A Bacterial Enzyme Tweak Produced the Strongest Biodegradable Plastic Yet - at Industrial Scale

Petroleum-based plastics are durable, cheap, and nearly impossible to eliminate from the environment once released. Biodegradable alternatives exist, but most face a frustrating trade-off: materials that degrade readily tend to be brittle and weak, while those with useful mechanical properties often resist biodegradation. A study from Shinshu University in Japan, published in Polymer Degradation and Stability, describes an engineered bacterial strain that breaks that pattern.

The material is a microbial copolyester called LAHB - poly[(D-lactate)-co-(R)-3-hydroxybutyrate] - which belongs to the polyhydroxyalkanoate (PHA) family of bioplastics. Unlike conventional plastics, PHAs are produced entirely by microorganisms through fermentation and can re-enter natural carbon cycles when discarded. LAHB specifically has been shown to biodegrade in soil, river water, coastal seawater, and even deep-sea environments.

The Engineering Problem

LAHB's mechanical properties depend strongly on how much lactate (LA) it contains. Higher lactate fractions improve toughness and make the material better suited for commercial applications. But increasing the lactate fraction while maintaining high molecular weight - which determines tensile strength - had been a persistent technical barrier. Previous attempts produced either high-lactate, low-molecular-weight material that was weak, or high-molecular-weight material with insufficient lactate.

Professor Seiichi Taguchi and colleagues at Shinshu University, working with researchers from Kaneka Corporation and Japan's National Institute of Advanced Industrial Science and Technology (AIST), traced the bottleneck to a specific enzyme: the lactate-polymerizing enzyme (LPE), responsible for incorporating lactate monomers into the growing polymer chain. The hypothesis was straightforward - if LPE activity was rate-limiting for lactate incorporation, then increasing its expression should raise the lactate fraction.

What the Engineered Strain Produced

The team introduced an LPE-overexpressing plasmid vector into a modified strain of the bacterium Cupriavidus necator, creating a strain designated GSXd147. In fed-batch fermentation using glucose as the carbon source, this strain reached 97 g/L dry cell weight with LAHB comprising 70 percent of that mass within 48 hours. The resulting polymer titer - 68 g/L - represents the highest LAHB production yield reported to date.

The improvement over the previous best strain was more than twofold in productivity. The lactate fraction reached 15.4 mol percent, a meaningful increase from previous production conditions. Crucially, the polymer maintained a high molecular weight of approximately 30 x 10^4 - the property that confers mechanical strength.

Mechanical Performance Comparable to Conventional Plastic

Mechanical testing of the high-molecular-weight LAHB films produced a result the researchers describe as unusual. Most LAHB variants cluster at either end of a strength-versus-flexibility trade-off: high elongation but low tensile strength, or high strength but brittle behavior. The new material achieved approximately 20 megapascals tensile strength with roughly 190 percent elongation at break - a combination the researchers characterize as comparable to polyethylene.

Polyethylene is the world's most widely produced plastic. A biodegradable alternative with similar physical properties would be applicable across a large fraction of polyethylene's current uses, from packaging to agricultural films to single-use consumer products.

Biodegradability Independent of Molecular Weight

The study tested biodegradation of both high- and low-molecular-weight LAHB samples in natural seawater. Both achieved more than 75 percent degradation within 5 weeks based on biochemical oxygen demand measurements, despite an approximately eightfold difference in molecular weight between the two samples. The finding is practically significant: it means the molecular weight engineering needed to achieve good mechanical performance does not compromise environmental degradability.

Some caveats apply. The study was conducted at laboratory and bench scale under controlled fermentation conditions. Industrial-scale fermentation introduces additional engineering challenges - temperature control, oxygen transfer, substrate feeding - that may affect yield and composition. The 68 g/L titer is a record for this specific polymer, but commercial competitiveness with petroleum-based plastics also depends on feedstock costs, downstream processing, and energy consumption, which were not assessed here.

The study was made available online on December 31, 2025, and will be published in Volume 246 of Polymer Degradation and Stability on April 1, 2026.