Unleashing Nature’s Plastic-Eating Marvels!

By Roshni Printer

 

Plastic waste management has been an ongoing global challenge, demanding both urgent attention and innovative solutions. Imagine, for a moment, that nature itself had provided us with such solutions! In the fight against plastic pollution, humans have discovered the incredible ability of worms and bacteria to naturally degrade plastic waste [1]. Let’s delve into how these creatures digest plastic, the biology behind it and the future of these discoveries.

 

Digestion of Polyethylene (PE) by Wax Worms

In 2017, Federica Bertocchini and her colleagues in Spain published a ground-breaking research paper highlighting the degradation of polyethylene (PE) plastic by a wax worm, Galleria mellonella [2]. As an amateur beekeeper, Bertocchini noticed that the wax worms, which are commonly found in beehives and feed on beeswax as pests, seemed to be able to chew through plastic bags she used to collect and dispose them. Intrigued by this observation, she decided to conduct a more systematic study to further explore the potential of wax worms as a solution for plastic waste degradation.

 

To ensure that the observation was not only due to the physical chewing motion of the worms, further experiments were conducted, in which the saliva extracted from the worms was spread on a PE film. The results showed a significant loss of mass of the PE within a few hours, which is comparable to the weathering effect generated by exposing the plastic to the environment for months or years [1]. This raised one major question: How could the wax worm saliva break the strong carbon-carbon bonds in plastic? The mechanism, which is still under scrutiny, can be attributed to enzymatic reactions [1]. PE is a plastic polymer, essentially a long-chain hydrocarbon (Figure 1). To initiate the degradation of PE, oxygen needs to be introduced into the polymeric chain to form carbonyl groups (C=O). Typically, abiotic factors like light or temperature are responsible for this crucial first step which is regarded as the bottleneck of the whole process. However, this can be accelerated by the two oxidases identified in wax worm saliva. In addition, the gut microbiome of wax worms also appears to involve in the digestion of PE, with the genus Acinetobacter suggested to be the major contributor to the effect [3].

 

Figure 1 Chemical structure of PE.

 

Degradation of Polyethylene Terephthalate (PET) by Bacteria

Around the time of Bertocchini’s discovery, a group of scientists in Japan also discovered the ability of bacteria to degrade a different type of plastic [4]. Named Ideonella sakaiensis, the bacterium was able to degrade polyethylene terephthalate (PET), the main component of plastic bottles. This bacterium produced two digestive enzymes known as PETase (or PET hydrolase) and MHETase to dismantle the polymer (Figure 2). The former acts on the ester bonds in PET, breaking the polymer down into its monomers, mono(2-hydroxyethyl) terephthalic acid (MHET); the latter further breaks down MHET into terephthalic acid (TPA) and ethylene glycol (EG). Further metabolisms enable the utilization of these compounds as energy and carbon sources by the bacterium.

 

Figure 2 Degradation pathway of PET.

To have any real impact on the degradation of plastic waste, the stability and efficiency of individual enzymes need to be tremendously enhanced – this is precisely what scientist Hal Alper has been working on [5]. Using artificial intelligence, his team ran through a database of enzymes to devise an optimal combination of mutations that would speed up the degradation of PET. When five mutations were introduced to the wild-type PETase, the resulting enzyme FAST-PETase could nearly completely degrade untreated, postconsumer-PET in one week, and work between 30 °C  and 50 °C and various pH levels. Furthermore, scientists have also successfully combined PETase and MHETase by physically connecting the two enzymes with a linker peptide to create a “super-enzyme” capable of degrading PET at a rate six times faster than using PETase alone [6, 7]. These approaches hold immense potential for accelerating the decomposition of PET, taking us one step closer to solving the real-world problem on plastic waste management.

 

More interestingly, there was another “delicious” breakthrough made by a team of scientists at Edinburgh [8]. They found an enzymatic pathway to convert post-consumer PET waste into vanillin – the main component in vanilla flavoring! Once the PET plastic was broken down into TPA and EG, genetically engineered Escherichia coli bacteria expressing five different enzymes were added to the degradation products, which results in a step-by-step synthesis of vanillin from TPA at a conversion rate of up to 79%. This biosynthetic pathway offers us with a way to upcycle plastic waste, creating a product with a higher value.

 

The race to further harness enzymes for plastic degradation is underway, and could open up the possibility for a cleaner future. With the aid of the power of nature, we are closer to turning the tables on plastic pollution.

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References:

[1] Sanluis-Verdes, A., Colomer-Vidal, P., Rodriguez-Ventura, F., Bello-Villarino, M., Spinola-Amilibia, M., Ruiz-Lopez, E., Illanes-Vicioso, R., Castroviejo, P., Aiese Cigliano, R., Montoya, M., Falabella, P., Pesquera, C., Gonzalez-Legarreta, L., Arias-Palomo, E., Solà, M., Torroba, T., Arias, C. F., & Bertocchini, F. (2022). Wax worm saliva and the enzymes therein are the key to polyethylene degradation by Galleria mellonella. Nature Communications, 13(1), 5568. https://doi.org/10.1038/s41467-022-33127-w

[2] Bombelli, P., Howe, C. J., & Bertocchini, F. (2017). Polyethylene bio-degradation by caterpillars of the wax moth Galleria mellonella. Current Biology, 27(8), R292–R293. https://doi.org/10.1016/j.cub.2017.02.060

[3] Cassone, B. J., Grove, H. C., Elebute, O., Villanueva, S. M. P., & LeMoine, C. M. R. (2020). Role of the intestinal microbiome in low-density polyethylene degradation by caterpillar larvae of the greater wax moth, Galleria mellonella. Proceedings of the Royal Society B: Biological Sciences, 287(1922), 20200112.
https://doi.org/10.1098/rspb.2020.0112

[4] Yoshida, S., Hiraga, K., Takehana, T., Taniguchi, I., Yamaji, H., Maeda, Y., Toyohara, K., Miyamoto, K., Kimura, Y., & Oda, K. (2016). A bacterium that degrades and assimilates poly(ethylene terephthalate). Science, 351(6278), 1196–1199. https://doi.org/10.1126/science.aad6359

[5] Lu, H., Diaz, D. J., Czarnecki, N. J., Zhu, C., Kim, W., Shroff, R., Acosta, D. J., Alexander, B. R., Cole, H. O., Zhang, Y., Lynd, N. A., Ellington, A. D., & Alper, H. S. (2022). Machine learning-aided engineering of hydrolases for PET depolymerization. Nature, 604(7907), 662–667. https://doi.org/10.1038/s41586-022-04599-z

[6] Knott, B. C., Erickson, E., Allen, M. D., Gado, J. E., Graham, R., Kearns, F. L., Pardo, I., Topuzlu, E., Anderson, J. J., Austin, H. P., Dominick, G., Johnson, C. W., Rorrer, N. A., Szostkiewicz, C. J., Copié, V., Payne, C. M., Woodcock, H. L., Donohoe, B. S., Beckham, G. T., & McGeehan, J. E. (2020). Characterization and engineering of a two-enzyme system for plastics depolymerization. Proceedings of the National Academy of Sciences of the United States of America, 117(41), 25476–25485. https://doi.org/10.1073/pnas.2006753117

[7] University of Portsmouth. (2020, September 28). New enzyme cocktail digests plastic waste six times faster. https://www.port.ac.uk/news-events-and-blogs/news/new-enzyme-cocktail-digests-plastic-waste-six-times-faster

[8] Sadler, J. C., & Wallace, S. (2021). Microbial synthesis of vanillin from waste poly(ethylene terephthalate). Green Chemistry, 23(13), 4665–4672. https://doi.org/10.1039/d1gc00931a