ABSTRACT
LuxR-type transcription factors detect acyl homoserine lactones (AHLs) and are typically used by bacteria to determine the population density of their own species. Escherichia coli and Salmonella enterica serovar Typhimurium cannot synthesize AHLs but can detect the AHLs produced by other bacterial species using the LuxR homolog, SdiA. Previously we determined that S. Typhimurium did not detect AHLs during transit through the gastrointestinal tract of a guinea pig, a rabbit, a cow, 5 mice, 6 pigs, or 12 chickens. However, SdiA was activated during transit through turtles colonized by Aeromonas hydrophila, leading to the hypothesis that SdiA is used for detecting the AHL production of other pathogens. In this report, we determined that SdiA is activated during the transit of S. Typhimurium through mice infected with the AHL-producing pathogen Yersinia enterocolitica. SdiA is not activated during transit through mice infected with a yenI mutant of Y. enterocolitica that cannot synthesize AHLs. However, activation of SdiA did not confer a fitness advantage in Yersinia-infected mice. We hypothesized that this is due to infrequent or short interactions between S. Typhimurium and Y. enterocolitica or that the SdiA regulon members do not function in mice. To test these hypotheses, we constructed an S. Typhimurium strain that synthesizes AHLs to mimic a constant interaction with Y. enterocolitica. In this background, sdiA(+) S. Typhimurium rapidly outcompetes the sdiA mutant in mice. All known members of the sdiA regulon are required for this phenotype. Thus, all members of the sdiA regulon are functional in mice.