High survival and stability rates ofEscherichia colidried in hydroxyectoine

Multiple Functions of Compatible Solute Ectoine and Strategies for Constructing Overproducers for Biobased Production

Ectoine and its derivative 5-hydroxyectoine are compatible solutes initially found in the hyperhalophilic bacterium Ectothiorhodospira halochloris, which inhabits the desert in Egypt. The habitat of ectoine producers implies the primary function of ectoine as a cytoprotectant against harsh conditions such as high salinity, drought, and high radiation. More extensive and in-depth studies have revealed the multiple functions of ectoine in its native producer bacterial cells and other types of cells and its biomolecular components (such as proteins and DNA) as a general protective agent. Its chemical properties as a bio-based amino acid derivative make it attractive for basic scientific research and related industries, such as the food/agricultural industry, cosmetic manufacturing, biologics, and therapeutic agent preparation. This article first discusses the functions and applications of ectoine and 5-hydroxyectoine. Subsequently, more emphasis was placed on advances in bio-based ectoine and/or 5-hydroxyectoine production. Strategies for developing more robust cell factories for highly efficient ectoine and/or 5-hydroxyectoine production are further discussed. We hope this review will provide a valuable reference for studies on the bio-based production of ectoine and 5-hydroxyectoine.

Accelerated water removal from frozen thin films containing bacteria

Freeze-drying, or lyophilization, is widely used to produce pharmaceutical solids that contain temperature-sensitive materials. Herein, using Escherichia coli as a model live organism, whose viability in dry powders is highly sensitive to the water content in the powders, we demonstrated that the drying rate from the frozen thin films generated by thin-film freezing (TFF) is significantly faster than from the bulk frozen solids in conventional shelf freeze-drying. This is likely because the loosely stacked frozen thin films provided a larger solid-air interface and the low thickness of the thin films provided a low mass transfer resistance. The highly porous microstructure and high specific surface area of the thin-film freeze-dried powders may also be related to the faster drying observed. Moreover, we demonstrated that TFF can be applied to produce dry powders of E. coli, a Gram-negative bacterium, or Lactobacillus acidophilus, a Gram-positive bacterium, with minimum bacterial viability loss (i.e., within one log reduction). It is concluded that the TFF technology is promising in accelerating water removal from frozen samples.

The ups and downs of ectoine: structural enzymology of a major microbial stress protectant and versatile nutrient

Ectoine and its derivative 5-hydroxyectoine are compatible solutes and chemical chaperones widely synthesized by Bacteria and some Archaea as cytoprotectants during osmotic stress and high- or low-growth temperature extremes. The function-preserving attributes of ectoines led to numerous biotechnological and biomedical applications and fostered the development of an industrial scale production process. Synthesis of ectoines requires the expenditure of considerable energetic and biosynthetic resources. Hence, microorganisms have developed ways to exploit ectoines as nutrients when they are no longer needed as stress protectants. Here, we summarize our current knowledge on the phylogenomic distribution of ectoine producing and consuming microorganisms. We emphasize the structural enzymology of the pathways underlying ectoine biosynthesis and consumption, an understanding that has been achieved only recently. The synthesis and degradation pathways critically differ in the isomeric form of the key metabolite N-acetyldiaminobutyric acid (ADABA). γ-ADABA serves as preferred substrate for the ectoine synthase, while the α-ADABA isomer is produced by the ectoine hydrolase as an intermediate in catabolism. It can serve as internal inducer for the genetic control of ectoine catabolic genes via the GabR/MocR-type regulator EnuR. Our review highlights the importance of structural enzymology to inspire the mechanistic understanding of metabolic networks at the biological scale.

Ectoines as novel anti-inflammatory and tissue protective lead compounds with special focus on inflammatory bowel disease and lung inflammation

The compatible solute ectoine is one of the most abundant and powerful cytoprotectant in the microbial world. Due to its unique ability to stabilize biological membranes and macromolecules it has been successfully commercialized as ingredient of various over-the-counter drugs, achieving primarily epithelial protection. While trying to elucidate the mechanism of its cell protective properties in in-vitro studies, a significant anti-inflammatory effect was documented for the small molecule. The tissue protective potential of ectoine considerably improved organ quality during preservation. In addition, ectoine and derivatives have been demonstrated to significantly decrease inflammatory cytokine production, thereby alleviating the inflammatory response following organ transplantation, and launching new therapeutic options for pathologies such as Inflammatory Bowel Disease (IBD) and Chronic Obstructive Pulmonary Disease (COPD). In this review, we aim to summarize the knowledge of this fairly nascent field of the anti-inflammatory potential of diverse ectoines. We also point out that this promising field faces challenges in its biochemical and molecular substantiations, including defining the molecular mechanisms of the observed effects and their regulation. However, based on their potent cytoprotective, anti-inflammatory, and non-toxic properties we believe that ectoines represent promising candidates for risk free interventions in inflammatory pathologies with steeply increasing demands for new therapeutics.

Boosting Escherichia coli's heterologous production rate of ectoines by exploiting the non-halophilic gene cluster from Acidiphilium cryptum

The compatible solutes ectoine and hydroxyectoine are synthesized by many microorganisms as potent osmostress and desiccation protectants. Besides their successful implementation into various skincare products, they are of increasing biotechnological interest due to new applications in the healthcare sector. To meet this growing demand, efficient heterologous overproduction solutions for ectoines need to be found. This study is the first report on the utilization of the non-halophilic biosynthesis enzymes from Acidiphilium cryptum DSM 2389^T for efficient heterologous production of ectoines in Escherichia coli. When grown at low salt conditions (≤ 0.5% NaCl) and utilizing the cheap carbon source glycerol, the production was characterized by the highest specific production of ectoine [2.9 g/g dry cell weight (dcw)] and hydroxyectoine (2.2 g/g dcw) reported so far and occurred at rapid specific production rates of up to 345 mg/(g dcw × h). This efficiency in production was related to an unprecedented carbon source conversion rate of approx. 60% of the theoretical maximum. These findings confirm the unique potential of the here implemented non-halophilic enzymes for ectoine production processes in E. coli and demonstrate the first efficient heterologous solution for hydroxyectoine production, as well as an extraordinary efficient low-salt ectoine production.

Proteome Comparison Between Natural Desiccation-Tolerant Plants and Drought-Protected Caspicum annuum Plants by Microbacterium sp. 3J1

Desiccation-tolerant plants are able to survive for extended periods of time in the absence of water. The molecular understanding of the mechanisms used by these plants to resist droughts can be of great value for improving drought tolerance in crops. This understanding is especially relevant in an environment that tends to increase the number and intensity of droughts. The combination of certain microorganisms with drought-sensitive plants can improve their tolerance to water scarcity. One of these bacteria is Microbacterium sp. 3J1, an actinobacteria able to protect pepper plants from drought. In this study, we supplemented drought-tolerant and drought-sensitive plant rhizospheres with Microbacterium sp. 3J1 and analyzed their proteomes under drought to investigate the plant-microbe interaction. We also compare this root proteome with the proteome found in desiccation-tolerant plants. In addition, we studied the proteome of Microbacterium sp. 3J1 subjected to drought to analyze its contribution to the plant-microbe interaction. We describe those mechanisms shared by desiccation-tolerant plants and sensitive plants protected by microorganisms focusing on protection against oxidative stress, and production of compatible solutes, plant hormones, and other more specific proteins.

Desiccation-induced cell damage in bacteria and the relevance for inoculant production

Plant growth-promoting bacteria show great potential for use in agriculture although efficient application remains challenging to achieve. Cells often lose viability during inoculant production and application, jeopardizing the efficacy of the inoculant. Since desiccation has been documented to be the primary stress factor affecting the decrease in survival, obtaining xerotolerance in plant growth-promoting bacteria is appealing. The molecular damage that occurs by drying bacteria has been broadly investigated, although a complete view is still lacking due to the complex nature of the process. Mechanic, structural, and metabolic changes that occur as a result of water depletion may potentially afflict lethal damage to membranes, DNA, and proteins. Bacteria respond to these harsh conditions by increasing production of exopolysaccharides, changing composition of the membrane, improving the stability of proteins, reducing oxidative stress, and repairing DNA damage. This review provides insight into the complex nature of desiccation stress in bacteria in order to facilitate strategic choices to improve survival and shelf life of newly developed inoculants. KEY POINTS: Desiccation-induced damage affects most major macromolecules in bacteria. Most bacteria are not xerotolerant despite multiple endogenous adaption mechanisms. Sensitivity to drying severely hampers inoculant quality.

The architecture of the diaminobutyrate acetyltransferase active site provides mechanistic insight into the biosynthesis of the chemical chaperone ectoine

Ectoine is a solute compatible with the physiologies of both prokaryotic and eukaryotic cells and is widely synthesized by bacteria as an osmotic stress protectant. Because it preserves functional attributes of proteins and macromolecular complexes, it is considered a chemical chaperone and has found numerous practical applications. However, the mechanism of its biosynthesis is incompletely understood. The second step in ectoine biosynthesis is catalyzed by l-2,4-diaminobutyrate acetyltransferase (EctA; EC 2.3.1.178), which transfers the acetyl group from acetyl-CoA to EctB-formed l-2,4-diaminobutyrate (DAB), yielding N-γ-acetyl-l-2,4-diaminobutyrate (N-γ-ADABA), the substrate of ectoine synthase (EctC). Here, we report the biochemical and structural characterization of the EctA enzyme from the thermotolerant bacterium Paenibacillus lautus (Pl). We found that (Pl)EctA forms a homodimer whose enzyme activity is highly regiospecific by producing N-γ-ADABA but not the ectoine catabolic intermediate N-α-acetyl-l-2,4-diaminobutyric acid. High-resolution crystal structures of (Pl)EctA (at 1.2-2.2 Å resolution) (i) for its apo-form, (ii) in complex with CoA, (iii) in complex with DAB, (iv) in complex with both CoA and DAB, and (v) in the presence of the product N-γ-ADABA were obtained. To pinpoint residues involved in DAB binding, we probed the structure-function relationship of (Pl)EctA by site-directed mutagenesis. Phylogenomics shows that EctA-type proteins from both Bacteria and Archaea are evolutionarily highly conserved, including catalytically important residues. Collectively, our biochemical and structural findings yielded detailed insights into the catalytic core of the EctA enzyme that laid the foundation for unraveling its reaction mechanism.

Salt stress and hydroxyectoine enhance phosphate solubilisation and plant colonisation capacity of Kosakonia radicincitans

Gram-negative bacterial endophytes have attracted research interest caused by their advantageous over epiphytic bacteria in plant nutrition and protection. However, research on these typically Gram-negative endophytes has deficiencies concerning the role of cultivation and pre-formulation strategies on further plant colonisation capabilities. Besides, the influence of cultivation conditions and osmotic stress within bacterial endophytes on their phosphate solubilising ability has not yet been addressed. By pre-conditioning cells with an osmoadaptation and a hydroxyectoine accumulation approach, this research aimed at enhancing the capability of the plant growth promoting bacterium Kosakonia radicincitans strain DSM 16656T to both solubilise phosphate and colonise plant seedlings. The results showed that halotolerant bacterial phenotypes increased the root-colonising capability by approximately 3-fold and presented growth-promoting effects in radish plants. Interestingly, findings also demonstrated that salt stress in the culture media along with the accumulation of hydroxyectoine led to an increase in the in vitro phosphate-solubilising ability by affecting the production of acid phosphatases, from 1.24 to 3.34 U mg-1 for non-salt stressed cells and hydroxyectoine-added cells respectively. Thus, this approach provides a useful knowledge upon which the salt stress and compatible solutes in bacteria endophytes can confer phenotypic adaptations to support the eco-physiological performance concerning phosphate-solubilising abilities and endosphere establishment.

Role of the Extremolytes Ectoine and Hydroxyectoine as Stress Protectants and Nutrients: Genetics, Phylogenomics, Biochemistry, and Structural Analysis

Fluctuations in environmental osmolarity are ubiquitous stress factors in many natural habitats of microorganisms, as they inevitably trigger osmotically instigated fluxes of water across the semi-permeable cytoplasmic membrane. Under hyperosmotic conditions, many microorganisms fend off the detrimental effects of water efflux and the ensuing dehydration of the cytoplasm and drop in turgor through the accumulation of a restricted class of organic osmolytes, the compatible solutes. Ectoine and its derivative 5-hydroxyectoine are prominent members of these compounds and are synthesized widely by members of the Bacteria and a few Archaea and Eukarya in response to high salinity/osmolarity and/or growth temperature extremes. Ectoines have excellent function-preserving properties, attributes that have led to their description as chemical chaperones and fostered the development of an industrial-scale biotechnological production process for their exploitation in biotechnology, skin care, and medicine. We review, here, the current knowledge on the biochemistry of the ectoine/hydroxyectoine biosynthetic enzymes and the available crystal structures of some of them, explore the genetics of the underlying biosynthetic genes and their transcriptional regulation, and present an extensive phylogenomic analysis of the ectoine/hydroxyectoine biosynthetic genes. In addition, we address the biochemistry, phylogenomics, and genetic regulation for the alternative use of ectoines as nutrients.

Protection of Pepper Plants from Drought by Microbacterium sp. 3J1 by Modulation of the Plant's Glutamine and α-ketoglutarate Content: A Comparative Metabolomics Approach

Drought tolerance of plants such as tomato or pepper can be improved by their inoculation with rhizobacteria such as Microbacterium sp. 3J1. This interaction depends on the production of trehalose by the microorganisms that in turn modulate the phyto-hormone profile of the plant. In this work we describe the characterization of metabolic changes during the interaction of pepper plants with Microbacterium sp. 3J1 and of the microorganism alone over a period of drought. Our main findings include the observation that the plant responds to the presence of the microorganism by changing the C and N metabolism based on its glutamine and α-ketoglutarate content, these changes contribute to major changes in the concentration of molecules involved in the balance of the osmotic pressure. These include sugars and amino-acids; the concentration of antioxidant molecules, of metabolites involved in the production of phytohormones like ethylene, and of substrates used for lignin production such as ferulic and sinapic acids. Most of the altered metabolites of the plant when inoculated with Microbacterium sp. 3J1 in response to drought coincided with the profile of altered metabolites in the microorganism alone when subjected to drought, pointing to a response by which the plant relies on the microbe for the production of such metabolites. To our knowledge this is the first comparative study of the microbe colonized-plant and microbe alone metabolomes under drought stress.

Tinkering with Osmotically Controlled Transcription Allows Enhanced Production and Excretion of Ectoine and Hydroxyectoine from a Microbial Cell Factory

Ectoine and hydroxyectoine are widely synthesized by members of the Bacteria and a few members of the Archaea as potent osmostress protectants. We have studied the salient features of the osmostress-responsive promoter directing the transcription of the ectoine/hydroxyectoine biosynthetic gene cluster from the plant-root-associated bacterium Pseudomonas stutzeri by transferring it into Escherichia coli, an enterobacterium that does not produce ectoines naturally. Using ect-lacZ reporter fusions, we found that the heterologous ect promoter reacted with exquisite sensitivity in its transcriptional profile to graded increases in sustained high salinity, responded to a true osmotic signal, and required the buildup of an osmotically effective gradient across the cytoplasmic membrane for its induction. The involvement of the -10, -35, and spacer regions of the sigma-70-type ect promoter in setting promoter strength and response to osmotic stress was assessed through site-directed mutagenesis. Moderate changes in the ect promoter sequence that increase its resemblance to housekeeping sigma-70-type promoters of E. coli afforded substantially enhanced expression, both in the absence and in the presence of osmotic stress. Building on this set of ect promoter mutants, we engineered an E. coli chassis strain for the heterologous production of ectoines. This synthetic cell factory lacks the genes for the osmostress-responsive synthesis of trehalose and the compatible solute importers ProP and ProU, and it continuously excretes ectoines into the growth medium. By combining appropriate host strains and different plasmid variants, excretion of ectoine, hydroxyectoine, or a mixture of both compounds was achieved under mild osmotic stress conditions.IMPORTANCE Ectoines are compatible solutes, organic osmolytes that are used by microorganisms to fend off the negative consequences of high environmental osmolarity on cellular physiology. An understanding of the salient features of osmostress-responsive promoters directing the expression of the ectoine/hydroxyectoine biosynthetic gene clusters is lacking. We exploited the ect promoter from an ectoine/hydroxyectoine-producing soil bacterium for such a study by transferring it into a surrogate bacterial host. Despite the fact that E. coli does not synthesize ectoines naturally, the ect promoter retained its exquisitely sensitive osmotic control, indicating that osmoregulation of ect transcription is an inherent feature of the promoter and its flanking sequences. These sequences were narrowed to a 116-bp DNA fragment. Ectoines have interesting commercial applications. Building on data from a site-directed mutagenesis study of the ect promoter, we designed a synthetic cell factory that secretes ectoine, hydroxyectoine, or a mixture of both compounds into the growth medium.

Engineering Rugged Field Assays to Detect Hazardous Chemicals Using Spore-Based Bacterial Biosensors

Bacterial whole cell-based biosensors have been genetically engineered to achieve selective and reliable detection of a wide range of hazardous chemicals. Although whole-cell biosensors demonstrate many advantages for field-based detection of target analytes, there are still some challenges that need to be addressed. Most notably, their often modest shelf life and need for special handling and storage make them challenging to use in situations where access to reagents, instrumentation, and expertise are limited. These problems can be circumvented by developing biosensors in Bacillus spores, which can be engineered to address all of these concerns. In its sporulated state, a whole cell-based biosensor has a remarkably long life span and is exceptionally resistant to environmental insult. When these spores are germinated for use in analytical techniques, they show no loss in performance, even after long periods of storage under harsh conditions. In this chapter, we will discuss the development and use of whole cell-based sensors, their adaptation to spore-based biosensors, their current applications, and future directions in the field.

EctD-mediated biotransformation of the chemical chaperone ectoine into hydroxyectoine and its mechanosensitive channel-independent excretion

Background

Ectoine and its derivative 5-hydroxyectoine are cytoprotectants widely synthesized by microorganisms as a defense against the detrimental effects of high osmolarity on cellular physiology and growth. Both ectoines possess the ability to preserve the functionality of proteins, macromolecular complexes, and even entire cells, attributes that led to their description as chemical chaperones. As a consequence, there is growing interest in using ectoines for biotechnological purposes, in skin care, and in medical applications. 5-Hydroxyectoine is synthesized from ectoine through a region- and stereo-specific hydroxylation reaction mediated by the EctD enzyme, a member of the non-heme-containing iron(II) and 2-oxoglutarate-dependent dioxygenases. This chemical modification endows the newly formed 5-hydroxyectoine with either superior or different stress- protecting and stabilizing properties. Microorganisms producing 5-hydroxyectoine typically contain a mixture of both ectoines. We aimed to establish a recombinant microbial cell factory where 5-hydroxyectoine is (i) produced in highly purified form, and (ii) secreted into the growth medium.

Results

We used an Escherichia coli strain (FF4169) defective in the synthesis of the osmostress protectant trehalose as the chassis for our recombinant cell factory. We expressed in this strain a plasmid-encoded ectD gene from Pseudomonas stutzeri A1501 under the control of the anhydrotetracycline-inducible tet promoter. We chose the ectoine hydroxylase from P. stutzeri A1501 for our cell factory after a careful comparison of the in vivo performance of seven different EctD proteins. In the final set-up of the cell factory, ectoine was provided to salt-stressed cultures of strain FF4169 (pMP41; ectD⁺). Ectoine was imported into the cells via the osmotically inducible ProP and ProU transport systems, intracellularly converted to 5-hydroxyectoine, which was then almost quantitatively secreted into the growth medium. Experiments with an E. coli mutant lacking all currently known mechanosensitive channels (MscL, MscS, MscK, MscM) revealed that the release of 5-hydroxyectoine under osmotic steady-state conditions occurred independently of these microbial safety valves. In shake-flask experiments, 2.13 g l⁻¹ ectoine (15 mM) was completely converted into 5-hydroxyectoine within 24 h.

Conclusions

We describe here a recombinant E. coli cell factory for the production and secretion of the chemical chaperone 5-hydroxyectoine free from contaminating ectoine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0525-4) contains supplementary material, which is available to authorized users.

Responses of yeast biocontrol agents to environmental stress

Biological control of postharvest diseases, utilizing wild species and strains of antagonistic yeast species, is a research topic that has received considerable attention in the literature over the past 30 years. In principle, it represents a promising alternative to chemical fungicides for the management of postharvest decay of fruits, vegetables, and grains. A yeast-based biocontrol system is composed of a tritrophic interaction between a host (commodity), a pathogen, and a yeast species, all of which are affected by environmental factors such as temperature, pH, and UV light as well as osmotic and oxidative stresses. Additionally, during the production process, biocontrol agents encounter various severe abiotic stresses that also impact their viability. Therefore, understanding the ecological fitness of the potential yeast biocontrol agents and developing strategies to enhance their stress tolerance are essential to their efficacy and commercial application. The current review provides an overview of the responses of antagonistic yeast species to various environmental stresses, the methods that can be used to improve stress tolerance and efficacy, and the related mechanisms associated with improved stress tolerance.

The hydroxyectoine gene cluster of the non-halophilic acidophile Acidiphilium cryptum

Acidiphilium cryptum is an acidophilic, heterotrophic α-Proteobacterium which thrives in acidic, metal-rich environments (e.g. acid mine drainage). Recently, an ectABCDask gene cluster for biosynthesis of the compatible solutes ectoine and hydroxyectoine was detected in the genome sequence of A. cryptum JF-5. We were able to demonstrate that the type strain A. cryptum DSM 2389(T) is capable of synthesizing the compatible solute hydroxyectoine in response to moderate osmotic stress caused by sodium chloride and aluminium sulphate, respectively. Furthermore, we used the A. cryptum JF-5 sequence to amplify the ectABCDask gene cluster from strain DSM 2389(T) and achieved heterologous expression of the gene cluster in Escherichia coli. Hence, we could for the first time prove metabolic functionality of the genes responsible for hydroxyectoine biosynthesis in the acidophile A. cryptum. In addition, we present information on specific enzyme activity of A. cryptum DSM 2389(T) ectoine synthase (EctC) in vitro. In contrast to EctCs from halophilic microorganisms, the A. cryptum enzyme exhibits a higher isoelectric point, thus a lower acidity, and has maximum specific activity in the absence of sodium chloride.

Genome Sequence of Arthrobacter siccitolerans 4J27, a Xeroprotectant-Producing Desiccation-Tolerant Microorganism

We report the first genome sequence for Arthrobacter siccitolerans 4J27, a newly described desiccation-tolerant species. The complete genome of A. siccitolerans 4J27 has been sequenced and is estimated to be around 5.3 Mb in size, with an average GC content of 65.13%. We predict 4,480 protein-coding sequences (CDSs).

Arthrobacter siccitolerans sp. nov., a highly desiccation-tolerant, xeroprotectant-producing strain isolated from dry soil

A novel desiccation-tolerant, xeroprotectant-producing bacterium, designated strain 4J27(T), was isolated from a Nerium oleander rhizosphere subjected to seasonal drought in Granada, Spain. Phylogenetic analysis based on 16S rRNA gene sequencing placed the isolate within the genus Arthrobacter, its closest relative being Arthrobacter phenanthrenivorans Shep3 DSM 18606(T), with which it showed 99.23 % 16S rRNA gene sequence similarity. DNA-DNA hybridization measurements showed less than 25 % relatedness between strain 4J27(T) and Arthrobacter phenanthrenivorans DSM 18606(T). The DNA base composition of strain 4J27(T) was 65.3 mol%. The main fatty acids were anteiso C15 : 0, anteiso C17 : 0, C16 : 0 and iso C16 : 0 and the major menaquinone was MK-9 (H2). The peptidoglycan type was A3α with an l-Lys-l-Ser-l-Thr-l-Ala interpeptide bridge. The bacterium tested positive for catalase activity and negative for oxidase activity. Phylogenetic, chemotaxonomic and phenotypic analyses indicated that the desiccation-tolerant strain 4J27(T) represents a novel species within the genus Arthrobacter, for which the name Arthrobacter siccitolerans is proposed. The type strain is 4J27(T) ( = CECT 8257(T) = LMG 27359(T)).

Xeroprotectants for the stabilization of biomaterials

With the advancement of science and technology, it is crucial to have effective preservation methods for the stable long-term storage of biological material (biomaterials). As an alternative to cryopreservation, various techniques have been developed, which are based on the survival mechanism of anhydrobiotic organisms. In this sense, it has been found that the synthesis of xeroprotectants can effectively stabilize biomaterials in a dry state. The most widely studied xeroprotectant is trehalose, which has excellent properties for the stabilization of certain proteins, bacteria, and biological membranes. There have also been attempts to apply trehalose to the stabilization of eukaryotic cells but without conclusive results. Consequently, a xeroprotectant or method that is useful for the stable drying of a particular biomaterial might not necessarily be suitable for another one. This article provides an overview of recent advances in the use of new techniques to stabilize biomaterials and compare xeroprotectants with other more standard methods.

Role of trehalose in salinity and temperature tolerance in the model halophilic bacterium Chromohalobacter salexigens

The disaccharide trehalose is considered as a universal stress molecule, protecting cells and biomolecules from injuries imposed by high osmolarity, heat, oxidation, desiccation and freezing. Chromohalobacter salexigens is a halophilic and extremely halotolerant γ-proteobacterium of the family Halomonadaceae. In this work, we have investigated the role of trehalose as a protectant against salinity, temperature and desiccation in C. salexigens. A mutant deficient in the trehalose-6-phosphate synthase gene (otsA::Ω) was not affected in its salt or heat tolerance, but double mutants ectoine- and trehalose-deficient, or hydroxyectoine-reduced and trehalose-deficient, displayed an osmo- and thermosensitive phenotype, respectively. This suggests a role of trehalose as a secondary solute involved in osmo- (at least at low salinity) and thermoprotection of C. salexigens. Interestingly, trehalose synthesis was osmoregulated at the transcriptional level, and thermoregulated at the post-transcriptional level, suggesting that C. salexigens cells need to be pre-conditioned by osmotic stress, in order to be able to quickly synthesize trehalose in response to heat stress. C. salexigens was more sensitive to desiccation than E. coli and desiccation tolerance was slightly improved when cells were grown at high temperature. Under these conditions, single mutants affected in the synthesis of trehalose or hydroxyectoine were more sensitive to desiccation than the wild-type strain. However, given the low survival rates of the wild type, the involvement of trehalose and hydroxyectoine in C. salexigens response to desiccation could not be firmly established.

Biotechnological uses of desiccation-tolerant microorganisms for the rhizoremediation of soils subjected to seasonal drought

Plant growth-promoting rhizobacteria (PGPR) increase the viability and health of host plants when they colonize roots and engage in associative symbiosis (Bashan et al. 2004). In return, PGPR viability is increased by host plant roots by the provision of nutrients and a more protective environment (Richardson et al. in Plant Soil 321:305-339, 2009). The PGPR have great potential in agriculture since the combination of certain microorganisms and plants can increase crop production and increase protection against frost, salinity, drought and other environmental stresses such as the presence of xenobiotic pollutants. But there is a great challenge in combining plants and microorganisms without compromising the viability of either microorganisms or seeds. In this paper, we review how anhydrobiotic engineering can be used for the formulation of biotechnological tools that guarantee the supply of both plants and microorganisms in the dry state. We also describe the application of this technology for the selection of desiccation-tolerant PGPR for polycyclic aromatic hydrocarbons bioremediation, in soils subjected to seasonal drought, by the rhizoremediation process.

A printed nanolitre-scale bacterial sensor array

The last decade has witnessed a significant increase in interest in whole-cell biosensors for diverse applications, as well as a rapid and continuous expansion of array technologies. The combination of these two disciplines has yielded the notion of whole-cell array biosensors. We present a potential manifestation of this idea by describing the printing of a whole-cell bacterial bioreporters array. Exploiting natural bacterial tendency to adhere to positively charged abiotic surfaces, we describe immobilization and patterning of bacterial "spots" in the nanolitre volume range by a non-contact robotic printer. We show that the printed Escherichia coli-based sensor bacteria are immobilized on the surface, and retain their viability and biosensing activity for at least 2 months when kept at 4 °C. Immobilization efficiency was improved by manipulating the bacterial genetics (overproducing curli protein), the growth and the printing media (osmotic stress and osmoprotectants) and by a chemical modification of the inanimate surface (self-assembled layers of 3-aminopropyl-triethoxysilane). We suggest that the methodology presented herein may be applicable to the manufacturing of whole-cell sensor arrays for diverse high throughput applications.

Whole-cell biochips for bio-sensing: integration of live cells and inanimate surfaces

Recent advances in the convergence of the biological, chemical, physical, and engineering sciences have opened new avenues of research into the interfacing of diverse biological moieties with inanimate platforms. A main aspect of this field, the integration of live cells with micro-machined platforms for high throughput and bio-sensing applications, is the subject of the present review. These unique hybrid systems are configured in a manner that ensures positioning of the cells in designated patterns, and enables cellular viability maintenance, and monitoring of cellular functionality. Here we review both animate and inanimate surface properties and how they affect cellular attachment, describe relevant modifications of both types of surfaces, list technologies for platform engineering and for cell deposition in the desired configurations, and discuss the influence of various deposition and immobilization methods on the viability and performance of the immobilized cells.

Natural and engineered hydroxyectoine production based on the Pseudomonas stutzeri ectABCD-ask gene cluster

We report on the presence of a functional hydroxyectoine biosynthesis gene cluster, ectABCD-ask, in Pseudomonas stutzeri DSM5190(T) and evaluate the suitability of P. stutzeri DSM5190(T) for hydroxyectoine production. Furthermore, we present information on heterologous de novo production of the compatible solute hydroxyectoine in Escherichia coli. In this host, the P. stutzeri gene cluster remained under the control of its salt-induced native promoters. We also noted the absence of trehalose when hydroxyectoine genes were expressed, as well as a remarkable inhibitory effect of externally applied betaine on hydroxyectoine synthesis. The specific heterologous production rate in E. coli under the conditions employed exceeded that of the natural producer Pseudomonas stutzeri and, for the first time, enabled effective hydroxyectoine production at low salinity (2%), with the added advantage of simple product processing due to the absence of other cosolutes.

Hydrophobic substances induce water stress in microbial cells

Ubiquitous noxious hydrophobic substances, such as hydrocarbons, pesticides and diverse industrial chemicals, stress biological systems and thereby affect their ability to mediate biosphere functions like element and energy cycling vital to biosphere health. Such chemically diverse compounds may have distinct toxic activities for cellular systems; they may also share a common mechanism of stress induction mediated by their hydrophobicity. We hypothesized that the stressful effects of, and cellular adaptations to, hydrophobic stressors operate at the level of water : macromolecule interactions. Here, we present evidence that: (i) hydrocarbons reduce structural interactions within and between cellular macromolecules, (ii) organic compatible solutes – metabolites that protect against osmotic and chaotrope‐induced stresses – ameliorate this effect, (iii) toxic hydrophobic substances induce a potent form of water stress in macromolecular and cellular systems, and (iv) the stress mechanism of, and cellular responses to, hydrophobic substances are remarkably similar to those associated with chaotrope‐induced water stress. These findings suggest that it may be possible to devise new interventions for microbial processes in both natural environments and industrial reactors to expand microbial tolerance of hydrophobic substances, and hence the biotic windows for such processes.

Rapid method for isolation of desiccation-tolerant strains and xeroprotectants

A novel biotechnological process has been developed for the isolation of desiccation-tolerant microorganisms and their xeroprotectants, i.e., compatible solutes involved in long-term stability of biomolecules in the dry state. Following exposure of soil samples to chloroform, we isolated a collection of desiccation-tolerant microorganisms. This collection was screened for the production of xeroprotectants by a variation of the bacterial milking (osmotic downshock) procedure and by a novel air-drying/rehydration ("dry milking") incubation method. The resultant solutes were shown to protect both proteins and living cells against desiccation damage, thereby validating them as xeroprotectants. Nuclear magnetic resonance (NMR) analytical studies were performed to identify the xeroprotectants; synthetic mixtures of these compounds were shown to perform similarly to natural isolates in drying experiments with proteins and cells. This new approach has biotechnological and environmental implications for the identification of new xeroprotectants of commercial and therapeutic value.

Ectoines in cell stress protection: uses and biotechnological production

Microorganisms produce and accumulate compatible solutes aiming at protecting themselves from environmental stresses. Among them, the wide spread in nature ectoines are receiving increasing attention by the scientific community because of their multiple applications. In fact, increasing commercial demand has led to a multiplication of efforts in order to improve processes for their production. In this review, the importance of current and potential applications of ectoines as protecting agents for macromolecules, cells and tissues, together with their potential as therapeutic agents for certain diseases are analyzed and current theories for the understanding of the molecular basis of their biological activity are discussed. The genetic, biochemical and environmental determinants of ectoines biosynthesis by natural and engineered producers are described. The major limitations of current bioprocesses used for ectoines production are discussed, with emphasis on the different microorganisms, environments, molecular engineering and fermentation strategies used to optimize the production and recovery of ectoines. The combined application of both bioprocess and metabolic engineering strategies, allowing a deeper understanding of the main factors controlling the production process is also stated. Finally, this review aims to summarize and update the state of the art in ectoines uses and applications and industrial scale production using bacteria, emphasizing the importance of reactor design and operation strategies, together with the metabolic engineering aspects and the need for feedback between wet and in silico work to optimize bioproduction.

Synthesis and uptake of the compatible solutes ectoine and 5-hydroxyectoine by Streptomyces coelicolor A3(2) in response to salt and heat stresses

Streptomyces coelicolor A3(2) synthesizes ectoine and 5-hydroxyectoine upon the imposition of either salt (0.5 M NaCl) or heat stress (39 degrees C). The cells produced the highest cellular levels of these compatible solutes when both stress conditions were simultaneously imposed. Protection against either severe salt (1.2 M NaCl) or heat stress (39 degrees C) or a combination of both environmental cues could be accomplished by adding low concentrations (1 mM) of either ectoine or 5-hydroxyectoine to S. coelicolor A3(2) cultures. The best salt and heat stress protection was observed when a mixture of ectoine and 5-hydroxyectoine (0.5 mM each) was provided to the growth medium. Transport assays with radiolabeled ectoine demonstrated that uptake was triggered by either salt or heat stress. The most effective transport and accumulation of [(14)C]ectoine by S. coelicolor A3(2) were achieved when both environmental cues were simultaneously applied. Our results demonstrate that the accumulation of the compatible solutes ectoine and 5-hydroxyectoine allows S. coelicolor A3(2) to fend off the detrimental effects of both high salinity and high temperature on cell physiology. We also characterized the enzyme (EctD) required for the synthesis of 5-hydroxyectoine from ectoine, a hydroxylase of the superfamily of the non-heme-containing iron(II)- and 2-oxoglutarate-dependent dioxygenases (EC 1.14.11). The gene cluster (ectABCD) encoding the enzymes for ectoine and 5-hydroxyectoine biosynthesis can be found in the genome of S. coelicolor A3(2), Streptomyces avermitilis, Streptomyces griseus, Streptomyces scabiei, and Streptomyces chrysomallus, suggesting that these compatible solutes play an important role as stress protectants in the genus Streptomyces.

Fermentation efficiency of thermally dried kefir

Three thermal drying methods (conventional, vacuum and convective) were used for drying of kefir biomass and their effect on cell viability, fermentation rate and other kinetic parameters of lactose and whey fermentation were studied. Convective drying rate was higher than conventional and even higher than vacuum at each studied temperature (28, 33 and 38 degrees C). After that, fermentations were performed by kefir biomass dried by the three drying methods. Ethanol concentration, ethanol productivity and ethanol yield are higher in whey fermentations performed by kefir biomass dried with convective drying method. Regarding lactic acid production, fermentation performed by kefir biomass dried with conventional drying method gave higher concentrations, compared to other drying methods. Storage of kefir biomass convectively dried at 33 degrees C for 4months, without any precaution decreases its fermentability and thus reduces ethanol (31%) and lactic acid productivity (20%), but remains a promising technology, since a significant part of its initial fermentative activity is retained.

Osmotically induced synthesis of the compatible solute hydroxyectoine is mediated by an evolutionarily conserved ectoine hydroxylase

By using natural abundance (13)C NMR spectroscopy, we investigated the types of compatible solutes synthesized in a variety of Bacilli under high salinity growth conditions. Glutamate, proline, and ectoine were the dominant compatible solutes synthesized by the various Bacillus species. The majority of the inspected Bacilli produced the tetrahydropyrimidine ectoine in response to high salinity stress, and a subset of these also synthesized a hydroxylation derivative of ectoine, 5-hydroxyectoine. In Salibacillus salexigens, a representative of the ectoine- and 5-hydroxyectoine-producing species, ectoine production was linearly correlated with the salinity of the growth medium and dependent on an ectABC biosynthetic operon. The formation of 5-hydroxyectoine was primarily a stationary growth phase phenomenon. The enzyme responsible for ectoine hydroxylation (EctD) was purified from S. salexigens to apparent homogeneity. The EctD protein was shown in vitro to directly hydroxylate ectoine in a reaction dependent on iron(II), molecular oxygen, and 2-oxoglutarate. We identified the structural gene (ectD) for the ectoine hydroxylase in S. salexigens. Northern blot analysis showed that the transcript levels of the ectABC and ectD genes increased as a function of salinity. Many EctD-related proteins can be found in data base searches in various Bacteria. Each of these bacterial species also contains an ectABC ectoine biosynthetic gene cluster, suggesting that 5-hydroxyectoine biosynthesis strictly depends on the prior synthesis of ectoine. Our data base searches and the biochemical characterization of the EctD protein from S. salexigens suggest that the EctD-related ectoine hydroxylases are members of a new subfamily within the non-heme-containing, iron(II)- and 2-oxoglutarate-dependent dioxygenase superfamily (EC 1.14.11).

Extremolytes: Natural compounds from extremophiles for versatile applications

Extremophilic microorganisms have adopted a variety of ingenious strategies for survival under high or low temperature, extreme pressure, and drastic salt concentrations. A novel application area for extremophiles is the use of "extremolytes," organic osmolytes from extremophilic microorganisms, to protect biological macromolecules and cells from damage by external stresses. In extremophiles, these low molecular weight compounds are accumulated in response to increased extracellular salt concentrations, but also as a response to other environmental changes, e.g., increased temperature. Extremolytes minimize the denaturation of biopolymers that usually occurs under conditions of water stress and are compatible with the intracellular machinery at high (>1 M) concentrations. The ectoines, as the first extremolytes that are produced in a large scale, have already found application as cell protectants in skin care and as protein-free stabilizers of proteins and cells in life sciences. In addition to ectoines, a range of extremolytes with heterogenous chemical structures like the polyol phosphates di-myoinositol-1,1'-phosphate, cyclic 2,3-diphosphoglycerate, and alpha-diglycerol phosphate and the mannose derivatives mannosylglycerate (firoin) and mannosylglyceramide (firoin-A) were characterized and were shown to have protective properties toward proteins and cells. A range of new applications, all based on the adaptation to stress conditions conferred by extremolytes, is in development.

Optimisation and comparison of liquid and dry formulations of the biocontrol yeast Pichia anomala J121

The biocontrol yeast Pichia anomala J121 can effectively reduce mould growth on moist cereal grains during airtight storage. Practical use of microorganisms requires formulated products that meet a number of criteria. In this study we compared different formulations of P. anomala. The best way to formulate P. anomala was freeze-drying. The initial viability was as high as 80%, with trehalose previously added to the yeast. Freeze-dried products could be stored at temperatures as high as 30 degrees C for a year, with only a minor decrease in viability. Vacuum-drying also resulted in products with high storage potential, but the products were not as easily rehydrated as freeze-dried samples. Upon desiccating the cells using fluidised-bed drying or as liquid formulations, a storage temperature of 10 degrees C was required to maintain viability. Dependent on the type of formulation, harvesting of cells at different nutritional stresses affected the initial viabilities, e.g. the initial viability for fluidised-bed-dried cells was higher when the culture was fed with excess glucose, but for freeze-drying it was superior when cells were harvested after depletion of carbon. Using micro-silos we found that the biocontrol activity remained intact after drying, storage and rehydration for all formulations.

The ectD gene, which is involved in the synthesis of the compatible solute hydroxyectoine, is essential for thermoprotection of the halophilic bacterium Chromohalobacter salexigens

The halophilic bacterium Chromohalobacter salexigens synthesizes and accumulates compatible solutes in response to salt and temperature stress. (13)C-nuclear magnetic resonance analysis of cells grown in minimal medium at the limiting temperature of 45 degrees C revealed the presence of hydroxyectoine, ectoine, glutamate, trehalose (not present in cells grown at 37 degrees C), and the ectoine precursor, Ngamma-acetyldiaminobutyric acid. High-performance liquid chromatography analyses showed that the levels of ectoine and hydroxyectoine were maximal during the stationary phase of growth. Accumulation of hydroxyectoine was up-regulated by salinity and temperature, whereas accumulation of ectoine was up-regulated by salinity and down-regulated by temperature. The ectD gene, which is involved in the conversion of ectoine to hydroxyectoine, was isolated as part of a DNA region that also contains a gene whose product belongs to the AraC-XylS family of transcriptional activators. Orthologs of ectD were found within the sequenced genomes of members of the proteobacteria, firmicutes, and actinobacteria, and their products were grouped into the ectoine hydroxylase subfamily, which was shown to belong to the superfamily of Fe(II)- and 2-oxoglutarate-dependent oxygenases. Analysis of the ectoine and hydroxyectoine contents of an ectABC ectD mutant strain fed with 1 mM ectoine or hydroxyectoine demonstrated that ectD is required for the main ectoine hydroxylase activity in C. salexigens. Although in minimal medium at 37 degrees C the wild-type strain grew with 0.5 to 3.0 M NaCl, with optimal growth at 1.5 M NaCl, at 45 degrees C it could not cope with the lowest (0.75 M NaCl) or the highest (3.0 M NaCl) salinity, and it grew optimally at 2.5 M NaCl. The ectD mutation caused a growth defect at 45 degrees C in minimal medium with 1.5 to 2.5 M NaCl, but it did not affect growth at 37 degrees C at any salinity tested. With 2.5 M NaCl, the ectD mutant synthesized 38% (at 37 degrees C) and 15% (at 45 degrees C) of the hydroxyectoine produced by the wild-type strain. All of these data reveal that hydroxyectoine synthesis mediated by the ectD gene is thermoregulated and essential for thermoprotection of C. salexigens.

Advances in preservation methods: keeping biosensor microorganisms alive and active

The ability of bacteria to sense their surroundings can be employed to measure the bioavailability and toxicity of pollutants. However, long-term maintenance of both viability and activity of the sensor bacteria is required for the development of cell-based devices for environmental monitoring. To meet these demands, various techniques to conserve such bacteria have been reported, including freeze drying, vacuum drying, continuous cultivation, and immobilisation in biocompatible polymers of organic or inorganic origin. Much effort has been invested in merging these bacterial preservation schemes with the construction of sensor cell arrays on platforms such as biochips or optic fibres, hopefully leading to effective miniaturised whole-cell biosensor systems. These approaches hold much promise for the future. Nevertheless, their eventual implementation in practical devices calls for significant enhancement of current knowledge on formulation of reporter microorganisms.

Ectoine and hydroxyectoine inhibit aggregation and neurotoxicity of Alzheimer's beta-amyloid

beta-Amyloid peptide (Abeta) is the major constituent of senile plaques, the key pathological feature of Alzheimer's disease. Abeta is physiologically produced as a soluble form, but aggregation of Abeta monomers into oligomers/fibrils causes neurotoxic change of the peptide. In nature, many microorganisms accumulate small molecule chaperones (SMCs) under stressful conditions to prevent the misfolding/denaturation of proteins and to maintain their stability. Hence, it is conceivable that SMCs such as ectoine and hydroxyectoine could be potential inhibitors against the aggregate formation of Alzheimer's Abeta, which has not been studied to date. The current work shows the effectiveness of ectoine and hydroxyectoine on the inhibition of Abeta42 aggregation and toxicity to human neuroblastoma cells. The characterization tools used for this study include thioflavin-T induced fluorescence, atomic force microscopy and cell viability assay. Considering that ectoine and hydroxyectoine are not toxic to cellular environment even at concentrations as high as 100 mM, the results may suggest a basis for the development of ectoines as potential inhibitors associated with neurodegenerative diseases.