Strains of bacteria characterized by exhibiting: (a) a 7.alpha.-dehydroxylase activity of less than 50%, and (b) bile acid deconjugation activity of less than 50%, and descendants, mutants and derivatives thereof preserving activities (a) and (b); and a pharmaceutical composition using one or more of such strains and use of same for preventing and treating diseases associated with or caused by an altered metabolism of bile acids.