The invention relates to novel 7β-hydroxysteroid dehydrogenases which are obtainable from bacteria of the genus Collinsella, especially of the strain Collinsella aerofaciens, to the sequences encoding said enzymes, to methods for producing said enzymes and to their use in the enzymatic conversion of cholic acid compounds, and especially in the production of ursodeoxycholic acid (UDCS). The invention also relates to novel methods for the synthesis UDCS.