The global proliferation of synthetic polymers has resulted in an environmental challenge of immense scale, with hundreds of millions of tons of plastic waste produced annually. The durability that makes plastic useful also ensures its persistence in the environment, leading to widespread pollution and the formation of pervasive microplastics. Scientists are exploring biological solutions, harnessing the natural capabilities of certain bacteria and fungi to fundamentally change how plastic waste is processed. This emerging field of microbial degradation offers an alternative recycling pathway using specialized biological agents.
Defining Plastic-Degrading Microbes
The scientific search for organisms capable of consuming plastic often focuses on environments where plastic waste has accumulated, such as landfills and recycling facilities. These locations provide a selective pressure that encourages the growth of microbes able to use synthetic polymers as a food source. A significant breakthrough occurred with the 2016 discovery of the bacterium Ideonella sakaiensis at a plastic bottle recycling site in Japan. This organism utilizes polyethylene terephthalate (PET)—the polyester commonly used in beverage bottles and clothing fibers—as its primary source of carbon and energy.
While PET is the most studied target, research has identified a diverse group of microbes that act on other common plastic types. Certain bacteria and fungi degrade polyester-polyurethane (PUR), a polymer used in items like foam insulation and synthetic leather. For example, species of the genus Pseudomonas employ different enzymes to break down PUR’s chemical bonds. These capabilities are often repurposed from processes microbes use to break down natural polymers, which share structural similarities with synthetic plastics.
The Science of Plastic Digestion
The mechanism by which these microbes consume plastic centers on specialized protein molecules called enzymes, which are secreted outside the cell to initiate the breakdown. In the case of Ideonella sakaiensis acting on PET, a two-enzyme system is employed to achieve complete degradation. The first enzyme, PETase, works directly on the solid plastic surface, initiating depolymerization by cleaving the long polymer chains and breaking the ester bonds. This releases soluble intermediate molecules.
The primary product of the PETase reaction is mono(2-hydroxyethyl) terephthalate (MHET). The second enzyme, MHETase, then acts upon the MHET molecule, completing the breakdown into its two fundamental building blocks. These final products are terephthalic acid (TPA) and ethylene glycol (EG), which are the original monomers used to manufacture PET plastic. Once these small, simple molecules are produced, the bacterial cell can absorb them to use as fuel for growth and metabolism.
Current Applications in Waste Management
The discovery of these enzymes has quickly transitioned to targeted biotechnological applications aimed at improving recycling. Scientists employ techniques like enzyme engineering and directed evolution to enhance the natural capabilities of enzymes such as PETase. Researchers create enzyme variants that are significantly more efficient, sometimes increasing the rate of PET degradation by several fold compared to the naturally occurring enzyme. This optimization focuses on making the enzymes more stable at higher temperatures and faster at their chemical reactions.
These optimized enzymes are being deployed in large-scale industrial settings, often within controlled environments called bioreactors. Companies use this enzymatic process to break down massive quantities of plastic waste, such as hundreds of kilograms of PET, within hours. The goal is to create a circular economy where the resulting monomers are purified and reused to manufacture new, high-quality plastic products. Researchers are also engineering microbes to convert these plastic building blocks into high-value chemicals, such as precursors for biofuels or flavorings like vanillin.
Real-World Limitations to Industrial Use
Despite the promising breakthroughs in the lab, several practical challenges must be overcome before plastic-eating microbes become a widespread solution to the global plastic crisis. One primary limitation is the relatively slow speed of the natural degradation process, which can take weeks or months to break down plastic film, contrasting with the massive volume of continuous plastic production. While engineered enzymes are much faster, the current technology primarily targets only a few types of plastic, such as PET and some polyurethanes. This leaves the majority of plastic waste streams, which are complex mixtures of different polymer types, largely unaddressed.
The economic viability of enzymatic recycling also presents a hurdle, as the high cost of producing, purifying, and maintaining large quantities of the engineered enzymes is substantial. Furthermore, these biological processes require precise environmental controls, including specific temperature ranges and pH levels, to ensure the enzymes remain active and efficient. Implementing these controls and the necessary logistics for collecting, sorting, and transporting plastic waste to centralized bioreactors adds considerable expense and complexity. Scaling up these systems from laboratory experiments to industrial-level operations remains a significant logistical and financial undertaking.