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Original- "Quorum Sensing "

Bacteria
Some of the best-known examples of quorum sensing come from studies of bacteria. Bacteria use quorum sensing to coordinate certain behaviors such as biofilm formation, virulence, and antibiotic resistance, based on the local density of the bacterial population. Quorum sensing can occur within a single bacterial species as well as between diverse species, and can regulate a host of different processes, in essence, serving as a simple indicator of population density or the diffusion rate of the cell's immediate environment. A variety of different molecules can be used as signals. Common classes of signaling molecules are oligopeptides in Gram-positive bacteria, N-acyl homoserine lactones (AHL) in Gram-negative bacteria, and a family of autoinducers known as autoinducer-2 (AI-2) in both Gram-negative and Gram-positive bacteria.

Mechanism
Bacteria that use quorum sensing constitutively produce and secrete certain signaling molecules (called autoinducers or pheromones). These bacteria also have a receptor that can specifically detect the signaling molecule (inducer). When the inducer binds the receptor, it activates transcription of certain genes, including those for inducer synthesis. There is a low likelihood of a bacterium detecting its own secreted inducer. Thus, in order for gene transcription to be activated, the cell must encounter signaling molecules secreted by other cells in its environment. When only a few other bacteria of the same kind are in the vicinity, diffusion reduces the concentration of the inducer in the surrounding medium to almost zero, so the bacteria produce little inducer. However, as the population grows, the concentration of the inducer passes a threshold, causing more inducer to be synthesized. This forms a positive feedback loop, and the receptor becomes fully activated. Activation of the receptor induces the up-regulation of other specific genes, causing all of the cells to begin transcription at approximately the same time. This coordinated behavior of bacterial cells can be useful in a variety of situations. For instance, the bioluminescent luciferase produced by Vibrio fischeri would not be visible if it were produced by a single cell. By using quorum sensing to limit the production of luciferase to situations when cell populations are large, V. fischeri cells are able to avoid wasting energy on the production of a useless product.

Edit- "Quorum Sensing "

Bacteria
Some of the best-known examples of quorum sensing come from studies of bacteria. Bacteria use quorum sensing to regulate certain phenotype expressions, which in turn, coordinate their behaviours. Some common phenotypes include biofilm formation, virulence factor expression, and motility. Certain bacteria are able to use quorum sensing to regulate bioluminescence, nitrogen fixation and sporulation.

Quorum sensing functions based on the local density of the bacterial population in the immediate environment. It can occur within a single bacterial species, as well as between diverse species. Both Gram-positive and gram-negative bacteria use quorum sensing, but there are some major differences in their mechanisms.

Mechanisms
In order for the bacteria to use quorum sensing constitutively, they must possess three characteristics: to secrete a signaling molecule, an autoinducer, to detect the change in concentration of signaling molecules, and to regulate gene transcription as a response. This process is highly dependent on the diffusion mechanism of the signaling molecules. Signaling molecules are usually constitutively secreted at a low level by individual bacteria. At low cell density, the molecules may just diffuse away. At high cell density, the local concentration of signaling molecules may exceed its threshold level, and trigger changes in gene expressions.

Gram-Positive Bacteria
Gram-positive bacteria use autoinducing peptide (AIP) as their autoinducers.

When gram-positive bacteria detect high concentration of AIP in the environment, AIP bind to a receptor to activate kinase. The kinase phosphorylates a transcription factor, which regulated gene transcription. This is called a two-component system.

Another possible mechanism is that AIP is transported into the cytosol, and binds directly to a transcription factor to initiate or inhibit transcription.

Gram- Negative Bacteria
Gram-negative bacteria produce N-acyl homoserine lactones (AHL) as their signaling molecule. Usually AHLs do not need additional processing, and bind directly to transcription factors to regulate gene expression.

Some gram-negative bacteria may use the two-component system as well.

Reference
Jodymao (talk) 07:48, 19 November 2017 (UTC)