Compost Bacteria Yield the Best Plastic Eating Enzyme Yet

A neon sign, of sorts, flicks on inside a bacterium the moment its enzyme meets the right chemical bond, and a laser sorts the glowing cells from the dark ones at up to 10,000 a second. The best plastic eating enzyme in the whole search did not come from the Kenyan landfill or the tropical zoo bacteria, but from an ordinary compost heap, because compost microbes already evolve tough, heat tolerant enzymes to break down waxy plant matter and wood lignin, and those same enzymes turn out to make short work of plastic too. Researchers at Aarhus University and the Danish Technological Institute screened 29 bacterial isolates gathered from landfill soil, zoo waste, larval guts and garden compost, and pulled 12 working enzymes out of the pile. One of them, nicknamed CCPUR1, is now the most heat tolerant plastic degrading enzyme on record.

Polyurethane and nylon are two of the toughest plastics to recycle, turning up in mattress foam, shoe soles, fridge insulation, clothing, ropes and fishing nets, and both are made from crude oil rather than anything renewable. Unlike the polyester in a drinks bottle, which one enzyme discovered back in 2012 can already break down at industrial scale, polyurethane and nylon have resisted a matching biological fix.

The team’s approach, called bioprospecting, skips the usual method of trawling gene databases for enzymes that merely resemble ones already known to work. Instead, they built fluorescent probes shaped like the chemical bonds inside polyurethane and nylon, so that any bacterium carrying an enzyme able to cleave those bonds would light up under a microscope. “It was like a three-stage rocket, where we gradually narrowed them down. We started by finding bacteria that could break down the fluorescent model material. Then we isolated the enzymes capable of breaking down exactly that fluorescent material. And finally, we tested the enzymes on increvasingly realistic materials,” says Malthe Kjaer Bendtsen, a co-author at Aarhus University. Each narrowing step traded a little speed for a lot more certainty that the enzyme found could actually do the job on real plastic, not just on a laboratory stand-in.

Most of the twelve enzymes turned out to be specialists, chewing through small fragments of plastic but stalling on anything larger. Only one earned the title of genuine all-rounder.

The Compost All-Rounder

CCPUR1 comes from Chelatococcus composti, a bacterium pulled from a compost heap about ten kilometers from campus, and its enzyme carries an unusually wide binding groove that lets bulky polymer chains slot inside it. It also holds its shape at higher temperatures than any previously known plastic degrading enzyme, tolerating heat up to 68.3 degrees Celsius, which matters because compost heaps run hot enough on their own to cook off less hardy enzymes. That heat tolerance turned out to matter for performance too, since CCPUR1 broke down 0.92 percent of a thermoset mattress foam sample in three days, edging out the previous best enzyme’s 0.49 percent over the same stretch.

Getting the raw material was less elegant than the lab work that followed. “I suggested looking for suitable enzymes in places where it is warm, because bacteria thrive and grow well there all year round. And preferably somewhere plastic has been present for a long time. A year and a half later, I was on a plane to Kenya, followed by half an hour on the back of a local student’s motorbike on the way to a huge landfill,” recalls Andreas Mollebjerg, who chased warm climates around the world before the compost heap up the road turned out to hold the winner.

Against soluble stand ins for polyurethane, all ten of the polyurethane active enzymes cleaved at least the first bond, and two of them pushed the reaction all the way through to a fully separated breakdown product. The nylon side of the search was harder going: most of the twelve enzymes showed at least some activity on nylon mimicking probes, but only two, CCPUR1 and CCPUR2, went on to release measurable amounts of free amine from actual nylon-6 and nylon-6,6 textile fibers. Nylon’s own manufacturing chemistry seems to be part of the problem, since the fibers pack tightly enough that an enzyme has trouble reaching the bonds it is built to cut. Heat tolerance again separated the contenders from the pretenders: the single most nylon hungry enzyme in isolation lost most of its activity within hours at temperatures the tougher enzymes shrugged off. None of this happened in a single afternoon either, with the slowest assays running for a week or more before a result was worth reading.

Under one percent of a foam sample in three days is not, by any reasonable measure, a recycling solution. It is closer to proof that the chemistry works at all in vitro, the same modest first step every industrial enzyme has had to clear before anyone thought to speed it up, the same playbook behind turning factory exhaust into a usable fuel feedstock or treating contaminated crop waste as ore rather than trash, where a waste stream stops being purely a liability once something learns to work on it.

The Limits of What Will Culture

The screen also has a built-in blind spot: it can only find enzymes carried by bacteria willing to grow, or culture, in a lab dish, so whatever plastic degrading talent exists among the many species that refuse to culture never got a chance to glow. The researchers themselves flag this as the main limitation of the whole approach, rather than something a reader has to infer.

Work on CCPUR1 has not stopped at the compost heap. A follow-up study with the University of Porto, published separately in the journal Chem Catalysis, used computer modeling and targeted gene edits to push an improved version of the enzyme to break down about 1.4 percent of the polyurethane in a real, untreated shoe sole foam within three days. “But the research highlights the strength of enzymes. Their real advantage when it comes to recycling is that they work under mild conditions, meaning at lower temperatures and pressures than those used in most recycling technologies today,” says Rosie Graham, the Aarhus postdoc who tested the enzymes’ appetite for real plastic. The team envisions something closer to a relay than a single silver bullet, one enzyme cocktail to strip away polyurethane, then a second, nylon specific mix to finish the job, each swapped in the way a chemical plant might switch catalysts between stages.

The bottle-recycling enzyme discovered in 2012 also came out of a compost heap, which either means compost is an unusually good place to look, or that nobody has looked hard enough anywhere else yet. Either way, the mattress foam sitting in a landfill somewhere right now has, for the first time, something patient enough to start eating it.

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