Fructooligosaccharides (FOSs) are short and medium chains of fructose in which two to nine fructosyl units are bound by β 2,1 glycosidic linkages and a glucose molecule is present at the end of the chain joined by an α 1,2 glycosidic bond. Fructooligosaccharides occur in various plants such as onions, asparagus, artichokes and garlic. 1)Short chain FOSs (sc FOSs), such as 1 kestose, nystose and 1F β fructofuranosylnystose can also be enzymatically synthesized from sucrose. 2,3) Fructooligosaccharides are prebiotics that have been studied in detail and they are widely used as a food ingredient. 4 6) A commercially available FOS product comprises a mixture of sc FOSs.4) The primary physiological function of FOSs is the preferential stimulation of colonic Bifidobacteria. The effect has been confirmed in experimental animal models and in humans.7 10) Other physiological functions of FOSs include the improvement of calcium bioavailability, the modulation of lipid metabolism and the inhibition of potential pathogen metabolism. 5,11,12) These functions are mainly due to the indigestibility 13) and selective fermentability of FOSs in the gastrointestinal system. 14)Studies have recently focused more on the immunomodulating effect of oligosaccharides.15,16) Nakamura et al . 17) reported that FOSs enhance the IgA response in the intestine of infant mice and Hosono et al . 18) also showed that the amounts of fecal IgA and of IgA secretion by Peyer s Patch cells are increased in FOS treated mice. Furthermore, Nicole et al . 19) and Pierre et al . 20) demonstrated that dietary FOSs influence gut associated lymphoid tissue.Although beneficial colonic bacteria proliferation and short chain fatty acid production in the gut are thought to be involved in the action mechanism of FOSs on the immune system, details have not been fully established. Furthermore, whether the prebiotic and immunomodulating activities differ among FOSs in vivo and in vitro remains obscure.We investigated the effects of 1 kestose and nystose, which are major components of a dietary FOS product, on intestinal microorganisms and on the splenocyte response to mitogenic stimulation in mice. MATERIALS AND METHODS Animals.Female 6 week old BALB c mice obtained from Charles River Japan (Yokohama, Japan) were maintained at 22 24 C and around 50% relative humidity under a 12 h light dark cycle. A commercial diet (CE 2, Clea Japan, Tokyo, Japan) and water were available ad libitum. The mice were separated by body weight into three experimental groups consisting of five animals. The experimental protocol was approved by the Animal Experiment Committee of Rakuno Gakuen University and the animals were managed in accordance with the Guide for the Care and Use of Laboratory Animals of the same institution. Bacterial strains. Lactobacillus reuteri JCM 1081, Lactobacillus murinus JCM 1717 T and Lactobacillus intestinalis JCM 7548T purchased from the Japan Collection of Microorganisms were cultured at 37 C in MRS medium for DNA extraction.Preparation and administration o...