Scientific interest has turned to plastic-degrading microbes as a potential solution to plastic pollution. These microorganisms, primarily bacteria and fungi, can break down complex plastic polymers into simpler substances. Their ability to use plastics as an energy source is being explored by scientists to harness their natural processes to address plastic waste.
Mechanisms of Microbial Plastic Degradation
The breakdown of plastic by microbes begins with colonization. Microorganisms first attach to the surface of a plastic item, forming a community encased in a self-produced slime, known as a biofilm. This attachment is influenced by the plastic’s surface properties, such as hydrophobicity. Within this biofilm, the microbes secrete specialized enzymes that act on the plastic’s polymer structure to initiate degradation.
Once the biofilm is established, the secreted enzymes break down the long, complex polymer chains that constitute the plastic. This process, which involves hydrolysis, targets the bonds holding the polymer together, cleaving them into smaller pieces like oligomers, dimers, and monomers. The efficiency of this enzymatic action depends on the plastic’s structure; amorphous regions are easier to degrade than highly crystalline regions.
The specific enzymes involved are tailored to the type of plastic. For instance, hydrolases like cutinases, lipases, and esterases are effective against polyesters. These enzymes sever the ester bonds common in plastics like polyethylene terephthalate (PET). The resulting monomers are absorbed by the microbial cells. Inside the cell, these molecules enter metabolic pathways for energy and growth, ultimately being converted into substances like carbon dioxide and water.
Key Plastic-Degrading Microbe Species
Numerous bacterial and fungal species have been identified with the ability to degrade various plastics. One is the bacterium Ideonella sakaiensis, discovered in 2016, which can completely break down PET plastic. This bacterium produces two specific enzymes, PETase and MHETase, that work in sequence to deconstruct the PET polymer into its fundamental building blocks.
Other bacteria from the Pseudomonas, Bacillus, and Rhodococcus genera have demonstrated plastic-degrading capabilities. Species like Pseudomonas aeruginosa and Bacillus megaterium degrade polyethylene (PE), one of the most common plastics. Pseudomonas species are versatile, with some strains also showing activity against PET. These bacteria are isolated from environments contaminated with plastic waste.
Fungi also play a role in the biological degradation of plastics. Species from the Aspergillus and Fusarium genera are examples. Aspergillus glaucus, Aspergillus niger, and Fusarium oxysporum have been found to degrade polyethylene. Fungi like Pestalotiopsis microspora have shown the ability to break down polyurethane (PU), even in low-oxygen environments.
Discovery and Natural Occurrence
Scientists discover plastic-degrading microbes by searching in environments where plastic waste is abundant. Landfills, industrial waste sites, and areas of the ocean with high plastic accumulation are locations for finding these organisms. Constant exposure to plastic pollution may drive microbial evolution, favoring those that can utilize plastic as a food source. The soil of mangrove forests, which act as traps for plastic debris, has been identified as a rich source of plastic-degrading bacteria and fungi.
The process of finding these microbes involves a technique called enrichment culture. Scientists take samples of soil or water from a plastic-polluted site and place them in a lab where a specific type of plastic is the only available source of carbon. Only the microorganisms capable of feeding on that plastic will survive and multiply. This method allows researchers to isolate and identify promising strains for further study.
Beyond landfills and oceans, these microbes have been found in places such as the digestive tracts of insects that consume plastic. For example, the larvae of the greater wax moth (Galleria mellonella) can chew through and digest polyethylene plastic, and research has pointed to the role of their gut microbes in this process. This suggests that symbiotic relationships between insects and microbes could be another avenue for discovering plastic-degrading organisms.
Applications in Plastic Bioremediation
The abilities of plastic-degrading microbes are being explored for bioremediation, the use of organisms in waste management. The primary goal is to use these organisms or their enzymes to break down plastic waste in controlled settings. One approach involves using large tanks called bioreactors, where microbes are mixed with plastic waste under optimized conditions to accelerate degradation.
Research is also focused on enhancing the natural efficiency of these microbes. Genetic engineering is used to modify enzymes, such as PETase, to make them more stable and faster at breaking down plastics. Another strategy involves combining different microbial species into a consortium. Such mixed communities can be more robust and effective, as different species might work together to break down complex plastic waste.
While these technologies show promise, they are currently at the laboratory or small pilot-scale stage. Scientists are working on scaling up these processes to handle the large volumes of plastic waste generated globally. The byproducts of this microbial degradation are also a consideration, with the ideal outcome being complete mineralization into harmless substances. Pre-treating plastics with heat or chemicals to make them more susceptible to microbial attack is another strategy being explored.