PHB Biosynthesis Counteracts Redox Stress in Herbaspirillum seropedicae

The role of NtrC in the adaptation of Herbaspirillum seropedicae SmR1 to nitrogen limitation and to nitrate

The RNA-Seq profiling of Herbaspirillum seropedicae SmR1 wild-type and ntrC mutant was performed under aerobic and three nitrogen conditions (ammonium limitation, ammonium shock, and nitrate shock) to identify the major metabolic pathways modulated by these nitrogen sources and those dependent on NtrC. Under ammonium limitation, H. seropedicae scavenges nitrogen compounds by activating transporter systems and metabolic pathways to utilize different nitrogen sources and by increasing proteolysis, along with genes involved in carbon storage, cell protection, and redox balance, while downregulating those involved in energy metabolism and protein synthesis. Growth on nitrate depends on the narKnirBDHsero_2899nasA operon responding to nitrate and NtrC. Ammonium shock resulted in a higher number of genes differently expressed when compared to nitrate. Our results showed that NtrC activates a network of transcriptional regulators to prepare the cell for nitrogen starvation, and also synchronizes nitrogen metabolism with carbon and redox balance pathways.

Inactivation of poly(3-hydroxybutyrate) (PHB) biosynthesis in 'Knallgas' bacterium Xanthobacter sp. SoF1

Aerobic hydrogen-oxidizing 'Knallgas' bacteria are promising candidates for microbial cell factories due to their ability to use hydrogen and carbon dioxide as the sole energy and carbon sources, respectively. These bacteria can convert atmospheric CO₂ to chemicals which could help to mitigate climate change by replacing fossil fuel-based chemicals. A known method to enhance the product yield is to disrupt competing metabolic pathways in the host organism. One such pathway in many 'Knallgas' bacteria is polyhydroxybutyrate (PHB) biosynthesis. In this study, the PHB biosynthesis genes of a non-model 'Knallgas' bacterium Xanthobacter sp. SoF1 were identified. Consequently, the phaA, phaB and phaC genes were individually deleted and the resulting knockouts were evaluated for their ability to produce PHB in autotrophic shake flask and small-scale bioreactor cultivations. The results demonstrate that PHB production was inactivated in the phaC1 knockout strain, which advances the development of Xanthobacter sp. SoF1 as a production host.

Construction of a Rhodobacter sphaeroides Strain That Efficiently Produces Hydrogen Gas from Acetate without Poly(β-Hydroxybutyrate) Accumulation: Insight into the Role of PhaR in Acetate Metabolism

The purple nonsulfur phototrophic bacterium Rhodobacter sphaeroides produces hydrogen gas (H₂) from acetate. An approach to improve the H₂ production is preventing accumulation of an intracellular energy storage molecule known as poly(β-hydroxybutyrate) (PHB), which competes with H₂ production for reducing power. However, disruption of PHB biosynthesis has been reported to severely impair the acetate assimilation depending on the genetic backgrounds and/or culture conditions. To solve this problem, we analyzed the relationship between PHB accumulation and acetate metabolism in R. sphaeroides. Gene deletion analyses based on the wild-type strain revealed that among the two polyhydroxyalkanoate synthase genes in the genome, phaC1, but not phaC2, is essential for PHB accumulation, and the phaC1 deletion mutant exhibited slow growth with acetate. On the other hand, a strain with the deletion of phaC1 together with phaR, which encodes a transcriptional regulator capable of sensing PHB accumulation, exhibited growth comparable to that of the wild-type strain despite no accumulation of PHB. These results suggest that PHB accumulation is required for normal growth with acetate by altering the expression of genes under the control of phaR. This hypothesis was supported by a transcriptome sequencing (RNA-seq) analysis revealing that phaR is involved in the regulation of the ethylmalonyl coenzyme A pathway for acetate assimilation. Consistent with these findings, deletion of phaC1 in a genetically engineered H₂-producing strain resulted in lower H₂ production from acetate due to growth defects, whereas deletion of phaR together with phaC1 restored growth with acetate and increased H₂ production from acetate without PHB accumulation. IMPORTANCE This study provides a novel approach for increasing the yield of photofermentative H₂ production from acetate by purple nonsulfur phototrophic bacteria. This study further suggests that polyhydroxyalkanoate is not only a storage substance for carbon and energy in bacteria, but may also act as a signaling molecule that mediates bacterial metabolic adaptations to specific environments. This notion will be helpful for understanding the physiology of polyhydroxyalkanoate-producing bacteria, as well as for their metabolic engineering via synthetic biology.

Adaptation to Varying Salinity in Halomonas elongata: Much More Than Ectoine Accumulation

The halophilic γ-proteobacterium Halomonas elongata DSM 2581 ^T thrives at salt concentrations well above 10 % NaCl (1.7 M NaCl). A well-known osmoregulatory mechanism is the accumulation of the compatible solute ectoine within the cell in response to osmotic stress. While ectoine accumulation is central to osmoregulation and promotes resistance to high salinity in halophilic bacteria, ectoine has this effect only to a much lesser extent in non-halophiles. We carried out transcriptome analysis of H. elongata grown on two different carbon sources (acetate or glucose), and low (0.17 M NaCl), medium (1 M), and high salinity (2 M) to identify additional mechanisms for adaptation to high saline environments. To avoid a methodological bias, the transcripts were evaluated by applying two methods, DESeq2 and Transcripts Per Million (TPM). The differentially transcribed genes in response to the available carbon sources and salt stress were then compared to the transcriptome profile of Chromohalobacter salexigens, a closely related moderate halophilic bacterium. Transcriptome profiling supports the notion that glucose is degraded via the cytoplasmic Entner-Doudoroff pathway, whereas the Embden-Meyerhoff-Parnas pathway is employed for gluconeogenesis. The machinery of oxidative phosphorylation in H. elongata and C. salexigens differs greatly from that of non-halophilic organisms, and electron flow can occur from quinone to oxygen along four alternative routes. Two of these pathways via cytochrome bo' and cytochrome bd quinol oxidases seem to be upregulated in salt stressed cells. Among the most highly regulated genes in H. elongata and C. salexigens are those encoding chemotaxis and motility proteins, with genes for chemotaxis and flagellar assembly severely downregulated at low salt concentrations. We also compared transcripts at low and high-salt stress (low growth rate) with transcripts at optimal salt concentration and found that the majority of regulated genes were down-regulated in stressed cells, including many genes involved in carbohydrate metabolism, while ribosome synthesis was up-regulated, which is in contrast to what is known from non-halophiles at slow growth. Finally, comparing the acidity of the cytoplasmic proteomes of non-halophiles, extreme halophiles and moderate halophiles suggests adaptation to an increased cytoplasmic ion concentration of H. elongata. Taken together, these results lead us to propose a model for salt tolerance in H. elongata where ion accumulation plays a greater role in salt tolerance than previously assumed.

Influence mechanism of C/N ratio on heterotrophic nitrification- aerobic denitrification process

A heterotrophic nitrification- aerobic denitrification (HNAD) bacterium, Acinetobacter junii ZHG-1, was isolated, meanwhile, the optimal conditions for the strain were evaluated, moreover, the influence mechanism of the C/N ratio on the HNAD process was investigated from the perspective of electron transport and energy level. The increasing of C/N ratio enhanced the reduced/oxidized nicotinamide adenine dinucleotide (NADH/NAD⁺) ratio, NADH concentration, electron transport system activity (ETSA), ATP content, as well as enzymes activities, consequently, the HNAD performance of the strain can be improved, however, when the C/N ratio was higher than 30, the activities of enzymes relating to the HNAD process and the ETSA had reached the maximum, which might limit the further improvement of the nitrogen removal with the increasing of C/N ratio. As the interaction between different biochemical reactions in HNAD process, more efforts should be devoted to the influent mechanism of different environmental factors on the HNAD process.

PHA Production from Cheese Whey and "Scotta": Comparison between a Consortium and a Pure Culture of Leuconostoc mesenteroides

It is urgent to expand the market of biodegradable alternatives to oil-derived plastics owing to (i) increasingly limited oil availability/accessibility, and (ii) the dramatic impact of traditional plastics on aquatic life, the food chain, all Earth ecosystems, and ultimately, human health. Polyhydroxyalkanoates (PHAs) are promising biodegradable polymers that can be obtained through microbial fermentation of agro-industrial byproducts, e.g., milk and cheese whey. Here, the PHA-accumulating efficiency of a mixed microbial culture (MMC, derived from activated sludges) grown on dairy byproducts (cheese and scotta whey) was measured. Bioreactor tests featuring temperature and pH control showed that both scotta and pre-treated Toma cheese whey could be used for PHA production by MMC, although scotta cheese whey supported higher PHA yield and productivity. The advantages of open MMCs include their plasticity and versatility to fast changing conditions; furthermore, no growth-medium sterilization is needed prior to fermentation. However, the use of pure cultures of efficient PHA producers may support better metabolic performances. Therefore, PHA-producing strains were isolated from a MMC, leading to the satisfactory identification of two bacterial strains, Citrobacter freundii and Leuconostoc spp., whose ability to accumulate PHAs in synthetic media was confirmed. A more detailed investigation by mass spectrometry revealed that the strain was L. mesenteroides. Although the validation of L. mesenteroides potential to produce PHA through fermentation of agro-industrial byproducts requires further investigations, this is the first study reporting PHA production with the Leuconostoc genus.

Contrasting transcriptomic responses of a microbial eukaryotic community to oil and dispersant

Dispersants can aid dispersion and biodegradation of oil in seawater, but the wider ecotoxicological effects of oil and dispersant to the base of marine food webs is unclear. Here we apply a metatranscriptomic approach to identify molecular responses of a natural marine microbial eukaryotic community to oil and chemically dispersed oil. Oil exposure stimulated the upregulation of ketogenesis in the eukaryotic community, which may alleviate carbon- and energy-limitation and reduce oxidative stress. In contrast, a chemically dispersed oil treatment stimulated eukaryotic genes and pathways consistent with nitrogen and oxygen depletion. These results suggest that the addition of dispersant may elevate bacterial biodegradation of crude oil, indirectly increasing competition for nitrogen between prokaryotic and eukaryotic communities as oxygen consumption induces bacterial anaerobic respiration and denitrification. Eukaryotic microbial communities may mitigate some of the negative effects of oil exposure such as reduced photosynthesis and elevated oxidative stress, through ketosis, but the addition of dispersant to the oil fundamentally alters the environmental and ecological conditions and therefore the biochemical response of the eukaryotic community.

An updated overview on the regulatory circuits of polyhydroxyalkanoates synthesis

Polyhydroxyalkanoates (PHA) are a promising and sustainable alternative to the petroleum‐based synthetic plastics. Regulation of PHA synthesis is receiving considerable importance as engineering the regulatory factors might help developing strains with improved PHA‐producing abilities. PHA synthesis is dedicatedly regulated by a number of regulatory networks. They tightly control the PHA content, granule size and their distribution in cells. Most PHA‐accumulating microorganisms have multiple regulatory networks that impart a combined effect on PHA metabolism. Among them, several factors ranging from global to specific regulators, have been identified and characterized till now. This review is an attempt to categorically summarize the diverse regulatory circuits that operate in some important PHA‐producing microorganisms. However, in several organisms, the detailed mechanisms involved in the regulation of PHA synthesis is not well‐explored and hence further research is needed. The information presented in this review might help researcher to identify the prevailing research gaps in PHA regulation.

Cyanobacterium Synechocystis sp. PCC 6803 lacking adc1 gene produces higher polyhydroxybutyrate accumulation under modified nutrients of acetate supplementation and nitrogen-phosphorus starvation

Increased polyhydroxybutyrate production in cyanobacterium Synechocystis sp. PCC 6803 lacking adc1 gene (Δadc1) is first-timely reported in this study. We constructed the mutant by disrupting adc1 gene encoding arginine decarboxylase, thereby exhibiting a partial blockade of polyamine synthesis. This Δadc1 mutant had a proliferative growth and certain contents of intracellular pigments including chlorophyll a and carotenoids as similar as those of wild type (WT). Highest PHB production was certainly induced by BG₁₁-N-P+A condition in both WT and Δadc1 mutant of about 24.9 %w/DCW at day 9 and 36.1 %w/DCW at day 7 of adaptation time, respectively. Abundant PHB granules were also visualized under both BG₁₁-N-P and BG₁₁-N-P+A conditions. All pha transcript amounts of Δadc1 mutant grown at 7 days-adaptation time were clearly upregulated corresponding to its PHB content under BG₁₁-N-P+A condition. Our finding indicated that this adc1 perturbation is alternatively achieved for PHB production in Synechocystis sp. PCC 6803.

Herbaspirillum seropedicae strain HRC54 expression profile in response to sugarcane apoplastic fluid

Bacterial transcriptome profiling in the presence of plant fluids or extracts during microbial growth may provide relevant information on plant-bacteria interactions. Here, RNA sequencing (RNA-Seq) was used to determine the transcriptomic profile of Herbaspirillum seropedicae strain HRC54 at the early stages of response to sugarcane apoplastic fluid. Differentially expressed gene (DEG) analysis was performed using the DESeq2 and edgeR packages, followed by functional annotation using Blast2GO and gene ontology enrichment analysis using the COG and KEGG databases. After 2 h of sugarcane apoplastic fluid addition to the H. seropedicae HRC54 culture, respectively, 44 and 45 genes were upregulated and downregulated. These genes were enriched in bacterial metabolism (e.g., oxidoreductase and transferase), ABC transporters, motility, secretion systems, and signal transduction. RNA-Seq expression profiles of 12 genes identified in data analyses were verified by RT-qPCR. The results suggested that H. seropedicae HRC54 recognized sugarcane apoplastic fluid as the host signal, and some DEGs were closely involved at the early stages of the establishment of plant-bacteria interactions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02848-y.

Redox Regulation in Diazotrophic Bacteria in Interaction with Plants

Plants interact with a large number of microorganisms that greatly influence their growth and health. Among the beneficial microorganisms, rhizosphere bacteria known as Plant Growth Promoting Bacteria increase plant fitness by producing compounds such as phytohormones or by carrying out symbioses that enhance nutrient acquisition. Nitrogen-fixing bacteria, either as endophytes or as endosymbionts, specifically improve the growth and development of plants by supplying them with nitrogen, a key macro-element. Survival and proliferation of these bacteria require their adaptation to the rhizosphere and host plant, which are particular ecological environments. This adaptation highly depends on bacteria response to the Reactive Oxygen Species (ROS), associated to abiotic stresses or produced by host plants, which determine the outcome of the plant-bacteria interaction. This paper reviews the different antioxidant defense mechanisms identified in diazotrophic bacteria, focusing on their involvement in coping with the changing conditions encountered during interaction with plant partners.

Polyhydroxybutyrate: A Useful Product of Chlorotic Cyanobacteria

Polyhydroxybutyrate (PHB) is a carbon polymer with diverse functions, varying greatly on the organism producing it. This microreview describes the current knowledge about PHB metabolism, structure, and different physiological roles with a special focus on cyanobacteria. Despite the physiological function of PHB in the cyanobacterial phylum still being unknown, these organisms provide the unique opportunity to directly convert atmospheric CO2 into bioplastic using a solar-based process. Recent research on PHB metabolism in the cyanobacterial model organism Synechocystis revealed a sophisticated control of PHB granule formation. Novel insights about the metabolic background of PHB synthesis resulted in the engineering of the first cyanobacterial superproducer strain.

The protective role of PHB and its degradation products against stress situations in bacteria

Many bacteria produce storage biopolymers that are mobilized under conditions of metabolic adaptation, for example, low nutrient availability and cellular stress. Polyhydroxyalkanoates are often found as carbon storage in Bacteria or Archaea, and of these polyhydroxybutyrate (PHB) is the most frequently occurring PHA type. Bacteria usually produce PHB upon availability of a carbon source and limitation of another essential nutrient. Therefore, it is widely believed that the function of PHB is to serve as a mobilizable carbon repository when bacteria face carbon limitation, supporting their survival. However, recent findings indicate that bacteria switch from PHB synthesis to mobilization under stress conditions such as thermal and oxidative shock. The mobilization products, 3-hydroxybutyrate and its oligomers, show a protective effect against protein aggregation and cellular damage caused by reactive oxygen species and heat shock. Thus, bacteria should have an environmental monitoring mechanism directly connected to the regulation of the PHB metabolism. Here, we review the current knowledge on PHB physiology together with a summary of recent findings on novel functions of PHB in stress resistance. Potential applications of these new functions are also presented.

Diazotrophic Bacteria and Their Mechanisms to Interact and Benefit Cereals

Plant-growth-promoting bacteria (PGPB) stimulate plant growth through diverse mechanisms. In addition to biological nitrogen fixation, diazotrophic PGPB can improve nutrient uptake efficiency from the soil, produce and release phytohormones to the host, and confer resistance against pathogens. The genetic determinants that drive the success of biological nitrogen fixation in nonlegume plants are understudied. These determinants include recognition and signaling pathways, bacterial colonization, and genotype specificity between host and bacteria. This review presents recent discoveries of how nitrogen-fixing PGPB interact with cereals and promote plant growth. We suggest adopting an experimental model system, such as the Setaria-diazotrophic bacteria association, as a reliable way to better understand the associated mechanisms and, ultimately, increase the use of PGPB inoculants for sustainable agriculture.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Metabolic circuits and gene regulators in polyhydroxyalkanoate producing organisms: Intervention strategies for enhanced production

Worldwide worries upsurge concerning environmental pollutions triggered by the accumulation of plastic wastes. Biopolymers are promising candidates for resolving these difficulties by replacing non-biodegradable plastics. Among biopolymers, polyhydroxyalkanoates (PHAs), are natural polymers that are synthesized and accumulated in a range of microorganisms, are considered as promising biopolymers since they have biocompatibility, biodegradability, and other physico-chemical properties comparable to those of synthetic plastics. Consequently, considerable research have been attempted to advance a better understanding of mechanisms related to the metabolic synthesis and characteristics of PHAs and to develop native and recombinant microorganisms that can proficiently produce PHAs comprising desired monomers with high titer and productivity for industrial applications. Recent developments in metabolic engineering and synthetic biology applied to enhance PHA synthesis include, promoter engineering, ribosome-binding site (RBS) engineering, development of synthetic constructs etc. This review gives a brief overview of metabolic routes and regulators of PHA production and its intervention strategies.

The First Insight into Polyhydroxyalkanoates Accumulation in Multi-Extremophilic Rubrobacter xylanophilus and Rubrobacter spartanus

Actinobacteria belonging to the genus Rubrobacter are known for their multi-extremophilic growth conditions-they are highly radiation-resistant, halotolerant, thermotolerant or even thermophilic. This work demonstrates that the members of the genus are capable of accumulating polyhydroxyalkanoates (PHA) since PHA-related genes are widely distributed among Rubrobacter spp. whose complete genome sequences are available in public databases. Interestingly, all Rubrobacter strains possess both class I and class III synthases (PhaC). We have experimentally investigated the PHA accumulation in two thermophilic species, R. xylanophilus and R. spartanus. The PHA content in both strains reached up to 50% of the cell dry mass, both bacteria were able to accumulate PHA consisting of 3-hydroxybutyrate and 3-hydroxyvalerate monomeric units, none other monomers were incorporated into the polymer chain. The capability of PHA accumulation likely contributes to the multi-extremophilic characteristics since it is known that PHA substantially enhances the stress robustness of bacteria. Hence, PHA can be considered as extremolytes enabling adaptation to extreme conditions. Furthermore, due to the high PHA content in biomass, a wide range of utilizable substrates, Gram-stain positivity, and thermophilic features, the Rubrobacter species, in particular Rubrobacter xylanophilus, could be also interesting candidates for industrial production of PHA within the concept of Next-Generation Industrial Biotechnology.

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.

Absence of KpsM (Slr0977) Impairs the Secretion of Extracellular Polymeric Substances (EPS) and Impacts Carbon Fluxes in Synechocystis sp. PCC 6803

Many cyanobacteria produce extracellular polymeric substances (EPS), composed mainly of heteropolysaccharides, that play a variety of physiological roles, being crucial for cell protection, motility, and biofilm formation. However, due to their complexity, the EPS biosynthetic pathways as well as their assembly and export mechanisms are still far from being fully understood. Here, we show that the absence of a putative EPS-related protein, KpsM (Slr0977), has a pleiotropic effect on Synechocystis sp. strain PCC 6803 physiology, with a strong impact on the export of EPS and carbon fluxes. The kpsM mutant exhibits a significant reduction of released polysaccharides and a smaller decrease of capsular polysaccharides, but it accumulates more polyhydroxybutyrate (PHB) than the wild type. In addition, this strain shows a light/cell density-dependent clumping phenotype and exhibits an altered protein secretion capacity. Furthermore, the most important structural component of pili, the protein PilA, was found to have a modified glycosylation pattern in the mutant compared to the wild type. Proteomic and transcriptomic analyses revealed significant changes in the mechanisms of energy production and conversion, namely, photosynthesis, oxidative phosphorylation, and carbon metabolism, in response to the inactivation of slr0977 Overall, this work shows for the first time that cells with impaired EPS secretion undergo transcriptomic and proteomic adjustments, highlighting the importance of EPS as a major carbon sink in cyanobacteria. The accumulation of PHB in cells of the mutant, without affecting significantly its fitness/growth rate, points to its possible use as a chassis for the production of compounds of interest.IMPORTANCE Most cyanobacteria produce extracellular polymeric substances (EPS) that fulfill different biological roles depending on the strain/environmental conditions. The interest in the cyanobacterial EPS synthesis/export pathways has been increasing, not only to optimize EPS production but also to efficiently redirect carbon flux toward the production of other compounds, allowing the implementation of industrial systems based on cyanobacterial cell factories. Here, we show that a Synechocystis kpsM (slr0977) mutant secretes less EPS than the wild type, accumulating more carbon intracellularly, as polyhydroxybutyrate. Further characterization showed a light/cell density-dependent clumping phenotype, altered protein secretion, and modified glycosylation of PilA. The proteome and transcriptome of the mutant revealed significant changes, namely, in photosynthesis and carbon metabolism. Altogether, this work provides a comprehensive overview of the impact of kpsM disruption on Synechocystis physiology, highlighting the importance of EPS as a carbon sink and showing how cells adapt when their secretion is impaired, and the redirection of the carbon fluxes.

Diverse Bacterial Genes Modulate Plant Root Association by Beneficial Bacteria

There is growing interest in the use of associative, plant growth-promoting bacteria (PGPB) as biofertilizers to serve as a sustainable alternative for agriculture application. While a variety of mechanisms have been proposed to explain bacterial plant growth promotion, the molecular details of this process remain unclear.

The plant rhizosphere harbors a diverse population of microorganisms, including beneficial plant growth-promoting bacteria (PGPB), that colonize plant roots and enhance growth and productivity. In order to specifically define bacterial traits that contribute to this beneficial interaction, we used high-throughput transposon mutagenesis sequencing (TnSeq) in two model root-bacterium systems associated with Setaria viridis: Azoarcus olearius DQS4^T and Herbaspirillum seropedicae SmR1. This approach identified ∼100 significant genes for each bacterium that appeared to confer a competitive advantage for root colonization. Most of the genes identified specifically in A. olearius encoded metabolism functions, whereas genes identified in H. seropedicae were motility related, suggesting that each strain requires unique functions for competitive root colonization. Genes were experimentally validated by site-directed mutagenesis, followed by inoculation of the mutated bacteria onto S. viridis roots individually, as well as in competition with the wild-type strain. The results identify key bacterial functions involved in iron uptake, polyhydroxybutyrate metabolism, and regulation of aromatic metabolism as important for root colonization. The hope is that by improving our understanding of the molecular mechanisms used by PGPB to colonize plants, we can increase the adoption of these bacteria in agriculture to improve the sustainability of modern cropping systems.

Production of Polyhydroxybutyrate (PHB) and Factors Impacting Its Chemical and Mechanical Characteristics

Plastic pollution is fueling the grave environmental threats currently facing humans, the animal kingdom, and the planet. The pursuit of renewable resourced biodegradable materials commenced in the 1970s with the need for carbon neutral fully sustainable products driving important progress in recent years. The development of bioplastic materials is highlighted as imperative to the solutions to our global environment challenges and to the restoration of the wellbeing of our planet. Bio-based plastics are becoming increasingly sustainable and are expected to substitute fossil-based plastics. Bioplastics currently include both, nondegradable and biodegradable compositions, depending on factors including the origins of production and post-use management and conditions. Among the most promising materials being developed and evaluated is polyhydroxybutyrate (PHB), a microbial bioprocessed polyester belonging to the polyhydroxyalkanoate (PHA) family. This biocompatible and non-toxic polymer is biosynthesized and accumulated by a number of specialized bacterial strains. The favorable mechanical properties and amenability to biodegradation when exposed to certain active biological environments, earmark PHB as a high potential replacement for petrochemical based polymers such as ubiquitous high density polyethylene (HDPE). To date, high production costs, minimal yields, production technology complexities, and difficulties relating to downstream processing are limiting factors for its progression and expansion in the marketplace. This review examines the chemical, mechanical, thermal, and crystalline characteristics of PHB, as well as various fermentation processing factors which influence the properties of PHB materials.

Novel unexpected functions of PHA granules

Polyhydroxyalkanoates (PHA), polyesters accumulated by numerous prokaryotes in the form of intracellular granules, have been for decades considered being predominantly storage molecules. However, numerous recent discoveries revealed and emphasized their complex biological role for microbial cells. Most of all, it was repeatedly reported and confirmed that the presence of PHA granules in prokaryotic cells enhances stress resistance and robustness of microbes against various environmental stress factors such as high or low temperature, freezing, oxidative, and osmotic pressure. It seems that protective mechanisms of PHA granules are associated with their extraordinary architecture and biophysical properties as well as with the complex and deeply interconnected nature of PHA metabolism. Therefore, this review aims at describing novel and unexpected properties of PHA granules with respect to their contribution to stress tolerance of various prokaryotes including common mesophilic heterotrophic bacteria, but also extremophiles or photo-autotrophic cyanobacteria. KEY POINTS: • PHA granules present in bacterial cells reveal unique properties and functions. • PHA enhances stress robustness of bacterial cells.

The Modification of Regulatory Circuits Involved in the Control of Polyhydroxyalkanoates Metabolism to Improve Their Production

Poly-(3-hydroxyalkanoates) (PHAs) are bacterial carbon and energy storage compounds. These polymers are synthesized under conditions of nutritional imbalance, where a nutrient is growth-limiting while there is still enough carbon source in the medium. On the other side, the accumulated polymer is mobilized under conditions of nutrient accessibility or by limitation of the carbon source. Thus, it is well known that the accumulation of PHAs is affected by the availability of nutritional resources and this knowledge has been used to establish culture conditions favoring high productivities. In addition to this effect of the metabolic status on PHAs accumulation, several genetic regulatory networks have been shown to drive PHAs metabolism, so the expression of the PHAs genes is under the influence of global or specific regulators. These regulators are thought to coordinate PHAs synthesis and mobilization with the rest of bacterial physiology. While the metabolic and biochemical knowledge related to the biosynthesis of these polymers has led to the development of processes in bioreactors for high-level production and also to the establishment of strategies for metabolic engineering for the synthesis of modified biopolymers, the use of knowledge related to the regulatory circuits controlling PHAs metabolism for strain improvement is scarce. A better understanding of the genetic control systems involved could serve as the foundation for new strategies for strain modification in order to increase PHAs production or to adjust the chemical structure of these biopolymers. In this review, the regulatory systems involved in the control of PHAs metabolism are examined, with emphasis on those acting at the level of expression of the enzymes involved and their potential modification for strain improvement, both for higher titers, or manipulation of polymer properties. The case of the PHAs producer Azotobacter vinelandii is taken as an example of the complexity and variety of systems controlling the accumulation of these interesting polymers in response to diverse situations, many of which could be engineered to improve PHAs production.

On the Role and Production of Polyhydroxybutyrate (PHB) in the Cyanobacterium Synechocystis sp. PCC 6803

The cyanobacterium Synechocystis sp. PCC 6803 is known for producing polyhydroxybutyrate (PHB) under unbalanced nutrient conditions. Although many cyanobacteria produce PHB, its physiological relevance remains unknown, since previous studies concluded that PHB is redundant. In this work, we try to better understand the physiological conditions that are important for PHB synthesis. The accumulation of intracellular PHB was higher when the cyanobacterial cells were grown under an alternating day–night rhythm as compared to continuous light. In contrast to previous reports, a reduction of PHB was observed when the cells were grown under conditions of limited gas exchange. Since previous data showed that PHB is not required for the resuscitation from nitrogen starvation, a series of different abiotic stresses were applied to test if PHB is beneficial for its fitness. However, under none of the tested conditions did cells containing PHB show a fitness advantage compared to a PHB-free-mutant (ΔphaEC). Additionally, the distribution of PHB in single cells of a population Synechocystis cells was analyzed via fluorescence-activated cell sorting (FACS). The results showed a considerable degree of phenotypic heterogeneity at the single cell level concerning the content of PHB, which was consistent over several generations. These results improve our understanding about how and why Synechocystis synthesizes PHB and gives suggestions how to further increase its production for a biotechnological process.

In silico prediction and expression profile analysis of small non-coding RNAs in Herbaspirillum seropedicae SmR1

BACKGROUND

Herbaspirillum seropedicae is a diazotrophic bacterium from the β-proteobacteria class that colonizes endophytically important gramineous species, promotes their growth through phytohormone-dependent stimulation and can express nif genes and fix nitrogen inside plant tissues. Due to these properties this bacterium has great potential as a commercial inoculant for agriculture. The H. seropedicae SmR1 genome is completely sequenced and annotated but despite the availability of diverse structural and functional analysis of this genome, studies involving small non-coding RNAs (sRNAs) has not yet been done. We have conducted computational prediction and RNA-seq analysis to select and confirm the expression of sRNA genes in the H. seropedicae SmR1 genome, in the presence of two nitrogen independent sources and in presence of naringenin, a flavonoid secreted by some plants.

RESULTS

This approach resulted in a set of 117 sRNAs distributed in riboswitch, cis-encoded and trans-encoded categories and among them 20 have Rfam homologs. The housekeeping sRNAs tmRNA, ssrS and 4.5S were found and we observed that a large number of sRNAs are more expressed in the nitrate condition rather than the control condition and in the presence of naringenin. Some sRNAs expression were confirmed in vitro and this work contributes to better understand the post transcriptional regulation in this bacterium.

CONCLUSIONS

H. seropedicae SmR1 express sRNAs in the presence of two nitrogen sources and/or in the presence of naringenin. The functions of most of these sRNAs remains unknown but their existence in this bacterium confirms the evidence that sRNAs are involved in many different cellular activities to adapt to nutritional and environmental changes.

Dynamics and potential roles of abundant and rare subcommunities in the bioremediation of cadmium-contaminated paddy soil by Pseudomonas chenduensis

Microbial bioremediation of heavy metal-contaminated soil is a potential technique to reduce heavy metals in crop plants. However, the dynamics and roles of the local microbiota in bioremediation of heavy metal-contaminated soil following microbial application are rarely reported. In this study, we used Pseudomonas chenduensis strain MBR for bioremediation of Cd-contaminated paddy soil and investigated its effects on the dynamics of the local soil bacterial community and Cd accumulation in rice. Cd accumulation in rice grains and roots were significantly reduced by the addition of the strain MBR. The addition of the strain MBR caused greater changes in bacterial communities in rhizosphere soil than in bulk soil. MBR enhanced the roles of microbial communities in transformation of Cd fractions, especially in rhizosphere soil. The strain MBR likely regulated abundant subcommunities more than rare subcommunities to improve Cd bioremediation, especially in rhizosphere soil. Consequently, the dynamics and functional roles of the local microbial communities differed significantly during bioremediation between abundant and rare subcommunities and between rhizosphere soil and bulk soil. This study provides new insight into the microbiota-related mechanisms underlying bioremediation.

PHB is Produced from Glycogen Turn-over during Nitrogen Starvation in Synechocystis sp. PCC 6803

Polyhydroxybutyrate (PHB) is a polymer of great interest as a substitute for conventional plastics, which are becoming an enormous environmental problem. PHB can be produced directly from CO₂ in photoautotrophic cyanobacteria. The model cyanobacterium Synechocystis sp. PCC 6803 produces PHB under conditions of nitrogen starvation. However, it is so far unclear which metabolic pathways provide the precursor molecules for PHB synthesis during nitrogen starvation. In this study, we investigated if PHB could be derived from the main intracellular carbon pool, glycogen. A mutant of the major glycogen phosphorylase, GlgP2 (slr1367 product), was almost completely impaired in PHB synthesis. Conversely, in the absence of glycogen synthase GlgA1 (sll0945 product), cells not only produced less PHB, but were also impaired in acclimation to nitrogen depletion. To analyze the role of the various carbon catabolic pathways (EMP, ED and OPP pathways) for PHB production, mutants of key enzymes of these pathways were analyzed, showing different impact on PHB synthesis. Together, this study clearly indicates that PHB in glycogen-producing Synechocystis sp. PCC 6803 cells is produced from this carbon-pool during nitrogen starvation periods. This knowledge can be used for metabolic engineering to get closer to the overall goal of a sustainable, carbon-neutral bioplastic production.

Defining the Genetic Basis of Plant⁻Endophytic Bacteria Interactions

Endophytic bacteria, which interact closely with their host, are an essential part of the plant microbiome. These interactions enhance plant tolerance to environmental changes as well as promote plant growth, thus they have become attractive targets for increasing crop production. Numerous studies have aimed to characterise how endophytic bacteria infect and colonise their hosts as well as conferring important traits to the plant. In this review, we summarise the current knowledge regarding endophytic colonisation and focus on the insights that have been obtained from the mutants of bacteria and plants as well as ‘omic analyses. These show how endophytic bacteria produce various molecules and have a range of activities related to chemotaxis, motility, adhesion, bacterial cell wall properties, secretion, regulating transcription and utilising a substrate in order to establish a successful interaction. Colonisation is mediated by plant receptors and is regulated by the signalling that is connected with phytohormones such as auxin and jasmonic (JA) and salicylic acids (SA). We also highlight changes in the expression of small RNAs and modifications of the cell wall properties. Moreover, in order to exploit the beneficial plant-endophytic bacteria interactions in agriculture successfully, we show that the key aspects that govern successful interactions remain to be defined.

Importance of Poly-3-Hydroxybutyrate Metabolism to the Ability of Herbaspirillum seropedicae To Promote Plant Growth

The application of bacteria as plant growth promoters is a sustainable alternative to mitigate the use of chemical fertilization in agriculture, reducing negative economic and environmental impacts. Several plant growth-promoting bacteria synthesize and accumulate the intracellular polymer polyhydroxybutyrate (PHB). However, the role of PHB in plant-bacterium interactions is poorly understood. In this study, applying the C4 model grass Setaria viridis and several mutants in the PHB metabolism of the endophyte Herbaspirillum seropedicae yielded new findings on the importance of PHB for bacterial colonization of S. viridis roots. Taken together, the results show that deletion of genes involved in the synthesis and degradation of PHB reduced the ability of the bacteria to enhance plant growth but with little effect on overall root colonization. The data suggest that PHB metabolism likely plays an important role in supporting specific metabolic routes utilized by the bacteria to stimulate plant growth.

Herbaspirillum seropedicae is an endophytic bacterium that establishes an association with a variety of plants, such as rice, corn, and sugarcane, and can significantly increase plant growth. H. seropedicae produces polyhydroxybutyrate (PHB), stored in the form of insoluble granules. Little information is available on the possible role of PHB in bacterial root colonization or in plant growth promotion. To investigate whether PHB is important for the association of H. seropedicae with plants, we inoculated roots of Setaria viridis with H. seropedicae strain SmR1 and mutants defective in PHB production (ΔphaP1, ΔphaP1 ΔphaP2, ΔphaC1, and ΔphaR) or mobilization (ΔphaZ1 ΔphaZ2). The strains producing large amounts of PHB colonized roots, significantly increasing root area and the number of lateral roots compared to those of PHB-negative strains. H. seropedicae grows under microaerobic conditions, which can be found in the rhizosphere. When grown under low-oxygen conditions, only the parental strain and ΔphaP2 mutant exhibited normal growth. The lack of normal growth under low oxygen correlated with the inability to stimulate plant growth, although there was no effect on the level of root colonization. The data suggest that PHB is produced in the root rhizosphere and plays a role in maintaining normal metabolism under microaerobic conditions. To confirm this, we screened for green fluorescent protein (GFP) expression under the control of the H. seropedicae promoters of the PHA synthase and PHA depolymerase genes in the rhizosphere. PHB synthesis is active on the root surface and later PHB depolymerase expression is activated.

IMPORTANCE The application of bacteria as plant growth promoters is a sustainable alternative to mitigate the use of chemical fertilization in agriculture, reducing negative economic and environmental impacts. Several plant growth-promoting bacteria synthesize and accumulate the intracellular polymer polyhydroxybutyrate (PHB). However, the role of PHB in plant-bacterium interactions is poorly understood. In this study, applying the C4 model grass Setaria viridis and several mutants in the PHB metabolism of the endophyte Herbaspirillum seropedicae yielded new findings on the importance of PHB for bacterial colonization of S. viridis roots. Taken together, the results show that deletion of genes involved in the synthesis and degradation of PHB reduced the ability of the bacteria to enhance plant growth but with little effect on overall root colonization. The data suggest that PHB metabolism likely plays an important role in supporting specific metabolic routes utilized by the bacteria to stimulate plant growth.

Survival Mechanisms of Campylobacter hepaticus Identified by Genomic Analysis and Comparative Transcriptomic Analysis of in vivo and in vitro Derived Bacteria

Chickens infected with Campylobacter jejuni or Campylobacter coli are largely asymptomatic, however, infection with the closely related species, Campylobacter hepaticus, can result in Spotty Liver Disease (SLD). C. hepaticus has been detected in the liver, bile, small intestine and caecum of SLD affected chickens. The survival and colonization mechanisms that C. hepaticus uses to colonize chickens remain unknown. In this study, we compared the genome sequences of 14 newly sequenced Australian isolates of C. hepaticus, isolates from outbreaks in the United Kingdom, and reference strains of C. jejuni and C. coli, with the aim of identifying virulence genes associated with SLD. We also carried out global comparative transcriptomic analysis between C. hepaticus recovered from the bile of SLD infected chickens and C. hepaticus grown in vitro. This revealed how the bacteria adapt to proliferate in the challenging host environment in which they are found. Additionally, biochemical experiments confirmed some in silico metabolic predictions. We found that, unlike other Campylobacter sp., C. hepaticus encodes glucose and polyhydroxybutyrate metabolism pathways. This study demonstrated the metabolic plasticity of C. hepaticus, which may contribute to survival in the competitive, nutrient and energy-limited environment of the chicken. Transcriptomic analysis indicated that gene clusters associated with glucose utilization, stress response, hydrogen metabolism, and sialic acid modification may play an important role in the pathogenicity of C. hepaticus. An understanding of the survival and virulence mechanisms that C. hepaticus uses will help to direct the development of effective intervention methods to protect birds from the debilitating effects of SLD.