bypassing the blood-brain barrier, which
normally keeps out bacteria. Theoretically,
the bacteria need only pass the intestinal
wall and get into the vagus nerve’s insula-
tion in order to reach the brain.
However, the bacteria also influence a
special type of nerve cell: astrocytes, which
feed neurons nutrients from the blood. The
astrocytes supply the food via outgrowths
known as feet, which are located close to the
brain’s blood vessels, absorbing nutrients.
And these feet contain lots of bacteria.
So some scientists believe that the bacte-
ria travel with the blood from the intestines
to the brain, where they sneak through the
blood-brain barrier and into the astrocytes.
The barrier has weak points in the brain-
stem’s vomit centre and olfactory bulb,
which is located right behind the nose,
where the brain needs to be able to detect
toxins in the blood or pheromones from the
air so it can react quickly by vomiting. The
brain has relaxed its safety measures there,
and the bacteria may have taken advantage.
Future studies aim to find out whether
the bacteria are particularly concentrated in
these locations around the olfactory bulb
and vomit centre. If this proves to be the
case, it will support the theory that bacteria
travel to the brain via the blood.
Messy intestinal flora
According to Rosalinda Roberts’ studies, the
intestinal bacteria live peacefully in the
brain without triggering acute infection. But
because she found bacteria particularly in
parts of the brain that are involved in mental
or physical brain diseases, the tiny microbes
probably mix things up in other ways,
affecting the brain cells’ workings and hence
also our thoughts and mental health.
Deep inside the brain are areas known as
the substantia nigra and the striatum, wheremotions are coordinated so they become
fluid, as when we lift a cup with a hand to
our mouth. In Parkinson's patients, these
areas do not function correctly, so that
motions become slow, clumsy or tremorous.
Both areas harbour bacteria, indicating that
the microbes affect the substantia nigra or
striatum directly through close contact.
Another brain area with many bacteria was
the hippocampus, which is involved inAlzheimer's and depression. In an article
from 2019, psychiatrist Elizabeth Sublette
from Columbia University in the US deter-
mines that some intestinal bacterium
species are particularly numerous in
patients with depression and so probably
boost disease development, whereas other
species exist mainly in healthy people, and
so might be contributing to protection of
the brain against depression.Intestinal flora could affect mood
Your mood and your quality of life are also
influenced by bacteria, according to a major
study from 2019. Microbiologist Jeroen Raes
from the University of Leuven in Belgium
studied the intestinal flora of more than
1000 people, finding striking relationships.
Faecalibacterium intestinal bacteria
were particularly numerous in people who
were emotionally balanced, whereas
Butyrivibrio was linked with the ability to
form positive social relationships. In the
intestines of people with lots of energy,
there were many Coprococcus bacteria.
Other scientists will now repeat Roberts’
experiment, firstly to double-check that the
bacteria really are present in the brain. Not
until then can a concrete relationship be
established between the bacteria and the
brain’s physical and mental diseases. If
successful, the discoveries could outline a
promising new direction for treating those
who suffer such forms of brain distress.92
% of the bacterium species
in the brain belong to
major bacterium classes.UN
IVE
RSI
TY
OF
ALA
BAMA (^) A
T (^) BI
RM
ING
HAM
Rosalinda Roberts (left) heads the
research into a large collection of
donated human brains.
64 | SCIENCE ILLUSTRATED
HUMANS BRAIN BACTERIA