## 4 Discussion
Strong bifidogenic effects have indeed been linked with consumption of fructan-type carbohydrates in both in vitro and in vivo studies (as reviewed by [PERSON] et al. [44]). _Bifidobacterium_ species are well-documented primary substrate degraders, able to produce high amounts of acetate and lactate [45; 46]. Several health-promoting properties have been associated with bifidobacteria, including the protection against pathogenic infection in the gastrointestinal tract [47]. Furthermore, both _Bifidobacterium longum_ and _Bifidobacterium adolescents_ strains have been correlated with immunomodulatory responses, including reduction of inflammation and supporting intestinal epithelial barrier integrity [48; 49; 50]. Next to strong stimulation of _Bifidobacteriaceae_ numbers, fermentation of fructan-type carbohydrates resulted in stimulation of the lactate-producing _Enterococcaceae_, _Streptococcaceae_ and _Lactobacillusaceae_ families. _Lactobacillusaceae_ levels were most strongly increased for the fructans categorized by a low DP (OF1, OF2 and OF/IN1), resulting in a more pronounced stimulation of lactate levels compared to the fructans with a higher DP. Fermentation of the fructan-type carbohydrates characterized by a high DP on the other hand more strongly stimulated propionate and butyrate levels, indicating a product-dependent microbial response. Indeed, [PERSON] et al. [51] have shown a different SCFA profile upon supplementing oligofructose and inulin in vitro. While the major end-metabolites upon fermentation of oligofructose were acetate and lactate, fermentation of long-chain inulin resulted in high concentrations of butyrate. In the present study, increased butyrate levels were mainly attributed to the enhancement of OTUs related to the _Blautia ficeis_ and _Faecalibacterium pransnitzii_. _Blautia_ species have been shown to exert a key role in fermentation of carbohydrates [52] and are therefore often associated with a healthy gut microbiome. Furthermore, anti-inflammatory effects have been attributed to the _Blautia_ genus [53]. Similarly, _Faecalibacterium pransnitzii_ is a potent butyrate-producing bacterium with well-documented anti-inflammatory properties [54]. Like _Blautia_, this bacterial species has been reported to be reduced in the gut microbiome of patients suffering from intestinal disorders, including colorectal cancer [55]. Overall, the strong bifidogenic effect as well as stimulation of other health-related bacterial species stresses the potent prebiotic potential of fructans, with differences in metabolic response related with the molecular structure of fructans, stressing the importance of proper selection. While co-administration of BB-12 with the different fructan-type carbohydrates resulted in similar metabolic effects compared to fruct supplementation, some additional changes (e.g., increased acetate, propionate and butyrate levels) were observed for specific symbiotic combinations, indicating that BB-12 was involved in the fermentation process. Furthermore, when co-administered with fructans, BB-12 decreased _Enterobacteriaceae_ numbers, a family known to contain several opportunistic pathogenic species [56], a decrease that reached statistical significance for most fructans tested. Overall, the observed metabolic changes and the successful engraftment of the probiotic strain in presence of the prebiotic fructan-type carbohydrates confirm the synergistic effect of co-administration of BB-12. As a remark, during a first series of non-reported experiments, in which no background sterile fecal suspension was co-administered, none of the fructans were fermented by BB-12. However, when the test was repeated in presence of sterile fecal suspension, each of the fructan-type carbohydrates were fermented by the BB-12 strain in a product-specific fashion, indicating that essential co-factors were likely present in the sterile fecal suspensions enabling metabolic activity of BB-12.