NMR studies of acetan and the related bacterial polysaccharide, CR1/4, produced by a mutant strain of Acetobacter xylinum

Systematic optimization of exopolysaccharide production by Gluconacetobacter sp. and use of (crude) glycerol as carbon source

The usage of polysaccharides as biodegradable polymers is of growing interest in the context of a sustainable and ecofriendly economy. For this, the production of exopolysaccharides (EPS) by Gluconacetobacter sp. was investigated. Glycerol as carbon source revealed to be beneficial compared to glucose. In addition, pure glycerol could be substituted by a crude glycerol waste stream from biodiesel production. Systematic analysis of the peptone and phosphate concentrations in glycerol-based media indicated a strong effect of peptone. Optimized parameters resulted in a titer of 25.4 ± 2.4 g/L EPS with a productivity of 0.46 ± 0.04 g*(L*h)^-1. With decreasing peptone, a variation in the monomer ratios was observed. An accompanying change in molecular size distribution indicated the production of two different polysaccharides. Intensified analysis revealed the main polysaccharide to be composed of glucose (Glc), galactose (Gal), mannose (Man) and glucuronic acid (GlcA), and the minor polysaccharide of Gal, Man, ribose (Rib).

Acetan and Acetan-Like Polysaccharides: Genetics, Biosynthesis, Structure, and Viscoelasticity

Bacteria produce a variety of multifunctional polysaccharides, including structural, intracellular, and extracellular polysaccharides. They are attractive for the industrial sector due to their natural origin, sustainability, biodegradability, low toxicity, stability, unique viscoelastic properties, stable cost, and supply. When incorporated into different matrices, they may control emulsification, stabilization, crystallization, water release, and encapsulation. Acetan is an important extracellular water-soluble polysaccharide produced mainly by bacterial species of the genera Komagataeibacter and Acetobacter. Since its original description in Komagataeibacter xylinus, acetan-like polysaccharides have also been described in other species of acetic acid bacteria. Our knowledge on chemical composition of different acetan-like polysaccharides, their viscoelasticity, and the genetic basis for their production has expanded during the last years. Here, we review data on acetan biosynthesis, its molecular structure, genetic organization, and mechanical properties. In addition, we have performed an extended bioinformatic analysis on acetan-like polysaccharide genetic clusters in the genomes of Komagataeibacter and Acetobacter species. The analysis revealed for the first time a second acetan-like polysaccharide genetic cluster, that is widespread in both genera. All species of the Komagataeibacter possess at least one acetan genetic cluster, while it is present in only one third of the Acetobacter species surveyed.

Genome sequences and description of novel exopolysaccharides producing species Komagataeibacter pomaceti sp. nov. and reclassification of Komagataeibacter kombuchae (Dutta and Gachhui 2007) Yamada et al., 2013 as a later heterotypic synonym of Komagataeibacter hansenii (Gosselé et al. 1983) Yamada et al., 2013

Strains T5K1 and AV446 isolated from apple cider vinegars during a submerged vinegar production in two separate vinegar facilities showed 94% 16S rRNA gene similarity to its closest neighbors Komagataeibacter maltaceti LMG 1529^T and Gluconacetobacter entanii LTH 4560^T. Further phylogenetic and phenotypic characterizations indicated that the isolates belonged to a novel species of the Komagataeibacter genus. Comparison based on 16S-23S rRNA gene ITS sequences and concatenated partial sequences of the housekeeping genes dnaK, groEL and rpoB, grouped both strains to a single phylogenetic cluster well separated from the other species of the Komagataeibacter genus. Average nucleotide identity of T5K1 and AV446 draft genome sequences compared to other Komagataeibacter type strains was below 94% and at the same time, in-silico DNA-DNA hybridization was below 70%. Both strains on the other hand showed approximately 98% (average nucleotide identity) and 87% (in silico DNA-DNA hybridization) similarity to each other. Strains T5K1 and AV446 can be differentiated from other Komagataeibacter type strains based on their ability to produce 2-keto-d-gluconic acid and at the same time inability to produce 5-keto-d-gluconic acid. Furthermore, strains of the new species do not grow on Asai medium supplemented with d-glucose or d-mannitol. The growth is also absent (T5K1) or weak (AV446) on Hoyer-Frateur medium supplemented with afore mentioned sugars. Both strains produce cellulose. In addition, draft genome analysis revealed that strains T5K1 and AV446 possess genes involved in the synthesis of acetan-like extracellular heteropolysaccharide. We propose the name Komagataeibacter pomaceti sp. nov. for the new species with LMG 30150^T [=CCM 8723^T=ZIM B1029^T] as the type strain. Data collected in this study and in a previous study also revealed that Komagataeibacter kombuchae is a later heterotypic synonym of Komagataeibacter hansenii.

Efficient one-pot per-O-acetylation–thioglycosidation of native sugars, 4,6-O-arylidenation and one-pot 4,6-O-benzylidenation–acetylation of S-/O-glycosides catalyzed by Mg(OTf)2

Sequential one-pot per-O-acetylation–S-/O-glycosidation under neat condition, regioselective 4,6-O-arylidenation and sequential one-pot benzylidenation–acetylation of Mg(OTf)₂as non-hygroscopic, recyclable catalyst are reported.

Isolation and characterization of two cellulose morphology mutants of Gluconacetobacter hansenii ATCC23769 producing cellulose with lower crystallinity

Gluconacetobacter hansenii, a Gram-negative bacterium, produces and secrets highly crystalline cellulose into growth medium, and has long been used as a model system for studying cellulose synthesis in higher plants. Cellulose synthesis involves the formation of β-1,4 glucan chains via the polymerization of glucose units by a multi-enzyme cellulose synthase complex (CSC). These glucan chains assemble into ordered structures including crystalline microfibrils. AcsA is the catalytic subunit of the cellulose synthase enzymes in the CSC, and AcsC is required for the secretion of cellulose. However, little is known about other proteins required for the assembly of crystalline cellulose. To address this question, we visually examined cellulose pellicles formed in growth media of 763 individual colonies of G. hansenii generated via Tn5 transposon insertion mutagenesis, and identified 85 that produced cellulose with altered morphologies. X-ray diffraction analysis of these 85 mutants identified two that produced cellulose with significantly lower crystallinity than wild type. The gene disrupted in one of these two mutants encoded a lysine decarboxylase and that in the other encoded an alanine racemase. Solid-state NMR analysis revealed that cellulose produced by these two mutants contained increased amounts of non-crystalline cellulose and monosaccharides associated with non-cellulosic polysaccharides as compared to the wild type. Monosaccharide analysis detected higher percentages of galactose and mannose in cellulose produced by both mutants. Field emission scanning electron microscopy showed that cellulose produced by the mutants was unevenly distributed, with some regions appearing to contain deposition of non-cellulosic polysaccharides; however, the width of the ribbon was comparable to that of normal cellulose. As both lysine decarboxylase and alanine racemase are required for the integrity of peptidoglycan, we propose a model for the role of peptidoglycan in the assembly of crystalline cellulose.

Exopolysaccharide production is required for biofilm formation and plant colonization by the nitrogen-fixing endophyte Gluconacetobacter diazotrophicus

The genome of the endophytic diazotrophic bacterial species Gluconacetobacter diazotrophicus PAL5 (PAL5) revealed the presence of a gum gene cluster. In this study, the gumD gene homologue, which is predicted to be responsible for the first step in exopolysaccharide (EPS) production, was insertionally inactivated and the resultant mutant (MGD) was functionally studied. The mutant MGD presented normal growth and nitrogen (N(2)) fixation levels but did not produce EPS when grown on different carbon sources. MGD presented altered colony morphology on soft agar plates (0.3% agar) and was defective in biofilm formation on glass wool. Most interestingly, MGD was defective in rice root surface attachment and in root surface and endophytic colonization. Genetic complementation reverted all mutant phenotypes. Also, the addition of EPS purified from culture supernatants of the wild-type strain PAL5 to the mutant MGD was effective in partially restoring wild-type biofilm formation and plant colonization. These data provide strong evidence that the PAL5 gumD gene is involved in EPS biosynthesis and that EPS biosynthesis is required for biofilm formation and plant colonization. To our knowledge, this is the first report of a role of EPS in the endophytic colonization of graminaceous plants by a nitrogen-fixing bacterium.

Characterization of cellulose production by a Gluconacetobacter xylinus strain from Kombucha

The aims of this work were to characterize and improve cellulose production by a Gluconoacetobacter xylinus strain isolated from Kombucha and determine the purity and some structural features of the cellulose from this strain. Cellulose yield in tea medium with both black tea and green tea and in Hestrin and Schramm (HS) medium under both static and agitated cultures was compared. In the tea medium, the highest cellulose yield was obtained with green tea (approximately 0.20 g/L) rather than black tea (approximately 0.14 g/L). Yield in HS was higher (approximately 0.28 g/L) but did not differ between static and agitated incubation. (1)H-NMR and (13)C-NMR spectroscopy indicated that the cellulose is pure (free of acetan) and has high crystallinity, respectively. Cellulose yield was improved by changing the type and level of carbon and nitrogen source in the HS medium. A high yield of approximately 2.64 g/L was obtained with mannitol at 20 g/L and corn steep liquor at 40 g/L in combination. In the tea medium, tea at a level of 3 g/L gave the highest cellulose yield and the addition of 3 g/L of tea to the HS medium increased cellulose yield to 3.34 g/L. In conclusion, the G. xylinus strain from Kombucha had different cellulose-producing characteristics than previous strains isolated from fruit. Cellulose was produced in a pure form and showed high potential applicability. Our studies extensively characterized cellulose production from a G. xylinus strain from Kombucha for the first time, indicating both similarities and differences to strains from different sources.

Novel glycosyltransferase genes involved in the acetan biosynthesis of Acetobacter xylinum

Novel aceQ and aceR genes involved in the acetan biosynthesis of Acetobacter xylinum were newly isolated. The homology search with DNA Data Bank of Japan indicated that aceQ and aceR were glycosyltransferases. Their gene-disrupted mutants were obtained by homologous recombination using the tetracycline resistance gene and the electroporation method. By NMR and ESI-MS analyses, aceQ-disrupted mutant DQ was found to secrete a water-soluble polysaccharide harboring the -Man-GlcUA side chain and the aceR-disrupted mutant DR was found to secrete an acetan analog, lacking the terminal Rha residue. These results suggested that aceQ and aceR encode a glucosyltransferase and a rhamnosyltransferase, respectively. It was indicated that acetan analogs harboring various side chains can be generated easily by genetic engineering.

Structure and conformation of a novel genetically engineered polysaccharide P2

A new exocellular polysaccharide (P2) has been produced by the manipulation of a glycosyl transferase gene (aceP) involved in the biosynthesis of the polysaccharide acetan by the bacterium Acetobacter xylinum strain CKE5. The P2 polysaccharide has been studied by methylation analysis, reductive cleavage, and 1H and 13C NMR spectroscopy. The data are consistent with the structure predicted when the aceP gene is deactivated: [Molecular structure: see text]. The effect of cooling on proton NMR line width indicates a coil-helix transition in P2 at about 70 degrees C.

Diffusion in Pseudomonas aeruginosa biofilms: a pulsed field gradient NMR study

A Pseudomonas aeruginosa biofilm is studied with pulsed field gradient echo nuclear magnetic resonance. Although not all spectral components are assigned yet, the experimental results show that a biofilm consists of components with very different diffusion coefficients. The various biofilm components that give motionally narrowed 1H NMR signals, can be grouped into five classes with diffusion coefficients, ranging from 2 x 10(-9) to 1 x 10(-13) m2 s-1. Investigation of the diffusion behavior of water in the biofilm shows three fractions with different diffusion coefficients. Besides the highly mobile bulk water at least two other fractions with much lower diffusion coefficients are detected. It is shown that one of the fractions with the low diffusion coefficient probably arises from intracellular water. Also for another component of the biofilm, glycerol, three fractions with diffusion coefficients that differ more than a factor ten are detected. Also a group of signals exists which result from practically immobile components.

Characterisation of the polysaccharide produced by Acetobacter xylinum strain CR1/4 by light scattering and atomic force microscopy

The molecular weight of the extracellular polysaccharide (CR1/4) produced by Acetobacter xylinum strain CR1/4 has been shown to be dependent upon growth conditions. Under normal growth conditions a high molecular weight polysaccharide ( > 1 x 10(6) Da) is produced. Maintaining the pH at 5 results in an order of magnitude increase in the total yield of polysaccharide, but also an order of magnitude decrease in molecular weight. Analysis of the CR1/4 polysaccharides by the techniques of atomic force microscopy and static light scattering suggests that they are double helices. In solution the molecules behave as stiff coils with a Kuhn statistical segment length of 325 nm.

Evidence for intermolecular binding between deacetylated acetan and the glucomannan konjac mannan

Binary mixtures of deacetylated acetan and konjac mannan form thermoreversible gels under conditions for which the individual components do not gel. Such synergistic behaviour is normally attributed to intermolecular binding between the two polysaccharides. X-ray diffraction data obtained from oriented fibres prepared from deacetylated acetan-konjac mannan gels provides direct evidence for intermolecular binding between the two polysaccharides. The novel heterotypic junction zones appear to be six-fold helices with a pitch of 5.6 +/- 0.1 nm.

Effect of deacetylation on the synergistic interaction of acetan with locust bean gum or konjac mannan

It has been discovered that deacetylation of the bacterial polysaccharide acetan promotes synergistic interactions with either locust bean gum (LBG) or konjac mannan (KM). Acetan is similar in structure to xanthan, and adopts a similar 5-fold conformation in the solid state. Like xanthan, it shows a thermally reversible order (helix)-disorder (coil) transition in solution. Both polymers have a cellulosic backbone with charged (anionic) sidechains attached at O-3 of alternate glucosyl residues, but the sidechains in acetan are longer (pentasaccharide rather than trisaccharide) and do not contain pyruvic substituents. Acetan has two sites of acetylation, one at O-6 of the inner mannosyl residue of the carbohydrate sidechains (as in xanthan) and the other on the polymer backbone (believed to be at O-6 of the branched glucosyl residues). Solutions of acetan or deacetylated acetan were equilibrated against 10 mM potassium chloride (to stabilise the ordered conformation) and were mixed (at 25 degrees C) with solutions of LBG or KM, also equilibrated against 10 mM potassium chloride. Unlike xanthan, native acetan showed no evidence of synergistic interaction with either LBG or KM. After deacetylation, however, large enhancements were observed in dilute-solution viscosity, and thermoreversible gels were formed at higher concentrations. With KM as co-synergist, gel melting was accompanied by an intense endotherm in differential scanning calorimetry. The magnitude of this endotherm increased with storage time at 25 degrees C, reaching a final value of delta H approximately 15.9 J/g (in comparison with delta H approximately 5.0 J/g for the order-disorder transition of deacetylated acetan alone). It is suggested that interaction occurs by formation of heterotypic junctions between the acetan backbone and unsubstituted regions of the plant polysaccharide, and that the acetate groups on native acetan promote solubility and hence inhibit association.

Cloning of the aceF gene encoding the phosphomannose isomerase and GDP-mannose pyrophosphorylase activities involved in acetan biosynthesis in Acetobacter xylinum

The aceF gene from Acetobacter xylinum was identified and cloned from a genomic DNA library. The complete DNA sequence was determined and computer analysis of the translated gene sequence revealed homology with the deduced amino acid sequence of xanB from Xanthomonas campestris. Therefore aceF is likely to encode a bifunctional enzyme with mannose-6-phosphate isomerase (PMI) and GDP-mannose pyrophosphorylase (GMP) activities. PMI and GMP activities were detected in strains of Escherichia coli expressing the cloned aceF gene.

High-field NMR as a technique for the determination of polysaccharide structures

NMR spectroscopy has played a developing role in the study of polysaccharide structures for over 30 years. Many new bacterial polysaccharide repeat unit structures have recently been published as a result of the application of modern NMR techniques. NMR can also be used to elucidate the structures of both regular and heterogeneous polysaccharides from fungal and plant sources, as well as complex glycosaminoglycans of animal origin. In addition to covalent structure, conformation and dynamics of polysaccharides are susceptible to NMR analysis, both in solution and in the solid state. Improvements in NMR technology with potential applications to polysaccharide studies hold promise for the future.

Structure and conformation of acetan polysaccharide

Acetan is an anionic bacterial polysaccharide. The chemical repeat unit consists of a cellobiose unit solubilised by attachment of a charged pentasaccharide sidechain to one of the glucose residues. The repeat unit contains two sites of acetylation. 1H and 13C NMR studies, coupled with both basic-methylation and mild-methylation studies, have shown that acetylation occurs at C6 on the (1,2)D-Man and the (1,34)D-Glc residues. A variety of techniques including NMR, optical rotation, circular dichroism and DSC show evidence for a thermoreversible conformational order (helix)-disorder (coil) transition for acetan in aqueous solution. The studies suggest that acetylation of the backbone does not prevent helix formation.

Identification, cloning and sequencing the aceA gene involved in acetan biosynthesis in Acetobacter xylinum

The aceA gene from Acetobacter xylinum was identified and cloned from a genomic DNA library. The complete DNA sequence was determined and computer analysis of the translated gene sequence revealed homology with the deduced amino acid sequence of gumD from Xanthomonas campestris. Therefore aceA is likely to encode the phosphate-prenyl glucose l-phosphate transferase catalyzing the first step in acetan biosynthesis in A. xylinum.

Genetic analysis of the acetan biosynthetic pathway in Acetobacter xylinum: nucleotide sequence analysis of the aceB, aceC, aceD and aceE genes

Sequence analysis of a 5.323 kb chromosomal DNA fragment from Acetobacter xylinum involved in the biosynthesis of the exopolysaccharide acetan, revealed the presence of four ace genes designated aceB, aceC, aceD and aceE. Comparison of translated gene sequences to the databanks was used to assign putative gene functions. AceB displayed strong homology to a glucose-diphosphoprenyl beta, D-glucose transferase from Xanthomonas campestris, while AceC was homologous to a cellobiosyl-diphosphoprenyl alpha, D-mannose transferase from the same organism. Thus these genes encode enzymes catalyzing the second and third steps of the acetan biosynthetic pathway. AceD and AceE were homologous to ExoP and ExoT respectively from Rhizobium meliloti and are likely to be involved in acetan polymerization and export.