Polyethylene is one of the most resistant and used plastics in the world. It is present in supermarket bags, packaging, plastic wrap, pipes… However, once discarded it is very difficult to remove. It can last for decades intact, posing a threat to the environment and the health of the planet. In recent years, scientists have turned to different microorganisms capable of breaking down plastic polymers and accelerating their degradation, but only a handful have been effective. Of all of them, the wax worm (Galleria mellonella) has been revealed as the fastest. The larvae get break down plastic in record time, barely an hour, and at room temperature. The secret, as researchers from the CSIC have just announced, is in their saliva.
This worm, parasite of hives, is already an old acquaintance. Its ability to break down plastic was discovered in 2017. But then researchers didn’t know how it did it. Some studies started from the assumption that the worms can use the plastic as food, so that the degradation would be the result of their metabolic activity and the digestive process. “But this assumption is highly questionable,” says Federica Bertocchini, from the Center for Biological Research (CIB-CSIC) and head of the study.
Bertocchini’s team focused from the start on the oral cavity of worms. They observed how they behave in the presence of polyethylene and discovered that the enzymes present in the worm’s saliva (that is, the liquid collected from the insect’s mouth) are capable of degrading it. “The polymer in contact with saliva oxidizes and depolymerizes in a few hours,” says the researcher. The results of the work, pending review, have been published in preprint in the BioRxiv repository.
without prior treatment
Furthermore, these worm saliva enzymes are the first and only known enzymes capable of degrading polyethylene without pre-treatment. “To degrade the plastic it is necessary for oxygen to penetrate the polymer (in the plastic molecule). This first oxidation step, which is normally the result of exposure to sunlight or high temperatures, is a bottleneck that slows down the degradation of plastics such as polyethylene, one of the most resistant”, explains Bertocchini. “For this reason, under normal environmental conditions, plastic takes months or years to degrade,” adds the researcher.
But the enzymes in the waxworm’s saliva perform this crucial step: they oxidize the plastic. “Thus, they allow to overcome the bottleneck of plastic degradation and accelerate its decomposition,” he adds.
In addition, the team has analyzed the saliva with electron microscopy and has observed a high protein content. These are two enzymes, named Demetra and Ceres, which belong to the family of phenol-oxidase enzymes. The first “showed a significant effect on the polyethylene, leaving marks (small craters) visible to the naked eye on its surface. The second also oxidizes the polymer, although without leaving visible marks.
Phenols are molecules that plants use as a defense against potential enemies, such as insect larvae. Therefore, insects could produce phenol oxidase enzymes as a way to oxidize plant phenols, thus neutralizing them, allowing them to feed on plants safely. Phenols are also present in many plastic additives, which could make them targets for these enzymes and create the necessary conditions for oxidation and depolymerization of the plastic. “To date, this is just speculation and further experiments will be needed to delve into the mechanism of enzymatic action,” the researchers warn.
How the wax bugs acquired this ability is also unclear. The researchers believe that it could be due to an evolutionary process. Wax worms feed on beehive wax and pollen from a wide variety of plant species. Taking into account that beehive wax is full of phenols, this type of enzyme would be very useful for worms. Indirectly, this would explain why waxworms can break down polyethylene. Whether this is so or not, the finding by the CSIC researchers could have numerous applications in the treatment or recycling of plastic waste.