Bacteria infused plastic designed to eat itself

Humans produce around 350 million tonnes of plastic waste every year. And, because of its chemical composition, some of that plastic will still be hanging around in our environment for centuries to come. But a team of scientists in the United States believe they have a solution - which involves "self-digesting plastic" that is impregnated with dormant bacteria that can reawaken when the plastic is dumped and break it down. Here’s Han Sol Kim at the University of California, San Diego…

Han Sol - We wanted to mitigate plastic pollution because plastic pollution is one of the pressing environmental problems. And many plastics are escaping our recycling or collection effort and reaching the environment. So we want to make inherently biodegradable plastics that can naturally break down once they are lead to the environment.

Chris - And so what's your solution? How might we be able to achieve that goal?

Han Sol - Inspiration for this study came from the idea of pairing plastics with bacteria because bacteria can program various functions into plastics such as biodegradability.

Chris - Say that again. So you've got bacteria that can change the way plastic behaves.

Han Sol - Yes. Because some bacteria are known to break down plastics. So we envision that if we can incorporate bacteria into plastic, the bacteria will break down the plastic at the end of its lifecycle.

Chris - Will this work for any plastic or are there specific types of plastic? Because plastics come in different formulations and types, don't they?

Han Sol - That's a really good question. But for this work, we only focused on a specific plastic, which is called thermoplastic polyurethane. We screened and engineered bacteria specifically for this plastic. But I believe that if we can screen and engineer bacteria to other plastics, we will be able to expand our work to other plastics beyond thermoplastic polyurethanes.

Chris - So the trick might work more broadly. In essence, how does it work then? Talk us through what you make, what bacteria you use and how that works.

Han Sol - We used Bacillus subtilis. It's a spore forming bacteria, which means that this bacteria will be transformed into a dormant form of life when it's exposed to harsh conditions. So under this spore form, Bacillus subtilis is very stable. So we were able to compound this Bacillus subtilis spore with plastic during the extrusion of plastic.

Chris - I see. So when you are making a plastic item, you're mixing up what makes the plastic with some of these bacteria, they form these inert spores and end up embedded in the plastic product.

Han Sol - Exactly.

Chris - So how do they then know when to come to life and start breaking down the plastic?

Han Sol - So spores can remain dormant for many years until they are exposed to favourable environments that enable them to thrive. For example, nutrients in soil are good triggers for returning spores to life. So spores have several proteins, which are called germinate receptors. And once the nutrients in soil bind into this receptor, they can trigger the germination of spores. This means that spores are likely to remain inactive until they detect these nutrients, which are scars during the useful life of plastic in our daily life. But once they are littered or buried in the soil, the nutrients will wake up spores.

Chris - Wow. And how quickly do the bacteria then degrade the plastic? So if you start with a certain amount of plastic, how quickly does it break down and what does it break down into?

Han Sol - Our study shows that more than 90% of mass was lost within five months, which is twice as fast as the degradation of plastic without spores. And more than 70% of plastic was biomineralised into CO2 within six months.

Chris - So it breaks down into CO2, obviously not ideal, but it's a gas which then just goes off and potentially becomes wood in a tree, I suppose, isn't it? It's critically not a hunk of plastic left in the landfill.

Han Sol - No. We believe this bacteria is environmentally promising because Bacillus subtilis is generally recognised as safe for humans, animals, and plants. Bacillus subtilis is ubiquitous in soil, and sometimes people even eat these Bacillus subtilis spores as probiotic supplements. And Bacillus subtilis can also help the growth of plants by suppressing the growth of other pathogenic microbes. So overall, we believe that this Bacillus subtilis is environmentally promising.