NEWS - Researchers report an enzyme that breaks down polyethylene terephthalate (PET) in a somewhat unlikely place: Comamonas testosteroni, a microbe that lives in sewage sludge. The enzyme could be used by wastewater treatment plants to break down microplastic particles and recycle plastic waste.
Plastic pollution is everywhere, and it mostly consists of PET. The polymer is used to make bottles, containers and even clothing. PET beads are an increasingly common microplastic found in places ranging from remote oceans to inside our bodies.
But the particles are so small that they can escape water treatment processes and end up in wastewater that re-enters the environment. On the other hand, wastewater also contains microorganisms that like to eat these plastic particles, including C. testosteroni, so named because it degrades sterols like testosterone.
“It’s important to note that PET plastic represents 12% of global plastic use. And it accounts for up to 50% of microplastics in wastewater,” says Ludmilla Aristilde of Northwestern University in Illinois.
“Most people think of nanoplastics entering wastewater treatment plants as nanoplastics. However, we show that microplastics and nanoplastics can form during wastewater treatment through microbial activity,” Aristilde said.
Other bacterial species, including Escherichia coli, have previously been engineered to convert plastic into other useful molecules. However, C. testosteroni naturally chews up polymers, such as those found in laundry detergents and terephthalate, a building block of PET monomers.
Aristilde and her team wanted to see if C. testosteroni could also produce enzymes that degrade PET polymers. The team incubated C. testosteroni strains with PET films and pellets. The microbes preferred the rougher surfaces of the pellets, breaking them down more than the smooth films.
To better simulate conditions in wastewater environments, the researchers also added acetate, an ion commonly found in wastewater. When acetate was present, the number of bacterial colonies increased significantly.
Although C. testosteroni produced some nano-sized PET particles, they also completely degraded the polymers into monomers, or compounds that can be used by C. testosteroni and other environmental microbes as a carbon source to grow and thrive or even converted into other useful molecules.
“With the presence of bacteria, microplastics are broken down into plastic nanoparticles. We found that wastewater bacteria have an innate ability to degrade plastic down to monomers. These small units are the carbon source that the bacteria use for growth,” Aristilde said.
The researchers then used protein analysis to identify the key enzyme that gives these microbes their plastic-eating ability. Although the enzyme differs from the PET-degrading enzyme in its protein sequence, it contains a similar binding pocket that is responsible for PET breakdown.
When the gene encoding this key enzyme was placed into microbes that do not naturally degrade PET, the engineered microbes gained the ability to do so, proving the enzyme’s functionality.
“We systematically show, for the first time, that wastewater bacteria can take the starting plastic material, degrade it, break it down and use it as a carbon source,” Aristilde said.
“It’s amazing that these bacteria can do this whole process, and we identified the key enzyme that is responsible for breaking down the plastic material. This could be used to help remove plastic from the environment,” Aristilde said.
Original research
Rebecca A. Wilkes, Nanqing Zhou, Austin L. Carroll, Ojaswi Aryal, Kelly P. Teitel, Rebecca S. Wilson, Lichun Zhang, Arushi Kapoor, Edgar Castaneda, Adam M. Guss, Jacob R. Waldbauer, and Ludmilla Aristilde (2024). Mechanisms of Polyethylene Terephthalate Pellet Fragmentation into Nanoplastics and Assimilable Carbons by Wastewater Comamonas. Environmental Science & Technology, DOI:10.1021/acs.est.4c06645
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