We isolated cell wall polysaccharides (PS-1 and PS-2) from Bifidobacterium infantisBifidobacteria are anaerobic gram-positive bacteria and are believed to be associated with health-promoting effects in the host intestine. Recent studies with several Bifidobacterium strains have offered structural information concerning the cell surface construction, such as lipoteichoic acids (4,5,10,18), teichoic acids (26), and cell wall polysaccharides (7,17). Op den Camp et al. have isolated from Bifidobacterium bifidum a new type of lipoteichoic acid, containing a 1,2-linked polyglycerophosphate backbone between a glycolipid moiety and a linear polysaccharide, and found a unique structure, 1,5-linked ␤-D-galactofuranose homopolymer (galactofuranan), in the polysaccharide chain (18). On the other hand, Fischer et al. have provided another structure for lipoteichoic acid-like amphipathic polymers, which consist of a glycolipid moiety, 1,6-linked ␤-glucan, and 1,5-linked ␤-galactofuranan with side chains of monomeric glycerophosphate (4, 5). Such macroamphiphiles do not link to peptidoglycan but are considered to be anchored to membranes through their lipid moiety and should be common in several species of Bifidobacterium (10).Previously, we reported the carbohydrate composition of purified cell walls of Bifidobacterium infantis Reuter ATCC 15697 (24). In the present study, we isolated two distinct neutral polysaccharides, PS-1 and PS-2, from the purified cell walls of the same strain and found that the backbone of PS-2 is a 1,3-linked galactan consisting of ␤-D-galactofuranose and ␣-Dgalactopyranose in which more than 90% of the ␤-D-galactofuranosyl residues and about 30% of ␣-D-galactopyranosyl residues are occupied by ␤-D-glucopyranose at position 6. The backbone structure, 33)-␤-D-Gal f -(133)-␣-D-Gal p -(13, has been identified also in the lipopolysaccharide of Klebsiella pneumoniae (12-15, 27, 28), Pasteurella haemolytica (22, 23), Serratia marcescens (19,20), and Serratia plymuthica (1). It is of interest that the cell wall polysaccharide from B. infantis contains the same structure as O-specific antigen of some gramnegative bacteria.Preparation and carbohydrate composition of cell wall polysaccharides, PS-1 and PS-2. We previously described the growth conditions of B. infantis and the isolation procedure of purified cell walls (24). Determinations of amino acids, monosaccharides, fatty acids, and nucleic acids of the cell walls were also carried out by the same methods as previously reported (24). B. infantis cell walls consisted of neutral sugars (glucose and galactose), amino sugars (glucosamine and muramic acid), peptidoglycan-derived amino acids (alanine, glutamic acid, threonine, serine, and ornithine), and a small amount of fatty acids, so that they were considered to be free from contamination with nucleic acids and proteins (24).The purified cell walls (300 mg) were treated with 5% trichloroacetic acid at 4ЊC overnight and then centrifuged, and the supernatant was thoroughly dialyzed against deionized water. T...