Purification and structural characterization of atypical dextran-type capsular exopolysaccharide from Lactobacillus gasseri ATCC 33323

Probiotic strain Lactobacillus gasseri (L. gasseri) is widely used in the food industry due to its diverse bioactive activities, in which the exopolysaccharide (EPS) is a key functional component. While the secreted EPS from L. gasseri has been previously explored, its capsular form remains much less characterized, limiting the comprehensive understanding of EPS-derived postbiotics in this species. This study aims to isolate, purify, and structurally characterize the capsular exopolysaccharide from L. gasseri ATCC 33323. A capsular exopolysaccharide, designated C-EPSL.g-G, was isolated and purified from L. gasseri ATCC 33323. Structural characterization revealed that C-EPSL.g-G (Mw 36.1 kDa) is a highly branched, atypical dextran with a branching degree of 76.9%. It consists of an α-(1→6) backbone substituted at the α-(1→2) position by monomeric glucose side chains. These side chains have a defined arrangement with an unsubstituted/sparse central core that is symmetrically flanked by dense branches. Notably, 52.4% of the side chains carry 1-phosphoglycerol modifications at the O-6 position. Additionally, minor acetyl (8.3%) and 3-hydroxypropionyl (10.5%) modifications are present in C-EPSL.g-G at variable acylation sites. In 0.15 M NaCl solution, C-EPSL.g-G adopts a spherical conformation characterized by a small Z-average hydrodynamic diameter (7.64 ± 0.20 nm), moderate intrinsic viscosity (11.61 ± 0.21 mL/g), and slightly negative zeta potential (– 8.71 ± 0.32 mV). Comparative genomic analysis revealed that the biosynthetic gene locus for this atypical dextran is conserved across most L. gasseri and Lactobacillus johnsonii species. This study reports the purification and comprehensive structural characterization of an atypical dextran-type capsular exopolysaccharide C-EPSL.g-G from L. gasseri ATCC 33323 for the first time. Resulting structural insights provide a foundation for future functional exploration and potential applications of this atypical dextran in the food industry.