- Pseudomonas aeruginosa is an environmental bacterium as well as an opportunistic pathogen that primarily infects immunocompromised individuals, including those suffering from cystic fibrosis. The density-dependent regulation of gene expression via cell-to-cell communication, also termed quorum sensing (QS), is an important virulence determinant in this organism. Generally, P. aeruginosa uses three hierarchically-arranged QS systems, las, rhl, and pqs, to coordinate the expression of hundreds of genes required for both virulence and nutrient acquisition. Each QS system consists of genes involved in autoinducer synthesis, lasI, rhlI, and pqsABCDH, and cognate regulatory genes lasR, rhlR, and pqsR. Although QS is required for infection, QS-deficient isolates, often carrying mutations in the central regulator lasR, have been isolated from clinical populations. Recent in vitro studies provide evidence indicating that lasR mutants can arise by social exploitation. As social cheaters, lasR mutants cease the production of public goods, such as extracellular proteases, and take advantage of their production by the QS-proficient majority. In this study, we sought to compare the social behavior of the lasR mutant to that of the rhlR and pqsR mutants in vitro. We also sought to understand whether social conflict plays a role in shaping the instantaneous QS diversity of both clinical and environmental populations. Under in vitro growth conditions requiring QS, both the lasR and pqsR mutants, but not the rhlR and signal-negative mutants, invaded wild-type populations. In contrast to the lasR and rhlR mutants, the pqsR mutant also grew well on its own. While the lasR mutant exhibited typical cheating behavior as previously reported, the pqsR and rhlR mutants demonstrated more complex behaviors that can be attributed to positive and negative pleiotropic effects through the differential regulation of pqs gene expression. Therefore, if the selective forces associated with each of these social phenotypes are similar in vivo, then lasR and pqsR, but not rhlR, mutants may thrive in both clinical and natural populations. Upon the analysis of clinical P. aeruginosa populations obtained from cystic fibrosis lung infections, we found that these predictions were partially substantiated. Generally, these populations demonstrated high QS diversity both within and between patients, suggesting diverse selection pressures within the lung. In contrast, environmental populations isolated from natural composts exhibited minimal QS diversity, indicating that social conflict may not play a predominant role in some natural populations. Overall, our study highlights the effects of social interactions on QS diversity within various P. aeruginosa populations.