How bacteria can help clean up plastic waste

Of the 7bn tonnes of plastic waste generated globally so far, less than 10% has been recycled, according to data from the UN Environment Programme.Millions of tonnes of plastic waste are lost to the environment, or sometimes shipped thousands of kilometres to destinations where it is mostly burned or dumped. The estimated annual loss in the value of plastic packaging waste during sorting and processing alone is $80-120bn. It is in this context that a discovery by a group of researchers offers a silver lining.Researchers have long observed that a common family of environmental bacteria, Comamonadacae, grow on plastics littered throughout urban rivers and wastewater systems. But exactly what these Comamonas bacteria are doing has remained a mystery. Now, Northwestern University-led researchers have discovered how cells of a Comamonas bacterium are breaking down plastic for food. First, they chew the plastic into small pieces, called nanoplastics. Then, they secrete a specialised enzyme that breaks down the plastic even further. Finally, the bacteria use a ring of carbon atoms from the plastic as a food source, the researchers found.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 is published in the journal Environmental Science & Technology. Northwestern’s Ludmilla Aristilde, who led the study, said: "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 optimised and exploited to help get rid of plastics in the environment.”The new study builds on previous research from Aristilde’s team, which unravelled the mechanisms that enable Comamonas testosteri to metabolise 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 (PET), a type of plastic commonly used in food packaging and beverage bottles. Because it does not break down easily, PET is a major contributor to plastic pollution. "It’s important to note that PET plastics represent 12% of total global plastics usage,” Aristilde said. "And it accounts for up to 50% of microplastics in wastewaters.”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 took bacterium — isolated 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. And, finally, the researchers looked inside the bacteria to pinpoint tools the bacteria used to help degrade the PET.After confirming that C. testosteroni can indeed break down plastics, Aristilde next wanted to learn how. 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.Although Aristilde imagines this discovery potentially 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 behaviour of plastics throughout its journey from wastewater to receiving rivers and lakes.”