The Mysterious Connection Between Bacterial Communities & Human Personalities

Ew, bacteria. For most people, that word just comes with a bad ring to it. Disease, pain and death may make bacteria appear like our biggest enemy. But as we know, not all bacteria behave this way and most colonize our bodies to actually benefit our wellbeing. We have a mutual relationship with millions of bacterial cells; in fact, bacteria and other microbes normally outnumber our own cells. We provide a home for aggregates of microbes, which together form the human microbiome. Interestingly there are many bacterial cells that, when given the chance, can disrespect our mutual agreement and cause disease. Escherichia coli is one such bacteria that is commonly found in the lining of our intestines. If our immune systems are unusually low, or if some other common body function is compromised, E. coli may take advantage of this and start behaving more like the bad bacteria we are so used to hearing about. 

Microbial cells outnumber our own cells in our bodies.

While that may be a gamble humans have evolved to accept, recent studies are shedding human bacterial communities in a whole new light – that they may go beyond merely impacting our health. Scientists have suggested and found strong correlations that the microbiome is strongly associated with human personalities. From introversion and extroversion, to anxiety and depression, and even the autism spectrum, the bacteria that call us home may have been playing a hidden role this entire time.

“From introversion and extroversion, to anxiety and depression, and even the autism spectrum, the bacteria that call us home may have been playing a hidden role this entire time.”

An Oxford study conducted by Johnson (2020) explores the relationship between the diversity of bacterial colonies in human guts and sociability. Some of their key takeaways indicate that extroverts tend to have more diverse microbiomes, with many different types of bacteria, than introverts. The study attributes this to a super cool phenomenon: microbiome mixing. Since highly social people have more interactions with others, they are more likely to have share part of their microbiomes together. Now, this isn’t as disgusting as it sounds. We have bacteria everywhere including our skin, which very slightly alters when in contact with another microbiome. When you consider this, kissing is quite biologically intimate…

Kissing is another way of sharing microbial diversity! Art by Katherine Streeter

Anyway, the intermingling of human microbiota explains why social people carry a diverse array of bacteria. On the flip side, unusually low diversity of microbes strongly correlated with anxiety and stress. Just the lifestyle of these individuals such as less contact with others, less food consumption, and less physical activity may have led to this altered microbiome composition. But this act is vice versa; the microbiome may also lead to these emotions. In fact, Johnson (2020) reiterates findings from another study (Azpiroz et al. 2016) that describes patients with irritable bowel syndrome expressing reduced feelings of anxiety. Irritable bowel syndrome is possibly caused by an imbalance of gut bacterial communities, so these results further support the correlation between microbiome composition and anxiety. Another study (Talbott et al. 2019) elucidates the importance of the gut microbiome in terms of mental health by observing the positive impacts of probiotics (supplements of good bacteria) on stressed adults. So similar to how hormones impact our moods, bacterial communities may play a role as well.

We carry a diverse array of microbes. Image from AMICI | Austrian Microbiome Initiative

Johnson (2020) further describes how the increase or decrease of certain genera of bacteria is linked to autism, especially in regards to social behavior. Once again, this could be attributed to less interaction with others (introversion) and other aspects such as frequent hospital visits, etc. Neurological correlations regarding autism have always faced some sort of scientific controversy. However, the strong associations identified are difficult to ignore. Scriven et al. 2018 break down the connection between the gut and the brain. They studied a communication path called the microbiome gut brain (MGB) axis that may explain why mental illnesses such as bipolar disorder and depression relate to intestinal infections such as irritable bowel syndrome.

The gut microbiota communicates with the brain via the MGB axis. Cartoon by the American Chemical Society

This information has brought in a whole new field of research for scientists to explore. The possibility of probiotic yogurt as a treatment for an illness such as depression? Breakthroughs in understanding autism in terms of human bacterial communities? Altering behavior by altering the microbiota? There is so much more to be discovered about the human microbiome and we are at the cusp of uncovering those mysteries!

We have a “mutual” relationship with most microbes in our body.


Azpiroz, F., Dubray, C., Bernalier‐Donadille, A., Cardot, J. M., Accarino, A., Serra, J., … & Dapoigny, M. (2017). Effects of sc FOS on the composition of fecal microbiota and anxiety in patients with irritable bowel syndrome: a randomized, double blind, placebo controlled study. Neurogastroenterology & Motility, 29(2), e12911.

Johnson, K. V. A. (2020). Gut microbiome composition and diversity are related to human personality traits. Human Microbiome Journal, 15, 100069.

Scriven, M., Dinan, T. G., Cryan, J. F., & Wall, M. (2018). Neuropsychiatric disorders: influence of gut microbe to brain signalling. Diseases, 6(3), 78.

Talbott, S. M., Talbott, J. A., Stephens, B. J., & Oddou, M. P. (2019). Effect of coordinated probiotic/prebiotic/phytobiotic supplementation on microbiome balance and psychological mood state in healthy stressed adults. Functional Foods in Health and Disease, 9(4), 265-275.

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