Finals: Midterm 2 Content Flashcards
toxins (2)
- kill cells
- alter host-cell functions without killing cells directly
type I toxins
- act extracellularly
type II toxins (3)
- act on the cell membrane and destroy cell membrane
- cytolytic
- can be enzymatic or non-enzymatic
type III toxins
- classical A/B toxins
cytolytic
- damage to membranes usually causes host cell lysis or death
type II toxins: non-enzymatic (2)
- form large pores/channels in membrane
- cholesterol-dependent cytolysins
type II toxins: how are non-enzymatic pores formed (2)
- toxin monomers can bind cholesterol and assemble on surface to form a pre-pore and then insert
- toxin monomer binds cholesterol and inserts into membrane, triggering monomers to bind and form a large pore
why does cell/phagosome lysis occur after non-enzymatic pore formation
- water enters the cell/phagosome which causes swelling
how does LLO function as a type II, non-enzymatic toxin (2)
- change in pH causes conformational change in the protein
- change allows toxin to insert into phagosome membrane
what do type III toxins do (2)
- alter metabolism of the host cell
- exploit or subvert normal host cell processes
what are A/B toxins (2)
- B is the Binding component of the toxin
- A is the enzymatically Active component of the toxin that binds to target inside host
what kinds of toxins are A/B toxins (5)
- toxins that target protein synthesis
- toxins that alter signal transduction
- toxins that alter actin polymerization
- neurotoxins
- anthrax toxins
A/B toxin: forms of B component (3)
- single unit that binds to receptor
- multi-meric structure that is preformed
- mulit-meric structure that forms on the membrane
type II toxins: enzymatic damage (3)
- caused by phospholipases
- enzyme removes polar head groups from phospholipid (PlcC activity)
- causes damage to the membrane, and instability leads to lysis
what is one way that toxins can alter signal transduction
- toxins can target or alter cAMP production
what is the purpose of the techniques for studying virulence factors
- they are used to investigate whether something is actually a virulence factor
Koch’s Postulate: First Postulate
- the microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms
Koch’s Postulates: Second Postulate
- the microorganism must be isolated from a diseased organism and grown in pure culture
Koch’s Postulate: Third Postulate
- the cultured microorganism should cause disease when introduced into a healthy host
Koch’s Postulates: Fourth Postulate
- microorganism must be re-isolated from the inoculated diseased experimental host and identified as being identical to the specific causative agent
Molecular Version of Koch’s Postulates: First Postulate
- gene for virulence should be present in the strain of bacteria that cause disease and absent in avirulent strains
Molecular Version of Koch’s Postulates: Second Postulate
- (i) knocking out or disruption the gene should reduce virulence, and (ii) introduction of the cloned gene into an avirulent strain should render the avirulent strain virulent
Molecular Version of Koch’s Postulates: Third Postulate
- expression of the gene should be demonstrated in human or a relevant model
Molecular Version of Koch’s Postulates: Fourth Postulate
- antibodies or a cell-mediated immune response to a virulence factor should be protective
what are two categories of techniques used to study virulence factors (2)
- biochemical; developed first
- genetic
supernatant
- liquid lying above a solid residue after crystallization, precipitation, centrifugation, or other process
what is the basis for biochemical techniques (2)
- to purify and identify the virulence factor (toxins, adhesins, etc)
- relies on having functional assay for the virulence trait
what are the disadvantages of using biochemical techniques (3)
- purified molecule may be missing co-factor that was removed during purification
- molecule may not have same function in the test tube compared to the cell
- does not tell us if our purified protein is the sole contributor to disease/virulence
what virulence factors work well in biochemical techniques (2)
- protein-based macromolecules
- factors that can be targeted for hydrophobicity, charge, mass, ligand binding, etc
what are some examples of biochemical techniques (6)
- centrifugation
- ion exchange
- size exclusion
- immunoprecipitation
- ligand binding
- Ni2+ affinity/6xHis
biochemical techniques: centrifugation (2)
- harvest bacteria by centrifugation
- separate supernatant into fractions and test for toxicity in animal model/cultured cell assay
biochemical techniques: ion exchange (2)
- use of positively or negatively charged beads in a column
- proteins attracted to the beads will remain in the column, while those repelled with leaves through the bottom of the column
biochemical techniques: size exclusion (2)
- use of beads with small aqueous channels in a column
- large molecules pass quickly, but small molecules are slow as they spend more time in the channels
biochemical techniques: immunoprecipitation (4)
- use of antibodies attached to beads in a column
- proteins are loaded in pH 7 buffer; proteins recognized by antibody bond to beads and other proteins remain in solution
- column washed with pH 7 buffer to remove proteins in solution
- column washed with pH 3 buffer to disrupt protein-antibody bonds and elute proteins
biochemical techniques: ligand binding (4)
- use of affinity bead with ligand attached in a column
- proteins are loaded; proteins recognized by ligand bond to beads and other proteins remain in solution
- column washed with buffer to remove proteins in solution
- column washed with low pH buffer or soluble ligand to elute proteins
biochemical techniques: Ni2+ affinity/6xHis (4)
- use of NTA-coated agarose bead coordinate Ni2+ ions in a column
- tagged and untagged proteins are loaded; tagged proteins bind to Ni2+ and other proteins remain in solution
- column washed with buffer to remove proteins in solution
- column washed with low pH buffer or imidazole (binds Ni2+) to elute proteins
how would you prove that a putative virulence factor (purified protein) caused disease (3)
- isolate toxin and investigate effects in other cells
- sequence toxin and knockout toxin to see if disease occurs or investigate for its presence in avirulent strain
- identify immune response in model to show that virulence factor grants immunity
molecular genetic techniques (2)
- gain of function experiments
- loss of function experiments
gain of function techniques (2)
- look for putative virulence factors by doing gain of function in an avirulent strain
- observing for restoration of virulence
loss of function techniques (2)
- knock out genes and look for reduction in virulence
- can be done through directed mutants or transposon mutants
gene expression experiments (3)
- reporter assays - promoter traps
- hybridization-based experiments
- sequencing (RNA-seq)
advantages of genetic approaches (3)
- allows us to find novel genes by screening mutants
- connection with virulence is established at the beginning of the experiment
- relatively biased
disadvantages of genetic approaches (2)
- usefulness of the information (results) depends on the screen
- you may not be able to identify the function of the gene product
disadvantages of gain of function experiments (3)
- there may be other accessory factors or genes involved
- many different restriction enzymes may need to be used
- virulence may not be expressed in an avirulent strain
GoF Salmonella invasion experiment: what are the steps to create the library (5)
- isolate virulent Salmonella DNA
- cut DNA with a restriction enzyme
- run on gel to separate DNA fragments according to size
- DNA between 1000-3000bp are isolated from the gel
- these DNA are cloned into plasmids
- plasmids are transformed into avirulent E. coli to create a library
GoF Salmonella invasion experiment: what are the steps to test the library against an assay (6)
- library of E. coli clones is used to infect tissue cells
- cells are washed and incubated
- gentamycin is added; kills extracellular bacteria, but not intracellular bacteria
- wash to remove gentamycin
- lyse the host cells to release the surviving bacteria
- plate surviving bacteria on lab media