## 3 Results
### Donor Screening (Experiment 2)  
The donor screening of six donors consisted of two types of colonic incubations with starch as main carbon source, i.e., (1) incubations with sterile fecal suspension of a given donor in presence of BB-12 to confirm engraftment of BB-12 in background suspension derived from a certain donor, and (2) incubations with non-sterile fecal suspension of each donor in absence of BB-12 to investigate the metabolic activity of the fecal microbiota derived from each donor as such. To focus on consistent observations, the averages over the six donors are presented in Figure 3, while individual data are reported in Figures S1 and S2.  
Figure 2: Metabolic activity of BB-12 upon dosing fructans with short (OF1, OF2), intermediate (OF/IN1, OF/IN2) and high degree of polymerization (IN1, IN2 and IN3) versus a blank incubation in monocultures. Bars represents the average changes (\(\pm\)SD) in pH (**A**), acetate ((**B**); mM) and lactate ((**C**); mM) levels between 0 and 6 h (light grey), 6 h and 24 h (dark grey) and 24 h and 48 h (stripes) as well as changes in _Bifidobacterium_ levels ((**D**); expressed as the ratio of their absolute abundance as measured via qPCR at the end versus the beginning of the incubation (48 h/0 h)) (\(n=3\)). Conditions that have no letter in common are statistically significantly different from one another (\(p<0.05\)). BB-12 = _Bifidobacterium animals_ subsp. _lactis_; SD = standard deviation; OF = oligofructose; IN = inulin.  
First, BB-12 grew in the fecal background of all donors tested as shown by a reduction in pH and increase of acetate, lactate and _Bifidobacterium_ spp. levels after 48 h of incubation (Figure 3). As was observed during experiment 1, propionate, butyrate and bCFA were not produced in the probiotic incubations, confirming sterility of the fecal suspensions. Supplementation of BB-12 generated the highest acetate concentrations for donors D and E, while the lowest acetate levels were observed for donor B (Figure S1). This was in line with the observations of bifidobacterial growth: while BB-12 was enriched for all donors, the strongest and mildest increases were observed for donors D/E and donor B, respectively. Overall, acetate was mainly produced during the 6-24 h timeframe upon supplementing BB-12. Further, BB-12 produced similar, yet low lactate quantities for all donors in the incubations with sterile fecal suspension.  
Next, the metabolic profiles of the fecal microbiota of all six donors revealed consistently reduced pH together with increased gas production and enhanced production of acetate, lactate, propionate, butyrate and bCFA (Figure 3). Upon its initial production, lactate was subsequently consumed. While some minor interindividual differences were observed, all donors revealed similar metabolic responses (Figure S2). Furthermore, enrichment of the Bifidobacterium population was observed for all donors during the colonic experiments, with strongest effects observed for donors C and F, while lowest enrichment was observed for donor E.  
Altogether, these data indicate that all donors could be selected for the final experiments. In the end, three representative donors with sufficient interindividual differences were selected, i.e., donors C (highest butyrate production), donor D (highest acetate production) and donor E (lowest bCFA production).