In the ocean, plastic waste collects to form large garbage patches that endanger marine life. This is partly because the synthetic plastic polymers used today can only be broken down biologically, that is, by microorganisms, at a very slow rate. Instead, the physical fragmentation of these materials leads to the formation of microplastics and nanoplastics. However, microorganisms do colonize plastic waste in the environment, forming biofilms and creating a unique microbial habitat that researchers refer to as the ‘plastisphere’.

A research team at the University of Konstanz has now identified a new enzyme that can not only degrade certain polyesters and bioplastics, but also might provide bacteria with resistance to antibiotics. The discovery raises hopes that microorganisms can adapt to plastic degradation more rapidly than previously thought. Due to its structure featuring a wide-open active site, the researchers have named this ‘plastic-eating’ enzyme the ‘Pac‑Man enzyme’. Accelerating the environmental breakdown of plastic would require the use of biodegradable plastics wherever possible, as these materials can be decomposed by microorganisms.

• For more background information, see the full-length version of the article in the online magazine campus.kn.

The research project
In their research project, biologists Harry Lerner and David Schleheck from Konstanz investigated the complete microbial degradation of bioplastics. At the same time, they analyzed both the composition and the complete genetic makeup (the metagenome) of the microbial community involved in this degradation. For the study, the researchers used bioplastic materials – long-chain aliphatic polyesters (LCAP) – developed by chemist Stefan Mecking's team. Their joint study has now been published in the renowned scientific journal The ISME Journal.

How did the research team conduct their study? "We buried small pieces of LCAP bioplastic film in the upper humus layer in the forest at the university’s botanical garden, about ten centimetres 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". In the laboratory, the team also mixed bioplastic powder into samples of the same forest soil. The forest samples were left untouched for a whole year, whilst in the laboratory, the CO₂ production – and thus the microbial respiration – was monitored in great detail over the course of one year too, in order to document the degradation of the 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.

A surprising discovery
The analysis of the films retrieved from the forest soil held a surprise for the researchers: electron microscopy revealed tiny holes in the material, each of which matched the size and shape of a single bacterial cell. "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.
In order to identify the microbes that accumulated in the soil samples during the degradation of the bioplastic materials, and the enzymes responsible for breaking them down, Harry Lerner extracted the entire DNA from the soil's microbial community, sequenced it, and analyzed the vast amount of data with great patience and care. The microbiologists identified a gene that was highly enriched exclusively in the forest soil containing LCAP. This gene encodes an esterase enzyme equipped with both a secretion signal, which directs the enzyme out of the cell, and a membrane-bound lipid anchor. This anchor ensures that the enzyme remains firmly attached to the bacterial cell surface. The ‘Pac-Man enzyme’ had been discovered.

The enzyme held yet another surprise. "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. And indeed, the researchers were able to demonstrate the enzyme’s dual biochemical function – breaking down polyesters into monomers on the one hand, and cleaving penicillin on the other – in the laboratory.

A turning point in the plastisphere?
"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."

At least, so far. The ‘Pac-Man enzyme’ could signal a turning point, indicating that certain microorganisms may be able to specialize in breaking down plastic. "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. Key facts:

• Original publication: Harry Lerner, Diego Casaburi, Nele Charlott Meier, Léa Bernabeu, Marcel Eck, Stefan Mecking, David Schleheck, Bacterial family-VIII esterase displays dual activities: hydrolysis of polyester bioplastics and β-lactam antibiotics, The ISME Journal, Volume 20, Issue 1, January 2026, wrag203
  DOI: 10.1093/ismejo/wrag203
  Link: https://academic.oup.com/ismej/article-lookup/doi/10.1093/ismejo/wrag203
• Authors:
  During the project, microbiologist Harry Lerner (first author) was a postdoctoral researcher in the team led by David Schleheck at the University of Konstanz. Now, he is a Marie Skłodowska-Curie Fellow at KU Leuven in Belgium.
  David Schleheck is a professor of Microbial Ecology and Limnic Microbiology in the Department of Biology at the University of Konstanz.
  Other authors include: Diego Casaburi and Nele Chalott Meier in the Department of Biology, as well as Lea Bernabeu, Marcel Eck and Stefan Mecking, a professor of Chemical Materials Science, in the Department of Chemistry at the University of Konstanz.
• Funding sources: Funding from the foundation Carl-Zeiss-Stiftung for the project INPEW awarded to Stefan Mecking, Helmut Cölfen and David Schleheck, funding period: 2020 bis 2026.