Modulation of genotoxic enzyme activities by nondigestible oligosaccharidemetabolism in in-vitro human gut bacterial ecosystems

Supplementation of the human diet with prebiotic substances such as inulin and nondigestible oligosaccharides (NDO), e.g., galacto-oligosaccharides (G OS), has been associated with various health benefits. However, little info rmation is available regarding the spatial location of their metabolism in human gut bacterial ecosystems. Therefore, the present study investigated t he metabolism of inulin and GOS with respect to bacterial growth, bifidobac terial stimulatory properties and anti-mutagenicity potential, in a three-s tage continuous culture model of the colon which reproduces the physicochem ical characteristics of the proximal (V1) and distal (V2, V3) colons. Ferme ntation of both carbohydrates was rapid, and occurred primarily in V1, as e videnced by acid formation. Inulin metabolism was associated with 10-fold s timulation of lactobacillus populations, together with smaller increases in bifidobacterial cell counts in V1. However, peptostreptococci, enterococci and Clostridium perfringens also increased in this fermentation vessel. In contrast, GOS was only weakly bifidogenic in V1, although these bacteria d id proliferate in V2. GOS also increased lactobacilli by an order of magnit ude in V1. However, overall changes in microbial populations resulting from inulin or GOS addition were minimal in V2 and V3. Potential beneficial eff ects of inulin metabolism included minor reductions in beta -glucosidase an d beta -glucuronidase, whereas GOS strongly suppressed these enzymes, toget her with arylsulphatase (AS). Growth of putatively health promoting micro-o rganisms was not only associated with reductions in enzymes linked to genot oxicity. For example, both carbohydrates stimulated synthesis of nitroreduc tase and azoreductase, throughout the fermentation system, while inulin inc reased AS. Colonic transit time is an important factor in bacterial metabol ism in the large bowel, and these data suggest that, in some circumstances, NDO fermentation will occur principally in the proximal colon.