Cell-density-encoding schemes in bacterial quorum-sensing systems. (A) Cells encode information about the cell density into the SM concentration and decode it to control a target response. (B) The quorum-sensing network can be divided into an encoder module (EM, left) and a decoder module (DM, right). The EM produces SMs (diamonds) at rate π inside the cell (volume Vc), which are exchanged (θdiff) with the environment (volume Ve) and are degraded both intra- and extracellular (λc, λe). The concentration of the SM detected by the receptor (diamond surrounded by a bold red line) determines the encoding behavior of the system. The DM transduces the signal from the receptor to regulate target gene expression. (C) Different encoding schemes. Left: abstract representation of EM shown in (B): intracellular encoding with diffusive SMs (Ac, Ae). Subscripts c and e denote cellular and extracellular concentrations, respectively. Middle: molecules A are actively exported (θout) and modified (γ) into molecules B, which are again imported (θin). [Bc] is detected by the receptor. Right: molecules A are modified during export (θγ) into molecules B. The extracellular concentration [Be] is detected by the receptor. To see this figure in color, go online.