Why talk about gut bacteria in a neuroscience lecture?
• Strong evidence that microbial products affect enteric neural function
• Stress responses can be modulated by the microbiome and vice versa
• Similar evidence about anxiety
• Various studies indicating that autism symptoms may be related to composition of microbiome
• Faecal transplants can alter symptoms of neurological disease?
-human behaviour in part depends on the bacteria in the gut
-stress modulates the biota, get diarrhea
-changing biota can change behaviour
-fecal transplants= remission in multiple sclerosis, permanent
-gut microbiota when happy you are happier
What is this?
-interactions in the gut
How many bacteria are there in the gut?
• The gut contains about 10X more bacteria (1013) than there are cells in the human body
• Estimates of the genetic diversity of the gut bacteria vary from there being 150X to 300X the number of different genes in microbiome that in the human genome
• Each individual has his/her own unique combination of intestinal “flora”
-born with almost no bacteria, in the first 5 years develop the biota that then doesn’t change that much
-the ecosystem is different between individuals, even twins reared together have differences
-more genes in the bacteria than in the individual, the more diverse the microbiota the more likely to be healthy and not to have diabetes, etc.
-different species go up and down in the mix but the no of species usually doesn’t change that much
How many bacterial families are present in the gut?
• At least 7 different families of bacteria present
– In colon, almost exclusively anaerobic
– In caecum, about 20% aerobic
What can affect the proportions of the bacteria in the gut?
• Diet, stress, probiotics and antibiotics can change proportions, but species mix remains relatively constant and proportions revert to original values when insult is removed.
What type of bacteria are there?
• Bacteria in mucus associated with epithelium differ from those in the lumen itself 60% microbes, 40% fibre in the fecal matter
-the mix of species doesn’t change that much
- veggie/ carnivorous diet, the vegetarian diet= looked like the biome in the herbivore, shifted in a week
-then go back and then the biome gets back to what it was
-there are also populations of bacteria that live in the mucus (so some special for lumen, some for mucus and some for both)
-continuous equilibrium between
What are the tool for study of gut microbiome?
•1. 16S ribosomal RNA sequencing and other next generation sequencing methods for identifying presence of different species
• 2. Metabolomics – Identification of the full suite of metabolic products produced by colonic bacteria -metabolomics= the products of the metabolism, part of the system by which the bacteria will talk to the host
• 3. Germ-free mice – Behavioural studies – Functional studies at single neuron level – Can introduce whole microbiome (faecal transplant) or single bacterial strains (probiotic)
- prokaryotes= bacteria, set or RNA , encodes 16S ribosomal RNA, differs from one species to another, so sequencing this allows for idnetification of bacteria and assign them to families
-once you have done that you can do whole gene sequencing for each gene in the entire sample
-have to have very good bioinformatics data
What is this?
-Families of bacteria in mouse faeces
-60% overlap between humans and mice, so can draw some conclusions but not all
How can you use metabolomics?
-Analysing the human microbiome, Sequencing and metabolomics
-from individuals for clostridium dificill who have it recurrent and non recurrent
-one of the metabolic products GABA is thought of as an inhibotory neurotransmitter, in gut it is excitatory, but here it is correlated with suscpetibilty to disease, it is produced by the gut micorbiota an dmaybe the way these can talk to the brain
What are the germ free mice?
- have no microbiome, completely sterile environment, separated from bacteria, can take it out of the box, give it bacteria and can control bacteria you are giving it and see what happens
-germ free mice are not healthy
-these are the the ways studies are done:
- can gave germ free animal
What are some of the key metabolites of the gut bacteria?
• Many metabolic products of microbiome have direct actions on nervous system
– GABA – excitatory for both enteric neurons and extrinsic primary afferent neurons
– Serotonin – excites enteric neurons and terminals of extrinsic primary afferent neurons
– Histamine – inflammatory mediator
– Lipopolysaccharide (LPS) cell wall component that is neuroactive, can act on enteroendocrine cells and enters the blood stream
– Short chain fatty acids (acetate, butyrate, proprionate) can excite intrinsic sensory neurons and EE cells via specific G- protein-coupled receptors
-these are the metabolites produced by the bacteria in the gut and via these they can have an effect on the system
What are the effects of probiotics on a myenteric neuron?
myenteric= sensory intrinsic neurons, on far left= the control, then exposed to large no of bacteria of same species, then after some time get a change in activation
-will change the bahviour in a mice= increase measure in anxiety
-probiotic bacteria= can survive the stomach
How do stress and microbiome interact?
• Stress modifies microbiome via cortisol – probably also via CRF receptors on enteric neurons
• Microbiome modifies central mood, etc via cytokines, tryptophan metabolites – Probably also via primary afferent neurons
How are germ free mice a powerful tool?
• Uterus is believed to be bacteria free, so mouse pups can be delivered under sterile conditions
– Caesarian delivery, isolated from dams, reared and held in germ free conditions
– Takes several generations as some bacteria are present in amniotic fluid
• Cannot achieve germ free status using antibiotics, however powerful.
• Problem is that these mice are metabolically handicapped
– Also changes in ENS development
-deliver the mice in a sterile environment
-about 10 generations of mice before can be sure of no bacteria in the animal
-fewer neurons in the distal small intestines= when germ free, and antibiotics
How do germ free mice differ in terms of anxiety?
-Germ free mice are less anxious than mice with a gut microbiome
• Studied using standard behavioural tests of anxiety – Elevated plus maze (right) – Light-dark box
• Anxiety measures increased by faecal transplant or instillation of a single probiotic strain
-anxiety levels higher when have germs
-unclear how that happens
What is the connection between autism and the microbiome?
• Suggested that altered microbiome due to prolonged antibiotics leads to over-representation of Clostridium sp leading to altered behaviour
– But syndrome itself is controversial
• Several small studies with selected patients support this
• Largest study gives different conclusion
– No difference in microbiomes between patients and “neurotypical” siblings
– No difference in microbiota between ASD patients with GI symptoms, neurotypical siblings and ASD patients without GI symptoms
• Could be chicken and egg problem
-small kid given antibiotics, then germs different, claim= autism is caused by this
-this is not true, larger data sets then dispute this