Biotransformation of host bile salts by gut microbes results in generation of secondary bile salt species that have biological and physicochemical properties that are distinct from the parent compounds. There is increased awareness that a bile salt-gut microbiome axis modulates various processes in the host, including innate and adaptive immunity, by interaction of microbial bile salt metabolites with host receptors. Omics and targeted approaches have vastly expanded the number and repertoire of secondary bile salt species. A new class of microbial bile salt metabolites was reported in 2020 and comprises bile salts that are conjugated by microbial enzymes. Amino acids other than those employed by host enzymes (glycine and taurine) are used as substrates in the formation of these microbial bile salt conjugates (MBSCs). Leucocholic acid, phenylalanocholic acid and tyrosocholic acid were the first MBSCs identified in mice and humans. The number of distinct MBSCs is now approaching 50, with variation both at the level of bile salt and amino acid employed for conjugation. Evidence is emerging that MBSC generation is a common feature of human gut bacteria, and initial links with disease states have been reported. In this review, we discuss this intriguing new class of secondary bile salts, with yet enigmatic function.

Department of General Visceral and Transplant Surgery University Hospital Aachen 52074 Aachen Germany

Department of Surgery NUTRIM School of Nutrition and Translational Research in Metabolism Maastricht University 6200 MD Maastricht The Netherlands

Institute of Medical Biochemistry and Laboratory Diagnostics Faculty General Hospital and 1st Faculty of Medicine Charles University 12808 Prague Czech Republic

Institute of Organic Chemistry RWTH Aachen University 52074 Aachen Germany

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