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.

Phasins are proteins located on the surface of poly-3-hydroxybutyrate (P3HB) granules that affect the metabolism of the polymer, the size and number of the granules, and some also have stress-protecting and growth-promoting effects. This study evaluated the effect of inactivating two new phasins (PhbP2 or PhbP3) on the cellular growth, production, and molecular mass of P3HB in cultures under low or high oxygen transfer rates (OTR). The results revealed that under high OTRₘₐₓ conditions (between 8.1 and 8.9 mmol L⁻¹ h⁻¹), the absence of phasins PhbP2 and PhbP3 resulted in a strong negative effect on the growth rate; in contrast, the rates of specific oxygen consumption increased in both cases. This behavior was not observed under a low oxygen transfer rate (3.9 ± 0.71 mol L⁻¹ h⁻¹), where cellular growth and oxygen consumption were the same for the different strains evaluated. It was observed that at high OTR, the absence of PhbP3 affected the production of P3HB, decreasing it by 30% at the end of cultivation. In contrast, the molecular weight remained constant over time. In summary, the absence of phasin PhbP3 significantly impacted the growth rate and polymer synthesis, particularly at high maximum oxygen transfer rates (OTRₘₐₓ).

Azotobacter vinelandii OP is a bacterium that can produce poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P3HBV), a biodegradable and biocompatible polymer with applications in the biomedical field. This study aimed to evaluate P3HBV production and its 3-hydroxyvalerate (3HV) fraction under different agitation rates and oxygen uptake rates (q_O2) in chemostat cultures of A. vinelandii OP. Steady-state conditions with either oxygen or carbon limitation were established by modulating the agitation rates. Under oxygen-limited conditions (low q_O2 values) biomass and P3HBV concentrations increased to 3.3 g L⁻¹ and 2.1 g L⁻¹, respectively. At higher q_O2 values, the chemostat cultures were limited by carbon, and P3HBV content decreased from 62% to 33% (w w⁻¹). The highest 3HV molar fractions, 33.7 and 36.4 mol %, were observed at both the lowest and highest q_O2 levels, possibly linked to comparable valeric acid consumption rates. An elevated NAD(P)H/NAD(P)⁺ ratio was also observed under oxygen limitation, favoring polymer accumulation by indicating a more favorable intracellular redox state. These findings highlight the impact of nutrient limitation and respiratory activity on the biosynthesis of P3HBV and the 3HV composition by Azotobacter vinelandii OP. Such insights can support the development of tailored bioprocesses to modulate polymer characteristics, enabling a broader range of potential biomedical applications for P3HBV.

Phasins are proteins, which are located on the surface of Polyhydroxybutyrate (P3HB) granules and that carry out activities that affect the polymer metabolism. This study aimed to evaluate the effect of inactivating phasins PhbP2 or PhbP3 on the growth rate, production and molecular mass of P3HB in cultures under low or high oxygen transfer rates (OTR). The results reveal that under high OTRₘₐₓ conditions (between 8.1 and 8.9 mmol L⁻¹ h⁻¹ depending on the strain evaluated), the absence of phasins PhbP2 and PhbP3 resulted in a strong negative effect on the growth rate. In contrast, the rates of specific oxygen consumption increased in the case of cultures with strains where phasins were absent. This behavior was not observed under conditions at a low oxygen transfer rate (3.9 ± 0.71 mol L⁻¹ h⁻¹); where both growth and oxygen consumption were the same for the different strains evaluated. It was observed, at high OTRₘₐₓ that the absence of phasin PhbP3 harms the P3HB synthesis, finding a decrease of 30 % in the polymer production at the end of cultivation; however, the molecular weight of the polymer remained constant during the course of cultivation. In summary, it can be concluded that both growth and polymer synthesis were affected when phasin PhbP3 was absent from the P3HB granule, especially at high OTRₘₐₓ.