Project Summary I am interested in the molecular biology and genetics of microbial social interactions. We use the saprophyte and opportunistic human pathogen Pseudomonas aeruginosa as a model to study social behaviors. P. aeruginosa uses a cell-cell signaling system called quorum sensing (QS) to coordinate gene expression. Because gene expression is coordinated at the population level, QS in P. aeruginosa, and other bacteria, is a means by which these single-celled organisms can cooperate and engage in specific group activities. In our prior NIGMS funded work, we established an experimental evolution system in which we demonstrated that P. aeruginosa QS can circuits diversify in response to changing environments. We also developed microbial co-culture systems, which allow us to ask about interspecies competition and how QS might impact interspecies interactions. The thrusts in this application ask 1) in cooperating populations of bacteria, what is the genetic basis for the diversification of the QS circuity, and does this diversification result in a division of labor where different individuals contribute separate functions to the group? and 2) how do the interactions between P. aeruginosa and other microbes influence P. aeruginosa behaviors, and how does QS contribute to these interactions? Bacteria, because of their rapid generation time and large population sizes, allow us to ask questions about social behavior that are not possible in organisms with smaller populations or longer generation times. The questions asked in this proposal will advance our knowledge of P. aeruginosa QS, cooperative behaviors in populations and interspecies interactions, and will inform efforts to target P. aeruginosa QS as a therapeutic approach.