Commensals, bacterial pathogens and intestinal inflammation: an intriguing ménage à trois.

According to a classical view of bacterial-host interactions at intestinal surfaces, the commensal microbiota establishes tolerance, and invasive pathogens cause stereotypic inflammation. The reality is more complex, marked by a "ménage à trois" situation encompassing three emerging concepts: (1) pathogens take advantage of inflammation to cross the epithelial barrier, (2) pathogens reduce the commensal flora to invade their niche, and (3) pathogens express dedicated effectors that modulate inflammation.