Scientifically Speaking | Will microbes engineer our future?
With effective commercialisation and scaling strategies, industries might be able to convert discarded plastics into eco-friendly products or raw materials
Plastic pollution is one of our most pressing environmental crises. The convenience of plastics, from take-out containers to smartphone cases, hides their dark side. These synthetic polymers can take centuries to degrade, causing a buildup that threatens marine life, terrestrial habitats, and human health.
The scale of the challenge is daunting. Every year, a staggering 359 million tonnes of plastic are produced worldwide. Of this, 150-200 million tonnes end up either in landfills or polluting the natural environment. One of the most widely used plastics is known as polyethylene terephthalate, or PET for short. Found in clothes and packaging materials, nearly 70 million tonnes of PET are made annually.
Can microbes break down plastic? This is a question that scientists have been trying to answer since the 1970s. Only recently have we had any success.
In 2001, at a rubbish dump, Japanese scientists led by Kohei Oda from the Kyoto Institute of Technology stumbled upon a film of previously unknown bacteria consuming plastic material. Rather than merely attacking the plastic's surface as Oda had anticipated, these bacteria were breaking down plastic entirely, converting it into basic nutrients. These bacteria were extracting carbon from plastic, utilizing it for energy and growth.
Yet, when Oda and his team made their discovery in 2001 the term “microplastic” hadn’t even been coined. Due to a perceived lack of interest, the team's initial research on the bacteria went unpublished.
Collaborating with his student Kazumi Hiraga, Oda resumed experiments and, in 2016, they published their findings in the journal, Science. The bacterium they discovered, which they named Ideonella sakaiensis after the city of Sakai where it was found, made a specific enzyme that helped to break down PET.
This was the impetus the field needed. Very soon, other plastic-degrading bacteria were uncovered around the planet – from a German cemetery compost heap to debris on Greek beaches.
Finding these plastic-chomping bacteria and their enzymes was only the beginning. To make them useful for commercialization they needed to be optimized to improve their efficiency, flexibility, and ability to function in different temperatures and environments.
In April 2020, a team of researchers led by I André, S Duquesne and A Marty published a significant advance towards this goal in the journal, Nature. The researchers engineered a super-efficient enzyme that could break down PET quickly and effectively. In simple terms, this enzyme could take a PET plastic and, in just 10 hours, break down about 90% of it into its basic building blocks.
This performance was much better than previous solutions, even outdoing Ideonella sakaiensis. The team also showed that once PET was broken down, it could be recycled into new, high-quality PET – just as good as the original. This meant that instead of always making new PET, companies could reuse it, creating a circular and sustainable system.
Then, in April 2022, a team of scientists led by Hal Alper in the US published an article in Nature demonstrating a new way to design enzymes that break down PET. They designed a new enzyme, which could degrade a wide variety of PET products in just a week using machine learning (a kind of artificial intelligence). Essentially, the team used machine learning to predict what modification to enzymes would make them more stable and effective. After testing, they found an enhanced enzyme with five changes from the original version, which broke down PET more efficiently than other versions. They also showed that they could convert coloured PET into near-equal amounts of clear PET. In other words, plastic waste could readily be turned back into useful, high-quality plastic.
By 2023, the field has advanced even further.
Last month a study in the journal Nature Communications by James Collins, Ting Lu, and their colleagues described a microbial community that efficiently degraded and upcycled PET. Upcycling is distinct from regular recycling since it turns waste material into something that is more beneficial or of higher quality.
PET plastic, when degraded, splits into two main chemicals: terephthalic acid and ethylene glycol. These chemicals have distinct characteristics, which pose challenges for microbes that typically struggle with breaking apart various food sources simultaneously.
The research team harnessed two strains of Pseudomonas putida, a bacterium frequently found in polluted water and soil. The researchers engineered one strain to consume terephthalic acid and the other to feast on ethylene glycol – two products that result from PET degradation. In comparison, a single strain designed to eat both PET components was slower and less efficient.
The team then incorporated various genes into the bacteria, enabling them to transform the digested plastic waste into new, useful materials. One of these materials was a biodegradable polymer. The team also got the bacteria to make a chemical frequently used in crafting plastics and nylon.
Reflecting on advancements from the discovery of Ideonella sakaiensis to engineering solutions, the future seems promising. With effective commercialisation and scaling strategies, industries might be able to integrate plastic-eating bacteria or enzymes into waste management systems, converting discarded plastics into eco-friendly products or raw materials.
And this is just the beginning. There are other kinds of plastics apart from PET which pollute the environment too. By integrating artificial intelligence with synthetic biology, scientists might soon be able to identify or engineer microbes and enzymes for these different plastics.
In the 1967 movie, The Graduate, there is a line that resonated with movie-goers of the day: "The future is plastics." Fast forward to today, and we could say that the future is in microbes that decompose plastics.
Anirban Mahapatra is a scientist by training and the author of a popular science book on COVID-19. The views expressed are personal.
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