In vivo relationship between intestinal bifidobacteria overgrowth and Bacteroides fragilis repression induced by consumption of bifidobacterial cell-free whey

Cell-free whey from a selected strain, Bifidobacterium breve C50, induced a n increase in bifidobacteria associated with a Bacteroides fragilis reducti on in the gut of conventional mice and humans. The purpose of our study was to investigate the mechanism of B. fragilis repression. C50 cell-free whey was given for 15 days to conventional or ex-germ-free mice mono-associated to the strain B, fragilis CFPL 358. Conventional and ex-germ-free control mice received whey which was incapable of promoting intestinal bifidobacter ia and of reducing B, fragilis. Bacterial counting was carried out in the i leum, caecum and colon of both mouse models. The C50 cell-free whey induced a significant increase in endogenous bifidobacteria in the ileum of conven tional mice, whereas B, fragilis was below detectable levels throughout the intestine. In ex-germ-free mice mono-associated with B. fragilis, the stra in was seen to be at a high level through the whole intestine and no signif icant difference in counts was observed according to the whey administered to animals. The data indicated that a prerequisite for C50 cell-free whey r epressive activity against B. fragilis is colonization of the mouse gut wit h complex bacterial microflora. With the exception of the distal ileum, the bifidobacterial overgrowth did not, however, support B. fragilis reduction . It is likely that in the caecum and colon some other bacteria participate d in the process. (C) 1999 Academic Press.