Formation of hyodeoxycholic acid from muricholic acid and hyocholic acid by an unidentified gram-positive rod termed HDCA-1 isolated from rat intestinal microflora

Abstract

From the rat intestinal microflora we isolated a gram-positive rod, termed HDCA-1, that is a member of a not previously described genomic species and that is able to transform the 3alpha,6beta, 7beta-trihydroxy bile acid beta-muricholic acid into hyodeoxycholic acid (3alpha,6alpha-dihydroxy acid) by dehydroxylation of the 7beta-hydroxy group and epimerization of the 6beta-hydroxy group into a 6alpha-hydroxy group. Other bile acids that were also transformed into hyodeoxycholic acid were hyocholic acid (3alpha, 6alpha,7alpha-trihydroxy acid), alpha-muricholic acid (3alpha,6beta, 7alpha-trihydroxy acid), and omega-muricholic acid (3alpha,6alpha, 7beta-trihydroxy acid). The strain HDCA-1 could not be grown unless a nonconjugated 7-hydroxylated bile acid and an unidentified growth factor produced by a Ruminococcus productus strain that was also isolated from the intestinal microflora were added to the culture medium. Germfree rats selectively associated with the strain HDCA-1 plus a bile acid-deconjugating strain and the growth factor-producing R. productus strain converted beta-muricholic acid almost completely into hyodeoxycholic acid.

FIG. 1

Scanning electron micrograph of a colony of strain HDCA-1, showing a monolayer of bacteria.

FIG. 2

Phylogenetic tree of HDCA-1 (ID3154) based on the alignment of the most similar 16S rDNA sequences, using the GeneCompar software package (Applied Maths). Type species are shown in bold, designations of type strains are followed by a T, and invalid species names are given in quotes.

FIG. 3

Transformation of β-muricholic acid (a) and hyocholic acid (b) by strain HDCA-1 as a function of time. □, β-muricholic acid (panel a) and hyocholic acid (panel b); ■, hyodeoxycholic acid; ▴, 3α,6α-dihydroxy-7-oxo-5β-cholanoic acid; and ●, 3α-hydroxy-6-oxo-5β-cholanoic acid.