Evaluation of various carbon substrates for the biosynthesis of polyhydroxyalkanoates bearing functional groups by Pseudomonas putida

Bio-conversion of organic wastes towards polyhydroxyalkanoates

The bio-manufacturing of products with substantial commercial value, particularly polyhydroxyalkanoates (PHA), using cost-effective carbon sources through microorganisms, has garnered heightened attention from both the scientific community and industry over the past few decades. Opting for industrial PHA production from various organic wastes, spanning industrial, agricultural, municipal, and food-based sources, emerges as a wiser choice. This strategy not only eases the burden of recycling organic waste and curbs environmental pollution but also trims down PHA production costs, rendering these materials more competitive in commercial markets. In addition, PHAs are a family of renewable, environmentally friendly, fully biodegradable and biocompatible polyesters with a multitude of applications. This review provides an overview of recent developments in PHA production from organic wastes. It covers the optimization of diverse metabolic pathways for producing various types of PHA from organic waste sources, pre-treatment and downstream processing for PHA using unrelated organic wastes, and challenges in industrial production of PHA using unrelated organic waste feedstocks and the challenges faced in industrial PHA production from organic wastes, along with potential solutions. Lastly, this study suggests underlying research endeavors aimed at further enhancing of the feasibility of industrial PHA production from organic wastes as an alternative to current petroleum-based plastics in the near future.

Polyhydroxyalkanoate production in Pseudomonas putida from alkanoic acids of varying lengths

Many studies have been conducted to produce microbial polyhydroxyalkanoates (PHA), a biopolymer, from Pseudomonas sp. fed with various alkanoic acids. Because this previous data was collected using methodologies that varied in critical aspects, such as culture media and size range of alkanoic acids, it has been difficult to compare the results for a thorough understanding of the relationship between feedstock and PHA production. Therefore, this study utilized consistent culture media with a wide range of alkanoic acids (C7-C14) to produce medium chain length PHAs. Three strains of Pseudomonas putida (NRRL B-14875, KT2440, and GN112) were used, and growth, cell dry weight, PHA titer, monomer distribution, and molecular weights were all examined. It was determined that although all the strains produced similar PHA titers using C7-C9 alkanoic acids, significant differences were observed with the use of longer chain feedstocks. Specifically, KT2440 and its derivative GN112 produced higher PHA titers compared to B-14875 when fed longer chain alkanoates. We also compared several analytical techniques for determining amounts of PHA and found they produced different results. In addition, the use of an internal standard had a higher risk of calculating inaccurate concentrations compared to an external standard. These observations highlight the importance of considering this aspect of analysis when evaluating different studies.

Closing the Gap between Bio-Based and Petroleum-Based Plastic through Bioengineering

Bioplastics, which are plastic materials produced from renewable bio-based feedstocks, have been investigated for their potential as an attractive alternative to petroleum-based plastics. Despite the harmful effects of plastic accumulation in the environment, bioplastic production is still underdeveloped. Recent advances in strain development, genome sequencing, and editing technologies have accelerated research efforts toward bioplastic production and helped to advance its goal of replacing conventional plastics. In this review, we highlight bioengineering approaches, new advancements, and related challenges in the bioproduction and biodegradation of plastics. We cover different types of polymers, including polylactic acid (PLA) and polyhydroxyalkanoates (PHAs and PHBs) produced by bacterial, microalgal, and plant species naturally as well as through genetic engineering. Moreover, we provide detailed information on pathways that produce PHAs and PHBs in bacteria. Lastly, we present the prospect of using large-scale genome engineering to enhance strains and develop microalgae as a sustainable production platform.

Emergent Approaches to Efficient and Sustainable Polyhydroxyalkanoate Production

Petroleum-derived plastics dominate currently used plastic materials. These plastics are derived from finite fossil carbon sources and were not designed for recycling or biodegradation. With the ever-increasing quantities of plastic wastes entering landfills and polluting our environment, there is an urgent need for fundamental change. One component to that change is developing cost-effective plastics derived from readily renewable resources that offer chemical or biological recycling and can be designed to have properties that not only allow the replacement of current plastics but also offer new application opportunities. Polyhydroxyalkanoates (PHAs) remain a promising candidate for commodity bioplastic production, despite the many decades of efforts by academicians and industrial scientists that have not yet achieved that goal. This article focuses on defining obstacles and solutions to overcome cost-performance metrics that are not sufficiently competitive with current commodity thermoplastics. To that end, this review describes various process innovations that build on fed-batch and semi-continuous modes of operation as well as methods that lead to high cell density cultivations. Also, we discuss work to move from costly to lower cost substrates such as lignocellulose-derived hydrolysates, metabolic engineering of organisms that provide higher substrate conversion rates, the potential of halophiles to provide low-cost platforms in non-sterile environments for PHA formation, and work that uses mixed culture strategies to overcome obstacles of using waste substrates. We also describe historical problems and potential solutions to downstream processing for PHA isolation that, along with feedstock costs, have been an Achilles heel towards the realization of cost-efficient processes. Finally, future directions for efficient PHA production and relevant structural variations are discussed.

Portable bacterial CRISPR transcriptional activation enables metabolic engineering in Pseudomonas putida

CRISPR-Cas transcriptional programming in bacteria is an emerging tool to regulate gene expression for metabolic pathway engineering. Here we implement CRISPR-Cas transcriptional activation (CRISPRa) in P. putida using a system previously developed in E. coli. We provide a methodology to transfer CRISPRa to a new host by first optimizing expression levels for the CRISPRa system components, and then applying rules for effective CRISPRa based on a systematic characterization of promoter features. Using this optimized system, we regulate biosynthesis in the biopterin and mevalonate pathways. We demonstrate that multiple genes can be activated simultaneously by targeting multiple promoters or by targeting a single promoter in a multi-gene operon. This work will enable new metabolic engineering strategies in P. putida and pave the way for CRISPR-Cas transcriptional programming in other bacterial species.

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.

Engineering glucose metabolism for enhanced muconic acid production in Pseudomonas putida KT2440

Pseudomonas putida KT2440 has received increasing attention as an important biocatalyst for the conversion of diverse carbon sources to multiple products, including the olefinic diacid, cis,cis-muconic acid (muconate). P. putida has been previously engineered to produce muconate from glucose; however, periplasmic oxidation of glucose causes substantial 2-ketogluconate accumulation, reducing product yield and selectivity. Deletion of the glucose dehydrogenase gene (gcd) prevents 2-ketogluconate accumulation, but dramatically slows growth and muconate production. In this work, we employed adaptive laboratory evolution to improve muconate production in strains incapable of producing 2-ketogluconate. Growth-based selection improved growth, but reduced muconate titer. A new muconate-responsive biosensor was therefore developed to enable muconate-based screening using fluorescence activated cell sorting. Sorted clones demonstrated both improved growth and muconate production. Mutations identified by whole genome resequencing of these isolates indicated that glucose metabolism may be dysregulated in strains lacking gcd. Using this information, we used targeted engineering to recapitulate improvements achieved by evolution. Deletion of the transcriptional repressor gene hexR improved strain growth and increased the muconate production rate, and the impact of this deletion was investigated using transcriptomics. The genes gntZ and gacS were also disrupted in several evolved clones, and deletion of these genes further improved strain growth and muconate production. Together, these targets provide a suite of modifications that improve glucose conversion to muconate by P. putida in the context of gcd deletion. Prior to this work, our engineered strain lacking gcd generated 7.0 g/L muconate at a productivity of 0.07 g/L/h and a 38% yield (mol/mol) in a fed-batch bioreactor. Here, the resulting strain with the deletion of hexR, gntZ, and gacS achieved 22.0 g/L at 0.21 g/L/h and a 35.6% yield (mol/mol) from glucose in similar conditions. These strategies enabled enhanced muconic acid production and may also improve production of other target molecules from glucose in P. putida.

Extrusion Coating of Paper with Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)—Packaging Related Functional Properties

Taking into account the current trend for environmentally friendly solutions, paper coated with a biopolymer presents an interesting field for future packaging applications. This study covers the application of the biopolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) on a paper substrate via extrusion coating. The intention of this study is to analyse the effect of a plasticiser on the processability (melting point, film thickness) and the final properties (crystallinity, elongation at break) of PHBV. Up to 15 wt.% of the plasticisers triethyl citrate (TEC) and polyethylene glycol (PEG) were used as additive. The processing (including melt flow rate) as well as the structural properties (melting and crystallisation temperature, surface structure by atomic force microscopy (AFM), polarisation microscopy, scanning electron microscopy (SEM)), mechanical properties (elongation at break, tensile strength, elastic modulus, adhesion), and barrier properties (grease) of these blends and their coating behaviour (thickness on paper), were tested at different extrusion temperatures. The melting temperature (Tm) of PHBV was reduced by the plasticisers (from 172 °C to 164 resp. 169 °C with 15 wt.% TEC resp. PEG). The minimal achieved PHBV film thickness on paper was 30 µm owing to its low melt strength. The elastic modulus decreased with both plasticisers (from 3000 N/mm2 to 1200 resp. 1600 N/mm2 with 15 wt.% TEC resp. PEG). At 15 wt.% TEC, the elongation at break increased to 2.4 length-% (pure PHBV films had 0.9 length-%). The grease barrier (staining) was low owing to cracks in the PHBV layers. The extrusion temperature correlated with the grease barrier, mechanical properties, and bond strength. The bond strength was higher for films extruded with a temperature profile for constant melt flow rate at different plasticiser concentrations. The bond strength was max. 1.2 N/15 mm. Grease staining occurs because of cracks induced by the low elongation at break and high brittleness. Extrusion coating of the used specific PHBV on paper is possible. In further studies, the minimum possible PHBV film thickness needs to be reduced to be cost-effective. The flexibility needs to be increased to avoid cracks, which cause migration and staining.

Biosynthesis and Characteristics of Aromatic Polyhydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are polyesters synthesized by bacteria as a carbon and energy storage material. PHAs are characterized by thermoplasticity, biodegradability, and biocompatibility, and thus have attracted considerable attention for use in medical, agricultural, and marine applications. The properties of PHAs depend on the monomer composition and many types of PHA monomers have been reported. This review focuses on biosynthesized PHAs bearing aromatic groups as side chains. Aromatic PHAs show characteristics different from those of aliphatic PHAs. This review summarizes the types of aromatic PHAs and their characteristics, including their thermal and mechanical properties and degradation behavior. Furthermore, the effect of the introduction of an aromatic monomer on the glass transition temperature (T_g) of PHAs is discussed. The introduction of aromatic monomers into PHA chains is a promising method for improving the properties of PHAs, as the characteristics of aromatic PHAs differ from those of aliphatic PHAs.

Synthesis of Short-Chain-Length and Medium-Chain-Length Polyhydroxyalkanoate Blends from Activated Sludge by Manipulating Octanoic Acid and Nonanoic Acid as Carbon Sources

The effects of octanoic acid/nonanoic acid and acclimation time on the synthesis of short-chain-length and medium-chain-length PHA blends from activated sludge were investigated. An increased concentration (847-1366 mg/L) of PHAs resulted from 4-month acclimation compared with the concentration derived from 2-month acclimation (450-1126 mg/L). The content of octanoic acid had a positive linear relationship with the content of even-numbered carbon monomers among the PHAs. The blending products were identified mainly with scl-PHAs during the 2-month acclimation period and were thereafter dominated by mcl-PHAs until 4 months of acclimation. Thermal properties analysis demonstrated that the products derived from 4-month acclimation were a mixture of scl-PHAs and mcl-PHAs rather than a copolymer of scl-PHAs and mcl-PHAs. High-throughput sequencing results indicated that Pseudofulvimonas, Paracoccus, and Blastocatella were the dominant genera that might be responsible for scl-PHAs production during the 2-month acclimation period, whereas Comamonas and Pseudomonas that were responsible for mcl-PHAs production then became the dominant genera after 4-months acclimation.

Potential for mcl-PHA production from nonanoic and azelaic acids

Greater than 65% of canola and high-oleic soy oil fatty acids is oleic acid, which is readily converted to nonanoic (NA) and azelaic (AzA) acids by ozonolysis. NA is an excellent substrate for medium-chain-length polyhydroxyalkanoate (mcl-PHA) production but AzA has few uses. Pseudomonas citronellolis DSM 50332 and Pseudomonas fluorescens ATCC 17400, both able to produce mcl-PHA from fatty acids and to grow on AzA as the sole source of carbon and energy, were assessed for the accumulation of mcl-PHA from AzA and NA. In N-limited shake flasks using NA, P. citronellolis produced 32% of its dry biomass as mcl-PHA containing 78% 3-hydroxynonanoate with 22% 3-hydroxyheptanoate. Pseudomonas fluorescens produced only 2% PHA. N-limited P. citronellolis on AzA produced 20% dry weight PHA containing 75% 3-hydroxydecanoate and 25% 3-hydroxyoctanoate, indicative of de novo synthesis. Although selective pressure, including β-oxidation inhibition, under well-controlled (chemostat) conditions was applied to P. citronellolis, no side-chain carboxyl groups were detected. It was concluded that one or more of FabG and PhaJ or the PHA synthase cannot catalyze reactions involving ω-carboxy substrates. However, a process based on oleic acid could be established if Pseudomonas putida was engineered to grow on AzA.

Production of a novel medium chain length poly(3-hydroxyalkanoate) using unprocessed biodiesel waste and its evaluation as a tissue engineering scaffold

This study demonstrated the utilization of unprocessed biodiesel waste as a carbon feedstock for Pseudomonas mendocinaCH50, for the production of PHAs. A PHA yield of 39.5% CDM was obtained using 5% (v/v) biodiesel waste substrate. Chemical analysis confirmed that the polymer produced was poly(3‐hydroxyhexanoate‐co‐3‐hydroxyoctanoate‐co‐3‐hydroxydecanoate‐co‐3‐hydroxydodecanoate) or P(3HHx‐3HO‐3HD‐3HDD). P(3HHx‐3HO‐3HD‐3HDD) was further characterized and evaluated for its use as a tissue engineering scaffold (TES). This study demonstrated that P(3HHx‐3HO‐3HD‐3HDD) was biocompatible with the C2C12 (myoblast) cell line. In fact, the % cell proliferation of C2C12 on the P(3HHx‐3HO‐3HD‐3HDD) scaffold was 72% higher than the standard tissue culture plastic confirming that this novel PHA was indeed a promising new material for soft tissue engineering.

Preparation and biocompatibility of crosslinked poly(3-hydroxyundecenoate)

A sticky polymer, poly(3-hydroxyundecenoate) (PHU), was produced by Pseudomonas oleovorans when nonanoate and undecenoate were used as carbon sources. Crosslinked PHU (CL-PHU) was prepared by heating using benzoyl peroxide as a crosslinker. According to the degree of crosslinking in the polymer, three types of CL-PHU were prepared: CL-PHU50, CL-PHU60 and CL-PHU70. Fourier transform-infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry results suggested that crosslinking of PHU was successfully achieved by heat, which increased the crosslinking density and decreased stiffness and flexibility of the polymer. Water contact angle measurements revealed no differences of hydrophilicity as the crosslinking density. Slight morphological changes of CL-PHU film surfaces were observed by atomic force microscopy. Chinese hamster ovary cells were used to investigate the biocompatibility of CL-PHU films using poly(l-lactide) surfaces as control. Surface properties of the film, such as roughness and adhesive force, enhanced the adhesion and proliferation of cells on the films. CL-PHU might be useful for cell compatible biomedical applications.

Biosynthesis of polyhydroxyalkanoates containing hydroxyl group from glycolate in Escherichia coli

Polyhydroxyalkanoates (PHAs) containing hydroxyl groups in a side chain were produced in recombinant Escherichia coli JM109 using glycolate as the sole carbon source. The propionate-CoA transferase (pct) gene from Megasphaera elsdenii and the β-ketothiolase (bktB) gene and phaCAB operon from Ralstonia eutropha H16 were introduced into E. coli JM109. A novel monomer containing a hydroxyl group, dihydroxybutyrate (DHBA), was the expected product of the condensation of glycolyl-CoA and acetyl-CoA by BktB. The recombinant strain produced a PHA containing 1 mol% DHBA. The incorporation of DHBA may have been restricted because the expression of phaAB1 competes for acetyl-CoA. The PHA containing DHBA units were evaluated regarding thermal properties, such as melting temperature, glass transition temperature and thermal degradation temperature. The current study demonstrates a potential use of PHA containing hydroxyl groups as renewable resources in biological materials.

Review of Bionanocomposite Coating Films and Their Applications

The properties of a composite material depend on its constituent materials such as natural biopolymers or synthetic biodegradable polymers and inorganic or organic nanomaterials or nano-scale minerals. The significance of bio-based and synthetic polymers and their drawbacks on coating film application is currently being discussed in research papers and articles. Properties and applications vary for each novel synthetic bio-based material, and a number of such materials have been fabricated in recent years. This review provides an in-depth discussion on the properties and applications of biopolymer-based nanocomposite coating films. Recent works and articles are cited in this paper. These citations are ubiquitous in the development of novel bionanocomposites and their applications.

The 'PHAome'

Polyhydroxyalkanoates (PHAs) are a family of polyesters synthesized by bacteria. Similarly to the genome, transcriptome, and proteome (the entire array of nucleic acids and proteins present in a cell or population of cells at a given time), the PHA spectrum exhibits diverse and dynamic modifications - the 'PHAome' - reflecting not only by the diversity of monomers, homopolymers, random and block copolymers, functional and graft polymers, molecular weights, and combinations of the above, but also the ranges of PHAs with various molecular weights and monomer ratios that are present at a particular timepoint in a bacterial cell. Echoing the Materials Genome Initiative (MGI) launched in 2011 to develop an infrastructure to accelerate advanced materials discovery and deployment, understanding the PHAome and ensuring an ample supply of PHAs based on it will promote the discovery of new properties and applications of this family of advanced materials.

Microbial synthesis of functional homo-, random, and block polyhydroxyalkanoates by β-oxidation deleted Pseudomonas entomophila

Functional polyhydroxyalkanoates (PHAs) allow chemical modifications to widen PHA diversity, promising to increase values of these biodegradable and biocompatible polyesters. Among functional PHAs, unsaturated PHA site chains can be easily grafted to add chemical groups, and to cross-link with other PHA polymer chains. However, it has been very difficult to obtain structurally controllable functional homo-, random, or block PHA. For the first time, a β-oxidation deleted Pseudomonas entomophila was used to successfully synthesize random copolymers of 3-hydroxydodecanoate (3HDD) and 3-hydroxy-9-decenoate (3H9D). Compositions of the random copolymers P(3HDD-co-3H9D) can be adjusted by ratios of dodecanoic acid (DDA) to 9-decenol (9DEO) fed to the culture of P. entomophila. Homopolymer P3H9D was formed when only 9DEO was added to the culture. Diblock copolymers of P3HDD-b-P3H9D were produced by feeding DDA as the first precursor to form a P3HDD block followed by adding 9DEO as the second precursor to form a second P3H9D block. It was demonstrated that random copolymers P(3HDD-co-3H9D) could be crossed-linked under UV-radiation due to the presence of the unsaturated bonds. Thermal and mechanical characterizations of the above homo-, random, and diblock PHA polymers were conducted. It was found that the diblock polymer P3HDD-b-P3H9D increased at least 2-fold on Young's modulus compared with its random copolymers consisting of similar 3HDD/3H9D ratios. This study demonstrates that PHA functionality could be controlled to meet various requirements.

Paclitaxel-incorporated nanoparticles using block copolymers composed of poly(ethylene glycol)/poly(3-hydroxyoctanoate)

Block copolymers composed of poly(3-hydroxyoctanoate) (PHO) and methoxy poly(ethylene glycol) (PEG) were synthesized to prepare paclitaxel-incorporated nanoparticle for antitumor drug delivery. In a (1)H-NMR study, chemical structures of PHO/PEG block copolymers were confirmed and their molecular weight (M.W.) was analyzed with gel permeation chromatography (GPC). Paclitaxel as a model anticancer drug was incorporated into the nanoparticles of PHO/PEG block copolymer. They have spherical shapes and their particle sizes were less than 100 nm. In a (1)H-NMR study in D2O, specific peaks of PEG solely appeared while peaks of PHO disappeared, indicating that nanoparticles have core-shell structures. The higher M.W. of PEG decreased loading efficiency and particle size. The higher drug feeding increased drug contents and average size of nanoparticles. In the drug release study, the higher M.W. of PEG block induced the acceleration of drug release rate. The increase in drug contents induced the slow release rate of drug. In an antitumor activity study in vitro, paclitaxel nanoparticles have practically similar anti-proliferation activity against HCT116 human colon carcinoma cells. In an in vivo animal study using HCT116 colon carcinoma cell-bearing mice, paclitaxel nanoparticles have enhanced antitumor activity compared to paclitaxel itself. Therefore, paclitaxel-incorporated nanoparticles of PHO/PEG block copolymer are a promising vehicle for antitumor drug delivery.

Production of medium-chain-length polyhydroxyalkanoates by activated sludge enriched under periodic feeding with nonanoic acid

The potential use of activated sludge for the production of medium-chain-length polyhydroxyalkanoates (MCL-PHAs) was investigated. The enrichment of bacterial populations capable of producing MCL-PHAs was achieved by periodic feeding with nonanoic acid in a sequencing batch reactor (SBR). Denaturing gradient gel electrophoresis analysis revealed Pseudomonas aeruginosa strains to be predominant in the bacterial community during the SBR process. The composition of PHA synthesized by the enriched biomass from nonanoic acid consisted of a large concentration (>89 mol%) of MCL monomer units and a small amount of short-chain-length monomer units. Under fed-batch fermentation with continuous feeding of nonanoic acid at a flow rate of 0.225 g/L/h and a C/N ratio of 40, a maximum PHA content of 48.6% dry cell weight and a conversion yield (Y(p/s)) of 0.94 g/g were achieved. These results indicate that MCL-PHA production by activated sludge is a promising alternative to typical pure culture approaches.

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) co-polymer production from a local isolate, Brevibacillus invocatus MTCC 9039

Accumulation of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] co-polymer by a local isolate, Brevibacillus invocatus MTCC 9039 under batch mode was investigated under glucose, acetate and propionate-supplemented conditions. Cells harvested at the stationary phase of growth depicted maximum accumulation of poly-3-hydroxybutyrate (PHB), i.e. 3% of dry cell weight (dcw) at pH 7.0 and temperature 30 degrees C at 48h of incubation. PHB accumulation reached up to 52% (dcw) under 3% glucose with 1% acetate supplementation. P(3HB-co-3HV) co-polymer synthesis was observed under propionate-supplemented condition, which reached up to 45% under 3% glucose with 1% propionate supplementation. Optimization of process parameters by response surface methodology (RSM) resulted into co-polymer accumulation up to 65% (dcw) at 2.08% glucose, 1.62% acetate, 0.75% propionate and 2.15 g l(-1) KH(2)PO(4) concentrations. This co-polymer exhibited comparable material properties with the commercial [P(3HB-co-3HV)] co-polymers, whereas the elasticity was tremendously high and could be comparable with polypropylene. Thus, B. invocatus MTCC 9039 is emerging as an interesting organism and could be exploited further for P(3HB-co-3HV) co-polymer production.

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%).

Enhanced production of SCL-LCL-PHA co-polymer by sludge-isolated Pseudomonas aeruginosa MTCC 7925

AIM

To boost short-chain-length-long-chain-length polyhydroxyalkanoate (SCL-LCL-PHA) co-polymer yield in Pseudomonas aeruginosa MTCC 7925 by manipulating culture conditions.

METHODS AND RESULTS

Stationary phase culture of P. aeruginosa MTCC 7925 was subjected to various doses of different carbons and, N and P deficiencies. Enhanced co-polymer yield was recorded under ethanol- and glucose-supplemented cultures. Interaction of ethanol with N-deficiency boosted co-polymer accumulation maximally.

CONCLUSIONS

The sludge-isolated P. aeruginosa MTCC 7925 demonstrated good capability to synthesize SCL-LCL-PHA co-polymer from unrelated carbon sources. Supplementation of ethanol under N-deficiency boosted the co-polymer yield up to 69% of dry cell weight, which is significantly higher when compared with other SCL-MCL-PHA co-polymer accumulating bacterial species.

SIGNIFICANCE AND IMPORTANCE OF THE STUDY

This is the first report on P. aeruginosa MTCC 7925, which is capable of accumulating LCL 3-hydroxyhexadecanoate and 3-hydroxyoctadecanoate units with SCL 3-hydroxybutyrate and 3-hydroxyvalerate as constituents of PHAs. This opens up new possibilities for various industrial applications owing to the superior properties of this new co-polymer.

Biosynthesis, properties and potential of natural–synthetic hybrids of polyhydroxyalkanoates and polyethylene glycols

Chemical conjugation with poly(ethylene glycols) (PEGs) are established procedures to facilitate solubilisation of hydrophobic compounds. Such techniques for PEGylation have been applied to polyhydroxybutyrate. 'BioPEGylation' of such polyhydroxyalkanoates (PHAs) to form natural-synthetic hybrids has been demonstrated through the addition of PEGs to microbial cultivation systems. The strategic addition of certain PEGs not only supports hybrid synthesis but may also provide a technique for control of PHA composition and molecular mass, and by extension, their physico-mechanical properties. PHA composition and molecular mass control by PEGs is dependent upon the polyethers' molecular mass, loading in the cultivation system, time of introduction and microbial species. Hybrid characterisation studies are in their infancy, but results to date suggest that PHA-PEG hybrids have subtle, but significant, differences in their physiochemical and material properties as a consequence of the PEGylation.

Fermentation process development for the production of medium-chain-length poly-3-hyroxyalkanoates

This paper presents a review of the existing fermentation processes for the production of medium-chain-length poly-3-hydroxyalkanoates (MCL-PHAs). These biodegradable polymers are usually produced most efficiently from structurally related carbon sources such as alkanes and alkanoic acids. Unlike alkanoic acids, alkanes exhibit little toxicity but their low aqueous solubility limits their use in high density culture. Alkanoic acids pose little mass transfer difficulty, but their toxicity requires that their concentration be well controlled. Using presently available technology, large-scale production of MCL-PHA from octane has been reported to cost from US $5 to 10 per kilogram, with expenditures almost evenly divided between carbon source, fermentation process, and the separation process. However, MCL-PHAs, even some with functional groups in their subunits, can also be produced from cheaper unrelated carbon sources, such as glucose. Metabolic engineering and other approaches should also allow increased PHA cellular content to be achieved. These approaches, as well as a better understanding of fermentation kinetics, will likely result in increased productivity and lower production costs.

Increased diversification of polyhydroxyalkanoates by modification reactions for industrial and medical applications

A wide range of diverse polyhydroxyalkanoates, PHAs, is currently available due to the low substrate specificity of PHA synthases and subsequent modifications by chemical reactions. These polymers are promising materials for a number of different applications due to their biocompatibility and biodegradability. This review summarizes the large variability of PHAs regarding chemical structure and material properties that can be currently produced. In the first part, in vivo and in vitro biosynthesis processes for production of a large variety of different PHAs will be summarized with regard to obtaining saturated and unsaturated copolyesters and side chain functionalized polyesters, including brominated, hydroxylated, methyl-branched polyesters, and phenyl derivatives of polyesters. In the second part, established chemical modifications of PHAs will be summarized as that by means of grafting reactions and graft/block copolymerizations, as well as by chlorination, cross-linking, epoxidation, hydroxylation, and carboxylation, reactions yield further functionalized PHAs.

Polyhydroxyalkanoate copolymers from forest biomass

The potential for the use of woody biomass in poly-beta-hydroxyalkanoate (PHA) biosynthesis is reviewed. Based on previously cited work indicating incorporation of xylose or levulinic acid (LA) into PHAs by several bacterial strains, we have initiated a study for exploring bioconversion of forest resources to technically relevant copolymers. Initially, PHA was synthesized in shake-flask cultures of Burkholderia cepacia grown on 2.2% (w/v) xylose, periodically amended with varying concentrations of levulinic acid [0.07-0.67% (w/v)]. Yields of poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) [P(3HB-co-3HV)] from 1.3 to 4.2 g/l were obtained and could be modulated to contain from 1.0 to 61 mol% 3-hydroxyvalerate (3HV), as determined by 1H and 13C NMR analyses. No evidence for either the 3HB or 4HV monomers was found. Characterization of these P(3HB-co-3HV) samples, which ranged in molecular mass (viscometric, Mv) from 511-919 kDa, by differential scanning calorimetry and thermogravimetric analyses (TGA) provided data which were in agreement for previously reported P(3HB-co-3HV) copolymers. For these samples, it was noted that melting temperature (Tm) and glass transition temperature (Tg) decreased as a function of 3HVcontent, with Tm demonstrating a pseudoeutectic profile as a function of mol% 3HV content. In order to extend these findings to the use of hemicellulosic process streams as an inexpensive carbon source, a detoxification procedure involving sequential overliming and activated charcoal treatments was developed. Two such detoxified process hydrolysates (NREL CF: aspen and CESF: maple) were each fermented with appropriate LA supplementation. For the NREL CF hydrolysate-based cultures amended with 0.25-0.5% LA, P(3HB-co-3HV) yields, PHA contents (PHA as percent of dry biomass), and mol% 3HV compositions of 2.0 g/l, 40% (w/w), and 16-52 mol% were obtained, respectively. Similarly, the CESF hydrolysate-based shake-flask cultures yielded 1.6 g/l PHA, 39% (w/w) PHA contents, and 4-67 mol% 3HV compositions. These data are comparable to copolymer yields and cellular contents reported for hexose plus levulinic acid-based shake-flask cultures, as reported using Alcaligenes eutrophus and Pseudomonas putida. However, our findings presage a conceivable alternative, forestry-based biorefinery approach for the production of value-added biodegradable PHA polymers. Specifically, this review describes the current and potential utilization of lignocellulosic process streams as platform precursors to PHA polymers including hemicellulosic hydrolysates, residual cellulose-derived levulinic acid, tall oil fatty acids (Kraft pulping residual), and lignin-derived aromatics.

UV-induced graft copolymerization of monoacrylate-poly(ethylene glycol) onto poly(3-hydroxyoctanoate) to reduce protein adsorption and platelet adhesion

Homogeneous solutions of poly(3-hydroxyoctanoate) (PHO) and the monoacrylate-poly(ethylene glycol) (PEGMA) monomer in chloroform were irradiated with UV light to obtain PEGMA-grafted PHO (PEGMA-g-PHO) copolymers. Variables affecting the degree of grafting (DG), such as the time of UV irradiation and the concentrations of the PEGMA monomer and initiator, were investigated. The PEGMA-g-PHO copolymers were characterized by measuring the water contact angle, molecular weight, thermal transition temperatures and mechanical properties, as well as by nuclear magnetic resonance spectroscopy. The results from all of these measurements indicate that PEGMA groups were present on the PHO polymer. The protein adsorption and platelet adhesion on the PEGMA-g-PHO surfaces were examined using poly(L-lactide) (PLLA) surfaces as the control. The proteins and platelets had a significantly lower tendency to adhere to the PEGMA-g-PHO copolymers than to PLLA. The graft copolymer with a high DG of PEGMA was very effective in reducing the protein adsorption and platelet adhesion and did not activate the platelets. The results obtained in this study suggest that PEGMA-g-PHO copolymers have the potential to be used as blood-contacting devices in a broad range of biomedical applications.

Preparation and cell compatibility of acrylamide-grafted poly(3-hydroxyoctanoate)

Poly(3-hydroxyoctanoate) (PHO) films were treated with plasma of different discharge powers (10-50 W) and then treated with acryl amide solutions in order to prepare films with surfaces that contained different amounts of amide groups. The surfaces were characterized by contact angle measurement, attenuated total reflectance infrared spectroscopy, electron spectroscopy for chemical analysis, and scanning electron microscopy. Results from all these measurements indicated that amide groups were present on the surfaces. The amount of amide groups increased in proportion to the discharge power of the plasma. The interaction of Chinese hamster ovary cells with these grafted surfaces was investigated. The number of cells that adhered to and grew on the surfaces was highest for films grafted at 30 W of plasma discharge power, indicating that the moderate hydrophilicity was optimal for cells to adhere and grow. The present results support the suggestion that acryl amide-grafted PHO could be used as cell-compatible biomedical applications.

Biosynthesis of polyhydroxyalkanoate copolyester containing cyclohexyl groups by Pseudomonas oleovorans

Production of polyhydroxyalkanoates (PHAs) substituted with cyclohexyl groups by Pseudomonas oleovorans grown with 4-cyclohexylbutyric acid (4-CHB) and its mixtures with nonanoic acid (NA) was investigated. Addition of NA to medium gave rise to an increase in the total concentration of 3-hydroxy-4-cyclohexylbutyrate repeating unit in the PHAs, indicating that the bioconversion rate of 4-CHB to polyester was significantly improved by the cometabolic effect. Increasing the proportion of NA from 1.0 to 7.5 mM at a concentration of 10 mM total carbon substrate also accelerated the uptake speed of 4-CHB by the organism and resulted in an increase of the ratio of 3-hydroxynonanoate to 3-hydroxyheptanoate from 1.28 to 2.05. Differential scanning calorimetric analysis of the PHAs bearing the corresponding functional groups showed one melting transition and one glass transition temperature varying according to the composition. These results indicated that random copolyesters were obtained from the carbon substrates used in this study.

Genetically engineered Pseudomonas: a factory of new bioplastics with broad applications

New bioplastics containing aromatic or mixtures of aliphatic and aromatic monomers have been obtained using genetically engineered strains of Pseudomonas putida. The mutation (-) or deletion (Delta) of some of the genes involved in the beta-oxidation pathway (fadA(-), fadB(-) Delta fadA or Delta fad BA mutants) elicits a strong intracellular accumulation of unusual homo- or co-polymers that dramatically alter the morphology of these bacteria, as more than 90% of the cytoplasm is occupied by these macromolecules. The introduction of a blockade in the beta-oxidation pathway, or in other related catabolic routes, has allowed the synthesis of polymers other than those accumulated in the wild type (with regard to both monomer size and relative percentage), the accumulation of certain intermediates that are rapidly catabolized in the wild type and the accumulation in the culture broths of end catabolites that, as in the case of phenylacetic acid, phenylbutyric acid, trans-cinnamic acid or their derivatives, have important medical or pharmaceutical applications (antitumoral, analgesic, radiopotentiators, chemopreventive or antihelmintic). Furthermore, using one of these polyesters (poly 3-hydroxy-6-phenylhexanoate), we obtained polymeric microspheres that could be used as drug vehicles.