A shovelful of ordinary compost from a recycling center ten kilometers from Aarhus University turned out to hold the most promising plastic-eating microbe of all. Scientists from Aarhus University and the Danish Technological Institute had traveled around the world hunting for bacteria that could break down some of the hardest-to-recycle plastics — and after digging through millions of microbes, their best discovery was practically in their own backyard.
The search was a huge experiment in bioprospecting, which just means searching nature for useful biological organisms. Researchers collected bacteria from a landfill in Kenya, a tropical zoo, the guts of wax moth larvae, and a compost heap near Aarhus. To find which microbes could actually chew through plastic, they used a clever tool called fluorescence-activated cell sorting, or FACS. They mixed the bacteria with molecules that mimicked the exact chemical bonds in polyurethane and nylon. When one of those molecules met a bacterium with a useful enzyme, it lit up like a tiny switch — so the bacteria essentially turned on the lights themselves.
From millions of candidates, the team found 12 working enzymes: ten that degrade polyurethane and two that break down nylon. Both plastics are notoriously stubborn — polyurethane shows up in foams, insulation, and shoes, while nylon is everywhere in fabric and ropes — and few natural enzymes are known to tackle them.
The winning enzyme, named CCPUR1, came from the compost heap near the university. It proved the most versatile, breaking down molecules that mimic polyurethane best. A fifth vial in the lab video, labeled PYNYL1, was also found in compost and stood out as the champion at degrading nylon, though it could not do as many jobs as CCPUR1. The researchers are now working to improve these enzymes so they can break down the plastics faster.
The journey to find them was an adventure in itself. Co-author Andreas Møllebjerg came up with the idea of looking in warm places where bacteria thrive all year round and where plastic had been sitting for a long time. A year and a half later he was in Kenya, riding half an hour on the back of a student's motorbike to a huge landfill where hundreds of people lived, collecting samples. He and co-author Malthe Kjær Bendtsen also drove 34 kilometers to Randers Regnskov Tropical Zoo and bought wax moth larvae from a pet food store to study the enzymes in their guts.
This is the first time bioprospecting has been done on such a large scale and across so many different environments to find plastic-eating enzymes. The old way was searching genome databases for genes that looked like known plastic degraders — which, as the researchers put it, is like looking for a lost key under a streetlamp because that is where the light is, rather than where you actually dropped it. By traveling to the real dirty places, the scientists may have found keys that could one day help recycle the plastics we throw away every day.