The water-soluble thione/thiol ergothioneine (ET) wasfirst isolated in 1909 by Charles Tanret [1], from theergot fungus Claviceps purpurea. This fungus is notorious for the toxicity of some of its metabolites tohumans, causing ergotism [2], which has even beenlinked to the Salem witch trials [3]. However, ergotismhas nothing to do with ET, which is instead very safefor human consumption and is synthesized by a rangeof other fungi and some bacteria (reviewed in [4–9]).Its biosynthetic pathways are reviewed in detail in [6].Indeed, as far as we know, humans and other animalsobtain all their ET from the diet [4,5,7–10], whereasplants seem to obtain it from fungi and other soilmicroorganisms.

The dietary thione-thiol, ergothioneine (ET), accumulates in human and animal tissues and may play important roles in disease prevention. ET biosynthesis has only been described in fungi and certain bacteria, and humans and animals are widely assumed to accumulate ET solely from diet. However, a recent study suggested that Lactobacillus/Limosilactobacillus reuteri , a commensal gut bacterium, may produce ET, thereby protecting the host against social defeat stress and sleep disturbances. Upon our further investigation, no evidence of ET biosynthesis was observed in L. reuteri when a heavy-labelled histidine precursor was administered. Instead, we discovered that L. reuteri avidly accumulates ET. This observation may indicate a possible mechanism by which the gut microbiota could influence tissue levels of ET in the host.