A four-environment bacterial screen turned up 12 enzymes that cut polyurethane and nylon

The conventional route for polyurethane and nylon is homology search: comb genome databases for gene sequences similar to those in enzymes already known to do the job. The release notes those are few ^[6].

The Danish group screened for the enzymes directly. Cells were mixed with molecules that imitate the chemical bonds in the two plastics and light up on contact with a bacterium carrying a suitable enzyme ^[9]. A cell sorter then pulled out the glowing cells at up to 10,000 per second ^[10]. Ten million cells pass through a machine at that rate in about 17 minutes ^[15].

Malthe Kjaer Bendtsen, a doctoral student and co-author, described the funnel. "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 increasingly realistic materials," he said ^[11].

One of those realistic materials was polyurethane mattress foam, in powdered form ^[12]. Grinding raises the surface area available to an enzyme per gram, so a foam block is the harder test ^[17]. The release counts the enzymes that worked. It does not report how fast they worked or how much material they degraded ^[18].

The sampling design came from a hunch about temperature. "I suggested looking for suitable enzymes in warm places 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. He helped me collect samples and communicate with the hundreds of people who lived on and off the landfill," said Andreas Moellebjerg, a co-author who handled collection ^[8]. The other trips were shorter: 34 kilometres to Randers Regnskov Tropical Zoo for plastic waste, a bag of wax moth larvae from a pet food store, and a shovelful of compost from a recycling centre roughly 10 kilometres from the university ^[14]. Of the four environments, the one nearest the lab produced the best candidate ^[16].

Both plastics are hard to degrade ^[13]. That is what makes the hit list interesting at all. This stage of the work leaves open whether the remaining obstacle is enzyme engineering and process economics; that depends on activity numbers on unground material. The team's stated next step is to improve the compost enzyme's ability to break down polyurethane and nylon ^[4]. The paper is in Angewandte Chemie ^[7].