RhlR-mediated cooperation in cystic fibrosis-adapted isolates of <i>Pseudomonas aeruginosa</i>

<i>Pseudomonas aeruginosa</i> uses quorum sensing (QS) to regulate the expression of dozens of genes, many of which encode shared products, called "public goods." <i>P. aeruginosa</i> possesses two complete acyl-homoserine lactone (AHL) QS circuits: the LasR-I and RhlR-I systems. Canonically, these systems are hierarchically organized: RhlR-I activity depends on LasR-I activation. However, in contrast to laboratory strains, isolates from people with cystic fibrosis can engage in AHL QS using only the transcription factor RhlR. In these isolates, RhlR regulates AHL QS and the production of secreted public goods, such as the exoprotease elastase, which are accessible to both producing and non-producing cells. When <i>P. aeruginosa</i> strains that use LasR to regulate elastase production are grown on casein as the sole carbon and energy source, LasR-null mutant "cheaters" commonly arise in populations due to a selective growth advantage. We asked if these social dynamics might differ in "RhlR cooperators": populations that use RhlR, not LasR, to regulate public goods. We passaged RhlR cooperators from several genetic backgrounds in casein broth. We found that cheaters emerged among most RhlR cooperators. However, in one isolate background, E90, RhlR-null mutants were dramatically outcompeted by RhlR cooperators. In this background, the mechanism by which RhlR mutants are outcompeted by RhlR cooperators is AHL-dependent and occurs in stationary phase but is not the same as previously described "policing" mechanisms. Our data suggest that cheating, or the lack thereof, does not explain the lack of RhlR mutants observed in most infection environments.IMPORTANCEQuorum sensing (QS) mutants arise in a variety of populations of bacteria, but mutants of the gene encoding the transcription factor RhlR in <i>Pseudomonas aeruginosa</i> appear to be infrequent. Our work provides insight on the mechanisms through which RhlR-mediated cooperation is maintained in a LasR-null population of <i>P. aeruginosa</i>. Characterizing the selective pressure(s) that disfavor mutations from occurring in RhlR may enhance our understanding of <i>P. aeruginosa</i> evolution in chronic infections and potentially guide the development of therapeutics targeting the RhlR-I QS circuit.