Structural characterization and immunomodulatory activity of an exopolysaccharide isolated from Bifidobacterium adolescentis.

Bifidobacterium adolescentis is a key probiotic that has been proven to possess various bioactivities. A water-soluble heteropolysaccharide (BEP-1A) was isolated from the probiotic and systematically investigated for the first time. The molecular weight of BEP-1A was calculated to be 9.69 × 106 Da. Combined with monosaccharide composition, Fourier transform infrared (FT-IR) spectroscopy, methylation and nuclear magnetic resonance (NMR) analysis, BEP-1A was composed of mannose, glucose and galactose at a molar ratio of 0.11⁚4.30⁚1.32. The backbone included β-1,2-Glcp, β-1,3-Glcp, α-1,4-Glcp, α-1,4-Galp, α-1,6-Galp and α-1,3-Manp, with the branch at the O-2 position of α-1,6-Galp, consisting of α-1,2-Galp and α-1-Glcp. Moreover, a filamentous structure of BEP-1A was detected by scanning electron microscopy (SEM). BEP-1A presented high thermal stability based on thermogravimetric analysis (TGA). X-ray diffractometry (XRD) results revealed that BEP-1A was an amorphous molecule without a crystal structure. Furthermore, BEP-1A significantly increased the viability of RAW 264.7 macrophages, improved phagocytosis, and promoted the secretion of nitric oxide (NO), reactive oxygen species (ROS), tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), interleukin-1β (IL-1β) and inducible nitric oxide synthase (iNOS). BEP-1A was also found to induce the nuclear translocation of the NF-κB subunit p65 and upregulate the phosphorylation of p65 and IκB-α, which suggested that the NF-κB pathway was involved in the BEP-1A-induced immunomodulatory effect. Overall, this study provides a theoretical basis for the development of BEP-1A as an immunomodulator in pharmaceuticals and functional foods.