Streptomyces Bacteria Emerge as Underexplored Factories for Biodegradable Plastic

A class of bacteria better known for producing antibiotics than plastics may hold an underappreciated answer to one of biotechnology’s most stubborn problems. Polyhydroxyalkanoates, or PHAs, are polyesters that microorganisms synthesize inside their cells as storage compounds when nutrients run short, and they behave like conventional plastics while degrading naturally in the environment. A new review published in Biotechnology for Sustainable Materials examines the state of PHA research with a particular focus on Streptomyces, a genus of soil-dwelling actinobacteria whose metabolic versatility has made them industrial workhorses for antibiotics and enzymes, yet whose capacity to accumulate these biopolymers remains poorly understood and largely untapped.

The review’s authors, Seemon Giri, Saswata Kabiraj and Lopamudra Ray, frame PHAs against the backdrop of an escalating plastic crisis. Conventional petroleum-derived plastics have become persistent pollutants because of their resistance to natural degradation, with millions of tonnes of plastic waste generated annually and a significant fraction inadequately managed, accumulating in terrestrial and aquatic ecosystems. Microplastics, now detected in water, soil, air and biological systems, add a further layer of concern. Projections cited in the review indicate that under a business-as-usual scenario, global plastic waste could increase drastically in the coming decades, intensifying environmental burden and ecological disruption.

PHAs come in several structural families defined by the length of their monomer units, and this architecture dictates their material behavior. Short-chain-length PHAs, built from three to five carbon monomers such as the units of polyhydroxybutyrate, are highly crystalline with melting temperatures around 170 to 180 degrees Celsius, but they are brittle and stiff, which limits their use in packaging and biomedical films. Medium-chain-length PHAs, with monomers of six to fourteen carbons, are flexible and elastomeric but suffer from low mechanical strength and lower melting temperatures. Long-chain-length variants, exceeding fourteen carbons, are rubber-like and rare. Copolymers that combine two or more monomer types, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate), offer tunable properties, greater flexibility and reduced brittleness compared with rigid homopolymers, broadening their industrial and biomedical applicability.

The classical biosynthetic route to polyhydroxybutyrate involves three enzymatic steps that link central carbon metabolism to polymer formation. Two molecules of acetyl-CoA, generated from glycolysis or fatty acid beta-oxidation, are condensed into acetoacetyl-CoA by the enzyme beta-ketothiolase, encoded by the phaA gene. Acetoacetyl-CoA reductase, encoded by phaB, then reduces this intermediate to (R)-hydroxybutyryl-CoA in a reaction dependent on a high cellular NADPH to NADP+ ratio. Finally, PHA synthase, encoded by phaC, polymerizes the hydroxyacyl-CoA monomers into granules. PHA synthases fall into four classes based on subunit structure and substrate specificity, with classes I and II being single-subunit enzymes typical of Gram-negative bacteria and classes III and IV two-subunit enzymes found in Gram-positive organisms.

In Streptomyces, this tidy picture breaks down in intriguing ways. In many bacteria the phaA, phaB and phaC genes sit together in clusters or operons, but in Streptomyces they are often scattered across the genome, complicating conventional gene identification. The phaC gene in the model organism Streptomyces coelicolor A3(2) was long thought to be absent until a database search revealed SCO7613 as a putative PHA synthase carrying the conserved lipase-box-like sequence in which the nucleophile is a serine residue. Deleting SCO7613 significantly reduced PHA accumulation but did not stop it entirely, suggesting that other homologous genes or alternative pathways contribute to polymer synthesis. Streptomyces aureofaciens NRRL 2209 carries a synthase with a serine instead of cysteine in its predicted active site, though that strain was not reported to produce PHA for validation. More strikingly, Streptomyces cavourensis BG2AG has been reported as a PHA producer that appears to follow a fatty acid beta-oxidation pathway involving a stereo-isomerisation step and a fatty-acyl-CoA oxidase, apparently compensating for a phaC gene whose lipase box sequence has changed.

Transcriptomics is now illuminating how these scattered genes are controlled. RNA-Seq allows global analysis of gene expression, showing how environmental signals feed into transcriptional regulation. Independent Component Analysis of more than 454 transcriptomic profiles of S. coelicolor has revealed 117 independently modulated groups of genes, termed iModulons, which represent the functional architecture of the genome, with genes involved in specific processes such as phosphate- and iron-dependent regulation being co-expressed. Transcriptomic studies have shown that upregulation of key primary metabolism genes, including glucokinase, phosphoglycerate mutase and pyruvate dehydrogenase, is essential for accelerating glucose consumption and building up precursors like acetyl-CoA for PHA synthesis. Meanwhile, genes in secondary metabolite biosynthetic gene clusters are often highly upregulated in liquid cultures, the standard setting for industrial PHA production, adding regulatory complexity.

Bioprocess optimization has delivered measurable gains across bacterial producers. Statistical experimental designs such as the Taguchi method and Response Surface Methodology have improved culture conditions for both Gram-positive and Gram-negative bacteria. Reported optimized titers include around 76.363 milligrams per milliliter for Pseudomonas aeruginosa using a three percent carbon source, 3.09 grams per liter for a Bacillus species on five grams per liter glucose, and 11.7 grams per liter of PHB for Cupriavidus necator. Among Streptomyces, optimization of Streptomyces toxytricini D2 yielded 23.64 grams per liter of PHA using eight percent tapioca molasses at pH 6.5, while Streptomyces nigra KDS4 reached 2.63 grams per liter on starch at pH 7.5 and S. cavourensis BG2AG produced 0.724 grams per liter on one percent glucose. The carbon-to-nitrogen ratio is a central lever: high ratios, above roughly 20 to 35, redirect excess carbon toward polymerization, while low ratios below 10 favor biomass growth over storage. In Bacillus megaterium, raising the ratio from 1 to 3 increased PHB synthesis by around 70 percent. Most producers, including Streptomyces, grow and synthesize PHA best at pH 6.5 to 7.5, with accumulation reaching up to 60 to 90 percent of cell dry weight under nutrient limitation, and mesophilic temperatures near 30 to 37 degrees Celsius generally outperform colder or hotter conditions.

The applications portfolio is broad. In packaging, PHAs offer biodegradability, non-toxicity, good water vapor barrier properties and compatibility with injection molding and extrusion, with commercial products including bottles, food-container films, drinking straws, trays, rigid containers and paperboard coatings. In medicine, they are used in dissolvable stitches, surgical meshes, wound dressings, tissue repair scaffolds, cardiac implants, dental membranes and nerve repair materials; electrospun PHBV scaffolds have supported osteoblast and fibroblast adhesion and proliferation, and in animal studies PHBV fibers loaded with the angiogenic factor R-spondin 1 significantly accelerated wound closure. In agriculture, PHA-based coatings serve controlled-release fertilizers and pesticide encapsulation, and one integrated biorefinery case study reported cucumber plants with up to 21 percent higher shoot biomass and tomato plants with up to 41 percent greater shoot biomass when co-produced protein hydrolysates were applied. PHA fibers are emerging in textiles, and mixed microbial cultures can convert volatile fatty acids from anaerobic fermentation into PHAs without sterile cultivation; raw PHA from such cultures has even served as a slow-release electron donor achieving roughly 92 percent conversion of trichloroethylene to non-toxic ethene in groundwater bioremediation.

Yet the economics remain the decisive obstacle. Substrates can account for up to 50 percent of total production expenses, with minimum selling prices for PHAs ranging from 4 to 8 dollars per kilogram compared with 0.23 to 0.48 dollars per kilogram for conventional petrochemical plastics. Downstream processing is energy-intensive and relies on environmentally hazardous solvents, most production systems remain confined to laboratory or pilot scale, and waste-derived feedstocks introduce variability that complicates standardization. For Streptomyces specifically, the cotton ball-like filamentous morphology impedes oxygen transfer and nutrient distribution, extraction from thick cell walls is costly, and the large, unstable genome with its dispersed genes makes engineering difficult. The review’s authors argue that CRISPR-based tools, which allow tunable downregulation of multiple genes rather than disruptive knockouts, together with synthetic biology, biorefinery integration and next-generation industrial biotechnology under non-sterile conditions, will be essential if these promising but underexplored bacteria are ever to make biodegradable plastics genuinely affordable.

Subject of Research: Microbial production of polyhydroxyalkanoate bioplastics by Streptomyces bacteria

Article Title: Microbial polyhydroxyalkanoates: advances, omics-driven insights and industrial applications with a focus on Streptomyces sp.

Article References: Giri, S., Kabiraj, S., & Ray, L. (2026). Microbial polyhydroxyalkanoates: advances, omics-driven insights and industrial applications with a focus on Streptomyces sp.. Biotechnology for Sustainable Materials, 3(1), Article 14. https://doi.org/10.1186/s44316-026-00063-4

Image Credits: AI Generated

DOI: 10.1186/s44316-026-00063-4

Keywords: polyhydroxyalkanoates, bioplastics, Streptomyces, biodegradable polymers, metabolic engineering, transcriptomics, bioprocess optimization, PHB, agro-industrial residues, sustainable materials, PHA synthase, circular bioeconomy