Bacterial 'Pac-Man enzyme' digests bioplastic in lab tests

Bury a film of bioplastic in a forest floor, and within 250 to 330 days it is gone. That is what the team at the University of Konstanz recorded when it buried polyester films about 10 centimeters deep in the humus of the university's botanical garden. The same soil, tested over the same year, left conventional HDPE almost untouched, with virtually no degradation.

The key actor is an enzyme that sits on the outside of a bacterial cell. Its active site, the pocket where a chemical reaction happens, is unusually wide open, which is why microbiologists Harry Lerner and David Schleheck nicknamed it the 'Pac-Man enzyme'. Because it is anchored to the cell surface, the plastic building blocks it produces form right beside the bacterium, which can absorb them before they drift away or are claimed by neighbors. Under the electron microscope, the buried film showed micrometer-sized holes, each matching the size of a single bacterial cell. The researchers hypothesize that bacteria digest their way into the plastic this way, which challenges the common assumption that microbes simply release such enzymes into their surroundings.

Searching the soil DNA, the team found a gene that appeared only in soil containing the polyester film. It codes for an esterase, an enzyme that cuts ester bonds, and its structure resembles beta-lactamases, the enzymes that disarm antibiotics such as penicillin by cleaving their beta-lactam ring. In the lab, the enzyme broke polyesters into their monomers and also cleaved penicillin, both through hydrolysis, a reaction with water. Schleheck said the surface position serves both jobs, since antibiotics are disarmed before they can enter the cell.

The finding opens a question the team does not fully answer. Lerner said scientists have been puzzled by how common antibiotic-resistance genes are in the plastisphere, and the dual function may explain part of it. Two explanations remain open: that organisms there evolve double-duty enzymes, or that some enzymes now specialize in plastic while their penicillin-cleaving activity is a leftover. The study does not decide between them.

For anyone designing plastics that escape recycling and end up in the environment, the study points to a concrete rule. Schleheck argues that materials should work with what microbes can do, which means choosing polymers with built-in chemical weak points, such as the ester bonds in this film. The study gives no timeline for when such materials could reach products, and it does not show how the enzyme would behave outside controlled soil and lab conditions. What it does show is that bacteria may adapt to polyester plastics faster than scientists expected.