When being alone is enough: noncanonical functions of canonical bacterial quorum-sensing systems

A number of bacterial pathogens are capable of detecting the presence of other bacteria located within their surrounding niche through a process of bacterial signaling and cell-to-cell communication commonly referred to as quorum sensing (QS). QS systems are commonly now described in the context of collective behaviors exhibited by groups of bacteria coordinating diverse arrays of physiological functions to enhance survival of the community. However, QS systems have also been implicated in a variety of processes distinct from the measure of bacterial cell density. This review will highlight noncanonical adaptations of canonical QS systems that have evolved to enable bacteria to detect nonself individuals within a population or to detect occupation of confined spaces.