Transient Accumulation of Glycine Betaine and Dynamics of Endogenous Osmolytes in Salt-Stressed Cultures of Sinorhizobium meliloti

Raman-Deuterium Isotope Probing and Metagenomics Reveal the Drought Tolerance of the Soil Microbiome and Its Promotion of Plant Growth

Drought has become a major agricultural threat leading crop yield loss. Although a few species of rhizobacteria have the ability to promote plant growth under drought, the drought tolerance of the soil microbiome and its relationship with the promotion of plant growth under drought are scarcely studied. This study aimed to develop a novel approach for assessing drought tolerance in agricultural land by quantitatively measuring microbial phenotypes using stable isotopes and Raman spectroscopy. Raman spectroscopy with deuterium isotope probing was used to identify the Raman signatures of drought effects from drought-tolerant bacteria. Counting drought-tolerant cells by applying these phenotypic properties to agricultural samples revealed that 0% to 52.2% of all measured single cells had drought-tolerant properties, depending on the soil sample. The proportions of drought-tolerant cells in each soil type showed similar tendencies to the numbers of revived pea plants cultivated under drought. The phenotype of the soil microbiome and plant behavior under drought conditions therefore appeared to be highly related. Studying metagenomics suggested that there was a reliable link between the phenotype and genotype of the soil microbiome that could explain mechanisms that promote plant growth in drought. In particular, the proportion of drought-tolerant cells was highly correlated with genes encoding phytohormone production, including tryptophan synthase and isopentenyl-diphosphate delta-isomerase; these enzymes are known to alleviate drought stress. Raman spectroscopy with deuterium isotope probing shows high potential as an alternative technology for quantitatively assessing drought tolerance through phenotypic analysis of the soil microbiome. IMPORTANCE Soil microbiome has played a critical role in the plant survival during drought. However, the drought tolerance of soil microbiome and its ability to promote plant growth under drought is still scarcely studied. In this study, we identified the Raman signature (i.e., phenotype) of drought effects from drought-tolerant bacteria in agricultural soil samples using Raman-deuterium isotope probing (Raman-DIP). Moreover, the number of drought-tolerant cells measured by Raman-DIP was highly related to the survival rate of plant cultivation under drought and the abundance of genes encoding phytohormone production alleviating drought stress in plant. These results suggest Raman-DIP is a promising technology for measuring drought tolerance of soil microbiome. This result give us important insight into further studies of a reliable link between phenotype and genotype of soil microbiome for future plant-bacteria interaction research.

Homeostasis and catabolism of choline and glycine betaine: lessons from Pseudomonas aeruginosa

Most sequenced bacteria possess mechanisms to import choline and glycine betaine (GB) into the cytoplasm. The primary role of choline in bacteria appears to be as the precursor to GB, and GB is thought to primarily act as a potent osmoprotectant. Choline and GB may play accessory roles in shaping microbial communities, based on their limited availability and ability to enhance survival under stress conditions. Choline and GB enrichment near eukaryotes suggests a role in the chemical relationships between these two kingdoms, and some of these interactions have been experimentally demonstrated. While many bacteria can convert choline to GB for osmoprotection, a variety of soil- and water-dwelling bacteria have catabolic pathways for the multistep conversion of choline, via GB, to glycine and can thereby use choline and GB as sole sources of carbon and nitrogen. In these choline catabolizers, the GB intermediate represents a metabolic decision point to determine whether GB is catabolized or stored as an osmo- and stress protectant. This minireview focuses on this decision point in Pseudomonas aeruginosa, which aerobically catabolizes choline and can use GB as an osmoprotectant and a nutrient source. P. aeruginosa is an experimentally tractable and ecologically relevant model to study the regulatory pathways controlling choline and GB homeostasis in choline-catabolizing bacteria. The study of P. aeruginosa associations with eukaryotes and other bacteria also makes this a powerful model to study the impact of choline and GB, and their associated regulatory and catabolic pathways, on host-microbe and microbe-microbe relationships.

Glycine betaine catabolism contributes to Pseudomonas syringae tolerance to hyperosmotic stress by relieving betaine-mediated suppression of compatible solute synthesis

Many bacteria can accumulate glycine betaine for osmoprotection and catabolize it as a growth substrate, but how they regulate these opposing roles is poorly understood. In Pseudomonas syringae B728a, expression of the betaine catabolism genes was reduced by an osmotic upshift to an intermediate stress level, consistent with betaine accumulation, but was increased by an upshift to a high stress level, as confirmed by an accompanying increase in degradation of radiolabeled betaine. Deletion of the gbcAB betaine catabolism genes reduced osmotolerance at a high osmolarity, and this reduction was due to the relief of betaine-mediated suppression of compatible solute synthesis. This conclusion was supported by the findings that, at high osmolarity, the ΔgbcAB mutant accumulated high betaine levels and low endogenous solutes and exhibited reduced expression of the solute synthesis genes. Moreover, the ΔgbcAB mutant and a mutant deficient in the synthesis of the compatible solutes NAGGN and trehalose exhibited similar reductions in osmotolerance and also in fitness on bean leaves. Activation of betaine catabolism at high osmotic stress resulted, in part, from induction of gbdR, which encodes the transcriptional activator GbdR. Betaine catabolism was subject to partial repression by succinate under hyperosmotic stress conditions, in contrast to strong repression in the absence of stress, suggesting that betaine functions both in nutrition and as an intracellular signal modulating solute synthesis under hyperosmotic stress conditions. Collectively, these results begin to provide a detailed mechanistic understanding of how P. syringae transitions from reliance on exogenously derived betaine to the use of endogenous solutes during adaptation to hyperosmotic conditions.

Role of trehalose in heat and desiccation tolerance in the soil bacterium Rhizobium etli

BACKGROUND

The compatible solute trehalose is involved in the osmostress response of Rhizobium etli, the microsymbiont of Phaseolus vulgaris. In this work, we reconstructed trehalose metabolism in R. etli, and investigated its role in cellular adaptation and survival to heat and desiccation stress under free living conditions.

RESULTS

Besides trehalose as major compatible solute, R. etli CE3 also accumulated glutamate and, if present in the medium, mannitol. Putative genes for trehalose synthesis (otsAB/treS/treZY), uptake (aglEFGK/thuEFGK) and degradation (thuAB/treC) were scattered among the chromosome and plasmids p42a, p42c, p42e, and p42f, and in some instances found redundant. Two copies of the otsA gene, encoding trehalose-6-P-synthase, were located in the chromosome (otsAch) and plasmid p42a (otsAa), and the latter seemed to be acquired by horizontal transfer. High temperature alone did not influence growth of R. etli, but a combination of high temperature and osmotic stress was more deleterious for growth than osmotic stress alone. Although high temperature induced some trehalose synthesis by R. etli, trehalose biosynthesis was mainly triggered by osmotic stress. However, an otsAch mutant, unable to synthesize trehalose in minimal medium, showed impaired growth at high temperature, suggesting that trehalose plays a role in thermoprotection of R. etli. Desiccation tolerance by R. etli wild type cells was dependent of high trehalose production by osmotic pre-conditioned cells. Cells of the mutant strain otsAch showed ca. 3-fold lower survival levels than the wild type strain after drying, and a null viability after 4 days storage.

CONCLUSIONS

Our findings suggest a beneficial effect of osmotic stress in R. etli tolerance to desiccation, and an important role of trehalose on the response of R. etli to high temperature and desiccation stress.

Studies of dimethylglycine oxidase isoenzymes in Arthrobacter globiformis cells

Glycine betaine (GB) could be used by Arthrobacter globiformis cells as a sole carbon source. The cells took up this molecule in the low as well as in the high salinity medium. Addition of GB to the mineral medium with high salt concentration revealed that GB was also used as an osmoprotectant. Dimethylglycine oxidase (DMGO) was involved in the catabolism of GB. Two genes for DMGO were detected in a cloned 26267 bp fragment of A. globiformis DNA. The genes involved in the tetrahydrofolate-dependent assimilation of methyl groups were located nearby the two of DMGO genes. Both cloned A. globiformis DMGO were active. The activity of DMGO was detected in A. globiformis cells and it depended on the addition of GB and the salinity of the medium. Reverse transcription-PCR demonstrated that the addition of GB influenced the transcription of dmg genes.

Osmotically induced synthesis of the dipeptide N-acetylglutaminylglutamine amide is mediated by a new pathway conserved among bacteria

The dipeptide N-acetylglutaminylglutamine amide (NAGGN) was discovered in the bacterium Sinorhizobium meliloti grown at high osmolarity, and subsequently shown to be synthesized and accumulated by a few osmotically challenged bacteria. However, its biosynthetic pathway remained unknown. Recently, two genes, which putatively encode a glutamine amidotransferase and an acetyltransferase and are up-regulated by osmotic stress, were identified in Pseudomonas aeruginosa. In this work, a locus carrying the orthologous genes in S. meliloti, asnO and ngg, was identified, and the genetic and molecular characterization of the NAGGN biosynthetic pathway is reported. By using NMR experiments, it was found that strains inactivated in asnO and ngg were unable to produce the dipeptide. Such inability has a deleterious effect on S. meliloti growth at high osmolarity, demonstrating the key role of NAGGN biosynthesis in cell osmoprotection. beta-Glucuronidase activity from transcriptional fusion revealed strong induction of asnO expression in cells grown in increased NaCl concentration, in good agreement with the NAGGN accumulation. The asnO-ngg cluster encodes a unique enzymatic machinery mediating nonribosomal peptide synthesis. This pathway first involves Ngg, a bifunctional enzyme that catalyzes the formation of the intermediate N-acetylglutaminylglutamine, and second AsnO, required for subsequent addition of an amide group and the conversion of N-acetylglutaminylglutamine into NAGGN. Interestingly, a strong conservation of the asnO-ngg cluster is observed in a large number of bacteria with different lifestyles, such as marine, symbiotic, and pathogenic bacteria, highlighting the ecological importance of NAGGN synthesis capability in osmoprotection and also potentially in bacteria host-cell interactions.

RpoE2 of Sinorhizobium meliloti is necessary for trehalose synthesis and growth in hyperosmotic media

Adaptation to osmotic stress can be achieved by the accumulation of compatible solutes that aid in turgor maintenance and macromolecule stabilization. The genetic regulation of solute accumulation is poorly understood, and has been described well at the molecular level only in enterobacteria. In this study, we show the importance of the alternative sigma factor RpoE2 in Sinorhizobium meliloti osmoadaptation. Construction and characterization of an S. meliloti rpoE2 mutant revealed compromised growth in hyperosmotic media. This defect was due to the lack of trehalose, a minor carbohydrate osmolyte normally produced in the initial stages of growth and in stationary phase. We demonstrate here that all three trehalose synthesis pathways are RpoE2 dependent, but only the OtsA pathway is important for osmoinducible trehalose synthesis. Furthermore, we confirm that the absence of RpoE2-dependent induction of otsA is the cause of the osmotic phenotype of the rpoE2 mutant. In conclusion, we have highlighted that, despite its low level, trehalose is a crucial compatible solute in S. meliloti, and the OtsA pathway induced by RpoE2 is needed for its accumulation under hyperosmotic conditions.

Importance of trehalose biosynthesis for Sinorhizobium meliloti Osmotolerance and nodulation of Alfalfa roots

The disaccharide trehalose is a well-known osmoprotectant, and trehalose accumulation through de novo biosynthesis is a common response of bacteria to abiotic stress. In this study, we have investigated the role of endogenous trehalose synthesis in the osmotolerance of Sinorhizobium meliloti. Genes coding for three possible trehalose synthesis pathways are present in the genome of S. meliloti 1021: OtsA, TreYZ, and TreS. Among these, OtsA has a major role in trehalose accumulation under all of the conditions tested and is the main system involved in osmoadaptation. Nevertheless, the other two systems are also important for growth in hyperosmotic medium. Genes for the three pathways are transcriptionally responsive to osmotic stress. The presence of at least one functional trehalose biosynthesis pathway is required for optimal competitiveness of S. meliloti to nodulate alfalfa roots.

An orphan LuxR homolog of Sinorhizobium meliloti affects stress adaptation and competition for nodulation

The Sin/ExpR quorum-sensing system of Sinorhizobium meliloti plays an important role in the symbiotic association with its host plant, Medicago sativa. The LuxR-type response regulators of the Sin system include the synthase (SinI)-associated SinR and the orphan regulator ExpR. Interestingly, the S. meliloti Rm1021 genome codes for four additional putative orphan LuxR homologs whose regulatory roles remain to be identified. These response regulators contain the characteristic domains of the LuxR family of proteins, which include an N-terminal autoinducer/response regulatory domain and a C-terminal helix-turn-helix domain. This study elucidates the regulatory role of one of the orphan LuxR-type response regulators, NesR. Through expression and phenotypic analyses, nesR was determined to affect the active methyl cycle of S. meliloti. Moreover, nesR was shown to influence nutritional and stress response activities in S. meliloti. Finally, the nesR mutant was deficient in competing with the wild-type strain for plant nodulation. Taken together, these results suggest that NesR potentially contributes to the adaptability of S. meliloti when it encounters challenges such as high osmolarity, nutrient starvation, and/or competition for nodulation, thus increasing its chances for survival in the stressful rhizosphere.

Effect of caspofungin on metabolite profiles of Aspergillus species determined by nuclear magnetic resonance spectroscopy

Invasive aspergillosis remains a potentially life-threatening infection, the incidence of which is increasing. Current methods used to determine the susceptibilities of Aspergillus strains to antifungal drugs are often unreliable. Nuclear magnetic resonance (NMR) spectroscopy can identify the metabolic complement of microorganisms while monitoring nutrient utilization from the incubation medium. We used 600-MHz (1)H NMR spectroscopy to monitor the metabolic responses of five Aspergillus species cultured in RPMI 1640-2% glucose-morpholinepropanesulfonate buffer to various concentrations of the antifungal drugs amphotericin B (AMB) and caspofungin. The metabolic endpoint (MEP) was determined from nutrient and metabolite resonances, measured as a function of the drug concentration, and was defined as a > or =50% reduction in nutrient consumption or metabolite production. MICs were evaluated by a modification of Clinical and Laboratory Standards Institute broth microdilution method M27-A, and minimal effective concentrations (MECs) were determined by microscopic examination of fungal hyphae. For AMB, the MEPs coincided with the MICs. For caspofungin, the MEPs agreed with the MECs for several Aspergillus strains, but the effect of drug pressure was more complex for others. Expansion of the MEP definition to include any significant changes in metabolite production resulted in agreement with the MEC in most cases. Paradoxical metabolic responses were observed for several Aspergillus strains at either high or low caspofungin concentrations and for one Aspergillus terreus strain with AMB. NMR spectroscopy proved to be a powerful tool for detecting the subtle effects of drug pressure on fungal metabolism and has the potential to provide an alternative method for determining the susceptibilities of Aspergillus species to antifungal drugs.

Medicago truncatula root nodule proteome analysis reveals differential plant and bacteroid responses to drought stress

Drought is one of the environmental factors most affecting crop production. Under drought, symbiotic nitrogen fixation is one of the physiological processes to first show stress responses in nodulated legumes. This inhibition process involves a number of factors whose interactions are not yet understood. This work aims to further understand changes occurring in nodules under drought stress from a proteomic perspective. Drought was imposed on Medicago truncatula 'Jemalong A17' plants grown in symbiosis with Sinorhizobium meliloti strain 2011. Changes at the protein level were analyzed using a nongel approach based on liquid chromatography coupled to tandem mass spectrometry. Due to the complexity of nodule tissue, the separation of plant and bacteroid fractions in M. truncatula root nodules was first checked with the aim of minimizing cross contamination between the fractions. Second, the protein plant fraction of M. truncatula nodules was profiled, leading to the identification of 377 plant proteins, the largest description of the plant nodule proteome so far. Third, both symbiotic partners were independently analyzed for quantitative differences at the protein level during drought stress. Multivariate data mining allowed for the classification of proteins sets that were involved in drought stress responses. The isolation of the nodule plant and bacteroid protein fractions enabled the independent analysis of the response of both counterparts, gaining further understanding of how each symbiotic member is distinctly affected at the protein level under a water-deficit situation.

Trehalose biosynthesis in Rhizobium leguminosarum bv. trifolii and its role in desiccation tolerance

Rhizobium leguminosarum bv. trifolii forms nitrogen-fixing root nodules on the pasture legume Trifolium repens, and T. repens seed is often coated with a compatible R. leguminosarum bv. trifolii strain prior to sowing. However, significant losses in bacterial viability occur during the seed-coating process and during storage of the coated seeds, most likely due to desiccation stress. The disaccharide trehalose is known to function as an osmoprotectant, and trehalose accumulation due to de novo biosynthesis is a common response to desiccation stress in bacteria. In this study we investigated the role of endogenous trehalose synthesis in desiccation tolerance in R. leguminosarum bv. trifolii strain NZP561. Strain NZP561 accumulated trehalose as it entered the stationary phase due to the combined actions of the TreYZ and OtsAB pathways. Mutants deficient in either pathway showed near-wild-type levels of trehalose accumulation, but double otsA treY mutants failed to accumulate any trehalose. The double mutants were more sensitive to the effects of drying, and their survival was impaired compared to that of the wild type when glass beads were coated with the organisms and stored at relative humidities of 5 and 32%. The otsA treY mutants were also less competitive for nodule occupancy. Gene expression studies showed that the otsA and treY genes were expressed constitutively and that expression was not influenced by the growth phase, suggesting that trehalose accumulation is controlled at the posttranscriptional level or by control of trehalose breakdown rates. Our results indicate that accumulated trehalose plays an important role in protecting R. leguminosarum bv. trifolii cells against desiccation stress and against stress encountered during nodulation.

Osmoadaptative responses in the rhizobia nodulating Acacia isolated from south-eastern Moroccan Sahara

Four strains of rhizobia nodulating Acacia were isolated from the Moroccan desert soil by trapping with seedlings of Acacia gummifera and Acacia raddiana, and were studied for their ability to tolerate high salinity and dryness conditions. The strains MDSMC 2, MDSMC 18 and MDSMC 50 were halotolerant (they tolerated up to 1 M NaCl) and they accumulated glutamate and mannosucrose. The synthesis of the latter solute, which is the major endogenous osmolyte, is partially repressed in the presence of glycine betaine. The strain MDSMC 34 was less halotolerant (growth inhibited by a concentration greater than 0.5 M NaCl), and accumulated trehalose (as the main endogenous osmolyte) and glutamate. Rhizobia from the Moroccan desert soil were highly resistant to desiccation and their tolerance to dryness was stimulated by osmotic pretreatment. Thus, the accumulation of mannosucrose or trehalose by desert rhizobia represents both an osmoadaptative response and a part of a desiccation tolerance mechanism.

Interrelations between glycine betaine catabolism and methionine biosynthesis in Sinorhizobium meliloti strain 102F34

Methionine is produced by methylation of homocysteine. Sinorhizobium meliloti 102F34 possesses only one methionine synthase, which catalyzes the transfer of a methyl group from methyl tetrahydrofolate to homocysteine. This vitamin B(12)-dependent enzyme is encoded by the metH gene. Glycine betaine can also serve as an alternative methyl donor for homocysteine. This reaction is catalyzed by betaine-homocysteine methyl transferase (BHMT), an enzyme that has been characterized in humans and rats. An S. meliloti gene whose product is related to the human BHMT enzyme has been identified and named bmt. This enzyme is closely related to mammalian BHMTs but has no homology with previously described bacterial betaine methyl transferases. Glycine betaine inhibits the growth of an S. meliloti bmt mutant in low- and high-osmotic strength media, an effect that correlates with a decrease in the catabolism of glycine betaine. This inhibition was not observed with other betaines, like homobetaine, dimethylsulfoniopropionate, and trigonelline. The addition of methionine to the growth medium allowed a bmt mutant to recover growth despite the presence of glycine betaine. Methionine also stimulated glycine betaine catabolism in a bmt strain, suggesting the existence of another catabolic pathway. Inactivation of metH or bmt did not affect the nodulation efficiency of the mutants in the 102F34 strain background. Nevertheless, a metH strain was severely defective in competing with the wild-type strain in a coinoculation experiment.

Succinate-mediated catabolite repression control on the production of glycine betaine catabolic enzymes in Pseudomonas aeruginosa PAO1 under low and elevated salinities

Glycine betaine (GB) and its immediate precursors choline and carnitine, dimethylsulfonioacetate, dimethylsulfoniopropionate, ectoine and proline were effective osmoprotectants for Pseudomonas aeruginosa, but pipecolate, trehalose and sucrose had no osmoprotective effect. GB was accumulated stably or transiently when succinate or glucose, respectively, was used as a carbon and energy source. The catabolite repression mediated by succinate occurred at both low and high salinities, and it did not involve the global regulators Vfr and Crc. A proteomic analysis showed that at least 21 proteins were induced when GB was used as a carbon and energy source, and provided evidence that succinate repressed the synthesis of all these proteins. Many of the proteins induced by GB (sarcosine oxidase, serine hydroxymethyltransferase and serine dehydratase) are involved in GB catabolism. In addition, GB uptake was stimulated at high medium osmolalities but it was insensitive to catabolite repression by succinate. Despite its ability to inhibit betaine catabolism, succinate did not allow any better growth of P. aeruginosa cells under hyperosmotic constraint. Conversely, as observed for cells supplied with glucose, a transient accumulation of GB was sufficient to provide a significant cell osmoprotection.

Ectoines as compatible solutes and carbon and energy sources for the halophilic bacterium Chromohalobacter salexigens

AIMS

To investigate the catabolism of ectoine and hydroxyectoine, which are the major compatible solutes synthesized by Chromohalobacter salexigens.

METHODS AND RESULTS

Growth curves performed in M63 minimal medium with low (0.75 mol l(-1) NaCl), optimal (1.5 mol l(-1) NaCl) or high (2.5 mol l(-1) NaCl) salinity revealed that betaine and ectoines were used as substrate for growth at optimal and high salt. Ectoine transport was maximal at optimal salinity, and showed 3- and 1.5-fold lower values at low and high salinity respectively. The salt-sensitive ectA mutant CHR62 showed an ectoine transport rate 6.8-fold higher than that of the wild type. Incubation of C. salexigens in a mixture of glucose and ectoine resulted in a biphasic growth pattern. However, CO(2) production due to ectoine catabolism was lower, but not completely abolished, in the presence of glucose. When used as the sole carbon source, glycine betaine effectively inhibited ectoine and hydroxyectoine synthesis at any salinity.

CONCLUSIONS

The catabolic pathways for ectoine and hydroxyectoine in C. salexigens operate at optimal and high (although less efficiently) salinity. Endogenous ectoine(s) may repress its own transport. Ectoine utilization was only partially repressed by glucose. Betaine, when used as carbon source, suppresses synthesis of ectoines even under high osmolarity conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is a previous step to the subsequent isolation and manipulation of the catabolic genes, so as to generate strains with enhanced production of ectoine and hydroxyectoine.

Ectoine-induced proteins in Sinorhizobium meliloti include an Ectoine ABC-type transporter involved in osmoprotection and ectoine catabolism

To understand the mechanisms of ectoine-induced osmoprotection in Sinorhizobium meliloti, a proteomic examination of S. meliloti cells grown in minimal medium supplemented with ectoine was undertaken. This revealed the induction of 10 proteins. The protein products of eight genes were identified by using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. Five of these genes, with four other genes whose products were not detected on two-dimensional gels, belong to the same gene cluster, which is localized on the pSymB megaplasmid. Four of the nine genes encode the characteristic components of an ATP-binding cassette transporter that was named ehu, for ectoine/hydroxyectoine uptake. This transporter was encoded by four genes (ehuA, ehuB, ehuC, and ehuD) that formed an operon with another gene cluster that contains five genes, named eutABCDE for ectoine utilization. On the basis of sequence homologies, eutABCDE encode enzymes with putative and hypothetical functions in ectoine catabolism. Analysis of the properties of ehuA and eutA mutants suggests that S. meliloti possesses at least one additional ectoine catabolic pathway as well as a lower-affinity transport system for ectoine and hydroxyectoine. The expression of ehuB, as determined by measurements of UidA activity, was shown to be induced by ectoine and hydroxyectoine but not by glycine betaine or by high osmolality.

Biosynthesis of exopolysaccharide by a Bacillus licheniformis strain isolated from ropy cider

A strain of Bacillus licheniformis displaying a ropy phenotype was isolated from a French ropy cider. The influence of culture conditions on the production of exopolysaccharide (EPS) was investigated. When B. licheniformis was grown in Man, Rogosa and Sharpe (MRS) medium, the highest amount of EPS was observed at mid exponential growth phase whatever the carbon source, glucose, fructose or sucrose. Interestingly at mid exponential growth phase, EPS amounts did not increase with increasing sugar concentrations. Incubation of B. licheniformis cells in media supplemented with ethanol (1-7%, v/v) revealed that EPS production was enhanced by the presence of ethanol, in exponential as well as in stationary phase. High Performance Liquid Chromatography (HPLC) and Nuclear Magnetic Resonance (NMR) analysis of EPS composition indicated that it was a heteropolymer in which mannose was the predominant monosaccharide as it constituted more than 80% of total polysaccharide.

Uptake of choline from salmon flesh and its conversion to glycine betaine in response to salt stress in Shewanella putrefaciens

When cultured in M63 minimal medium plus 0.6 M NaCl, the growth of Shewanella putrefaciens was strongly inhibited. The addition of an extract from smoked salmon to this medium restored the growth almost to the unstressed level. A comparison of the 13C NMR spectra of intracellular solutes extracted from S. putrefaciens cells cultured in both conditions revealed the accumulation of glycine betaine (GB) from the smoked salmon extract (SSE). Analysis of the osmoprotective properties of this extract for several strains of Escherichia coli (which differ from each other in their ability to accumulate GB (i) from the surrounding environment, and (ii) from its hydroxylated precursor choline), demonstrated the absence of GB in the SSE. From the overall results, we inferred that salt-stressed S. putrefaciens cells accumulated GB from choline present in the SSE. Furthermore, the use of [14C]-labeled betaines gave evidence that S. putrefaciens (i) oxidised choline to GB, (ii) accumulated GB as a non-metabolisable osmolyte (up to 1300 nmol (mg dw)(-1) when cultured in a medium containing 0.5 M NaCl and either 1 mM choline or 1 mM GB), and (iii) both choline and GB uptake activities were osmotically upregulated (both activities were increased more than 50-fold in media containing 0.4 to 0.6 M NaCl). In all, our results suggest that in salted smoked salmon, S. putrefaciens imports and oxidises choline, leading to the intracellular accumulation of GB.

Synthesis, accumulation and hydrolysis of trehalose during growth of peanut rhizobia in hyperosmotic media

We examined and compared the activities of synthetic and hydrolytic enzymes involved in trehalose metabolism, in three peanut rhizobia strains grown in control, hypersaline, and non-ionic hyperosmotic media. Results indicated that the effects of hyperosmolarity on the synthesis and the degradation of the disaccharide were diverse. In the salt-tolerant slow-growing strain Bradyrhizobium sp. ATCC 10317, we observed increased synthesis and accumulation of trehalose under hyperosmolarity imposed by either NaCl or PEG-8000. In the other two peanut rhizobia strains, the disaccharide level did not change under hypersalinity. In the salt-sensitive slow-growing strain Bradyrhizobium sp. USDA 3187, intracellular trehalose diminished in late stationary phase-cells grown with PEG, this reduction was accompanied by both an increased activity of synthetic enzymes and a decreased activity of trehalase. In the salt-tolerant fast-growing strain Rhizobium sp. TAL 1000, we also observed a reduction of intracellular trehalose under PEG-mediated growth, this decrease was early and transiently accompanied by an enhancement of trehalase activity, afterwards, the activity of synthetic enzymes augmented.

Osmoprotection by pipecolic acid in Sinorhizobium meliloti: specific effects of D and L isomers

DL-Pipecolic acid (DL-PIP) promotes growth restoration of Sinorhizobium meliloti cells facing inhibitory hyperosmolarity. Surprisingly, D and L isomers of this imino acid supplied separately were not effective. The uptake of L-PIP was significantly favored in the presence of the D isomer and by a hyperosmotic stress. Chromatographic analysis of the intracellular solutes showed that stressed cells did not accumulate radiolabeled L-PIP. Rather, it participates in the synthesis of the main endogenous osmolytes (glutamate and the dipeptide N-acetylglutaminylglutamine amide) during the lag phase, thus providing a means for the stressed cells to recover the osmotic balance. (13)C nuclear magnetic resonance analysis was used to determine the fate of D-PIP taken into the cells. In the absence of L-PIP, the imported D isomer was readily degraded. Supplied together with its L isomer, D-PIP was accumulated temporarily and thus might contribute together with the endogenous osmolytes to enhance the internal osmotic strength. Furthermore, it started to disappear from the cytosol when the L isomer was no longer available in the culture medium (during the late exponential phase of growth). Together, these results show an uncommon mechanism of protection of osmotically stressed cells of S. meliloti. It was proved, for the first time, that the presence of the two isomers of the same molecule is necessary for it to manifest an osmoprotective activity. Indeed, D-PIP seems to play a major role in cellular osmoadaptation through both its own accumulation and improvement of the utilization of the L isomer as an immediate precursor of endogenous osmolytes.

Glycine betaine, carnitine, and choline enhance salinity tolerance and prevent the accumulation of sodium to a level inhibiting growth of Tetragenococcus halophila

Natural-abundance (13)C-nuclear magnetic resonance was used to probe the intracellular organic solute content of the moderately halophilic bacterium Tetragenococcus halophila. When grown in complex growth media supplemented or not with NaCl, T. halophila accumulates glycine betaine and carnitine. Unlike other moderate halophiles, T. halophila was not able to produce potent osmoprotectants (such as ectoines and glycine betaine) through de novo synthesis when cultured in defined medium under hyperosmotic constraint. Addition of 2 mM carnitine, glycine betaine, or choline to defined medium improved growth parameters, not only at high salinity (up to 2.5 M NaCl) but also in media lacking NaCl. These compounds were taken up when available in the surrounding medium. The transport activity occurred at low and high salinities and seems to be constitutive. Glycine betaine and carnitine were accumulated by T. halophila in an unmodified form, while exogenously provided choline led to an intracellular accumulation of glycine betaine. This is the first evidence of the existence of a choline-glycine betaine pathway in a lactic acid bacterium. An assay showed that the compatible solutes strikingly repressed the accumulation of glutamate and slightly increased the intracellular potassium level only at high salinity. Interestingly, osmoprotectant-treated cells were able to maintain the intracellular sodium concentration at a relatively constant level (200 to 300 nmol/mg [dry weight]), independent of the NaCl concentration of the medium. In contrast, in the absence of osmoprotectant, the intracellular sodium content increased sharply from 200 to 2,060 nmol/mg (dry weight) when the salinity of the medium was raised from 1 to 2 M. Indeed, the imported compatible solutes play an actual role in regulating the intracellular Na(+) content and confer a much higher salt tolerance to T. halophila.

Disaccharides as a new class of nonaccumulated osmoprotectants for Sinorhizobium meliloti

Sucrose and ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidine carboxylic acid) are very unusual osmoprotectants for Sinorhizobium meliloti because these compounds, unlike other bacterial osmoprotectants, do not accumulate as cytosolic osmolytes in salt-stressed S. meliloti cells. Here, we show that, in fact, sucrose and ectoine belong to a new family of nonaccumulated sinorhizobial osmoprotectants which also comprises the following six disaccharides: trehalose, maltose, cellobiose, gentiobiose, turanose, and palatinose. Also, several of these disaccharides were very effective exogenous osmoprotectants for strains of Rhizobium leguminosarum biovars phaseoli and trifolii. Sucrose and trehalose are synthesized as endogenous osmolytes in various bacteria, but the other five disaccharides had never been implicated before in osmoregulation in any organism. All of the disaccharides that acted as powerful osmoprotectants in S. meliloti and R. leguminosarum also acted as very effective competitors of [14C]sucrose uptake in salt-stressed cultures of these bacteria. Conversely, disaccharides that were not osmoprotective for S. meliloti and R. leguminosarum did not inhibit sucrose uptake in these bacteria. Hence, disaccharide osmoprotectants apparently shared the same uptake routes in these bacteria. Natural-abundance 13C nuclear magnetic resonance spectroscopy and quantification of cytosolic solutes demonstrated that the novel disaccharide osmoprotectants were not accumulated to osmotically significant levels in salt-stressed S. meliloti cells; rather, these compounds, like sucrose and ectoine, were catabolized during early exponential growth, and contributed indirectly to enhance the cytosolic levels of two endogenously synthesized osmolytes, glutamate and the dipeptide N-acetylglutaminylglutamine amide. The ecological implication of the use of these disaccharides as osmoprotectants is discussed.

Toxicity and osmoprotective activities of analogues of glycine betaine obtained by solid phase organic synthesis towards Sinorhizobium meliloti

Seven analogues of the bacterial osmoprotectant glycine betaine (GB, trimethylammonioacetate), in which the methyl groups of the Me3N+ moiety are replaced by various substituents, were obtained by SPOS using Wang resin. Their biological activities (osmoprotection vs toxicity), appeared closely related to their uptake efficiency and their catabolism in the betaine-demethylating model bacterium Sinorhizobium meliloti.

Sucrose is a nonaccumulated osmoprotectant in Sinorhizobium meliloti

Intracellular accumulation of sucrose in response to lowered water activity seems to occur only in photosynthetic organisms. Here we demonstrate, for the first time, the potent ability of this common sugar, supplied exogenously, to reduce growth inhibition of Sinorhizobium meliloti cells in media of inhibitory osmolarity. Independently of the nature of the growth substrates and the osmotic agent, sucrose appears particularly efficient in promoting the recovery of cytoplasmic volume after plasmolysis. Surprisingly, sucrose is not accumulated by the bacteria at an osmotically efficient level. Instead, it strongly stimulates the accumulation of the main endogenous osmolytes glutamate and N-acetylglutaminylglutamine amide (NAGGN). Examining cell volume changes during the hyperosmotic treatment, we found a close correlation between the enhancement of the osmotically active solute pool and the increase in cell volume. Sucrose shares several features with ectoine, another nonaccumulated osmoprotectant for S. meliloti. Overall, osmoregulation in S. meliloti appears to be strongly divergent from that in most bacteria.

Intracellular changes in ions and organic solutes in halotolerant brevibacterium sp. Strain JCM 6894 after exposure to hyperosmotic shock

In the present study we aimed to observe the intracellular responses when there was a hyperosmotic shock with a large shift in ionic strength in nutrient-rich and nutrient-poor external environments in order to clarify the availability of substrates. To do this, we used the halotolerant organism Brevibacterium sp. strain JCM 6894, which is able to grow in the presence of a wide range of salt concentrations. Hyperosmotic shock was induced by transferring cells in the late exponential phase of growth in a complex medium containing 0.5 M NaCl into either old or fresh culture medium containing 2 M NaCl. Changes in the growth rate, in the pH of the medium, and in the internal cation or organic solute concentrations in the cytosol after an upshock were analyzed as a function of incubation time. The cells exhibited very different responses to upshocks in fresh culture medium and in old culture medium; in fresh culture medium, growth was stimulated and the medium became more acidic, whereas the old culture medium repressed growth and the medium became more alkaline. The intracellular free Na+ concentrations remained low (80 nmol mg of protein-1) after an upshock in fresh culture medium, although they quickly increased twofold in the old culture medium. In contrast, K+ ions immediately accumulated in the cells in fresh culture medium, whereas K+ ions were taken up quite slowly in old culture medium. Furthermore, the cells placed in fresh culture medium transiently accumulated alanine and glutamine in response to the upshock, but the cells placed in old culture medium did not. Growth of the Brevibacterium strain at higher levels of salinity was supported by ectoine synthesis but was not observed after the shift to high-osmolarity conditions in the old culture. In the fresh culture, however, ectoine was vigorously synthesized in cells for more than 5 h after the upshock; the concentration of ectoine in cells was more than 3,500 nmol mg of protein-1 at 10 h, which corresponded to a ninefold increase compared to the concentration before the shock. These findings are consistent with the results of an analysis of the extracellular medium composition before and after the upshock.

Differential Effects of Dimethylsulfoniopropionate, Dimethylsulfonioacetate, and Other S-Methylated Compounds on the Growth of Sinorhizobium meliloti at Low and High Osmolarities

An extract from the marine alga Ulva lactuca was highly osmoprotective in salt-stressed cultures of Sinorhizobium meliloti 102F34. This beneficial activity was due to algal 3-dimethylsulfoniopropionate (DMSP), which was accumulated as a dominant compatible solute and strongly reduced the accumulation of endogenous osmolytes in stressed cells. Synthetic DMSP also acted as a powerful osmoprotectant and was accumulated as a nonmetabolizable cytosolic osmolyte (up to a concentration of 1,400 nmol/mg of protein) throughout the growth cycles of the stressed cultures. In contrast, 2-dimethylsulfonioacetate (DMSA), the sulfonium analog of the universal osmoprotectant glycine betaine (GB), was highly toxic to unstressed cells and was not osmoprotective in stressed cells of wild-type strains of S. meliloti . Nonetheless, the transport and accumulation of DMSA, like the transport and accumulation of DMSP and GB, were osmoregulated and increased fourfold in stressed cells of strain 102F34. Strikingly, DMSA was not toxic and became highly osmoprotective in mutants that are impaired in their ability to demethylate GB and DMSA. Furthermore, 2-methylthioacetate and thioglycolic acid (TGA), the demethylation products of DMSA, were excreted, apparently as a mechanism of cellular detoxification. Also, exogenous TGA and DMSA displayed similar inhibitory effects in strain 102F34. Thus, on the basis of these findings and other physiological and biochemical evidence, we infer that the toxicity of DMSA in wild-type strains of S. meliloti stems from its catabolism via the GB demethylation pathway. This is the first report describing the toxicity of DMSA in any organism and a metabolically stable osmoprotectant (DMSP) in S. meliloti.