232 Moreover, subsets of mice could be identiﬁed based on microbiota composition that clustered together corre- sponding to effectiveness of the BFMP treatment. These results reinforce the notion that the composition of the endogenous intestinal microbiota plays an important role in the host response to the probiotic intervention, thereby inﬂuencing the effectiveness of probiotic intervention. In this study, it was also observed that BFMP consumption resulted in a metabolic shift; a decreased caecal pH and alteration in short- chain fatty acid proﬁles. It must be noted, however, that these beneﬁcial effects cannot be directly linked to the activity of this speciﬁc Biﬁdobacte- rium strain since a non-fermented milk product was used as control product. Overall, the data support the hypothesis that probiotics are not only able to inﬂuence the compo- sition, but also the metabolic activity of the intestinal microbiota (De Preter et al. 2010). Both of these effects need to be studied separately to get a complete picture of the inﬂuence of probiotic intervention on the intestinal microbiota. This is also emphasised in a recent study in which the effect of a speciﬁc synbiotic product on the intestinal microbiota was examined (Vitali et al. 2010). This study showed no inﬂuence on the composition of dominant faecal microbiota, but signiﬁcant changes in faecal metabolic proﬁles were observed. These results suggest that synbiotic intervention is able to affect the metabolic activity of the intestinal microbiota while maintaining microbiota composition with respect to its predominant components. Conclusions Knowledge on the composition and diversity of a healthy microbiota and on how changes in the intestinal microbiota lead to or are associated with disease, is far from complete. More research is needed to examine the species and strain diversity in the GI tract, the diversity of microbial genes (microbiome) in the GI tract and the activity of these genes (mRNA, protein and metabolite production). For future probiotic research it is important to determine the level of compositional and functional microbial dysbiosis in rele- vant target populations and identify potential members of the healthy microbiota to counteract the dysbiosis. Understanding the molecular mechanisms of action attrib- uted to commensal and pathogenic bacteria will contribute to better designed probiotic products. In the future, this knowledge can be applied in the development of tailor- made probiotics designed for clearly characterised target populations. 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