## 2 Materials and Methods
### Strain Preservation and Quality Control  
The provided probiotic strain was grown in liquid culture medium (reinforced clostridial medium--RCM) under anaerobic conditions for 24 h at 37 \({}^{\circ}\)C. A subculture was made on a selective and differentiating solid growth medium ((transgalactosylated oligosaccharidesadide agar, containing 12.5 mg/L muiprocin) and the strain was grown for a period of 96 h at 37 \({}^{\circ}\)C. A second subculture was made by picking up a colony and growing in RCM under anaerobic conditions for 24 h at 37 \({}^{\circ}\)C. The obtained culture was aliquoted and cryopreserved at \(-80\)\({}^{\circ}\)C by mixing equal volumes of the culture and a pre-reduced 40% glycerol suspension. A sample of the preserved bacterial culture was subjected to DNA extraction (performed according to [PERSON] et al. [28] with minor modifications as reported by [PERSON] et al. [29]) and send out to LGC Genomics GmbH (Berlin, Germany) for 16S rRNA gene sequencing using the primers and PCR conditions as described by [PERSON] et al. [30]. The obtained sequence was searched against a curated 16S rRNA gene dataset of the ribosomal database project (RDP), using the Seqmatch tool, to confirm strain identity. Further, the cryopreserved strain was grown in a standardized fashion under optimal conditions (RCM medium inoculated at 1%, 37 \({}^{\circ}\)C, anaerobic conditions) prior to inoculation in each experiment. The optimal incubation period for growth of the preserved strain was determined at 16 h.  
Fermentation of Fractans by BB-12 in Monoculture during Short-Term Colonic Incubations (Experiment 1)  
To evaluate the symbiotic potential of combining the BB-12 test strain with each of the seven fractans, first, a short-term colonic experiment was performed in the absence of indigenous colon microbiota. Briefly, each of the seven fractans were dissolved in water, filtered through a 0.22 \(\mu\)m filter and dosed aseptically to reactors containing sterile carbohydrate-depleted nutritional medium (5.2 g/L K\({}_{2}\)HPO\({}_{4}\), 16.3 g/L KH\({}_{2}\)PO\({}_{4}\), 2.0 g/L NaHCO\({}_{3}\) (Chem-lab NV, Zedelgem, Belgium), 2.0 g/L yeast extract, 2.0 g/L peptone (Oxoid, Aalst, Belgium), 1.0 g/L mucin ([PERSON], Karlsruhe, Germany), 0.5 g/L L-cysteine and 2.0 mL/L Tween 80; pH = 6.5) in order to reach a concentration of 5 g/L of each fractan at the start of the incubation. A dose corresponding with 10\({}^{9}\) CFU of the BB-12 strain was co-administered, reaching a concentration 1.5 \(\times\) 10\({}^{7}\) CFU/mL at start of the incubation. A sterile anaerobic fecal suspension was prepared from a freshly collected fecal sample of a healthy human donor as described by [PERSON] et al. [31] and inoculated at 10% (_v/v_) into the reactors to provide the BB-12 strain with necessary co-factors to perform substrate breakdown. A blank incubation was included, where BB-12 strain was administered without co-supplementation of fractans. All reactors were anaerobically incubated at 37 \({}^{\circ}\)C for 48 h under continuous mixing (90 rpm). All experiments were performed in biological triplicate. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Code**} & **Commercial** & \multirow{2}{*}{**Supplier**} & **Purity Fractan** & **Sugars 1** & **DP** \\  & **Name** & & & & \\ \hline OF1 & Frutalose® OFP & Sensus, The Netherlands & 89 & 8 & 4 \\ \hline OF2 & Orafti®P95 & Beneo, Belgium & 93.2-97.5 & 2.5–6.8 & 4-5 \\ \hline OF/IN1 & Fibulose® P97 & Cosucra, Belgium & 97 \(\pm\) 2 & 3 \(\pm\) 2 & 5.5 \\ \hline OF/IN2 & Orafti® Synergy1 & Beneo, Belgium & 92 \(\pm\) 2 & 8 \(\pm\) 2 & 14.5 \\ \hline IN1 & Fibruline® Instant & Cosucra, Belgium & \(>\)90 & \(<\)10 & 9 \\ \hline IN2 & Orafti® GR & Beneo, Belgium & \(>\)90 & \(<\)10 & 10 \\ \hline IN3 & Frutafit® IQ & Sensus, The Netherlands & \(>\)90 & \(<\)10 & 8–13 \\ \hline \end{tabular}  
* including fructose, glucose and sucrose.  
\end{table}