11: Quorum Sensing Flashcards
What features of an auto inducer molecule make it a good cell-cell communication signal?
It is produced by the bacterium is induces - a molecule produced by an organism to induce expression of its own genes, and also induces the production of more auto inducer in a positive feedback loop
What simple experiment was used to demonstrate that a secreted molecule was used for signalling in Vibrio fisheri?
It was found that bioluminescence doesn’t get switched on until a specific cell density is reached. The addition of spent media to young cultures was able to circumvent the lag in expression and bioluminescence was able to be seen before cell density had been reached, demonstrating the cause was something that had been secreted into the media.
Explain why bacteria might use quorum sensing to coordinate a process in the population
Working together in a group provides more fitness to the individuals compared with them acting alone. Secretion of extracellular enzyme to degrade food. Acting in concert to attack the immune system. Forming biofilm that resists the immune system and drugs. Moving in a group across a surface, swarming.
How does the auto inducer molecule system allow bacteria to estimate population density?
An auto inducer can act as a marker of population density - as the auto inducer must be able to pass in and out of the cell, when the cell density is low the inducer will be lost through diffusion into the surrounding environment. However if the cell density is high the auto inducer will accumulate. Concentration of the auto inducer is dependent on cell density and the rate of diffusion of the auto inducer into the external environment.
When one cell in the population becomes active for quorum sensing, why do all the rest of the cells quickly become activated also?
Quorum sensing triggers coordinated and collective actions, such as all cells in the population turning on the same gene once the cell population density is above a certain threshold value.
What other factor affects the concentration of auto inducer in the surrounding environment?
The rate of diffusion of the auto inducer in the external environment
N-acyl-homoserine lactose’s can vary greatly between species. What proteins control the specific synthesis of and the specific response to AHLs
LuxI/TraI family proteins control synthesis - attaches the homoserine lactose to fatty acid
LuxR/TraR family proteins control response - composed of ligand binding protein it binds AHL’s and the DNA binding protein which turns on a gene
Gram positive bacteria don’t use AHLs. What do they signal with and how is it sensed by the cell?
Gram positive bacteria produce peptides as long chain of precursors that get cleaved off to make small molecules which are actively exported out of the cell at a constant rate. Once they’ve accumulated to a certain level they’re able to bind a sensor kinase system which then goes on to activate target genes and increase production of the peptide synthase.
Why are pigmented strains useful in QS research?
Mutants that can’t produce AHL’s can be used to detect AHL molecules
Describe three phenotypes controlled by quorum sensing
Biofilm formation - in pseudomonas aeruginosa quorum sensing positively regulates biofilm formation
Virulence factors - quorum sensing up regulates exoenzyme production but negatively regulates toxin production
Surfactant production and swarming motility - cells become hyper flagellated and secrete surfactants which are secreted ahead of swarming (front) and increase wetting of agar and movement across the surface
How does plasmid pRL1JI turn the QS system of rhizobium leguminosarum into a pheromone system to detect plasmid less cells to mate with?
The plasmidless recipient cell is constantly producing auto inducer which is a long chain homoserine lactose. In the donor cell, production of the same auto inducer is repressed by the plasmid, suppressing autoinduction process. However when the donor cells in the presence of the recipient cell, inhibition of auto inducer process by the plasmid is overcome and therefore it knows there is a plasmidless recipient nearby activating conjuagtive transfer.
Describe three ways that QS can be disrupted
Antiactivation
Lactonase production
In some systems other AHL’s can bind and block LuxR proteins
