Bridging microbiology to psychology.
The microbiota-gut-brain or gut-brain imaginary axis is well-received with surmounting research supporting its reality and importance. Even I had authored a review article about it in the Frontiers of Neuroscience.
Given the border between psychology as a soft science and biology as a hard science, can we still take this axis from biology to psychology? To date, only three studies — published in the Brain, Behaviour, and Immunity journal — had attempted this idea. That’s 3 out of ~20,000 publications identified in the PubMed database by searching ‘gut microbiota’ or ‘gut microbiome’ keywords. That’s 0.015%. They are valuable and might set the foundation for future research on the psychological aspects of the gut-brain axis.
Big Five is the most research-backed personality test to date. It classifies personality into 5 domains, the O-C-E-A-N:
- Openness to experience: ‘inventive/curious vs. consistent/cautious.’
- Conscientiousness: ‘efficient/organized vs. easy-going/careless.’
- Extraversion: ‘outgoing/energetic vs. solitary/reserved.’
- Agreeableness: ‘friendly/compassionate vs. challenging/detached.’
- Neuroticism: ‘sensitive/nervous vs. secure/confident.’
In 2018, a group of Korean researchers investigated whether the gut microbiota profile has any correlation with the Big Five personality traits in 672 adults aged 23–69. These participants were not users of antibiotics or probiotics and were mentally healthy. After controlling for possible confounds — age, sex, BMI, and dietary intake — they unearthed the following:
- High neuroticism correlated with a high abundance of Gammaproteobacteria, Pateurellaceae, and Haemophilus species.
- Low conscientiousness correlated with a high abundance of Proteobacteria.
- High conscientiousness correlated with a high abundance of Lachnospiraceae, the universal butyrate-producing bacteria.
- High agreeableness correlated with a little higher alpha diversity of gut bacteria.
- Other correlations became non-significant after adjusting for confounds.
Infant and Children Temperament
Temperament differs from personality in that it categorizes children or infant behaviors. Like personality, temperaments have been observed to be stable and, in turn, affect personality in later life. Psychologists split temperament into three types:
- Effortful control: Attention/inhibitory control.
- Negative affectivity: Frustration, fear, sadness.
- Extraversion/surgency: Motor activity, impulsivity, laughter, low shyness, desire for closeness with others, and positive excitement.
Anna Aatsinki and her colleagues from the University of Turku, Finland collected data on the temperament and gut microbiota profile from 303 infants of about 2.5 months of age. They controlled for sex, delivery mode, gestational age, infant age, antibiotic use, and breastfeeding status in their analysis. Their findings were published in 2019, showing that:
- Surgency (positive emotionality) correlated with high Bifidobacterium and Streptococcus species and low Atopobium species.
- Negative emotionality and fear reactivity correlated with low alpha diversity of gut bacteria.
- Negative emotionality correlated with high Erwinia, Rothia, and Serratia species.
- Fear reactivity correlated with high Peptinophilus and Atopobium species.
Previously in 2015, a similar kind of research was performed in the United States by Lisa Christian and her collaborators, except that they examined 77 children aged 18–27 months. Their discoveries, controlled for age and dietary patterns, were as follows:
- Surgency/Extraversion correlated with a higher phylogenic diversity of gut bacteria.
- In boys only, surgency/extraversion correlated with differences in the abundances of Dialister, Rikenellaceae, Ruminococcaceae, and Parabacteroides species.
You may notice that the gut microbiota profiles are different between infants, children, and adults. This is because the gut microbiota changes as we age. The elderly would also have a distinct gut microbiota composition that other age groups. You may also ask why those bacteria are linked to personality or temperament. “Although we discovered connections between diversity and temperament traits, it is not certain whether early microbial diversity affects disease risk later in life. It is also unclear what are the exact mechanisms behind the association,” Aatsinki explains.
So no direct evidence exists for now. Academics, however, have proposed that those bacteria could initiate or fuel gut dysbiosis, a condition linked to negative aspects of mood like depression and anxiety. A gut dysbiosis means there are imbalances in the gut microbial community wherein ‘bad’ bacteria are overwhelming the ‘good’ ones. This creates systemic inflammation that could reach the brain and disharmonized the vagus nerve, the central parasympathetic (‘rest-and-digest’) conduit connecting the gut to the brainstem. Feel free to visit this one for the current knowns and unknowns of the impact of gut microbes on health and diseases.
Although correlational studies do not ascertain the direction of causality, the Koreans speculate that “personality traits could influence behaviors such as eating habits or lifestyles, and such interactions might have a greater effect on gut microbiota.” Yes, they did control for nutrient intake, but they also admitted that questionnaires come with limitations.
Likewise, Lisa Christian writes “although parents control what foods are offered, children with certain temperamental characteristics may accept different quantities, varieties, or types of food. In addition, parental feeding behavior may be influenced by child temperament; for example, parents may use food to soothe or reward fussy children.” These innovative researchers support the causality direction of personality/temperament > gut bacteria. Could the opposite be true as well?
Researchers have learned to make depressed rats by colonizing their gut microbiota with that derived from patients with major depressive disorder (MDD). Perhaps they should also try that with the Big Five personality traits or temperaments to demonstrate causality. I think that would attract a lot of attention. While most personality and temperamental traits can be inferred from animal behaviors to some extent — e.g., extroverted mice play with strangers — I wonder what a conscientious or agreeable mice look like. There is no such mouse research model just yet. Comment if you think you know!
This article was previously published in Microbial Instincts with minor changes.
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