strange as this sounds, it’s actually how some
antidepressants are initially studied and tested.
In a unique twist on such an experiment, mice had their
microbiomes populated with a certain type of probiotic
called Lactobacillus rhamnosus, and then they were thrown
into a tank. Compared to mice that didn’t get the probiotic,
those that did seemed more eager to stay afloat. They even
showed a marked increase in anti-anxiety GABA receptors
in certain parts of the brain. This effect was also absent in
mice who were fed the probiotic but had their vagus nerves
severed—the vagus nerve innervates the intestines and is
connected directly to the brain. This suggests that the
mechanism of action was direct microbial communication
with the brain.^4
If probiotics helped depressed mice, what are the odds
they might help with other psychiatric symptoms? Mice
born to mothers that ate the mouse equivalent of fast food (a
deadly combination of fat and sugar) multiple times a day
showed social behavior symptoms similar to autism. Upon
inspection of their gut bacteria populations, these autistic
mice had nine times less of another probiotic species,
Lactobacillus reuteri, present. By restoring L. reuteri with a
probiotic supplement, scientists were able to “correct” these
social behavioral deficits, and the mice even showed
increased production of the social hormone oxytocin, which
acts like a neurotransmitter in the brain.
Interestingly, the amount of L. reuteri found in our
systems has declined in tandem with the increases seen in
rates of autism and consumption of fast food. In the 1960s,
when the bacterium was discovered, L. reuteri was present