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

Cholic acid (CA) transport was investigated in nine intestinal Bifidobacterium strains. Upon energization with glucose, all of the bificlobacteria accumulated CA. The driving force behind CA accumulation was found to be the transmembrane proton gradient (DeltapH, alkaline interior). The levels of accumulated CA generally coincided with the theoretical values, which were calculated by the Henderson-Hasselbalch equation using the measured internal pH values of the bificlobacteria, and a pK(a) value of 6.4 for CA. These results suggest that the mechanism of CA accumulation is based on the diffusion of a hydrophobic weak acid across the bacterial cell membrane, and its dissociation according to the DeltapH 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 1192 T. These short-chain fatty acids, which are weak acids, reduced the DeltapH, thereby decreasing CA accumulation in a dose-dependent manner. The bificlobacteria did not alter or modify the CA molecule. The probiotic potential of CA accumulation in vivo is discussed in relation to human bile acid metabolism.