Microbiota Flashcards Preview

GI/Nutrition - Victor > Microbiota > Flashcards

Flashcards in Microbiota Deck (36)
Loading flashcards...
1
Q

Architecture of the intestinal mucosal surface

A
2
Q

What are the extrisnic barrier defenses?

A
  1. Mechanical/Involuntary Reflexes
    • Cough
    • Gag
    • Peristalsis
  2. Structural
    • Mucus
  3. Chemical
    • Acid
    • Enzymes
    • Antimicrobial peptides and polypeptides
  4. Microbiological
    • Commensal microbiota
3
Q
  1. What area do mucus/mucins define?
  2. What secretes mucus/mucins?
  3. What do mucus/mucins form?
A
  1. Viscoelastic gel that defines structure of extrinsic barrier
  2. Secreted by specialized Goblet cells
    • Secretion can be constitutive or regulated
    • Humans secrete 10 liters of mucus/day
  3. 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
  4. –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
  1. degradation of mucin
  2. avoidance of mucus
  3. alterations in host cells
6
Q

What are the classes of mucosal defense mechanisms?

A
  1. Acdification (pH ~ 3.5-4.0)
  2. Toxic oxygen-derived products (O2-; H2O2; etc.)
  3. Toxic nitrogen products (NO)
  4. AMPs (defensins and cationinc proteins)
  5. Enzymes (lysozyme; acid hydrolase)
  6. 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:
    1. 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
    2. Lysozyme C
      • glycosidase
      • hydrolyzes peptidoglycan (bacterial cell wall)
    3. Phospholipase A2
      • catalyzes hydrolysis of fatty acids (bacterial cell membrane)
      • bactericidal against G+ bacteria
    4. RegIIIγ
      • C-type lectin
      • binds to peptidoglycan
      • bactericidal against Gram-positive bacteria
      • inducibly expressed upon Toll-like receptor (TLR) activation
    5. Cryptdin related sequences (CRS)
      • antibacterial activity comparable to cryptdins
8
Q

How do pathogens evade AMPs?

A
  1. Protease secretion
  2. Surface charge modification
  3. Capsule formation
  4. Modulate AMP expression
  5. 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
  1. Symbionts
    • Share mutual relationship with the host, have known health promoting functions
  2. Commensals
    • Permanent residents of this ecosystem and provide no benefit or detriment to the host
  3. 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
17
Q

______ ______ regulates microbiota composition

  • What is the evidence for this?
A

Oxygen tension regulates microbiota composition

  • Hyperbaric oxygen treatment raised tissue oxygen tension
  • Changes in oxygen tension were associated with increased abundance of facultative anaerobic bacteria
  • Changes in composition were limited to mucosal associated bacteria
18
Q

______ immune factors modulate intestinal colonization

A

Innate immune factors modulate intestinal colonization

19
Q

____ _______ selects for microbial composition

A

Host nutrition selects for microbial composition

20
Q

What is associated with distinct enterotypes?

A

Long-term dietary habits

21
Q

What are the protective functions of the intestinal microbiota?

A
  • Pathogen displacement
  • Nutrient composition
  • Receptor competition
  • Production of anti-microbial factors
    • bactieriocins & lactic acids
22
Q

What is the result of an absence of bacterial colonization (in mice)?

A
  • Mice lack a mature mucosal immune system
    • Underdevelopment of lymphatic tissues
    • Delayed B cell migration in response to bacterial antigen
    • Reduced antibody diversity
    • Reduced lymphocyte responsiveness
23
Q

What was the result of** reconstitution of a normal microbiota** (in mice)?

A
  • Mice develop normal mucosal immune function
    • Increased lymphocyte infiltration of gut mucosa
    • Germinal center formation in Peyer’s Patches
    • Induction of innate antimicrobial effector molecules
    • Treatment with bacterial polysaccharide from bacterial symbiont (B. fragilis) restores many immune functions
24
Q

Describe microbiota induction of epithelial antimicrobials:

A
  • Luminal:
    1. TLR-dependent expression
    2. Activation of MYD88
    3. Upregulation of REG3γ
    4. Killing of bacteria
  • Basolateral
    1. AMPs block bacteria
    2. Epithelium secretes IL6, IL23 and TGFβ
    3. Dendritic cells phagocytose microbe ⇒ secrete IL23
    4. DC IL23 secretion ⇒ induces IL22 secretion
    5. IL22 ⇒ causes epithelium to secrete chemokines
    6. Chemokines ⇒ activate endothelium
25
Q

Which T cell subsets does the microbiota specifically affect?

A
  • iNKT cells
  • Innate lymphoid cells
  • Th17 cells
  • Tregs
26
Q

Describe the production and function of short chain fatty acids (SCFAs):

A
  • Commensal bacteria ferment nondigestible dietary polysaccharides to produce short chain fatty acids (SCFAs)
  • SCFAs regulate:
    • PMNs, dendritic cells, macrophages/monocytes, and intestinal epithelial cells
  • SCFAs induce:
    • regulatory T cell differentiation
  • SCFAs regulate expression of virulence factors on bacterial pathogens
27
Q

What human diseases are caused by a disruption in the microbiome?

A
  • Antibiotic associated diarrhea:
    • C. difficile colitis
  • Systemic infection:
    • VRE
28
Q

**Commensal microbiota prevent pathogen colonization via (6): **

A
  1. Bacteriocin production
  2. SCFA production
  3. Consumption of oxygen
  4. Competition for nutrients and attachment sites
  5. Induction of epithelial antimicrobials
  6. Induction of mucus production and secretion
29
Q

Describe the effect of antibiotics on the gut microbiota:

A
30
Q
  1. Commensal bacteria regulate digestion by:
  2. What is the role of commensal fermentation?
A
  1. Commensal bacteria regulate digestion by:
    • mediation bile acid synthesis
    • lipid absorption
    • amino acid metabolism
    • vitamin synthesis
    • SCFA production
  2. Byproducts of commensal fermentation (metabolites) regulate the immune system
31
Q

(digestible/nondigestible) carbohydrates provide the most energy to the host

A

digestible carbohydrates provide the most energy to the host

32
Q

Be generally familiar with how the microbiota influences overall health. (it’s a very dense chart)

A
33
Q

What are the disease associations with the microbiome?

A
  1. Inflammatory Bowel Disease (Crohn’s Disease)
  2. Obesity and obesity related diseases, including diabetes and non-alcoholic fatty liver disease
  3. Cancer
  4. Allergy/Asthma
34
Q

IBD patients show both abnormal ….

A

bacterial colonization and immune function

  • characteristic shift in microbial colonization moves from obligate anaerobic bacteria to facultative anaerobic species
  • predominantly proteobacteria
  • most likely associated with increased oxygen tension caused by inflammation
35
Q

A probiotic is ….

A

“a viable microbial food supplement which beneficially influences the health of the host”

36
Q

Why are probiotics used in treatment of immune disorders?

A
  • Restoration of a “healthy” biota
  • Restoration of barrier function (prevent excess antigen transfer across skin and gut barriers)
  • Skew T cell immune responses to Th1 type