The human gastrointestinal tract harbours trillions of microorganisms, consisting of up to 1,000 or so different bacterial species. These bacteria, known collectively as the gut microbiota, perform a number of vital functions in our body. They defend against pathogens, aid in digestion and nutrient absorption, produce vitamins (K and B), and boost our immune system. The gut microbiota also has the potential to influence our brain development and behaviour. Our gut and the central nervous system constantly communicate with each other by releasing signalling molecules. The gut microbiota is also involved in this communication process, known as the microbiota-gut–brain axis.