Polyethylene, which is a widely used plastic and the toughest to be degraded, has met its match. Larvae of wax moth Galleria mellonella has been shown to degrade polyethylene into ethylene glycol at an unbelievably fast rate.
Other methods of biodegrading polyethylene using a culture of fungus Penicillium simplicissimum after treating with nitric oxide and bacterium Nocardia asteroids take a very long time (three months and 4-7 months respectively) and are not efficient either.
Polyethylene is the world’s most widely used plastic (over 60 million tonnes are produced worldwide annually), primarily used to make films used in packaging and plastic bags. Very small amounts of plastic bags get recycled; they mostly end up in the ocean. It takes a very long time — 100-400 years — for polyethylene bags to be degraded.
In a paper published on April 24 in the journal Current Biology, Federica Bertocchini from the Institute of Biomedicine and Biotechnology of Cantabria (CSIC), Spain and her team found that wax worms kept in a polyethylene shopping bag formed holes in about 40 minutes. In about 12 hours, nearly 100 wax worms kept in the bag were able to reduce the mass of plastic by 92 mg. About 2.2 holes were made per worm per hour.
To confirm that the breakdown of polyethylene was not due to mechanical action of chewing, the researchers meshed the caterpillars and applied the paste on a polyethylene film. Compared with films that were not treated with the caterpillar paste, there was 13% loss of mass at the end of 14 hours in the case of the film treated with the paste. Though the loss of mass using the paste is less than when the worms were in direct contact with the polyethylene film, the average degradation rate of 0.23 mg per cm per hour is “markedly higher” than what was reported earlier.
Spectroscopic analysis carried out by the researchers showed that untreated parts of the film showed signatures of polyethylene while the treated parts of the film carried ethylene glycol signature, thus confirming the biodegradation of polyethylene into ethylene glycol.
Further studies showed that parts of the film that were treated with the paste had greater surface roughness, suggesting that the physical contact of the paste changed the integrity of the polymer surface.
“It was a chance discovery,” Dr. Bertocchini told me over telephone. “I am a beekeeper and was clearing the beehives as they were infested with worms. I put the worms in a plastic shopping bag. But soon I found the bag was full of holes and the worms were outside the bag.”
Wax worms live as parasites in bee colonies. They lay their eggs inside the hives which then hatch and grow eating the beeswax. “There is a lot of similarity in chemical structure of polyethylene and beeswax. The caterpillars are breaking down the chemical bonds of polyethylene like they do with beeswax,” she says.
“There is a possibility that the one or more enzymes of the caterpillar is causing the degradation,” she says. “We have so far not conducted any studies to analyse the faeces of the worms that made the holes. We have also not analysed the metabolism of the worms eating beeswax.”
The only catch is that the worms remain as caterpillars for only a few days. So using the caterpillars to biodegrade polyethylene might not be possible. Though the caterpillar paste can be used for biodegradation, isolating the enzymes and using them is the way to go forward for commercial-scale biodegration of polyethylene.
The team is planning to isolate the molecules responsible for degradation and study the biodegradation efficiency.