompose them at an effective scale.

“Plastics have only been around since about the 1950s really, so microorganisms haven’t really evolved to live with these plastics,” said Gavin Lear, a microbial researcher in New Zealand, who was not involved in the new study. “It might be that they’ve got some enzymes that are good at degrading polymers … but they’ve not really had the time, if they are capable at all, to be able to work well with these plastic polymers.”

The Northwestern study involved one of the most commonly used plastics, the type seen in everyday products like water bottles, cosmetic packaging and textiles. Comamonas bacteria were grown on two forms of these plastics: pellets and films. At the end of the six week trial period, both the pellets and films showed some etching, the beginning signs of degradation. 

“We found that this has to do with the structure,” said Aristilde. “It has to do with the morphology. The plastic pellets have more grooves for the bacteria to kind of go in. They get a little help to start.”

Plastics are typically difficult to biodegrade because the long strings of molecules making up the material are organized in a way that is hard for microbial enzymes to access, Lear said. Microbes can at least partially degrade some plastics, like PET, but, Lear said, most plastics in circulation have carbon-only backbones that are particularly challenging to break down. 

While it is exciting to see that Comamonas bacteria can break down both larger pieces of plastic and the chemical structure of plastic itself, it doesn’t completely get rid of the plastic pellets or films.

“We didn’t see that plastic disappear. We didn’t see substantial weight loss,” Lear said. “We need to be mindful that we don’t have a solution to the plastic problem that’s immediately solved by microorganisms.”

The particular plastics used in the Northwestern study, PET plastics, are also among the easiest plastics to recycle, Lear said, so as of right now, there isn’t a huge market for bacterial decomposition. Lear said it is still important to study, however, because recycling systems are far from perfect, and a lot of plastic which isn’t properly recycled ends up polluting the environment. 

After verifying that Comamonas could degrade the plastic, the team at Northwestern focused on identifying which specific enzyme was responsible. They found that one hydrolase enzyme was the primary enzyme involved in breaking down the plastic. The enzyme is distinct from previously discovered enzymes. 

Even though the study didn’t show complete degradation, it’s fascinating that these enzymes exist, said Jay Mellies, a biodegradation researcher at Reed College in Oregon, who was not involved in the study. There is a large family of these hydrolysis enzymes and Mellies wonders how they are evolving to better deal with plastic. 

“Microbes can metabolize any carbon-based material on the planet. This one’s difficult to degrade because it’s crystalline. It’s hard to chew up. But they’re doing [it] and it’s on their time scale,” Mellies said. “They’ve been around for six billion years, and they’ll be here long after we’re gone.”

Future work regarding plastic-degrading bacteria depends on applications and scalability. Aristilde’s team tested one possible application by genetically modifying different bacteria to start producing the enzyme in question. Once they did, the other bacteria were able to degrade plastic, too, confirming the enzyme is actually responsible for the changes. 

It might be that, with some time and work, either humans or bacteria will figure out how to degrade plastic efficiently. But we aren’t there yet.