Cholic acid accumulation and its diminution by short-chain fatty acids in bifidobacteria

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Cholic acid (CA) transport was investigated in nine intestinal Bifidobacterium strains. Uponenergization with glucose, all of the bifidobacteria accumulated CA. The driving force behind CAaccumulation was found to be the transmembrane proton gradient (ΔpH, alkaline interior). Thelevels of accumulated CA generally coincided with the theoretical values, which were calculatedby the Henderson–Hasselbalch equation using the measured internal pH values of thebifidobacteria, and a pKa value of 6・4 for CA. These results suggest that the mechanism of CAaccumulation is based on the diffusion of a hydrophobic weak acid across the bacterial cellmembrane, and its dissociation according to the ΔpH value. A mixture of short-chain fatty acids(acetate, propionate and butyrate) at the appropriate colonic concentration (117 mM in total)reduced CA accumulation in Bifidobacterium breve JCM 1192T. These short-chain fatty acids,which are weak acids, reduced the ΔpH, thereby decreasing CA accumulation in a dose-dependentmanner. The bifidobacteria did not alter or modify the CA molecule. The probiotic potential ofCA accumulation in vivo is discussed in relation to human bile acid metabolism.
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