Methods in cell-to-cell signaling in Salmonella.

Many bacteria can sense their population density. This has been termed "quorum sensing." The bacteria use this information to coordinate their behavior, essentially behaving as multicellular organisms. The paradigm of Gram-negative quorum sensing is the LuxL/LuxR-type system employed by Vibriofischeri to regulate luminescence. The LuxR transcription factor detects the presence of N-acylhomoserine lactones (AHLs) produced by LuxI. The AHL diffuses freely across the cell wall, and its accumulation signals a high population density within a confined space. Upon binding AHL, the LuxR transcription factor activates the luminescence genes. Homologous systems are used by numerous Gram-negative pathogens to regulate host interaction genes. The AHLs produced by different LuxI homologs can vary in the length and modification of their acyl side chain. In the first section of this chapter, we describe the use of bacterial biosensors to determine whether a particular bacterial species synthesizes AHLs. The second section describes how to identify AHL-responsive genes in Salmonella typhimurium, an organism that detects but does not synthesize AHLs. The approach described can be modified for use with any organism that responds to AHLs but does not synthesize them. The third section describes the use of recombination-based in vivo expression technology (RIVET) to study AHL detection in vitro and in vivo, in this case the mouse gut.