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Plastic recycling: waste-eating bacteria emerges as new solution

© Shutterstock / Teerasak LadnongkhunA pile of discarded plastic bottles.
A pile of discarded plastic bottles.

Researchers from Northwestern University have identified the metabolic processes that enable naturally occurring bacteria to recycle plastic waste.

  • The bacteria feed on carbon atoms contained within plastic waste and convert it into polymers that could be used to produce virgin materials. 
  • Previous attempts to develop biological recycling have relied on costly and laborious interventions. 
  • With its unique metabolic processes, the researchers believe that C. testosteroni could enable biological recycling to be adopted at scale. 

A team of researchers, co-led by Rebecca Wilkes and Ludmilla Aristilde of Northwestern University in Illinois, has identified the metabolic pathways through which plastic waste can be broken down by a common form of soil bacteria. With this new information in hand, the researchers believe that biological recycling platforms could be developed as a scalable solution to the plastics crisis. 

“Soil bacteria provide an untapped, underexplored, naturally occurring resource of biochemical reactions that could be exploited to help us deal with the accumulating waste on our planet,” said Aristilde. “The power of microbiology is amazing and could play an important role in establishing a circular economy.” 

Nature’s plastic recycling 

Wilkes and Aristilde worked alongside researchers from the University of Chicago, Oak Ridge National Laboratory and the Technical University of Denmark to study the metabolic pathways of C. testosteroni – a species of bacteria that is abundantly present in soils and sewage sludge. Their findings, published in Nature Chemical Biology, explore how these natural processes could be leveraged in the recycling of complex waste.

C. testosteroni feed on carbon atoms, including those found within plastics or lignin – the fibrous component of woody plants. As these atoms are contained within complex compounds, which cannot be broken down by most types of bacteria, the researchers sought to identify the specific pathways that enable C. testosteroni to do so. By combining a number of analytical techniques, they were able to map the process from beginning to end.

“We started with a plastic or lignin compound that has seven or eight carbons linked together through a core six-carbon circular shape forming the so-called benzene ring,” Aristilde explained. “Then, they break that apart into shorter chains that have three or four carbons. In the process, the bacteria feed those broken-down products into their natural metabolism, so they can make amino acids or DNA to help them grow.”   

They went on to discover that C. testosteroni can direct the carbon it consumes through a variety of metabolic routes, each of which results in different by-products that could prove useful in industrial applications. For example, certain metabolic pathways produced biopolymers that could replace the fossil fuel-derived chemicals used in the production of virgin plastics. 

This discovery suggests C. testosteroni could be immensely valuable in the development of a circular plastics ecosystem, through which waste is not only broken down but also converted into recycled ingredients. The researchers’ next steps will be to investigate the factors that determine which metabolic pathway the bacteria use, with the goal of developing certain ‘triggers’ that prompt it to take the most desirable option. 

Building on previous research 

The concept of bacterial recycling is nothing new, as it has long been known that certain forms of bacteria can feed on the carbon component of plastic. Even Escherichia Coli, more commonly known as E. Coli, can perform this function when its preferred carbon source is unavailable. 

In its natural state, however, E. Coli feeds on different types of sugar. If there are any sugars within its surrounding environment, then they will be consumed while plastic waste is left behind. 

Scientists have attempted to navigate this issue through biological engineering, which enables them to modify the behaviour of bacterial cells. Although this approach has shown great promise, it can often be complex, costly and laborious. 

These problems could be sidestepped with the use of C. testosteroni. Its metabolism is uniquely designed for the digestion of complex forms of carbon, such as those found within plastic waste, rather than simple sugars. As such, no human intervention is necessary. 

“Engineering bacteria for different purposes is a laborious process,” Airstilde explained. “It is important to note that C. testosteroni cannot use sugars, period. It has natural genetic limitations that prevent competition with sugars, making this bacterium an attractive platform.” 

A scalable solution to plastic waste? 

With recycling rates standing at just 9%, plastics pollution has become one of the greatest challenges of the 21st Century. Our plastic waste is finding its way into every sphere of the ecosystem, with drastic consequences for the health of animals and humans alike. 

Still, we continue to produce more plastic. This not only adds to the ever-increasing burden of waste, it also requires the extraction and consumption of fossil fuels. Indeed, suggest that plastics production accounts for around 4% of annual oil and gas demand. 

As the plastics crisis has continued to evolve, a number of recycling technologies have been developed. According to the latest research, however, even the most advanced of these methods has its limitations. 

Evidently, there is an urgent need for scalable solutions that are environmentally sustainable, technically feasible and economically viable. Given its natural abundance, the researchers claim that C. testosteroni may well be the answer. 

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