PROJECT SUMMARY The health of animals often depends on their ability to form symbioses with bacteria that are acquired from environmental reservoirs. Bacteria that express specific traits that increase host fitness, and in many cases, hosts that lack these beneficial symbionts are at increased risk for developing disease. As part of our long- term goal to understand how these symbioses are established and maintained, this proposal will investigate several factors that impact the ability of the beneficial symbiont Vibrio fischeri to colonize and be maintained by its natural host the Hawaiian bobtail squid Euprymna scolopes. The focus will be on three areas: 1) interference competition among beneficial symbionts, 2) the role of sulfur metabolism in symbiont physiology, and 3) evolution of signaling systems in bacterial symbionts. Progress in understanding these areas will provide new insights into the mechanisms enabling bacterial symbionts to colonize and be maintained by their hosts. The findings associated with this project may contribute to the strategies employed to improve host health by manipulating beneficial symbionts.