Quorum sensing in microbial biosensors

Small signaling molecules are utilized by several unicellular organisms to assess the cell density of other organisms of the same species in their vicinity. Together, the biochemical pathways involved in the production, secretion and recognition of these diffusible signals are known as quorum sensing (QS). Upon establishing that their local concentration has reached a threshold, the unicellular organisms collectively undertake a change in their transcriptional profiles, initiating complex activities which benefit the group as a whole but would have had limited relevance at a lower population count. In bacteria, QS regulates diverse functions such as formation of biofilms, onset of virulence, competence and bioluminescence. Researchers have developed whole-cell microbial biosensors that detect the presence of QS signals in clinical and environmental isolates. These biosensors enhance our understanding of microbial ecosystems present in diverse locations including the mammalian gut and lake sediments. Components of bacterial QS machinery have found widespread application in the emerging field of synthetic biology for the engineering of complex genetic circuits with novel functionalities-for example, production of biochemicals, spatio-temporal control of gene expression and creation of synthetic ecosystems. Engineered QS-based devices have been used to create microbial biosensors that localize to cancer cells or serve as improved live attenuated vaccines. In this chapter, we will discuss bacterial QS, and its usage in synthetic biology, followed by an overview of the applications of QS-based microbial biosensors in health and environment.