The enzyme, named BhEstB, was isolated and characterized by a team of researchers working at the University of Hamburg in Germany in collaboration with partners in Algeria and at Heinrich-Heine-University Düsseldorf and Forschungszentrum Jülich. Its source is Bacillus halotolerans, a salt-tolerant member of a bacterial genus famous for its metabolic versatility and resilience in harsh environments. Writing in the journal Applied Microbiology and Biotechnology, lead author Hadjira Bounabi and colleagues, including corresponding author Wolfgang R. Streit, describe BhEstB as a versatile carboxylesterase whose behavior defies easy categorization within the established families of PET-degrading enzymes. Structurally, it is closely related to an esterase from Bacillus subtilis, yet it diverges markedly from canonical PETases and from most well-characterized MHETases, including the MHETase of Ideonella sakaiensis. Instead, it clusters more closely with carboxylesterases such as TfCa, placing it in a distinct branch of the hydrolase family tree.

That phylogenetic distance turns out to matter enormously for function. When the team measured BhEstB’s activity across a panel of artificial substrates—p-nitrophenyl esters of varying chain lengths—the enzyme displayed broad esterase activity, with optimal performance at a mildly alkaline pH of 8 and at temperatures between 20 and 30 degrees Celsius. Those are gentle, ambient conditions, a striking contrast to the elevated temperatures often required by PET-active enzymes that must approach the polymer’s glass transition temperature to gain access to its chains. The modest thermal optimum initially suggested BhEstB might be a niche player, but the kinetic data told a very different story when the researchers turned to the soluble intermediates of PET breakdown.

Against BHET, BhEstB delivered results that place it among the most efficient BHET-hydrolyzing enzymes characterized to date. The enzyme achieved complete conversion of 50 millimolar BHET within just one hour at 30 degrees Celsius. The underlying kinetics are equally impressive: a turnover number, or kcat, of 15.57 plus or minus 1.25 per second, a Michaelis constant, or Km, of 6.54 plus or minus 1.01 millimolar, and a corresponding catalytic efficiency of 2.38 times ten to the third per molar per second. In practical terms, this means each enzyme molecule processes roughly fifteen BHET molecules every second under saturating conditions, with a substrate affinity that allows meaningful conversion even at moderate concentrations. For a bioprocess engineer, those numbers translate directly into smaller reactors, shorter residence times, and lower costs.

The enzyme’s performance on MHET, the other major soluble intermediate of PET hydrolysis, was nearly as striking. BhEstB converted 39.8 millimolar MHET into terephthalic acid with a yield of 77 percent, plus or minus 0.7 percent, within 24 hours. Terephthalic acid is the prize at the end of this enzymatic relay: it is the monomer from which virgin PET is made, and recovering it in pure form from waste plastic is the central goal of enzymatic recycling. By efficiently finishing the job that depolymerizing enzymes begin, BhEstB functions as a second-stage catalyst that could be paired with PETases or cutinases in a two-step biocatalytic cascade, converting plastic waste all the way back to its building blocks.

Perhaps the most telling comparison came from head-to-head benchmarking. BhEstB significantly outperformed not only the engineered LCC variant known as LCC ICCG—one of the most intensively optimized PET-depolymerizing enzymes in the field—but also PET46, a recently published archaeal feruloyl esterase that had itself drawn attention for its PET-intermediate activity. That a naturally occurring Bacillus enzyme, without the benefit of directed evolution or rational engineering, could surpass these heavily studied catalysts on BHET and MHET underscores how much unexplored enzymatic diversity remains hidden in soil microorganisms. It also validates the authors’ framing of BhEstB as a blueprint: a structural and mechanistic template from which future engineered variants could be derived.

The blueprint idea carries real weight because BhEstB’s sequence and structure differ from the canonical PETase fold that has dominated the literature since Ideonella sakaiensis was described in 2016. Carboxylesterases of this type appear to have evolved for hydrolyzing ordinary ester bonds in plant and soil chemistry, yet their active-site architecture accommodates the aromatic diesters of PET breakdown with unexpected ease. Understanding which structural features grant BhEstB its extraordinary BHET turnover—features that canonical PETases and MHETases lack—could guide protein engineers in designing hybrid enzymes that combine polymer attack with rapid intermediate processing, eliminating the accumulation of BHET and MHET that can limit the throughput of current enzymatic recycling schemes.

On the bulk polymer itself, the team was careful to keep expectations measured. BhEstB showed modest overall activity on commercial semi-crystalline PET powder, with some activity observed at elevated temperatures, but it is clearly not a polymer-depolymerizing specialist on the order of LCC or the best engineered PETases. The authors frame this honestly: BhEstB is a versatile BHET- and MHET-active carboxylesterase with modest activity toward the bulk polymer, contributing to the expanding diversity of PET-intermediate-degrading enzymes rather than replacing the depolymerization workhorses. In a multi-enzyme recycling process, that division of labor is not a weakness but a design principle—each catalyst doing what it does best under conditions that favor the overall reaction.

The broader significance of the work lies in what it reveals about the ecology and evolution of plastic-degrading chemistry. PET has existed for less than a century, far too short a time for enzymes to have evolved specifically for it, so the catalysts being discovered today are almost certainly repurposed from older metabolic functions such as cutin degradation, ferulate ester hydrolysis, or general ester scavenging. BhEstB, with its carboxylesterase scaffold and ambient-temperature optimum, fits this pattern perfectly and suggests that the search space for PET-intermediate enzymes extends well beyond the known PETase lineages into the vast and poorly sampled diversity of bacterial esterases. Metagenomic and activity-based screening of soil and marine microbiomes, the approach exemplified by the Hamburg group’s broader research program, is likely to yield many more such catalysts.

For a world drowning in PET waste—hundreds of millions of tonnes produced annually, with only a small fraction chemically recycled—enzymes like BhEstB represent a practical step toward genuine circularity. A complete enzymatic recycling process would combine a depolymerase that shreds the polymer into soluble fragments with an intermediate-processing enzyme that finishes the conversion to terephthalic acid and ethylene glycol, both of which can feed back into virgin-quality PET production. By demonstrating complete BHET conversion in an hour and efficient MHET turnover at 30 degrees Celsius, BhEstB shows that the second stage of that process can be fast, mild, and economical. The study, funded in part by Algeria’s Directorate-General for Scientific Research and Technological Development and Germany’s Federal Ministry of Education and Research, and published open access, adds a powerful new name to the growing roster of plastic-eating enzymes—and a reminder that some of biotechnology’s best tools are still waiting quietly in the soil.

Subject of Research: Enzymatic degradation of PET plastic intermediates by a Bacillus carboxylesterase

Article Title: A versatile Bacillus esterase reveals a carboxylesterase blueprint for PET intermediate degradation

Article References: A versatile Bacillus esterase reveals a carboxylesterase blueprint for PET intermediate degradation. (n.d.). https://doi.org/10.1007/s00253-026-14056-w

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14056-w

Keywords: PET biodegradation, Bacillus halotolerans, carboxylesterase, BHET, MHET, terephthalic acid, enzyme kinetics, plastic recycling, biocatalysis, hydrolases, PETase, microbial enzymes

The scale of the problem the authors address is staggering. Since the 1950s, humanity has produced billions of tonnes of plastic, the vast majority of which has never been recycled and instead accumulates in landfills, rivers, and oceans. Microplastics and nanoplastics are now documented in marine and terrestrial ecosystems, in the guts of animals ranging from whales to earthworms, and even in the human digestive tract, where recent studies have identified PET-degrading enzyme candidates in the gut microbiome. Beyond the physical contamination, the plastic lifecycle contributes substantially to greenhouse gas emissions, linking the pollution crisis directly to climate change. Conventional mechanical recycling degrades polymer quality with each cycle, and chemical recycling often demands intense heat and petrochemical inputs, which is precisely why biological routes have attracted such intense scientific attention.

The turning point in plastic biodegradation came in 2016, when Japanese researchers described Ideonella sakaiensis, a bacterium capable of degrading and assimilating polyethylene terephthalate, the ubiquitous plastic known as PET. The bacterium deploys two enzymes in sequence: PETase, which hydrolyzes the polymer into mono-(2-hydroxyethyl) terephthalate (MHET) and related intermediates, and MHETase, which cleaves those intermediates into the monomers terephthalic acid and ethylene glycol. That discovery ignited a global hunt for plastic-degrading enzymes, and subsequent surveys have found PETase-like enzymes with functional motifs distributed across the world’s oceans, in glaciers, in deep-sea sediments, and among uncultured marine microorganisms. Machine learning and multi-omics databases such as PAZy and PlasticEnz are now accelerating the discovery of previously hidden candidate enzymes, dramatically expanding the known catalytic repertoire.

Wild-type enzymes, however, are far too slow for industrial use, and this is where enzyme engineering has transformed the field. Directed evolution campaigns have produced thermostable PET depolymerases that operate at temperatures approaching the glass transition of PET, where the polymer chains become more mobile and accessible. Computational redesign strategies, including the GRAPE approach and machine learning-aided hydrolase engineering published in Nature, have yielded variants capable of nearly complete PET depolymerization at industrially relevant high-solids loading. Researchers have also engineered the expression side of the equation, optimizing signal peptides such as pelB and SPamy to boost secretion of PETase from Escherichia coli and Bacillus subtilis, and displaying engineered PETase and MHETase on bacterial cell surfaces to create efficient dual-enzyme cascade systems. Multi-enzyme cascade strategies and self-assembled complexes now push degradation rates further, and PET enzymatic recycling has reached pilot scale, marking a genuine milestone on the road to commercialization.

PET is not the only target. Polyurethanes, found in foams, adhesives, and coatings, are attacked by enzymes including urethanases, esterases, and lipases, with recent multi-omics studies revealing how marine fungi degrade these polymers. Polyethylene, the most produced plastic and chemically the most recalcitrant, is far harder: its carbon-carbon backbone resists hydrolysis, and degradation typically requires oxidative enzymes such as alkane hydroxylases and, more recently, small laccases, including one discovered in Acinetobacter dijkshoorniae that shows potential for polyethylene depolymerization and bio-upcycling. Polystyrene degradation has been documented in bacteria such as Exiguobacterium, with evidence for atypical oxygenase-mediated pathways, and multi-omic analyses of plastic-associated microbial communities continue to reveal novel enzymes, dubbed plastizymes, with unexplored biotransformation potential. Pretreatment technologies, including UV irradiation, plasma, and thermal conditioning, are increasingly recognized as key enablers that make polymers vulnerable to enzymatic attack.

Degrading plastic is only half of the circular equation; the other half is turning the resulting monomers and renewable carbon into new materials. Here the review highlights polyhydroxyalkanoates (PHAs), a family of microbially synthesized polyesters that behave like conventional plastics yet biodegrade readily in soil and marine environments, and that already show promise in biomedical applications such as wound treatment and food packaging. Metabolic engineering has been decisive in making PHA production economically viable. Researchers have knocked out competing fermentation pathways in E. coli to channel carbon into PHB, disrupted PHA depolymerase genes in Rhodobacter sphaeroides to prevent the polymer from being consumed, and overexpressed biosynthetic gene clusters to raise yields. In halophilic chassis such as Halomonas bluephagenesis, scientists have even manipulated the size of intracellular PHA granules, which influences polymer processing properties, while fine-tuning monomer composition to tune material performance from rigid to elastic.

Perhaps the most exciting frontier is the direct coupling of the two halves of the cycle: converting plastic waste itself into new bioplastic. Pseudomonas putida KT2440 has emerged as the leading chassis for this kind of plastic upcycling, engineered to metabolize both terephthalic acid and ethylene glycol, the two monomers released from PET, and to funnel them into value-added products including PHAs and beta-ketoadipic acid. Tandem chemical deconstruction followed by biological upcycling has been demonstrated for PET, and engineered strains have been built for concurrent PET monomer metabolism and hydrolase expression, effectively creating synthetic PETrophy. Even more striking, I. sakaiensis itself has been shown to directly convert PET into PHA fermentatively, and co-cultivation strategies pair a PET-degrading strain with a PHB-producing strain to achieve one-step degradation and biosynthesis. Similar logic applies to polyurethane monomers, which defined microbial mixed cultures can convert into bioplastic, and to mixed microbial communities that produce PHA from terephthalic acid.

Synthetic consortium design adds another layer of sophistication. Engineered microbial division of labor, in which different strains specialize in depolymerization, monomer transport, and product synthesis, has been shown to improve plastic upcycling beyond what single strains achieve. Artificial consortia have also been used to convert inexpensive feedstocks such as inulin and glucose-xylose mixtures into medium-chain-length PHAs, and photoheterotrophic communities can co-produce biohydrogen alongside PHA. These systems mirror the modular logic of industrial chemistry but operate at ambient temperature and pressure in water, offering a potentially far lower energy footprint than thermochemical recycling routes such as pyrolysis, which nonetheless remain part of the broader circular economy toolkit.

The authors are candid about the bottlenecks that remain. Enzyme stability and catalytic efficiency still fall short of the throughput needed for bulk waste streams, particularly for polyolefins like polyethylene and polypropylene, where degradation rates remain orders of magnitude below those for PET. Process economics, downstream separation, feedstock logistics, and the cost of engineered strains all weigh against bioplastic prices that must compete with commodity petroplastics. The review also notes that some researchers caution microbial enzymes will offer limited solutions to the global pollution crisis on their own, underscoring that biology must be integrated with policy, sorting infrastructure, and system-level design rather than seen as a silver bullet. The authors call for interdisciplinary collaboration spanning strain engineering, protein design, bioprocess innovation, and environmental systems analysis to overcome these barriers.

What emerges from the BRIN team’s synthesis is a coherent vision: microorganisms as the connective tissue of a circular plastics economy, digesting yesterday’s bottles and packaging and secreting tomorrow’s materials. With PET enzymatic recycling already demonstrated at pilot scale, PHA strains genetically fine-tuned for yield and composition, and upcycling routes from waste plastic to new bioplastic proven in the laboratory, the pieces of the puzzle are increasingly in place. The remaining challenge is less one of discovery than of integration and scale, of turning elegant metabolic pathways into robust industrial processes. If that integration succeeds, the microbes that evolved in a world saturated with plastic may become the very tool humanity uses to unsaturate it.

Subject of Research: Microbial metabolic engineering for plastic degradation and bioplastic synthesis in a circular plastics economy

Article Title: The role of microbial metabolic engineering for circular plastics economy

Article References: The role of microbial metabolic engineering for circular plastics economy. (n.d.). https://doi.org/10.1007/s13205-026-05050-0

Image Credits: AI Generated

DOI: 10.1007/s13205-026-05050-0

Keywords: metabolic engineering, plastic degradation, PETase, polyhydroxyalkanoates, bioplastics, circular economy, Ideonella sakaiensis, enzyme engineering, Pseudomonas putida, biodegradation, plastic recycling, microbial consortia