Evolution of bacterial communication systems

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PROJECT SUMMARY/ ABSTRACT Quorum sensing (QS) is an important form of bacterial communication used to coordinate gene expression and group behaviors in a cell density-dependent manner. Many bacteria use a form of QS in which a signal synthase produces a membrane-permeable small molecule to which a paired cytosolic receptor responds. Hundreds of QS systems with diverse properties have been identified, many of which are important for the virulence of pathogenic bacteria. How this diversity evolved is a question of fundamental importance to the field. In my early postdoctoral work on the model QS system LasI-LasR from Pseudomonas aeruginosa, I observed that the receptor, LasR, has not evolved to maximal signal sensitivity and that variants of both LasI and LasR tend to be less selective than wildtype. Further, QS systems are often tightly regulated to ensure activity only occurs at a specific cell density. Based on these observations, I hypothesize that QS systems evolve to balance signal sensitivity, selectivity, and fitness. This proposal tests that hypothesis using the LasI-LasR model system. Aim 1 will investigate the costs and benefits of signal sensitivity using hyper- and hypo-sensitive LasR variants I identified previously. I will measure the timing and level of QS activity in these mutants, the susceptibility of the mutants to signal interference, and ultimately the fitness of the mutants in monoculture and in competition with wildtype P. aeruginosa. Aim 2 will evaluate how P. aeruginosa responds to pressure on LasI-LasR signal selectivity. I will use my recently identified non-selective LasI and LasR variants as a starting point for the in vitro evolution of new signal selectivity. This aim will illuminate additional determinants of QS selectivity and will generate tools for future research on the trajectory by which QS signal selectivity evolves. Together these aims will deepen our understanding of the evolution of bacterial communication systems, are expected to facilitate future research into the roles of these systems in polymicrobial communities, and will lead to new avenues for understanding and treating infections. This research proposal will form the foundation of my applications for an independent faculty position and the results I obtain are expected to help me successfully compete for future funding. These experiments will be initiated during the K99 phase of the award and will include training in bacterial co-culture, in vitro evolution, and biostatistical methods. Additionally, this proposal includes a career development and training plan to complement my prior experience. I have assembled a multidisciplinary advisory team to help me achieve my goals and my mentor, Dr. Greenberg, is a pioneer and expert in bacterial communication with a long track record of successful mentorship. The training and support provided by this award will enable me to achieve my long-term goal of establishing an independent research program focused on bacterial signaling and physiology in complex environments.