A new study from the University of Turku has uncovered interesting associations between an infant's gut microbiome composition at the age of 10 weeks, and the development of certain temperament traits at six months age. The research does not imply causation but instead adds to a compelling growing body of evidence connecting gut bacteria with mood and behavior.

It is still extraordinarily early days for many scientists investigating the broader role of the gut microbiome in humans. While some studies are revealing associations between mental health conditions such as depression or schizophrenia and the microbiome, these are only general correlations. Evidence on these intertwined connections between the gut and brain certainly suggest a fascinating bi-directional relationship, however, positive mental health is certainly not a simple a matter of taking a certain probiotic supplement.

Even less research is out there examining associations between the gut microbiome and behavior in infants. One 2015 study examined this relationship in toddlers aged between 18 and 27 months, but this new study set out to investigate the association at an even younger age. The hypothesis being, if the early months in a young life are so fundamental to neurodevelopment, and our gut bacteria is fundamentally linked with the brain, then our microbiome composition could be vital in the development of basic behavioral traits.

The study recruited 303 infants. A stool sample was collected and analyzed at the age of two and half months, and then at around six months of age the mothers completed a behavior questionnaire evaluating the child's temperament. The most general finding was that greater microbial diversity equated with less fear reactivity and lower negative emotionality.

"It was interesting that, for example, the Bifidobacterium genus including several lactic acid bacteria was associated with higher positive emotions in infants," says Anna Aatsinki, one of the lead authors on the study. "Positive emotionality is the tendency to experience and express happiness and delight, and it can also be a sign of an extrovert personality later in life."

On a more granular level the study homed in on several specific associations between certain bacterial genera and infant temperaments. High abundance of Bifidobacterium and Streptococcus, and low levels of Atopobium, were associated with positive emotionality. Negative emotionality was associated with Erwinia, Rothia and Serratia bacteria. Fear reactivity in particular was found to be specifically associated with an increased abundance of Peptinophilus and Atopobium bacteria.

The researchers are incredibly clear these findings are merely associational observations and no causal connection is suggested. These kinds of correlational studies are simply the first step, pointing the way to future research better equipped to investigate the underlying mechanisms that could be generating these associations.

"Although we discovered connections between diversity and temperament traits, it is not certain whether early microbial diversity affects disease risk later in life," says Aatsinki. "It is also unclear what are the exact mechanisms behind the association. This is why we need follow-up studies as well as a closer examination of metabolites produced by the microbes."

The new study was published in the journal Brain, Behavior, and Immunity.