The microbiota of the GI tract Flashcards
How does the quantity of bacteria change along the GI tract?
Bacterial density increases
Stomach ~10^4
Colon ~10^11
What is a facultative anaerobic bacteria?
An obligate anaerobe?
FAB - can grow in the presence of oxygen AND in the absence of oxygen
OA - cannot grow in the presence of oxygen
What factors influence the type of bacteria that grow in the gut tube?
Oxygen concentrations
pH levels
Transit time (off food through structure)
How many bacterial cells are in the average human gut?
Up to 100 trillion
More bacteria than human cells in the body
What is an OTU? What does it indicate?
OTU - operational taxonomic unit
Indicates diversity of bacteria in the gut flora, indicates health
Functions of the bacterial microbiota?
- Metabolism of some dietary components
- Defence against pathogens
- Produce metabolites
- Develop immune system (immune priming)
- Host signalling (gut-brain axis)
Digestion of junk food vs fibre with regard to gut microbiota?
Junk food digested in stomach/small intestine - doesn’t feed gut microbes
Fibre feeds gut microbes, energy absorbed in the colon after their action
Benefits of including fibre in your diet?
- Improves faecal bulking, eases passage, shorter transit time
- Contains phytochemicals, anti-oxidants and vitamins
Benefits of bacterial fermentation of fibre in the gut?
- Additional phytochemicals
- Maintains slightly acidic pH
- Increased commensal population and maintenance of pH protects against pathogens
- Essential supply of short chain FA’s
Three main short chain fatty acids provided by microbial fermentation and their functions?
- Butyrate: epithelial cell regrowth and regeneration
- Propionate: gluconeogenesis & satiety signalling
- Acetate: supplies peripheral tissues, lipogenesis
Main products of bacterial carbohydrate metabolism?
- Short chain fatty acids
- Gases (CO2, H2, CH4)
Main products of microbiota protein metabolism? When is this type of metabolism high?
- Branched short chain fatty acids
- Gases (NH3, H2S)
- Phenols, indoles, amines
High at pH’s above 6 - non healthy microbiota
How does what we eat influence gut microbiota? What sort of diet does this encourage?
Certain bacteria are primed to digest certain foods - these thrive if your diet focuses on a few tings
Want to be eating a diverse, balanced diet
Ways in which microbiota contributes to colonization resistance?
- Barrier effect
2. Active competitive exclusion
How does the lowering of gut pH by bacterial fermentation help?
- Higher fermentation rates, more short chain FA production/absorption
- Pathogen exclusion: typically pathogens grow at pH’s over 6
- Quicker transit & high epithelial cell turnover
What is the barrier effect?
Commensal bacteria lie within the lumen and outer mucous layer of the gut - prevents adhesion/colonization by pathogens
What happens when a bacteria from the gut microbiota penetrates the inner mucous layer and gets past epithelial cells?
IgA mediated immune response leads to the bacteria being killed off
More bacteria constantly entering gut, more being killed off - homeostasis
What happens when the immune response to a penetrating gut bacteria is not regulated?
Inflammation - results in destruction of the mucous layer and conflict between immune system and newly exposed bacteria
Why do the gut microbiota and immune system need to co-evolve? What happens if they don’t?
Evolve together so that the immune system is tolerant of the gut microbiota and can regulate it so that it doesn’t grow out of control
Autoimmune diseases
How does the gut microbiota influence host signalling?
By producing short chain FA’s - important signalling molecules. Act on receptors on gut epithelial cells
Some of the short chain fatty acid receptors and their functions?
- GPR43/FFAR2: activated by acetate/propionate. Results in GLP-1 secretion, inhibits fat accumulation
- GPR/FFAR3: activated by acetate/propionate. Results in PPY secretion - improves insulin sensitivity and signals satiety
- GPR109A: activated by butyrate, suppresses colonic inflammation and carcinogenesis
Is the gut microbiota the same throughout life?
No - changes constantly depending on age and diet
How does microbial diversity change throughout life?
Infant - low diversity, good
Healthy adult - high diversity, good
Elderly - declines a bit, want it to stay as high as possible
