One if by land, two if by sea: signalling to the ranks with CSP and XIP.

In many streptococci, quorum sensing utilizes secreted, linear peptides that engage cognate receptors to coordinate gene expression among members of a local population. Streptococcus mutans employs the secreted peptides CSP and XIP to stimulate production of antimicrobial bacteriocins and to induce development of competence for genetic transformation. Recent progress in the field reveals that these pathways not only monitor the presence of signal emitters but also sense environmental factors. Both kinds of information are integrated by regulatory networks that then generate multiple outcomes, even among parallel cells growing in identical conditions. In this issue of Molecular Microbiology, Son and co-workers investigate how two medium types shape cellular responses to CSP and XIP pheromones in individuals across a population. Their findings characterize restrictive properties of media differing in peptidic fragment content and reveal unusual signalling properties that contribute to bimodal responses of gene expression.