Tim Newman
Sept. 7, 2016
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| How much sway can a microbe hold? |
Although the interaction between our brain and gut has been studied for years, its complexities run deeper than initially thought. It seems that our minds are, in some part, controlled by the bacteria in our bowels.
The gut has defenses against
pathogens, but, at the same time, it encourages the survival and growth of
"healthy" gut bacteria.
The vast majority of these
single-celled visitors are based in the colon, where no less than 1 trillion reside in each
gram of intestinal content.
Estimating the number of bacterial
guests in our gut is challenging; to date, the best guess is that 40 trillion
bacteria call our intestines home - partially dependent on the size of your last
bowel movement (poop's major ingredient is bacteria).
To put that unwieldy number into
perspective, our bodies consist of roughly 30 trillion cells. So, in a very
real sense, we are more bacteria than man.
Most of our gut bacteria belong to
30 or 40 species, but there can be up to 1,000 different species in all.
Collectively, they are termed the micro biome.
Of course, bacteria do benefit
from the warmth and nutrition
in our bowels, but it is not a one-way relationship - they also give back.
Some species benefit us by
breaking dietary fiber down into short-chain fatty acids that we can then
absorb and use. They metabolize a number of compounds on our behalf and play a
role in the synthesis of vitamins
B and K.
On the other side of the fence,
recent research infers that dysregulation
of gut bacteria might be an important factor in inflammatory
and autoimmune conditions.
The microbiome's role in health
and disease is only slowly giving up its secrets. The latest and perhaps most
remarkable finding is the ability that gut bacteria have to moderate our brain
and behavior.
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Why should the gut and brain be
linked?
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The goings on in our guts are a
matter of life or death. If the gut is empty, our brain must be told; if there
is a problem with our gut that will hinder food processing and therefore
nutrition absorption, the brain will need to be informed. If our gut is facing
a pathogen attack, our brain should be kept in the loop.
The links between our gut and
brain are hormonal, immunological, and neural, via the central
nervous system and the enteric nervous system, which governs the
function of the gut. Collectively, they are termed the gut-brain axis.
Although, at first glance, the
connections between the gut and brain might seem surprising, we have all
experienced it in action. The relationship between stress,
anxiety, and a
swift bowel movement are no stranger to anyone.
These gut-brain conversations have
been studied for some time. However, a new level to this partnership has
recently been glimpsed; researchers are now considering the influence of our
microbiome on the gut-brain axis. In other words, researchers are asking: do
the bacteria in our gut affect our psychology and behavior?
Termed, rather clumsily, the
brain-gut-enteric microbiota axis or microbiome-gut-brain axis, researchers are
only beginning to scratch its surface.
Stress and the gut
In humans, the
hypothalamic-pituitary-adrenal (HPA)axis is the primary responder to stresses
of any kind. It is one of the major players in the limbic system and is heavily
involved in emotions and memory.
Stress activates the HPA axis and
eventually results in the release of cortisol - the "stress hormone"
- which has a variety of effects on many organs, including the brain and gut.
In this way, the brain's response
to stress has a direct influence on the cells of the gut, including epithelial
and immune cells, enteric neurons, interstitial cells of Cajal (the pacemakers
of the bowels), and enterochromaffin cells (serotonin
synthesizing cells).
Conversely, these cell types are
also under the influence of our resident army of bacteria. Although the
mechanisms by which the microbiota regulate the brain are less clear, evidence
is mounting that there is, indeed, a two-way dialogue.
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What a difference a microbe makes
The
first clues that microbes might have some control over our mental activity came
more than 20 years ago.
Patients with hepatic encephalopathy - a decline in brain function due to
severe liver disease - were found to improve substantially after taking oral antibiotics.
Later studies provided further
hints that the microbiome had more than a passing influence on states of mind;
it was found to impact
anxiety and depressive-like behaviors.
Another key observation linked
dysbiosis (microbial
imbalance) with autism. Children with
autism often have abnormal and less diverse communities of bacteria in their
gut. One
researcher concluded:
"We suspect that gut microbes
may alter levels of neurotransmitter-related metabolites, affecting
gut-to-brain communication and/or altering brain function. [...] Correlations
between gut bacteria and neurotransmitter-related metabolites are stepping
stones for a better understanding of the crosstalk between gut bacteria and
autism."
Researchers in 2004 noted that
mice bred to have no gut bacteria had an exaggerated HPA axis response to
stress. Further investigations using similar germ-free mice have demonstrated
that their lack of gut bacteria alters memory function.
Germ-free mice have been a useful
tool to study the microbiome-gut-brain axis. They have helped prove that something
is going on, but the results are impossible to extrapolate into humans. They
replicate no natural situation known to man - there is no such thing as a
germ-free human.
Other studies have used different
approaches; some investigated the effects of the neuroactive compounds that gut
flora produce; others still have looked at the differences in the gut flora of
individuals with psychiatric or neurological differences.
Research, in general, has not been
conclusive. Even if changes in gut flora are seen, the eternal chicken or egg
question persists: was the psychiatric condition caused by the change in gut
flora, or did the psychiatric condition and its altered behavior patterns cause
the gut flora to change? Or, is there a two-way interaction?
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How can gut flora moderate the
brain?
Bacterial influence over human
psychology is slowly coming into focus.
Bacterial influence over human psychology is slowly coming into focus.
Stress is known to increase the
permeability of the intestinal lining; this gives bacteria easier access to
both the immune system and the neuronal cells of the enteric nervous system.
This may be one of the ways in
which bacteria find a way to influence us. However, another, more direct route
has also been demonstrated.
One study, using food-borne
pathogens, provided evidence that bacteria in the intestines can activate
stress circuits by directly activating the vagus nerve
- a cranial nerve supplying a number of organs, including the upper digestive
tract.
A more direct route still might
involve direct contact of the microbiome with the sensory neurons of the
enteric nervous system. Studies have shown that these sensory neurons are less active
in germ-free mice, and, once the mice have been given probiotics to restock
their microbiome, the activity levels of the neurons return to normal.
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Probiotics influencing psychology
If
germ-free mice show differences in behavior, the next question is whether
adding gut bacteria to an animal can make similar changes. A meta-analysis, published
in the Journal of Neurogastroenterology and Motility, collated the results of
studies looking at the effects of probiotics on central nervous system function
in both humans and animals.
They examined 25 animal and 15
human studies, most of which used Bifidobacterium and Lactobacillus over a
2-4-week period. Although, as the authors mention, translating animal studies
like this into human terms is a dodgy game. They concluded:
"These probiotics showed
efficacy in improving psychiatric disorder-related behaviors including anxiety,
depression,
autism spectrum disorder, obsessive-compulsive
disorder, and memory abilities, including spatial and non-spatial
memory."
Another study, published in PLOS
One, found that age-related decline in memory could be reversed in rats
by altering the levels of Actinobacteria and Bacterioidetes in their gut with
probiotics.
The authors conclude: "The
data support the notion that intestinal microbiota can be manipulated to
positively impact on neuronal function."
http://www.medicalnewstoday.com/articles/312734.php
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