Plastic pollution, one of the most pressing environmental challenges, could soon be whittled away with a new study finding a family of bacteria that can break down plastic for food.
According to Northwestern University, a typical family of bacteria, Comamonadacae, grows on plastics littered throughout urban rivers and wastewater systems, degrading the synthetic material.
The study revealed that the environmental bacteria chew the plastic into small pieces, called nanoplastics. Then, they secrete a specialized enzyme that further breaks the plastic. Finally, the bacteria use a ring of carbon atoms from the plastic as a food source.
The discovery opens new possibilities for developing bacteria-based engineering solutions to help clean up difficult-to-remove plastic waste, which pollutes drinking water and harms wildlife.
The study was published on Oct. 3 in the Environmental Science & Technology journal.
“We have systematically shown, for the first time, that a wastewater bacterium can take a starting plastic material, deteriorate it, fragment it, break it down and use it as a source of carbon,” said Northwestern’s Ludmilla Aristilde, who led the study, in the statement.
“It is amazing that this bacterium can perform that entire process, and we identified a key enzyme responsible for breaking down the plastic materials. This could be optimized and exploited to help eliminate plastics in the environment.”
An expert in the dynamics of organics in environmental processes, Aristilde is an associate professor of environmental engineering at Northwestern’s McCormick School of Engineering.
The study’s co-first authors are Rebecca Wilkes, a former doctorate student in Aristilde’s lab, and Nanqing Zhou, a current postdoctoral associate in Aristilde’s lab.
The pollution problem
According to the statement, the new study builds on previous research from Aristilde’s team, which unraveled the mechanisms that enable Comamonas testosteroni to metabolize simple carbons generated from broken-down plants and plastics.
In the new research, Aristilde and her team again looked to C. testosteroni, which grows on polyethylene terephthalate, a plastic commonly used in food packaging and beverage bottles. Because it does not break down quickly, PET is a significant contributor to plastic pollution.
“It’s important to note that PET plastics represent 12 percent of total global plastics usage,” Aristilde said. “And it accounts for up to 50 percent of microplastics in wastewaters.”
Innate ability to degrade plastics
To better understand how C. testosteroni interacts with and feeds on the plastic, Aristilde and her team used multiple theoretical and experimental approaches.
First, they isolated bacterium from wastewater and grew it on PET films and pellets. Then, they used advanced microscopy to observe how the surface of the plastic material changed over time.
Next, they examined the water around the bacteria, searching for evidence of plastic broken down into smaller nano-sized pieces. Finally, the researchers looked inside the bacteria to pinpoint tools the bacteria used to help degrade the PET.
“In the presence of the bacterium, the microplastics were broken down into tiny nanoparticles of plastics,” Aristilde said.
“We found that the wastewater bacterium has an innate ability to degrade plastic all the way down to monomers, small building blocks which join together to form polymers. These small units are a bioavailable source of carbon that bacteria can use for growth.”
After confirming that C. testosteroni can break down plastics, Aristilde wanted to learn how it did.
Through omics techniques that can measure all enzymes inside the cell, her team discovered one specific enzyme the bacterium expressed when exposed to PET plastics.
To further explore this enzyme’s role, Aristilde asked collaborators at Oak Ridge National Laboratory in Tennessee to prepare bacterial cells without the ability to express the enzyme.
Remarkably, without that enzyme, the bacteria’s ability to degrade plastic was lost or significantly diminished.
How plastics change in water
Although Aristilde imagines this discovery could be harnessed for environmental solutions, she also says this new knowledge can help people better understand how plastics evolve in wastewater.
“Wastewater is a huge reservoir of microplastics and nanoplastics,” Aristilde said.
“Most people think nanoplastics enter wastewater treatment plants as nanoplastics. But we’re showing that nanoplastics can be formed during wastewater treatment through microbial activity. That’s something we need to pay attention to as our society tries to understand the behavior of plastics throughout its journey from wastewater to receiving rivers and lakes.”
