Collaboration between a temperate phage and <i>Pseudomonas aeruginosa</i> quorum sensing constrains social cheats

Quorum sensing (QS) enables bacteria such as <i>Pseudomonas aeruginosa</i> to coordinate cooperative activities. How bacteria in cooperating groups can resist infiltration by non-cooperating variants is an emerging area of interest in sociobiology and molecular biology. There have been several recent reports on how QS and certain bacteriophage interact. In some strains of <i>P. aeruginosa</i>, QS can activate phage defense systems. At least one bacteriophage can repress <i>P. aeruginosa</i> QS. Here, we show that a previously undescribed bacteriophage can help cooperating groups of <i>P. aeruginosa</i> resist infiltration by non-cooperating QS mutants. This represents a mutualism in which both the bacteriophage and the <i>P. aeruginosa</i> host benefit at least under certain conditions.