## 4 Discussion  
This study evaluated combinations of _Bifidobacterium animals_ subsp. _lactis_ BB-12 with seven fructans ranging from oligofructose (OF1-OF2; low degree of polymerization (DP)), mixtures of oligofructose/inulin (OF/IN1-OF/IN2) up to inulin (IN1-IN2-IN3; high DP), using a combination of monoculture incubations and short-term colonic incubations incorporating a simulation of the complex human colon microbiota. The conducted experiments not only revealed a marked modulation of microbial activity and composition upon fructan administration but it also pointed out several additional effects due to co-administration of BB-12. First, monoculture experiments revealed that the fermentation of fructans by BB-12 was substrate-dependent, with BB-12 most strongly fermenting fructans with low DP (OF1 and OF2). This followed from the strong growth of BB-12 and strong increase in acetate/lactate levels for OF1 and OF2. Other studies have indeed already demonstrated that fructan fermentation by BB-12 depends on the molecular structure of the fructan [20; 36; 37], with enhanced growth of BB-12 for fructans with low DP [20; 38; 39]. Furthermore, inability of BB-12 to metabolize long-chain inulins, has been observed before as well [37; 38; 40]. Overall, these monoculture-derived data confirmed the potential of developing symbiotic combinations between BB-12 and the tested fructan-type carbohydrates, where specifically selected fructans would support the growth of BB-12. A peculiar finding was that, in presence of a complex microbiota, fructan administration boosted the growth of BB-12 for all fructans tested across all three donors (as measured via abundance of OTU5 that was exclusively detected when BB-12 was dosed). While being most pronounced for OF1, this effect was also observed for fructans with high DP that were hardly fermented by BB-12 in monoculture, suggesting that BB-12 has high potential to compete with the human colon microbiota when fructans are present, independently of the molecular structure of the fructan. As a potential explanation, when focusing on the degradation of ratfiling, [PERSON] et al. [20] observed preferential fermentation of shorter fructan fractions. Such fermentation pattern has previously been linked with intracellular fructan fermentation [41; 42]. By importing short oligofructose fractions, BB-12 could thus have a competitive advantage as opposed to other lactic acid bacteria that ferment fructans via extracellular mechanisms by which they have to import fructose that could meanwhile already have been fermented by other gut microbes [43]. While during monoculture experiments, such oligosaccharides were already present in some test products as such (OF1-OF2 and to a lesser extent OF/IN1-OF/IN2), in presence of a mixed microbiota, members of the indigenous microbiota could release oligosaccharides from long fructans (IN1-IN2-IN3), which BB-12 could then ferment intracellularly. This could explain the observed competitiveness of BB-12 to grow within the complex colon microbiota in presence of fructans. As a side remark, the fitness of BB-12 also followed from the donor screening (experiment 2), where BB-12 was capable to grow on starch in presence of sterile fecal suspensions of all six donors tested as followed from increased _Bifidobacterium_ numbers and a reduction in pH due to increased acetate/lactate levels. Prior to elaborating on the effects of symbiotic administration on the microbial activity and composition, it has to be noted that treatment with the fructan-type carbohydrates as such already exerted strong effects. This involved a markedly decreased pH due to increased production of acetate and lactate, while proteolytic fermentation was lowered. At community level, the increased production of acidic end-metabolites was associated with a strong bifidogenic effect, mainly attributed to stimulation of _Bifidobacteriaceae_ OTUs related to _Bifidobacterium adolescents_ and _Bifidobacterium longum_.