In this issue of Diabetes Care , Simon et al. (1) report the results of a study testing the hypothesis that the administration of Lactobacillus reuteri in subjects without diabetes improves insulin sensitivity. A quick review of the background may help us put this work in context. Our digestive tract hosts some 2 kg of microbes, made up of ∼100 trillion microorganisms. Present in sparse colonies in the upper intestine, this indigenous microbial community (microbiota) jams into the cecum, colon, and rectum; the appendix is the ultimate shelter when antibiotics wage a mass killing. The taxonomy lists hundreds of species of bacteria (and minority groups of yeasts and viruses), whose collective genome is called microbiome (ninefold larger than our genome). Having coevolved with Homo sapiens , these microbes are so adapted to the human gut environment that they do not grow in culture as easily as do other germs and are therefore identified by their genetic material that is usually retrieved from feces. Microbiota have long been known to train our immune system, process indigestible foodstuffs, manufacture vitamins, and break down toxins and medications. In the past decade, variants of gut microbiome have been associated with diverse human diseases ranging from enterocolitis to psychiatric disorders. Of special interest to us is the association with obesity (2 …