Evolution of polyhydroxyalkanoate synthesizing systems toward a sustainable plastic industry

Waste to wealth: Polyhydroxyalkanoates (PHA) production from food waste for a sustainable packaging paradigm

The growing demand for sustainable food packaging and the increasing concerns regarding environmental pollution have driven interest in biodegradable materials. This paper presents an in-depth review of the production of Polyhydroxyalkanoates (PHA), a biodegradable polymer, from food waste. PHA-based bioplastics, particularly when derived from low-cost carbon sources such as volatile fatty acids (VFAs) and waste oils, offer a promising solution for reducing plastic waste and enhancing food packaging sustainability. Through optimization of microbial fermentation processes, PHA production can achieve significant efficiency improvements, with yields reaching up to 87 % PHA content under ideal conditions. This review highlights the technical advancements in using PHA for food packaging, emphasizing its biodegradability, biocompatibility, and potential to serve as a biodegradable alternative to petroleum-based plastics. However, challenges such as high production costs, mechanical limitations, and the need for scalability remain barriers to industrial adoption. The future of PHA in food packaging hinges on overcoming these challenges through further research and innovation in production techniques, material properties, and cost reduction strategies, along with necessary legislative support to promote widespread use.

Development of hybrid bionanocomposites of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with zinc oxide and silicon-doped hydroxyapatite nanocrystals and machine learning for predicting dynamic mechanical properties

The development of hybrid materials that integrate bioactive and antimicrobial properties within a biodegradable and biocompatible polymer matrix is a key focus in current biomedical research and applications. A significant research gap exists in the field of PHBV nanocomposites, particularly concerning those that simultaneously incorporate both ZnO and HAP particles. This study focuses on the fabrication and characterization of innovative hybrid bionanocomposites composed of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) combined with zinc oxide (ZnO) and silicon-doped hydroxyapatite (SiHAP) nanocrystals. The hybrid nanocomposite with 5 wt% ZnO and 0.1 wt% SiHAP exhibited the highest storage modulus, suitable for load-bearing applications. DMA analysis at 20 °C showed significant increases in storage (50.8 %) and loss (92 %) moduli for this composition. This particular group demonstrated cellular viability of approximately 100 %. Our results suggest that these newly developed novel composites demonstrate exceptional biocompatibility, bioactivity, and antimicrobial properties. As a result, they show significant potential as tissue engineering tools for addressing bone tissue disorders. Various Machine learning (ML) algorithms were applied to model the dynamic mechanical properties of nanocomposites based on experimental data. The study shows that these models provide accurate insights into the dynamic mechanical behavior of nanocomposites, offering a reliable method for optimizing their properties.

Recent Trends in the Production and Recovery of Bioplastics Using Polyhydroxyalkanoates Copolymers

Polyhydroxyalkanoates (PHAs) are polyesters synthesized as a carbon and energy reserve material by a wide number of bacteria. These polymers are characterized by their thermoplastic properties similar to those of plastics derived from the petrochemical industry, such as polyethylene and polypropylene. PHAs are widely used in the medical field and have the potential to be used in other applications due to their biocompatibility and biodegradability. Among PHAs, P(3HB-co-3HV) copolymers are thermo-elastomeric polyesters that are typically soft and flexible with low to no crystallinity, which can expand the range of applications of these bioplastics. Several bacterial species, such as Cupriavidus necator, Azotobacter vinelandii, Halomonas sp. and Bacillus megaterium, have been successfully used for P(3HB-co-3HV) production, both in batch and fed-batch cultures using different low-cost substrates, such as vegetable and fruit waste. Nevertheless, in recent years, several fermentation strategies using other microbial models, such as methanotrophic bacterial strains as well as halophilic bacteria, have been developed in order to improve PHA production in cultivation conditions that are easily implemented on a large scale. This review aims to summarize the recent trends in the production and recovery of PHA copolymers by fermentation, including different cultivation modalities, low-cost raw materials, as well as downstream strategies that have recently been developed with the purpose of producing copolymers, such as P(3HB-co-3HV), with suitable mechanical properties for applications in the biomedical field.

Advances in Microbial Biotechnology for Sustainable Alternatives to Petroleum-Based Plastics: A Comprehensive Review of Polyhydroxyalkanoate Production

The industrial production of polyhydroxyalkanoates (PHAs) faces several limitations that hinder their competitiveness against traditional plastics, mainly due to high production costs and complex recovery processes. Innovations in microbial biotechnology offer promising solutions to overcome these challenges. The modification of the biosynthetic pathways is one of the main tactics; allowing for direct carbon flux toward PHA formation, increasing polymer accumulation and improving polymer properties. Additionally, techniques have been implemented to expand the range of renewable substrates used in PHA production. These feedstocks are inexpensive and plentiful but require costly and energy-intensive pretreatment. By removing the need for pretreatment and enabling the direct use of these raw materials, microbial biotechnology aims to reduce production costs. Furthermore, improving downstream processes to facilitate the separation of biomass from culture broth and the recovery of PHAs is critical. Genetic modifications that alter cell morphology and allow PHA secretion directly into the culture medium simplify the extraction and purification process, significantly reducing operating costs. These advances in microbial biotechnology not only enhance the efficient and sustainable production of PHAs, but also position these biopolymers as a viable and competitive alternative to petroleum-based plastics, contributing to a circular economy and reducing the dependence on fossil resources.

Complete sequence randomness of lactate-based copolymers (LAHBs) with varied lactate monomer fractions employing a series of propionyl-CoA transferases

Previously, we biosynthesized an evolved version of a bio-based polylactide (PLA) on microbial platforms using our engineered lactate-polymerizing enzyme (LPE). This lactate (LA)-based copolyester, LAHB, has advantages over PLA, including improved flexibility and biodegradability, and its properties can be regulated through the LA fraction. To expand the LA-incorporation capacity and improve polymer properties, in the state of in vivo LAHB production, propionyl-CoA transferases (PCTs) that exhibited enhanced production of LA-CoA than the conventional PCTs were selected. Here, the present study has demonstrated that the LA fraction of LAHB could be altered using various PCTs. Enhanced PCT performance was achieved by balancing polymer production and cell growth. Both events are governed by the use of acetyl-CoA, a commonly shared key metabolite. This could be attributed to the different reactivities of individual PCTs towards acetyl-CoA, which serves both as a CoA donor and a leading compound in the TCA cycle. Interestingly, we found complete sequence randomness in the LAHB copolymers, independent of the LA fraction. The mechanism of LA fraction-independent sequence randomness is discussed. This new PCT-based strategy synergistically combines with the evolution of LPE to advance the LAHB project, and enables us to perform advanced applications other than LAHB production utilizing CoA-linked substrates.

Copolymers as a turning point for large scale polyhydroxyalkanoates applications

Traditional plastics reshaped the society thanks to their brilliant properties and cut-price manufacturing costs. However, their protracted durability and limited recycling threaten the environment. Worthy alternatives seem to be polyhydroxyalkanoates, compostable biopolymers produced by several microbes. The most common 3-hydroxybutyrate homopolymer has limited applications calling for copolymers biosynthesis to enhance material properties. As a growing number of researches assess the discovery of novel comonomers, great endeavors are dedicated as well to copolymers production scale-up, where the choice of the microbial carbon source significantly affects the overall economic feasibility. Diving into novel metabolic pathways, engineered strains, and cutting-edge bioprocess strategies, this review aims to survey up-to-date publications about copolymers production, focusing primarily on precursors origins. Specifically, in the core of the review, copolymers precursors have been divided into three categories based on their economic value: the costliest structurally related ones, the structurally unrelated ones, and finally various low-cost waste streams. The combination of cheap biomasses, efficient pretreatment strategies, and robust microorganisms paths the way towards the development of versatile and circular polymers. Conceived to researchers and industries interested in tackling polyhydroxyalkanoates production, this review explores an angle often underestimated yet of prime importance: if PHAs copolymers offer advanced properties and sustainable end-of-life, the feedstock choice for their upstream becomes a major factor in the development of plastic substitutes.

Recent advancements in bio-based dielectric and piezoelectric polymers and their biomedical applications

The advent of polymer-based dielectrics marked a significant breakthrough in dielectric materials. However, despite their many advantages, they pose serious environmental threats. Therefore, in recent years, there has been growing interest in bio-based polymers as a sustainable alternative to traditional petroleum-based polymers. Their renewable nature and reduced environmental impact can fulfil the rising demand for eco-friendly substitutes. Beyond their ecological benefits, bio-based polymers also possess distinctive electrical properties that make them extremely attractive in a variety of applications. Considering these, herein, we present recent advancements in bio-based dielectric polymers and nanocomposites. First, the fundamental concepts of dielectric and polymer-based dielectric materials are covered. Then, we will delve into the discussion of recent advancements in the dielectric properties and thermal stability of bio-based polymers, including polylactic acid, polyhydroxyalkanoates, polybutylene succinate, starch, cellulose, chitosan, chitins, and alginates, and their nanocomposites. Other novel bio-based dielectric polymers and their distinct dielectric characteristics have also been pointed out. In an additional section, the piezoelectric properties of these polymers and their recent biomedical applications have been highlighted and discussed thoroughly. In conclusion, this paper thoroughly discusses the recent advances in bio-based dielectric polymers and their potential to revolutionize the biomedical industry while cultivating a more sustainable and greener future.

Modification of poly(lactate) via polymer blending with microbially produced poly[(R)-lactate-co-(R)-3-hydroxybutyrate] copolymers

This study investigated the effect of polymer blending of microbially produced poly[(R)-lactate-co-(R)-3-hydroxybutyrate] copolymers (LAHB) with poly(lactate) (PLA) on their mechanical, thermal, and biodegradable properties. Blending of high lactate (LA) content and high molecular weight LAHB significantly improved the tensile elongation of PLA up to more than 250 % at optimal LAHB composition of 20-30 wt%. Temperature-modulated differential scanning calorimetry and dynamic mechanical analysis revealed that PLA and LAHB were immiscible but interacted with each other, as indicated by the mutual plasticization effect. Detailed morphological characterization using scanning probe microscopy, small-angle X-ray scattering, and solid-state NMR confirmed that PLA and LAHB formed a two-phase structure with a characteristic length scale as small as 20 nm. Because of mixing in this order, the polymer blends were optically transparent. The biological oxygen demand test of the polymer blends in seawater indicated an enhancement of PLA biodegradation during biodegradation of the polymer blends.

(R/S)-lactate/2-hydroxybutyrate dehydrogenases in and biosynthesis of block copolyesters by Ralstonia eutropha

Bacterial polyhydroxyalkanoates (PHAs) are promising bio-based biodegradable polyesters. It was recently reported that novel PHA block copolymers composed of (R)-3-hydroxybutyrate (3HB) and (R)-2-hydroxybutyrate (2HB) were synthesized by Escherichia coli expressing PhaCAR, a chimeric enzyme of PHA synthases derived from Aeromonas caviae and Ralstonia eutropha. In this study, the sequence-regulating PhaCAR was applied in the natural PHA-producing bacterium, R. eutropha. During the investigation, (R/S)-2HB was found to exhibit strong growth inhibitory effects on the cells of R. eutropha. This was probably due to formation of excess 2-ketobutyrate (2KB) from (R/S)-2HB and the consequent L-valine depletion caused by dominant L-isoleucine synthesis attributed to the excess 2KB. Deletion analyses for genes of lactate dehydrogenase homologs identified cytochrome-dependent D-lactate dehydrogenase (Dld) and [Fe-S] protein-dependent L-lactate dehydrogenase as the enzymes responsible for sensitivity to (R)-2HB and (S)-2HB, respectively. The engineered R. eutropha strain (phaCAR+, ldhACd-hadACd+ encoding clostridial (R)-2-hydroxyisocaproate dehydrogenase and (R)-2-hydoroxyisocaproate CoA transferase, ∆dld) synthesized PHA containing 10 mol% of 2HB when cultivated on glucose with addition of sodium (RS)-2HB, and the 2HB composition in PHA increased up to 35 mol% by overexpression phaCAR. The solvent fractionation and NMR analyses showed that the resulting PHAs were most likely to be block polymers consisting of P(3HB-co-3HV) and P(2HB) segments, suggesting that PhaCAR functions as the sequence-regulating PHA synthase independently from genetic and metabolic backgrounds of the host cell. KEY POINTS: (R/S)-2-hydroxubutyrates (2HB) caused l-valine deletion in Ralstonia eutropha (R)- and (S)-lactate/2HB dehydrogenases functional in R. eutropha were identified The engineered R. eutropha synthesized block copolymers of 2HB-containing polyhydroxyalkanoates on glucose and 2HB.

Biotransformation of starch-based wastewater into bioplastics: Optimization of poly(3-hydroxybutyrate) production by Cupriavidus necator DSM 545 using potato wastewater hydrolysate

Biodegradable biopolymers, such as polyhydroxyalkanoates (PHAs), have emerged as an alternative to petrochemical-based plastics. The present work explores the production of PHAs based on the biotransformation of potato processing wastewater and addresses two different strategies for PHA recovery. To this end, culture conditions for PHA synthesis by Cupriavidus necator DSM 545 were optimized on a laboratory scale using a response surface methodology-based experimental design. Optimal conditions rendered a PHB, poly(3-hydroxybutyrate), accumulation of 83.74 ± 2.37 % (5.1 ± 0.2 gL-1), a 1.4-fold increase compared to the initial conditions. Moreover, polymer extraction with non-halogenated agent improved PHB recovery compared to chloroform method (PHB yield up to 78.78 ± 0.57 %), while maintaining PHB purity. (99.83 ± 4.95 %). Overall, the present work demonstrated the potential valorization of starch-based wastewater by biotransformation into PHBs, a high value-added product, and showed that recovery approaches more eco-friendly than the traditional treatments could be applied to PHB recovery to some extent.

Recent Biotechnological Applications of Polyhydroxyalkanoates (PHA) in the Biomedical Sector-A Review

Petroleum-derived plastics are materials of great importance for the contemporary lifestyle, and are widely used commercially because they are low cost, resistant, malleable, and weightless, in addition to their hydrophobic character. However, some factors that confer the qualities of these materials also cause problems, mainly environmental, associated with their use. The COVID-19 pandemic aggravated these impacts due to the high demand for personal protective equipment and the packaging sector. In this scenario, bioplastics are environmentally positive alternatives to these plastics due to their applicability in several areas ranging from packaging, to biomedicine, to agriculture. Polyhydroxyalkanoates (PHAs) are biodegradable biopolymers usually produced by microorganisms as an energy reserve. Their structural variability provides a wide range of applications, making them a viable option to replace polluting materials. PHAs can be applied in various biotechnology sectors, such as producing drug carriers and scaffolds for tissue engineering. This review aimed to survey works published in the last five years on the study and biotechnological application of PHAs in the biomedical sector, exploring the versatility and advantages of their use and helping to understand how to enhance their application.

Impact of Multiple Reprocessing on Properties of Polyhydroxybutyrate and Polypropylene

Biobased plastics have the potential to be sustainable, but to explore their circularity further, current end-of-life options need to be broadened. Mechanical recycling is one of the most accepted methods to bring back plastics into the loop. Polyhydroxybutyrates (PHBs) are biobased and biodegradable in nature with promising properties and varied applications in the market. This study focuses on their potential for mechanical recycling by multiple extrusion cycles (E1-E5) and multi-faceted characterization of the virgin (V) and reprocessed materials from E1 to E5. The behavior is compared to polypropylene (PP) as a reference with a similar property profile, which has also been reprocessed five times. The thermal properties of both series showed a stable melting point and thermal decomposition temperature from thermal analyses (differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)). However, a steady increase in the degree of crystallinity was observed which could counterbalance the decrease in molecular weight due to repeated extrusion measured by gel permeation chromatography and resulted in similar values of tensile strength across the cycles. The strain at break was impacted after the first extrusion, but no significant change was observed thereafter; the same was observed for impact strength. Even in scanning electron microscopy (SEM) images, virgin and E5 samples appeared similar, showing the stability of morphological characteristics. Fourier transform infrared spectroscopy (FTIR) results revealed that no new groups are being formed even on repeated processing. The deviation between the PHB and PP series was more predominant in the melt mass flow rate (MFR) and rheology studies. There was a drastic drop in the MFR values in PHB from virgin to E5, whereas not much difference was observed for PP throughout the cycles. This observation was corroborated by frequency sweeps conducted with the parallel plate method. The viscosity dropped from virgin to E1 and E2, but from E3 to E5 it presented similar values. This was in contrast to PP, where all the samples from virgin to E5 had the same values of viscosity. This paper highlights the possibilities of mechanical recycling of PHB and explains why future work with the addition of virgin material and other additives is an area to be explored.

Bacterial Population Changes during the Degradation Process of a Lactate (LA)-Enriched Biodegradable Polymer in River Water: LA-Cluster Preferable Bacterial Consortium

The lactate-based polyester poly[lactate (LA)-co-3-hydroxybutyrate (3HB)], termed LAHB, is a highly transparent and flexible bio-based polymeric material. There are many unknowns regarding its degradation process in riverine environments, especially the changes in bacterial flora that might result from its degradation and the identities of any LAHB-degrading bacteria. LAHB were immersed in the river water samples (A and B), and LAHB degradation was observed in terms of the weight change of the polymer and the microscopic changes on the polymer surfaces. A metagenomic analysis of microorganisms was conducted to determine the effect of LAHB degradation on the aquatic environment. The bacterial flora obtained from beta diversity analysis differed between the two river samples. The river A water sample showed the simultaneous degradation of LA and 3HB even though the copolymer was LA-enriched, suggesting preferable hydrolysis of the LA-enriched segments. In contrast, only 3HB degraded for the LAHB in the river B water sample. The linear discriminant analysis effect size (LEfSe) analysis revealed 14 bacteria that were significantly increased in the river A water sample during LAHB degradation, suggesting that these bacteria preferentially degraded and assimilated LA-clustering polymers. Our metagenomic analysis provides useful insights into the dynamic changes in microbial communities and LA-clustering polymer-degrading bacteria.

Production of polyhydroxyalkanoate (PHA) copolymer from food waste using mixed culture for carboxylate production and Pseudomonas putida for PHA synthesis

Production of polyhydroxyalkanoates (PHAs) with high concentration of carboxylate, that was accumulated from solid state fermentation (SSF) of food waste (FW), was tested using Pseudomonas putida strain KT2440. Mixed-culture SSF of FW supplied in a high concentration of carboxylate, which caused a high PHA production of 0.56 g PHA/g CDM under nutrients control. Interestingly, this high PHA fraction in CDM was almost constant at 0.55 g PHA/g CDM even under high nutrients concentration (25 mM NH₄⁺), probably due to high reducing power maintained by high carboxylate concentration. PHA characterization indicated that the dominant PHA building block produced was 3-hydroxybutyrate, followed by 3-hydroxy-2-methylvalerate and 3-hydroxyhenxanoate. Carboxylate profiles before and after PHA production suggested that acetate, butyrate, and propionate were the main precursors to PHA via several metabolic pathways. Our result support that mixed culture SSF of FW for high concentration carboxylate and P. putida for PHA production enables sustainable production of PHA in cost-effective manners.

Recent progress and challenges in biotechnological valorization of lignocellulosic materials: Towards sustainable biofuels and platform chemicals synthesis

Lignocellulosic materials (LCM) have garnered attention as feedstocks for second-generation biofuels and platform chemicals. With an estimated annual production of nearly 200 billion tons, LCM represent an abundant source of clean, renewable, and sustainable carbon that can be funneled to numerous biofuels and platform chemicals by sustainable microbial bioprocessing. However, the low bioavailability of LCM due to the recalcitrant nature of plant cell components, the complexity and compositional heterogeneity of LCM monomers, and the limited metabolic flexibility of wild-type product-forming microorganisms to simultaneously utilize various LCM monomers are major roadblocks. Several innovative strategies have been proposed recently to counter these issues and expedite the widespread commercialization of biorefineries using LCM as feedstocks. Herein, we critically summarize the recent advances in the biological valorization of LCM to value-added products. The review focuses on the progress achieved in the development of strategies that boost efficiency indicators such as yield and selectivity, minimize carbon losses via integrated biorefinery concepts, facilitate carbon co-metabolism and carbon-flux redirection towards targeted products using recently engineered microorganisms, and address specific product-related challenges, to provide perspectives on future research needs and developments. The strategies and views presented here could guide future studies in developing feasible and economically sustainable LCM-based biorefineries as a crucial node in achieving carbon neutrality.

Enhanced production of biobased, biodegradable, Poly(3-hydroxybutyrate) using an unexplored marine bacterium Pseudohalocynthiibacter aestuariivivens, isolated from highly polluted coastal environment

The production and disposal of plastics from limited fossil reserves, has prompted research for greener and sustainable alternatives. Polyhydroxyalkanoates (PHAs) are biocompatible, biodegradable, and thermoprocessable polyester produced by microbes. PHAs found several applications but their use is limited due to high production cost and low yields. Herein, for the first time, the isolation and characterization of Pseudohalocynthiibacter aestuariivivens P96, a marine bacterium able to produce surprising amount of PHAs is reported. In the best growth condition P96 was able to reach a maximum production of 4.73 g/L, corresponding to the 87 % of total cell dry-weight. Using scanning and transmission microscopy, lab-scale fermentation, spectroscopic techniques, and genome analysis, the production of thermoprocessable polymer Polyhydroxybutyrate P(3HB), a PHAs class, endowed with mechanical and thermal properties comparable to that of petroleum-based plastics was confirmed. This study represents a milestone toward the use of this unexplored marine bacterium for P(3HB) production.

Effect of Different Plasticizers on Thermal, Crystalline, and Permeability Properties of Poly(3–hydroxybutyrate–co−3–hydroxyhexanoate) Films

Poly(3−hydroxybutyrate−co−3−hydroxyhexanoate) (PHBH) films were prepared using a cast film technique. Dioxane was chosen over other polymer solvents as it resulted in homogenous films with better morphology. Several plasticizers with different molecular weights and concentrations were added to the biopolymer solution prior to casting. Thermal, crystalline, and permeability properties were analyzed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X−ray diffraction (XRD), and both water vapor and oxygen transmission rate analysis. In general, the addition of plasticizers decreased the glass transition temperature (Tg), cold crystallization temperatures (Tcc), melting temperatures, as well as crystallinity degrees and increased the crystallite sizes and water vapor and oxygen transmission rates. The use of isosorbide and low-molecular-weight poly(ethylene glycol) (PEG) lowered the Tg around 30 °C at the highest used concentration, also being the most effective in increasing the crystallite size. When considering isosorbide and low-molecular-weight poly(ethylene glycol) (PEG) as very good plasticizers for PHBH, the question of which plasticizer to use strongly relies on the desired PHBH application.

Continuous feeding strategy for polyhydroxyalkanoate production from solid waste animal fat at laboratory- and pilot-scale

Bioconversion of waste animal fat (WAF) to polyhydroxyalkanoates (PHAs) is an approach to lower the production costs of these plastic alternatives. However, the solid nature of WAF requires a tailor-made process development. In this study, a double-jacket feeding system was built to thermally liquefy the WAF to employ a continuous feeding strategy. During laboratory-scale cultivations with Ralstonia eutropha Re2058/pCB113, 70% more PHA (45 g_PHA L^-1 ) and a 75% higher space-time yield (0.63 g_PHA L^-1  h^-1 ) were achieved compared to previously reported fermentations with solid WAF. During the development process, growth and PHA formation were monitored in real-time by in-line photon density wave spectroscopy. The process robustness was further evaluated during scale-down fermentations employing an oscillating aeration, which did not alter the PHA yield although cells encountered periods of oxygen limitation. Flow cytometry with propidium iodide staining showed that more than two-thirds of the cells were viable at the end of the cultivation and viability was even little higher in the scale-down cultivations. Application of this feeding system at 150-L pilot-scale cultivation yielded in 31.5 g_PHA L^-1 , which is a promising result for the further scale-up to industrial scale.

Class I Polyhydroxyalkanoate (PHA) Synthase Increased Polylactic Acid Production in Engineered Escherichia Coli

Polylactic acid (PLA), a homopolymer of lactic acid (LA), is a bio-derived, biocompatible, and biodegradable polyester. The evolved class II PHA synthase (PhaC1_Ps6-19) was commonly utilized in the de novo biosynthesis of PLA from biomass. This study tested alternative class I PHA synthase (PhaC_Cs) from Chromobacterium sp. USM2 in engineered Escherichia coli for the de novo biosynthesis of PLA from glucose. The results indicated that PhaC_Cs had better performance in PLA production than that of class II synthase PhaC1_Ps6-19. In addition, the sulA gene was engineered in PLA-producing strains for morphological engineering. The morphologically engineered strains present increased PLA production. This study also tested fused propionyl-CoA transferase and lactate dehydrogenase A (fused Pct_Cp/LdhA) in engineered E. coli and found that fused Pct_Cp/LdhA did not apparently improve the PLA production. After systematic engineering, the highest PLA production was achieved by E. coli MS6 (with PhaC_Cs and sulA), which could produce up to 955.0 mg/L of PLA in fed-batch fermentation with the cell dry weights of 2.23%, and the average molecular weight of produced PLA could reach 21,000 Da.

Poly(3-mercapto-2-methylpropionate), a Novel α-Methylated Bio-Polythioester with Rubber-like Elasticity, and Its Copolymer with 3-hydroxybutyrate: Biosynthesis and Characterization

A new polythioester (PTE), poly(3-mercapto-2-methylpropionate) [P(3M2MP)], and its copolymer with 3-hydroxybutyrate (3HB) were successfully biosynthesized from 3-mercapto-2-methylpropionic acid as a structurally-related precursor. This is the fourth PTE of biological origin and the first to be α-methylated. P(3M2MP) was biosynthesized using an engineered Escherichia coli LSBJ, which has a high molecular weight, amorphous structure, and elastomeric properties, reaching 2600% elongation at break. P(3HB-co-3M2MP) copolymers were synthesized by expressing 3HB-supplying enzymes. The copolymers were produced with high content in the cells and showed a high 3M2MP unit incorporation of up to 77.2 wt% and 54.8 mol%, respectively. As the 3M2MP fraction in the copolymer increased, the molecular weight decreased and the polymers became softer, more flexible, and less crystalline, with lower glass transition temperatures and higher elongations at break. The properties of this PTE were distinct from those of previously biosynthesized PTEs, indicating that the range of material properties can be further expanded by introducing α-methylated thioester monomers.

Controllable secretion of multilayer vesicles driven by microbial polymer accumulation

Membrane vesicles (MVs) are formed in various microorganisms triggered by physiological and environmental phenomena. In this study, we have discovered that the biogenesis of MV took place in the recombinant cell of Escherichia coli BW25113 strain that intracellularly accumulates microbial polyester, polyhydroxybutyrate (PHB). This discovery was achieved as a trigger of foam formation during the microbial PHB fermentation. The purified MVs were existed as a mixture of outer MVs and outer/inner MVs, revealed by transmission electron microscopy. It should be noted that there was a good correlation between MV formation and PHB production level that can be finely controlled by varying glucose concentrations, suggesting the causal relationship in both supramolecules artificially produced in the microbial platform. Notably, the controllable secretion of MV was governed spatiotemporally through the morphological change of the E. coli cells caused by the PHB intracellular accumulation. Based on a hypothesis of PHB internal-pressure dependent envelope-disorder induced MV biogenesis, here we propose a new Polymer Intracellular Accumulation-triggered system for MV Production (designated "PIA-MVP") with presenting a mechanistic model for MV biogenesis. The PIA-MVP is a promising microbial platform that will provides us with a significance for further study focusing on biopolymer capsulation and cross-membrane transportation for different application purposes.

Optimization of Culture Conditions for Secretory Production of 3-Hydroxybutyrate Oligomers Using Recombinant Escherichia coli

Poly(3-hydroxybutyrate) [P(3HB)] is the most representative polyhydroxyalkanoate (PHA), which is a storage polyester for prokaryotic cells. P(3HB)-producing recombinant Escherichia coli secretes diethylene glycol (DEG)-terminated 3HB oligomers (3HBO-DEG) through a PHA synthase-mediated chain transfer and alcoholysis reactions with externally added DEG. The purpose of this study was to optimize the culture conditions for the secretory production of 3HBO-DEG with jar fermenters. First, the effects of culture conditions, such as agitation speed, culture temperature, culture pH, and medium composition on 3HBO-DEG production, were investigated in a batch culture using 250-ml mini jar fermenters. Based on the best culture conditions, a fed-batch culture was conducted by feeding glucose to further increase the 3HBO-DEG titer. Consequently, the optimized culture conditions were reproduced using a 2-L jar fermenter. This study successfully demonstrates a high titer of 3HBO-DEG, up to 34.8 g/L, by optimizing the culture conditions, showing the feasibility of a new synthetic strategy for PHA-based materials by combining secretory oligomer production and subsequent chemical reaction.

Recent advances in the microbial synthesis of lactate-based copolymer

Due to the increasing environmental pollution of un-degradable plastics and the consumption of non-renewable resources, more attention has been attracted by new bio-degradable/based polymers produced from renewable resources. Polylactic acid (PLA) is one of the most representative bio-based materials, with obvious advantages and disadvantages, and has a wide range of applications in industry, medicine, and research. By copolymerizing to make up for its deficiencies, the obtained copolymers have more excellent properties. The development of a one-step microbial metabolism production process of the lactate (LA)-based copolymers overcomes the inherent shortcomings in the traditional chemical synthesis process. The most common lactate-based copolymer is poly(lactate-co-3-hydroxybutyrate) [P(LA-co-3HB)], within which the difference of LA monomer fraction will cause the change in the material properties. It is necessary to regulate LA monomer fraction by appropriate methods. Based on synthetic biology and systems metabolic engineering, this review mainly focus on how did the different production strategies (such as enzyme engineering, fermentation engineering, etc.) of P(LA-co-3HB) optimize the chassis cells to efficiently produce it. In addition, the metabolic engineering strategies of some other lactate-based copolymers are also introduced in this article. These studies would facilitate to expand the application fields of the corresponding materials.

Recent developments in short- and medium-chain- length Polyhydroxyalkanoates: Production, properties, and applications

Developing renewable resource-based plastics with complete biodegradability and a minimal carbon footprint can open new opportunities to effectively manage the end-of-life plastics waste and achieve a low carbon society. Polyhydroxyalkanoates (PHAs) are biobased and biodegradable thermoplastic polyesters that accumulate in microorganisms (e.g., bacterial, microalgal, and fungal species) as insoluble and inert intracellular inclusion. The PHAs recovery from microorganisms, which typically involves cell lysis, extraction, and purification, provides high molecular weight and purified polyesters that can be compounded and processed using conventional plastics converting equipment. The physio-chemical, thermal, and mechanical properties of the PHAs are comparable to traditional synthetic polymers such as polypropylene and polyethylene. As a result, it has attracted substantial applications interest in packaging, personal care, coatings, agricultural and biomedical uses. However, PHAs have certain performance limitations (e.g. slow crystallization), and substantially more expensive than many other polymers. As such, more research and development is required to enable them for extensive use. This review provides a critical review of the recent progress achieved in PHAs production using different microorganisms, downstream processing, material properties, processing avenues, recycling, aerobic and anaerobic biodegradation, and applications.

Biosynthesis of Random-Homo Block Copolymer Poly[Glycolate-ran-3-Hydroxybutyrate (3HB)]-b-Poly(3HB) Using Sequence-Regulating Chimeric Polyhydroxyalkanoate Synthase in Escherichia coli

Glycolate (GL)-containing polyhydroxyalkanoate (PHA) was synthesized in Escherichia coli expressing the engineered chimeric PHA synthase PhaC AR and coenzyme A transferase. The cells produced poly[GL-co-3-hydroxybutyrate (3HB)] with the supplementation of GL and 3HB, thus demonstrating that PhaC AR is the first known class I PHA synthase that is capable of incorporating GL units. The triad sequence analysis using 1H nuclear magnetic resonance indicated that the obtained polymer was composed of two distinct regions, a P(GL-ran-3HB) random segment and P(3HB) homopolymer segment. The random segment was estimated to contain a 71 mol% GL molar ratio, which was much greater than the value (15 mol%) previously achieved by using PhaC1 P s STQK. Differential scanning calorimetry analysis of the polymer films supported the presence of random copolymer and homopolymer phases. The solvent fractionation of the polymer indicated the presence of a covalent linkage between these segments. Therefore, it was concluded that PhaC AR synthesized a novel random-homo block copolymer, P(GL-ran-3HB)-b-P(3HB).