Science 338, 264–266 (2012)

Credit: CDC/JANICE HANEY CARR

Bacteria use quorum sensing to time the expression of certain genes, such as extracellular proteases, with population density. Within Pseudomonas aeruginosa, which uses the LasR transcriptional activator to sense the acylhomoserine lactone C12-HSL, some cells develop mutations in LasR that exempt them from responding to community signals. These 'cheaters' thus exploit the products of the community without investing effort to generate those 'public goods' and so must be kept at low enough levels such that the community can continue to flourish. Dandekar et al. speculated that the 'private goods', or intracellular pathways activated by LasR, might explain how cheaters are kept under control. For example, LasR positively regulates nucleoside hydrolase, a key degradative enzyme in adenosine catabolism. The authors observed that the proportion of cheaters in groups grown with combinations of casein and adenosine as carbon sources were lower at higher concentrations of adenosine. This effect was not observed when adenosine was replaced with glucose, the catabolism of which is not regulated by LasR. Adenosine-dependent cheater suppression was observed across five of six other P. aeruginosa isolates, suggesting that quorum-sensing metabolic cheater restraint is a common mechanism. Though it is unclear whether the coregulation of public and private goods arose specifically as a mechanism to limit cheating, these results have immediate implications for the further development of antimicrobials and quorum sensing–based synthetic circuits.