Thermophilic Bacterial Exopolysaccharides: From Bio-Physicochemical Characterization to Biotechnological Applications

1. Introduction

Over the last few decades, vast numbers of extracellular polysaccharides or exopolysaccharides from extremophiles have been extensively studied all over the world. In the year 1972 Sutherland, proposed the word ‘exopolysaccharide’ (EPS) which stands for a variety of bacterial and microalgal heterogeneous long-chain polysaccharides that are synthesized and released outside the cell wall into their ambience during growth. Hot spring microorganisms produce a large number of structurally diverse extracellular polymeric substances i.e. EPSs. Diverse macromolecules e.g. peptidoglycan, lipopolysaccharide, and exopolysaccharide are the most important component of bacterial polysaccharides and these polysaccharides make the structure of their cell wall (e.g. Peptidoglycan) and Poly N-acetyl glucosamine plays a major role in bacteria to survive in the unfavourable environments (Kazak et al., 2010; Nichols et al., 2005; Ruffing & Chen, 2006). The thermophilic bacterium Geobacillus sp. (WSUCF1) has a great production rate in the production of two exopolysaccharides with prominent quantities. Following purification of these two exopolysaccharides, it was discovered that EPS-1 is composed of glucomannan with a 1:0.21 molar ratio of mannose and glucose, whereas EPS-2 was made up entirely of mannan. Both EPSs have molecular weights of around 1000 kDa, and their FTIR and NMR spectra revealed the existence of α-type glycosidic linkages in a linear structure. XRD examination revealed a low degree of crystallinity i.e. for EPS-1 and EPS-2 is 0.11 and 0.27 respectively (Wang et al., 2021). Most of the exopolysaccharides have their applications in food, pharmaceutical, and other industries. Exopolysaccharides produced by thermophiles have been used for various biotechnological processes like fermentation and food emulsification (Ruffing & Chen, 2006). A novel exopolysaccharide (EPS-B3-15) isolated from marine thermo-tolerant Bacillus licheniformis strain B3-15 showed high-temperature stability up to 80ºC. Depending on this property of thermal stability it can be used for nano-medicine development (Caccamo et al., 2020). High-temperature-loving (thermophilic) bacteria from different classes of Archaea and Bacteria have been discovered from a variety of thermal habitats, including both deep and shallow marine hot springs, as well as terrestrial hot springs, which have provided the genesis for the separation of microbial EPS producers. Some thermophilic bacteria e.g. Archaeoglobus fulgidus, Thermococcus litoralis, Pseudomonas aeruginosa are good EPSs producers (Lapaglia & Hartzell, 1997; Nicolaus et al., 1993).