What's in a name? The semantics of quorum sensing

Abstract

The expression of many bacterial phenotypes is regulated according to the concentration of chemical cues that they or other bacteria produce, a process often termed quorum sensing (QS). Many aspects of the environment can affect cue concentration. Thus these molecules might be indirect proxies for any one or combination of environmental factors. Recent research suggests that the adaptive significance of QS varies depending on its evolutionary and ecological context. Consequently, some researchers have proposed new terms, each emphasizing different adaptive functions, for the QS process. However, these new terms generate potential for a semantic quagmire and perpetuate the questionable notion that we can identify a single, dominant environmental feature to which the microbes respond. In fact, the ecological context of QS regulation, like the process itself, is complex and impacted by multiple aspects of natural environments.

Figure 1

Biotic and abiotic factors that bacterial cells might indirectly monitor using quorum sensing cues. The adaptive significance of quorum sensing regulation could relate to any one or a combination of these factors. (a) A cell at low local population density without barriers to diffusion or under environmental conditions under which quorum sensing cues are relatively unstable, experiences low intracellular concentrations of the quorum sensing cue. Consequently this cell is not active for quorum sensing regulated functions (cell is red). (b) When cue producing cells are at high local density the concentration of the cue that each cell experiences is higher due to the cumulative effects of neighboring cells. Thus high local population density is likely to be associated with elevated cue concentrations and thereby activation of quorum sensing regulated target functions (cell is green). (c) Limited diffusion or confinement results in high concentrations of quorum sensing cues in and around cells, leading to active quorum sensing regulated target functions (cell is green). (d) Environmental conditions under which quorum sensing cues are relatively stable or produced at high rates. A cell is more likely to experience high intracellular concentrations of the quorum sensing cue, and hence activate target functions (cell is green).