New Scientist - 07.09.2019

7 September 2019 | New Scientist | 37

gut-brain axis. Sudo’s discovery fascinated
researchers across a wide range of fields and
launched a spate of studies. Some of these
uncovered differences in the brain chemistry
of germ-free mice compared with normal ones,
including a dramatic reduction in serotonin, a
neurotransmitter linked to depression. Others
showed that mice bred to model depression
in humans appeared to benefit from being fed
certain types of bacteria. A study by John Cryan
and Ted Dinan, both at University College
Cork in Ireland, found that the bacterium
Lactobacillus rhamnosus, which is in some
live yogurt, has potent anti-anxiety effects
in mice. “They became a lot more chilled out
and relaxed. They behaved as if they were on
Valium or Prozac,” says Cryan. “We looked at
their brains and found widespread changes.”
It isn’t possible to do germ-free research
in humans. However, Cryan and Dinan also
found that giving normal mice a faecal
transplant from a person with depression
led to the mice developing depression-like
symptoms. This convinced them that the
findings in mice have relevance to people.
The pair also realised that if gut bacteria
influence our emotions this would have huge
implications for how we understand and treat a
variety of mental health issues. In 2013, together
with their colleague Catherine Stanton, they
proposed the idea of psychobiotics – a new
class of probiotics that could improve people’s
mood. Not least, the new line of research
suggested ways in which we could use our
diet to positively influence our mood (see
“Eat your way to happiness”, left).
It was an audacious idea, not least because,
at that point, nobody was sure how the
gut-brain axis might work. “The question

is, how can bacteria in your gut communicate

with your brain?” says Cryan.

A clue seemed to lie in the astonishing

discovery, in the early 2000s, that microbes

can produce almost every neurotransmitter

found in the human brain, including serotonin

and dopamine, which is involved in motivation

and reward. But there was a problem: the brain

is designed to insulate itself from most outside

influences with a “blood-brain barrier” that

prevents cells, particles and certain molecules –

including neurotransmitters – from getting

in. How could these chemicals produced by

microbes in the gut affect the brain?

Hotline to the brain

A breakthrough came in 2017 with the

discovery of special cells in the gut lining.

These enterochromaffin cells can detect

neurotransmitters produced by microbes,

resulting in a pulse being triggered in the

vagus nerve, which connects the gut to the

brain. What’s more, experiments in mice

show that cutting this nerve terminates

many psychobiotic actions. For example,

Dinan and Cryan found that L. rhamnosus

didn’t alleviate stress in mice when the vagus

nerve was severed. More evidence suggests

that gut microbes and the molecules they

produce can directly modulate the integrity

of the blood-brain barrier too.

This apparent communication between

gut microbes and the brain may even affect

the organ’s development. The brain is full of

structures with specialised functions. Those

involved with stress and mood include the

amygdala, prefrontal cortex, hippocampus

and hypothalamus. They are connected by

Gut health from the inside out

See Megan Rossi speak at New Scientist Live on 12 October

newscientistlive.com

CHUNG SUNG-JUN/GETTY IMAGES

“ Microbes can

produce almost

every neuro-

transmitter

found in the

human brain”

spaghetti-like axons running helter-skelter

throughout the brain, in a complex wiring

pattern that is unique for each of us. That

wiring starts getting laid down in the uterus

and continues in earnest during the first

three years of a baby’s life. Amazingly, gut

microbes can influence this process. Germ-free

mice show abnormal development in their

amygdala, hippocampus and prefrontal cortex.

How gut microbes do this, and influence

the brain thereafter, is less clear, but we do

know that these structures aren’t static.

For example, depression can cause your

amygdala – which is responsible for the crucial

fight-or-flight response in life-threatening

situations – to become more active and

swell up. The condition can also cause the

hippocampus to shrink, potentially affecting

your memory. These physical changes explain

why you can’t simply “think yourself happy”.

The exact extent to which microbes are

involved is unknown, but a link has been

found between gut bacteria and the way

healthy people process emotions. In 2017,

Kirsten Tillisch and Emeran Mayer, both

at the University of California, Los Angeles,

examined the gut microbes of 40 women

and found the volunteers could be divided into

two groups: those with lots of bacteria from

the genus Prevotella and those with lots from

the genus Bacteroides. The researchers then

used functional MRI scans to look at activity

in specific parts of the women’s brains while

they viewed emotionally disturbing images.

Each group had distinct brain activity, which

was different enough to indicate which an

Probiotics like kimchi

could boost your mood

>