Plastics, pollution and a new bacterial hope for recycling

There have been growing concerns over the amount of waste plastic that is being released into the environment, especially into our oceans, causing enormous harm to aquatic life and contaminating our food chain. For instance, tiny plastic particles, known as microplastics, are ingested by sea life and by this route enter the human food chain.

Plastics are used globally for food and goods packaging on an immense scale. Despite attempts to recycle plastic waste products, the difficulty and associated costs, combined with the length of time it takes for plastic to naturally degrade, have led to vast areas being polluted by plastic waste. Measures are being introduced in many countries to reduce the amount of disposable plastics used by the general population. However, with already so much plastic amassed, which will not naturally decompose for hundreds of years, more is needed to be done to tackle this environmental crisis.

The plastic commonly used to make items such as disposable plastic bottles, and synthetic fibres for clothing is polyethylene terephthalate (PET). PET is produced from raw materials extracted from crude oil, and is composed of water-insoluble polymer chains. PET can be recycled, although current recycling methods only convert these plastic bottles into opaque fibres for use in carpets or clothing, rather than back into clear plastic bottles. Therefore to produce new plastic bottles, more crude oil is used and additional plastic is generated only to be eventually discarded.

Fortunately, a breakthrough discovery was made in 2016 at a Japanese plastic recycling plant. A strain of bacteria was discovered by scientists which was living on the plastic held within the plant. This bacteria named Ideonella sakaiensis 201-F6 had evolved to be able to enzymatically digest PET, a completely man-made synthetic product, to produce smaller water soluble molecules it could metabolize. This action is accomplished by two enzymes contained within the bacteria, PETase and MHETase. PETase splits the bonds holding the polymer together to produce mono(2-hydroxyethyl) terephthalic acid (MHET). MHET is then further broken down to terephthalic acid and ethylene glycol by MHETase.⁽¹⁾

This finding has led scientists to harness these enzymes, which give the bacteria its plastic-eating properties, in order to speed up the degradation of our plastic waste. Ideonella sakaiensis can break down PET in a couple of days, but ideally this would still need to be accelerated in order for it to be used on an industrial scale. Structural studies on the enzymes PETase and MHETase have been performed by groups across the globe^(2,3,4) and researchers have demonstrated that modifications engineered into PETase are able to speed up the chemical breakdown of PET⁽⁵⁾. If PET can be broken down into its constituent parts (terephthalic acid and ethylene glycol) inside bio-reactors and recycled back into PET then in theory we can end further PET generation from crude oil, and reduce the amount of waste plastic entering the ecosystem.

References and further reading:

Yoshida, S. et al (2016) A bacterium that degrades and assimilates polyethylene terephthalate). Science. 351 (6278):1196-1199.
Palm, G.J. el al (2019) Structure of the plastic-degrading Ideonella sakaiensis MHETase bound to a substrate. Nat Commun. 10:1717.
Joo, S. et al (2018) Structural insight into molecular mechanism of polyethylene terephthalate) degradation. Nat Commun. 9:382.
Han, X. et al (2017) Structural insight into catalytic mechanism of PET hydrolase. Nat Commun. 8:2106.
Austin H.P. et al (2018) Characterization and engineering of a plastic-degrading aromatic polyesterase. Proc Natl Acad Sci. 115 (19):E4350-E4357.

This article originally appeared on Pivotal Scientific.