AbstractsBiology & Animal Science

Many Gram-negative bacteria determine their population density by synthesizing N -acylhomoserine lactones (AHLs) using enzymes of the LuxI family, and detecting them with transcription factors of the LuxR family. The Escherichia, Salmonella , and Klebsiella genera all encode a luxR homolog named sdiA , but do not encode a luxI homolog or synthesize AHLs. Chromosomal lacZY fusions were previously isolated in srgE and the rck operon that respond to plasmid-encoded sdiA . The rck operon includes genes that are involved in complement resistance and adhesion to extracellular matrix, and genes that regulate the expression and folding of plasmid-encoded fimbria. The function of srgE is unknown. We found that these fusions do not respond to chromosomal sdiA and AHL in vitro unless the bacteria are grown in 0.2% to 0.5% motility agar. Interestingly, plasmid-based fusions to these same promoters do not require these special conditions. Once these conditions were identified, it was determined that SdiA responds to certain AHLs with much higher sensitivity than others. It was also determined that the rck and srgE promoters are temperature sensitive. Both are active at 37°C but not at room temperature. The srgE promoter, but not the rck promoter, can also be activated at 26°C. This requirement for high termperatures suggested that this AHL detection system is used within the gastrointestinal tract of host animals. To test this hypothesis, we used recombination-based in vivo expression technology (RIVET). Using this system, we demonstrated that SdiA is not active in several healthy animals (mice, guinea pigs, rabbits, pigs, chicks, turtles, and calves) suggesting that the normal flora of these animals do not synthesize AHLs of the proper type or at high enough concentrations to be detected by SdiA. However, SdiA activity was observed in animals infected with AHL producing pathogens, Aeromonas hydrophila in turtles or Yersinia enterocolitica in mice. Therefore, we hypothesize that SdiA detects other pathogens and takes advantage of the presence this second pathogen. The mechanism(s) by which Salmonella gains an advantage from the second pathogen are not yet known.