Bacterial Quorum Sensing and Metabolic Incentives to Cooperate

Cheat Control In quorum-sensing induction, a Pseudomonas aeruginosa population growing on a single carbon source, such as casein, will reach a density where the levels of signaling molecules they collectively secrete triggers the cells to synthesize and secrete proteases to digest the casein. However, it is metabolically costly to secrete proteases, and the system is prone to mutant “cheats.” These cheats do not respond to quorum sensing and do not go to the cost of synthesizing protease, but they do profit from the breakdown products that allow all the cells—cheats and cooperators alike—to grow. Dandekar et al. (p. 264 ) found that quorum signaling–insensitive P. aeruginosa cheats could not synthesize nucleotide hydrolase and were thus unable to grow if casein was replaced by adenosine. This allowed cooperators to outgrow the cheats, leading to a stable equilibrium between cheats and cooperators. This principle of regulation may be applicable to other bacterial quorum-sensing systems and might be exploited in the development of drugs that disrupt bacterial cooperation.