These microplastics are increasingly being seen as a major threat to environmental health, their accumulation is thought to be responsible for the death and destruction of marine life and is even being found in fish for human consumption. It is therefore suspected that we are eating microplastics and it is not yet known what effect that will have on our health; however it is unlikely to be good!
So what does this have to do with microbiology?
Well, I was getting to that!!
In 2016 a team in Japan lead by Yoshida discovered a new bacterium, Ideonella sakaiensis. This discovery was exciting for a number of reasons; firstly it is always exciting to us Microbiologists when a new bacterium is discovered (yep, it really is!), secondly of all the places this bacterium could be discovered it was in a waste reprocessing plant, and thirdly the bacterium was growing on and actually breaking down PET.
This is really exciting as it may represent a way of reprocessing PET without it being discharged into the environment and causing such massive concerns. Bacteria, you’ gotta love um!
I. sakaiensis is able to breakdown PET because it produces 2 enzymes which degrade the PET compound.
PET is made from ethylene glycol and terephthalic acid (TPA). The enzymes produced by I. sakaiensis reverse this process to produce the original parent compounds ethylene glycol and TPA. Of course science needs some long-winded compound names and lots of acronyms for this process when in fact it has just two-steps but I think the process is actually really quite neat.
The first enzyme is called PETase, or PET-digesting enzyme. This breaks PET into mono(2-hydroxyethyl)-TPA (MHET) with additional trace amounts of TPA and bis(2-hydroxyethyl)-TPA. I did warn you about the names! The second enzyme MHETase, or MHET-digesting enzyme, breaks MHET down into TPA and ethylene glycol.
So the whole process from production of PET to its final breakdown results in the very same molecules that you started with, and that is very neat as there are no additional new compounds that might be of further harm to the environment.
In the original Japanese study a colony of I. Sakaiensis took about 6 weeks to degrade a thin film of PET; okay it’s not a rapid process at the moment but it’s a start and much better than the hundreds of years which it would normally take to breakdown plastics in the environment.
What happened next… a lucky accident?
Like all good scientific discoveries (think penicillin)… a mishap occurred! A group from Portsmouth University in the UK was trying to expand upon the Japanese discovery. Whilst studying the mechanism by which I. Sakaiensis breaks down PET and trying to establish the exact structure of the enzymes it produces using X-ray scanning, the team from Portsmouth “accidentally” created a mutant form of the PETase (first enzyme) which had enhanced activity against PET, although they don’t say by how much the activity was enhanced. I particularly like the quote from the Lead scientist, Professor John McGeehan, who said "serendipity often plays a significant role in fundamental scientific research, and our discovery here is no exception”. I would add that whilst the creation of the new enzyme might have been a lucky accident, the recognition of this mutation and its relevance was no accident and shows the skills of the researchers at Portsmouth; but humility is always a worthy trait.
The new enzyme created in Portsmouth not only had enhanced activity against PET but it was also active against poly(ethylene furanoate) (PEF). PEF is currently being pursued as the replacement for PET. It is hailed as the “environmentally friendly” version of PET as it is produced using a different starting block to TPA (which is a petroleum product) called 2,5-Furandicarboxylic acid. This is made from naturally occurring carbohydrates and, as it doesn’t use TPA, PEF is said to produce less greenhouse gases than PET hence its “green credentials”. As with many manmade things, it’s not really that “green”; a discarded plastic bottle made of PEF will still cause the same environmental pollution as PET but at least the PETase enzyme from Portsmouth can now also break this plastic down.
What does the future hold?
The researchers from Portsmouth say that their accidental creation of a more active PETase shows that there is more scope to further enhance the ability of PETase to breakdown PET. They believe with a more systematic and structured approach to modifying its structure the enzyme could become even more active. This might allow an industrial scale process whereby the PETase enzyme is used to breakdown PET in recycling centres in a more cost effective way. It may also allow for the processing of existing PET in the environment through the “deliberate contamination” with I. Sakaiensis to speed up the removal of PET. HOWEVER I’m not convinced by this last argument, from what I can see every time humans have tried to “modify” their surrounding it has almost always back-fired (e.g. the “planned” introduction of grey squirrels in the UK, rabbits in Australia, and ferrets, stoats and weasels in New Zealand).
1. A bacterium that degrades and assimilates poly(ethylene terephthalate) Yoshida S, Hiraga K, Takehana T, et al. Science 11 Mar 2016:Vol. 351, Issue 6278, pp. 1196-1199
2. Characterization and engineering of a plastic-degrading aromatic polyesterase Austin H, Allen M, Donohue B, et al. PNAS published ahead of print April 2018.