Plastic-eating bacteria: how natures recycler works and what it means

01What plastic-eating bacteria are

Plastic-eating bacteria are microorganisms capable of breaking down synthetic polymers that were previously considered non-biodegradable. The most notable discovery was made in 2016, when researchers in Japan identified a bacterium called Ideonella sakaiensis that can digest polyethylene terephthalate, or PET, the plastic commonly used in water bottles and food containers. The bacterium uses specialized enzymes to degrade PET into its component monomers, which it then uses as a carbon source for growth.

The discovery was significant because PET is one of the most produced plastics in the world, and its resistance to biological degradation is a major contributor to plastic pollution. Before this discovery, the only known mechanisms for breaking down PET were chemical and thermal processes, which are energy-intensive and produce lower-quality outputs. The identification of a biological pathway opened the possibility of enzymatic recycling, which could operate at lower temperatures and produce higher-quality recovered monomers.

02How the enzymes break down plastic

Ideonella sakaiensis produces two key enzymes that work in sequence to break down PET. The first, called PETase, cleaves the long PET polymer chains into smaller units, primarily producing mono(2-hydroxyethyl) terephthalic acid, or MHET. The second enzyme, called MHETase, further breaks down MHET into the two constituent monomers of PET: ethylene glycol and terephthalic acid. These monomers can then be used by the bacterium as nutrients or recovered for industrial reuse.

The enzymes work by targeting the ester bonds that link the repeating units of the PET polymer. This is a hydrolytic process, meaning it uses water to break chemical bonds. The enzymes are secreted by the bacterium and act on the surface of the plastic, gradually etching and degrading the material. The process is relatively slow compared to chemical recycling, but research has focused on engineering the enzymes to improve their speed, stability, and range of plastic types they can process.

03Which plastics can be degraded

The plastics that can be degraded by biological organisms are currently limited. Ideonella sakaiensis and its enzymes primarily target PET, which is one of the easier plastics to break down because its ester bonds are susceptible to hydrolysis. Other plastics, such as polyethylene, polypropylene, and polystyrene, have stronger carbon-carbon bonds in their backbones that are much more resistant to enzymatic attack.

Research has identified other microorganisms and enzymes that can degrade different plastics. Some fungi can break down polyurethane. Certain bacteria have been found to degrade nylon. Engineered variants of PETase have shown activity against other polyesters. However, the major commodity plastics that dominate global production and pollution, particularly polyethylene and polypropylene, remain largely resistant to biological degradation with current technology.

04The speed and efficiency of degradation

The degradation speed of naturally occurring PETase is relatively slow, taking days to weeks to break down a small piece of PET. This is one of the main limitations for practical application. Since the discovery of Ideonella sakaiensis, researchers have engineered improved variants of PETase that are significantly faster and more thermostable. A 2020 study described an engineered PETase variant that was more effective at degrading PET than the natural enzyme.

In 2022, researchers at the University of Texas at Austin reported a machine-learning-engineered PETase variant that could degrade PET in as little as 24 hours at elevated temperatures. This represented a significant improvement over the natural enzyme and brought enzymatic recycling closer to commercial viability. However, scaling from laboratory conditions to industrial throughput remains a substantial engineering challenge.

05The challenges of scaling up

Scaling up enzymatic plastic recycling from laboratory demonstrations to industrial processes faces several challenges. The enzymes need to be produced at large scale and low cost. The plastic waste needs to be sorted, cleaned, and pretreated to make it accessible to the enzymes. The reaction conditions, including temperature, pH, and enzyme concentration, need to be optimized for throughput rather than yield. And the recovered monomers need to be purified to a standard that allows repolymerization into new plastic.

Several companies are working on commercializing enzymatic recycling. Carbios, a French company, has built a demonstration plant that uses an engineered enzyme to depolymerize PET at scale. The process operates at higher temperatures than the natural enzyme can tolerate, using a thermostable variant developed through protein engineering. The economic viability depends on the cost of enzymes, the throughput of the process, and the market value of recovered monomers compared to virgin plastic.

06The environmental implications

The environmental implications of plastic-eating bacteria are potentially significant but must be understood in context. Enzymatic recycling is not a replacement for reducing plastic production or improving mechanical recycling. It is a complementary technology that could handle plastic waste that is difficult or impossible to recycle mechanically, such as contaminated or mixed PET waste. The recovered monomers could reduce demand for virgin plastic produced from fossil fuels.

There are also potential risks to consider. Engineered enzymes or bacteria released into the environment could have unintended consequences, though most industrial processes use contained bioreactors rather than environmental release. The focus of commercial development is on controlled industrial processes, not on releasing organisms into the environment to degrade plastic waste in situ. The environmental benefit comes from replacing energy-intensive chemical recycling and reducing landfill and incineration of plastic waste.

07What the future of bacterial recycling looks like

The future of bacterial recycling is likely to involve a combination of engineered enzymes, optimized bioreactor designs, and integration with existing waste management infrastructure. The technology is most promising for PET recycling in the near term, with potential expansion to other plastics as new enzymes are discovered or engineered. The use of machine learning to design improved enzyme variants is accelerating development, and commercial plants are expected to increase in number and scale over the coming years.

The broader question is whether biological recycling can become economically competitive with virgin plastic production. The answer depends on factors including enzyme cost, process efficiency, energy prices, and regulatory policies that internalize the environmental costs of plastic pollution. If the economics work, enzymatic recycling could become a significant component of the circular economy for plastics. If they do not, the technology may remain a niche solution for specific waste streams. Either way, the discovery of plastic-eating bacteria has opened a new approach to one of the most persistent environmental challenges.