The connection between intestinal microbes and human health has been appreciated since the 1880s with Theodor Escherich’s investigation of Escherichia coli and other fecal bacteria. Escherich hypothesized that indigenous micoorganisms play roles in both digestion and intestinal diseases . In the last century, our understanding of the bacteria, viruses, archaea, and eukaryotes that normally inhabit the gut has expanded alongside the rest of the field of microbiology, and numerous fundamental roles have been established for this community, now termed the microbiome. As speculated by Escherich, these roles definitively include nutrient digestion [2, 3] and protection from invading pathogens  but also extend to short- and long-term instruction of the immune system [5–7] and production of a wide range of metabolites that are unable to be produced by human physiology.
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Citation Glowacki RWP, Martens EC (2020) In sickness and health: Effects of gut microbial metabolites on human physiology. PLoS Pathog 16(4): e1008370. https://doi.org/10.1371/journal.ppat.1008370