The microbiome of the gut, steroid metabolism and the environment

Steroids are produced by all members of the eukaryotic kingdom (except insects) and have a wide range of functions. Vertebrates form cholesterol, which is a major component of cell membranes and serves as precursor for other steroids, such as bile acids, vitamin D and steroid hormones. Steroids are largely metabolized in the liver and excreted via the urine and faeces - before faecal excretion, some of them are reabsorbed and transported to the liver again (enterohepatic circulation). During gut passage, they are in contact with micro-organisms. Although bacteria and archaea cannot form steroids, some bacterial species can metabolize them to produce metabolites that may be inactive or may show activities other than those of the substance formed by the vertebrate. Some bacterial steroid metabolites are thought to be involved in the onset of diseases such as elevated blood pressure and colonic adenoma. After excretion, steroids (endogenous or synthetic steroids such as oestrogens or glucocorticoids) can be degraded by aerobic or anaerobic pathways but some metabolites, for example the bacterial cholesterol metabolite coprostanol, may persist in the environment for a long time. Some excreted steroids are biologically active, so it is important to ensure that they do not contaminate surface and ground water or the soil.