38 | New Scientist | 7 September 2019
individual belonged to with an accuracy of
87 per cent. In particular, the group with plenty
of Prevotella bacteria, which was much smaller,
had less activity in the hippocampus. This is
also found in people with depression.
Tillisch and Mayer have also found that
they can influence the way people’s brains
process emotions by using probiotics. They
gave 36 healthy women probiotic yogurt
containing four types of bacteria twice daily
for four weeks. Brain scans revealed that this
affected the activity and physical connectivity
in the emotion centres of the women’s brains,
producing changes associated with healthier
emotional processing.
Ingesting microbes may even help people
at risk of depression and anxiety. Several
studies focus on pregnant women because
severe depression affects around 15 per cent
of women during and after pregnancy, and can
interfere with their ability to bond with their
babies. In a 2017 study, more than 200 women
were given L. rhamnosus from early pregnancy
to six months after delivery. It found they
had better scores than a control group on
tests of depression and anxiety. The finding
is particularly welcome because many women
are reluctant to take antidepressants during
pregnancy or while breastfeeding. Likewise,
healthy college students who took off-the-shelf
probiotics for a month showed improvements
on several measures of anxiety, including
panic, worry and negative mood. Students with
the highest stress levels, who took the most
probiotics, showed the biggest improvement.
“I do believe the data supports a role for
microbes modulating how we feel,” saysTillisch. However, she cautions that the studies
in people show correlation, not causation, so it
is possible that mood is affecting the microbes
rather than the other way around.
This is all very exciting, but there is still
another hurdle before psychobiotics can
become mainstream treatments. Currently, we
have only the vaguest notion of which bacteria
might have a good influence on moods. In fact,
we don’t even know exactly which microbes
are in our gut, because many can’t be cultured.
Thousands of gut microbes have been
identified by analysing samples for a small
sequence of genetic material called 16S rRNA.
But this can only reliably distinguish bacteria
down to the genus level. However, a new
technique is revolutionising microbial
research. Called whole-genome shotgun
sequencing, it can find species and even
subspecies by looking at every gene in a
sample taken from any environment.
Using this technique, Jeroen Raes and
colleagues at KU Leuven in Belgium wereable to get a fine-grain view of the microbes
in the guts of over 1000 people. In April,
they reported that people diagnosed with
depression had reduced numbers of bacteria in
two genera, Coprococcus and Dialister. These,
then, are potential human psychobiotics.
The subjects also completed questionnaires
about their quality of life. This revealed that
people with a better quality of life tended to
have more microbes that produce butyrate,
a short-chain fatty acid that nourishes and
heals the gut. Their gut microbes also produced
more of a precursor to the neurotransmitter
dopamine. One of the team, Mireia Valles-
Colomer, subsequently identified a bacterium
called Butyricicoccus pullicaecorum 25-3T as
another potential psychobiotic.
This large-scale human study was possible
because Belgium keeps extensive electronic
medical records that are available for scientific
analysis. Several northern European countries
have similar data sets and, with whole-genome
shotgun analysis, they are likely to reveal
the identity of more possible psychobiotics.
However, we may be able to do more than
simply finding “good bacteria”. Dinan and
Cryan suspect that just as pathogenic bacteria
share certain genes, there may also be genes
connected with mental health benefits. If
so, then the ideal psychobiotic might be a
genetically modified organism containing
genes from several different bacteria, they say.
That is for the future. There are things we
can all do now to boost the psychobiotics
we already possess. This is such a hot topic
it can surely only be a matter of time before
psychobiotics offer an alternative treatment
for people diagnosed with a variety of mood
disorders. Instead of targeting the head, we
could go for the gut. “This new way of looking
at brain health is literally turning things
upside down,” says Cryan. ❚Scott C. Anderson is the author,
with John Cryan and Ted Dinan,
of The Psychobiotic Revolution
published by National Geographic.
Follow him on Twitter @Psychobiotic“ Ingesting
microbes
may even help
people at risk
of depression
and anxiety”
Bacteria such as Prevotella
(left) and Bacteroides (right)
influence how your brain
processes emotionsBOTH IMAGES: DENNIS KUNKEL MICROSCOPY/SCIENCE PHOTO LIBRARY