Quorum Sensing in Microorganisms Explained

Quorum sensing can be defined as the type of intercellular communication generally seen in bacteria. Quorum sensing is often associated with the behavioural action of a large population of bacteria, which when are not in groups are considered to be not evolved to show such behavioural psychology. In this article, we will learn about what is quorum sensing, the components of quorum sensing, the description of sensing molecules, and quorum sensing in bacteria. 

What is Quorum Sensing?

Quorum sensing meaning can be described as the cell to cell communication pathway utilized generally by microbes such as bacteria. Quorum sensing alternatively can also be described as the gene expression regulation in the response to the fluctuations in cell-population density. Microbes communicate with each other using this communication pathway but this chemical communication like the cellular signalling pathway of eukaryotes requires a signaling molecule. The chemical signaling molecule that is produced by the bacteria is known as the autoinducers, these perform the quorum sensing in the bacterial population. An increase in the concentration of the chemical signaling molecules, autoinducers is the function of the cell density. It can be described in simple terms as when the concentration of the autoinducers reaches a critical threshold, the bacterial population can sense the autoinducers and lead to a change of expression of genes in the complete population of the bacteria. Qurum sensing in bacteria allows behavioural alterations on a wide-scale population in response to changes in the number and/or species present in a community. Quorum sensing in bacteria is used for the regulation of various aspects of the physiological function. Some of the functions that are being regulated by the bacteria are as follows symbiosis, virulence, conjugation, antibiotic production, competence, motility, sporulation, and quorum sensing biofilm formation.

Components of Quorum Sensing 

It is important to understand each of the individual components of quorum sensing to understand the mechanism of action of such intercellular communication pathways and understand its role in the regulation of physiological activities. The components of quorum sensing in bacteria involve 2 major components, they are as follows, autoinducers and receptors.

Autoinducers- They are the quorum sensing molecules, they are the chemical compounds produced by the bacteria, it the autoinducer whose concentration when crosses the critical threshold in the population starts a signaling cascade to alter the gene expression. It is important to note that similar to the ligand and, receptor interaction. The autoinducers also bind to the receptors with high specificity. The complementarity in the shape of receptor and autoinducer is the key to their high affinity interaction. They also follow cooperative binding similar to eukaryotic signaling molecules. There are some important criteria for a signaling molecule to be termed as autoinducer, they are as follows,

1. Autoinducers must be synthesized by the bacteria during a specific developmental stage or in response to the change in the environmental conditions.

2. Quorum signaling molecules must accumulate in the extracellular environment until the critical threshold is reached. 

3. The autoinducers must bind to a specific receptor.

4. The cellular response must be concerted in the population, that is it the signaling cascade must work in the complete population.

Autoinducer release by quorum sensing mechanism is the function of the population density.

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Autoinducer can be broadly categorized into three classes,

1. Acylated homoserine lactone, commonly known as AHL. they are produced by gram-negative bacteria. They are neutral lipid molecules composed of a homoserine lactone ring joined with the acyl chain. The receptors of AHL are known as R regulators. They generally act on the regulation of the transcription.

2. Peptides include oligonucleotides. Quorum sensing in gram-positive bacteria follows the quorum sensing mechanism by using oligonucleotides as an autoinducer, oligonucleotides are generally involved in the post-transcriptional modification. 

3. Furanosyl borate diester, the most common example of this class is Autoinducer-2, they are secreted by both gram-positive and gram-negative, they are often called the evolutionary link between these two classes of the group of bacteria.

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Quorum Sensing in Bacteria

Quorum sensing in bacteria has three key steps in the mechanism. It is important to note that quorum sensing is insignificant in bacteria which has low population density or individual bacteria. It is used to manipulate gene expression at a large population density only. The three main principles of this mechanism are as follows, the signaling molecules known as autoinducers are secreted by the bacterial cell when the co\ncentration of these autoinducers cross the critical threshold of concentration, they can be sensed by other bacterial cells. The bacterial cell can belong to the same group or the different groups of the bacteria. This starts a signaling cascade resulting in the alteration of the expression of the gene. It is important to note that this alteration in the gene is not limited to singular bacterial ce4ll, all the cells of the population work in a concerted manner to induce change at the level of population. A common example of this mechanism is seen in the case of Vibrio fischeri quorum sensing which results in bioluminescence. Another example includes quorum sensing biofilm production.

Vibrio Fischeri Quorum Sensing

Vibrio fischeri uses quorum sensing to produce bioluminescence. It is the most common and widely studied example of quorum sensing in bacteria. The bacteria live in a symbiotic association with marine animals such as squid. The luxICDABEG operon in the bacteria codes for the proteins necessary for bioluminescence. The protein is named luciferase, it is enzymatic in nature. It has two subunits named alpha and beta and these subunits are encoded by genes luxA and luxB, respectively. Light is released as a byproduct of the oxidation reaction performed by luciferase.

Quorum sensing directly downregulates the bioluminescence property of the bacteria by controlling the expression of the luxA and luxB genes. The mechanism can be explained in the following steps,

Synthesis of autoinducer N-3-oxohexanoyl-homoserine lactone (3-oxo-C6-HSL) by luxI

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Binding of luxI to LuxR at the threshold concentration 100-200nM

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Activation of luxR

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Binding of LuxR/3-oxo-C6-HSL dimer upstream of the luxICDABEG operon 

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Recruitment of RNA polymerase at the site

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Transcription of the operon leads to transcription of luxA and, luxB gene

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Expression of alpha and beta subunit of luciferase

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Quaternary folding of the protein

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The catalytic reaction that is the oxidation of the substrate yields the light or bioluminescence.

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Application of Quorum Sensing in Bacteria

There are following applications of it, they are as follows

1. It regulates bacterial communication

2. It regulates the symbiotic relationship of bacteria

3. It regulates virulence

4. It regulates conjugation

5. It regulates the competence of the bacteria.

6. It also regulates the quorum sensing biofilm production

7. It also regulates the production of secondary metabolites

8. It also sometimes play role in the secretion of exopolysaccharides.