Background: A high proportion of type 2 diabetes cases are associated with host genetic and environmental factors. During the past decade, microorganisms that inhabit the gut have emerged as contributors to the pathogenesis of obesity and type 2 diabetes. Therefore, manipulation of the human gut microbiota will provide essential clues regarding new therapeutic targets for diabetes. Summary: Several studies have established the presence of gut dysbiosis in patients with type 2 diabetes mellitus, even though there are some differences among the studies that could be explained by differences in ethnicity, diet, and methodology. Gut dysbiosis affects the quality and quantity of short-chain fatty acids and secondary bile acids that act as signaling molecules in energy, glucose, and lipid metabolism. In addition, gut dysbiosis affect intestinal permeability. In particular, a high-fat diet can lead to changes in the gut microbiota that strongly reduce intestinal permeability due to the malfunction of tight junction proteins, such as occludin and ZO-1 [<citeref rid="ref1">1</citeref>]. The formation of leaky gut results in increased plasma levels of lipopolysaccharide, which activate Toll-like receptor 4 and result in innate and adaptive immune responses [<citeref rid="ref2">2</citeref>]. Key messages: Gut dysbiosis play an important role in the pathogenesis of obesity and diabetes, for example, via chronic low-grade inflammation. Normalizing gut dysbiosis could be a new approach to overcome diseases of insulin resistance, such as diabetes mellitus.