Nesse et al. Acta Veterinaria Scandinavica 2011, 53:44 http://www.actavetscand.com/content/53/1/44 Page 4 of 5 Figure 2 Results from semi quantitative PCR run 28 cycles. a) rck b) srgE. In both pictures: lane 1: 1 kb ladder, lane 2: 14028 wild type with AHL-C6, lane 3: 14028 wild type with AHL-C8, lane 4: 14028 wild type with dH2O, lane 5: 14028 sdiA mutant with AHL-C6, lane 6: 14028 sdiA mutant with AHL-C8, lane 7: lane 5: 114028 sdiA mutant with dH2O. invasion at 37°C in vitro, most probably through activa- tion of SdiA. The exact mechanisms behind the increased cell inva- sion that we observed are not known. However, several genes regulated by SdiA are believed to be involved in bacteria-host interactions. Three genes in the SdiA regu- lated rck operon play a role in adhesion to host tissues [8]. It has earlier been shown that Rck promotes adher- ence to epithelial cells and the extracellular matrix pro- teins fibronectin and laminin [9], and Rck has recently also been reported to mediate a zipper-like internaliza- tion of S. Enteritidis into cells in vitro [10]. Two other genes in the rck operon, pefI and srgA, appear to affect the expression and function of the pef operon which encodes socalled plasmid-encoded fimbriae [9,11,12]. The function of the rest of the genes in this operon is unknown. Very little is also known about srgE, but recently a computerized analysis suggested that SrgE may be a secreted substrate of a type III secretion sys- tem [13]. It has earlier been suggested that the mammalian gas- trointestinal tract may be the location where SdiA detects and responds to AHLs [5]. Although compounds that can activate AHL biosensors have been detected in the bovine rumen [14] and the avian craw (Flodgaard, Nesse, Bergsjø & Kaldhusdahl, unpublished results), lit- tle is yet known about the AHL producing potential of the intestinal flora of different hosts under varying con- ditions. Using a RIVETmethod (Recombination-based In Vivo Expression Technology) to record SdiA activity in vivo, Smith et al. observed SdiA activation during the transit of the S. Typhimurum RIVET strain through tur- tles colonized by the AHL-producing species Aeromonas hydrophila [15]. On the other hand, SdiA activation was not observed during the transit through the gastrointest- inal tract of a guinea pig, a rabbit, a cow, five mice, six pigs, or 12 chickens [15]. Interestingly, SdiA was acti- vated in mice that were infected with the AHL-produ- cing pathogen Yersinia enterocolitica [16]. These results indicate that S. Typhimurium can respond to AHLs pre- sent in the intestinal environment of these animals through the activation of SdiA. Conclusions The results from the present study show that S. Typhi- murium can respond to two different AHL quorum sen- sing signals (C6-AHL and C8-AHL) with increased cell invasion at 37°C in vitro, and that this response most likely is sdiA mediated. This indicates that if AHLs are present in the intestinal environment, they may increase the invasiveness of S. Typhimurium into epithelial cells. However, any possible effects on virulence are yet to be elucidated. Acknowledgements The study was funded by the Norwegian Veterinary Institute. Author details 1Norwegian Veterinary Institute, P.O.Box 750 Sentrum, N-0106 Oslo, Norway. 2Norwegian School of Veterinary Science, P.O.Box 8146 Dep, N-0033 Oslo, Norway. Authors’ contributions LLN was responsible for the study design, organisation of the work, analyses of the data and the preparation of the manuscript. KB carried out the cell invasion studies and PCR studies. LKV participated in the study design and the data analyses. IO was responsible for primer design and contributed to the PCR analyses. BD contributed to study design and participated in the data analyses. All authors have contributed to the writing of the manuscript, and read and approved the final manuscript. Competing interests The authors declare that they have no competing interests. Received: 11 March 2011 Accepted: 28 June 2011 Published: 28 June 2011 References 1. Miller MB, Bassler BL: Quorum sensing in bacteria. Annual Review of Microbiology 2001, 55:165-199. 2. Whitehead NA, Barnard AML, Slater H, Simpson NJL, Salmond GPC: Quorum-sensing in gram-negative bacteria. Fems Microbiol Rev 2001, 25:365-404. 3. Michael B, Smith JN, Swift S, Heffron F, Ahmer BMM: SdiA of Salmonella enterica is a LuxR homolog that detects mixed microbial communities. J Bacteriol 2001, 183:5733-5742. 4. Gray KM, Garey JR: The evolution of bacterial LuxI and LuxR quorum sensing regulators. Microbiol-Sgm 2001, 147:2379-2387. 5. Smith JN, Ahmer BMM: Detection of other microbial species by Salmonella: Expression of the SdiA regulon. J Bacteriol 2003, 185:1357-1366. 6. Lissner CR, Swanson RN, O’Brien AD: Genetic control of the innate resistance of mice to Salmonella typhimurium: expression of the Ity