How a Special Type of Worm Can Quickly Break Down Plastics

The saliva of wax worms has enzymes that can quickly start the breakdown of polyethylene, according to research done by the CSIC.

The wax worm saliva breaks down plastic, according to a CSIC study team. Numerous ways to handle or recycle plastic trash might be used in conjunction with the finding. In 2017, the group made the initial discovery that this lepidopteran worm species, Galleria mellonella, could break down polyethylene. Its saliva includes phenol oxidase family enzymes that may rapidly begin polyethylene breakdown at ambient temperature, and this is how they have now discovered how it achieves it. These enzymes, according to the study's principal investigator Federica Bertocchini, a CSIC researcher at the CIB-CSIC (Centre for Biological Research), are the first and only ones currently known to be able to degrade polyethylene plastic without the requirement for pre-treatment. While it is being reviewed, the study's results have been made available online on the BioRxiv library. The funding for the study came from the German Roechling Foundation.

“For plastic to degrade, oxygen must penetrate the polymer (the plastic molecule). This is the first step in oxidation, which is usually a result of exposure to sunlight or high temperatures, and represents a bottleneck that slows down the degradation of plastics like polyethylene, one of the most resistant polymers,” according to Bertocchini. “That is why, under normal environmental conditions, plastic takes months or even years to degrade,” she continues.

“Now we have found out that enzymes in the wax worm’s saliva perform this crucial step: they oxidize the plastic. This means they can overcome the bottleneck in the plastic degradation process and accelerate its decomposition,” she claims.

Polyethylene is one of the most durable and widely used polymers. When paired with polypropylene and polystyrene, it accounts for 70% of the production of all plastics. There is a pressing need for answers to the plastic waste problem since plastic pollution threatens global health and the environment. One of the areas of research with the most promise and potential is the biological breakdown of plastics. Microorganisms like fungus and bacteria are involved in this process, which is referred to as biodegradation. However, it is known that very few microbes can break down the robust plastic polymers that make up polyethylene. Additionally, significant pre-treatment is typically necessary to assure oxidation and let the microbes to have some (although slow) impact on the plastic.

Plastic-eating worms

The finding that some bug species of the Lepidoptera and Coleoptera groups are capable of degrading polyethylene and polystyrene opened up a new area of study a few years ago.  “In our lab, we discovered the insect that seems to be the fastest of all: the larvae of the lepidopteran Galleria mellonella, commonly known as the wax worm,” according to Bertocchini. “These larvae are able to oxidize and break down the polymers in the plastic really quickly,” (after just one hour’s exposure).

“In recent years, efforts have been made to find out how these insects manage to do this. Numerous studies have focused on the microorganisms inhabiting the digestive system of these worms, based on the assumption that the worms can use plastic as food and that its degradation would be the result of their metabolic activity and digestive processes,” the researcher says. “But this assumption is highly questionable so, from the start, our research has focused on the worm’s oral cavity,” she adds.

“We have scrutinized the wax worm’s behavior when it comes into contact with polyethylene and found that the enzymes present in the worm’s saliva (i.e., the liquid inside the insect’s mouth) can degrade polyethylene,” Bertocchini tells us. “On coming into contact with saliva, the polymer oxidizes and depolymerizes within a few hours. We have identified degraded residues that form in the presence of the worm’s saliva,” she claims.

The researchers also used electron microscopy to investigate the saliva and found that it contained a lot of protein. “We have isolated two enzymes from the saliva that can reproduce the oxidation produced by the saliva as a whole,” she continues. These two proteins, Demetra and Ceres, are members of the phenol oxidase enzyme family.

“We found that the Demetra enzyme had a significant effect on polyethylene, leaving marks (small craters) on the surface of the plastic, visible to the naked eye; this effect was also confirmed by the appearance of degradation products formed after exposure of the polyethylene to this enzyme. The Ceres enzyme oxidizes the polymer too, but does not leave visible marks, suggesting that the two enzymes have a different effect on polyethylene,” she concludes.

How phenol oxidase enzymes work

Phenols are chemicals that plants utilize to protect themselves against possible adversaries like insect larvae. In order to oxidize plant phenols and so neutralize them, insects may therefore generate phenol oxidase enzymes, allowing them to safely feed on the plants. Many plastic additives include phenols as well, which might make them targets for these enzymes and establish the prerequisites for the oxidation and depolymerization of the plastic. 

“So far, this is just speculation and further experiments will be needed to investigate the enzyme’s mechanisms of action in-depth,” researchers warn.

How wax worms have developed this skill is an even more intriguing subject. It may be caused by evolution, according to researchers' conjecture.

“Wax worms feed on hive wax and pollen from a wide variety of plant species. Considering that hive wax is full of phenols, this type of enzyme would be very useful to these bugs. Indirectly, this would explain why wax worms can break down polyethylene. However, so far this theory is only speculation and we must carry out more research combining insect biology with biotechnology."

Reference: “Wax worm saliva and the enzymes therein are the key to polyethylene degradation by Galleria mellonella” by A. Sanluis-Verdes, P. Colomer-Vidal, F. Rodríguez-Ventura, M. Bello-Villarino, M. Spinola-Amilibia, E. Ruiz-López, R. Illanes-Vicioso, P. Castroviejo, R. Aiese Cigliano, M. Montoya, P. Falabella, C. Pesquera, L. González-Legarreta, E. Arias-Palomo, M. Solà, T. Torroba, C.F. Arias and F. Bertocchini, 8 April 2022, BioRxiv.

DOI: 10.1101/2022.04.08.487620