By now, everyone should be aware of the alarming levels of plastic polluting our oceans. Every year, upwards of 100 million tonnes of plastic are manufactured, with 10-20 million tonnes finding their way into our oceans. Ocean plastic is expected to triple within the next decade. It is estimated that of the 8.3 billion tonnes of plastic produced worldwide since 1953, only 9% has been recycled.

The recycling of plastics is crucial to the reduction of virgin plastic use. Not only will it save energy (it takes 88% less energy than producing plastics from raw materials) and reduce demand for petroleum-based raw materials, but it will help divert plastic streams away from landfill and oceans. Of the six main plastics used for high-rate consumables, PVC (Plasticised Polyvinyl chloride), HDPE (High-density polyethylene) and PET (Polyethylene terephthalate) are regularly recycled.

PET is the plastic commonly used to create bottles for soft drinks. Because they usually take 500-1000 years to degrade and their use is increasing at a frightening rate, they present one of the bigger plastic challenges we face. Upwards of 480 billion plastic drinking bottles were sold in 2016, an increase of 180 billion from less than a decade previous. Of these, less than half were collected for recycling and 7-14% of those collected were actually recycled, suggesting a lot of room for improvement.

Enter Ideonella sakaiensis 201-F6. Catchy name, right? Well maybe not, but don’t let that fool you. This is the first bacterium to evolve that can eat plastic, more specifically PET. A team of Japanese researchers from the University of Kyoto made the discovery in 2016 whilst sifting through plastic waste. The colony found had begun using PET as a food source.

Defined as being complex polymers, plastics are made up of long, repeating chains. Because of this, they take a long time to degrade as they don’t dissolve in water. 201-F6 works by releasing an enzyme (a specialised protein that accelerates chemical reactions), coined PETase to break down chemical bonds (called ester bonds) within these polymer chains. The purpose of this? Bacteria can then use the free carbon left over as a food source.

When this was discovered in 2016, it wasn’t the most efficient of reactions; it ate PET slowly and the ability to use it effectively to recycle PET plastics wasn’t feasible. Whilst investigating the structure of PETase this year, however, Professor John McGeehan and his colleagues tweaked the enzyme to create a much more effective version which could also degrade polyethylenefuranoate (PEF) as well as PET.

On this improvement Professor McGeehan said, “Although the improvement is modest, this unanticipated discovery suggests that there is room to further improve these enzymes, moving us closer to a recycling solution for the ever-growing mountain of discarded plastics”. Also adding that their insights had given them the blueprints to engineer even more effective enzymes.

This is just the beginning. Using PETase breaks PET back down into its original building blocks (monomers), making it more effective to recreate PET products from. Depending on the method, this has the potential to be more energy efficient than current ones such as melt filtration. However, if not undertaken in a controlled setting, this can create microplastics which are another worrying issue and may be happening to polluting plastic globally right now.

A likely method of implementing PETase would be within a bioreactor. Here, PET-based plastics will be put into a closed, controlled aqueous environment with a seed of the tweaked 201-F6. Conditions such as temperature (early papers suggest the enzyme works best at roughly 30 °C) and oxygen levels (201-F6 is aerobic) will be adjusted to the optimal level for the bacterium to grow, multiply and digest PET. The original study observed the breakdown of PET by 201-F6 within 96 hours. This will have to be improved greatly for practical purposes. Additionally, some semi-crystalline PET structures make it hard for the bacteria to access due to the compactness of the molecules. This suggests some pre-treatment for the plastics would be needed initially.

As well as reducing the demand for petroleum-based raw materials for manufacture, it will help to reduce the number of plastic bottles polluting landfill and oceans. A recent study discovered that waste plastic, under radiation from the sun, caused the release of the potent greenhouse gas methane into the atmosphere. As if we didn’t have incentive enough to sort out this problem.

The discovery of this plastic eating bacteria does, however, pose a danger. If there are enzymes that could break down PET, then there is a legitimate possibility of bacteria evolving to break down other plastics. This could become a serious problem. Imagine how detrimental it would be if bacteria started eating plastics used in the medical industry such as polypropylene (syringes), or in the construction industry such as acrylic? Not controlled properly, their effects would be undesirable, to say the least.

We still have a long way to go. The recycling of PET will require even greater optimisation of PETase, the creation of the right infrastructure to carry out the process, more stringent policies on plastic recycling and better social education. But the future is bright. Plastic eating bacteria are an exciting prospect. The ability to engineer enzymes for the better is rare, and with the opportunity to improve on progress made so far, the ceiling is high.