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AI - Gut Microbiota > Role of Gut Microbiota > Flashcards

Role of Gut Microbiota Flashcards

1
Q

Where is gut microbiota found?

A

At the mucosal surfaces of the gut.

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2
Q

What is a mucosal surface composed of?

A

An epithelial layer - acts as the key protective barrier - and a mucus layer.

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3
Q

Where is the majority of gut microbiota found?

A

In the colon.

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4
Q

What proportion of the body’s immune cells are found in the gut?

A

70%

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5
Q

What is the role of immune cells within the gut?

A

To maintain homeostasis of the commensal bacteria population.

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6
Q

Does a baby have commensal bacteria before birth?

A

It is unclear whether a baby is completely sterile within the womb or whether it has a small amount of commensal bacteria.

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7
Q

How is a baby colonised in vaginal delivery?

A

Colonised by faecal bacteria, and bacteria within the vaginal mucosal surface.

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8
Q

How is a baby delivered by C-section first exposed to bacteria?

A

Delivery method is much more sterile - first exposure mainly comes from interaction with medical staff and from the skin of the mother.

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9
Q

Why should a baby be breast-fed?

A

Breast milk contains bacteria, allowing further colonisation of the gut.

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10
Q

Describe the diversity of the gut microbiota during development.

A

During the first year of development, there is high diversity in the commensal bacteria present in different babies. This stabilises in the young adult.

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11
Q

What causes changes to the microbiota during development?

A

Depends on lifestyle, diet, genetic factors, and changes to gut physiology.

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12
Q

Why has very little information been known about which species colonise the gut, until recently?

A

It is difficult to culture bacteria from the gut.

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13
Q

What has allowed identification of the bacterial species in the gut?

A

Sequencing of the 16S rRNA gene.

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14
Q

Why does sequencing of the 16S rRNA gene identify a bacterial species?

A

Has a highly conserved region and 9 hypervariable regions that are unique to each bacterial species.

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15
Q

What are the two major phyla of bacteria found in the gut?

A

Bacteroidetes and firmicutes.

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16
Q

How many different bacterial species are thought to be present within the gut?

A

Up to 35,000

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17
Q

Other than sequencing of 16S rRNA, what else can be used to characterise the bacterial species in the gut?

A

The metabolome - metabolites produced by the bacteria.

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18
Q

Describe the conditions in the stomach.

A

Low pH, not a good environment for bacterial growth. Sparse bacterial load with low/moderate diversity.

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19
Q

Give the bacterial phyla that dominate in the stomach.

A

Firmicutes and actinobacteria.

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20
Q

Describe the conditions in the duodenum.

A

Neutral pH, increased bacterial load with moderate diversity.

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21
Q

Give the bacterial phyla that dominate in the duodenum.

A

Firmicutes and proteobacteria.

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22
Q

Describe the conditions in the colon.

A

Neutral pH, high bacterial load with high diversity.

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23
Q

Give the bacterial phyla that dominate in the colon.

A

Firmicutes and bacteroidetes.

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24
Q

Give the bacterial load in the stomach, duodenum and colon.

A

Stomach - 10^1 /g
Duodenum - 10 ^3 /g
Colon - 10^12 /g

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25
Q

How is a large absorptive surface achieved in the gut?

A

Through villi and microvilli structures.

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26
Q

Why must there be high levels of vasculature in the gut?

A

Allows nutrient transport and good blood supply.

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27
Q

Give evidence for immune cell maintenance of homeostasis in the gut, and the requirement of the microbiota for gut development.

A

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.

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28
Q

Give the cells that produce the mucins to form the mucus layer.

A

Epithelial cells - goblet cells in particular.

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29
Q

Where in are immune cells found in the gut?

A

Beneath the epithelial layer - especially within the lamina propria.

30
Q

What are the epithelial cells required for?

A

Nutrient absorption and to act as a physical barrier against commensal bacteria.

31
Q

Where are dendritic cells found and why?

A

Closest to the epithelial layer as they can pass pseudopods between epithelial cells to collect antigens.

32
Q

What is the role of thick muscle layers in the gut?

A

Enable gut movement, whilst also acting as a protective barrier.

33
Q

Why is the enteric nervous system required?

A

For movement of the gut.

34
Q

Give the 3 ways in which the immune system controls the microbiota to prevent inflammation and disease.

A
  • 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.
35
Q

How is entry of bacteria into the epithelium prevented?

A

Limiting contact of microbiota and potential pathogens with host cells, controlling location of bacteria and immune cells.

36
Q

Give evidence for the importance of the mucus layer in gut homeostasis.

A

Mice deficient in mucin-2 (main component of mucus layer) have gut dysfunction.

37
Q

Describe the function of the mucin layer.

A

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.

38
Q

Why do epithelial cells constantly produce antimicrobial peptides?

A

Allows epithelial cells to control microbiota and fight any bacteria that try to cross the epithelial layer.

39
Q

Give examples of antimicrobial peptides produced by epithelial cells.

A

Lipocalin-1, a-defensin, RegIIIy.

40
Q

What does lipocalin-1 do?

A

Deprives the environment of iron - a crucial nutrient for bacteria.

41
Q

What is a-defensin?

A

A pore forming protein - causes bacterial cell lysis.

42
Q

What is RegIIIy?

A

A C-type lectin - binds gram positive bacteria.

43
Q

What induces RegIIIy production in epithelial cells?

A

TLR and NLR activation - further increases production.

44
Q

Give the main antibody produced in the gut.

A

IgA.

45
Q

Describe the role of Peyer’s Patches.

A

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.

46
Q

How is the immune response regulated in the gut?

A

By Tregs.

47
Q

Why must there be a balance of activation and regulation of the immune response?

A

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.

48
Q

Give evidence for the important of Tregs in gut homeostasis.

A

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.

49
Q

Give examples of activatory T cells present in the gut.

A

Th1, Th2, Th17 cells.

50
Q

Give examples of regulatory T cells present in the gut.

A

Tregs and Tr1 cells (simialr to Tregs).

51
Q

Give the role of ILC1s.

A

Resistance against intracellular pathogens and viruses.

52
Q

Give the role of ILC2s.

A

Resistance against helminth infection, and aid tissue repair.

53
Q

Give the role of ILC3s.

A

Control commensal bacteria, resistance against bacterial/fungal infections, repair of lymphoid tissues.

54
Q

How are ILC1s stimulated to produce IFNy? And what does this result in?

A

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.

55
Q

How does RA produced by DCs affect ILC1s?

A

Drives conversion to ILC3 - as RA drives expression of RORyt.

56
Q

How are ILC2s activated? And what does this result in?

A

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.

57
Q

What is the role of amphiregulin?

A

Involved in tissue repair.

58
Q

What is the role of IL-13?

A

Aids mucin production - giving increased mucus production in the damaged area.

59
Q

How are ILC3s stimulated, and what is the outcome of their stimulation?

A

IL-7 produced by epithelial cells, stimulates ILC3s to produce IL-17a.
Results in recruitment of other immune cell, including neutrophils.

60
Q

How are ILC3s stabilised?

A

By release of IL-23 and IL-1b from DCs - stabilise RORyt expression.

61
Q

How can ILC3s affect myeloid cell proliferation?

A

Can release GM-CSF.

62
Q

Where do immune responses in the gut occur?

A

Locally in specialised compartments.

63
Q

Where are Peyer’s patches found?

A

Localised to gut tissue - nexrt to crypts.

64
Q

Where is the MLN located?

A

Further away from the gut tissue - close to the wall of the small intestine.

65
Q

Why is compartmentalisation important?

A

Prevents leakage into other parts of the body which could cause sepsis and systemic immune responses.

66
Q

How are effector T cells localised to the gut?

A

By a4b7 integrins expressed on their cell surface.

67
Q

How does vitamin A (retinoic acid) in the diet affect the mucosal immune system in the gut?

A
  • increases production of IgA
  • promotes Tregs
  • decreases Th17 responses
  • promotes ILC1 -> ILC3 switch
68
Q

What is the effect of saturated fatty acids on the gut immune system?

A

Increases production of pro-inflammatory cytokines in macrophages.

69
Q

What is the effect of unsaturated fatty acids, e.g. omega 3, on the gut immune system?

A

Decreases levels of pro-inflammatory cytokines - signals via GPR120.

70
Q

What is the effect of cholesterol on the gut immune system?

A

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.

AI - Gut Microbiota (2 decks)

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