A soil-bacteria enzyme helped bioplastics vanish in 250 to 330 days while regular plastic barely broke down

Bioplastic film buried in forest soil at the University of Konstanz disappeared completely within roughly 250 to 330 days, while high-density polyethylene, a plastic common in bottles and trash bags, showed virtually no degradation at all. Cellulose, the natural benchmark, broke down in about 80 days. Behind the vanishing act is a bacterial enzyme that biologists nicknamed the Pac-Man esterase.

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The finding comes with a twist about antibiotics that complicates the good news.

Buried film and soil microcosms in a botanical garden

The experiments paired two approaches, according to the University of Konstanz release. In the field, long-chain aliphatic polyester film, called LCAP, was buried about 10 centimeters deep in the humus layer of the forest in the university’s botanical garden. In the laboratory, bioplastic powder was mixed into soil from the same forest and the carbon dioxide output was tracked for a year as a measure of microbial respiration.

The LCAP material was developed by the chemist Stefan Mecking and his team at Konstanz, and it was tested alongside other materials. The controls were cellulose, the bioplastics PHBV and PCL, HDPE and untreated soil. All of the bioplastics fully degraded, in the range of 250 to 330 days, according to first author Harry Lerner, as ScienceDaily summarized the study. The paper in The ISME Journal adds that HDPE converted to carbon dioxide at 3 percent at most, so the conventional plastic barely moved.

The setting matters. Plastic that reaches the environment becomes a surface that microbes colonize, forming biofilms that researchers call the plastisphere, and the Konstanz team studied that habitat in intact forest soil instead of in a flask of purified enzyme. David Schleheck, a Konstanz biologist, described the plastisphere as a new habitat in the environment, and the soil results suggest microbial communities respond to it quickly. Electron microscopy of the buried film, taken after about 15 months in the ground, showed cavities that were absent from film that had never been exposed to soil.

A cell-surface esterase with a wide-open active site

The researchers then asked which microbes and genes did the work. Metagenomic sequencing found one gene strongly enriched in soil that contained LCAP, encoding an esterase with a secretion signal and a lipid anchor that is predicted to hold it on the bacterial surface. Lerner said the team had hypothesized that bacteria are coated with plastic-depolymerases anchored to their cell surfaces, and electron microscopy backed the idea by showing tiny holes in the film matching the size and shape of single bacterial cells.

The enzyme is a family-VIII esterase, and structural modeling suggests its active site is a wide, open groove rather than a narrow tunnel, which earned the “Pac-Man” label. In the lab, a soluble version released building-block molecules from the polyester. Mecking’s chemistry group designed LCAP with ester bonds that water can cut, and those bonds are what the enzyme targets.

Penicillin cleavage and the antibiotic resistance question

Lerner said the enzyme’s structure resembles that of esterases but also that of beta-lactamases, the enzymes that disable antibiotics such as penicillin. In tests, the soluble enzyme did cleave penicillin G and ampicillin. Lerner told the university that “This prevalence could act as a vector for the spread of antibiotic resistance in the environment,” a statement framed around how common such enzymes may be in soil.

The authors are cautious about their own result. Cleaving an antibiotic in a test tube does not prove it confers resistance inside living bacteria, and the attempt to express the full-length, membrane-anchored enzyme in E. coli produced no protein, leaving the anchoring hypothesis untested. The structural picture comes from AlphaFold3 and AutoDock 4 predictions rather than a solved crystal structure, and the plastic and antibiotic activity was tested only with the soluble construct under laboratory conditions, which limits how far the findings can be extended to living soil communities.

Conventional plastic remains out of reach

The result does not mean bacteria can eat the plastic already in circulation. Polyethylene stayed largely unaffected, and the tested bioplastics degrade because their polyester bonds are breakable. Plastic made of long, stable carbon chains is a different problem. The EPA’s materials data counts 35.7 million tons of plastics in U.S. municipal solid waste in 2018, the latest year in its tables, and about 27 million tons of it went to landfills.

Other labs are chasing similar outcomes by different routes. Flinders University researchers reported a milk-protein packaging film that fully broke down in soil in about 13 weeks, as ScienceDaily reported earlier this year. David Schleheck, the Konstanz microbial ecologist who led the work with Mecking’s team, said bacteria can adapt to breaking down polyester plastics more quickly than the group expected, and argued that tackling plastic pollution means working with the capabilities of microbes, a goal that depends on designing more products with breakable bonds. Whether manufacturers adopt such polyesters at scale, and whether the soil bacteria that digest them carry an antibiotic-resistance side effect, are questions the paper leaves open for further work.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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