Molecular mass of poly-3-hydroxybutyrate (P3HB) produced by Azotobacter vinelandii is determined by the ratio of synthesis and degradation under fixed dissolved oxygen tension

Production of poly-3-hydroxybutyrate (PHB) nanoparticles using grape residues as the sole carbon source

The production of poly-3-hydroxybutyrate (PHB) on an industrial scale remains a major challenge due to its higher production cost compared to petroleum-based plastics. As a result, it is necessary to develop efficient fermentative processes using low-cost substrates and identify high-value-added applications where biodegradability and biocompatibility properties are of fundamental importance. In this study, grape residues, mainly grape skins, were used as the sole carbon source in Azotobacter vinelandii OP cultures for PHB production and subsequent nanoparticle synthesis based on the extracted polymer. The grape residue pretreatment showed a high rate of conversion into reducing sugars (fructose and glucose), achieving up to 43.3 % w w-1 without the use of acid or external heat. The cultures were grown in shake flasks, obtaining a biomass concentration of 2.9 g L-1 and a PHB accumulation of up to 37.7 % w w-1. PHB was characterized using techniques such as Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The formation of emulsified PHB nanoparticles showed high stability, with a particle size between 210 and 240 nm and a zeta potential between -12 and - 15 mV over 72 h. Owing to these properties, the produced PHB nanoparticles hold significant potential for applications in drug delivery.

Relation of 3HV fraction and thermomechanical properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) produced by Azotobacter vinelandii OP

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) has attracted substantial attention as a promising material for industrial applications. In this study, different PHBV films with distinct 3-hydroxyvalerate (3HV) contents produced by Azotobacter vinelandii OP were evaluated. The 3HV fraction ranged from 18.6 to 36.7 mol%, and the number-average molecular weight (Mn) was between 238 and 434 kDa. In the bioreactor, a 3HV fraction (36.7 mol%) and an Mn value of 409 kDa were obtained with an oxygen transfer rate (OTR) of 12.5 mmol L-1 h-1. Thermal analysis measurements showed decreased melting (Tm) and glass transition (Tg) temperatures, and values with relatively high 3HV fractions indicated improved thermomechanical properties. The incorporation of the 3HV fraction in the PHBV chain improved the thermal stability of the films, reduced the polymer Tm, and affected the tensile strength. PHBV film with 36.7 mol% 3HV showed an increase in its tensile strength (51.8 MPa) and a decrease in its Tm (170.61 °C) compared with PHB. Finally, scanning electron microscopy (SEM) results revealed that the PHBV film with 32.8 mol% 3HV showed a degradation upon contact with soil, water, or soil bacteria, showing more porous surfaces after degradation. The latter phenomenon indicated that thermomechanical properties played an important role in biodegradation.

Extended batch cultures for poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) production by Azotobacter vinelandii OP growing at different aeration rates

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a polymer produced by Azotobacter vinelandii OP. In the bioreactor, PHBV production and its molar composition are affected by aeration rate. PHBV production by A. vinelandii OP was evaluated using extended batch cultures at different aeration rates, which determined different oxygen transfer rates (OTR) in the cultures. Under the conditions evaluated, PHBV with different 3-hydroxyvalerate (3HV) fractions were obtained. In the cultures with a low OTR (6.7 mmol L-1 h-1, at 0.3 vvm), a PHBV content of 38% w w-1 with 9.1 mol % 3HV was achieved. The maximum PHBV production (72% w w-1) was obtained at a high OTR (18.2 mmol L-1 h-1, at 1.0 vvm), both at 48 h. Thus, PHBV production increased in the bioreactor with an increased aeration rate, but not the 3HV fraction in the polymer chain. An OTR of 24.9 mmol L-1 h-1 (at 2.1 vvm) was the most suitable for improving the PHBV content (61% w w-1) and a high 3HV fraction of 20.8 mol % (at 48 h); and volumetric productivity (0.15 g L-1 h-1). The findings indicate that the extended batch culture at 2.1 vvm is the most adequate mode of cultivation to produce higher biomass and PHBV with a high 3HV fraction. Overall, the results have shown that the PHBV production and 3HV fraction could be affected by the aeration rate and the proposed approach could be applied to implement cultivation strategies to optimize PHBV production for different biotechnological applications.

Evaluation of Poly-3-Hydroxybutyrate (P3HB) Scaffolds Used for Epidermal Cells Growth as Potential Biomatrix

Advances in tissue engineering have made possible the construction of organs and tissues with the use of biomaterials and cells. Three important elements are considered: a specific cell culture, an adequate environment, and a scaffold. The present study aimed to develop P3HB scaffolds by 3D printing and evaluate their biocompatibility with HaCaT epidermal cells, as a potential model that allows the formation of functional tissue. By using a method of extraction and purification with ethanol and acetone, a biopolymer having suitable properties for use as a tissue support was obtained. This polymer exhibited a higher molecular weight (1500 kDa) and lower contact angle (less than 90°) compared to the material obtained using the conventional method. The biocompatibility analysis reveals that the scaffold obtained using the ethanol–acetone method and produced by 3D printing without pores was not cytotoxic, did not self-degrade, and allowed high homogenous cell proliferation of HaCaT cells. In summary, it is possible to conclude that the P3HB scaffold obtained by 3D printing and a simplified extraction method is a suitable support for the homogeneous development of HaCaT keratinocyte cell lineage, which would allow the evaluation of this material to be used as a biomatrix for tissue engineering.

Competitive Biosynthesis of Bacterial Alginate Using Azotobacter vinelandii 12 for Tissue Engineering Applications

This study investigated the effect of various cultivation conditions (sucrose/phosphate concentrations, aeration level) on alginate biosynthesis using the bacterial producing strain Azotobacter vinelandii 12 by the full factorial design (FFD) method and physicochemical properties (e.g., rheological properties) of the produced bacterial alginate. We demonstrated experimentally the applicability of bacterial alginate for tissue engineering (the cytotoxicity testing using mesenchymal stem cells (MSCs)). The isolated synthesis of high molecular weight (M_w) capsular alginate with a high level of acetylation (25%) was achieved by FFD method under a low sucrose concentration, an increased phosphate concentration, and a high aeration level. Testing the viscoelastic properties and cytotoxicity showed that bacterial alginate with a maximal M_w (574 kDa) formed the densest hydrogels (which demonstrated relatively low cytotoxicity for MSCs in contrast to bacterial alginate with low M_w). The obtained data have shown promising prospects in controlled biosynthesis of bacterial alginate with different physicochemical characteristics for various biomedical applications including tissue engineering.

Recent progress and challenges in microbial polyhydroxybutyrate (PHB) production from CO2 as a sustainable feedstock: A state-of-the-art review

The recalcitrance of petroleum-based plastics causes severe environmental problems and has accelerated research into production of biodegradable polymers from inexpensive and sustainable feedstocks. Various microorganisms are capable of producing Polyhydroxybutyrate (PHB), a representative biodegradable polymer, under nutrient-limited conditions, among which CO₂-utilizing microorganisms are of primary interest. Herein, we discuss recent progress on bacterial strains including proteobacteria, purple non-sulfur bacteria, and cyanobacteria in terms of CO₂-containing carbon sources, PHB-production capability, and genetic modification. In addition, this review introduces recent technical approaches used to improve PHB production from CO₂ such as two-stage bioprocesses and bioelectrochemical systems. Challenges and future perspectives for the development of economically feasible PHB production are also discussed. Finally, this review might provide insights into the construction of a closed-carbon-loop to cope with climate change.

Polyhydroxyalkanoates biopolymers toward decarbonizing economy and sustainable future

Polymers are synonymous with the modern way of living. However, polymers with a large carbon footprint, especially those derived from nonrenewable petrochemical sources, are increasingly perceived as detrimental to the environment and a sustainable future. Polyhydroxyalkanoate (PHA) is a microbial biopolymer and a plausible alternative for renewable sources. However, PHA in its monomeric forms has very limited applications due to its limited flexibility, tensile strength, and moldability. Herein, the life cycle of PHA molecules, from biosynthesis to commercial utilization for diverse applications is discussed. For clarity, the applications of this bioplastic biocomposite material are further segregated into two domains, namely, the industrial sector and the medical sector. The industry sectors reviewed here include food packaging, textiles, agriculture, automotive, and electronics. High-value addition of PHA for a sustainable future can be foreseen in the medical domain. Properties such as biodegradability and biocompatibility make PHA a suitable candidate for decarbonizing biomaterials during tissue repair, organ reconstruction, drug delivery, bone tissue engineering, and chemotherapeutics.

Production of Poly-3-Hydroxybutyrate (P3HB) with Ultra-High Molecular Weight (UHMW) by Mutant Strains of Azotobacter vinelandii Under Microaerophilic Conditions

Poly-3-hydroxybutyrate (P3HB) is a biopolymer, which presents characteristics similar to those of plastics derived from the petrochemical industry. The thermomechanical properties and biodegradability of P3HB are influenced by its molecular weight (MW). The aim of the present study was to evaluate the changes of the molecular weight of P3HB as a function of oxygen transfer rate (OTR) in the cultures using two strains of Azotobacter vinelandii, a wild-type strain OP, and PhbZ1 mutant with a P3HB depolymerase inactivated. Both strains were grown in a bioreactor under different OTR conditions. An inverse relationship was found between the average molecular weight of P3HB and the OTR_max, obtaining a polymer with a maximal MW (8000-10,000 kDa) from the cultures developed at OTR_max of 5 mmol L^-1 h^-1 using both strains, with respect to the cultures conducted at 8 and 11 mmol L^-1 h^-1, which produced a P3HB between 4000 and 5000 kDa. The increase in MW of P3HB was related to the activity of enzymes involved in the synthesis and depolymerization. Overall, our results show that it is possible to modulate the average molecular weight of P3HB by manipulating oxygen transfer conditions with both strains (OP and PhbZ1 mutant) of A. vinelandii.

Facile method based on 19F-NMR for the determination of hydroxyl value and molecular weight of hydroxyl terminated polymers

Two-steps method to determine hydroxyl value and molecular weight of hydroxyl terminated polymers by ¹⁹F-NMR measurements.

Poly(3-hydroxybutyrate) accumulation by Azotobacter vinelandii under different oxygen transfer strategies

Azotobacter vinelandii OP is a bacterium that produces poly(3-hydroxybutyrate) (PHB). PHB production in a stirred bioreactor, at different oxygen transfer strategies, was evaluated. By applying different oxygen contents in the inlet gas, the oxygen transfer rate (OTR) was changed under a constant agitation rate. Batch cultures were performed without dissolved oxygen tension (DOT) control (using 9% and 21% oxygen in the inlet gas) and under DOT control (4%) using gas blending. The cultures that developed without DOT control were limited by oxygen. As result of varying the oxygen content in the inlet gas, a lower OTR (4.6 mmol L^-1 h^-1) and specific oxygen uptake rate (11.6 mmol g^-1 h^-1) were obtained using 9% oxygen in the inlet gas. The use of 9% oxygen in the inlet gas was the most suitable for improving the intracellular PHB content (56 ± 6 w w^-1). For the first time, PHB accumulation in A. vinelandii OP cultures, developed with different OTRs, was compared under homogeneous mixing conditions, demonstrating that bacterial respiration affects PHB synthesis. These results can be used to design new oxygen transfer strategies to produce PHB under productive conditions.

Oxygen uptake rate in alginate producer (algU+) and nonproducer (algU-) strains of Azotobacter vinelandii under nitrogen-fixation conditions

AIMS

The sigma E (AlgU) in Azotobacter vinelandii has been shown to control the expression of cydR gene, a repressor of genes of the alternative respiratory chain, and alginate has been considered a barrier for oxygen diffusion. Therefore, the aim of the present study was to compare the respiratory activity of an alginate nonproducing strain, lacking the sigma factor E (algU-), and polymer-producing strains (algU+) of A. vinelandii under diazotrophic conditions at different aeration conditions.

METHODS AND RESULTS

Our results reveal that under diazotrophic and high aeration conditions, A. vinelandii strain OP (algU-) had a specific oxygen consumption rate higher (30 and 54%) than those observed in the OP algU+-complemented strain, named OPAlgU+, and the ATCC 9046 respectively. However, the specific growth rate and biomass yields (based on oxygen and sucrose) were lower for OP cultivations as compared to the algU+ strains. These differences were partially explained by an increase in 1·5-fold of cydA relative expression in the OP strain, as compared to that obtained in the isogenic OPAlgU+ strain.

CONCLUSIONS

Overall, our results confirm the important role of algU gene on the regulation of respiratory metabolism under diazotrophic growth when A. vinelandii is exposed to high aeration.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights the role of AlgU to control respiration of A. vinelandii when exposed to diazotrophy.

Molecular mass of Poly-3-hydroxybutyrate (P3HB) produced by Azotobacter vinelandii is influenced by the polymer content in the inoculum

Poly-3-hydroxybutyrate (P3HB) is a biopolymer produced by Azotobacter vinelandii. The physicochemical properties and applications of P3HB are strongly influenced by its weight-average molecular mass (Mw), and in A. vinelandii, it could be influenced by the culture conditions. The aim of this study was to evaluate the effect of the P3HB content of the inoculum on the Mw of the polymer produced by A. vinelandii OP in bioreactor cultures. A. vinelandii cells containing 20, 50 and 70% of P3HB were used as inoculum. The P3HB content in the inoculum affected the volumetric P3HB productivity (q_P3HB) and the Mw of P3HB. Those cultures inoculated with cells containing 20% of P3HB, achieved the highest q_P3HB (0.17±0.018g_P3HBL^-1h^-1); whereas a P3HB content of 70% was reflected as a low q_P3HB (0.021±0.002g_P3HBL^-1h^-1). On the other hand, using an inoculum with 70% of polymer content, the Mw of the biopolymer remained stable at values close to 3200kDa; whereas, when an inoculum with 20% of P3HB was used, the Mw decreased drastically during early stages of cultivation. These results show that manipulating the P3HB content of the inoculum is possible to produce biopolymers with a suitable Mw.