**3. Decipher the bacterial dialog: quorum sensing**

Gene expression in bacteria can be regulated by a wide array of intra- and extracellular cues. On top of the common chemical inducers that are most often introduced manually to initiate protein overexpression, bacteria are actually capable of producing their own extracellular signals for intercellular communication. The term "quorum sensing (QS)" was coined by EP Greenberg and colleagues decades ago, to describe the phenomena where the secretion and perception of small signaling molecules are transduced to coordinate behavior of a minimal unit (quorum) of microorganisms. Since then, there's been an explosion in understanding how bacteria communicate with themselves. In this section, well-characterized quorumsensing systems and types of signals, receptors, mechanisms of signal transduction, and target outputs of each system are introduced. In addition, since quorum sensing in many bacteria is also shown to control gene expression in a global manner, several regulons will be introduced, again with the focus on *E. coli* and their potential application. Lastly, beyond controlling gene expression on a global scale, quorum sensing allows bacteria to communicate within and between species. Common pathways and inducers of interspecies communication will be introduced, and we will highlight some of the many applications built upon this ability to communicate not only between species, but also between kingdoms and non-biological substances. That is, by introducing QS phenomena, we develop its potential for keying protein expression via genetic or other means to cue its signaling processes.
