Processes 2019, 7,97
propagation through nutrient competition and may help explain how healthy individuals can be
asymptomatically colonized.
Figure 1. Predicted cross-feeding of byproducts between the four species. (A) Species exchange rates
specified in mmol/gDW/h. Secretion rates are positive, and uptake rates are negative. (B) Byproduct
cross-feeding patterns identified from the species uptake and secretion fluxes in (A).
The biofilm model also was tuned for healthy nutrient levels to produce acetate:propionate:
butyrate fractions of 60%:20%:20% when averaged across the biofilm to be consistent with in vivo
studies [5,59] (see Materials and Methods). The model predicted the total SCFA concentration to be
32.5 mmol/L (Figure 2C), which was in reasonable agreement with an in vivo study with a control
diet that yielded 41.1 mmol/L of total SCFAs [60]. One possible explanation for the lower SCFA levels
predicted by our model is the simplified diet (glucose, eight amino acids) compared to the control diet
used experimentally.
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