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- The History, Biochemistry, and Potential of Quorum Sensing

Quorum sensing is the ability of cells to sense cell density in a population. Cells use molecules called Auto-Inducers as transcriptional activators for density-dependent genes. As cells grow, they produce small concentrations of auto-inducers. In low concentrations, the auto-inducers do not have any activity. Once a certain cell-density is reached, enough auto-inducer is produced to trigger the expression of density dependent phenotypes. These new phenotypes contribute to factors that only help survival of the bacteria when large cell numbers are present. If these genes were expressed in small cell density, the cells would waste energy without exhibiting a use for the phenotype.

The first evidence of quorum sensing came from Myxobacterium in the 1960s. This experiment suggested that a secondary metabolite [substrate] was produced that triggered the differentiation of cells (8). In 1972, the classical QS system came from bioluminescence in the marine bacteria Photobacterium fischeria (now reclassified as Vibrio fischeri) (2). Until this research, cell-to-cell signaling was believed only to be a trait of multicellular organisms; these studies were initially taken with much skepticism. However, since the study of V. fischeri, many more density-dependent phenotypes have been identified in a wide range of bacteria.

QS is capable of controlling a large variety of phenotypes, many of which are of great interest in medicine and pathology. For example, some virulence factors in Streptococcus and Staphylococcus are only expressed once the population of bacteria is large enough to defeat the host immune system.

In Streptomyces species, mupirocin (a potent antibiotic) is only produced in late exponential and stationary phases (6). QS regulates the antibiotic production (15), so that they are only produced when surrounding cell count is high. By killing the cells around it, Streptomyces increases its own available nutrients.

A phenomenon known as swarming, the coordinated movement across a substrate by a population is also controlled by QS (7). Swarming motility is a virulence factor for some species, and also provides antibiotic resistance to the swarming cells (10). By inhibiting the QS of swarming cells, these virulence factors may be repressible.

It's believed that most prokaryotes have some sort of signaling mechanism for cell-to-cell communication. As experiments that attempt to elude QS mechanisms increasingly become more popular, so are the promises of QS targeting molecules for industry and medicine.

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