Regulation of Genes Coding for Bacterial Virulence Factors Flashcards
What are the adaptive challenges faced by pathogenic bacteria in water and in humans?
How do bacteria survive these?
Water: low temperature. low nutrient and osmotic strength. pH near neutral. O2 available. respiratory metabolism. free iron available. adhesins to stick to rocks.
Human: higher temperature (37oC). higher osmotic strength, C and energy sources abundant. low pH then higher pH. low or no O2. fermentative metabolism. iron bound to haem. adhesins to stick to cells. exposure to bile salts.
(Need to regulate genes at transcriptional level to survive these).
What are the types of regulation?
Change in number of transcripts (transcriptional regulation)-activators, repressors.
DNA rearrangements-Slipped-strand mispairing, Gene conversions, Gene inversions, Insertions and excisions, Epigenetic changes (cytosine methylation)- can affect binding of transcription factors.
Change in the number of proteins (translational regulation)-RNA topology.
How are genes organised?
Gene arrangement:
promoter (binds RNAP) and operator (binds regulatory protein).
Operon: several genes transcribed as part of a single transcript controlled by a single promoter.
Regulon: set of genes/operons all of which have a promoter region which responds to same regulatory proteins.
Stimulon: set of genes that respond to the same regulatory signal but not necessarily in the same way.
This is the most common way virulence factors are regulated.
What do activators proteins do?
Stimulate transcription.
Normally can’t bind to the promoter alone- need a ligand.
Activators either help to recruit RNA polymerase or help melt DNA, which forces it apart.
(Eg. E.coli MalT- Binds maltose and actiaves genes for maltose catabolism.)
What do repressor proteins do?
Prevent transcription.
Two methods: eg. E.coli TrpR- repressor can only bind with ligand. Gene is repressed. TrpR can only repress when it binds tryptophan. Negative feedback.
eg. E.coli LacI- repressor is bound- ligand binding changes conformation and releases it. LacI binds in absence of lactose. When lactose is present, it falls off DNA.
How can cells sense the extracellular environment?
Two component signal transduction systems.
- Transmembrane sensor kinase-binds signal and uses ATP to autophosphorylate a His..
- Autophosphorylation of sensor.
- Transfer of phosphate to response regulator.
- Response regulator binds target operator or activator sequence.
- Gene expression stimulated or inhibited.
Some signals only present outside of the cell, so need a sensing protein.
Only activated when a signal is present.
Describe some two component systems in different bacteria.
B.pertussis:
Sensor/regulator- BvgS/BvgA.
Signal- nicotinic acid, [Mg2+], temperature.
Regulates- toxin and colonisation genes, biofilm formation.
V.cholerae:
Sensor/regulator- ToxS/ToxR.
Signal- amino acids, CO2, bile salts, osmolarity.
Regulates- cholera toxin and colonisation genes.
S.enterica:
Sensor/regulator- PhoQ/PhoP.
Signal- [Mg2+], [Ca2+].
Regulates- Mg2+ uptake, transcriptional regulator genes ssrB, hilA, slyA, other virulence genes.
N.gonhorreae:
Sensor/regulator- PilB/PilA.
Signal- unknown.
Regulates- pilin genes (pilE).
E.coli:
Sensor/regulator- QseB/QseC, QseE/QseF.
Signal- noradrenalin.
Regulates- LEE (locus of enterocyte effacement).
Describe the pathogenesis of B.pertussis.
Kills ciliated cells.
May have an intracellular phase, not seen by the immune system.
Pathogen can hide in human cells.
Encounter:
- Exclusively human pathogens.
- Exceptionally contagious.
Entry:
- Trachea and bronchi by inhalation.
- Bind to ciliated epithelial cells and multiply there.
Spread and Multiplication:
- 3 weeks after entry begin to get “whooping” cough.
- Lasts for 2-3 months.
Damage:
-Caused by a range of toxins and other virulence determinants.
What are the virulence factors of B.pertussis?
Adhesins:
- filamentous haemagglutinin (Fha)- unusual structure, sits on surface, it is sticky so binds to cell tightly.
- pertussis toxin (Ptx)- afimbriate adhesion.
Toxins:
- pertussis toxin.
- Invasive Acase (CyaA)
- Tracheal cytotoxin- constitutively produced, peptidoglycan fragment which is secreted- this is toxic to ciliated cells, so kills them, stimulates release of IL-1.
How are B.pertussis virulence properties transcriptionally regulated?
Exclusively by the two component regulatory system (BvgS and BvgA).
fha, cyaA and ptx expression is greatest at 37oC, suppressed at low temperatures and high Mg2+.
Some virulence genes are activated, others are repressed by Bvg.
Control occurs in a regulatory cascade (rheostat).
Describe the B.pertussis virulence regulatory cascade.
The sensor BvgS responds to a signal.
Histidine is phosphorylated.
Phosphate is transferred to another residue (asparagine), then to another histidine.
Phosphate is then transferred to BvgA which activates 4 classes of genes (virulence repressor genes- switchof things that would give the cell away or things it doesnt need). Tight binding to these genes and they are always switched off.
Fha is switched on. BvgA-phosphate regulates the toxin. BipA is activated at low levels of BvgA and produces intimin. It is switched off at higher levels of BvgA- high affinity sites activate, low affinity sites act as a repressor- these fill up at higher concentrations of BvgA.
What is noradrenaline (norepinephrine) and what does it do?
Hormone and neurotransmitter.
Found in microM concentrations in the gut. Concentration increases in time of stress.
Modulates immune responses - important role in the innate response to bacterial infections.
Also regulates peristalsis.
(E.coli EHEC adheres to ileal mucosa in the presence of noradrenaline).
Describe the QseC and QseE signalling cascades in EHEC.
They allow integration of signal from inside and out.
Both systems autophosphorylate in response to their signals.
QseE/QseF switches on T3SS.
Both systems can cross talk.
E/F results in actin polymerisation, AE lesions (LEE activation). C/B results in inhibition of ER protein export, flagella regulation, potassium uptake and osmolarity, HUS.
Describe the pathogenesis of V.cholerae.
V.cholerae ingested.
Virulence genes expressed (pH shock in stomach, rise in temp).
Adheres to and colonises small intestine (flagella, toxin coregulated pilus (type 4 pilus for sticking to intestinal cells)).
Produces cholera toxin.
Cholera toxin acts on mucosal cells.
Extensive fluid and ion loss from tissues leading to hypotension, electrolyte imbalance and death.
How is virulence gene expression in V.cholerae regulated?
toxS and toxR:
- “ancestral” chromosomal genes.
- both proteins reside in the cytoplasmic membrane.
- ToxS lacks significant cytoplasmic domain.
- ToxR (regulator) has both periplasmic and cytoplasmic domains.
- ToxR activates transcription of toxT and partially activates ctxAB gene transcription.
- ToxT required for full activation of ctxAB, tcpA-F and acf (role in chemotaxis) genes.
Doesn’t phosphorylate but regulates by direct protein protein interactions.
