1. Bacterial Quorum Sensing Many species of bacteria use quorum sensing to coordinate their gene expression according to the local density of their population. APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Jan. 2008, p. 437–445

2. Quorum sensing was first observed in Vibrio fischeri, a bioluminiscent bacterium that lives as a mutualistic symbiont in the photophore (or light-producing organ) of the Hawaiian bobtail squid. When V. 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 11 cells/ml) transcription of luciferase is induced, leading to bioluminescence.

3. Lux I 3-oxohexanoyl- DL -homoserine lactones (3OC6HSL) acylhomoserine lactone (acyl-HSL synthase) Lux R synthesize To stabilize the transcription factor

11. Results and Discussion The modeling suggests: (i) A threshold response results from positive feed back on a single, noncooperatively bound regulatory element (either a LuxR or LuxI homolog)

13. (ii) Bistability results from either positive feedback on two noncooperatively bound regulatory elements or positive feedback on one or more cooperatively bound regulatory elements

17. In Drosophila, temporal gradients of regulatory proteins drive differential patterning of the embryo by using, for example, bistability generated by positive feedback on a signaling receptor In mammalian cells, interferon molecules secreted by virus-infected cells lead uninfected cells to up- regulated antiviral defenses via interferon- stimulated positive feedback.