Structural analysis of succinoglycan oligosaccharides from Sinorhizobium meliloti strains with different host compatibility phenotypes

Rhizobial oxidized 3-hydroxylbutanoyl glycan-based gelatin hydrogels with enhanced physiochemical properties for pH-responsive drug delivery

Rhizobial exopolysaccharide (EPS) is an acidic polysaccharide involved in nitrogen fixation-related signal transduction in the rhizosphere, serving as a structural support for biofilms, and protecting against various external environmental stresses. Rhizobial EPS as a hydrogel biomaterial was used for a pH-responsive drug delivery system combing with gelatins. Pure gelatin (GA) hydrogels have limited practical applications due to their poor mechanical strength and poor thermal stability. We developed new GA hydrogels using oxidized 3-hydroxylbutanoyl glycan (OHbG) as a polymer cross-linking agent to overcome these limitations. OHbG was synthesized from sodium periodate oxidation of 3-hydroxylbutanoyl glycan directly isolated from Rhizobium leguminosarum bv. viciae VF39. The newly fabricated OHbG/GA hydrogels exhibited 21-fold higher compressive stress and 4.7-fold higher storage modulus (G') than GA at the same strain. This result suggested that OHbG provided mechanical improvement. In addition, these OHbG/GA hydrogels showed effective pH-controlled drug release for 5-fluorouracil, self-healable, and self-antioxidant capacity by uronic acids of OHbG. Cell viability tests using HEK-293 cells in vitro also showed that the OHbG/GA hydrogels were non-toxic. This suggests that the new OHbG/GA hydrogels can be used as a potentially novel biomaterial for drug delivery based on its self-healing ability, antioxidant capacity, and pH-responsive drug delivery.

Physicochemical and Rheological Properties of Succinoglycan Overproduced by Sinorhizobium meliloti 1021 Mutant

Commercial bacterial exopolysaccharide (EPS) applications have been gaining interest; therefore, strains that provide higher yields are required for industrial-scale processes. Succinoglycan (SG) is a type of bacterial anionic exopolysaccharide produced by Rhizobium, Agrobacterium, and other soil bacterial species. SG has been widely used as a pharmaceutical, cosmetic, and food additive based on its properties as a thickener, texture enhancer, emulsifier, stabilizer, and gelling agent. An SG-overproducing mutant strain (SMC1) was developed from Sinorhizobium meliloti 1021 through N-methyl-N'-nitro-N-nitrosoguanidine (NTG) mutation, and the physicochemical and rheological properties of SMC1-SG were analyzed. SMC1 produced (22.3 g/L) 3.65-fold more SG than did the wild type. Succinoglycan (SMC1-SG) overproduced by SMC1 was structurally characterized by FT-IR and 1H NMR spectroscopy. The molecular weights of SG and SMC1-SG were 4.20 × 105 and 4.80 × 105 Da, respectively, as determined by GPC. Based on DSC and TGA, SMC1-SG exhibited a higher endothermic peak (90.9 °C) than that of SG (77.2 °C). Storage modulus (G') and loss modulus (G″) measurements during heating and cooling showed that SMC1-SG had improved thermal behavior compared to that of SG, with intersections at 74.9 °C and 72.0 °C, respectively. The SMC1-SG's viscosity reduction pattern was maintained even at high temperatures (65 °C). Gelation by metal cations was observed in Fe3+ and Cr3+ solutions for both SG and SMC1-SG. Antibacterial activities of SG and SMC1-SG against Escherichia coli and Staphylococcus aureus were also observed. Therefore, like SG, SMC1-SG may be a potential biomaterial for pharmaceutical, cosmetic, and food industries.

Novel succinoglycan dialdehyde/aminoethylcarbamoyl-β-cyclodextrin hydrogels for pH-responsive delivery of hydrophobic drugs

β-Cyclodextrin cross-linked succinoglycan dialdehyde hydrogels was prepared for hydrophobic drug delivery. Succinoglycan dialdehyde (SGDA) was synthesized from sodium periodate oxidation of succinoglycan isolated from Sinorhizobium meliloti Rm1021. Aminoethylcarbamoyl-β-cyclodextrin (ACD) was crosslinked with SGDA to form a succinoglycan dialdehyde/aminoethylcarbamoyl-β-cyclodextrin (SGDA/ACD) hydrogels. The SGDA/ACD hydrogels exhibited a 65.7 % improvement in storage modulus (G') and a 5.7-fold higher compressive strain than the SGDA/poly(ethylene glycol) diamine (PEG) hydrogels as controls. A hardly soluble drug, baicalein was used for the drug loading and release properties of SGDA/ACD hydrogels. Baicalein was released about 98 % within 48 h at pH 7.4, but not completely released even after 48 h at pH 2.0. In addition, at pH 7.4, only about 56 % of the baicalein loaded on the SGDA/PEG hydrogels was released within 48 h, while about 98 % of the baicalein loaded on the SGDA/ACD hydrogels was released within 48 h. It indicates that ACD significantly improved the solubilization efficacy of the baicalein. In vitro testing of cell viability using HEK-293 cells also showed that the SGDA/ACD hydrogels were suitable for the cells. In conclusion, SGDA/ACD hydrogels significantly enhance the utilization of baicalein and provide potential applications in drug delivery systems for hardly soluble drugs.

pH-Responsive Succinoglycan-Carboxymethyl Cellulose Hydrogels with Highly Improved Mechanical Strength for Controlled Drug Delivery Systems

Carboxymethyl cellulose (CMC)-based hydrogels are generally superabsorbent and biocompatible, but their low mechanical strength limits their application. To overcome these drawbacks, we used bacterial succinoglycan (SG), a biocompatible natural polysaccharide, as a double crosslinking strategy to produce novel interpenetrating polymer network (IPN) hydrogels in a non-bead form. These new SG/CMC-based IPN hydrogels significantly increased the mechanical strength while maintaining the characteristic superabsorbent property of CMC-based hydrogels. The SG/CMC gels exhibited an 8.5-fold improvement in compressive stress and up to a 6.5-fold higher storage modulus (G') at the same strain compared to the CMC alone gels. Furthermore, SG/CMC gels not only showed pH-controlled drug release for 5-fluorouracil but also did not show any cytotoxicity to HEK-293 cells. This suggests that SG/CMC hydrogels could be used as future biomedical biomaterials for drug delivery.

From Binary Model Systems to the Human Microbiome: Factors That Drive Strain Specificity in Host-Symbiont Associations

Microbial symbionts are ubiquitous and can have significant impact on hosts. These impacts can vary in the sign (positive or negative) and degree depending on the identity of the interacting partners. Studies on host-symbiont associations indicate that subspecies (strain) genetic variation can influence interaction outcomes, making it necessary to go beyond species-level distinction to understand host-symbiont dynamics. In this review, we discuss examples of strain specificity found in host-symbiont associations, from binary model systems to the human microbiome. Although host and bacterial factors identified as mediators for specificity could be distinct at the molecular level, they generally fall into two broad functional categories: (1) those that contribute a required activity in support of the association and (2) those involved in antagonistic interactions with organisms outside of the association. We argue here based on current literature that factors from these two categories can work in concert to drive strain specificity and that this strain specificity must be considered to fully understand the molecular and ecological dynamics of host-symbiont associations, including the human microbiome.

Fabrication and Characterization of Polysaccharide Metallohydrogel Obtained from Succinoglycan and Trivalent Chromium

In the present study, a polysaccharide metallohydrogel was successfully fabricated using succinoglycan and trivalent chromium and was verified via Fourier transform infrared spectroscopy, differential scanning calorimetry analysis, thermogravimetric analysis (TGA), field emission scanning electron microscopy, and rheological measurements. Thermal behavior analysis via TGA indicated that the final mass loss of pure succinoglycan was 87.8% although it was reduced to 65.8% by forming a hydrogel with trivalent chromium cations. Moreover, succinoglycan-based metallohydrogels exhibited improved mechanical properties based on the added concentration of Cr³⁺ and displayed a 10 times higher compressive stress and enhanced storage modulus (G′) of 230% at the same strain. In addition, the pore size of the obtained SCx could be adjusted by changing the concentration of Cr³⁺. Gelation can also be adjusted based on the initial pH of the metallohydrogel formulation. This was attributed to crosslinking between chromium trivalent ions and hydroxyl/carboxyl groups of succinoglycan, each of which exhibits a specific pH-dependent behavior in aqueous solutions. It could be used as a soft sensor to detect Cr³⁺ in certain biological systems, or as a soft matrix for bioseparation that allows control of pore size and mechanical strength by tuning the Cr³⁺ concentration.

Biocompatible and self-recoverable succinoglycan dialdehyde-crosslinked alginate hydrogels for pH-controlled drug delivery

We fabricated polysaccharide-based hydrogels, which are biocompatible, self-recoverable and pH-sensitive. Succinoglycan dialdehyde (SGDA) was first synthesized from bacterial succinoglycan directly isolated from Sinorhizobium meliloti and then hydrazine-functionalized alginate (HZ-Alg) was prepared to form SGDA-crosslinked alginate hydrogels (SGDA/HZ-Alg) without any catalyst. Due to structural characteristics of SGDA, SGDA/HZ-Alg were effectively obtained in a short time even at low concentrations (0.94-1.57 wt%) where they exhibited self-recoverable and tunable rheological properties corresponding to efficiency of recovery from 93.2%-97.9%. Moreover, SGDA/HZ-Alg showed the pH-responsive degradation as well as pH-controlled release behavior for 5-fluorouracil. 5-Fluorouracil was released approximately 98 % at pH 2.0 within 12 h, but not completely released even after 24 h at pH 7.4. The WST-8 assay results also demonstrated that SGDA/HZ-Alg did not show any cytotoxicity against HEK-293 cells. Since the suggested hydrogels are biocompatible, rheologically self-recoverable and tunable, and pH-controllable, they would be potential biomaterials for the hydrogel-based drug delivery systems.

Exploring the Role of Bacterial Extracellular Polymeric Substances for Sustainable Development in Agriculture

The incessant need to increase crop yields has led to the development of many chemical fertilizers containing NPK (nitrogen-phosphorous-potassium) which can degrade soil health in the long term. In addition, these fertilizers are often leached into nearby water bodies causing algal bloom and eutrophication. Bacterial secondary metabolites exuded into the extracellular space, termed extracellular polymeric substances (EPS) have gained commercial significance because of their biodegradability, non-toxicity, and renewability. In many habitats, bacterial communities faced with adversity will adhere together by production of EPS which also serves to bond them to surfaces. Typically, hygroscopic, EPS retain moisture in desiccating conditions and modulate nutrient exchange. Many plant growth-promoting bacteria (PGPR) combat harsh environmental conditions like salinity, drought, and attack of pathogens by producing EPS. The adhesive nature of EPS promotes soil aggregation and restores moisture thus combating soil erosion and promoting soil fertility. In addition, these molecules play vital roles in maintaining symbiosis and nitrogen fixation thus enhancing sustainability. Thus, along with other commercial applications, EPS show promising avenues for improving agricultural productivity thus helping to address land scarcity as well as minimizing environmental pollution.

Symbiotic Outcome Modified by the Diversification from 7 to over 700 Nodule-Specific Cysteine-Rich Peptides

Legume-rhizobium symbiosis represents one of the most successfully co-evolved mutualisms. Within nodules, the bacterial cells undergo distinct metabolic and morphological changes and differentiate into nitrogen-fixing bacteroids. Legumes in the inverted repeat lacking clade (IRLC) employ an array of defensin-like small secreted peptides (SSPs), known as nodule-specific cysteine-rich (NCR) peptides, to regulate bacteroid differentiation and activity. While most NCRs exhibit bactericidal effects in vitro, studies confirm that inside nodules they target the bacterial cell cycle and other cellular pathways to control and extend rhizobial differentiation into an irreversible (or terminal) state where the host gains control over bacteroids. While NCRs are well established as positive regulators of effective symbiosis, more recent findings also suggest that NCRs affect partner compatibility. The extent of bacterial differentiation has been linked to species-specific size and complexity of the NCR gene family that varies even among closely related species, suggesting a more recent origin of NCRs followed by rapid expansion in certain species. NCRs have diversified functionally, as well as in their expression patterns and responsiveness, likely driving further functional specialisation. In this review, we evaluate the functions of NCR peptides and their role as a driving force underlying the outcome of rhizobial symbiosis, where the plant is able to determine the outcome of rhizobial interaction in a temporal and spatial manner.

Succinoglycan dialdehyde-reinforced gelatin hydrogels with toughness and thermal stability

Pure gelatin hydrogel (PG) has limited practical applications due to their thermal instability and unfavorable mechanical properties. To overcome these limitations, dually crosslinked hydrogels were developed by imparting chemical crosslinking to existing physically crosslinked gelatin hydrogel networks using succinoglycan dialdehyde (SGDA) as a macromolecular crosslinker. SGDA-reinforced gelatin hydrogels (SGDA/Gels) displayed an 11 times higher compressive stress under identical deformation strain and a 1040% improvement in storage modulus (G') than PG. In addition, chemical crosslinking induced by SGDA increased the thermal stability of SGDA/Gels, such that they did not decompose at 60 °C, as confirmed by oscillatory temperature ramp experiments. The newly synthesized SGDA/Gels with reinforced networks and thermal stability exhibit potential for long-term use as controlled drug delivery carriers and 3D cell culture scaffolds for tissue engineering.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2013-2014

This review is the eighth update of the original article published in 1999 on the application of Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2014. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly- saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 37:353-491, 2018.

Synthesis of Rhizobial Exopolysaccharides and Their Importance for Symbiosis with Legume Plants

Rhizobia dwell and multiply in the soil and represent a unique group of bacteria able to enter into a symbiotic interaction with plants from the Fabaceae family and fix atmospheric nitrogen inside de novo created plant organs, called nodules. One of the key determinants of the successful interaction between these bacteria and plants are exopolysaccharides, which represent species-specific homo- and heteropolymers of different carbohydrate units frequently decorated by non-carbohydrate substituents. Exopolysaccharides are typically built from repeat units assembled by the Wzx/Wzy-dependent pathway, where individual subunits are synthesized in conjunction with the lipid anchor undecaprenylphosphate (und-PP), due to the activity of glycosyltransferases. Complete oligosaccharide repeat units are transferred to the periplasmic space by the activity of the Wzx flippase, and, while still being anchored in the membrane, they are joined by the polymerase Wzy. Here we have focused on the genetic control over the process of exopolysaccharides (EPS) biosynthesis in rhizobia, with emphasis put on the recent advancements in understanding the mode of action of the key proteins operating in the pathway. A role played by exopolysaccharide in Rhizobium-legume symbiosis, including recent data confirming the signaling function of EPS, is also discussed.

Structural and Functional Properties, Biosynthesis, and Patenting Trends of Bacterial Succinoglycan: A Review

The exopolysaccharide succinoglycan is produced mainly by a large number of soil microbes of Agrobacterium, Rhizobium or Pseudomonas genera etc. Structural properties of succinoglycan are unique in terms of its thermal stability and superior viscosifying property. Unlike the other highly commercialized bacterial exopolysaccharides like dextran or xanthan, mass scale application of succinoglycan has not been that much broadly explored yet. Bacterial succinoglycan is found suitable as a viscosifying and emulsifying agent in food industry, in gravel packing or fluid-loss control agent etc. In this present review, the key aspects of succinoglycan study, in particular, developments in structural characterizations, exo/exs operon system involved in biosynthesis pathway, commercial applications in food and other industries and patenting trends have been discussed.

Function of Succinoglycan Polysaccharide in Sinorhizobium meliloti Host Plant Invasion Depends on Succinylation, Not Molecular Weight

The acidic polysaccharide succinoglycan produced by the rhizobial symbiont Sinorhizobium meliloti 1021 is required for this bacterium to invade the host plant Medicago truncatula and establish a nitrogen-fixing symbiosis. S. meliloti mutants that cannot make succinoglycan cannot initiate invasion structures called infection threads in plant root hairs. S. meliloti exoH mutants that cannot succinylate succinoglycan are also unable to form infection threads, despite the fact that they make large quantities of succinoglycan. Succinoglycan produced by exoH mutants is refractory to cleavage by the glycanases encoded by exoK and exsH, and thus succinoglycan produced by exoH mutants is made only in the high-molecular-weight (HMW) form. One interpretation of the symbiotic defect of exoH mutants is that the low-molecular-weight (LMW) form of succinoglycan is required for infection thread formation. However, our data demonstrate that production of the HMW form of succinoglycan by S. meliloti 1021 is sufficient for invasion of the host M. truncatula and that the LMW form is not required. Here, we show that S. meliloti strains deficient in the exoK- and exsH-encoded glycanases invade M. truncatula and form a productive symbiosis, although they do this with somewhat less efficiency than the wild type. We have also characterized the polysaccharides produced by these double glycanase mutants and determined that they consist of only HMW succinoglycan and no detectable LMW succinoglycan. This demonstrates that LMW succinoglycan is not required for host invasion. These results suggest succinoglycan function is not dependent upon the presence of a small, readily diffusible form.

Sinorhizobium meliloti is a bacterium that forms a beneficial symbiosis with legume host plants. S. meliloti and other rhizobia convert atmospheric nitrogen to ammonia, a nutrient source for the host plant. To establish the symbiosis, rhizobia must invade plant roots, supplying the proper signals to prevent a plant immune response during invasion. A polysaccharide, succinoglycan, produced by S. meliloti is required for successful invasion. Here, we show that the critical feature of succinoglycan that allows infection to proceed is the attachment of a “succinyl” chemical group and that the chain length of succinoglycan is much less important for its function. We also show that none of the short-chain versions of succinoglycan is produced in the absence of two chain-cleaving enzymes.