Application of random mutagenesis to enhance the production of polyhydroxyalkanoates by Cupriavidus necator H16 on waste frying oil

Identification of Oil-Loving Cupriavidus necator BM3-1 for Polyhydroxyalkanoate Production and Assessing Contribution of Exopolysaccharide for Vegetable Oil Utilization

Polyhydroxyalkanoates (PHA) have received attention owing to their biodegradability and biocompatibility, with studies exploring PHA-producing bacterial strains. As vegetable oil provides carbon and monomer precursors for poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(3HB-co-3HHx)), oil-utilizing strains may facilitate PHA production. Herein, Cupriavidus necator BM3-1, which produces 11.1 g/L of PHB with 5% vegetable oil, was selected among various novel Cupriavidus necator strains. This strain exhibited higher preference for vegetable oils over sugars, with soybean oil and tryptone determined to be optimal sources for PHA production. BM3-1 produced 33.9 g/L of exopolysaccharides (EPS), which was three-fold higher than the amount produced by H16 (10.1 g/L). EPS exhibited 59.7% of emulsification activity (EI24), higher than that of SDS and of EPS from H16 with soybean oil. To evaluate P(3HB-co-3HHx) production from soybean oil, BM3-1 was engineered with P(3HB-co-3HHx) biosynthetic genes (phaCRa, phaARe, and phaJPa). BM3-1/pPhaCJ produced 3.5 mol% of 3HHx and 37.1 g/L PHA. BM3-1/pCB81 (phaCAJ) produced 32.8 g/L PHA, including 5.9 mol% 3HHx. Physical and thermal analyses revealed that P(3HB-co-5.9 mol% 3HHx) was better than PHB. Collectively, we identified a novel strain with high vegetable oil utilization capacity for the production of EPS, with the option to engineer the strain for P(3HB-co-3HHx).

Polyhydroxyalkanoate involvement in stress-survival of two psychrophilic bacterial strains from the High Arctic

An ever-growing body of literature evidences the protective role of polyhydroxyalkanoates (PHAs) against a plethora of mostly physical stressors in prokaryotic cells. To date, most of the research done involved bacterial strains isolated from habitats not considered to be life-challenging or extremely impacted by abiotic environmental factors. Polar region microorganisms experience a multitude of damaging factors in combinations rarely seen in other of Earth's environments. Therefore, the main objective of this investigation was to examine the role of PHAs in the adaptation of psychrophilic, Arctic-derived bacteria to stress conditions. Arctic PHA producers: Acidovorax sp. A1169 and Collimonas sp. A2191, were chosen and their genes involved in PHB metabolism were deactivated making them unable to accumulate PHAs (ΔphaC) or to utilize them (Δi-phaZ) as a carbon source. Varying stressors were applied to the wild-type and the prepared mutant strains and their survival rates were assessed based on CFU count. Wild-type strains with a functional PHA metabolism were best suited to survive the freeze-thaw cycle - a common feature of polar region habitats. However, the majority of stresses were best survived by the ΔphaC mutants, suggesting that the biochemical imbalance caused by the lack of PHAs induced a permanent cell-wide stress response thus causing them to better withstand the stressor application. Δi-phaZ mutants were superior in surviving UV irradiation, hinting that PHA granule presence in bacterial cells is beneficial despite it being biologically inaccessible. Obtained data suggests that the ability to metabolize PHA although important for survival, probably is not the most crucial mechanism in the stress-resistance strategies arsenal of cold-loving bacteria. KEY POINTS: • PHA metabolism helps psychrophiles survive freezing • PHA-lacking psychrophile mutants cope better with oxidative and heat stresses • PHA granule presence enhances the UV resistance of psychrophiles.

Nonsterilized Fermentation of Crude Glycerol for Polyhydroxybutyrate Production by Metabolically Engineered Vibrio natriegens

Polyhydroxybutyrate (PHB) is an attractive biodegradable polymer that can be produced through the microbial fermentation of organic wastes or wastewater. However, its mass production has been restricted by the poor utilization of organic wastes due to the presence of inhibitory substances, slow microbial growth, and high energy input required for feedstock sterilization. Here, Vibrio natriegens, a fast-growing bacterium with a broad substrate spectrum and high tolerance to salt and toxic substances, was genetically engineered to enable efficient PHB production from nonsterilized fermentation of organic wastes. The key genes encoding the PHB biosynthesis pathway of V. natriegens were identified through base editing and overexpressed. The metabolically engineered strain showed 166-fold higher PHB content (34.95 wt %) than the wide type when using glycerol as a substrate. Enhanced PHB production was also achieved when other sugars were used as feedstock. Importantly, it outperformed the engineered Escherichia coli MG1655 in PHB productivity (0.053 g/L/h) and tolerance to toxic substances in crude glycerol, without obvious activity decline under nonsterilized fermentation conditions. Our work demonstrates the great potential of engineered V. natriegens for low-cost PHB bioproduction and lays a foundation for exploiting this strain as a next-generation model chassis microorganism in synthetic biology.

Genetic engineering of low-temperature polyhydroxyalkanoate production by Acidovorax sp. A1169, a psychrophile isolated from a subglacial outflow

In recent years, extremophilic microorganisms have been employed as producers of the microbial bioplastics polyhydroxyalkanoates (PHA), which are of great biotechnological value. Nevertheless, cold-loving or psychrophilic (cryophilic) bacteria have been neglected in this regard. Here, we present an investigation of the Arctic glacier-derived PHA producer Acidovorax sp. A1169. Biolog GEN III Microplates were used as a screening tool to identify the most suitable carbon substrate concerning PHA synthesis. The strain produced homopolymer poly(3-hydroxybutyrate) (PHB) most efficiently (2 g/L) at a temperature of 15 °C when supplied with fructose or mannitol as carbon sources with a substantial decrease of PHB biosynthesis at 17.5 °C. The PHB yield did not increase considerably or even decreased when carbon source concentration exceeded 10 g/L hinting that the strain is oligotrophic in nature. The strain was also capable of introducing 3-hydroxyvalerate (3HV) into the polymer structure, which is known to improve PHA thermoplastic properties. This is the first investigation providing insight into a PHA biosynthesis process by means of a true psychrophile, offering guidelines on polar-region bacteria cultivation, production of PHA and also on the methodology for genetic engineering of psychrophiles.

Enhancement of cyanobacterial PHB production using random chemical mutagenesis with detection through FACS

Poly-hydroxy-butyrate (PHB) bioplastic resin can be made directly from atmospheric CO₂ using cyanobacteria. However, higher PHB productivities are required before large-scale production is economically viable. Random mutagenesis offers a way to create new production strains with increased PHB yields and increased biomass densities without complex technical manipulation associated with genetically modified organisms. This study used staining with lipid fluorescent dye (BODIPY 493/593) and fluorescence-activated cell sorting (FACS) to select high lipid content mutants and followed this with a well plate growth screen. Thirteen mutants were selected for flask cultivation and two strains produced significantly higher PHB yields (29% and 26% higher than wild type), biomass accumulation (36% and 33% higher than wild type) and volumetric PHB density (75% and 67% higher than wild type). The maximum PHB yielding strain (% dcw) was 12.0%, which was 43% higher than the wild type (8.3% in this study). The highest volumetric PHB density was 18.8 mg PHB/L compared to 10.7 mg PHB/L by the wild type. To develop cyanobacterial strain with higher PHB productivities, the combination of random chemical mutagenesis and FACS holds great potential to promote cyanobacteria bioplastic production becoming economically viable.

A Review on Enhancing Cupriavidus necator Fermentation for Poly(3-hydroxybutyrate) (PHB) Production From Low-Cost Carbon Sources

In the context of a circular economy, bioplastic production using biodegradable materials such as poly(3-hydroxybutyrate) (PHB) has been proposed as a promising solution to fundamentally solve the disposal issue of plastic waste. PHB production techniques through fermentation of PHB-accumulating microbes such as Cupriavidus necator have been revolutionized over the past several years with the development of new strategies such as metabolic engineering. This review comprehensively summarizes the latest PHB production technologies via Cupriavidus necator fermentation. The mechanism of the biosynthesis pathway for PHB production was first assessed. PHB production efficiencies of common carbon sources, including food waste, lignocellulosic materials, glycerol, and carbon dioxide, were then summarized and critically analyzed. The key findings in enhancing strategies for PHB production in recent years, including pre-treatment methods, nutrient limitations, feeding optimization strategies, and metabolism engineering strategies, were summarized. Furthermore, technical challenges and future prospects of strategies for enhanced production efficiencies of PHB were also highlighted. Based on the overview of the current enhancing technologies, more pilot-scale and larger-scale tests are essential for future implementation of enhancing strategies in full-scale biogas plants. Critical analyses of various enhancing strategies would facilitate the establishment of more sustainable microbial fermentation systems for better waste management and greater efficiency of PHB production.

Engineered Mutants of a Marine Photosynthetic Purple Nonsulfur Bacterium with Increased Volumetric Productivity of Polyhydroxyalkanoate Bioplastics

Polyhydroxyalkanoates (PHAs) are green and sustainable bioplastics that could replace petrochemical synthetic plastics without posing environmental threats to living organisms. In addition, sustainable PHA production could be achieved using marine photosynthetic purple nonsulfur bacteria (PNSBs) that utilize natural seawater, sunlight, carbon dioxide gas, and nitrogen gas for growth. However, PHA production using marine photosynthetic PNSBs has not been economically feasible yet due to its high cost and low productivity. In this work, strain improvement, using genome-wide mutagenesis coupled with high-throughput screening via fluorescence-activated cell sorting, we were able to create Rhodovulum sulfidophilum mutants with enhanced volumetric PHA productivity, with an up to 1.7-fold increase. The best selected mutants (E6 and E6M4) reached the stationary growth phase 1 day faster and accumulated the maximum PHA content 2 days faster than the wild type. Maximizing volumetric PHA productivity before the stationary growth phase is indeed an additional advantage for R. sulfidophilum as a growth-associated PHA producer.

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.

Recent Advances in the Biosynthesis of Polyhydroxyalkanoates from Lignocellulosic Feedstocks

Polyhydroxyalkanoates (PHA) are biodegradable polymers that are considered able to replace synthetic plastic because their biochemical characteristics are in some cases the same as other biodegradable polymers. However, due to the disadvantages of costly and non-renewable carbon sources, the production of PHA has been lower in the industrial sector against conventional plastics. At the same time, first-generation sugar-based cultivated feedstocks as substrates for PHA production threatens food security and considerably require other resources such as land and energy. Therefore, attempts have been made in pursuit of suitable sustainable and affordable sources of carbon to reduce production costs. Thus, in this review, we highlight utilising waste lignocellulosic feedstocks (LF) as a renewable and inexpensive carbon source to produce PHA. These waste feedstocks, second-generation plant lignocellulosic biomass, such as maize stoves, dedicated energy crops, rice straws, wood chips, are commonly available renewable biomass sources with a steady supply of about 150 billion tonnes per year of global yield. The generation of PHA from lignocellulose is still in its infancy, hence more screening of lignocellulosic materials and improvements in downstream processing and substrate pre-treatment are needed in the future to further advance the biopolymer sector.

Established and Emerging Producers of PHA: Redefining the Possibility

The polyhydroxyalkanoate was discovered almost around a century ago. Still, all the efforts to replace the traditional non-biodegradable plastic with much more environmentally friendly alternative are not enough. While the petroleum-based plastic is like a parasite, taking over the planet rapidly and without any feasible cure, its perennial presence has made the ocean a floating island of life-threatening debris and has flooded the landfills with toxic towering mountains. It demands for an immediate solution; most resembling answer would be the polyhydroxyalkanoates. The production cost is yet one of the significant challenges that various corporate is facing to replace the petroleum-based plastic. To deal with the economic constrain better strain, better practices, and a better market can be adopted for superior results. It demands for systems for polyhydroxyalkanoate production namely bacteria, yeast, microalgae, and transgenic plants. Solely strains affect more than 40% of overall production cost, playing a significant role in both upstream and downstream processes. The highly modifiable nature of the biopolymer provides the opportunity to replace the petroleum plastic in almost all sectors from food packaging to medical industry. The review will highlight the recent advancements and techno-economic analysis of current commercial models of polyhydroxyalkanoate production. Bio-compatibility and the biodegradability perks to be utilized highly efficient in the medical applications gives ample reason to tilt the scale in the favor of the polyhydroxyalkanoate as the new conventional and sustainable plastic.

What Is New in the Field of Industrial Wastes Conversion into Polyhydroxyalkanoates by Bacteria?

The rising global consumption and industrialization has resulted in increased food processing demand. Food industry generates a tremendous amount of waste which causes serious environmental issues. These problems have forced us to create strategies that will help to reduce the volume of waste and the contamination to the environment. Waste from food industries has great potential as substrates for value-added bioproducts. Among them, polyhydroxyalkanaotes (PHAs) have received considerable attention in recent years due to their comparable characteristics to common plastics. These biodegradable polyesters are produced by microorganisms during fermentation processes utilizing various carbon sources. Scale-up of PHA production is limited due to the cost of the carbon source metabolized by the microorganisms. Therefore, there is a growing need for the development of novel microbial processes using inexpensive carbon sources. Such substrates could be waste generated by the food industry and food service. The use of industrial waste streams for PHAs biosynthesis could transform PHA production into cheaper and more environmentally friendly bioprocess. This review collates in detail recent developments in the biosynthesis of various types of PHAs produced using waste derived from agrofood industries. Challenges associated with this production bioprocess were described, and new ways to overcome them were proposed.

The underexplored role of diverse stress factors in microbial biopolymer synthesis

Polyhydroxyalkanoates (PHA) are microbial polyesters which, apart from their primary storage role, enhance the stress robustness of PHA accumulating cells against various stressors. PHA also represent interesting alternatives to petrochemical polymers, which can be produced from renewable resources employing approaches of microbial biotechnology. During biotechnological processes, bacterial cells are exposed to various stressor factors such as fluctuations in temperature, osmolarity, pH-value, elevated pressure or the presence of microbial inhibitors. This review summarizes how PHA helps microbial cells to cope with biotechnological process-relevant stressors and, vice versa, how various stress conditions can affect PHA production processes. The review suggests a fundamentally new strategy for PHA production: the fine-tuned exposure to selected stressors, which might be used to boost PHA production and even to tailor their structure.

A comprehensive overview and recent advances on polyhydroxyalkanoates (PHA) production using various organic waste streams

Polyhydroxyalkanoates (PHA) are appealing as an important alternative to replace synthetic plastics owing to its comparable physicochemical properties to that of synthetic plastics, and biodegradable and biocompatible nature. This review gives an inclusive overview of the current research activities dealing with PHA production by utilizing different waste fluxes generated from food, milk and sugar processing industries. Valorization of these waste fluxes makes the process cost effective and practically applicable. Recent advances in the approaches adopted for waste treatment, fermentation strategies, and genetic engineering can give insights to the researchers for future direction of waste to bioplastics production. Lastly, synthesis and application of PHA-nanocomposites, research and development challenges, future perspectives for sustainable and cost-effective PHB production are also discussed. In addition, the review addresses the useful information about the opportunities and confines associated with the sustainable PHA production using different waste streams and their evaluation for commercial implementation within a biorefinery.

Utilization of food waste streams for the production of biopolymers

Uncontrolled decomposition of agro-industrial waste leads to extensive contamination of water, land, and air. There is a tremendous amount of waste from various sources which causes serious environmental problems. The concern in the disposal problems has stimulated research interest in the valorization of waste streams. Valorization of the wastes not only reduces the volume of waste but also reduces the contamination to the environment. Waste from food industries has great potential as primary or secondary feedstocks for biopolymer production by extraction or fermentation with pre-treatment or without pre-treatment by solid-state fermentation to obtain fermentable sugars. Various types of waste can be used as substrates for the production of biomaterials but recently more focus has been observed on the agro-industrial wastes which have a high rate of production worldwide. This review collates in detail the different food wastes used for biopolymer, technologies for the production and characterization of the biopolymers, and their economic/technical viability.

Anabolism of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) by Cupriavidus necator DSM 545 from spent coffee grounds oil

Oil extracted from spent coffee grounds (SCG) [yield 16.8 % (w/w)] was discovered to be a highly suitable carbon substrate for the biosynthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3 HV)] copolymers by Cupriavidus necator DSM 545 in the absence of any traditional 3 HV precursors. Cells cultivated in a 3 L bioreactor (batch) reached a total biomass concentration of 8.9 g L^-1 with a P(3HB-co-3 HV) (6.8 mol% 3 HV) content of 89.6 % (w/w). In contrast, cells grown on sunflower oil reached a total biomass concentration of 9.4 gL^-1 with a P(3HB-co-3 HV) (0.2 mol% 3 HV) content of 88.1 % (w/w). It is proposed that the organism could synthesize 3 HV monomers from succinyl CoA, an intermediate of the tricarboxylic acid (TCA) cycle, via the succinate-propionate metabolic pathway.

Enzymatic Hydrolysis of Poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate) Scaffolds

Polyhydroxyalkanoates (PHAs) are hydrolyzable bio-polyesters. The possibility of utilizing lignocellulosic waste by-products and grape pomace as carbon sources for PHA biosynthesis was investigated. PHAs were biosynthesized by employing Cupriavidus necator grown on fructose (PHBV-1) or grape sugar extract (PHBV-2). Fifty grams of lyophilized grape sugar extract contained 19.2 g of glucose, 19.1 g of fructose, 2.7 g of pectin, 0.52 g of polyphenols, 0.51 g of flavonoids and 7.97 g of non-identified rest compounds. The grape sugar extract supported the higher production of biomass and modified the composition of PHBV-2. The biosynthesized PHAs served as matrices for the preparation of the scaffolds. The PHBV-2 scaffolds had about 44.2% lower crystallinity compared to the PHBV-1 scaffolds. The degree of crystallinity markedly influenced the mechanical behavior and enzymatic hydrolysis of the PHA scaffolds in the synthetic gastric juice and phosphate buffer saline solution with the lipase for 81 days. The higher proportion of amorphous moieties in PHBV-2 accelerated enzymatic hydrolysis. After 81-days of lasting enzymatic hydrolysis, the morphological changes of the PHBV-1 scaffolds were negligible compared to the visible destruction of the PHBV-2 scaffolds. These results indicated that the presence of pectin and phenolic moieties in PHBV may markedly change the semi-crystalline character of PHBV, as well as its mechanical properties and the course of abiotic or enzymatic hydrolysis.

Production of polyhydroxyalkanoates on waste frying oil employing selected Halomonas strains

The aim of this work was to study the potential of selected Halomonas species for conversion of waste frying oil into polyhydroxyalkanoates (PHA). In total nine Halomonas strains were experimentally screened for their capability of PHA production. Among them, Halomonas neptunia and Halomonas hydrothermalis were identified as potent PHA producers. Initial concentration of NaCl was identified as parameter influencing PHA yields as well as molecular weight of the polymer. In addition, H. hydrothermalis was capable of biosynthesis of a copolymer of 3-hydroxybutyrate and 3-hydroxyvalerate P(3HB-co-3HV). When valerate was utilized as a precursor, the 3HV fraction in the copolymer reached high values of 50.15 mol.%. PHA production on lipid substrates by Halomonas has not been reported so far. Bearing in mind all the positive aspects of employing extremophiles in industrial biotechnology, H. hydrothermalis seems to be a very interesting halophilic strain for production of PHA using lipid substrates.

Metabolomic insights into polyhydroxyalkanoates production by halophilic bacteria with acetic acid as carbon source

A metabolomics method was established to analyze changes of intracellular metabolites and study the mechanism for enhancing polyhydroxyalkanoates production by halotolerant bacteria, Bacillus cereus strain HY-3, using acetic acid as carbon source. Maximum poly(3-hydroxybutyrate) (PHB) contents for the medium with 0.5 g/L and 5.0 g/L of acetic acid were 41.0 ± 0.415% and 49.2 ± 1.21%. Principal components analysis revealed clear metabolic differences in different growth stages and different concentrations of carbon source. According to statistical analysis, 3-hydroxybutyrate (3-HB), serine, threonine, malate, and pyruvate were determined as potential biomarkers for PHB production. Moreover, metabolic pathways analysis indicated that high level of 3-HB in death phase was due to the limitation of carbon source. Metabolism of glycine, serine, and threonine was influential pathway for PHB production among amino acid metabolisms. High levels of organic acids from the TCA cycle could stimulate the carbon source flux into PHB biosynthetic pathway.

Production of bioplastic through food waste valorization

The tremendous amount of food waste from diverse sources is an environmental burden if disposed of inappropriately. Thus, implementation of a biorefinery platform for food waste is an ideal option to pursue (e.g., production of value-added products while reducing the volume of waste). The adoption of such a process is expected to reduce the production cost of biodegradable plastics (e.g., compared to conventional routes of production using overpriced pure substrates (e.g., glucose)). This review focuses on current technologies for the production of polyhydroxyalkanoates (PHA) from food waste. Technical details were also described to offer clear insights into diverse pretreatments for preparation of raw materials for the actual production of bioplastic (from food wastes). In this respect, particular attention was paid to fermentation technologies based on pure and mixed cultures. A clear description on the chemical modification of starch, cellulose, chitin, and caprolactone is also provided with a number of case studies (covering PHA-based products) along with a discussion on the prospects of food waste valorization approaches and their economic/technical viability.

Extremophiles - Platform Strains for Sustainable Production of Polyhydroxyalkanoates

Polyhydroxyalkanoates (PHA) are biodegradable polyesters, which are produced by various bacteria including numerous halophiles. Employment of halophilic strain for PHA production brings numerous benefits such as robustness of the process against contamination by ubiquitous mesophiles or possibility to isolate polymer from bacterial biomass via hypotonic lysis. In this work, we screened three moderate halophiles – Halomonas halophila, Halomonas organivorans and Halomonas salina for the presence of phaC gene encoding for PHA synthase and, subsequently, we have investigated their PHA production potential on various sugars. Among tested strains, H. organivorans demonstrated the extraordinary capacity of PHA production in particular on galactose and mannose since on these saccharides PHA content in dried bacterial cells reached 83 and 90 wt. % on mannose and galactose, respectively. Therefore, H. organivoras can be considered being promising PHA producing strain in particular suitable for the valorization of lignocellulose materials rich in galactomannans such as spent coffee grounds.

PHA granules help bacterial cells to preserve cell integrity when exposed to sudden osmotic imbalances

Polyhydroxyalkanoates (PHA) are microbial polyesters which accumulate as intracellular granules in numerous prokaryotes and mainly serve as storage materials; beyond this primary function, PHA also enhance the robustness of bacteria against various stress factors. We have observed that the presence of PHA in bacterial cells substantially enhances their ability to maintain cell integrity when suddenly exposed to osmotic imbalances. In the case of the non-halophilic bacterium Cupriavidus necator, the presence of PHA decreased plasmolysis-induced cytoplasmic membrane damage during osmotic up-shock, which subsequently enabled the cells to withstand subsequent osmotic downshock. In contrast, sudden induction of osmotic up- and subsequent down-shock resulted in massive hypotonic lysis of non-PHA containing cells as determined by Transmission Electron Microscopy and Thermogravimetrical Analysis. Furthermore, a protective effect of PHA against hypotonic lysis was also observed in the case of the halophilic bacterium Halomonas halophila; here, challenged PHA-rich cells were capable of retaining cell integrity more effectively than their PHA-poor counterparts. Hence, it appears that the fact that PHA granules, as an added value to their primary storage function, protect halophiles from the harmful effect of osmotic down-shock might explain why PHA accumulation is such a common feature among halophilic prokaryotes. The results of this study, apart from their fundamental importance, are also of practical biotechnological significance: because PHA-rich bacterial cells are resistant to osmotic imbalances, they could be utilized in in-situ bioremediation technologies or during enrichment of mixed microbial consortia in PHA producers under conditions of fluctuating salinity.

Development of a plasmid-based, tunable, tolC-derived expression system for application in Cupriavidus necator H16

Cupriavidus necator H16 gains increasing attention in microbial research and biotechnological application due to its diverse metabolic features. Here we present a tightly controlled gene expression system for C. necator including the pBBR1-vector that contains hybrid promoters originating from C. necator native tolC-promoter in combination with a synthetic tetO-operator. The expression of the reporter gene from these plasmids relies on the addition of the exogenous inducer doxycycline (dc). The novel expression system offers a combination of advantageous features as; (i) high and dose-dependent recombinant protein production, (ii) tight control with a high dynamic range (On/Off ratio), which makes it applicable for harmful pathways or for toxic protein production, (iii) comparable cheap inducer (doxycycline, dc), (iv) effective at low inducer concentration, that makes it useful for large scale application, (v) rapid, diffusion controlled induction, and (vi) the inducer does not interfere within the cell metabolism. As applications of the expression system in C. necator H16, the growth ability on glycerol was enhanced by constitutively expressing the E. coli glpk gene-encoding for glycerol kinase. Likewise, we used the system to overcome the expression toxicity of mevalonate pathway in C. necator H16. With this system, the mevalonate-genes were successfully introduced in the host and the recombinant strains could produce about 200 mg/l mevalonate.

Fed-Batch Synthesis of Poly(3-Hydroxybutyrate) and Poly(3-Hydroxybutyrate-co-4-Hydroxybutyrate) from Sucrose and 4-Hydroxybutyrate Precursors by Burkholderia sacchari Strain DSM 17165

Based on direct sucrose conversion, the bacterium Burkholderia sacchari is an excellent producer of the microbial homopolyester poly(3-hydroxybutyrate) (PHB). Restrictions of the strain’s wild type in metabolizing structurally related 3-hydroxyvalerate (3HV) precursors towards 3HV-containing polyhydroxyalkanoate (PHA) copolyester calls for alternatives. We demonstrate the highly productive biosynthesis of PHA copolyesters consisting of 3-hydroxybuytrate (3HB) and 4-hydroxybutyrate (4HB) monomers. Controlled bioreactor cultivations were carried out using saccharose from the Brazilian sugarcane industry as the main carbon source, with and without co-feeding with the 4HB-related precursor γ-butyrolactone (GBL). Without GBL co-feeding, the homopolyester PHB was produced at a volumetric productivity of 1.29 g/(L·h), a mass fraction of 0.52 g PHB per g biomass, and a final PHB concentration of 36.5 g/L; the maximum specific growth rate µ_max amounted to 0.15 1/h. Adding GBL, we obtained 3HB and 4HB monomers in the polyester at a volumetric productivity of 1.87 g/(L·h), a mass fraction of 0.72 g PHA per g biomass, a final PHA concentration of 53.7 g/L, and a µ_max of 0.18 1/h. Thermoanalysis revealed improved material properties of the second polyester in terms of reduced melting temperature T_m (161 °C vs. 178 °C) and decreased degree of crystallinity X_c (24% vs. 71%), indicating its enhanced suitability for polymer processing.

Application of flow cytometry for monitoring the production of poly(3-hydroxybutyrate) by Halomonas boliviensis

In this study, a flow cytometry (FC) protocol was implemented to measure poly(3-hydroxybutyrate) (PHB) content in a halophilic bacterium, to have a faster and easier control of the process. The halophilic bacterium Halomonas boliviensis was stained with BODIPY 493/503 and analyzed using FC. Bacterial polymer accumulation induced by two different nutrient limitations during the operation of a 2 L bioreactor was studied using traditional gas chromatography (GC) analysis and FC. The application of this rapid and straightforward method is useful to obtain complex and precise information about PHB accumulation that could be used for the monitoring, control and optimization of the production of PHB. A clear correlation between the PHB concentration determined by GC and the fluorescence signal obtained from stained bacteria by using FC was observed. Additionally, the heterogeneity of bacterial population as a function of PHB content was measured. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 33:276-284, 2017.

US132 Cyclodextrin Glucanotransferase Engineering by Random Mutagenesis for an Anti-Staling Purpose

The use of the cyclodextrin glucanotransferase (CGTase) of the US132 strain, which is an effective anti-staling agent, has been hampered by its high cyclization activity. Since that random mutagenesis using error-prone PCR is nowadays a method of choice for enzymes engineering, we have optimized this method by adjusting manganese concentration in order to obtain a high percentage of active CGTase mutants. Therefore, the amplification of the gene encoding the US132 CGTase was performed using a MnCl2 concentration ranging between 0 and 0.5 mM. The finding showed that a manganese concentration of 0.04 mM allowed for 90 % of active mutants. A simple method to rapidly screen the obtained mutants was also developed. After the examination of a small library (of less than 1000 clones), the active mutant named MJ13 was selected for a significant decrease in the cyclization activity, thereby showing a remarkable change in the enzyme specificity towards starch dextrinizing. Sequence analysis showed that MJ13 is a triple mutant with two mutations in the catalytic domain (K47E and S382P) and one substitution in the starch binding domain (N655S).