Current trends in medium-chain-length polyhydroxyalkanoates: Microbial production, purification, and characterization

Polyhydroxyalkanoates (PHAs) have gained interest recently due to their biodegradability and versatility. In particular, the chemical compositions of medium-chain-length (mcl)-PHAs are highly diverse, comprising different monomers containing 6-14 carbon atoms. This review summarizes different feedstocks and fermentation strategies to enhance mcl-PHA production and briefly discusses the downstream processing. This review also provides comprehensive details on analytical tools for determining the composition and properties of mcl-PHA. Moreover, this study provides novel information by statistically analyzing the data collected from several reports on mcl-PHA to determine the optimal fermentation parameters (specific growth rate, PHA productivity, and PHA yield from various structurally related and unrelated substrates), mcl-PHA composition, molecular weight (MW), and thermal and mechanical properties, in addition to other relevant statistical values. The analysis revealed that the median PHA productivity observed in the fed-batch feeding strategy was 0.4 g L-1 h-1, which is eight times higher than that obtained from batch feeding (0.05 g L-1 h-1). Furthermore, 3-hydroxyoctanoate and -decanoate were the primary monomers incorporated into mcl-PHA. The investigation also determined the median glass transition temperature (-43°C) and melting temperature (47°C), which indicated that mcl-PHA is a flexible amorphous polymer at room temperature with a median MW of 104 kDa. However, information on the monomer composition or heterogeneity and the associated physical and mechanical data of mcl-PHAs is inadequate. Based on their mechanical values, the mcl-PHAs can be classified as semi-crystalline polymers (median crystallinity 23%) with rubber-like properties and a median elongation at break of 385%. However, due to the limited mechanical data available for mcl-PHAs with known monomer composition, identifying suitable processing tools and applications to develop mcl-PHAs further is challenging.

Copolymers and Blends Based on 3-Hydroxybutyrate and 3-Hydroxyvalerate Units

This review presents a comprehensive update of the biopolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), emphasizing its production, properties, and applications. The overall biosynthesis pathway of PHBV is explored in detail, highlighting recent advances in production techniques. The inherent physicochemical properties of PHBV, along with its degradation behavior, are discussed in detail. This review also explores various blends and composites of PHBV, demonstrating their potential for a range of applications. Finally, the versatility of PHBV-based materials in multiple sectors is examined, emphasizing their increasing importance in the field of biodegradable polymers.

Polyhydroxybutyrate synthesis in Camelina: Towards coproduction of renewable feedstocks for bioplastics and fuels

Plant-based co-production of polyhydroxyalkanoates (PHAs) and seed oil has the potential to create a viable domestic source of feedstocks for renewable fuels and plastics. PHAs, a class of biodegradable polyesters, can replace conventional plastics in many applications while providing full degradation in all biologically active environments. Here we report the production of the PHA poly[(R)-3-hydroxybutyrate] (PHB) in the seed cytosol of the emerging bioenergy crop Camelina sativa engineered with a bacterial PHB biosynthetic pathway. Two approaches were used: cytosolic localization of all three enzymes of the PHB pathway in the seed, or localization of the first two enzymes of the pathway in the cytosol and anchoring of the third enzyme required for polymerization to the cytosolic face of the endoplasmic reticulum (ER). The ER-targeted approach was found to provide more stable polymer production with PHB levels up to 10.2% of the mature seed weight achieved in seeds with good viability. These results mark a significant step forward towards engineering lines for commercial use. Plant-based PHA production would enable a direct link between low-cost large-scale agricultural production of biodegradable polymers and seed oil with the global plastics and renewable fuels markets.

Poly(3-hydroxybutyrate) Production from Lignocellulosic Wastes Using Bacillus megaterium ATCC 14581

This study aimed to analyze the production of poly(3-hydroxybutyrate) (PHB) from lignocellulosic biomass through a series of steps, including microwave irradiation, ammonia delignification, enzymatic hydrolysis, and fermentation, using the Bacillus megaterium ATCC 14581 strain. The lignocellulosic biomass was first pretreated using microwave irradiation at different temperatures (180, 200, and 220 °C) for 10, 20, and 30 min. The optimal pretreatment conditions were determined using the central composite design (CCD) and the response surface methodology (RSM). In the second step, the pretreated biomass was subjected to ammonia delignification, followed by enzymatic hydrolysis. The yield obtained for the pretreated and enzymatically hydrolyzed biomass was lower (70.2%) compared to the pretreated, delignified, and enzymatically hydrolyzed biomass (91.4%). These hydrolysates were used as carbon substrates for the synthesis of PHB using Bacillus megaterium ATCC 14581 in batch cultures. Various analytical methods were employed, namely nuclear magnetic resonance (1H-NMR and13C-NMR), electrospray ionization mass spectrometry (EI-MS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA), to identify and characterize the extracted PHB. The XRD analysis confirmed the partially crystalline nature of PHB.

Continuous polyhydroxybutyrate production from biogas in an innovative two-stage bioreactor configuration

Biogas biorefineries have opened up new horizons beyond heat and electricity production in the anaerobic digestion sector. Added-value products such as polyhydroxyalkanoates (PHAs), which are environmentally benign and potential candidates to replace conventional plastics, can be generated from biogas. This work investigated the potential of an innovative two-stage growth-accumulation system for the continuous production of biogas-based polyhydroxybutyrate (PHB) using Methylocystis hirsuta CSC1 as cell factory. The system comprised two turbulent bioreactors in series to enhance methane and oxygen mass transfer: a continuous stirred tank reactor (CSTR) and a bubble column bioreactor (BCB) with internal gas recirculation. The CSTR was devoted to methanotrophic growth under nitrogen balanced growth conditions and the BCB targeted PHB production under nitrogen limiting conditions. Two different operational approaches under different nitrogen loading rates and dilution rates were investigated. A balanced nitrogen loading rate along with a dilution rate (D) of 0.3 day-1 resulted in the most stable operating conditions and a PHB productivity of ~53 g PHB m-3 day-1 . However, higher PHB productivities (~127 g PHB m-3 day-1 ) were achieved using nitrogen excess at a D = 0.2 day-1 . Overall, the high PHB contents (up to 48% w/w) obtained in the CSTR under theoretically nutrient balanced conditions and the poor process stability challenged the hypothetical advantages conferred by multistage vs single-stage process configurations for long-term PHB production.

Heterologous constitutive production of short-chain-length polyhydroxyalkanoates in Pseudomonas putida KT2440: the involvement of IbpA inclusion body protein

Designing cell factories for the production of novel polyhydroxyalkanoates (PHAs) via smart metabolic engineering is key to obtain à la carte materials with tailored physicochemical properties. To this end, we used the model medium-chain-length-PHA producing bacterium, P. putida KT2440 as a chassis, which is characterized by its metabolic versatility and stress tolerance. Different PHA biosynthetic modules were assembled in expression plasmids using the Golden gate/MoClo modular assembly technique to implement an orthogonal short-chain-lengh-PHA (scl-PHA) switch in a "deaf" PHA mutant. This was specifically constructed to override endogenous multilevel regulation of PHA synthesis in the native strain. We generated a panel of engineered approaches carrying the genes from Rhodospirillum rubrum, Cupriavidus necator and Pseudomonas pseudoalcaligenes, demonstrating that diverse scl-PHAs can be constitutively produced in the chassis strain to varying yields from 23% to 84% PHA/CDW. Co-feeding assays of the most promising engineered strain harboring the PHA machinery from C. necator resulted to a panel of PHBV from 0.6% to 19% C5 monomeric incorporation. Chromosomally integrated PHA machineries with high PhaCCn synthase dosage successfully resulted in 68% PHA/CDW production. Interestingly, an inverse relationship between PhaC synthase dosage and granule size distribution was demonstrated in the heterologous host. In this vein, it is proposed the key involvement of inclusion body protein IbpA to the heterologous production of tailored PHA in P. putida KT2440.

Biodegradable Polyhydroxyalkanoates with a Different Set of Valerate Monomers: Chemical Structure and Physicochemical Properties

The properties, features of thermal behavior and crystallization of copolymers containing various types of valerate monomers were studied depending on the set and ratio of monomers. We synthesized and studied the properties of three-component copolymers containing unusual monomers 4-hydroxyvalerate (4HV) and 3-hydroxy-4-methylvalerate (3H4MV), in addition to the usual 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) monomers. The results showed that P(3HB-co-3HV-co-4HV) and P(3HB-co-3HV-co-3H4MV) terpolymers tended to increase thermal stability, especially for methylated samples, including an increase in the gap between melting point (Tmelt) and thermal degradation temperature (Tdegr), an increase in the melting point and glass transition temperature, as well as a lower degree of crystallinity (40-46%) compared with P(3HB-co-3HV) (58-66%). The copolymer crystallization kinetics depended on the set and ratio of monomers. For terpolymers during exothermic crystallization, higher rates of spherulite formation (Gmax) were registered, reaching, depending on the ratio of monomers, 1.6-2.0 µm/min, which was several times higher than the Gmax index (0.52 µm/min) for the P(3HB-co-3HV) copolymer. The revealed differences in the thermal properties and crystallization kinetics of terpolymers indicate that they are promising polymers for processing into high quality products from melts.

Biosynthesis and Properties of Sulfur-Containing Polyhydroxyalkanoates (PHAs) Produced by Wild-Type Strain Cupriavidus necator B-10646

The study addresses the growth of the wild-type strain Cupriavidus necator B-10646 and the synthesis of sulfur-containing polyhydroxyalkanoates (PHA) by this strain on media containing fructose and three different precursors (3-mercaptopropionic acid, 3',3'-dithiodipropionic acid and 3',3'-thiodipropionic acid). By varying the concentration and number of doses of the precursors added into the bacterial culture, it was possible to find conditions that ensure the formation of 3-mercaptopropionate (3MP) monomers from the precursors and their incorporation into the C-chain of poly(3-hydroxybutyrate). A series of P(3HB-co-3MP) copolymer samples with different content of 3MP monomers (from 2.04 to 39.0 mol.%) were synthesized and the physicochemical properties were studied. The effect of 3MP monomers is manifested in a certain decrease in the molecular weight of the samples and an increase in polydispersity. Temperature changes are manifested in the appearance of two peaks in the melting region with different intervals regardless of the 3MP content. The studied P(3HB-co-3MP) samples, regardless of the content of 3MP monomers, are characterized by equalization of the ratio of the amorphous and crystalline phases and have a close degree of crystallinity with a minimum of 42%, = and a maximum of 54%.

Complete genome sequence of Aquitalea pelogenes USM4 (JCM19919), a polyhydroxyalkanoate producer

Polyhydroxyalkanoate (PHA) is a type of biopolymer produced by most bacteria and archaea, resembling thermoplastic with biodegradability and biocompatibility features. Here, we report the complete genome of a PHA producer, Aquitalea sp. USM4, isolated from Perak, Malaysia. This bacterium possessed a 4.2 Mb circular chromosome and a 54,370 bp plasmid. A total of 4067 predicted protein-coding sequences, 87 tRNA genes, and 25 rRNA operons were identified using PGAP. Based on ANI and dDDH analysis, the Aquitalea sp. USM4 is highly similar to Aquitalea pelogenes. We also identified genes, including acetyl-CoA (phaA), acetoacetyl-CoA (phaB), PHA synthase (phaC), enoyl-CoA hydratase (phaJ), and phasin (phaP), which play an important role in PHA production in Aquitalea sp. USM4. The heterologous expression of phaC1 from Aquitalea sp. USM4 in Cupriavidus necator PHB^-4 was able to incorporate six different types of PHA monomers, which are 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), 4-hydroxybutyrate (4HB), 5-hydroxyvalerate (5HV), 3-hydroxyhexanoate (3HHx) and isocaproic acid (3H4MV) with suitable precursor substrates. This is the first complete genome sequence of the genus Aquitalea among the 22 genome sequences from 4 Aquitalea species listed in the GOLD database, which provides an insight into its genome evolution and molecular machinery responsible for PHA biosynthesis.

A review on microbial synthesis of lactate-containing polyesters

Degradable polylactic acids (PLA) have been widely used in agriculture, textile, medicine and degradable plastics industry, and can completely replace petroleum-based plastics in the future. At present, polylactic acid was chemically synthesized by ring-opening polymerisation or the direct polycondensation of lactic acid, which inevitably leads to chemical and heavy metal catalyst pollution. The current research focus has gradually shifted to the development of recombinant industrial strains for the efficiently production of lactate-containing polyesters from renewable resources. This review summarizes various explorations of metabolic pathway optimization and production cost control in the industrialization of lactate-containing polyesters bio-production. In particular, the effects of key enzymes, including CoA transferase, polyhydroxyalkanoate synthase, and their mutants, culture conditions, low-cost carbon sources, and recombinant strains on the yield and composition of lactate-containing polyesters are summarized and discussed. Future prospects and challenges for the industrialization of lactate-containing polyesters are also pointed out.

Role of carbon-dioxide sequestering bacteria for clean air environment and prospective production of biomaterials: a sustainable approach

The increase in demand of fossil fuel uses for developmental activity and manufacturing of goods have resulted a huge emission of global warming gases (GWGs) in the atmosphere. Among all GWGs, CO₂ is the major contributor that inevitably causes global warming and climate change. Mitigation strategies like biological CO₂ capture through sequestration and their storage into biological organic form are used to minimize the concentration of atmospheric CO₂ with the goal to control climate change. Since increasing atmospheric CO₂ level supports microbial growth and productivity thus microbial-based CO₂ sequestration has remarkable advantages as compared to plant-based sequestration. This review focuses on CO₂ sequestration mechanism in bacteria through different carbon fixation pathways, involved enzymes, their role in calcite, and other environmentally friendly biomaterials such as biofuel, bioplastic, and biosurfactant.

Post-Synthetic Enzymatic and Chemical Modifications for Novel Sustainable Polyesters

Because of their biodegradability, compostability, compatibility and flexible structures, biodegradable polymers such as polyhydroxyalkanoates (PHA) are an important class of biopolymers with various industrial and biological uses. PHAs are thermoplastic polyesters with a limited processability due to their low heat resistance. Furthermore, due to their high crystallinity, some PHAs are stiff and brittle. These features result sometimes in very poor mechanical characteristics with low extension at break values which limit the application range of some natural PHAs. Several in vivo approaches for PHA copolymer modifications range from polymer production to enhance PHA-based material performance after synthesis. The methods for enzymatic and chemical polymer modifications are aiming at modifying the structures of the polyesters and thereby their characteristics while retaining the biodegradability. This survey illustrates the efficient use of enzymes and chemicals in post-synthetic PHA modifications, offering insights on these green techniques for modifying and improving polymer performance. Important studies in this sector will be reviewed, as well as chances and obstacles for their stability and hyper-production.

Production of a newly discovered PHA family member with an isobutyrate-fed enrichment culture

Abstract

Using microbial enrichment cultures for the production of waste-derived polyhydroxyalkanoates (PHAs) is a promising technology to recover secondary resources. Volatile fatty acids (VFAs) form the preferred substrate for PHA production. Isobutyrate is a VFA appearing in multiple waste valorization routes, such as anaerobic fermentation, chain elongation, and microbial electrosynthesis, but has never been assessed individually on its PHA production potential. This research investigates isobutyrate as sole carbon source for a microbial enrichment culture in comparison to its structural isomer butyrate. The results reveal that the enrichment of isobutyrate has a very distinct character regarding microbial community development, PHA productivity, and even PHA composition. Although butyrate is a superior substrate in almost every aspect, this research shows that isobutyrate-rich waste streams have a noteworthy PHA-producing potential. The main finding is that the dominant microorganism, a Comamonas sp., is linked to the production of a unique PHA family member, poly(3-hydroxyisobutyrate) (PHiB), up to 37% of the cell dry weight. This is the first scientific report identifying microbial PHiB production, demonstrating that mixed microbial communities can be a powerful tool for discovery of new metabolic pathways and new types of polymers.

Key points

• PHiB production is a successful storage strategy in an isobutyrate-fed SBR

• Isomers isobutyrate and butyrate reveal a very distinct PHA production behavior

• Enrichments can be a tool for discovery of new metabolic pathways and polymers

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11742-9.

Engineering Native and Synthetic Pathways in Pseudomonas putida for the Production of Tailored Polyhydroxyalkanoates

Growing environmental concern sparks renewed interest in the sustainable production of (bio)materials that can replace oil-derived goods. Polyhydroxyalkanoates (PHAs) are isotactic polymers that play a critical role in the central metabolism of producer bacteria, as they act as dynamic reservoirs of carbon and reducing equivalents. PHAs continue to attract industrial attention as a starting point toward renewable, biodegradable, biocompatible, and versatile thermoplastic and elastomeric materials. Pseudomonas species have been known for long as efficient biopolymer producers, especially for medium-chain-length PHAs. The surge of synthetic biology and metabolic engineering approaches in recent years offers the possibility of exploiting the untapped potential of Pseudomonas cell factories for the production of tailored PHAs. In this article, an overview of the metabolic and regulatory circuits that rule PHA accumulation in Pseudomonas putida is provided, and approaches leading to the biosynthesis of novel polymers (e.g., PHAs including nonbiological chemical elements in their structures) are discussed. The potential of novel PHAs to disrupt existing and future market segments is closer to realization than ever before. The review is concluded by pinpointing challenges that currently hinder the wide adoption of bio-based PHAs, and strategies toward programmable polymer biosynthesis from alternative substrates in engineered P. putida strains are proposed.

Microbial Polyhydroxyalkanoates and Nonnatural Polyesters

Microorganisms produce diverse polymers for various purposes such as storing genetic information, energy, and reducing power, and serving as structural materials and scaffolds. Among these polymers, polyhydroxyalkanoates (PHAs) are microbial polyesters synthesized and accumulated intracellularly as a storage material of carbon, energy, and reducing power under unfavorable growth conditions in the presence of excess carbon source. PHAs have attracted considerable attention for their wide range of applications in industrial and medical fields. Since the first discovery of PHA accumulating bacteria about 100 years ago, remarkable advances have been made in the understanding of PHA biosynthesis and metabolic engineering of microorganisms toward developing efficient PHA producers. Recently, nonnatural polyesters have also been synthesized by metabolically engineered microorganisms, which opened a new avenue toward sustainable production of more diverse plastics. Herein, the current state of PHAs and nonnatural polyesters is reviewed, covering mechanisms of microbial polyester biosynthesis, metabolic pathways, and enzymes involved in biosynthesis of short-chain-length PHAs, medium-chain-length PHAs, and nonnatural polyesters, especially 2-hydroxyacid-containing polyesters, metabolic engineering strategies to produce novel polymers and enhance production capabilities and fermentation, and downstream processing strategies for cost-effective production of these microbial polyesters. In addition, the applications of PHAs and prospects are discussed.

Beyond Intracellular Accumulation of Polyhydroxyalkanoates: Chiral Hydroxyalkanoic Acids and Polymer Secretion

Polyhydroxyalkanoates (PHAs) are ubiquitous prokaryotic storage compounds of carbon and energy, acting as sinks for reducing power during periods of surplus of carbon source relative to other nutrients. With close to 150 different hydroxyalkanoate monomers identified, the structure and properties of these polyesters can be adjusted to serve applications ranging from food packaging to biomedical uses. Despite its versatility and the intensive research in the area over the last three decades, the market share of PHAs is still low. While considerable rich literature has accumulated concerning biochemical, physiological, and genetic aspects of PHAs intracellular accumulation, the costs of substrates and processing costs, including the extraction of the polymer accumulated in intracellular granules, still hampers a more widespread use of this family of polymers. This review presents a comprehensive survey and critical analysis of the process engineering and metabolic engineering strategies reported in literature aimed at the production of chiral (R)-hydroxycarboxylic acids (RHAs), either from the accumulated polymer or by bypassing the accumulation of PHAs using metabolically engineered bacteria, and the strategies developed to recover the accumulated polymer without using conventional downstream separations processes. Each of these topics, that have received less attention compared to PHAs accumulation, could potentially improve the economy of PHAs production and use. (R)-hydroxycarboxylic acids can be used as chiral precursors, thanks to its easily modifiable functional groups, and can be either produced de-novo or be obtained from recycled PHA products. On the other hand, efficient mechanisms of PHAs release from bacterial cells, including controlled cell lysis and PHA excretion, could reduce downstream costs and simplify the polymer recovery process.

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Production by Rhodospirillum rubrum Using a Two-Step Culture Strategy

Polyhydroxyalkanoates (PHAs) are microbially synthesized biopolyesters which have attracted great attentions as a new biological material, potential alternative to traditional fossil fuel-based plastic due to their biodegradability and biocompatibility. Poly-3-hydroxybutyrate (PHB) and poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) are the most common members of PHAs. In this study, the nonsulfur and facultatively phototrophic bacterium Rhodospirillum rubrum was cultivated to accumulate PHA by a two-step culture strategy. Gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), and nuclear magnetic resonance spectroscopy (NMR) analyses showed that PHAs synthesized from fructose was PHBV, in which the 3HV content was 46.5 mol%, which means the better mechanical property. The molecular weight, distribution, and thermal features were characterized by gel permeation chromatography (GPC), differential scanning calorimeter (DSC), and thermo gravimetric analysis (TGA), respectively. The low PDI of 1.08 revealed the narrow and evenly molar mass distribution which shows the stable features. The high melting temperature and their other physical properties implied their potential applications. The traditional process of producing PHBV involves related carbon sources such as valeric acid. However, our study clearly described a new medium formula with fructose and a complete fermentation method to produce PHBV with a high 3HV faction and low molecular distribution.

Cellular architecture and migration behavior of fibroblast cells on polyhydroxyoctanoate (PHO): A natural polymer of bacterial origin

Biodegradable and biocompatible novel materials of natural origin are gaining more and more attention in recent years. These so called biopolymers, characterized by their biointegrity and biocompatibility, find completely new and promising applications in biomedical sciences. The presented work focuses on the medium chain length elastomeric polyhydroxyalkanoate biopolymer-polyhydroxyoctanoate (PHO). This biopolymer is fully biodegradable without formation of harmful byproducts.We investigated PHO's physical properties with nanoindentation technique and scratch testing to determine Young's modulus and friction coefficient. Further, the work focused on the impact of PHO, used as growth substrate, on the physiology and morphology of mouse embryonic fibroblast cells (MEF 3T3). Application of fluorescent staining protocols and advanced microscopic techniques allowed to study the morphological changes in the cytoskeletons of cells grown on PHO and also gave an insight into their migration strategies on the polymer surface. We found that PHO exhibits no cellular cytotoxicity, similarly to a glass substrate. MEF cells spread better on glass surface than on each tested PHO substrate though there was almost no difference between PHO substrates cast from different solvents. However, a detailed analysis of actin and microtubule cytoskeletal architecture reveals changes in the density of actin and microtubular networks. Migration of MEF cells on PHO substrates was slower than on the glass substrate. To elucidate the molecular mechanisms of observed changes in cytoskeletal architecture and migration parameters can be of special interest for future medical application of PHO polymer.

Systems Metabolic Engineering Strategies for Non-Natural Microbial Polyester Production

Plastics, used everyday, are mostly synthetic polymers derived from fossil resources, and their accumulation is becoming a serious concern worldwide. Polyhydroxyalkanoates (PHAs) are naturally produced polyesters synthesized and intracellularly accumulated by many different microorganisms. PHAs are good alternatives to petroleum-based plastics because they possess a wide range of material properties depending on monomer types and molecular weights. In addition, PHAs are biodegradable and can be produced from renewable biomass. Thus, producing PHAs through the development of high-performance engineered microorganisms and efficient bioprocesses gained much interest. In addition, non-natural polyesters comprising 2-hydroxycarboxylic acids as monomers have been produced by fermentation of metabolically engineered bacteria. For example, poly(lactic acid) and poly(lactic acid-co-glycolic acid), which have been chemically synthesized using the corresponding monomers either fermentatively or chemically produced, can be produced by metabolically engineered bacteria by one-step fermentation. Recently, PHAs containing aromatic monomers could be produced by fermentation of metabolically engineered bacteria. Here, metabolic engineering strategies applied in developing microbial strains capable of producing non-natural polyesters in a stepwise manner are reviewed. It is hoped that the detailed strategies described will be helpful for designing metabolic engineering strategies for developing diverse microbial strains capable of producing various polymers that can replace petroleum-derived polymers.

Recovery of polyhydroxyalkanoates from municipal secondary wastewater sludge

In the current study, the feasibility of utilizing municipal secondary wastewater sludge for Polyhydroxyalkanoate (PHA) extraction was improved by optimization of various parameters (temperature, duration and concentration of sludge solids). Optimized process parameters resulted in PHA recovery of 0.605 g, significantly higher than un-optimized conditions. The characterization of PHA was carried out by GC-MS, FT-IR and NMR (¹H and ¹³C) spectroscopy. The PHA profile was found to be dominated by mcl PHA (58%) along with other diverse PHA. The results of the present study show rich diversity of PHA extracted from a raw material which is readily available at minimal cost. In conclusion, exploring the potential of wastes for production of bioplastics not only reduces the cost of bioplastic production, but also provides a sustainable means for waste management.

Controlling microbial PHB synthesis via CRISPRi

Microbial polyhydroxyalkanoates (PHA) are a family of biopolyesters with properties similar to petroleum plastics such as polyethylene (PE) or polypropylene (PP). Polyhydroxybutyrate (PHB) is the most common PHA known so far. Clustered regularly interspaced short palindromic repeats interference (CRISPRi), a technology recently developed to control gene expression levels in eukaryotic and prokaryotic genomes, was employed to regulate PHB synthase activity influencing PHB synthesis. Recombinant Escherichia coli harboring an operon of three PHB synthesis genes phaCAB cloned from Ralstonia eutropha, was transformed with various single guided RNA (sgRNA with its guide sequence of 20-23 bases) able to bind to various locations of the PHB synthase PhaC, respectively. Depending on the binding location and the number of sgRNA on phaC, CRISPRi was able to control the phaC transcription and thus PhaC activity. It was found that PHB content, molecular weight, and polydispersity were approximately in direct and reverse proportion to the PhaC activity, respectively. The higher the PhaC activity, the more the intracellular PHB accumulation, yet the less the PHB molecular weights and the wider the polydispersity. This study allowed the PHB contents to be controlled in the ranges of 1.47-75.21% cell dry weights, molecular weights from 2 to 6 millions Dalton and polydispersity of 1.2 to 1.43 in 48 h shake flask studies. This result will be very important for future development of ultrahigh molecular weight PHA useful to meet high strength application requirements.

Footprint area analysis of binary imaged Cupriavidus necator cells to study PHB production at balanced, transient, and limited growth conditions in a cascade process

Statistical distribution of cell and poly[3-(R)-hydroxybutyrate] (PHB) granule size and number of granules per cell are investigated for PHB production in a five-stage cascade (5CSTR). Electron microscopic pictures of cells from individual cascade stages (R1-R5) were converted to binary pictures to visualize footprint areas for polyhydroxyalkanoate (PHA) and non-PHA biomass. Results for each stage were correlated to the corresponding experimentally determined kinetics (specific growth rate μ and specific productivity π). Log-normal distribution describes PHA granule size dissimilarity, whereas for R1 and R4, gamma distribution best reflects the situation. R1, devoted to balanced biomass synthesis, predominately contains cells with rather small granules, whereas with increasing residence time τ, maximum and average granule sizes by trend increase, approaching an upper limit determined by the cell's geometry. Generally, an increase of intracellular PHA content and ratio of granule to cell area slow down along the cascade. Further, the number of granules per cell decreases with increasing τ. Data for μ and π obtained by binary picture analysis correlate well with the experimental results. The work describes long-term continuous PHA production under balanced, transient, and nutrient-deficient conditions, as well as their reflection on the granules size, granule number, and cell structure on the microscopic level.

Self-Assembled Protein-Coated Polyhydroxyalkanoate Beads: Properties and Biomedical Applications

Polyhydroxyalkanoates (PHAs) are biological polyesters that can be naturally produced by a range of bacteria as water-insoluble inclusions composed of a PHA core coated with PHA synthesis, structural, and regulatory proteins. These naturally self-assembling shell-core particles have been recently conceived as biomaterials that can be bioengineered as biologically active beads for medical applications. Protein engineering of PHA-associated proteins enabled the production of PHA-protein assemblies exhibiting biologically active protein-based functions relevant for applications as vaccines or diagnostics. Here we provide an overview of the recent advances in bioengineering of PHA particles toward the display of biomedically relevant protein functions such as selected disease-specific antigens as diagnostic tools or for the design of particulate subunit vaccines against infectious diseases such as tuberculosis, meningitis, pneumonia, and hepatitis C.

Impact of Ralstonia eutropha's poly(3-Hydroxybutyrate) (PHB) Depolymerases and Phasins on PHB storage in recombinant Escherichia coli

The model organism for polyhydroxybutyrate (PHB) biosynthesis, Ralstonia eutropha H16, possesses multiple isoenzymes of granules coating phasins as well as of PHB depolymerases, which degrade accumulated PHB under conditions of carbon limitation. In this study, recombinant Escherichia coli BL21(DE3) strains were used to study the impact of selected PHB depolymerases of R. eutropha H16 on the growth behavior and on the amount of accumulated PHB in the absence or presence of phasins. For this purpose, 20 recombinant E. coli BL21(DE3) strains were constructed, which harbored a plasmid carrying the phaCAB operon from R. eutropha H16 to ensure PHB synthesis and a second plasmid carrying different combinations of the genes encoding a phasin and a PHB depolymerase from R. eutropha H16. It is shown in this study that the growth behavior of the respective recombinant E. coli strains was barely affected by the overexpression of the phasin and PHB depolymerase genes. However, the impact on the PHB contents was significantly greater. The strains expressing the genes of the PHB depolymerases PhaZ1, PhaZ2, PhaZ3, and PhaZ7 showed 35% to 94% lower PHB contents after 30 h of cultivation than the control strain. The strain harboring phaZ7 reached by far the lowest content of accumulated PHB (only 2.0% [wt/wt] PHB of cell dry weight). Furthermore, coexpression of phasins in addition to the PHB depolymerases influenced the amount of PHB stored in cells of the respective strains. It was shown that the phasins PhaP1, PhaP2, and PhaP4 are not substitutable without an impact on the amount of stored PHB. In particular, the phasins PhaP2 and PhaP4 seemed to limit the degradation of PHB by the PHB depolymerases PhaZ2, PhaZ3, and PhaZ7, whereas almost no influence of the different phasins was observed if phaZ1 was coexpressed. This study represents an extensive analysis of the impact of PHB depolymerases and phasins on PHB accumulation and provides a deeper insight into the complex interplay of these enzymes.

Improved productivity of poly (4-hydroxybutyrate) (P4HB) in recombinant Escherichia coli using glycerol as the growth substrate with fed-batch culture

Background

The most successful polyhydroxyalkanoate (PHA) in medical applications is poly(4-hydroxybutyrate) (P4HB), which is due to its biodegradability, biocompatibility and mechanical properties. One of the major obstacles for wider applications of P4HB is the cost of production and purification. It is highly desired to obtain P4HB in large scale at a competitive cost.

Results

In this work, we studied the possibility to increase P4HB productivity by using high cell density culture. To do so, we investigated for the first time some of the most relevant factors influencing P4HB biosynthesis in recombinant Escherichia coli. We observed that P4HB biosynthesis correlated more with limitations of amino acids and less with nitrogen depletion, contrary to the synthesis of many other types of PHAs. Furthermore, it was found that using glycerol as the primary carbon source, addition of acetic acid at the beginning of a batch culture stimulated P4HB accumulation in E. coli. Fed-batch high cell density cultures were performed to reach high P4HB productivity using glycerol as the sole carbon source for cell growth and 4HB as the precursor for P4HB synthesis. A P4HB yield of 15 g L⁻¹ was obtained using an exponential feeding mode, leading to a productivity of 0.207 g L⁻¹ h⁻¹, which is the highest productivity for P4HB reported so far.

Conclusions

We demonstrated that the NZ-amines (amino acids source) in excess abolished P4HB accumulation, suggesting that limitation in certain amino acid pools promotes P4HB synthesis. Furthermore, the enhanced P4HB yield could be achieved by both the effective growth of E. coli JM109 (pKSSE5.3) on glycerol and the stimulated P4HB synthesis via exogenous addition of acetic acid. We have developed fermentation strategies for P4HB production by using glycerol, leading to a productivity of 0.207 g L⁻¹ h⁻¹ P4HB. This high P4HB productivity will decrease the total production cost, allowing further development of P4HB applications.

Microbial production of lactate-containing polyesters

Due to our increasing concerns on environmental problems and limited fossil resources, biobased production of chemicals and materials through biorefinery has been attracting much attention. Optimization of the metabolic performance of microorganisms, the key biocatalysts for the efficient production of the desired target bioproducts, has been achieved by metabolic engineering. Metabolic engineering allowed more efficient production of polyhydroxyalkanoates, a family of microbial polyesters. More recently, non-natural polyesters containing lactate as a monomer have also been produced by one-step fermentation of engineered bacteria. Systems metabolic engineering integrating traditional metabolic engineering with systems biology, synthetic biology, protein/enzyme engineering through directed evolution and structural design, and evolutionary engineering, enabled microorganisms to efficiently produce natural and non-natural products. Here, we review the strategies for the metabolic engineering of microorganisms for the in vivo biosynthesis of lactate-containing polyesters and for the optimization of whole cell metabolism to efficiently produce lactate-containing polyesters. Also, major problems to be solved to further enhance the production of lactate-containing polyesters are discussed.

Versatile metabolic adaptations of Ralstonia eutropha H16 to a loss of PdhL, the E3 component of the pyruvate dehydrogenase complex

A previous study reported that the Tn5-induced poly(3-hydroxybutyric acid) (PHB)-leaky mutant Ralstonia eutropha H1482 showed a reduced PHB synthesis rate and significantly lower dihydrolipoamide dehydrogenase (DHLDH) activity than the wild-type R. eutropha H16 but similar growth behavior. Insertion of Tn5 was localized in the pdhL gene encoding the DHLDH (E3 component) of the pyruvate dehydrogenase complex (PDHC). Taking advantage of the available genome sequence of R. eutropha H16, observations were verified and further detailed analyses and experiments were done. In silico genome analysis revealed that R. eutropha possesses all five known types of 2-oxoacid multienzyme complexes and five DHLDH-coding genes. Of these DHLDHs, only PdhL harbors an amino-terminal lipoyl domain. Furthermore, insertion of Tn5 in pdhL of mutant H1482 disrupted the carboxy-terminal dimerization domain, thereby causing synthesis of a truncated PdhL lacking this essential region, obviously leading to an inactive enzyme. The defined ΔpdhL deletion mutant of R. eutropha exhibited the same phenotype as the Tn5 mutant H1482; this excludes polar effects as the cause of the phenotype of the Tn5 mutant H1482. However, insertion of Tn5 or deletion of pdhL decreases DHLDH activity, probably negatively affecting PDHC activity, causing the mutant phenotype. Moreover, complementation experiments showed that different plasmid-encoded E3 components of R. eutropha H16 or of other bacteria, like Burkholderia cepacia, were able to restore the wild-type phenotype at least partially. Interestingly, the E3 component of B. cepacia possesses an amino-terminal lipoyl domain, like the wild-type H16. A comparison of the proteomes of the wild-type H16 and of the mutant H1482 revealed striking differences and allowed us to reconstruct at least partially the impressive adaptations of R. eutropha H1482 to the loss of PdhL on the cellular level.

Elucidation of beta-oxidation pathways in Ralstonia eutropha H16 by examination of global gene expression

Ralstonia eutropha H16 is capable of growth and polyhydroxyalkanoate production on plant oils and fatty acids. However, little is known about the triacylglycerol and fatty acid degradation pathways of this bacterium. We compare whole-cell gene expression levels of R. eutropha H16 during growth and polyhydroxyalkanoate production on trioleate and fructose. Trioleate is a triacylglycerol that serves as a model for plant oils. Among the genes of note, two potential fatty acid β-oxidation operons and two putative lipase genes were shown to be upregulated in trioleate cultures. The genes of the glyoxylate bypass also exhibit increased expression during growth on trioleate. We observed that single β-oxidation operon deletion mutants of R. eutropha could grow using palm oil or crude palm kernel oil as the sole carbon source, regardless of which operon was present in the genome, but a double mutant was unable to grow under these conditions. A lipase deletion mutant did not exhibit a growth defect in emulsified oil cultures but did exhibit a phenotype in cultures containing nonemulsified oil. Mutants of the glyoxylate shunt gene for isocitrate lyase were able to grow in the presence of oils, while a malate synthase (aceB) deletion mutant grew more slowly than wild type. Gene expression under polyhydroxyalkanoate storage conditions was also examined. Many findings of this analysis confirm results from previous studies by our group and others. This work represents the first examination of global gene expression involving triacylglycerol and fatty acid catabolism genes in R. eutropha.

Impact of multiple beta-ketothiolase deletion mutations in Ralstonia eutropha H16 on the composition of 3-mercaptopropionic acid-containing copolymers

beta-Ketothiolases catalyze the first step of poly(3-hydroxybutyrate) [poly(3HB)] synthesis in bacteria by condensing two molecules of acetyl coenzyme A (acetyl-CoA) to acetoacetyl-CoA. Analyses of the genome sequence of Ralstonia eutropha H16 revealed 15 isoenzymes of PhaA in this bacterium. In this study, we generated knockout mutants of various phaA homologues to investigate their role in and contributions to poly(3HB) metabolism and to suppress biosynthesis of 3HB-CoA for obtaining enhanced molar 3-mercaptopriopionate (3MP) contents in poly(3HB-co-3MP) copolymers when cells were grown on gluconate plus 3-mercaptopropionate or 3,3'-dithiodipropionate. In silico sequence analysis of PhaA homologues, transcriptome data, and other aspects recommended the homologues phaA, bktB, H16_A1713/H16_B1771, H16_A1528, H16_B1369, H16_B0381, and H16_A0170 for further analysis. Single- and multiple-deletion mutants were generated to investigate the influence of these beta-ketothiolases on growth and polymer accumulation. The deletion of single genes resulted in no significant differences from the wild type regarding growth and polymer accumulation during cultivation on gluconate or gluconate plus 3MP. Deletion of phaA plus bktB (H16Delta2 mutant) resulted in approximately 30% less polymer accumulation than in the wild type. Deletion of H16_A1713/H16_B1771, H16_A1528, H16_B0381, and H16_B1369 in addition to phaA and bktB gave no differences in comparison to the H16Delta2 mutant. In contrast, deletion of H16_A0170 additionally to phaA and bktB yielded a mutant which accumulated about 30% poly(3HB) (wt/wt of the cell dry weight [CDW]). Although we were not able to suppress poly(3HB) biosynthesis completely, the copolymer compositions could be altered significantly with a lowered percentage ratio of 3HB constituents (from 85 to 52 mol%) and an increased percentage ratio of 3MP constituents (from 15 to 48 mol%), respectively. In this study, we demonstrated that PhaA, BktB, and H16_A0170 are majorly involved in poly(3HB) synthesis in R. eutropha H16. A fourth beta-ketothiolase or a combination of several of the other beta-ketothiolases contributed to a maximum of only 30% (wt/wt of CDW) of the remaining (co)polymer.

Zobellella denitrificans strain MW1, a newly isolated bacterium suitable for poly(3-hydroxybutyrate) production from glycerol

AIMS

To search for new bacteria for efficient production of polyhydroxyalkanoates (PHAs) from glycerol.

METHODS AND RESULTS

Samples were taken from different environments in Germany and Egypt, and bacteria capable of growing in mineral salts medium with glycerol as sole carbon source were enriched. From a wastewater sediment sample in Egypt, a Gram-negative bacterium (strain MW1) was isolated that exhibited good growth and that accumulated considerable amounts of polyhydroxybutyrate (PHB) from glycerol and also from other carbon sources. The 16S rRNA gene sequence of this isolate exhibited 98.5% and 96.2% similarity to Zobellella denitrificans strain ZD1 and to Zobellella taiwanensis strain ZT1 respectively. The isolate was therefore affiliated as strain MW1 of Z. denitrificans. Strain MW1 grows optimally on glycerol at 41 degrees C and pH 7.3 and accumulated PHB up to 80.4% (w/w) of cell dry weight. PHB accumulation was growth-associated. Although it was not an absolute requirement, 20 g l(-1) sodium chloride enhanced both growth (5 g cell dry weight per litre) and PHB content (87%, w/w). Zobellella denitrificans strain MW1 is also capable to accumulate the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer if sodium propionate was used as cosubstrate in addition to glycerol.

CONCLUSIONS

A new PHB-accumulating strain was isolated and identified. This strain is able to utilize glycerol for growth and PHB accumulation to high content especially in the presence of NaCl that will enable the utilization of waste glycerol from biodiesel industry.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is the first report on accumulation of PHA in a member of the new genus Zobellella. Furthermore, utilization of glycerol as the sole carbon source for fast growth and PHB biosynthesis, growth in the presence of NaCl and high PHB contents of the cells will make this newly isolated bacterium a potent candidate for industrial production of PHB from crude glycerol occurring as byproduct during biodiesel production.

Plants as bioreactors: Recent developments and emerging opportunities

In recent years, the use of plants as bioreactors has emerged as an exciting area of research and significant advances have created new opportunities. The driving forces behind the rapid growth of plant bioreactors include low production cost, product safety and easy scale up. As the yield and concentration of a product is crucial for commercial viability, several strategies have been developed to boost up protein expression in transgenic plants. Augmenting tissue-specific transcription, elevating transcript stability, tissue-specific targeting, translation optimization and sub-cellular accumulation are some of the strategies employed. Various kinds of products that are currently being produced in plants include vaccine antigens, medical diagnostics proteins, industrial and pharmaceutical proteins, nutritional supplements like minerals, vitamins, carbohydrates and biopolymers. A large number of plant-derived recombinant proteins have reached advanced clinical trials. A few of these products have already been introduced in the market.

Disruption of the 2-methylcitric acid cycle and evaluation of poly-3-hydroxybutyrate-co-3-hydroxyvalerate biosynthesis suggest alternate catabolic pathways of propionate inBurkholderia sacchari

The objective of the present work was to evaluate the relevance of the 2-methylcitric acid cycle (2MCC) to the catabolism of propionate in Burkholderia sacchari . Two B. sacchari mutants unable to grow on propionate were obtained: one disrupted in acnM, and the other in acnM and prpC deleted. An operative 2MCC significantly reduces the bacterial ability to incorporate 3-hydroxyvalerate (3HV) into a biodegradable copolyester accumulated from carbohydrates plus propionate. The efficiency of the mutants in converting propionate to 3HV units (Y3HV/prp) increased from 0.09 g·g–1to 0.81–0.96 g·g–1, indicating that acnM and prpC are both essential for growth on propionate. None of the mutations resulted in achievement of the maximum theoretical Y3HV/prp(1.35 g·g–1). When increasing concentrations of propionate were supplied, decreasing values of Y3HV/prpwere observed. The results obtained corroborate the hypothesis of the presence of other propionate catabolic pathways in B. sacchari. The 2MCC would be the more operative pathway, but a second pathway, which remains to be elucidated, would assume more importance under propionate concentrations of 1 g·L–1or higher. The efficiency in converting propionate to 3HV units can be improved by decreasing the propionate concentrations, owing to the role of the 2MCC.

Large-scale production of poly(3-hydroxyoctanoic acid) by Pseudomonas putida GPo1 and a simplified downstream process

The suitability of Pseudomonas putida GPo1 for large-scale cultivation and production of poly(3-hydroxyoctanoate) (PHO) was investigated in this study. Three fed-batch cultivations of P. putida GPo1 at the 350- or 400-liter scale in a bioreactor with a capacity of 650 liters were done in mineral salts medium containing initially 20 mM sodium octanoate as the carbon source. The feeding solution included ammonium octanoate, which was fed at a relatively low concentration to promote PHO accumulation under nitrogen-limited conditions. During cultivation, the pH was regulated by addition of NaOH, NH(4)OH, or octanoic acid, which was used as an additional carbon source. Partial O(2) pressure (pO(2)) was adjusted to 20 to 40% by controlling the airflow and stirrer speed. Under the optimized conditions, P. putida GPo1 was able to grow to cell densities as high as 18, 37, and 53 g cells (dry mass) (CDM) per liter containing 49, 55, and 60% (wt/wt) of PHO, respectively. The resulting 40 kg CDM from these three cultivations was used directly for extraction of PHO. Three different methods of extraction of PHO were applied. From these, only acetone extraction showed better performance and resulted in 94% recovery of the PHO contents of cells. A novel mixture of precipitation solvents composed of 70% (vol/vol) methanol and 70% (vol/vol) ethanol was identified in this study. The ratio of PHO concentrate to the mixture was 0.2:1 (vol/vol) and allowed complete precipitation of PHO as white flakes. However, at a ratio of 1:1 (vol/vol) of the solvent mixture to PHO concentrate, a highly purified PHO was obtained. Precipitation yielded a dough-like polymeric material which was cast into thin layers and then shredded into small strips to allow evaporation of the remaining solvents. Gas chromatographic analysis revealed a purity of about 99% +/- 0.2% (wt/wt) of the polymer, which consisted mainly of 3-hydroxyoctanoic acid (96 mol%).