Lecture 8 - Microbiota Flashcards
Describe the trends of human disease in the Western world since 1950
Decreased incidence of:
• Infectious disease
Increase incidence of: • Autoimmune and allergic disease: • Asthma • Allergy • MS • Crohn's disease • T1DM
Describe the hygiene hypothesis for increased incidence of allergic and autoimmune disease
The increased sanitation has meant that we are not exposed to the same pathogens:
• Increased cleanliness
• Vaccination
• Antibiotic use
Our immune system becomes dysregulated
Describe the role of microbes in the development and function of the immune system
We know this through studying germ free mice:
• Underdeveloped Peyer’s patches
Describe the ‘Old Friends’ hypothesis
The vital microbial exposure is not infectious, rather our natural commensal microbiota
Humans co-evolved with these harmless organisms
Microbes are vital for the development and function of the immune system
What is the most important factor in the composition of the microbiota?
Diet; esp. fibre
There has been a profound transition in diet in the last 50 years
This is leading to profound changes in the microbiota, and hence in disease
Describe the presence of the microbes along the alimentary canal
Non-uniform presence
Increased density as one progresses from stomach to colon
Describe the Human Microbiome Project
- First phase
• 2007-2012
• Characterisation of composition of various locations (nose, mouth, skin, GIT, UGT)
• Evaluation of genetic metabolic potential - Second phase:
• 2013-2015
• Creation of integrated dataset of biological properties from microbiome and host
Describe the genetic implications of the microbiome
Human genome: 23,000 genes
Human microbiome: > 1 million genes
Is there such thing as a healthy microbiome?
Each person has a unique microbiome
Two individuals may have different micro biomes, but still have a ‘healthy’ microbiome
Describe the variability of the microbiome and the implications of this
The gut microbiota is highly variable (on a phylum level)
However, the metabolic capacity does not vary that much
Describe the study of microbiota in children from Burkina Faso and Florence
Diet:
Burkina faso:
• Pre-industrialisation diet
Florence:
• Modern, western diet
Composition of microbiota:
• Vastly different composition of microbiome
Conclusion:
• Diet is a dominant driver of the composition of the microbiome
Compare dominant bacteria genera found in children from Burkina Faso and Florence
Burkina faso:
• Predominantly Bacteroidetes
Florence:
• Predominantly Firmicutes
Define the following:
• Probiotics
• Autobionts
• Pathobionts
Give examples of each
Probiotics:
• Transient
• Beneficial bacteria consumed
• e.g. Lactobacillus spp., Bifidobacterium spp.
Autobionts: • Permanent • Symbiotic • Immunomodulatory • e.g. Bacteriodetes fragilis
Pathobionts: • Permanent • Parasitic / infectious • Cause opportunistic infections • e.g. C. difficile
Describe the role of autobionts in digestion
Autobionts are adapted to life in the gut
Express PUL (polysaccharide utilisation loci) when there is polysaccharide present in the diet
PUL products are capable of digesting many plant polysaccharides present in dietary fibre
Humans do not have enzymes that can break down dietary fibre
Describe Bacteroides ovatus’ break down of xyloglucan
B. ovatus expresses a PUL capable of breaking down xyloglucan
Xyloglucan PUL:
• 8 glycosyl hydrolyses
• 2 glycan transporters
• 2 glycan chaperones
Xyloglucan is a complex carbohydrate present in dietary fibre that humans cannot digest on their own
B. ovatus has a complex system of breaking down xyloglucan:
• Chaperones on outer membrane capture sugar
• Endoglycosylases on OM that cleave within the polysaccharide
• Transporter on OM moves oligosaccharides into periplasmic space
• Further digestion
• Monosaccharides move through transporter on inner membrane into bacterial cell
Which molecules are largely produced in the break down of dietary fibre?
Give specific examples
What happens then to these breakdown products?
SCFA: short chain fatty acids
Examples:
• Acetate (Bacteroides)
• Propionate (Bacteroides)
• Butyrate (Firmicutes)
SCFAs move into host cells through a variety of mechanisms:
• Passive diffusion
• Transporters
• Binding to GPRC
List some roles of SCFAs
- Stimulation of mucous secretion
- IgA production
- Tissue repair & epithelial integrity
- Inhibition of inflammation (through inhibition of NFKB)
- Promotion of Tregs
List the various roles of gut microbiota
- Digestion of dietary fibre
- Production of vitamins (B and K)
- Gut epithelial development
- Immune system development, tolerance development
- Barrier function (occupation of niche)
Describe the observations in the gut microbiota in obesity and T2DM
Significant changes in gut microbiota
(this could underlie increased chronic inflammation)
- Low diversity
- Dysbiosis
- Reduced butyrate production
- Increased opportunistic pathogens
Describe studies in microbiota transplant studies in mice, looking at obesity, T2DM and pregnancy
Obesity & T2DM:
• Gut microbiota from obese/lean human twins transferred to mice
• Mice who received ‘obese’ microbiota had greater body mass and adiposity than those who received ‘lean’ microbiota
Pregnancy:
• Gut microbiota from pregnant women in first and third trimester transferred to mice
• ‘Third trimester’ microbiota mice had increased body mass and insulin insensitivity than ‘first trimester’ microbiota
NB Women in third trimester of pregnancy experience: • Weight gain • Insulin insensitivity • Dysbiosis • Increased inflammation
Describe bacteria can modulate host metabolism
SCFA products from bacteria can signal in a variety of mechanisms to lead to changes in host metabolism:
1. GPRC41 • SCFA binds GPRC41 on enterocytes • S-T pathway • Enterocytes release PYY • PYY regulates gut motility and suppresses hunger
- GPCR43
• SCFA binds GPRC43 on enterocytes and adipocytes
• Results in release of GLP1 from enterocytes
• Inhibits insulin signalling in adipocytes → inhibits uptake and storage of glucose in adipocytes - Intestinal gluconeogenesis
• SCFA diffuse into enterocytes and are used as a carbon source for intestinal gluconeogenesis - Fiaf (fasting induced adipose factor
• Dietary fibre inhibits Fiaf (?*)
• Fiaf normally inhibits lipoprotein lipase and fat accumulation in adipocytes
What is PYY?
What are its actions?
What triggers its release?
PYY: peptide-YY
Actions:
• Regulation of gut motility & intestinal transit time
• Suppresses hunger
What is GLP1?
What triggers its release?
Glucagon-like peptide 1
Released by enterocytes through GPRC43 signalling
Function:
• Potent anti-hyperglycaemic hormone
• Stimulates insulin secretion
• Inhibits glucagon secretion
What is the effect of signalling through GPRC43 on adipocytes?
Inhibition of insulin signalling, inhibiting glucose uptake and storage in adipocytes
What is IGN?
What is the role of SCFAs in IGN?
Intestinal gluconeogenesis
SCFAs are used as a carbon source for IGN
Describe the action of Fiaf
Inhibits lipoprotein lipase
i.e. inhibits fat digestion in intestine
Describe the importance of GPRC43
What happens when it is KO’d or over expressed?
GPRC43:
• SCFAs signal through this receptor on enterocytes and adipocytes
• Leads to release of GLP-1 in enterocytes
• Leads to inhibition of insulin in adipocytes
KO:
• Mice are obese on lean diet
Over-expression:
• Mice are lean on high calorie diet
Describe the effects of dysbiosis on inflammation and immunity
Changes during dysbiosis:
• Increased local and systemic inflammation
- Preponderance of Th17 over Tregs
- Increased presence of pathobionts and release of LPS
- Decreased barrier function
- Increased gut bacteria in blood
Describe how the gut microbiota regulates Treg / Th17 balance
What are the effects of both of these subtypes?
Treg:
• Stimulated by butyrate
• ‘Immunosuppressive’
• Releases IL-10
Th17:
• Stimulated by pathobionts
• Inflammatory
• Releases IL-17
Compare the following in children from Burkina Faso and Florence:
• SCFA
• Pathogenic bacteria
Burkina Faso:
• Increased SCFA levels
• Decreased pathobiont levels
Florance:
• Decreased SCFAs
• Increased pathobionts
Pathobionts: • Escherichia spp. • Salmonella • Shigella • Klebsiella
Compare healthy and disrupted microbial communities in the gut
Healthy: • Substantial microbiota population occupying the niche • IgA production • Maintenance of epithelial integrity • Mucous production
Disrupted: • Depleted microbiota • Overgrowth of pathiobionts • Epithelial amage • Dissemination of pathogens and commensals into blood
Describe the effect of dysbiosis on systems around the body
Brain:
• Decreased satiety
• Behavioural problems
Lungs:
• Asthma
Liver:
• Development of NAFLD
Pancreas:
• Diabetes
Allergy
Joints:
• Autoimmune arthritis
Describe ways that microbiota can be used as immunotherapy
Diet:
• Less fat, more fibre
Prebiotics:
• Compounds that are good for microbiota
Probiotics:
• Beneficial bacteria
Faecal transplant
Targeted manipulation
Describe trends in fibre intake currently
Both men and women consume roughly half of recommended fibre
Women require less, but still aren’t getting enough
Describe the importance of early life in terms development of immunoallergic diseases
Early life is the most important phase of life
Evidence:
• Populations moving from regions of low risk of these diseases to regions of high risk (e.g. India to Europe / US)
• The earlier an individual moves, the more likely they are to develop the disease
Rationale:
• The interaction between microbiota and host immune system is established early in
life
Implications:
• Antibiotic use early in life is v. risky
Describe the use of faecal transplants to treat C. difficile infections
Faecal transplants from healthy donors are effective at treating C. difficile infections
What is the trouble with faecal transplant?
How could this be averted?
Must be certain that there are no pathogens in the transplant
Other approaches:
• Cultured autobionts
• Inclusion of microbiota small molecules in diet
