Alterations in (small) intestinal microbiota are associated with obesity and insulin resistance, with the latter usually characterized by low-grade endotoxemia. Recent prospective epidemiological data showed that specific bacterial species in fecal samples (Lactobacillus Gasseri and Streptococcus mutans) can predict development of type 2 diabetes mellitus in obese postmenopausal women (Karlsson, Nature 2013). We recently showed that fecal transplantation (infusing intestinal microbiota from lean donors) in male recipients with metabolic syndrome has beneficial effects on the recipients’ microbiota composition and glucose metabolism via lowering plasma endotoxin levels (Vrieze, Gastroenterology 2012). Moreover, preliminary data suggest that 4 weeks daily oral gavage with one of the identified small intestinal bacterial strains (butyrate producer Eubacterium hallii) has dose-dependent beneficial effects on insulin sensitivity and liver steatosis in male db/db mice. Combined, our data suggest that specific intestinal bacterial strains might be developed as therapeutic targets to normalize inflammatory tone and insulin sensitivity in humans.