From “gut feelings” to “having some guts”, English is full of phrases where our bowels exert an influence upon our behaviour. But these are more than metaphors. There are open lines of communication between brains and bowels and, in mice at least, these channels allow an individual’s gut bacteria to steer their behaviour.

The latest evidence for this “gut-brain axis” comes from Javier Bravo at University College Cork. He fed mice with a probiotic bacterium called Lactobacillus rhamnosus, often found in yoghurts and dairy products. The bacterial menu changed the levels of signalling chemicals in the rodents’ brains, and reduced behaviours associated with stress, anxiety and depression.

Probiotic bacteria – those that benefit their host – are the subject of