Polysaccharides exopolysaccharides

Plant stmctural material is the polysaccharide cellulose, which is a linear P (1 — 4) linked polymer. Some stmctural polysaccharides iacorporate nitrogen iato thek molecular stmcture an example is chitin, the material which comprises the hard exoskeletons of kisects and cmstaceans. Chitki is a cellulose derivative whereki the OH at C-2 is replaced by an acetylated amino group (—NHCOCH ). Microbial polysaccharides, of which the capsular or extracellular (exopolysaccharides) are probably the most important class, show more diversity both ki monomer units and the nature of thek linkages. 

Commercial applications for polysaccharides include their use as food additives, medicines and industrial products. Although plant polysaccharides (such as starch, agar and alginate) have been exploited commercially for many years, microbial exopolysaccharides have only become widely used over the past few decades. The diversity of polysaccharide structure is far greater in micro-organisms compared to plants and around 20 microbial polysaccharides with market potential have been described. However, microorganisms are still considered to be a rich and as yet underexploited source of exopolysaccharides. 

The presence of organic acid substituents in exopolysaccharides increases the lipophilidty of the molecule. In addition, for some exopolysaccharides with relatively high organic acid contents, their interaction with cations and with other polysaccharides may be influenced. Several amino adds have also been found in bacterial exopolysaccharides, including serine and L-glutamic add (Figure 7.1). 

As with all polysaccharides, microbial exopolysaccharides can be divided into... 

For food and pharmaceutical applications, the microbial count must be reduced to less than 10,000 viable cells per g exopolysaccharide. Treatment with propylene oxide gas has been used for reducing the number of viable cells in xanthan powders. The patented process involves propylene oxide treatment for 3 h in a tumbling reactor. There is an initial evacuation step before propylene oxide exposure. After treatment, evacuation and tumbling are alternated and if necessary the reactor is flushed with sterile nitrogen gas to reduce the residual propylene oxide level below the Food and Drug Administration permitted maximum (300 mg kg 1). The treated polysaccharide is then packaged aseptically. 

The alternative large scale recovery method to precipitation is ultrafiltration. For concentration of viscous exopolysaccharides, ultrafiltration is only effective for pseudoplastic polymers (shearing reduces effective viscosity see section 7.7). Thus, pseudoplastic xanthan gum can be concentrated to a viscosity of around 30,000 centipoise by ultrafiltration, whereas other polysaccharides which are less pseudoplastic, are concentrated only to a fraction of this viscosity and have proportionally lower flux rates. Xanthan gum is routinely concentrated 5 to 10-fold by ultrafiltration. 

Different polysaccharides change the perception of flavour, thus xanthan is superior to gum guar in the perception of sweetness. Mixtures of xanthan and locust bean gum have improved flavour release and texture when used in pies and pat s compared to starch. Many foods are emulsions, examples being soups, sauces and spreads. Exopolysaccharides are used to stabilise these emulsions and prevent the phases from... 

Many polysaccharides of eukaryotic origin show non-specific anti-viral activity and this property may be shared by some of the exopolysaccharides. The structural requirements for activity are not immediately evident as the polysaccharides exhibiting this activity are very diverse. 

A considerable amount of extracellular polysaccharides is produced in the process of cultivation of certain plant suspension cultures and the spent culture medium has proved to be an accessible source for their production (1-3). The interest in investigating these extracellular polysaccharides has been quite strong over the past 10-15 years, motivated by their biological activity (4,5). Plants of the Asteraceae family, as well as their cell cultures, have been established to contain polysaccharides with immunostimulating activity (1-6). The object of our research was Helianthus annuus 1805 cell culture (Asteraceae), which according to the preliminary investigation produces a considerable amount of exopolysaccharides. 

A new possibility for isolation of the exopolysaccharides in deep freezing of the culture medium (-18°C) was arrived at in the course of our research (Sample 2, Polysaccharide fraction I). 

A second polysaccharide fraction (Sample 3) was isolated from the filtrates by concentration and following precipitation with ethanol (1 3). The yield and characteristics of the obtained polysaccharides (Samples 2 and 3) are given in Table 1. It is evident that the sum of the yields for Samples 2 and 3 is almost equal to the yield for Sample 1. The polyuronic content data are also well balanced. This fact indicates that the suggested method is suited for fractional isolation of the polysaccharides from the spent culture medium of H.annuus 1805 cell suspension. As can be seen from Table 1, the main part of the exopolysaccharide was in fraction I. 

The first report of the occurrence of KDO in a structure other than LPS was that of Taylor47 on the capsular polysaccharide (K-antigen) of a clinical isolate, Escherichia coli LP1092. Almost simultaneously, Bhattachaijee and coworkers83 described an exopolysaccharide from Neisseria meningitidis serogroup 29e. This material contains KDO... 

Fig. 1 shows decomposition of monosaccharides liberated by hydrolysis with 0.5 M H2SO4 of an algal exopolysaccharide. There appeared to be some decomposition of xylose and mannose however, to obtain accurate rates of decomposition, the hydrolysis should have been continued for a much longer time, as monosaccharides are still being liberated from the polysaccharide and there are not enough data points to extrapolate back accurately to the origin in those cases where there is apparent decomposition. 

In the biosynthesis of the capsular polysaccharide from Xanthomonas campestris, the modification was shown265 to occur at the level of a polyprenyl pentasaccharide diphosphate intermediate prior to polymerization of the repeating units, and enolpyruvate phosphate was a precursor of the pyruvic acid residues. A similar observation was made during a study of the biosynthesis of Rhizobium meliloti exopolysaccharide.266... 

Microbial polysaccharides may contain substituted glucuronic acids. 4-0-(L-l-Carboxyethyl)-D-glucuronic acid has been identified121 as component of the capsular polysaccharide from Klebsiella type 37, and 3-0-(l-1-carboxyethyl)-D-glucuronic acid has been found in the exopolysaccharide produced by Altermonas sp. 1644.122... 

Macromolecules such as exopolysaccharides and humic condensates affect the distribution of microorganisms and nutrients within the water column by adding microstructure. Many planktonic bacteria and algae secrete polysaccharides. In some circumstances, exopolysaccharide production is a substantial portion of gross production. The adaptive significance of this activity is unclear. Possibilities include the dumping of excess photosynthate,... 

Most of the research on bacteria from the genus Rhizobium is conducted in the field of genetics and bacteria-host plant simbiotic interactions. Little is known about the production of extracellular polysaccharides produced by Rhizobium as well as their properties in solution in particular, no studies have been conducted on the effect of substrate concentration, agitation, and aeration as relevant parameters to monitor in the process of exopolysaccharide (EPS) production. The present work aimed to determine the extent to which some variables affect the production of polysaccharides by Rhizobium sp. 

Exopolysaccharide Polysaccharides that are secreted by an organism into the environment... 

Bacterial Polysaccharides. Many bacterial species release exopolysaccharides into their environment. The synthesis of capsular and slime polysaccharides or similar sticky surface materials serves as an adhesive to attach the bacteria to solid substrates, even in a marine environment. These polysaccharides range from compositionally simple homopolymers to very complex heteropolymers composed of several individual sugars linked in a variety of ways. 

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