Analysis of faecal substrates sheds light into coprostanol origin, preservation and diagenesis

Cholesterol plays an essential role in eukaryotic life as a structural building block of cell membranes and for signalling1-2, and it is the precursor of vital biomolecules including bile acids or vitamin D3. While the mechanisms of cholesterol homeostasis have been largely investigated but the role of microbiome-related functions on cholesterol metabolism are still poorly understood. The gastro-intestinal tract hosts millions of bacteria, viruses, archaea, parasites and fungi that influence the metabolic abilities of their host. Many of these microbes generate metabolites that the enzymes from the host are not capable of producing4. In most individuals, much of cholesterol in the gut undergoes microbial conversion to its major metabolite in faeces, coprostanol. Coprostanol, a 5β-stanol, is uniquely formed through microbial saturation of its precursor Δ5 - sterol cholesterol by specific bacteria present in the gut of mammals5, and it's generation depends primarily on the diet, the endogenous cholesterol biosynthesis and the efficiency of the gut microbial action, which produces a specific chemical signature6-7. Coprostanol, is the result of the conversion of dietary and de novo synthesized cholesterol6. This microbial conversion occurs in most humans, and prevents the reabsorption of cholesterol in the colon, which can be an advantage against cholesterol-related cardiovascular diseases4. Nevertheless, the microbial actors behind this conversion are yet to be elucidated, and only a few cholesterol reducing bacteria, mainly Bacteroides and Eubacterium, have proven to be able to convert cholesterol into coprostanol in vitro8-9. Further, the genes or enzymes involved in this conversion remain poorly investigated. Once excreted, coprostanol is believed to remain mostly intact during early diagenesis, which together with the characteristic distribution of coprostanol and its homolog