Lecture 5 - evading immune system, phase variation, experimental approaches to understand regulatory mechanisms, effects of phage an antigenic variation Flashcards
Give some examples of some physical chemical and anatomical barrier to resisting infection
- Normal flora compete with pathogens
- Removal of particles including microorganisms by rapid passage of over over cillia in the nasopharynx
- Flushing of the urinary tract prevents colonisation
- Stomach acidity (pH=2) inhibits microbial growth
What are the features of helicobacter pylori?
- Colonises gastric mucosa
- only active at low pH
- causes stomach ulcers and gastritis
- asjust the pH of their microoenvironment by the pH dependent activity of urease (hydrolyses urea to ammonia, raising the pH)
What is the chemical reaction for the hydrolysis of urea to ammonia?
H2N-C(=O)-NH2 + 3H2O -> 2NH4+ + 2OH- + CO2
What pathogens other than helicobacter pylori utilise urease and why?
To exploit nitrogen sources
- Proteus mirabilis: inducible (urea)
- Klebsiella sp: inducible (nitrogen starvation)
- Precipitates, kidney stones
What strategies do bacteria use to evade the host defenses?
- Hide, enter cell and stay intracellular
- Modify, blok, resist host defences
- Evade innate immune response
- Evade adaptive immune response
- Molecular mimicry
- Phase vairation
- Antigenic variation
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Once infected, how does the human body respond to infection?
If have no innate immunity -> disease
Otherwise leads to adaptive immunity, following recovery and reinfection the specific immunological memory is triggered so that no disease
What is the involvement of pathogen PAMPs in innate immunity?
- All pathogens that enter our body have unique pathogen associated molecular patterns (PAMPs)
- Related to the innate immune resonse
- Macrophages, dendritic cells survey for PAMPs
- Non-specific approach to dealing with the pathogen
- e.g. peptidoglycan of gram- bacteria
How does the capsule protect against phagocytosis?
Blocks recognition of PAMPs by the innate immune system
e.g. The Vi capsular polysaccharide enables salmonella enterica serovar typhi to evade the immune system
How do pathogens use mimicry to evade the immune system
- mimic host cells
- either by host protein function mimicry or surface decoration to mimic the host
e.g. Human neutrophils recognise sialic acid as self.
- GroupB Streptococci sialylate capsule
- Haemophilus influenzae sialylates LPS (LPS major PAMP, TLR4)
What are the basic features of the adaptive immune system?
- Highly specific and retains memory
- Antibodies are raised in response to specific antigens
- T Cells: regonise fragments of pathogens (particular epitopes trigger immune response - if antigen looks different or is no longer present there is no longer an antigenic response, by translational fusion)
- B cells: recognise intact pathogen molecules
Why can we not say that all cells in a clonal population are identical even thoug derived from same parent?
Stochastic variation (due to weak promoter, some cells may express and some may not)/threshold responses
Antigenic and phase variation (digital repsonses)
How can you experimentally determine whether a gene is being expressed?
Use reporter genes
Transcriptional fusion: place the reporter gene downstream of rbs
Translational fusion: place the reporter next to the rbs so that the proteins are fused together
This experiment is dependent on the regulatory information, can determine whether regulation occurs at the mRNA or the translational level
What reporters can be used in a reporter fusion experiment?
LacZ: enzymatic assay, liquid (population average); enzymeatic reaction, colony
Gfp: microscopy or flow cytometry (individual cells) fluorimeter (population average)
What are the features of heterogeneous expression in a population and how is it identified experimentally?
Experimentally
- Uses reporter fusion (transcriptional reporter), and at the intermediate level of inducer the bacterial (e.g. A.tumefaciens) gene (e.g. VirE) is differentially expressed in a population
Features
- Stochastic
- Not heritable
- Not phase variation
What is antigenic variation and how is it used to evade the immune system?
- Variants of a protein each with differnet antigenicity
- Allow bacteria to retain function as different antigens ellicit different immune response
- Often cell surface structures/proteins
Describe the involvement of antigenic variation in escaping the immune system?
- Immune system recognises one protein and begins to clear the infection
- The population is 1/2 one protein and 1/2 its antigenic variant
- Once the first is gone the second dominates
Pathogens can have thousands of different antigenic variants for evading the immune system, need a large population for this to be possible