Structural analysis of the α-D-glucan produced by the sourdough isolate Lactobacillus brevis E25

Cereal-associated Lactic Acid Bacteria (LAB) are well known for homopolymeric exopolysaccharide (EPS) production. Herein, the structure of an EPS isolated from sourdough isolate Lactobacillus brevis E25 was determined. A modified BHI medium was used for production of EPS-E25 in order to eliminate potential contaminants. Analysis of sugar monomers in EPS revealed that glucose was the only sugar present. Structural characterisation of EPS by NMR and methylation analysis revealed that E25 produced a highly branched α-glucan with (α1 → 3) and (α1 → 6) glycosidic linkages, and was similar in structure to a previously reported EPS from Lactobacillus reuteri 180. The ¹H and ¹³C NMR data were contrasted with newly recorded data for known polysaccharides (alternan, commercial dextran) which also contain α-(1,3,6)Glc branch points. It was found in both E25 EPS and alternan that NMR parameters could be used to distinguish glucose residues that had the same substitution pattern but occupied different positions in the structure.