Cheng Zhong

Effects of light wavelengths on extracellular and capsular polysaccharide production by Nostoc flagelliforme.

The influences of different wavelengths of light (red 660nm, yellow 590nm, green 520nm, blue 460nm, purple 400nm) and white light on extracellular polysaccharide (EPS) and capsular polysaccharide (CPS) production by Nostoc flagelliforme in liquid culture were demonstrated in this study. The results showed that, compared with white light, red and blue lights significantly increased both EPS and CPS production while yellow light reduced their production; purple and green lights stimulated EPS production but inhibited CPS formation. Nine constituent monosaccharides and one uronic acid were detected in both EPS and CPS, and their ratios showed significant differences among treatment with different light wavelengths. However, the advanced structure of EPS and CPS from various light conditions did not present obvious difference through Fourier transform infrared spectroscopy and X-ray diffraction characterization. These findings establish a basis for development of high-yielding polysaccharide production process and understanding their regulation.

Revealing Differences in Metabolic Flux Distributions between a Mutant Strain and Its Parent Strain Gluconacetobacter xylinus CGMCC 2955

A better understanding of metabolic fluxes is important for manipulating microbial metabolism toward desired end products, or away from undesirable by-products. A mutant strain, Gluconacetobacter xylinus AX2-16, was obtained by combined chemical mutation of the parent strain (G. xylinus CGMCC 2955) using DEC (diethyl sulfate) and LiCl. The highest bacterial cellulose production for this mutant was obtained at about 11.75 g/L, which was an increase of 62% compared with that by the parent strain. In contrast, gluconic acid (the main byproduct) concentration was only 5.71 g/L for mutant strain, which was 55.7% lower than that of parent strain. Metabolic flux analysis indicated that 40.1% of the carbon source was transformed to bacterial cellulose in mutant strain, compared with 24.2% for parent strain. Only 32.7% and 4.0% of the carbon source were converted into gluconic acid and acetic acid in mutant strain, compared with 58.5% and 9.5% of that in parent strain. In addition, a higher flux of tricarboxylic acid (TCA) cycle was obtained in mutant strain (57.0%) compared with parent strain (17.0%). It was also indicated from the flux analysis that more ATP was produced in mutant strain from pentose phosphate pathway (PPP) and TCA cycle. The enzymatic activity of succinate dehydrogenase (SDH), which is one of the key enzymes in TCA cycle, was 1.65-fold higher in mutant strain than that in parent strain at the end of culture. It was further validated by the measurement of ATPase that 3.53–6.41 fold higher enzymatic activity was obtained from mutant strain compared with parent strain.

Surfactant-free emulsions stabilized by tempo-oxidized bacterial cellulose.

In order to seek a safe, biodegradable, and sustainable solid stabilizer for food, topical and pharmaceutical emulsions, individualized cellulose nanofibers were prepared by oxidizing bacterial cellulose (BC) in a Tempo-mediated system; their ability to stabilize oil/water interface was investigated. Significant amounts of C6 carboxylate groups were selectively formed on each cellulose microfibril surface, so that the hydrophilicity was strengthened, leading to lower contact angles. Meanwhile, both the length and width of fibrils were decreased significantly, by partial cleavage of numerous numbers of inter- and intra-fibrillar hydrogen bonds. Tempo-oxidized BC (TOBC) was more effective than BC in stabilizing oil-water interface, attributing to the much smaller size. Fibril dosage and oxidation degree exerted a great influence on the stability and particle size distribution of emulsion samples. When the fibril dosage was 0.7wt.%, the sample was so stable that it did not experience creaming and coalescence over 8 months. The 2-TOBC coated droplets showed the greatest stability, although both the zeta potential and the electric repulsion were the largest for the 10-TOBC analogue, which was manipulated by the wettability of fibrils. In addition, the stability of samples was analyzed from the viewpoint of particle size distribution. Consequently, fibril size and wettability are two counterbalanced factors influencing the stability of TOBC-stabilized emulsions; a combination of suitable wettability and size imparts TOBC-stabilized emulsion high stability. As a kind of biomass-based particle stabilizer, TOBC showed great potential applications in food, topical and pharmaceutical formulations.

Facile fabrication of moldable antibacterial carboxymethyl chitosan supramolecular hydrogels cross-linked by metal ions complexation

Herein, carboxymethyl chitosan (CMCh) supramolecular hydrogels cross-linked by metal ions (Ag+, Cu2+ and Zn2+) are reported. The hydrogels were formed within a few seconds by simple mixing of CMCh solution with metallic salt solutions at an appropriate pH. The prepared hydrogels were characterized by using FT-IR, XRD, SEM and rotational rheometery. FT-IR measurements suggested that the facile complexation of metal ions with carboxylic, amino and hydroxyl groups of CMCh chains promoted the rapid hydrogelation. SEM images revealed a cross-linked structure of hydrogels, while microstructural openings were observed by cross-sectional studies of the freeze-dried hydrogels. Moreover, the hydrogels showed a remarkable moldability to form free standing objects. The antibacterial activities of the hydrogels were also studied against Escherichia coli and Staphylococcus aureus by agar well diffusion method. The results demonstrated an excellent antibacterial activity of the supramolecular hydrogels. Therefore, the developed CMCh supramolecular hydrogels might be used effectively in biomedical field.

Preparation and characterization of a novel bacterial cellulose/chitosan bio-hydrogel:

Composites of chitosan chloride and bacterial cellulose were successfully prepared by in situ method. Composites of bacterial cellulose/chitosan and pristine bacterial cellulose were investigated by means of scanning electron microscope, atomic force microscope, Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray diffraction, and bacteriostatic test. The crystallization of bacterial cellulose was interfered and weakened by the chitosan chloride included in the growth media, resulting in lower crystallinity index and thermal stability. And interaction between two polymers is verified by the thermal gravimetric analysis. The ultrafine nanofibril network structure of bacterial cellulose was retained by the composites, while the diameters were larger and the aperture inside were smaller than those of pristine bacterial cellulose, as shown through scanning electron microscope and atomic force microscope figures. The antimicrobial effects were enhanced by the increasing concentration of chitosan in composites. All the characteristics of the composites provide evidence for the miscibility of chitosan and cellulose. Their biocompatibility is proved through our published data. It is strongly indicated that bacterial cellulose–chitosan nanocomposites have great potential in tissue engineering or pharmaceutical applications in the near future.

The Effect of Growth, Migration and Bacterial Cellulose Synthesis of Gluconacetobacter xylinus in Presence of Direct Current Electric Field Condition

In this study, the movement and orientation of bacteria cells were controlled by direct current(DC) electric fields, result in altering alignment of bacterial cellulose nanofiber and further changing the 3-dimensional network structure of bacterial cellulose. A modified swarm plate assay was performed to investigate the migration of Gluconacetobacter xylinus cells which exposed in DC electric field. It suggested that the cells moved toward to negative pole and with the increasement of the electric field strength the velocity will also increase. The SEM analysis demonstrated that the cellulose fiber bundles which synthesized at 1V/cm have lager diameter and a trend toward one direction. Meanwhile the growth state of G.xylinus in the presence of DC electric field was also being observed.

Enhanced bacterial cellulose production by Gluconacetobacter xylinus via expression of Vitreoscilla hemoglobin and oxygen tension regulation

Oxygen plays a key role during bacterial cellulose (BC) biosynthesis by Gluconacetobacter xylinus. In this study, the Vitreoscilla hemoglobin (VHb)-encoding gene vgb, which has been widely applied to improve cell survival during hypoxia, was heterologously expressed in G. xylinus via the pBla-VHb-122 plasmid. G. xylinus and G. xylinus-vgb+ were statically cultured under hypoxic (10 and 15% oxygen tension in the gaseous phase), atmospheric (21%), and oxygen-enriched conditions (40 and 80%) to investigate the effect of oxygen on cell growth and BC production. Irrespective of vgb expression, we found that cell density increased with oxygen tension (10–80%) during the exponential growth phase but plateaued to the same value in the stationary phase. In contrast, BC production was found to significantly increase at lower oxygen tensions. In addition, we found that BC production at oxygen tensions of 10 and 15% was 26.5 and 58.6% higher, respectively, in G. xylinus-vgb+ than that in G. xylinus. The maximum BC yield and glucose conversion rate, of 4.3xa0g/L and 184.7xa0mg/g, respectively, were observed in G. xylinus-vgb+ at an oxygen tension of 15%. Finally, BC characterization suggested that hypoxic conditions enhance BC’s mass density, Young’s modulus, and thermostability, with G. xylinus-vgb+ synthesizing softer BC than G. xylinus under hypoxia as a result of a decreased Young’s modulus. These results will facilitate the use of static culture for the production of BC.

Complete genome analysis of Gluconacetobacter xylinus CGMCC 2955 for elucidating bacterial cellulose biosynthesis and metabolic regulation

Complete genome sequence of Gluconacetobacter xylinus CGMCC 2955 for fine control of bacterial cellulose (BC) synthesis is presented here. The genome, at 3,563,314u2009bp, was found to contain 3,193 predicted genes without gaps. There are four BC synthase operonsxa0(bcs), among which only bcsI is structurally complete, comprising bcsA, bcsB, bcsC, and bcsD. Genes encoding key enzymes in glycolytic, pentose phosphate, and BC biosynthetic pathways and in the tricarboxylic acid cycle were identified. G. xylinus CGMCC 2955 has a complete glycolytic pathway because sequence data analysis revealed that this strain possesses a phosphofructokinase (pfk)-encoding gene, which is absent in most BC-producing strains. Furthermore, combined with our previous results, the data on metabolism of various carbon sources (monosaccharide, ethanol, and acetate) and their regulatory mechanism of action on BC production were explained. Regulation of BC synthase (Bcs) is another effective method for precise control of BC biosynthesis, and cyclic diguanylate (c-di-GMP) is the key activator of BcsA–BcsB subunit of Bcs. The quorum sensing (QS) system was found to positively regulate phosphodiesterase, which decomposed c-di-GMP. Thus, in this study, we demonstrated the presence of QS in G. xylinus CGMCC 2955 and proposed a possible regulatory mechanism of QS action on BC production.

The Cells of Gluconacetobacter xylinus Response to Exposure

Via direct current (DC) electric field, bacterial cellulose (BC), a biopolymer generated by Gluconacetobacter xylinus, was altered into its nanofibers alignment and 3-dimensional network structure in its synthesized process. In this investigation, cultures of G. xylinus cells exposed at DC electric field and the sham-exposed controls were respectively investigated for: the culture able count near anode and cathode, the morphological analysis of cells, BC yield, and FTIR analysis of it. Exposed samples and controls showed significant difference in cells morphology, whereas they were similar at the two poles in same condition. An exposure to DC electric field of 0, 0.25, 0.5, 0.75, and 1 V/cm produced a obviously tendency on the ratio of culture able count near anode and cathode. BC yield showed a slight increase and then sharp decrease as the electric field intensity increased. In addition, essentially unchanged in FTIR analysis of BC formed at 0 V/cm and 1 V/cm. These results combined with our previous studies can establish a conjectural theory of cells response to exposure of DC electric field which has a stimulate factor.

Scale-up of 5-keto-Gluconic Acid Production by Gluconobacter oxydans HGI-1

Gluconobacter oxydans is known to oxidize glucose to gluconic acid (GA), and subsequently, to 2-keto-gluconic acid (2KGA) and 5-keto-gluconic acid (5KGA). 5KGA can be converted to L-(+)-tartaric acid which is important in industry. In order to increase the production of 5KGA, G. oxydans HGI-1 which converts GA only to 5KGA was chosen in this study. The fermentation process occurred in shake flask was studied in different inoculation, and 10 % inoculation amount was chosen. Furthermore, it was scaled-up to a 5-L stirred-tank fermenter, limitation of oxygen could not happen and pH could be adjusted by automated titration of 6 M KOH. 5KGA was accumulated up to 106.33 g/L in fermentation broth, while the strain was fed-batch cultured, the concentration of 5KGA was up to 179.4 g/L and the conversion rate of glucose was 89.0 %. G. oxydans HGI-1 was suitable for industrial production.