Rats that received gut microbiomes from energetic human infants seem more inclined to explore their surroundings. This research suggests that the bacteria present in our guts during childhood may influence our personalities.

“This indicates that our microbes play an active role in emotional development, rather than merely being passive presence,” explains Harriet Scherekens from University College Cork, Ireland, who was not involved in the study.

Increasing research identifies a link between the microbial communities in our guts and various aspects of our well-being, emotions, and mood. For instance, individuals lacking certain gut bacteria types seem to face a higher risk of depression and anxiety.

While it remains uncertain whether bacteria are the cause of these emotional shifts or if the microbial communities alter in response to user actions, some evidence suggests that changes in the microbiome can influence an individual’s mood. For example, fecal transplants from depressed individuals to rats appear to induce depressive behavior. Conversely, depressed rodents receiving fecal transplants have shown improved symptoms upon preliminary examination.

To delve deeper into how the gut microbiome relates to temperament, Anna Artshinki and colleagues at the University of Turku in Finland conducted fecal transplants from infants into young rats.

Initially, the team evaluated the personalities of 27 toddlers aged 2.5 years using standard temperament assessments and an activity that encouraged play with bubble guns.

“Although we couldn’t study anxiety in 2-year-olds directly, we anticipated assessing behavioral differences, such as levels of inhibition versus sociability,” notes Artosinki.

From their evaluations, researchers classified 10 infants as energetic and 8 as inhibited and withdrawn. They then selected four energetic and four restrained infants (split evenly between genders) for fecal sample collection.

Fecal samples spiked with glycerol, alongside control samples, were transplanted into 53 22- or 23-day-old rats whose intestines had been pre-cleaned.

Artshinki’s team then subjected the rats to a variety of behavioral tests. They discovered that rats with microbiomes from energetic infants displayed a greater exploratory tendency compared to those receiving control implants or feces from inhibited infants.

To investigate how gut bacteria might influence the brain, the researchers also examined rat brain tissues for gene activity changes. This analysis indicated that rats receiving microbiomes from inhibited infants showed reduced activity in dopamine-producing neurons, a neurotransmitter linked to rewarding risk-taking behavior.

“This study effectively illustrates how the early childhood gut microbiome contributes to shaping behavioral tendencies,” Scherekens remarks. “By transferring microbiomes from children to rodents, researchers have created a valuable translation between microbes, human temperament, and brain function.”

This indicates a gut-brain pathway that impacts curiosity, reward, and motivation through the dopamine system, Scherekens adds.

Nonetheless, Artshinki cautions against overstating the implications. “Overall, adult temperament traits are strongly correlated with genetics, yet environmental factors—potentially including the microbiome—may play a role in certain behavioral distinctions.”

Artosinki emphasizes that whether microbes drive the differences in children’s behaviors remains an open question. It’s possible that children exhibiting more active traits interact with their environment and new foods in unique ways, thereby developing distinct microbiomes as a result.