Bacterial conversion of waste into polyhydroxybutyrate (PHB): A new approach of bio-circular economy for treating waste and energy generation

Production of polyhydroxyalkanoates using sewage and cheese whey

Recently, polyhydroxyalkanoates (PHAs) have been produced using raw sewage in our laboratory; however, the production concentrations are low. Therefore, this study aimed to enhance PHA production by applying different strategies. PHA production was higher in sewage-containing medium than in mineral salt medium and was enhanced 22-fold after glucose supplementation. A relatively high degree of glucose consumption (83.6 ± 1.59 %) was also achieved. Bacteria incubated with cheese whey diluted with sewage showed higher PHA production than bacteria incubated with cheese whey diluted with distilled water did. The expression of the PHA synthase gene (phaC) was evaluated via real-time polymerase chain reaction using low- and high-carbon-containing sewage. Relatively higher phaC expression levels were observed in high-carbon-containing sewage but at lower nitrogen concentrations. The characteristics of the produced PHA were comparable to those of standard PHA. Therefore, this study revealed that the bacterium Bacillus sp. CYR1 can produce PHA from low- or high-carbon-containing wastewater.

Additive Free Crosslinking of Poly-3-hydroxybutyrate via Electron Beam Irradiation at Elevated Temperatures

When applying electron or gamma irradiation to poly-3-hydroxybutyrate (P3HB), main chain scissions are the dominant material reactions. Though propositions have been made that crosslinking in the amorphous phase of P3HB occurs under irradiation, a conclusive method to achieve controlled additive free irradiation crosslinking has not been shown and no mechanism has been derived to the best of our knowledge. By applying irradiation in a molten state at 195 °C and doses above 200 kGy, we were able to initiate crosslink reactions and achieved gel formation of up to 16%. The gel dose Dgel was determined to be 200 kGy and a range of the G values, the number of scissions and crosslinks for 100 eV energy deposition, is given. Rheology measurements, as well as size exclusion chromatography (SEC), showed indications for branching at doses from 100 to 250 kGy. Thermal analysis showed the development of a bimodal peak with a decrease in the peak melt temperature and an increase in peak width. In combination with an increase in the thermal degradation temperature for a dose of 200 kGy compared to 100 kGy, thermal analysis also showed phenomena attributed to branching and crosslinking.

Metabolomic analysis of halotolerant endophytic bacterium Salinivibrio costicola isolated from Suaeda maritima (L.) dumort

In this study, the Salinivibrio costicola strain was isolated from Suaeda maritima (L.) Dumort. collected in Sinan, Republic of Korea. The endophytic characteristics of the Gram-negative bacterium S. costicola were verified with metagenomics sequencing of S. maritima. S. costicola was cultivated for 3 days in a liquid medium with 3.3% sea salt and analyzed the metabolites produced by the strain cultured in five different bacterial cultivation media. From the bacterial cultures, polyhydroxybutyrate derivatives were detected using high-resolution mass spectrometry, and three major compounds were isolated by high-performance liquid chromatography. The chemical structures of the compounds were elucidated using nuclear magnetic resonance and MS analyses. The relationship between the compounds was confirmed with Global Natural Product Social Molecular Networking, which showed clustering of the compounds. From the S. maritima extract, polyhydroxybutyrate derivatives produced by S. costicola were detected as being accumulated in the host plant.

Review of the Developments of Bacterial Medium-Chain-Length Polyhydroxyalkanoates (mcl-PHAs)

Synthetic plastics derived from fossil fuels—such as polyethylene, polypropylene, polyvinyl chloride, and polystyrene—are non-degradable. A large amount of plastic waste enters landfills and pollutes the environment. Hence, there is an urgent need to produce biodegradable plastics such as polyhydroxyalkanoates (PHAs). PHAs have garnered increasing interest as replaceable materials to conventional plastics due to their broad applicability in various purposes such as food packaging, agriculture, tissue-engineering scaffolds, and drug delivery. Based on the chain length of 3-hydroxyalkanoate repeat units, there are three types PHAs, i.e., short-chain-length (scl-PHAs, 4 to 5 carbon atoms), medium-chain-length (mcl-PHAs, 6 to 14 carbon atoms), and long-chain-length (lcl-PHAs, more than 14 carbon atoms). Previous reviews discussed the recent developments in scl-PHAs, but there are limited reviews specifically focused on the developments of mcl-PHAs. Hence, this review focused on the mcl-PHA production, using various carbon (organic/inorganic) sources and at different operation modes (continuous, batch, fed-batch, and high-cell density). This review also focused on recent developments on extraction methods of mcl-PHAs (solvent, non-solvent, enzymatic, ultrasound); physical/thermal properties (Mw, Mn, PDI, Tm, Tg, and crystallinity); applications in various fields; and their production at pilot and industrial scales in Asia, Europe, North America, and South America.

Poly(3-hydroxybutyrate) Nanocomposites with Cellulose Nanocrystals

Poly(3-hydroxybutyrate) (PHB) is one of the most promising substitutes for the petroleum-based polymers used in the packaging and biomedical fields due to its biodegradability, biocompatibility, good stiffness, and strength, along with its good gas-barrier properties. One route to overcome some of the PHB's weaknesses, such as its slow crystallization, brittleness, modest thermal stability, and low melt strength is the addition of cellulose nanocrystals (CNCs) and the production of PHB/CNCs nanocomposites. Choosing the adequate processing technology for the fabrication of the PHB/CNCs nanocomposites and a suitable surface treatment for the CNCs are key factors in obtaining a good interfacial adhesion, superior thermal stability, and mechanical performances for the resulting nanocomposites. The information provided in this review related to the preparation routes, thermal, mechanical, and barrier properties of the PHB/CNCs nanocomposites may represent a starting point in finding new strategies to reduce the manufacturing costs or to design better technological solutions for the production of these materials at industrial scale. It is outlined in this review that the use of low-value biomass resources in the obtaining of both PHB and CNCs might be a safe track for a circular and bio-based economy. Undoubtedly, the PHB/CNCs nanocomposites will be an important part of a greener future in terms of successful replacement of the conventional plastic materials in many engineering and biomedical applications.

Two-Stage Polyhydroxyalkanoates (PHA) Production from Cheese Whey Using Acetobacter pasteurianus C1 and Bacillus sp. CYR1

Cheese whey (CW) can be an excellent carbon source for polyhydroxyalkanoates (PHA)-producing bacteria. Most studies have used CW, which contains high amounts of lactose, however, there are no reports using raw CW, which has a relatively low amount of lactose. Therefore, in the present study, PHA production was evaluated in a two-stage process using the CW that contains low amounts of lactose. In first stage, the carbon source existing in CW was converted into acetic acid using the bacteria, Acetobacter pasteurianus C1, which was isolated from food waste. In the second stage, acetic acid produced in the first stage was converted into PHA using the bacteria, Bacillus sp. CYR-1. Under the condition of without the pretreatment of CW, acetic acid produced from CW was diluted at different folds and used for the production of PHA. Strain CYR-1 incubated with 10-fold diluted CW containing 5.7 g/L of acetic acid showed the higher PHA production (240.6 mg/L), whereas strain CYR-1 incubated with four-fold diluted CW containing 12.3 g/L of acetic acid showed 126 mg/L of PHA. After removing the excess protein present in CW, PHA production was further enhanced by 3.26 times (411 mg/L) at a four-fold dilution containing 11.3 g/L of acetic acid. Based on Fourier transform infrared spectroscopy (FT-IR), and ¹H and ¹³C nuclear magnetic resonance (NMR) analyses, it was confirmed that the PHA produced from the two-stage process is poly-β-hydroxybutyrate (PHB). All bands appearing in the FT-IR spectrum and the chemical shifts of NMR nearly matched with those of standard PHB. Based on these studies, we concluded that a two-stage process using Acetobacter pasteurianus C1 and Bacillus sp. CYR-1 would be applicable for the production of PHB using CW containing a low amount of lactose.

Poly-β-Hydroxybutyrate Production by Rhodopseudomonas sp. Grown in Semi-Continuous Mode in a 4 L Photobioreactor

The synthesis of polyhydroxybutyrate (PHB) by photosynthetic non-sulfur bacteria is a potential approach for producing biodegradable plastics. In this work, acetate was used as a single carbon source to study the effect on PHB formation in Rhodopseudomonas sp. cultured in a cylindrical four-liter photobioreactor under semi-continuous mode. The cultivation process is divided into a symmetrical growth phase and a PHB accumulation phase separated temporally. The symmetrical growth phase (nutrient sufficient conditions) was followed by a sulfur-limited phase to promote PHB accumulation. The main novelty is the progressive lowering of the sulfur concentration into Rhodopseudomonas culture, which was obtained by two concomitant conditions: (1) sulfur consumption during the bacterial growth and (2) semi-continuous growth strategy. This caused a progressive lowering of the sulfur concentration into Rhodopseudomonas culturedue to the sulfur-free medium used to replace 2 L of culture (50% of the total) that was withdrawn from the photobioreactor at each dilution. The PHB content ranged from 9.26% to 15.24% of cell dry weight. At the steady state phase, the average cumulative PHB was >210 mg/L. Sulfur deficiency proved to be one of the most suitable conditions to obtain high cumulative PHB in Rhodopseudomonas culture.

Utilization of Agave durangensis leaves by Bacillus cereus 4N for polyhydroxybutyrate (PHB) biosynthesis

Lignocellulosic wastes may provide a means to economize polyhydroxybutyrate (PHB) production. This study has proposed the use of Agave durangensis leaves obtained from the artisanal mezcal industry as a novel substrate for this aim. Results revealed an increase in PHB biosynthesis (0.32 g/L) and improvement in %PHB (16.79-19.51%) by Bacillus cereus 4N when A. durangensis leaves used as carbon source were physically pre-treated by ultrasound for 30 min (ADL + US30') and thermally pre-treated (ADL + Q). Chemical analyses and SEM studies revealed compositional and morphological changes when A. durangensis leaves were physically pre-treated. Also, elemental analysis of growth media showed that carbon/nitrogen ratios of 14-21, and low nitrogen, hydrogen, and protein content were well-suited for PHB biosynthesis. Confocal microscopy revealed morphological changes in the bacterial cell and carbonosome structure under the influence of different substrates. Finally, Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) analyses showed that homopolymeric PHB with a high thermal-resistance (271.94-272.89 °C) was produced. Therefore, the present study demonstrates the potential use of physically pre-treated A. durangensis leaves to produce PHB. These results promote the development of a circular economy in Mexico, where lignocellulosic wastes can be employed to produce value-added biotechnological products.

A new practical approach for the biological treatment of a mixture of cheese whey and white wastewaters using Kefir grains

Kefir grains are a microbial consortium of different genera of bacteria and yeasts. In this study, the performance of Tunisian Kefir grains during the biological treatment of a mixture of Gouda cheese whey and white wastewaters (GCW) in ratio 1:1 with very high organic matter concentration is investigated. The biological process was evaluated and optimized through the response surface methodology. Under the optimum conditions, Kefir grains concentration of 1.02%, temperature at 36.68 °C, and incubation time of 5.14 days, the removal efficiencies of COD, PO₄^3-, and NO₃^- were 87, 37.48, and 39.5%, respectively. Interestingly, the reusability tests of the grains proved not only their high resistance to harsh environmental conditions but also their great potential for more practical applications. Particularly, different strains were isolated from the grains and identified as Kluyveromyces marxianus, Lactoccocus lactis, Lactobacillus kefiri, and Bacillus spp. using 16S rDNA sequence analysis and rep-PCR fingerprinting. At the biological level, the raw GCW (RGCW) has a negative impact on the Hordeum vulgare both on seed germination, and on the growth parameters of seedlings. Interestingly, after Kefir grains treatment, the treated GCW (TGCW) allow a seedlings growth and germination rate similar to those soaked in water.

Biological conversion of methane to polyhydroxyalkanoates: Current advances, challenges, and perspectives

Methane emissions and plastic pollution are critical global challenges. The biological conversion of methane to poly-β-hydroxybutyrate (PHB) not only mitigates methane emissions but also provides biodegradable polymer substitutes for petroleum-based materials used in plastics production. This work provides an early overview of the methane-based PHB advances and discusses challenges and related strategies. Recent advances of PHB, including PHB biosynthetic pathways, methanotrophs, bioreactors, and the performances of PHB materials are introduced. Major challenges of methane-based PHB production are discussed in detail; these include low efficiency of methanotrophs, low gas-liquid mass transfer efficiency, and poor material properties. To overcome these limitations, various approaches are also explored, such as feast-famine regimes, engineered microorganisms, gas-permeable membrane bioreactors, two-phase partitioning bioreactors, poly-β-hydroxybutyrate-co-hydroxyvalerate synthesis, and molecular weight manipulation.