Plants and animals are consistently colonized by bacteria, and these symbiotic microorganisms often contribute to beneficial processes and/or disease in their hosts. Many symbiotic bacteria communicate using pheromone signals, allowing them to coordinate group behaviors during infection. Such pheromone signaling is typically called ?quorum sensing?, because high cell density is necessary for pheromones to build up. However, a ?quorum? is typically not sufficient to induce pheromone-mediated behaviors fully, and bacterial pheromones are not simply census-taking signals. Importantly, the pheromones may also function to communicate aspects of the host environment among the bacteria. This project will use genetic, molecular, microscopic, and microbiological approaches in exploiting the tractable model symbiosis between the bacterium Vibrio fischeri and its natural host squid Euprymna scolopes to examine the relationship between conditions in the host and the symbiont?s use of pheromones. Pheromone signaling was discovered in V. fischeri and it remains among the best-understood model systems for such study. The symbiotic infection occurs in transparent tissue that is only colonized by V. fischeri, and pheromone signaling during infection results in the emission of bioluminescence by the bacteria, making the infection relatively simple and the regulatory output easy to monitor. The scientific aims of this project are to determine what aspects of the host environment lead to the induction of bacterial bioluminescence and to determine what role pheromones play in communicating conditions in the host. A third aim is to integrate research on bacteria-host symbioses into undergraduate education, both through direct emersion in research and through the generation of materials to facilitate in-class discussion of scientific papers. The project will provide insight into the environment symbiotic bacteria encounter in a host, and how their perception of the host environment can be integrated with their use of pheromones. The project will also help train young scientists.