## 2 Biologically Active Polysaccharides--Structure and Function
### Bacterial polysaccharides  
Bacteria have the ability to synthesize biopolymers, which are divided into intracellular polysaccharides (CPS) and extracellular polymeric substances (EPS) according to their localization and function in the cell [47]. The main function of CPS is the structural support of the bacterial cell wall, but EPS have strong antioxidant properties and application in cancer therapy, antimicrobial and antiviral activities, and antioxidant and cholesterol-lowering effects [48]. K5 PS is a heparin-like envelope polysaccharide of _Escherichia coli_ that inhibits the metastasis of human breast cancer and murine melanoma cells [49].  
The source of bioactive polymers showing antiproliferative activity is _Paenibacillus polymyxa_ rods, which secrete polysaccharides of the levan-type, consisting mainly of \(\beta\)-D-fructofuranosyl residues connected by 2,6-bonds with 2,1-branches [50]. Moreover, _Rhizobium_ papillary bacteria contain polysaccharides with the structure of a-glucans and a skeleton composed of glucose molecules linked by 1,4-bonds with 1,6-branches, which inhibit the formation of neoplasms in mice [51].  
In recent years, there has been an increased interest in curdlan, insoluble in water linear beta-1,3-glucan, a high-molecular-weight polymer of glucose, obtained for the first time from _Alcaligenes faecalis_[52]. Curdlan has anti-cancer and anti-inflammatory properties and also supports wound healing [53]; its reaction mechanism is based on the interaction with the lectin-1 receptor present in dendric cells, macrophages, and monocytes. Further, it stimulates the immune system and enhances the phagocytosis process and production of TNF-\(\alpha\), IL-6, and ROS [54]. In addition, symbiotic polysaccharides of _Bacteroides fragilis_, presented in intestinal dendritic cells, activate CD4 T cells and stimulate the production of cytokines [55], while the polysaccharide from _Paenibacillus polymyxa_ has an anti-proliferative effect, and its derivatives formed as a result of acetylation show even stronger biological activity [28].  
A commercially used biopolymer is dextran, a high molecular weight water-soluble glucose polymer released by _Leuconstoc mesenteroides_. Dextran is used as a drug carrier to inhibit cancer cells and reduce the toxic effects of drugs [56].  
Many cyanobacteria are able to release easily recoverable polysaccharides into the culture medium. Most cyanobacterial polysaccharides show an anionic nature, due to the presence of uronic acids and other charged groups such as pyruvyl or sulfate [57]. Numerous publications describe excopolsysaccharides produced by cyanobacteria _Spirulina platensis_, considered by some researchers to be microalgae. Polysaccharides extracted from of _S. platensis_ are water-soluble; it has been proven that they have antitumor, antioxidant, antiaging, and antivirus properties [58]. [PERSON] et al. investigated the structural characterization and bioactive potential of sulfated polysaccharides from _S. platensis_ and isolated a fraction containing glucose, rhamnose, xylose, fucose, mannose, and galactose as main sugars with Mw of 1016 kDa [59].