Microbiota Flashcards
1
Q
Architecture of the intestinal mucosal surface
A
2
Q
What are the extrisnic barrier defenses?
A
-
Mechanical/Involuntary Reflexes
- Cough
- Gag
- Peristalsis
-
Structural
- Mucus
-
Chemical
- Acid
- Enzymes
- Antimicrobial peptides and polypeptides
-
Microbiological
- Commensal microbiota
3
Q
- What area do mucus/mucins define?
- What secretes mucus/mucins?
- What do mucus/mucins form?
A
- Viscoelastic gel that defines structure of extrinsic barrier
-
Secreted by specialized Goblet cells
- Secretion can be constitutive or regulated
- Humans secrete 10 liters of mucus/day
-
Forms selectively permeable mucus blanket
- Variable thickness depending on anatomic site and pathophysiology
- Comprised of a variety of mucins, water, ions, proteins, and lipids
- Contains antibodies, antimicrobial peptides, and bacteria
- –Continuous turnover and changes in content contribute to dynamic system
4
Q
What is mucins role in host defense?
A
-
Mucus-commensal interactions
- Specific binding of some commensals via adhesins
- “grazing” on mucus-cleaving specific sugars from tips of oligosaccharides
- Small subset of commensals digest mucins
-
Bacterial exclusion
- Thickness and viscosity contribute to exclude bacteria
- Bacteria and LPS have been shown to induce MUC gene expression
- Pathogens have developed specific mechanisms to evade barrier (flagella, interference with exocytosis)
-
Containment of secreted antibodies and antimicrobials
- IgA and other secreted antibodies bind mucus through low affinity bonds, and interact with commensals and pathogens
- Cationic AMP may be contained via electrostatic interactions with mucins
5
Q
How do microbes evade mucus?
A
- degradation of mucin
- avoidance of mucus
- alterations in host cells
6
Q
What are the classes of mucosal defense mechanisms?
A
- Acdification (pH ~ 3.5-4.0)
- Toxic oxygen-derived products (O2-; H2O2; etc.)
- Toxic nitrogen products (NO)
- AMPs (defensins and cationinc proteins)
- Enzymes (lysozyme; acid hydrolase)
- Competitors (microbiota)
7
Q
- Where are AMPs found?
- What are the different AMPs?
- What is the function of each?
long card, sorry : /
A
- AMPs are found in paneth cells
-
Paneth cell AMPs:
-
α-Defensins (cryptdins in mouse)
- constitutively expressed cationic AMPs
- anti-microbial and chemo-attractant properties
- kill target microbes by forming pores in their cell membrane
-
Lysozyme C
- glycosidase
- hydrolyzes peptidoglycan (bacterial cell wall)
-
Phospholipase A2
- catalyzes hydrolysis of fatty acids (bacterial cell membrane)
- bactericidal against G+ bacteria
-
RegIIIγ
- C-type lectin
- binds to peptidoglycan
- bactericidal against Gram-positive bacteria
- inducibly expressed upon Toll-like receptor (TLR) activation
-
Cryptdin related sequences (CRS)
- antibacterial activity comparable to cryptdins
-
α-Defensins (cryptdins in mouse)
8
Q
How do pathogens evade AMPs?
A
- Protease secretion
- Surface charge modification
- Capsule formation
- Modulate AMP expression
- Efflux pumps
9
Q
What are the secreted immunological defenses?
A
-
sIgA
- Predominant immunoglobulin in mucosal secretions, monomeric & polymeric
-
IgM
- Also associated with SC in mucosal secretions
- May not be transported as well due to MW restrictions in SC dependent transport
- Compensatory increase with IgA deficiency
-
IgG
- Found at same levels as IgM
- Proportion of IgA to IgG varies by site and time of collection (ie: proportion varies through menstrual cycle)
- Not selectively transported in humans
- **IgE **- Found in low concentration, associated with mucosal allergic responses
- IgD - Found in low concentration in milk and saliva
10
Q
Describe the IgA structure and biosynthesis:
A
-
Serum IgA
- Predominantly monomeric
- Polymeric IgA-j chain containing polymers and variable but low amount of SIgA.
-
Mucosal IgA
- Predominantly polymeric
- Structure consists of alpha chain, J chain (only associated with pIgA), and secretory component
- Synthesized as monomer and forms pIgA prior to secretion
- Approximately 4 grams of IgA secreted daily
- Metabolized and cleared by liver
11
Q
How is mucosal IgA induced?
A
- Migrating DCs can induce B and T cell activation in the mesenteric lymph node
- DCs do not recirculate through lymph and blood
- Recirculation of B cells through lymph and blood
12
Q
What are the biological activities of IgA? (5)
A
-
Inhibition of adherence
- Surrounds microbe and repels attachment to mucosal surface
- Agglutination of microbes by Fc-Fc interactions
-
Mucus trapping
- May associate with mucins, and trap microbes in mucus blanket
-
Virus neutralization
- Mechanism may depend on antibody specificity, isotype, and concentration
- Inhibition of cellular attachment
- Neutralization within epithelial cells
-
Enzyme and toxin neutralization
- In saliva, inhibition of enzymes from oral bacteria
- In gut, neutralization of bacterial toxins
-
Inhibition of antigen penetration
- IgA deficient subjects show increased absorption of food antigens
13
Q
How do microbes evade IgA?
A
-
Specific IgA proteases
- Cleave one of several prolyl-seryl or prolyl-threonyl peptide bonds in hinge region. Cleave off intact Fab fragments that retain binding activity
- Exquisitely substrate specific, not inhibited by protease inhibitors
- Cause local IgA deficiency in vivo
- Meningitis association (H. influenzae, N. meningitidis, S. pneumoniae)
-
Other proteases
- Wide spectrum protease activity can cleave IgA, noted in peridontal pathogen Porphyromonas gingivalis, and some intestinal Enterobacteriaceae
-
Glycosidases
- IgA is heavily glycosylated
- subject to damage by bacterial glycosidases,
- disrupting conformation, net charge, and resistance to proteolysis
-
IgA binding proteins
- Cell surface proteins that bind IgA non-specifically (ie: Fc)
- Lectin binding of O-linked carbohydrate in IgA hinge region
14
Q
Healthy microbiota contains a balanced composition of three major classes of bacteria:
A
-
Symbionts
- Share mutual relationship with the host, have known health promoting functions
-
Commensals
- Permanent residents of this ecosystem and provide no benefit or detriment to the host
-
Pathobionts
- Live as commensals but have the potential to induce pathology
- Note: Altered microbiota is associated with diseases (IBD, autoimmunity, obesity diabetes, etc.)
15
Q
Describe the duality of the commensal microbiome:
A
- Mucosal surfaces are in
- constant contact with microbes
- Primary role of mucosal surfaces:
- allow normal physiological function while protecting the host from infection
- Mucosal immune system:
- protects the host from the microbiota
- Microbiota has a ….
- symbiotic role in host protection and host physiology
16
Q
Microbial composition of the lower GI tract:
A
24
Q
Describe microbiota induction of epithelial antimicrobials:
A
- Luminal:
- TLR-dependent expression
- Activation of MYD88
- Upregulation of REG3γ
- Killing of bacteria
- Basolateral
- AMPs block bacteria
- Epithelium secretes IL6, IL23 and TGFβ
- Dendritic cells phagocytose microbe ⇒ secrete IL23
- DC IL23 secretion ⇒ induces IL22 secretion
- IL22 ⇒ causes epithelium to secrete chemokines
- Chemokines ⇒ activate endothelium
25
Q
Which T cell subsets does the microbiota specifically affect?
A
- iNKT cells
- Innate lymphoid cells
- Th17 cells
- Tregs
29
Q
Describe the effect of antibiotics on the gut microbiota:
A
30
Q
- Commensal bacteria regulate digestion by:
- What is the role of commensal fermentation?
A
-
Commensal bacteria regulate digestion by:
- mediation bile acid synthesis
- lipid absorption
- amino acid metabolism
- vitamin synthesis
- SCFA production
- Byproducts of commensal fermentation (metabolites) regulate the immune system
32
Q
Be generally familiar with how the microbiota influences overall health. (it’s a very dense chart)
A
