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Bacteria have always been considered to act independently of a population. Until …

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Biochemistry of Quorum Sensing
- The History, Biochemistry, and Potential of Quorum Sensing

One of the most understood signaling mechanisms for prokaryotes is that of the bioluminescent bacteria Vibrio fisheri. V. fischeri is a Gram negative bacteria that lives in salt water either as non-luminous free plankton, or inside the light organs in marine fish where they are luminous. In 1972, it was shown that luminescence is a density-dependent phenotype; it is only expressed once a high enough density of cells is reached (2). In the experiments, it was shown that there is both an inhibitor and inducer (as the gene regulation mechanism was not investigated, the terms "repressor" and "activator" were not used) that are responsible for the regulation of the luminescent enzyme Luciferase. Cells were non-luminescent when first placed in fresh medium, where the inhibitor was present. The optical density of cells increased as expected; however luminescence did not change significantly until the threshold density was reached. This occurred when the bacteria were enough to consume the inhibitor and produce an activator, triggering luminescence. K.  Nealson described the activator molecule responsible for activation as an "Auto-inducer"; a bacterial signaling molecule (13). In 1977, Nealson grew V. fischeri cells in medium with a contained cell density of 107 CFU/ml. At this density in normal growth medium, the cells produced no significant luminescence. However, when the presumed auto-inducer was artificially added, the cells did show considerable luminescence; showing that the auto-inducer is responsible for luminescence (14). When compared to the in situ life of V. fischeri, the results correspond with the evolutionary benefit for sensing neighboring cells.    A QS system prevents single cells from producing the energy-consuming luminescence while not in the presence of a population; when the luminescence can not be seen. When the cells grow in symbioses with marine fish, nutrients are rich, and the cell density can trigger bioluminescence. New quorum sensing mechanisms are often compared to that of V. fischeri, and are often named "lux operon-like", after the auto-inducer regulatory mechanism of Luciferase.

Cells can use a variety of signaling molecules for QS systems; however some are much more common than others. Short peptides and amino acids, coded by gene sequences previously thought to be insignificant due to their short length, can be used as signaling molecules. These oligopeptides are used by Gram-positive bacteria by triggering phosphocascades. However, Homoserine Lactones (HSLs) are used by a wide variety of organisms. Bacteria also use a class of acylated HSLs (Acyl-Homoserine Lactone, or AHL) as their signaling molecule. HSLs and AHLs are found in Gram-negative bacteria. AHLs are hydrophobic, and able to diffuse through cell membranes to affect gene expressions. AHLs have a carbon side-chain with a variable length which is responsible for its effector selectivity.

Some signal molecules use transduction cascades to activate their target genes. And peptides generally use transport machinery in the membrane to interact with an intracellular target. Cells can even use more than one type of signal molecule to target gene expression. Pseudomonas aeruginus is a bacterial pathogen which causes severe infections in cystic fibrosis patients, often leading to death. P. aeruginus uses 2 AHLs (C4-HSL and C12-HSL) to trigger the formation of bio-film (11). These 2 AHLs are packaged into vesicles to convey their signal to neighboring cells. The packaging of the molecules is controlled by a molecule 2-heptyl-3-hydroxy-4-quinoline (Pseudomonas quinolone signal), which is also controlled by a quorum sensory molecule. Studies like that of Nakamura in P. aeruginus have shown how widespread quorum sensing is, and how important it is to the survival of a species.

Not only can cells respond to cells of the same species, but some experiments have shown cells to be able to respond to cells of other species (20). One of the most common signaling molecules is auto-inducer 2 (AI-2). AI-2 is highly conserved throughout bacteria; Gram-negative and Gram-positive bacteria both contain AI-2 producing genes. Many of these organisms use AI-2 as a growth regulatory molecule. The conservation of AI-2 suggests that cells can use it as an inter-species communication molecule. It's been hypothesized that different species in a population can use AI-2, but have reciprocal effects. A cell of one species can induce itself to enter logarithmic growth, while sending neighboring cells of different species into stationary phase.

QS systems are also capable of inter-kingdom communication. Agrobacterium tumefaciens is a bacteria that produces tumors in plants by transferring of a virulent Ti plasmid. A. tumefaciens can also transfer the Ti plasmid to other bacteria via conjugation. Conjugation requires high cell density. The Ti plasmid codes for conjugation genes which are expressed by a lux-like QS system activated by an AHL. The Ti plasmid forces tumor cells in the plant to produce opines, which trigger the conjugation to occur. (19)

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