Vibrio parahaemolyticus is the leading cause of gastroenteritis in Asia and the most frequent cause of seafood-associated gastroenteritis in the USA. Our long-term goal is to understand how environmental signals relevant to pathogenesis regulate gene expression in V. parahaemolyticus. This information will address fundamental questions related to bacterial physiology and might lead to the development of better therapies. A remarkable feature of V. parahaemolyticus is that it has two strikingly different cell types. In liquid media, it grows as a "swimmer cell," a short rod with a single polar flagellum. On surfaces it differentiates into a "swarmer cell," an elongated rod with numerous lateral flagella. It is widely believed that the pathogenic form of the bacterium is the swarmer cell, but the mechanisms of V. parahaemolyticus pathogenesis remain unclear. Our hypothesis is that swarmer cell differentiation and virulence factor genes share many signal transduction pathways and transcriptional regulatory proteins. This hypothesis is based on the observations that (i) many bacterial pathogens exhibit swarmer cell differentiation, (ii) iron limitation, which contributes to induction of virulence genes in many pathogens, is one of the key signals for swarmer cell differentiation in V. parahaemolyticus, and (iii) the OpaR protein of V. parahaemolyticus is a global regulator for quorum sensing, a type three secretion system (T3SS1), capsular polysaccharide, and swarmer cell differentiation. Based on these observations, we propose to: 1. Develop and validate an Affymetrix antisense expression array for V. parahaemolyticus. 2. Use the microarray to identify genes that respond to surface sensing and OpaR. We will analyze global gene expression under various growth conditions in a wild type strain and an opaR mutant that is blocked in quorum sensing. Comparison of these transcription profiles will reveal the extent of overlap between these systems and begin to define the quorum-sensing regulon in V. parahaemolyticus. Our experiments will use novel methods of inducing swarmer cell differentiation to minimize indirect effects that would otherwise complicate a simple comparison of liquid- to plate-grown cells.