<P> Quorum sensing was first observed in Aliivibrio fischeri, a bioluminescent bacterium that lives as a mutualistic symbiont in the photophore (or light - producing organ) of the Hawaiian bobtail squid . When A. fischeri cells are free - living (or planktonic), the autoinducer is at low concentration, and, thus, cells do not luminesce . However, when they are highly concentrated in the photophore (about 10 cells / ml), transcription of luciferase is induced, leading to bioluminescence . </P> <P> In the Gram - negative bacterium Escherichia coli (E. coli), cell division may be partially regulated by AI - 2 - mediated quorum sensing . This species uses AI - 2, which is produced and processed by the lsr operon . Part of it encodes an ABC transporter, which imports AI - 2 into the cells during the early stationary (latent) phase of growth . AI - 2 is then phosphorylated by the LsrK kinase, and the newly produced phospho - AI - 2 can be either internalized or used to suppress LsrR, a repressor of the lsr operon (thereby activating the operon). Transcription of the lsr operon is also thought to be inhibited by dihydroxyacetone phosphate (DHAP) through its competitive binding to LsrR . Glyceraldehyde 3 - phosphate has also been shown to inhibit the lsr operon through cAMP - CAPK - mediated inhibition . This explains why, when grown with glucose, E. coli will lose the ability to internalize AI - 2 (because of catabolite repression). When grown normally, AI - 2 presence is transient . </P> <P> E. coli and Salmonella enterica do not produce AHL signals commonly found in other Gram - negative bacteria . However, they have a receptor that detects AHLs from other bacteria and change their gene expression in accordance with the presence of other "quorate" populations of Gram - negative bacteria . </P> <P> Salmonella encodes a LuxR homolog, SdiA, but does not encode an AHL synthase . SdiA detects AHLs produced by other species of bacteria including Aeromonas hydrophila, Hafnia alvei, and Yersinia enterocolitica . When AHL is detected, SdiA regulates the rck operon on the Salmonella virulence plasmid (pefI - srgD - srgA - srgB - rck - srgC) and a single gene horizontal acquisition in the chromosome srgE . Salmonella does not detect AHL when passing through the gastrointestinal tracts of several animal species, suggesting that the normal microbiota does not produce AHLs . However, SdiA does become activated when Salmonella transits through turtles colonized with Aeromonas hydrophila or mice infected with Yersinia enterocolitica . Therefore, Salmonella appears to use SdiA to detect the AHL production of other pathogens rather than the normal gut flora . </P>

Is the idea of quorum sensing applicable to other forms of life