Lecture A6-8, pt1 - the mucosal microbiota: mutualists of the gut

What is the densest site of microbial colonization in the gut, and why?

The colon, due to slow transit and anaerobic conditions that support microbial growth.

How does the gut microbiota outnumber human cells?

Microbial cells outnumber human cells at a ratio of ~1.3:1.

What does the host provide to the gut microbiota?

Nutrients (e.g., dietary glycans) and a stable environment.

What are three key benefits the gut microbiota provides?

1. Nutrient metabolism (SCFAs like butyrate, and vitamins B12 and K).

2. Immune regulation (Tregs and SIgA production).

3. Pathogen defense (outcompetes harmful microbes).

Name five dominant phyla in the gut microbiota and a key species from each.

1. Firmicutes (Gram+): Faecalibacterium prausnitzii (anti-inflammatory).

2. Bacteroidetes (Gram-): Bacteroides fragilis (immune modulation).

3. Verrucomicrobia: Akkermansia muciniphila (mucin degrader).

4. Proteobacteria: Escherichia coli (pathobiont in dysbiosis).

5. Actinobacteria: Bifidobacterium spp. (dominant in infants).

What are alpha and beta diversity in the context of gut microbiota?

• Alpha diversity: Richness within an individual microbiota.

• Beta diversity: Variations between individuals' microbiota.

How does delivery mode influence microbiota colonization at birth?

• Vaginal birth: Transfers maternal vaginal and gut microbes (e.g., Lactobacillus).

• C-section: Reduced diversity; higher risk of immune disorders.

Which microbe is enriched by breastfeeding, and how?

Bifidobacterium spp., promoted by human milk oligosaccharides (HMOs).

What are the three main SCFAs produced by gut microbes, and their roles?

1. Butyrate: Energy for colonocytes; strengthens gut barrier.

2. Propionate: Regulates hepatic gluconeogenesis.

3. Acetate: Precursor for systemic lipogenesis.

Name a species and molecule that enhance Treg activity.

Bacteroides fragilis produces polysaccharide A to enhance Treg activity.

How does the gut microbiota defend against pathogens?

• Competes for nutrients and binding sites.

• Produces antimicrobials (e.g., bacteriocins, H₂O₂).

• Stimulates epithelial cells to release AMPs like defensins.

What is saccharolytic fermentation, and where does it occur?

The fermentation of dietary carbohydrates (e.g., inulin, resistant starch) in the proximal colon, producing SCFAs with anti-inflammatory effects.

What harmful metabolites are produced during proteolytic fermentation?

Ammonia, amines, and hydrogen sulfide (H₂S), linked to inflammation and colonic diseases.

What diet increases proteolytic fermentation?

Low-fiber, high-protein diets.

What are the three key characteristics of dysbiosis?

1. Loss of beneficial microbes (e.g., Faecalibacterium prausnitzii, Akkermansia muciniphila).

2. Overgrowth of pathobionts (e.g., E. coli, Clostridium difficile).

3. Reduced diversity due to diet, antibiotics, or disease.

Name three diseases associated with dysbiosis.

1. IBD: Loss of anti-inflammatory microbes exacerbates inflammation.

2. Obesity and Type 2 diabetes: Altered SCFA production promotes inflammation.

3. Neurological conditions: Affects gut-brain signaling, contributing to depression and anxiety.

What is the microbiota-gut-brain axis?

Bidirectional communication between the gut microbiota and the brain.

Name three pathways involved in the gut-brain axis.

1. Neural signaling (via the vagus nerve).

2. Metabolite signaling (SCFAs, tryptophan derivatives).

3. Immune signaling (cytokines crossing the blood-brain barrier).

How does the gut microbiota contribute to systemic health beyond the gut?

• SCFAs like butyrate influence systemic inflammation and energy homeostasis.

• Gut microbes synthesize vitamins (e.g., B12, K).

Why does majority of the microbial diversity reside in the colon?

The colon contains the majority of the gut microbiota due to favourable anaerobic conditions.

Why is beta diversity important in microbiota studies?

Beta diversity highlights differences in microbiota composition between individuals, which can explain variations in disease susceptibility or dietary responses.

How does the gut microbiota stabilize during adulthood?

By age three, the gut microbiota resembles the adult profile, with composition influenced by long-term diet and lifestyle.

What is the impact of breastfeeding vs. formula feeding on infant microbiota?

• Breastfeeding: Enriches Bifidobacterium spp. via HMOs.

• Formula feeding: Increases diversity but includes more E. coli and Clostridium spp., which may increase disease risk.

How do SCFAs promote immune tolerance?

SCFAs like butyrate enhance Treg differentiation and induce anti-inflammatory cytokines like IL-10.

What is the role of SIgA in gut immunity?

Secretory IgA binds to pathogens, preventing them from adhering to epithelial surfaces and invading tissues.

What is the primary benefit of saccharolytic fermentation?

Produces SCFAs (e.g., butyrate) that strengthen gut barrier integrity and reduce inflammation.

Why does proteolytic fermentation increase in low-fiber diets?

A lack of dietary carbohydrates forces microbes to ferment proteins, resulting in harmful by-products like H₂S.

How do antibiotics contribute to dysbiosis?

Antibiotics reduce microbial diversity, allowing pathobionts like Clostridium difficile to overgrow.

Name two neurological conditions linked to dysbiosis.

Depression and Alzheimer’s disease.

How does the vagus nerve mediate communication in the gut-brain axis?

The vagus nerve transmits signals from gut microbes to the brain, influencing mood and behavior.

Which microbial metabolites impact the blood-brain barrier?

SCFAs like butyrate enhance the integrity of the blood-brain barrier.

What are microbiota-accessible carbohydrates (MACs), and how are they used by gut microbes?

MACs are dietary glycans (e.g., resistant starch, inulin) fermented by gut microbes into short-chain fatty acids (SCFAs).

Name the three primary SCFAs produced by gut microbes and their key roles.

1. Butyrate: Energy source for colonocytes; strengthens the gut barrier.

2. Propionate: Regulates hepatic gluconeogenesis.

3. Acetate: Precursor for systemic lipogenesis; acts as a metabolic signaling molecule.

Which SCFA provides ~70% of energy for colonocytes?

Butyrate.

What essential vitamins are synthesized by the gut microbiota?

1. Vitamin B12: Supports DNA synthesis and red blood cell production.

2. Vitamin K: Necessary for blood clotting.

How do SCFAs influence immune regulation?

• SCFAs promote regulatory T cell (Treg) differentiation.

• Enhance anti-inflammatory cytokine production (e.g., IL-10).

What is the role of polysaccharide A from Bacteroides fragilis in immune regulation?

Polysaccharide A stimulates Treg activity, reducing inflammation and maintaining immune balance.

What is the function of secretory IgA (SIgA) in gut immunity?

• SIgA binds to pathogens to prevent their adherence to epithelial surfaces.

• Strengthens mucosal barrier defenses.

How does the gut microbiota defend against pathogens?

1. Competes for nutrients and adhesion sites.

2. Produces antimicrobials like bacteriocins and hydrogen peroxide (H₂O₂).

3. Stimulates epithelial cells to release antimicrobial peptides (AMPs), such as defensins.

What are bacteriocins, and how do they function?

Bacteriocins are small peptides produced by gut microbes that inhibit the growth of competing bacteria.

What is the role of antimicrobial peptides (AMPs) like defensins?

Defensins create pores in bacterial membranes, killing pathogens.

What are the local and systemic effects of butyrate?

• Local: Provides energy to colonocytes, strengthens tight junctions, and enhances mucus secretion.

• Systemic: Reduces inflammation by modulating immune responses.

How does propionate affect the liver?

Propionate regulates hepatic gluconeogenesis, maintaining glucose homeostasis.

Why is acetate considered a key systemic SCFA?

• Acetate serves as a precursor for fat synthesis.

• It acts as a signaling molecule, influencing metabolic and immune pathways.

How does butyrate contribute to gut barrier integrity?

Butyrate promotes tight junction formation between epithelial cells, reducing gut permeability and preventing bacterial translocation.

What systemic effect does acetate have beyond lipogenesis?

Acetate enhances the uptake and utilization of other SCFAs, such as butyrate, in colonocytes.

How do SCFAs regulate inflammation?

SCFAs inhibit histone deacetylases (HDACs), leading to epigenetic changes that increase anti-inflammatory cytokines like IL-10.

Why is vitamin B12 production by gut microbes significant?

Vitamin B12 is essential for DNA synthesis, energy production, and nervous system function.

Which gut bacteria are known to synthesize vitamin K?

Members of the Firmicutes and Bacteroidetes phyla contribute to vitamin K production.

What is the relationship between SCFAs and epithelial cells in the immune response?

SCFAs stimulate epithelial cells to produce signaling molecules that enhance Treg differentiation and SIgA secretion.

How does Bacteroides fragilis polysaccharide A interact with the host immune system?

Polysaccharide A is recognized by antigen-presenting cells, which then promote Treg activity to suppress inflammation.

What dual roles do antimicrobial peptides (AMPs) like defensins play in pathogen defense?

1. Directly kill pathogens by disrupting their membranes.

2. Signal immune cells to strengthen the local immune response.

Why is competition for nutrients and adhesion sites a key strategy for microbiota in pathogen defense?

By occupying niches and consuming resources, beneficial microbes prevent pathogens from establishing themselves in the gut.

How do SCFAs link nutrient metabolism to immune regulation?

SCFAs provide metabolic energy to colonocytes and regulate immune cells like Tregs, creating a feedback loop that supports both epithelial and immune health.

Why are SCFAs central to the gut’s role in systemic health?

SCFAs modulate both local (gut barrier and inflammation) and systemic (glucose metabolism, lipid synthesis) processes.