Engineering Bacteria to Degrade Plastic in Salt Water

6 min read

Written by Yejin Lhee

February 27, 2024


The Great Pacific Garbage Patch. Photo taken from Forbes

Adapted from:

Plastic is everywhere. It’s what often houses our food and makes up a lot of the items that we own - it's even found in the air that we breathe and the water that we drink in the form of small bits of plastic that we can’t see with the naked eye.

The grasp plastic has on our lives can be seen in the 400 million metric tons of plastic that the world produces annually; and this number is projected to increase. From the 400 million metric tons, 0.5% (over 1 million tons) of plastic ends up in the ocean. A widely known example of plastic accumulation can be seen in the Great Pacific Garbage Patch.

Getting rid of ocean plastic

Plastic is hard to get rid of. Most of the commonly used plastics are non biodegradable and do not decompose when accumulated in landfills or the natural environment. Instead, the only way that we can deal with this huge amount of plastic is to burn it, which releases harmful toxins and gasses into the already-delicate atmosphere.

Along with the visible plastics that we can see floating in the ocean, microplastics exist as well. Microplastics are small pieces of plastic debris that are less than five millimeters in length. The small bits of plastics can float on the vast ocean water, sink to the bottom, or be eaten by marine life. To haul and get rid of the sheer amount of plastics that come in different sizes would take a mass effort that would only do more harm than good.

A glimmer of hope

Polyester terephthalate (PET) is an everyday plastic material that is 100% recyclable and are made out of long polymers, or chains of individual plastic molecules called monomers. A chain of monomers are classified as polymers. PETs are being extensively studied for their biodegradable characteristics.

To complement this biodegradable characteristic of PET plastics, scientists have also found bacteria that have PET degrading enzymes (PHEs); enzymes are proteins that help speed up the chemical reactions in an organism. Bacteria that have these PHEs are able to break the chains of plastic molecules existing in the PETs, a process called depolymerization. From these broken pieces, we can re-link the monomers to make plastics for a new purpose, which allows PET to be 100% recyclable. Repurposing already existing plastic is an energy-saving and environmentally-friendly alternative to current methods, such as burning plastic.

Scientists have since been trying to engineer PHEs in bacteria to degrade PET plastics further than they already can. However, scientists have hit some roadblocks. Current PHE holding bacteria are unable to survive in environments with high concentrations of salt, which would be a problem for plastics that are in the ocean. As well, in order for PHE to degrade plastics, the bacteria must be lysed, or broken, to release the PHEs into the environment.

Using synthetic biology

In September, Li and his team were able to solve these problems by engineering a bacteria strain with PHE to survive in salty environments by using a new bacteria and by engineering the PHEs to go to the surface of the cell, rather than inside of it.

Previously, scientists were working with Escherichia coli, a commonly used bacteria in synthetic biology due to its compliance to take up plasmids and fast growth. Li and his fellow scientist decided to use the even faster reproducing bacteria Vibrio natriegens (Vn), which is able to grow well in salt concentrations similar to that of the ocean, allowing it to be a suitable candidate for marine remediation.

The scientists then started working on engineering, or modifying the PHE itself. They first took a native PHE and modified it to include a signal molecule to send it to the surface of the bacterial cell. This modification was placed in a plasmid and inserted into the bacteria. A plasmid is a circular form of DNA that, when inserted into a host, will replicate and express itself independently; through this method, the bacteria will express the genetic instructions in the plasmid’s DNA. By sending the PHEs to the surface of the cell, the previous method of having to break the cell to release PHE is not needed anymore; this allows for the bacteria to lead a normal life cycle that includes reproduction for a more sustaining medium in breaking down PET plastics.

By introducing the PHE + surface signal modification to Vn, the researchers found that this engineered species was able to break down PET in salt water conditions without having to break the cells; this is a small step towards remediating the invasive plastics that live in our oceans.

There is still more work to be done…

While Li and his team has made significant progress in the ocean-plastic journey by making the first genetically modified organism to break down PET in ocean-like conditions, there are still many roadblocks to making this a tangible solution. The modified bacteria is not enough to completely degrade PET particles efficiently; it would take around 24 years for this engineered bacteria to completely degrade 1g/L of PET particles. As well, PETs are widely used today, but this still does not mean there are other plastic types that reside in the ocean. Another direction would be finding ways to translate these findings to degrading non-PET plastics.

Building Science

The development of PHEs that are able to withstand salt water is a very good example of how science is built upon the success of others. Science is based on collective learning. Within this story, there were scientists before Li’s team that found the PHEs necessary for Li and his team to incorporate them into a new bacterial medium. As well, without the previously developed engineering techniques, Li’s team would not have been able to find the tools they needed to carry out the engineering process. Similarly, Li’s team’s success and limitations also poses a new platform for future scientists and engineers to launch off of to hopefully create a solution to our current problems.

Science and engineering are based on using the understanding, knowledge, and questions of our predecessors that we use to create answers that ultimately, will create more questions and start the scientific cycle all over again.