Abstract
Arabinogalactan (AG) has been studied as a potential prebiotic in view of stimulating bifidobacteria presence in the gut microbiota. However, bifidobacteria prefer fermentation of oligosaccharides to that of polysaccharides. The contribution of other gut bacteria may allow better growth of bifidobacteria on AG. β-galactanases and β-galactosidases are the main enzymes for the degradation of AG. Additional enzymes such as α-L-arabinofuranosidase and β-L-arabinopyranosidase are required to remove the arabinose side chains. All of these predicted functions are encoded by the genomes of both Bifidobacterium longum subsp. longum NCC 2705 and Bacteroides caccae ATCC 43185. However, neither strain was able to grow significantly on AG, with 25% (B. longum subsp. longum NCC 2705) and 39% (Bac. caccae ATCC 43185) of AG degraded after 48-h fermentation, respectively. In this study, the β-galactanase, β-galactosidase, α-L-arabinofuranosidase, and β-L-arabinopyranosidase from both strains were investigated. The extracellular β-galactosidases of both B. longum subsp. longum NCC 2705 and Bac. caccae ATCC 43185 were able to cleave the β-1,3; 1,4 and 1,6 linkages. However, the β-galactosidase activity of B. longum subsp. longum NCC 2705 was weaker for the β-1,4 linkage, compared with the β-1,3 and 1,6 linkages. The arabinose side chains of AG inhibited the cleavage of β-1,3 and 1,6 linkages by the endo-β-galactanase from both strains, and partially inhibited the cleavage of β-1,4 linkages by the endo-β-1,4 galactanase from Bac. caccae ATCC 43185. The α-L-arabinofuranosidase and β-L-arabinopyranosidase from both strains were unable to cleave arabinose from AG under the conditions used. These results show limited breakdown of AG by these two strains in monoculture. When cocultured with Bac. caccae ATCC 43185, B. longum subsp. longum NCC 2705 grew significantly better than in monoculture on AG after 6 h of fermentation (p < 0.05). The coculture showed 48% AG degradation after 48 h of fermentation, along with reduced pH. Furthermore, compared to monoculture of Bac. caccae ATCC 43185, the concentration of succinate significantly increased from 0.01 ± 0.01 to 4.41 ± 0.61 mM, whereas propionate significantly decreased from 13.07 ± 0.37 to 9.75 ± 2.01 mM in the coculture (p < 0.05). These results suggest that the growth and metabolic activities of Bac. caccae ATCC 43185 were restrained in the coculture, as the pH decreased due to the metabolism of B. longum subsp. longum NCC 2705.
Highlights
Colon microbes ferment dietary fiber to produce short chain fatty acids (SCFAs) that have many roles in promoting gut health [1]
There was no difference in the number of viable Bac. caccae ATCC 43185 between monoculture and coculture (Figure 4b). These results indicate that compared to monoculture, B. longum subsp. longum NCC 2705 showed higher abundance when it was cocultured with Bac. caccae ATCC 43185 in YC-AG
We demonstrate why neither strain was able to completely degrade AG in pure culture even though annotated gene functions are present in the genome of both B. longum subsp. longum NCC 2705 and Bac. caccae ATCC 43185
Summary
Colon microbes ferment dietary fiber to produce short chain fatty acids (SCFAs) that have many roles in promoting gut health [1]. The ratio of SCFAs produced varies with the metabolism of each gut bacterial species and carbohydrate state [2]. The SCFAs may reduce pH in the colon and results in shifting of the composition of gut microbiota, which is determined by the acid tolerance of gut bacterial species [2]. The backbone of type I AG and side chains of type II AG are substituted with α-arabinofuranose and/or, less frequently, β-arabinopyranose at the non-reducing terminus of side chains of both types of AG [7,8]. Gum arabic (GA) has a similar structure to type II AG, which includes a β-1,3 linked galactan backbone and β-1,6 linked galactan side chains with some arabinose substitutions [10]
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