Role of Gut Microbiota Flashcards
Where is gut microbiota found?
At the mucosal surfaces of the gut.
What is a mucosal surface composed of?
An epithelial layer - acts as the key protective barrier - and a mucus layer.
Where is the majority of gut microbiota found?
In the colon.
What proportion of the body’s immune cells are found in the gut?
70%
What is the role of immune cells within the gut?
To maintain homeostasis of the commensal bacteria population.
Does a baby have commensal bacteria before birth?
It is unclear whether a baby is completely sterile within the womb or whether it has a small amount of commensal bacteria.
How is a baby colonised in vaginal delivery?
Colonised by faecal bacteria, and bacteria within the vaginal mucosal surface.
How is a baby delivered by C-section first exposed to bacteria?
Delivery method is much more sterile - first exposure mainly comes from interaction with medical staff and from the skin of the mother.
Why should a baby be breast-fed?
Breast milk contains bacteria, allowing further colonisation of the gut.
Describe the diversity of the gut microbiota during development.
During the first year of development, there is high diversity in the commensal bacteria present in different babies. This stabilises in the young adult.
What causes changes to the microbiota during development?
Depends on lifestyle, diet, genetic factors, and changes to gut physiology.
Why has very little information been known about which species colonise the gut, until recently?
It is difficult to culture bacteria from the gut.
What has allowed identification of the bacterial species in the gut?
Sequencing of the 16S rRNA gene.
Why does sequencing of the 16S rRNA gene identify a bacterial species?
Has a highly conserved region and 9 hypervariable regions that are unique to each bacterial species.
What are the two major phyla of bacteria found in the gut?
Bacteroidetes and firmicutes.
How many different bacterial species are thought to be present within the gut?
Up to 35,000
Other than sequencing of 16S rRNA, what else can be used to characterise the bacterial species in the gut?
The metabolome - metabolites produced by the bacteria.
Describe the conditions in the stomach.
Low pH, not a good environment for bacterial growth. Sparse bacterial load with low/moderate diversity.
Give the bacterial phyla that dominate in the stomach.
Firmicutes and actinobacteria.
Describe the conditions in the duodenum.
Neutral pH, increased bacterial load with moderate diversity.
Give the bacterial phyla that dominate in the duodenum.
Firmicutes and proteobacteria.
Describe the conditions in the colon.
Neutral pH, high bacterial load with high diversity.
Give the bacterial phyla that dominate in the colon.
Firmicutes and bacteroidetes.
Give the bacterial load in the stomach, duodenum and colon.
Stomach - 10^1 /g
Duodenum - 10 ^3 /g
Colon - 10^12 /g
How is a large absorptive surface achieved in the gut?
Through villi and microvilli structures.
Why must there be high levels of vasculature in the gut?
Allows nutrient transport and good blood supply.
Give evidence for immune cell maintenance of homeostasis in the gut, and the requirement of the microbiota for gut development.
Zebrafish raised in a germ-free environment die due to impaired macromolecule absorption.
Germ-free mice have a lack of Th17 and a reduced number of Treg cells.
Give the cells that produce the mucins to form the mucus layer.
Epithelial cells - goblet cells in particular.
Where in are immune cells found in the gut?
Beneath the epithelial layer - especially within the lamina propria.
What are the epithelial cells required for?
Nutrient absorption and to act as a physical barrier against commensal bacteria.
Where are dendritic cells found and why?
Closest to the epithelial layer as they can pass pseudopods between epithelial cells to collect antigens.
What is the role of thick muscle layers in the gut?
Enable gut movement, whilst also acting as a protective barrier.
Why is the enteric nervous system required?
For movement of the gut.
Give the 3 ways in which the immune system controls the microbiota to prevent inflammation and disease.
- Limiting direct contact between bacteria and the epithelium.
- Rapid detection and killing of pathogenic bacteria.
- Minimising exposure of resident bacteria to the immune system, using compartmentalisation of immune cells.
How is entry of bacteria into the epithelium prevented?
Limiting contact of microbiota and potential pathogens with host cells, controlling location of bacteria and immune cells.
Give evidence for the importance of the mucus layer in gut homeostasis.
Mice deficient in mucin-2 (main component of mucus layer) have gut dysfunction.
Describe the function of the mucin layer.
Large glycosylated mucins act as a thick barrier, limiting bacteria’s ability to reach the epithelial layer. Composed of 2 layers; inner layer is more dense, outer layer is less dense and houses commensal bacteria.
Why do epithelial cells constantly produce antimicrobial peptides?
Allows epithelial cells to control microbiota and fight any bacteria that try to cross the epithelial layer.
Give examples of antimicrobial peptides produced by epithelial cells.
Lipocalin-1, a-defensin, RegIIIy.
What does lipocalin-1 do?
Deprives the environment of iron - a crucial nutrient for bacteria.
What is a-defensin?
A pore forming protein - causes bacterial cell lysis.
What is RegIIIy?
A C-type lectin - binds gram positive bacteria.
What induces RegIIIy production in epithelial cells?
TLR and NLR activation - further increases production.
Give the main antibody produced in the gut.
IgA.
Describe the role of Peyer’s Patches.
M cells endocytose material from the gut lumen and release it into PPs. DCs in the PP collects antigen and presents it to B/T cells in the PP or in the MLN. Activated B cells produce IgA which is transcytosed into the gut lumen.
How is the immune response regulated in the gut?
By Tregs.
Why must there be a balance of activation and regulation of the immune response?
There needs to be a good enough level of activation if a pathogen is encountered, and there must be regulation to avoid overactivity of immune cells and inflammation.
Give evidence for the important of Tregs in gut homeostasis.
KO mice for IL-10, TGFb and Foxp3 - all susceptible to gut inflammation.
Naive T cells transferred into immunodeficient mice -> IBS
Recovered by transfer of Tregs - homeostasis restored, overinflammation prevented.
Give examples of activatory T cells present in the gut.
Th1, Th2, Th17 cells.
Give examples of regulatory T cells present in the gut.
Tregs and Tr1 cells (simialr to Tregs).
Give the role of ILC1s.
Resistance against intracellular pathogens and viruses.
Give the role of ILC2s.
Resistance against helminth infection, and aid tissue repair.
Give the role of ILC3s.
Control commensal bacteria, resistance against bacterial/fungal infections, repair of lymphoid tissues.
How are ILC1s stimulated to produce IFNy? And what does this result in?
Epithelial cells produce IL-15, which stimulates ILC1 cells to produce IFNγ.
Results in increased expression of chemokines, affecting immune cell recruitment – Th1 and ILC1 recruitment in particular.
How does RA produced by DCs affect ILC1s?
Drives conversion to ILC3 - as RA drives expression of RORyt.
How are ILC2s activated? And what does this result in?
Production of IL-25 and IL-33 by epithelial cells activates ILC2 upon pathogen-induced damage of epithelial cells.
Results in production of amphiregulin and IL-13 by ILC2s.
What is the role of amphiregulin?
Involved in tissue repair.
What is the role of IL-13?
Aids mucin production - giving increased mucus production in the damaged area.
How are ILC3s stimulated, and what is the outcome of their stimulation?
IL-7 produced by epithelial cells, stimulates ILC3s to produce IL-17a.
Results in recruitment of other immune cell, including neutrophils.
How are ILC3s stabilised?
By release of IL-23 and IL-1b from DCs - stabilise RORyt expression.
How can ILC3s affect myeloid cell proliferation?
Can release GM-CSF.
Where do immune responses in the gut occur?
Locally in specialised compartments.
Where are Peyer’s patches found?
Localised to gut tissue - nexrt to crypts.
Where is the MLN located?
Further away from the gut tissue - close to the wall of the small intestine.
Why is compartmentalisation important?
Prevents leakage into other parts of the body which could cause sepsis and systemic immune responses.
How are effector T cells localised to the gut?
By a4b7 integrins expressed on their cell surface.
How does vitamin A (retinoic acid) in the diet affect the mucosal immune system in the gut?
- increases production of IgA
- promotes Tregs
- decreases Th17 responses
- promotes ILC1 -> ILC3 switch
What is the effect of saturated fatty acids on the gut immune system?
Increases production of pro-inflammatory cytokines in macrophages.
What is the effect of unsaturated fatty acids, e.g. omega 3, on the gut immune system?
Decreases levels of pro-inflammatory cytokines - signals via GPR120.
What is the effect of cholesterol on the gut immune system?
Activation of the inflammasome in macrophages - characterised in zebrafish where cholesterol diet caused increased macrophage production and reduced gut movement was seen after a week.
