Isolates from normal human intestinal flora but not lactic acid bacteria exhibit 7α- and 7β-hydroxysteroid dehydrogenase activities

Abstract

Ursodeoxycholic acid (UDCA)-producing bacteria are of clinical and industrial interest due to the multiple beneficial effects of this bile acid on human health. UDCA is the 7β-OH epimer of the primary (i.e. synthesized by the liver) bile acid chenodeoxycholic acid (CDCA). Epimerization proceeds in two subsequent and reversible steps, catalysed by a 7α- and a 7β-hydroxysteroid dehydrogenase (7α- and 7β-HSDH), with 7oxo-lithocholic acid (7oxo-LCA) as the intermediate product. The aim of this study was to test the 7α- and 7β-HSDH activities of anaerobic whole cell cultures of a number of lactic acid bacteria and human intestinal isolates, using CDCA, UDCA and 7oxo-LCA as the substrates. Among 140 strains tested, 21 exhibited at least one of both 7-HSDH activities. 7α-HSDH activity was detected in six strains, 7β-HSDH in nine strains, and both activities in six other strains. All active strains were isolated from normal human and infant faeces. They belonged to the genera Clostridium, Eubacterium and Ruminococcus, whereas no strain of Lactobacillus, Bifidobacterium or Streptococcus was found to be active under our study conditions. The present study therefore revealed, for the first time, a number of normal human intestinal isolates supporting the epimerization of CDCA to UDCA, and further extended our knowledge of those intestinal bacteria which are responsible for 7α- or 7β-HSDH activity. Key words: screening, bile acids, epimerization, intestinal microflora, lactic acid bacteria, probiotics.