Plastic pollution is one of the world’s most persistent environmental problems. Vast amounts of discarded plastic accumulate in the oceans, sometimes forming enormous garbage patches that threaten marine life. Most conventional plastics break down extremely slowly because microorganisms struggle to digest the synthetic polymers from which they are made. Instead of disappearing, these materials gradually fragment into increasingly smaller particles, including microplastics and nanoplastics.
Yet plastic waste is not completely beyond the reach of nature. Bacteria and other microorganisms can colonize its surfaces, forming communities known as biofilms. These communities create a distinctive microbial ecosystem that scientists call the “plastisphere.”
Researchers at the University of Konstanz have now discovered an unusual enzyme that could reveal how some bacteria are becoming better equipped to break down certain types of plastic. The enzyme can digest particular polyesters and biodegradable plastics, but it has another unexpected ability: it can also break down penicillin, suggesting a possible connection to antibiotic resistance.
Scientists have nicknamed it the “Pac-Man enzyme” because its molecular structure features a remarkably wide opening where chemical reactions take place, known as the active site. The discovery suggests that bacteria may be adapting to digest certain plastic materials more quickly than researchers previously believed.
However, the findings also highlight an important limitation. For microorganisms to make a meaningful difference in reducing plastic pollution, more plastic products would need to be made from biodegradable materials that contain chemical bonds microbes can break apart.
Testing How Bacteria Break Down Bioplastics
To investigate how microorganisms digest biodegradable plastics, University of Konstanz biologists Harry Lerner and David Schleheck studied the entire process of microbial decomposition. They wanted to determine not only whether the materials could be completely broken down, but also which microorganisms were responsible and what biological mechanisms allowed them to do it.
The researchers examined the composition of the microbial communities involved and analyzed their collective genetic information (the metagenome). This approach allowed them to search for genes that might produce enzymes capable of breaking apart plastic polymers.
Their experiments focused on long-chain aliphatic polyesters (LCAP), a class of biodegradable plastic materials developed by a research team led by chemist Stefan Mecking. These materials contain chemical bonds that certain microorganisms can potentially break down, unlike the more resistant structures found in many conventional plastics.
The collaborative findings were published in The ISME Journal.
To observe how the bioplastics behaved in a natural environment, the team placed samples beneath the surface of forest soil at the university’s botanical garden.
“We buried small pieces of LCAP bioplastic film in the upper humus layer in the forest at the university’s botanical garden, about ten centimeters deep,” explains Harry Lerner.
“This layer is where the breakdown of cellulose and other natural polymers, such as cutin – a plant-based polyester – takes place.”
Humus is the upper layer of soil rich in decomposing organic matter, where microorganisms play a major role in recycling nutrients. Cellulose is the main structural component of plant cell walls, while cutin is a naturally occurring polyester that helps form the protective outer covering of plants.
The researchers selected this environment because its microorganisms already possess the biological machinery needed to break down complex natural materials.
Bioplastics Disappeared Within About a Year
Alongside the outdoor experiment, the scientists carried out laboratory tests using soil collected from the same forest. They mixed powdered bioplastics into the soil and monitored microbial activity over the course of an entire year.
One important measurement was the amount of carbon dioxide, or CO2, released from the samples. As microorganisms consume and metabolize organic materials, they produce carbon dioxide through respiration. Tracking this gas helped the researchers determine how actively the microbes were breaking down the plastics.
The buried plastic films remained in the forest soil for a full year, allowing the team to compare decomposition under natural conditions with the carefully monitored laboratory experiments.
To assess the results, the scientists also tested cellulose, other biodegradable plastics, conventional polyethylene plastic, and soil samples without added materials.
“Cellulose, other types of bioplastics such as PHBV and PCL, as well as high-density plastic (HDPE) and untreated soil were used as controls in the laboratory. We found that all bioplastic materials were completely degraded within roughly 250 to 330 days. Cellulose broke down after about 80 days, whereas virtually no degradation occurred for HDPE,” says Lerner.
The difference was striking. While the biodegradable plastics were completely broken down within approximately nine to eleven months, conventional high-density polyethylene showed almost no decomposition.
These results demonstrate why the chemical makeup of plastic matters so much. Some polymers contain bonds that microbial enzymes can attack, allowing bacteria to dismantle them into smaller molecules. Others, including widely used polyethylene, are far more resistant to biological degradation.
Mysterious Holes Reveal How Bacteria Eat Plastic
When the researchers recovered the plastic films from the forest soil, they discovered something unexpected.
Using electron microscopy, which can reveal structures far too small to see with an ordinary light microscope, the team examined the surfaces of the buried materials. They found numerous tiny holes, each approximately matching the dimensions and shape of an individual bacterial cell.
The unusual pattern suggested that bacteria were doing more than simply attaching themselves to the plastic. They appeared to be breaking into the material itself.
“We hypothesized that bacteria are coated with plastic-depolymerases anchored to their cell surfaces. This would enable them to digest their way into the material and become embedded within the film, leaving behind microscopic holes of exactly this type,” explains Lerner.
Depolymerases are enzymes that break large molecules called polymers into smaller chemical units. If these enzymes are attached directly to the outside of a bacterium, they could allow the microorganism to digest the surrounding material while remaining in close contact with it.
This would help explain the microscopic cavities observed in the plastic films. Rather than relying entirely on enzymes released into the environment, the bacteria might be able to break down the material directly at their surfaces.
Scientists Identify the Plastic-Eating “Pac-Man Enzyme”
To find the biological machinery behind this unusual behavior, Lerner extracted DNA from the entire community of microorganisms living in the soil samples.
He then sequenced the genetic material and carefully analyzed the extensive data to determine which microbes and genes became especially abundant as the bioplastics decomposed.
One gene stood out. It was strongly enriched specifically in forest soil containing LCAP, suggesting that it could play an important role in breaking down the biodegradable polyester.
Further analysis revealed that the gene produces an esterase, a type of enzyme capable of splitting certain chemical bonds found in esters. These bonds are present in many naturally occurring substances as well as in various biodegradable polyesters.
What made this particular enzyme especially interesting was how it was positioned on bacterial cells.
Its genetic instructions included a secretion signal that directs the enzyme outside the cell, along with a lipid anchor that holds it firmly to the cell membrane.
This arrangement means the enzyme can remain attached to the bacterium’s surface while attacking nearby plastic molecules. Its unusually exposed active site also provides space for the enzyme to interact with polymer chains.
The researchers had identified the molecular mechanism they were looking for, along with the enzyme they named “Pac-Man.”
A Surprising Connection to Antibiotic Resistance
The discovery became even more intriguing when scientists examined the enzyme’s molecular structure.
Although it resembled esterases, which are known for breaking down ester bonds, it also shared structural similarities with another important family of bacterial enzymes: beta-lactamases.
These enzymes are well known for their role in antibiotic resistance. Some bacteria produce beta-lactamases that disable antibiotics such as penicillin by breaking open a critical part of their chemical structure, known as the beta-lactam ring.
“Its structure resembles that of esterases, but also that of beta-lactamases, which are bacterial enzymes that are capable of cleaving the beta-lactam ring of certain antibiotics, such as penicillin, thereby making bacteria resistant to antibiotics,” says Lerner.
Laboratory experiments confirmed that the newly discovered enzyme was capable of performing both types of chemical reactions.
It could break polyester chains into their individual building blocks, known as monomers, while also cleaving penicillin.
This dual activity was particularly surprising because plastic degradation and antibiotic breakdown are usually considered separate biological functions.
The finding raises the possibility that some enzymes involved in plastic decomposition could also contribute to antibiotic resistance. However, demonstrating that an enzyme can chemically break down penicillin in the laboratory does not establish that it provides antibiotic resistance to living bacteria.
Could Plastic-Eating Bacteria Help Solve Plastic Pollution?
The discovery also raises a broader question about how rapidly microorganisms can adapt to synthetic materials introduced into their environment.
For most of Earth’s history, plastic pollution did not exist. Microorganisms evolved enzymes to digest naturally occurring substances, including plant fibers and other complex organic molecules.
Modern plastics have introduced an enormous new collection of potential carbon sources into the environment, but many of these materials remain chemically inaccessible to microbes.
“The plastisphere is a new habitat in our environment,” explains Schleheck.
“Humans have only been introducing plastic into the environment in significant quantities for around 50 to 75 years. Since then, it has theoretically been available to microbial communities – such as bacteria, yeasts and fungi – as an additional source of carbon and energy for their growth. By ‘theoretically’, I mean that they would certainly like to use the plastic as a growth substrate – but they cannot, because the materials are actually indigestible to microbial metabolism and are therefore hardly degraded.”
The Pac-Man enzyme suggests that this barrier may not be absolute for every type of plastic.
Some bacteria appear capable of using specialized enzymes to attack biodegradable polyesters, potentially allowing them to exploit these materials as sources of energy and nutrients.
For scientists studying plastic pollution, the finding offers a promising clue about how biological degradation might be improved. It also suggests that microbial communities could be adapting to certain synthetic polymers more rapidly than previously anticipated.
However, the results do not mean that bacteria are suddenly capable of consuming the enormous amounts of conventional plastic already accumulating in the environment. The experiments showed that polyethylene remained largely unaffected, even as the biodegradable plastics decomposed.
Instead, the researchers emphasize the importance of designing materials that microorganisms can actually digest.
“I find this encouraging, because it seems that bacteria can adapt to breaking down polyester plastics more quickly than we expected. To tackle the environmental problem of plastic pollution, we humans need to work with the capabilities of microbes. Ideally, this would involve using only polymers with biochemical breaking points, such as the hydrolysable ester bonds in polyesters like LCAP or other types of bioplastics,” concludes Schleheck.
The discovery offers a glimpse of a potentially more sustainable approach to plastic production. Rather than expecting microorganisms to break down materials that are naturally resistant to decomposition, scientists could develop plastics with chemical structures that make biological recycling possible.
The Pac-Man enzyme illustrates how bacteria can exploit these molecular weak points, offering new insight into the relationship between plastic pollution, microbial adaptation, and the future of biodegradable materials.
Source: www.sciencedaily.com
