Eating a strain of gut-loving bacteria reduces anxiety-like behaviour in mice - a finding that suggests a spoonful of microbes could help stress levels go down in people too.
The human gut is home to about 1000 trillion bacteria, which prevent pathogenic bacteria infiltrating our bowels. Once thought to simply prevent diarrhoea, gut bacteria have recently been linked to autism, reducing the symptoms of Parkinson's disease, and treating eczema. Now it seems these microbes can also reduce stress in animals and humans.
Last year, Philip Sherman and colleagues at the Hospital for Sick Children in Toronto, Canada, found that mice infected with the gut bacteria Citrobacter rodentium experienced more memory problems following the stress of being dropped in water than uninfected mice.
Earlier this year, meanwhile, John Bienenstock and colleagues at the McMaster University in Hamilton, Ontario, Canada, discovered that mice bred without specific gut bugs, including C. rodentium, have more anxiety-like behaviour than mice born in a germ-free habitat.
And giving mice antibiotics which change the composition of their gut bacteria alters mouse behaviour - making oft cautious rodents more adventurous.
On the back of these discoveries, Bienenstock joined forces with John Cryan at the Alimentary Pharmabiotic Centre in Cork, Ireland, to work out how probiotics are affecting behaviour.
They fed mice for 28 days on a broth laced with Lactobacillus rhamnosus - a bacterium often used in probiotics - and subjected them to a barrage of stress tests. As expected, bacteria-fed mice showed less anxious behaviour and had lower levels of the stress hormone corticosterone than mice fed regular broth. The brains of these rodents showed predictable alterations, such as reduced expression of receptors in the amygdala - a region of the brain associated with anxiety. So how do probiotics affect the brain?
"The bacteria rely on the vagus nerve," says Cryan. This nerve connects the gastrointestinal tract to the central nervous system. When the team severed the nerve in the bacteria-fed mice, their behaviour - and the expression of brain receptors - became just like the mice fed regular broth. Cryan says it's possible that chemicals produced by bacteria interfere with the nerve's activity.
Mark Lyte at Texas Tech University in Lubbock, who was not involved in the study, believes these findings are relevant to humans and could lead to treatments for depression and anxiety. Small-scale studies have already found that probiotics can reduce anxiety in humans, he says.