Bifidobacterium bifidum presents on the cell surface a complex mixture of glucans and galactans with different immunological properties

The chemical structure of cell surface polysaccharides isolated from Bifidobacterium bifidum strain PRI1, an important member of the gut microbiota of breast-fed infants, has been elucidated by chemical and NMR spectroscopy analysis. Results demonstrated that the bacterium produces a complex mixture of polysaccharides that could be classified in two main groups: a phospho-glycero--galactofuranan, PGG, and a mixture composed of four neutral polysaccharides named as (CSGG), composed of -(1→6)-glucan, -(1→4)-galactan, -(1→6)-galactan, -galactofuranan and starch. These two fractions exerted different immune responses when assayed on dendritic cells: PGG enhanced pro-inflammatory immune responses by increasing interferon- levels while CSGG induced immunosuppressive regulatory T cells and interleukin-10. These findings demonstrate that bacterial polysaccharides have a distinct role depending on their chemical structure in regulation of the host/bacterium interaction. Our findings suggest that polysaccharides may differentially regulate the host immunity depending on the composition of this complex mixture, either enhancing immunity or inducing immune tolerance.