Investigation of dextran oligosaccharides as a potential prebiotic

In this study, a range of novel and commercially available dextran-based gluco- oligosaccharides with different structures (percentage of a-l,2 linkages) and molecular weights were investigated to establish a link between structure and function of candidate colonic prebiotics. Their fermentation properties were tested in pH-controlled faecal batch cultures and the response of the healthy microbiota was monitored through fluorescent in situ hybridization, enumerating numerically and clinically significant bacterial groups and their metabolic activities through the determination of short chain fatty acids and rate of gas generation. An inversely proportional relationship between molecular weight and bifidogenicity/selectivity was observed in most cases. There appeared to be a threshold for this effect as 1 kDa and 0.5 kDa dextrans mediated similar responses in terms of bacteriology. Interestingly, most of the test dextrans produced high levels of propionate which may be beneficial for obese individuals. For this, we investigated the response of the obese faecal microbiota under the same experimental conditions. No differences in major faecal bacterial groups and phyla were observed between the lean and obese inocula and in general, the response to the test substrates was similar between the lean and obese fermentations. The prebiotic potential of highly branched 1 kDa dextran, which combined selectivity and bifidogenicity, was further investigated in continuous culture models simulating the human colon inoculated with lean faeces. We observed that Bifidobacterium sp. was selectively stimulated and SCF A production increased in the vessel corresponding to the distal colon, suggesting persistence of the gastrointestinal tract. We have established that low molecular weight gluco-oligosaccharides show prebiotic potential in vitro that should be further confirmed in in vivo human studies.