Bacteria levans

The above chemicals can be obtained by fermentation (qv) of other sugars. However, some compounds require sucrose as a unique feedstock. Examples are the polysaccharides dextran, alteman, andlevan, which are produced by specific strains of bacteria (48,54—56). Dextrans are used to make chromatographic separation media, and sulfated dextran derivatives are used as plasma extenders (41). Levans show promise as sweetness potentiators and, along with alteman, have potential as food thickeners and bulking agents in reduced-caloric foods (55,56) (see Carbohydrates). 

Two extracellular D-fructans, (2- 6)-linked S-D-fructofuranan or levan and the less common corresponding (2 l)-linked polysaccharide, of the inulin type, are elaborated by different bacteria. These polysaccharides are formed from sucrose by the action of sucrose fructosyltransferases. Terminal )S-D-fructofuranosyl groups are present in some bacterial heteropolysacchar-... 

Hehre, Genghof and Neill106 c have shown that levans isolated from sucrose cultures of Streptococcus salivarius (and of a spore-forming bacterium) exhibit precipitation and complement fixation when tested against antisera produced by immunization with bacteria obtained from sucrose cultures of either species. 

It is of interest that similar enzymes, capable of exchanging glycosidic linkages, appear to be involved in the synthesis of some polysaccharides. Thus the polysaccharide dextran is formed from sucrose by enzyme preparations from Leuconostoc46 while the polysaccharide levan is produced from sucrose or raffinose by enzymes of other bacteria.49 The reactions may be written as follows ... 

Levan p (2-6) Linear, branched Plant, bacteria, fungi... 

Levansucrase is a transfructosylase which converts sucrose to levan , a largely p(2 6)-linked fructofuranose polymer, and has been investigated because of its synthesis by oral bacteria. The levans and dextrans produced from dietary sucrose are sparingly water soluble and contribute to the build-up of dental plaque on the teeth. The enzyme from Bacillus subtilis was shown to have ping-pong kinetics,and the intermediacy of a covalent, fructosylated... 

The polymers such as cellulose, curdlan, xanthan, levan, and dextran that are obtained from the respective bacteria as described in section 10.2 are utilized in various food applications. Cellulose is used in food (nata de coco), and curdlan is used in food as a thickener and gelling agent. Dextran is used in healthy beverages, xanthan is used as a rheology modifier and food additive, and levan is used in food biotechnology. 

Particular difficulties exist, however, in purifying dextrans synthesized by many streptococci, because the bacteria simultaneously elaborate exocellular fructans (levansDextrans and levans synthesized concomitantly by Leuconostoc micro-organisms have been separated by fractionally precipitating the polysaccharides from aqueous ethanol. These separative procedures are, however, only effective when considerable differences in molecular weight exist between the main fractions of the two polysaccharides, and, as a result, they are of limited value in effecting separation of levans from many streptococcal dextrans. Reports that some oral bacteria are able to metabolize levans suggest that... 

P-D-Fru/joins with a-D-Glcp via l<- 2 to form sucrose. Fructan (reserve foods in roots, stems, leaves and seeds e.g. inuhn, irisan, asparagosan) is D-Fru/in a-2— 1 and a-2—>6 linkages. Levan from various grasses is a linear fructan with D-Fru in P-2—>6, and levan from bacteria is branched with d-Fru in P-2—J-6 and P-2 1. 

The type of biosynthesis, exemplified by the levans and some dextrans, is generally associated with some Gram-positive organisms, usually cocci, and is the simplest type of saccharide synthesis to occur in bacteria. The enzymes involved are extracellular and do not require sugar nucleotide or lipid-phosphate sugars. 

Levans and Allied Polymers Organisational Aspects of Glycosyl Transfers in Bacteria... 

For composite materials applications, the main useful polysaccharides are cellulose and starch, but more complex carbohydrate polymers originated from bacteria and fungi (e.g., exo-polysaccharides such as xanthan, curdlan, puUulan, levan and hyaluronic acid) have attracted increased interest in the last years due to their outstanding potential for various industrial areas [10-14]. 

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