Sykepleiepluss.com [creativecommons license] via Wikimedia CommonsThe blood–brain barrier is a selectively permeable barrier between the brain extracellular fluid and the blood formed by capillary endothelial cells connected by tight junctions. The blood–brain barrier prevents potentially harmful molecules and cells from entering the brain and maintains microenvironmental conditions suitable for neuron growth. Its integrity is therefore crucial to proper brain function. A recently published article (Sci. Transl. Med. 6, 263ra158; 2014) identifies a surprising regulator of the integrity of the blood–brain barrier: the microorganisms that colonize the gut.