glucose, galactose)
Biopolymers of microbial origin have been studied lately due to their improved properties and easy production when compared with other natural polymers [4,78]. Microorganisms usually have higher growth rates than algae and plants, and their production processes can easily be manipulated to improve yields and productivity [79]. Moreover, the production process is not climate or seasonal dependent and can rely on the use of low‐cost by‐products or wastes as raw materials [80]. Microbial polysaccharides have unique features and properties that make them suitable to a wide range of applications. More specifically, these biopolymers have been extensively used in food, pharmaceutical, medical, and cosmetic products due to their unique performances as thickening, stabilizing, and binding agents. Some of these biopolymers (e.g. bacterial alginate, gellan, FucoPol) can also have intermolecular interactions that could result in polymeric matrices, allowing the physical manipulation of polysaccharides into structured materials such as gels (e.g. hydrogels) or films that could be used in biomedical applications [4,81]. Adding to this property, the ability of polysaccharides to interact with different inorganic materials represents an important feature for the encapsulation of bioactive substances (e.g. pharmaceuticals for their controlled release) and for the incorporation of nanostructures (such as carbon nanotubes or metallic nanoparticles [MNPs]) to produce enhanced biomaterials (with synergetic conductive or magnetic properties, respectively) [82].
2.3 Commercially Relevant Microbial Polysaccharides: Established Uses and Novel/Prospective Applications
The great diversity of microbial polysaccharide composition and functional properties enables their application in several industrial fields (e.g. medical, food products, pharmaceutical, biomedicine). Although only few are commercialized, among them are pullulan, scleroglucan, xanthan gum, dextran, levan, gellan gum, and hyaluronic acid.
2.3.1 Pullulan
Pullulan is a fungal polymer produced by A. pullulans. It is a neutral, linear glucose homopolysaccharide composed of α‐(1,6) maltotriose units [83,84]. Pullulan presents a unique linkage pattern showing remarkable physical properties, such as high solubility, adhesiveness, forming fibers, and thin biodegradable film capacity, which are transparent and impermeable to oxygen [15,85]. Therefore, pullulan offers a variety of potential industrial and medical applications. For a long time, pullulan membranes/films have been used as coating and packaging material in the food industry, but nowadays they are also being used in dietary capsule formulations. Pullulan‐based oral care products are also being commercialized. Due to the easy decomposition, it is used as coating in the paper industry. Modified pullulan is used as raw material in pharmaceutical applications, namely, nanoparticles, bioimaging, plasma expander, tissue engineering, etc. [84,86].
The cancer therapy and bioremediation are emerging markets for pullulan, due to its bioactivity with some cytotoxic molecules and the adsorption capacity for some heavy metals [84,85]. Moreover, currently several research groups are studying novel pullulan composites blended with other biodegradable materials (e.g. pullulan/dextran, pullulan/rice starch gel, and pullulan/cellulose) in order to obtain desired properties, such as thermal stability, high tensile strength, and emulsion stability [84]. Hydrogels of pullulan have been studied for three‐dimensional (3D) printing of scaffolds [87]. Hayashibara Co., Ltd. (Japan), is one of the companies that commercialized pullulan to be applied mainly as a thickening agent and edible coating.
2.3.2 Scleroglucan
Scleroglucan is a water‐soluble homopolymer of β‐glucans produced by filamentous fungi, especially of the genus Sclerotium as part of the adhesion mechanism to plant tissues [16,88]. Scleroglucan was first commercialized in the 1970s, being currently available under different trademarks (e.g. Clearogel, Polytetran, Polytran FS, Actigum) [88,89].
Scleroglucan is a thermostable biopolymer and, due to its nonionic nature, is stable over a wide range of pH (2.5–12). Scleroglucan solution exhibits shear thinning behavior; it acts as foam stabilizer and has a good emulsifying capacity. Further, it also exhibits biological activity. The interesting physicochemical and biological properties enable the use of scleroglucan on several industrial applications. Initially, scleroglucan was used in the oil industry [90,91]. Nowadays
