The more we find out about the human microbiome, the more we discover just how many facets of our existence are influenced by the invisible microscopic organisms that dwell inside us.
Now, scientists have found that even the fear response in infants could be partially determined by the makeup of bacteria living inside the human gut.
In a new study, scientists found babies with less balanced gut microbiomes – reflecting greater abundances of certain bacteria in the gut – tended to show increased fear behavior in an experiment, as compared to infants whose gut bacteria were more balanced overall.
In the experiment, a group of over 30 babies (each aged about one year old) had their fear response analyzed when a researcher wore a series of Halloween-style masks in front of them, including a horse mask, a monkey mask, and an alien mask.
For each child, their facial fear, vocal distress, bodily fear, escape behavior, and startle response were rated. While it sounds a bit unkind for the infants involved, the researchers made sure the experience wasn't too threatening for the young participants.
"We really wanted the experience to be enjoyable for both the kids and their parents," explains pediatrician and neuroscientist Rebecca Knickmeyer from Michigan State University.
"The parents were there the whole time and they could jump in whenever they wanted."
Interestingly, when the results were compared against another dataset from the same group – an analysis of the infants' stool samples, taken at both one month and one year of age – a link was found between the makeup of their gut microbiome and their fear levels in the mask experiment.
Specifically, lower abundance of Bacteroides and increased abundance of Veillonella, Dialister, Bifidobacterium, Lactobacillus and an unnamed genus of Clostridiales were linked with increased fear behavior, which was also more likely to be evident in babies who exhibited increased richness and reduced evenness in their microbiome at the time they were newborns.
Not that there's anything necessarily wrong with the fear response of these very young study participants, the researchers emphasize – although at some point in life, learning how to regulate fear is an important element of mental health.
"Fear reactions are a normal part of child development," Knickmeyer says.
"Children should be aware of threats in their environment and be ready to respond to them. But if they can't dampen that response when they're safe, they may be at heightened risk to develop anxiety and depression later on in life."
As for how and why gut microbiome distribution might affect levels of fear response like this, the researchers aren't entirely sure, although similar kinds of variance in bacterial proportions have previously been linked to fear in animal studies.
A neuroimaging analysis using MRI scans offered some support evidence to suggest that the volume of the amygdala (which processes fear in the brain, in addition to other emotional responses) at one year of age might also be tied to microbiome composition, although they acknowledge more research is needed to explore that lead further.
It is important to note that this was just a pilot study, with a small and not very diverse sample group, designed to see if it is worth pursuing a more thorough examination of this potential link between the development of human fear behaviors and our microbiome.
"As such, these findings should be treated with caution until replicated," the team advises.
For now, all we know is that the beginnings of human fear may be somehow tied to gut microbiome makeup at the start of life – and the paths this emotion takes in later life may also depend on how these bacterial populations thrive or falter.
"Our results suggest that the infant gut microbiome may contribute to the developmental trajectory of fear reactivity and that this relationship may involve the amygdala," the researchers write in their study.
"With further research, the gut microbiome may emerge as a key modulator of fear development and as such may become a means to prevent or ameliorate psychiatric disorders and behavioral problems characterized by abnormal fear reactivity."
The findings are reported in Nature Communications.