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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-β-galactofuranan, PGβG, and a mixture composed of four neutral polysaccharides named as (CSGG), composed of β-(1 → 6)-glucan, β-(1 → 4)-galactan, β-(1 → 6)-galactan, β-galactofuranan and starch. These two fractions exerted different immune responses when assayed on dendritic cells: PGβG enhanced pro-inflammatory immune responses by increasing interferon-γ levels while CSGG induced immunosuppressive regulatory T cells and interleukin-10. These findings demonstrate that bacterial polysaccharides have a …