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Rats given gut microbiome transplants from exuberant human toddlers seem to be more willing to explore their environment. This finding hints that the bacteria inhabiting our guts when we are children play a role in shaping our personalities.

“It suggests our microbes are active participants in emotional development, not just passive passengers,” says at University College Cork in Ireland, who wasn’t involved in the study.

A growing body of research has linked the communities of microbes that reside in our guts to our health, emotions and moods. For example, people who lack certain types of gut bacteria seem to face a higher risk of depression �ǰ���anxiety.

It isn’t entirely clear if the bacteria cause these changes or if the microbial community alters as a result of behaviour, but there are some signs that altering the make-up of the microbiome can influence one’s mood. For example, faecal transplants from people with depression to rats  in the rodents, and people with depression treated with faecal transplants have had in preliminary trials.

To shed more light on how the gut microbiome may be linked to temperament, at the University of Turku in Finland and her colleagues transplanted faeces from toddlers to young rats.

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First, the team evaluated the personalities of 27 2.5-year-old toddlers using a standard temperament assessment and an exercise in which children were invited to play with a bubble gun.

“We couldn’t really study things like anxiety disorder in 2-year-olds, but we thought there might be behaviour differences we could look at; if they are, for instance, behaviourally inhibited versus very outgoing and extroverted,” says Aatsinki.

Based on these assessments, the researchers judged 10 of the toddlers as exuberant, and eight as inhibited and introverted. From these groups, they selected four exuberant and four inhibited toddlers – half boys, half girls – and collected samples of their faeces.

Faecal samples with added glycerol or control samples of glycerol were transferred to 53 rats aged 22 or 23 days old, which had already had their bowels cleansed.

Aatsinki and her colleagues then put the rats through a series of behavioural tests in different situations. They found that rats with microbiomes from toddlers with high exuberance traits showed more exploratory behaviour than rats with a control transplant or rats receiving faeces from inhibited toddlers.

To explore how gut microbes might exert their influence on the brain, the researchers also analysed brain tissue from the rats, looking for changes in gene activity. This showed that rats given transplants from inhibited toddlers had less activity in neurons that produce dopamine, a brain chemical linked to reward for risk-taking behaviour.

“This study beautifully shows how the gut microbiome in early life may help shape behavioural tendencies,” says Schellekens. “By transferring microbiota from children to rodents, the researchers create a rare translational link between microbes, human temperament and brain function.”

This points to a gut–brain route that influences curiosity, reward and motivation via the dopamine system, says Schellekens.

The influence shouldn’t be overstated though, says Aatsinki. “Overall, adults’ temperament traits are relatively strongly correlated with genetics, but environmental factors, potentially including the microbiome, could influence the variance of some behaviours.”

Whether the microbes are behind behaviour differences in the children is still an open question, adds Aatsinki. It could also be that children who develop exuberant phenotypes interact with their environment and new foods differently, and so develop a different microbiome, she says.