Depressed? Anxious? What If Your Bacteria Are Playing a Part?

The fascinating world of the gut-brain axis.


Although we’re used to science advancing at the speed of light these days, it hasn’t always been like this.

Take the Greek physician Hippocrates, for example, born around 460 BC.

He proposed that the human body contained four fluids or “humors”—blood, phlegm, yellow bile, and black bile, usually in balance.

When one of them became more or less present, illness could ensue.

He believed that an excess of black bile, for example, could cause someone to become melancholic.

What we now call depressed.

In fact the term “melancholic” comes from the ancient Greek words “melas” and “kholé”, meaning black bile.

This theory of “humorism,” certainly nothing to do with comedy, stuck around for almost 2,000 years.

Jumping ahead to the 19th century, many scientists argued that a buildup of waste matter in the colon could trigger something they called “auto intoxication”, poisoning the gut and producing infections linked with depression, anxiety and psychosis.

So if you were depressed in those days you might get treated with a colonic purge, which doesn’t exactly sound like something guaranteed to put a smile on your face.

This poisoning idea was eventually dismissed as quackery after direct observation of the colon during surgery and autopsies showed zero evidence for hardened feces accumulating on the walls of the intestine.

However, although it’s clear that both of these schools of thought were distinctly not on the right track, there’s increasing (and exciting) evidence of a very real connection between the gut and the brain.

You see, it’s now widely believed that there is a “gut-brain axis”, which is in fact a two-way street.

Your brain acts on your gut, shaping its microbial makeup, while your gut is busy manufacturing neurotransmitters including dopamine, gamma-aminobutyric acid (GABA) and serotonin.

In fact the vast majority of your serotonin originates in your intestine.

Talk about gut feelings, eh?

There have been many remarkable experiments showing ways in which the function of the brain can be affected by the gut microbiome.

Nobuyki Sudo and colleagues at Japan’s Kyushu University published pioneering work in 2004 by experimenting with mice that had been specially bred to be germ-free giving them, effectively, sterile guts.

When these mice were placed in a tube that restricted their movement, their levels of stress hormones became far higher than those of mice in the same situation whose microbiomes were not germ-free.

Fascinatingly, the scientists were able to re-engineer these germ-free mice, turning them into relaxed rodents simply by adding one species of bacteria to their guts: Bifidobacterium infantis (now viewed as a subspecies of Bifidobacterium longum) which is one of the very first types of bacteria acquired by a baby, particularly one born vaginally.

The amazing studies kept coming.

Researchers at McMaster University in Ontario were able to change the behavior of germ-free mice by colonizing their intestines with bacteria from other mice—giving them what you might call a poop-personality transplant.

This led naturally daring mice to become apprehensive and shy, for example, leading scientists to suggest that microbial interactions with the brain could induce psychological change.

Although some are understandably uncomfortable with experiments such as these, other researchers from McMaster later joined forces with scientists from University College Cork to show that mice fed a broth containing Lactobacillus rhamnosus were far less likely to relapse into “behavioral despair” when dropped into a tall cylinder, from which there was no escape, than mice without these microbes.

The same experiment has been used to test antidepressants, leading one scientist to suggest that the broth-fed mice were behaving as though they were on Prozac.

Lactobacillus rhamnosus is a bacterium commonly found in the human body, and also used in the fermentation of milk as it is turned into yogurt.

It hasn’t all been about mice, however.

A 2013 proof-of-concept study at UCLA showed through fMRI scans that women who ate yogurt containing active probiotics twice a day for a month showed a reduced reflexive response to photos of actors with frightened or angry faces.

The researchers warned that their results were rudimentary, but there was at least an indication that consumption of probiotic bacteria such as Bifidobacterium animalis subsp Lactis, Streptococcus thermophiles, Lactobacillus bulgaricus, and Lactococcus lactis subsp Lactis could have been making the participants less prone to anxiety.

Another study, in Norway in 2014, found significant correlations between bacteria in stool samples and depression, with elevated levels of the order Bacteroidales and a reduced abundance of the family Lachnospiraceae.

Advances in the mental health/microbiota world are certainly moving faster than the science of Hippocrates’ day, but there’s still a lot of work to be done.

It’s an area of great promise, though.

Who knows?

When the bottom’s fallen out of your world, the day may come when microbes, delivered in various ways, are used instead of standard prescription medicines.


Further reading

Can the Bacteria in Your Gut Explain Your Mood?

Changing gut bacteria through diet affects brain function 

Consumption of Fermented Milk Product With Probiotic Modulates Brain Activity

Correlation between the human fecal microbiota and depression



Mental Health May Depend on Creatures in the Gut

Postnatal microbial colonization programs the hypothalamic-pituitary-adrenal system for stress response in mice

The gut microbiome and diet in psychiatry: focus on depression 

The tantalizing links between gut microbes and the brain