A University of Konstanz study offers a reality check on "biodegradable" claims: a plastic's chemistry and where it ends up decide whether microbes can finish the job.

Biodegradable plastics buried in forest soil and mixed into soil samples were completely broken down by microbes within about 250 to 330 days, while conventional high-density polyethylene showed virtually no breakdown, according to researchers at the University of Konstanz in Germany. The team traced part of that work to a bacterial enzyme with an unusually wide-open active site, which they nicknamed the "Pac-Man enzyme."

The peer-reviewed study appeared in The ISME Journal in September and drew renewed attention this month through a ScienceDaily summary published Oct. 16. Its central lesson is practical — microbes in soil can dismantle some plastics, but only those built with chemical bonds the microbes' enzymes can attack.

That matters for U.S. shoppers who see "biodegradable" on cutlery, bags and packaging. The study tested specific materials under specific conditions, and none of its results show that a typical labeled product will vanish in a backyard, a landfill or a roadside ditch.

Bacteria Tunneled Into the Film, Cell by Cell

Biologists Harry Lerner and David Schleheck buried thin films of long-chain aliphatic polyesters (LCAP), a plant-oil-based bioplastic developed by chemist Stefan Mecking's group, about 10 centimeters (4 inches) deep in the humus layer of the university's botanical garden forest. In parallel, they mixed powdered plastics into soil from the same site in the lab and tracked carbon dioxide released by microbes for a year. Because microbes exhale carbon dioxide as they consume material, that measurement shows how much of the plastic was actually used as food rather than simply crumbling.

Lerner said "all bioplastic materials were completely degraded within roughly 250 to 330 days." The comparison materials included two other bioplastics, PHBV and PCL, along with cellulose, which broke down in about 80 days. High-density polyethylene, the plastic used in milk jugs and many bottles, showed virtually no degradation over the same period.

Electron microscope images of the recovered films revealed tiny holes, each roughly the size and shape of a single bacterial cell. The researchers proposed that bacteria carry plastic-cutting enzymes anchored to their surfaces, letting them digest their way into the material rather than relying only on enzymes released into the soil.

A Wide-Open Active Site and an Antibiotic Twist

To find the machinery, Lerner sequenced DNA from the whole soil microbial community and looked for genes that became abundant only where LCAP was present. One stood out. It encodes a family-VIII esterase, an enzyme that splits ester bonds, with a signal that sends it outside the cell and a lipid anchor that keeps it attached to the cell membrane.

The enzyme's broad, exposed active site gave it room to grip long polymer chains, which inspired the Pac-Man name. It also resembled beta-lactamases, enzymes some bacteria use to break the ring structure of penicillin-type antibiotics. In lab tests, the enzyme cut polyester into its building blocks and also cleaved penicillin.

That dual activity is a lab finding, not evidence of a new public-health threat. The researchers noted that showing an enzyme can break down penicillin in a test tube does not establish that it makes living bacteria resistant to antibiotics. The study did not test whether biodegradable plastics in soil increase antibiotic resistance.

Reading the Label: 'Biodegradable' Depends on Where It Lands

The Federal Trade Commission's environmental marketing guidance has long held that an unqualified biodegradable claim requires evidence that the whole product will completely decompose within one year after customary disposal, which the agency defines as a landfill, incinerator or recycling facility. The FTC has brought cases against companies that claimed additive-treated plastics were biodegradable without that proof.

California goes further. State law restricts selling plastic products labeled "biodegradable," "degradable" or "decomposable," and sets standards for "compostable" claims, according to CalRecycle's labeling guidance. Compostable plastic products sold there must meet the ASTM D6400 standard, which is designed around industrial composting, not backyard piles.

The Konstanz results line up with that caution — even the bioplastics needed most of a year in biologically rich forest humus, a far more active setting than a compacted landfill cell. Schleheck framed the takeaway as a design problem, saying "we humans need to work with the capabilities of microbes."

For households, the reasonable steps are simple. Check whether your city's curbside or drop-off compost program accepts certified compostable plastics before putting them in a green bin. Do not bury bioplastic packaging in garden beds expecting it to disappear, and keep conventional plastics out of compost entirely.

Limits of a Single Forest-Soil Study

The work is a peer-reviewed field and laboratory study from one forest soil in southern Germany over roughly one year. LCAP is a research material, not a common packaging plastic, so breakdown rates for products sold in the United States could differ. Soil type, moisture and temperature all shape microbial activity. The team also did not test conventional plastics sold with additives marketed as making them biodegradable, the type of claim the FTC has challenged.

The study also does not show that bacteria are adapting to consume conventional polyethylene, polypropylene or polystyrene, which make up much of the plastic waste in rivers and oceans. Schleheck said microbes would like to use plastic as food but mostly cannot, because standard plastics are largely indigestible to them.

Next steps include learning how widespread the enzyme is in other soils and whether similar enzymes act on other polyesters. Nature World News will report on follow-up research and any changes to federal or state labeling rules.

What Readers Want to Know

What is the "Pac-Man enzyme"? It is a bacterial esterase found in forest soil that breaks down certain polyester bioplastics. Its wide-open active site reminded researchers of the video game character.

Did the bioplastics really disappear? In the lab tests, all the bioplastics tested were completely degraded within about 250 to 330 days. Conventional high-density polyethylene showed virtually no breakdown.

Does this mean "biodegradable" products break down in my backyard? Not necessarily. The study used specific materials in rich forest soil. Breakdown depends on the material and the environment, and labels are not a guarantee.

Is the enzyme's link to penicillin a health risk? The study did not show that. It found the enzyme can cleave penicillin in the lab, but it did not test antibiotic resistance in living bacteria or people.

What should I do with compostable or biodegradable packaging? Follow your local compost or trash program's rules. Many programs accept only certified compostable items, and some accept none.

Will bacteria solve plastic pollution? Not for conventional plastics. The researchers say the bigger opportunity is designing plastics that microbes can actually digest.