Plastics revolutionised the way we live. Plastics are food-safe, easy to manufacture, and sterile. They are also durable and long-lasting. This has made plastic an economical and attractive choice in a wide range of applications.

However, the persistence of plastics is also their drawback. Some plastics take hundreds of years to begin breaking down. Despite many plastics being recyclable, it is estimated that over 80% of the plastic manufactured will end up in landfill or as pollution, choking our oceans and terrestrial ecosystems. The COVID-19 pandemic has also increased the demand for single-use plastics, further exacerbating the problem. By 2050, it is estimated that there will be more plastic in the ocean than fish by weight.

Finding more sustainable, effective methods of dealing with our plastic waste is critical to our efforts to conserve the environment.

Seven main types of plastic are manufactured globally. This includes PET, which is a clear, strong, and lightweight plastic often used to make food packaging and drinks bottles. Many PET products are single-use, contributing to the catastrophic levels of plastic pollution the planet is now experiencing.

Fortunately, some microbes can break down the molecules – called ‘polymers’ – in PET, thanks to the similarity of these compounds to naturally occurring substances. The discovery of a bacterial species that produces enzymes capable of breaking down PET opened up a world of possibility for degrading this ubiquitous type of plastic. Since then, several bacteria have been identified that can use PET as their food source.

In their research, Dr Jay Mellies and his colleagues at Reed College and Willamette University identified a group of five bacterial species that together can break down PET and the byproducts produced during its degradation.

In a more recent study, they continued this work by comparing the genomes of 232 bacterial species that are related to the five bacteria they had previously studied. By comparing the genomes of the plastic-eating bacteria to their non-plastic-eating counterparts, the researchers isolated genes that confer this ability. These genes contain the instructions for building enzymes that break down PET and the byproducts produced during its degradation.

One of the team’s discoveries was an enzyme-producing gene that plays an important role in breaking down a specific byproduct of PET degradation. They achieved this by creating a strain of bacteria without a functioning version of this gene, and comparing it to the original bacterial strain.

Confirming their predictions, only the original bacteria broke down the PET byproduct. This enabled the researchers to identify and conduct a 3D structural analysis of the enzyme responsible for this process.

In addition to identifying several genes and their associated enzymes that degrade PET, Dr Mellies and his colleagues also discovered genes implicated in degrading other types of plastics and plasticisers.

Plasticisers are substances that are added to a range of hard plastics to make them softer and more flexible. For example, PVC is a type of hard plastic that can be softened with plasticisers to make shower curtains, flexible plastic tubing, and electrical wire insulation. Plasticisers are potentially harmful substances that have been implicated in a range of human, animal, and environmental health issues. Because plasticisers are not bound within the chemical structure of plastic polymers, they can leach into the surrounding environment.

As such, developing processes that can also break down the plasticisers within plastics is imperative. The researchers identified a range of genes in their group of bacteria that have the potential to break down plasticisers. The full suite of bacteria studied exhibited an incredible level of functional diversity and ability to degrade a range of polymers and plasticisers.

An important function involved in the effective degradation of PET is the ability of bacteria to adhere to a plastic surface and form a layer. This thin layer of bacteria is called a ‘biofilm’. Dr Mellies and his colleagues measured biofilm production in the bacteria they studied, and discovered a range of abilities.

The ability to produce biofilms is related to the production of a substance called ‘surfactin’, which triggers biofilm formation. The team recommend that including bacterial strains with enhanced surfactin production could increase the efficiency of processes developed to degrade mixed-plastic waste.

Dr Mellies suggests that the full suite of bacteria they investigated could together possess the ability to degrade the full range of plastic types, thanks to their functional diversity. His team’s research holds promise for developing mixed-plastic waste degradation using a specially selected group of bacterial species.

Plastic-eating bacteria could help us to achieve complete and safe plastic degradation, which is vital for solving the mounting global problem of plastic pollution.