Neuroscientists come round to probiotic thinking

The Lactobacillis bacteria is a common 'probiotic'. Now neuroscientists are beginning to believe there may indeed be a link between gut bacteria and mental health

There is growing evidence that the claims made by the "probiotic" food industry, that sort of bacteria we harbour in our gut can affect our mental health, may have something to them.

Neuroscientists have been sceptical until now, but, as Nature reports, hard evidence linking conditions such as autism and depression to the microbiome – as the microbial contents of our gut is known – has caused a re-think.

“The field is going to another level of sophistication,” says Sarkis Mazmanian, a microbiologist at the California Institute of Technology in Pasadena. “Hopefully this will shift this image that there’s too much commercial interest and data from too few labs.”

This year, the US National Institute of Mental Health spent more than US$1 million on a new research programme aimed at the microbiome–brain connection. And on 19 November, neuroscientists will present evidence for the link in a symposium at the annual Society for Neuroscience meeting in Washington DC called ‘Gut Microbes and the Brain: Paradigm Shift in Neuroscience’.

Although correlations have been noted between the composition of the gut microbiome and behavioural conditions, especially autism, neuroscientists are only now starting to understand how gut bacteria may influence the brain. The immune system almost certainly plays a part, Mazmanian says, as does the vagus nerve, which connects the brain to the digestive tract. Bacterial waste products can also influence the brain — for example, at least two types of intestinal bacterium produce the neurotransmitter γ-aminobutyric acid (GABA).

One theory is that the microbiome is likely to have its greatest impact on the brain early in life. Pharmacologist John Cryan at University College Cork in Ireland found that mice born by caesarean section, which were unable to pick up their mothers’ vaginal microbes during birth, were significantly more anxious and had symptoms of depression.

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