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Quorum sensing in Bioreactor Operation(s)
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Introduction:
In most simple terms, Quorum Sensing refers to change in gene-expression in response to change in the cell density/cell population. It's exhibited only by those bacteria, which have the capability to release special signal molecules called auto-inducers, whose concentration is directly proportional to the cell density. For, the cells to become physiologically active and divide, they need to sense a minimum threshold of the auto-inducer, in absence of which, cells donot divide, and remain in the phase of lag/slow growth and eventually die.
In gram-positive bacteria, forms of oligo-peptides are used as auto-inducers, while gram-negative bacteria use acylated homoserine lactones as auto-inducers.

Role in Bioreactor Operation(s)

Deciding factor for Inoculum Density
Refer any literature on bacterial biomass production in bioreactors, and you'll observe that a fixed volume of reactor operation needs an optimal concentration of inoculum biomass to initiate the run. Where as higher concentrations can work, any concentration of biomass below a fixed value won't be able to successfully inoculate the bioreactor. This is a direct consequence of QS. Auto-inducers are the signal molecules that synchronize the physiological activity of the bacteria and enable them to express the house keeping genes, including those responsible for the synthesis of auto-inducer. But any bacterium never senses it's own auto-inducer, and always need an alien auto-inducer (synthesized by other cell) to activate the genes. When, the concentration of cells in the inoculum is very less, then upon transferring it into bulk volume of the reactor, diffusion limitations unable the interaction of auto-inducer with the complementary receptors on bacterial cells. But when the concentration is high, every cell can detect auto-inducer of each other's cell, further amplifying the expression of more auto-inducer synthesizing genes. This synchronization leads to a loop of auto-inducer synthesis and high expression of house keeping genes, leading to fast multiplication of the cells.

For Enhanced Biofouling of Membrane Biorecators in Waste Water Treatment
Waste water treatment is done normally by using Membrane Bioreactors, in which biofilms of bacterial cells are grown on reactor membranes, which degrade the toxic wastes of the water to be treated. And, to increase the cell density in the biofilms, synthetic analogs of auto-inducers are supplied, which trigger the growth and multiplication of the bacterial community responsible for the density of biofilm. A recent report on biofouling control in membrane reactors has proved that if the inhibitor(Porcine kidney acylase I) of N-acyl homoserine lactone (AHL) auto-inducer is applied to the culture, it prevented the biofouling of the membranes, thus establishing the quenching effect on auto-inducers and hence the dependency of biofouling on auto-inducers.

Bioreactor Clean-up in Industrial Set-up
Biofilming of contaminating microbes in the pipes and orifices of bioreactors in industrial set-ups is very common. And, considering the fact that Biofilms are highly toxic and tolerant to most antimicrobials, their clean-up is very difficult. QS quenching by the use of inhibitors, comes handy under such circumstances to clean-up the biofilms growing in sensitive locations in industrial fermenters.

Wall effect
It's very common to have observed the growth of bacterial culture (being cultivated in bioreactor vessel), on the walls of the reactor. It leads to very high consumption of nutrients by the wall attached bacterial growth, but no effective increase in the biomass grown in the reactor (which is sampled regularly). This effect is called as wall effect and is a direct consequence of high QS among the wall attached cells which tend to form a biofilm/aggregate there through concerted activity induced by auto-inducers.


Spent Media Induced Activity

It has been observed by many scientific groups, that supply of spent media to the batch reactors elicits the growth and productivity. And, it was quite evident and has been proved that the elicitation is actually an effect of auto-inducers present in the spent media, which were synthesized by the cells growing in that media. It's thus a novel and cost effective way of eliciting the response of cells towards growth and productivity.

It's thus evident that, QS plays a significant role in successful operation (and even maintenance) of bioreactors. Many scientific groups tend to forget the involvement of auto-inducers in routine cultivation of cells in bioreactors, which if realized can significantly enhance growth & productivity through as simple measure as using the spent media as elicitor! I hope this article added/updated your knowledge about QS!

Following is a simple animated depiction of QS:

Thanks!
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