In vitro metabolism of estrogens by isolated intestinal micro-organisms and by human faecal microflora

The effect of intestinal micro-organisms on estrogen metabolism has been studied by in vitro experiments. Estrone, estradiol and 16α-hydroxyestrone were incubated with isolated intestinal microorganisms and with human faecal flora. 16-oxoestradiol, 15α-hydroxyestrone and estriol were also used as substrates in incubations with human faecal flora. After a simple purification step the identification of the metabolites was carried out by capillary gas chromatography and mass spectrometry. Alcatigenes faecalis, Pseudomonas aeruginosa and Staphylococcus aureus were able to convert estrone to estradiol and vice versa. The anaerobe Bacteroides fragilis could only reduce estrone to estradiol, Streptococcus faecalis performed the reverse reaction but both were able to convert estrone to 16α-hydroxyestrone. Mixed human faecal flora was able to interconvert estrone and estradiol both in aerobic and anaerobic conditions. In addition it reduced 16α-hydroxyestrone to estriol, 16-oxoestradiol to 16-epiestriol and 15α-hydroxyestrone to 15α-hydroxyestradiol. The significance of these results is discussed in the light of present knowledge of estrogen metabolism.