Physicochemical characteristics and in vitro and in vivo antioxidant activity of a cell-bound exopolysaccharide produced by Lactobacillus fermentum S1

A cell-bound exopolysaccharide (c-EPS) from Lactobacillus fermentum S1 was isolated and purified to near homogeneity by anion exchange and gel filtration chromatography. The c-EPS is a homogeneous heteropolysaccharide with an average molecular weight of 7.19 × 105 Da and comprises mainly mannose, rhamnose, glucose, and galactose. Fourier transform infrared spectroscopy spectrum of the c-EPS exhibited typical characteristic absorption peaks of polysaccharides. Methylation and NMR analyses showed that the c-EPS had a backbone of α-D-Galp-(1 → 3), α-L-Rhap-(1 → 2), α-D-Glcp-(1 → 3), β-D-Galp-(1 → 3), β-D-Glclp-(1 → 2), and β-L-Rhap-(1 → 3,4) residues, terminated with α-D-Manp-(1 → residue. The advanced structure study indicated the c-EPS not to have a triple-helical conformation, while the microstructural study revealed a hollow porous structure for c-EPS. Further, the thermal analysis showed that the degradation temperature for the c-EPS was 288.0 °C; its peak temperature was 89.4 °C with an enthalpy value of 273.1 J/g. Moreover, the c-EPS exhibited potent DPPH, hydroxyl, and ABTS+ radicals scavenging activities, as well as FRAP in a dose-dependent manner, which could significantly enhance the T-AOC and SOD activity and reduce MDA level in Caenorhabditis elegans. Therefore, this c-EPS could be utilized as a promising natural antioxidant for application in functional foods.