"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.