GUEST COMMENTARY Layers of Signaling in a Bacterium-Host Association

Quorum sensing, the monitoring of population density by bacteria, is used to coordinately control gene expression and therefore particular behaviors under conditions of high cell density. Such group behaviors provide advantages to organisms under certain conditions, such as during pathogenic coloniza-tion when virulence traits are induced by a group of bacteria. In the accompanying paper, Lupp and Ruby describe the re-quirement for a quorum-sensing system at relatively low cell density during colonization by Vibrio fischeri of its symbiotic host, Euprymna scolopes (18). Quorum-sensing systems in Vibrio fischeri and Vibrio har-veyi. Quorum sensing is a field built from laboratory-based investigations of luminous marine bacteria, namely, Vibrio fis-cheri and Vibrio harveyi (25). It was in V. fischeri that the quorum-sensing regulators LuxR (a transcriptional activator