34 | New Scientist | 7 September 2019
our gut to change our mood and feelings.
It is early days, but the promise is astounding.
The World Health Organization rates depression
and anxiety as the number one cause of
disability, affecting at least 300 million people
worldwide. The new findings challenge the
whole paradigm of mental illness being caused
by a chemical imbalance in the brain, and
offer an alternative to drug treatment. You’ve
probably heard of probiotics, but these are their
new incarnation – psychobiotics. They could
be about to change the mood of the planet.
Bacteria have been associated with sickness
almost since they were discovered 350 years
ago by Dutch biologist and microscope pioneer
Antonie van Leeuwenhoek. Only recently
have we begun to understand that microbes
also contribute to our health. They produce
vitamins and help us eke out extra energy
from otherwise indigestible food, for example.
Most importantly, by outcompeting and
directly battling pathogens, our home-grown
microbes protect us from disease.
It wasn’t until the 21st century that we
got the first hint that microbes might also
influence how we feel. It started with very
clean mice. In 2004, Nobuyuki Sudo at Kyushu
University, Japan, found that mice lacking
microbes had an abnormal response to stress.
These so-called germ-free mice are born
through antiseptic caesarean sections and
raised in a sterile environment. Such animals
don’t exist in nature because microbes
are inescapable: they coat the skin and are
especially fond of the mucous lining of the gut.
They accumulate in the colon, which has the
right conditions to support a dense population
of bacteria, fungi and viruses. Without a
protective set of microbes, germ-free mice
are particularly vulnerable to pathogens.
A normal mouse isn’t finicky about food, and
can consume millions of pathogens without
a hiccup, but a germ-free mouse can die from
eating a mere dozen or so harmful bacteria.
Sudo and his team expected physical disease
in their mice, but they weren’t prepared for
the behavioural differences they observed.
Compared with their more germy cousins,
the germ-free mice devoted more time to
inanimate objects than to other mice and had
an exaggerated response to stress. They also
had less developed brains. It is hard to know
what is going on in the mind of a mouse, but
they acted like they were depressed. Tellingly,
when Sudo fed them a pathogen-free microbial
concoction, they developed a normal stress
response within days.
This surprising connection between
microbes and mood was dubbed theThe
psychobiotic
revolution
The discovery that gut bacteria
influence our emotions
should make us all happy.
Scott Anderson reports
>Features Cover story
R
EMEMBER the last time you had a
stomach bug and just wanted to
crawl into bed and pull up the covers?
That is called “sickness behaviour” and it is
a kind of short-term depression. The bacteria
infecting you aren’t just making you feel
nauseous, they are controlling your mood
too. It sounds absurd: they are in your gut
and your feelings are generated in your brain.
In fact, this is just an inkling of the power that
microbes have over our emotions.
In recent years, such organisms in the gut
have been implicated in a range of conditions
that affect mood, especially depression and
anxiety. The good news is that bacteria
don’t just make you feel low; the right ones
can also improve your mood. That has an
intriguing implication: one day we may be
able to manipulate the microbes living within