ISIS Report 20/11/13
How Microbes Influence our Minds
We think of ourselves as autonomous individuals with a will of our
own, but microbes in our gut turn out to have more say on how we feel and
behave than we know Dr Eva Sirinathsinghji
A fully referenced version of this article is posted on ISIS members
website and is otherwise available for download here.
between the gut and the brain has long been acknowledged and has led to the gut
being dubbed the ‘second brain’. This gut-brain communication was previously thought
to be regulated by neural, endocrine and immunological signalling, but now
research is focusing on how the gut microbiota impact
such signalling in what is now being termed the microbiome-gut-brain axis (see below).
The role of the gut microbiota in gut-brain signalling is well
evidenced by the simultaneous presence of mental health-related illnesses such
as anxiety with gastrointestinal disorders such as irritable bowel syndrome
(IBS) or inflammatory bowel disease (IBD) . Research dating back to the
1970s showed that stress alters the composition of the gut flora in adult mice
. Since then, newborn mice suffering maternal separation-induced stress [3, 4]
were found to have reduced level of Lactobacilli, which makes the animals
more susceptible to infection. Further, hepatic encephalopathy (the occurrence
of confusion, altered level of consciousness, and coma as a result of liver
failure) is successfully treated with antibiotics or laxatives, thereby serving
as a reminder that gut bacteria do send signals to the brain (albeit under
pathological conditions in this case) . Indeed, the gut is thought to
harbour the majority of the body’s microbes and recent work from the Human Microbiome Project
reveals large variability in microbiota profiles between individuals, an average gut carrying around 1000 different
species of microbes and more than 7000 strains.
How do gut microbes signal to the brain?
are many ways in which the gut microbiota signal the brain (see ):
The composition of the
micriobiota itself is one very dynamic factor that changes with diet, age,
location, disease and so on. The micr0biota composition determines competition
for dietary ingredients as growth substrates, conversion of sugar into
inhibitory fermentation products, production of growth substrates, release of
bacteriocins (molecules toxic to other bacterial species), stimulation of the
innate immune system, and competition against microbes colonizing the gut wall
and gut-barrier function.
Another mechanism is through immune
activation as the immune system signals bi-directionally with the brain.
The circulation of pro- or anti-inflammatory cytokines is indirectly mediated
by microbiotic influence on the innate immune system, which can then impact
directly on brain function. The innate and adaptive immune system is also
crucial for gut health in maintaining homeostasis of the intestinal
The vagus nerve or cranial nerve is
the main nerve of the parasympathetic nervous system and mediates many organ
functions including bronchial constriction, heart rate and gut mobility; transmitting information on the luminal
environment such as hyperosmolarity, carbohydrate levels and presence of
bacterial products. Vagotomy studies have linked certain gut-brain signalling
to this nerve. Its activation also has anti-inflammatory effects.
Many microbes produce neurometabolites that
are either neurotransmitters or modulators of neurotransmission
including GABA (γ-aminobutyric acid) produced by Lactobacillus spp.
and Bifidobacterium spp, noradrenaline produced by Escherichia spp,
Bacillus spp and Saccharomyces spp, serotonin from Candida spp,
Streptococcus spp, Escherichia spp and Enterococcus spp.],
dopamine from Bacillus spp, and acetylcholine from Lactobacillus spp.
These could act directly on nerve terminals in the gut or via ‘transducer’
cells such as enterochromaffin cells present throughout the intestinal tract
and are accessible to microbes and in contact with afferent and efferent
nerve terminals. Some of these cells may also
signal and therefore modulate immune cell activity.
Microbial metabolites are produced by
microbes when modulating the host metabolic reactions, including bile acid,
short chain fatty acids and choline. Carbohydrates from dietary fibre are also
broken down by microbes, resulting in the production of neuro-active chemicals
such as n‑butyrate, acetate,
hydrogen sulfide and propionate. Alterations or overproduction of certain
metabolites are associated with brain disorders such as autism (see below).
Tryptophan metabolism is thought to be
dysregulated in many digestive and brain disorders.
Tryptophan is an essential amino acid that cannot be produced by the human body
and must be provided by diet or gut bacteria. It is the precursor to many neuroactive
agents including serotonin, which regulates gut motility and is also an
important neurotransmitter that mediates mood and wellbeing. The kynurenine arm
of tryptophan metabolism mediates 95 % of peripheral tryptophan levels. The
level is dysregulated
in germ free animals as well as models of depression that are successfully
treated with probiotics. Kynerenine metabolism can be induced by inflammatory
mediators and stress hormones. A study has shown how probiotic bacteria can alter
Microbiome influence on behaviour
newer studies of the microbiome-gut-brain axis have looked at the regulation of
the hypothalamic-pituitary-adrenal (HPA) that regulates the body’s reaction to
stress. The HPA axis is a major part of the endocrine system that also
regulates many other processes including
digestion, immune response, mood and emotion, sexuality and energy storage
& expenditure. Chronic over-activation of the HPA axis can have knock-on
effects for learning and memory, anxiety and depression.
Early experiments comparing germ-free and germ-colonised mice
showed that the activity of the HPA axis is exaggerated in germ-free mice,
which is reversed following colonisation with the probiotic bacteria
Bifidobacterium infantis ; and the earlier the colonisation, the fuller
the reversal of effects.
However, the exact opposite observations have also been
documented, with reduced anxiety being reported in germ-free mice [8, 9]. The
reason for such discrepancies remains unclear, but may be due to the
composition of the microbiota varying in the mice in different studies. At the
behavioural level, studies suggest germ-free animals are bolder and
show less anxiety on anxiety-tests such as the elevated plus maze or light-dark
boxes, which test the animals’ aversion to open or light spaces . These
changes were accompanied by altered NMDA and serotonin neurotransmitter
receptor expression in the brain.
Studies have also been done by infecting animals with microbes and
assessing physiology and behaviour. Infection with Trichuris muris,
which is closely related to the human parasite Trichuris
trichiura known as whipworm, leads to chronic inflammation . Infected mice
showed increased anxiety-like behaviour, a rise in plasma kynurenine:tryptophan
ratio and plasma levels of the pro-inflammatory cytokines tumour necrosis
factor‑α and interferon‑γ. The mice also showed reduced
expression of brain derived neuronal growth factor (BDNF) in the hippocampus,
the area of the brain involved in learning and memory. Lactobacillus rhamnosus
(JB‑1) ingestion improved anxiety-like behaviour in mice, reduced the
levels of the stress hormone corticosterone and also altered the expression of
GABA neurotransmitter receptors in the brain, as consistent with findings in
anxiety and depression-related disorders.
Anxiety levels can differ between strains of lab mice, a trait
that has been exploited in microbiome studies. The Balb/c strain tends to be
more timid and anxious compared with the NIH Swizz strain that is described as
more gregarious. By generating germ-free animals of each strain and then
infecting them with faecal content from colonised non-germ-free animals of the
opposing strain, the behavioural phenotype was transferred between strains .
This example of how behavioural traits can be transferred highlights the
importance of the gut microbiota on behaviour and personality. Animal studies
are being backed up by research on humans. A study published in 2013 showed
that ingestion of a probiotic-containing fermented milk beverage by healthy
women attenuated emotive stress-induced changes in brain activity and
connectivity as assessed by functional magnetic resonance imaging (fMRI) .
Another study of 66 healthy volunteers found that ingestion of probiotic
combination for 30 days reduced psychological distress .
Learning and memory experiments have also shown how the gut
microbiota can impact cognitive function. Germ-free mice showed stress-induced
memory dysfunction  while diabetic mice with impairments in learning and
memory were improved through probiotic supplementation . Whether
these impairments in learning are a direct result of microbiome disturbance or
an indirect result of stress or diabetes respectively remains unclear however.
Disruption of the microbiome and disease
all this research revealing the importance of the gut microbiome on the
gut-brain axis, it seems likely that many nervous system disorders such as
pain, autism and multiple sclerosis may result from
dysbiosis (microbial imbalance) in the gut.
Some of the most compelling evidence of the
importance of the micriobiome-gut-brain axis comes from studies of the
microbiota’s mediation of pain, particularly visceral pain. Probiotics were
able to reduce pain symptoms associated with inflammatory bowel syndrome-induced
pain as well as abdominal pain in animals [17, 18]. This has been associated
with changes in the expression of opioid and cannabinoid receptors in the gut,
which may be mediated by direct excitation of enteric neurones in the gut that
control colonic motility.
autism, observations of temporary improvements in symptoms following doses of antibiotics or dietary alterations have been noted
since the 1990s. Autistic people show alterations in sulphur metabolism and urinary
peptide profiles as well as increases in short chain fatty acids and ammonia in
the gut. As production of short-chain fatty acids are by-products of anaerobic fermentation,
it suggests an overgrowth of anaerobic bacteria such as Clostridia,
Bacteriodetes, and Desulfovibrio. Clostridium has
indeed been found in excess in the faeces of autistic children.
is increasing evidence that exposure to Monsanto’s herbicide Roundup, may be an
underlying cause of autism spectrum disorders (see
). Glyphosate, the active ingredient, acts through inhibition of the
5-enolpyruvylshikimic acid-3-phosphate synthase (EPSPS synthase) enzyme in the
shikimate pathway that catalyses the production of aromatic amino acids. This
pathway does not exist in animals, but it does exist in bacteria, including
those that live in the gut and are now known to be as much a part of our body
as our own cells. A widely accepted dogma is that glyphosate is safe due to the
lack of the EPSPS enzyme in our body. This however does not hold water now that
the importance of our microbiota to our physiology is clear.
It has been found that autistic children in the United States are
far more likely to be formula-fed, with some milk formulas containing soy and
therefore it is most likely GM soy contaminated with glyphosate . Further,
dysbiosis occurs in cattle and poultry exposed to glyphosate, with increases in
pathogenic strains such as Salmonella and Clostridium and decreases in
beneficial bacteria [21, 22]. Roundup has also been shown to be toxic to three
beneficial bacterial starter cultures used for the dairy industry .
Autistic patients show decreased tryptophan metabolism, while tryptophan
depletion leads to exacerbation of autistic symptoms as well as feelings of
reduced happiness and calmness. Increased ammonia in autistic patients may be
explained by glyphosate’s ability to activate phenylalanine ammonia lyase (PAL),
an enzyme in animals as well as gut bacteria, backing up the strong link between autism and hepatic encephalopathy that is also
characterised by excessive ammonia. Glyphosate’s ability to impair liver
function and therefore the clearance of xenobiotics from the body through
inhibition of cytochrome P450 enzymes on top of impairment of serum sulphate
transport could further exacerbate the problem. Glyphosate’s disruption of
liver function, sulphate transport and the gut flora has also been suggested to
contribute to many other diseases including Alzheimer’s, obesity, depression,
cancer, infertility, diabetes and heart disease (see ).
disease of the nervous system that may be affected by the gut microbiota is multiple sclerosis (MS), a devastating autoimmune disease
of the nervous system. Mouse autoimmune encephalomyelitis (EAE) models of MS,
show reduced symptoms when they are kept germ-free, suggesting an underlying
role of the gut microbiota . Further, mice susceptible to autoimmune
encephalomyelitis (EAE) do not get the disease if they are kept in
germ-free or specific pathogen-free conditions .
gut may make up more than 98 % of the genes in/on our
body that are from the resident microbiota. The microbiota play a crucial role
in our physiology. Disruption of their function and composition can have far
reaching effects making it critical for us to understand exactly how
environmental factors, whether it be antibiotic use, chemical exposure, diet
and lifestyle are impacting this incredible symbiosis. The excessive use of
antibiotics, particularly in agriculture, as well as harmful chemicals need to
be reduced or even banned immediately if we are to curb the rise in chronic
diseases in much of the world.