fig2

Figure 2. Competitive environmental fitness parameters that influence synbiotic mechanisms. (A) Exemplary prebiotic utilization strategies by a co-administered probiotic; left side of the cell: extracellular hydrolysis of the oligo- or polymeric substrate (e.g., FOS or inulin) with the subsequent import of released mono-saccharides or small oligosaccharides (i.e., sucrose and 1-kestose) that enter intracellular metabolism, with partial release of the substrate into the environment as “public goods”; right side of the cell: direct import of small oligosaccharides (e.g., tri- and tetra-saccharides in GOS) with subsequent intracellular hydrolysis and metabolization, i.e., “privatized goods”; (B) exemplary scenarios of endogenous microbiome members that can compete with the co-administered probiotic for the utilization of the prebiotic substrates. These competing capacities may be redundantly present in multiple members of the microbiome. Moreover, these redundant utilization pathways may have varying affinities and utilization rates for the prebiotic substrates, indicated by variations in arrow thickness; (C) In addition to the microbiome’s redundancy in prebiotic utilization capacities, their variable substrate affinities and utilization rates, the high relative abundance of competing microbes in comparison to the introduced probiotic favors complementary synbiotic effects rather than synergistic synbiotic effects, especially when the prebiotic substrate can readily be utilized by multiple members in the ecosystem and thus has poor species- or strain-specific selectivity. Consequently, the competing capacity of the dominating members of the microbiome (blue, pink, and orange) prevents the utilization of the prebiotic by, and the resulting growth stimulation of, the co-administered probiotic (green). FOS: Fructo-oligosaccharides; GOS: galacto-oligosaccharides.