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44 Cards in this Set
- Front
- Back
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TB problem
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75% of ppl exposed don't get infected
25% get infected Of those 25%, 90% get latent TB, 10% active tb Of the 90%, 5-10% develop active TB |
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Describe the TB life cycle.
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TB infection
Macrophage engulfs bacteria Bact replicated in mac phagosome Actiavted mac have enhanced lysosome and phagosome ->Greater chance o bacterial spread Secretion of different cytokines Granuloma: get containment or reactivation -5-10% of these containment cases get disease beacuse granuloma breaks down |
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TB genome
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250 genes involved in lipid metabolism (48 in E. coli)
150 genes involved in lipid/FA synthesis, remainder involved in biosynthesis 30% of M. tb dry-weight is lipid (12.5% in E.coli) .: Lipids are rly important in TB biology |
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FA synthesis
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-Synthesized from acteyl CoA repeated cycles of addition
-FAS-II of bacteria is a multienz complex (7 polypep) -FAS-I of vertebrates is a single, multi-fctnal polypep |
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Bacterial cell wall: G-
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Outer and inner cell Mb
LPS in outer layer Thin p/g Overall negative charge |
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Bacterial cell wall G+
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No outer mb
No LPS Thick p/g layer Teichoic acids Overall neg charge Polymers of glycerol-P or ribitol-P |
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Mycobacterial Cell wall
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Actinobacteria
Rod-shaped bacillus Slow growing Colourless, rough colonies Cell mb: 3 distinct layers No LPS No teichoic acid Thin p/g layer |
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What is the electron transparent zone (ETZ)?
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Very hydrophobic zone
Dominated by mycolic acid residues (FA=60% of cell wall core) Fluidity is very low and powerful hydrophobic interactions form a wax-like shell around the bacterium PORIN molec are required to allow movement of small, hydrophobic molec across the cell wall |
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What are the physical properties of Mycobacterium (due to the "unusual" cell wall)?
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Hydrophobicity and low permeability contributes to resistance to most common antibiotics
-> Most TB drugs aren't general antibacterials Resistance to dehydration, treatment with alkali, many chemical disinfectants and Gram's stains |
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What stain is used for mycobact?
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Acid fast stain
-Cell wall penetrated by phenol based stains (carbolfichsin) but resists decolorization by acid-alcohol (TB bacilli remain pink, others are colourless) |
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What does the cell wall core consist of?
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Mycolyl-arabinogalactan p/g complex (mAGP)
1) p/g covalently linked to arabino-galactan chains (polysac) 2) arabinogalactan esterified to mycolic acids (long chain FA) |
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What does the outer capsule consist of?
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Polar head gps of free (non-covalently attached) glycolipids + capsular polysac (glucans, mannans, arabinoglycans)
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What are the lipids unique to mycobacterium tb critical for?
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Virulence
-Often methyl branched for improved stability |
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What is the difference btw mycobacterium and G-/+?
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Myco: No OM, thin p/g, inner mb
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What are the 2 first line anti-TB drugas that inhibit cell wall synthesis?
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1) Isoniazid (INH)
2) Ethambutol (EMB) |
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What does isoniazid do?
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Been around since 1952
Inhibits mycolic acid biosynthesis leading to cell death Active only vs dividing bacteria |
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What does ethambutol do?
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1961
Inhibits arabinogalactan biosynthesis Active only vs dividing bacteria |
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What are the other 2 mycobact inhibiting drugs?
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RIF (vs RNA pol)
PZA |
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Describe mycolic acids.
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High molecular weight (2-alkyl, 3 hydroxy FA)
70-90 carbon atoms Also occurs as freely extractable esters of trehalose (di-glucose) |
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How many FAS required for myolic acid synthesis?
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2
-FAS-I (single polypep, vertbrate type) synthesizes C20-C26, which feeds into FAS-II -FAS-II (multienz complex, bact type) elongates from C20 to form the long FA chain |
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What form does the mycolic acid take?
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Oxygenated and non oxygenated
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How many major classes of M. tb are there?
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3
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What are they?
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a-mycolic acid
methoxy-mycolic acid keto-mycolic acid |
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What does INH inhibit?
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Binds somewhere in FAS II
Inhibition of MA synthesis Loss of acid-fastness Cell death |
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Describe how lipds are transferred from infected to unifected host
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1) TB capsule lipids released inside the host macrophages and taken up in vesicles lacking bacteria
2) Vesicles + TB lipids released from cell surface via exocytosis 3) Vesicles/TB lipids taken up by neighboring cells |
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When are lipids released by the bact?
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When in the macrophage
Taken up by uninfected APCs -> Tremendous potential to influence the host immune response |
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What are LAMs?
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Lipoglycans
(lipoarabinomannan) |
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What are the 3 domains LAMs are composed of?
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1) Phosphatidylinosotol mannoside anchor (PIM)
-phosholipd, with up to 4 FA -inserted into cell mb or mycoluc acid layer with polar oligosac chains on outer surface 2) Polysac domain composed of mannan and arabinan -Mannan core, very branched, has 30-35 mannose residues, arabinan domain contains ~ 60 arabinose units 3) Capping motifs -In slow growin Mycobact cap structure, consist of monnose oligosac: ManLAM - in fast growing myco, cap structure is absent or consists of phospho-inositol: PILAM |
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What is LAM structurally related to?
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LPS in G- organisms
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What is the endosomal system?
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Network of mb-enclosed vesicles that transports endo/phagocytosed material to the lysosome for degradation
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What is required for hydrolytic capacity of lysosomal enz and generation of H2O2?
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Vacuoloar pH increasingly acidic
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Where does M. tb reside?
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In endosomal vacuoles inside macs (phagosome)
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How does M. tb block phagosomal maturation?
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Prevents fusion with late endosomes/lysosomes (resides in an early endosomal compartment; pH 6.3)
Some might escape lysosome and replicate in the cytoplasm |
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What do M. tb do to [Ca2+] when they infect macs?
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Inhibits [Ca2+] increase required for fusion of phagosomes containing bacteria with late endosomes/lysosomes
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What increases Ca2+ in macs?
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Calcium ionophore
->allows phagosome/lysosome fusion ->bacteria ends up in lysosom |
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Does LAM from all myco species inhibit [Ca2+] increase?
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No, only from M. tb (not from others like M. smegmatis)
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What part of M. tb is essential for this inhibition of Ca2+ inc?
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LAM capping motifs
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What is TDM?
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Trehalose 6,6' dimycolate
-Extractable lipid in outer capsule -one of two forms of mycolic acids ass't with cell wall |
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What is TDM ass't with?
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Granuloma formation
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What is LAM ass't with?
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Phagosome
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What does injection of TDM cause?
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Accumulation of macrophages and granuloma formation in he lungs
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What does the immune response to purified TDM mimic?
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Certain aspects of nature TB pathology
(dev'p of granuloma and production of proinflammatory cytokines: IL-1B, IL-6, TNF-a) |
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What is the moost bioactive of the cell wall ass't lipids?
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TDM
-> majority of the host inflammatory response is directed toward TDM Causes profound recruitment of a range of infl;ammatory cells -> granuloma-like structure |
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Summary
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Unique structure of the cell wall
-mAGP, mycolic acids, capsule -unique physical characteristics due to cell wall structure INH and ETH act at the level of cell wall synthesis Fine structure of lipids critical for fct Role of LAM in modifying the host response and protecting bacilli from lysosomal killing Role of TDM in inflammation and granuloma formation |
