text: Salmonella encodes a LuxR homolog, SdiA, but does not encode an AHL synthase. SdiA detects AHLs produced by other species of bacteria including Aeromonas hydrophila, Hafnia alvei, and Yersinia enterocolitica. When AHL is detected, SdiA regulates the rck operon on the Salmonella virulence plasmid (pefI-srgD-srgA-srgB-rck-srgC) and a single gene horizontal acquisition in the chromosome srgE. Salmonella does not detect AHL when passing through the gastrointestinal tracts of several animal species, suggesting that the normal microbiota does not produce AHLs. However, SdiA does become activated when Salmonella transits through turtles colonized with Aeromonas hydrophila or mice infected with Yersinia enterocolitica. Therefore, Salmonella appears to use SdiA to detect the AHL production of other pathogens rather than the normal gut flora.