2020-04-02_Science_Illustrated

lactic-acid bacteria which make up a natural
part of the human intestinal flora. The ‘food’
resulted in lower concentrations of the
corticosterone stress hormone in the blood,
and influenced the brain’s ability to process
impulses of anxiety, fear and stress. After six
weeks on the high-bacterium diet, the mice
became less nervous, and they spent more
time exploring open areas of the cage,
where they would have been vulnerable to
enemies in nature.
When John Cryan cut the mice’s vagus
nerves, he discovered a possible explana-
tion of how the intestinal flora control the
brain. This nerve links the brain and the
intestines, informing the brain that the
intestines are full. On the other hand, the
brain can inform the intestines that it is the
middle of the night, so they should take it
easy. In mice without the vagus nerve
connected, the lactic-acid bacteria lost
their antidepressive effect,and the animals
remained just as nervous as they would
have been without the special ‘feed’.

Scientists found the bacteria with a type of

microscope that is rarely used on brains.

BACTEROIDETES
Neurotransmitter
protects against depression
An intestinal flora with a low level of
Bacteroides bacteria affects nerve signal
flows n the brain in a pattern linked with
depression. The depressive thoughts disappear with
an increased number of bacteria in the intestines.
BACTERIUM ROLE: secretes GABA, which regulates
signals between nerve endings in the brain.

1

PROTEOBACTERIA
Protein triggers Alzheimer's

2

Every second person has Helicobacter pylori
bacteria in their stomach, from where they
send neurotransmitters that affect intestinal
flora and the brain. Alzheimer's patients have more H.
pylori bacteria in their stomachs than healthy people,
and the bacterium is known to cause dementia in mice.
BACTERIUM ROLE: emits peptides that make brain
cells produce harmful protein lumps.

FIRMICUTES
Substance in the blood
aggravates schizophrenia

3

Schizophrenia patients often have more
Lactobacillus bacteria – related to lactic
acid bacteria – in their intestines. The
symptoms typically deteriorate with the increased
number of this type of bacteria.
BACTERIUM ROLE: increases the blood's content
of kynurenine that blocks receptors in the brain.

Since then, many scientists have shown

that different intestinal bacteria can affect

the human brain so that it becomes more or

less receptive to schizophrenia, depression,

anxiety, Alzheimer's, and Parkinson’s. But

they still don’t know exactly how the intes-

tinal flora influences the brain.

So far, the predominant theory has been

that intestinal bacteria activate the vagus

nerve’s outermost ramifications in the

intestinal wall, making it send nerve signals

to the brain. But Rosalinda Roberts’ discov-

ery indicates that the microbes reduce this

extended chain of command by migrating

through the vagus nerve into the brain itself.

By nerve or by blood?

The images from Roberts’ electron

microscope clearly show that the

brain’s bacteria accumulate in the

myelin sheaths that function as

insulation and surround most

neurons. This same myelin

fatty substance also

surrounds the vagus

nerve that passes

directly into the

lower brain, the

brainstem,

KYNURENINE

BACTERIA

PEPTIDES

GABA

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Protein lump