Our gastro-intestinal tract is colonized by billions of microorganisms that constitute the intestinal flora or microbiota. These microorganisms, primarily bacteria, are required for our development and physiology; however, changes in the "normal" composition of the microbiota, referred to as dysbiosis, have been linked to chronic inflammatory diseases in humans. Currently, a major research effort is to understand what these commensal bacteria are, how they maintain health, and how their dysregulation causes disease.