Nesse et al. Acta Veterinaria Scandinavica 2011, 53:44 http://www.actavetscand.com/content/53/1/44 R E S E A R C H Open Access Salmonella Typhimurium invasion of HEp-2 epithelial cells in vitro is increased by N- acylhomoserine lactone quorum sensing signals Live L Nesse1*, Kristin Berg1, Lene K Vestby1, Ingrid Olsaker2 and Berit Djønne1 Abstract Background: In Gram-negative bacteria, the most commonly studied quorum sensing signals are the N- acylhomoserine lactones (AHLs). In Salmonella, AHLs are recognized by SdiA, which is believed to be a sensor of AHLs produced by other bacteria, since Salmonella does not produce AHLs itself. It has been speculated that AHLs produced by the gastrointestinal flora may influence the regulation of virulence traits in Salmonella. The aim of the present work was to study the effect of AHLs on epithelial cell invasion by Salmonella in vitro. Methods: Invasion by Salmonella enterica subspecies enterica serovar Typhimurium (S. Typhimurium) strain and its isogenc sdiA mutant was studied using a conventional gentamycin invasion assay with HEp-2 cells at 37°C. Gene expression was studied using a semi-quantitative PCR. Results: The S. Typhimurium strain, but not its isogenic sdiA mutant, displayed increased in vitro invasion after addition of both N-hexanoyl-DL-homoserine lactone (C6-AHL) and N-octanoyl-DL-homoserine lactone (C8-AHL). Increased expression of two of the genes in the SdiA regulon (rck and srgE) was observed in the wild type strain, but not in the sdiA mutant. Conclusions: The results from the present study show that S. Typhimurium can respond to two different AHL quorum sensing signals (C6-AHL and C8-AHL) with increased cell invasion at 37°C in vitro, and that this response most likely is sdiA mediated. These results indicate that if AHLs are present in the intestinal environment, they may increase the invasiveness of Salmonella. Introduction Bacteria can communicate through quorum sensing sig- nals, and they use quorum sensing to regulate a number of physiological activities, e.g. symbiosis, virulence, com- petence, conjugation, antibiotic production, motility, sporulation, and biofilm formation (for review, see [1,2]). In Gram-negative bacteria, the most commonly studied quorum sensing signals are the N-acylhomoser- ine lactones (AHLs). A variety of AHL molecules have been discovered which differ primarily in acyl chain length and the nature of the substituents at the C-3 position. AHLs are synthesized by proteins encoded by luxI gene homologues. * Correspondence: live.nesse@vetinst.no 1Norwegian Veterinary Institute, P.O.Box 750 Sentrum, N-0106 Oslo, Norway Full list of author information is available at the end of the article Salmonella enterica is a facultative intracellular patho- gen that can cause diseases ranging from mild gastroen- teritis to systemic infections. The AHL sensor of Salmonella is sdiA [3]. It is believed that Salmonella acquired the sdiA gene through lateral transfer of a pseudomonad homologue to an early ancestor [4]. How- ever, searches in the existing databases have failed to identify any luxI homologue in available sequences [3], indicating that Salmonella do not synthesize AHLs. This is supported by studies showing that Salmonella did not activate any AHL reporter systems tested under the con- ditions employed [3]. Consequently, Salmonella appear to be able to recog- nize AHL signals, but not to produce them. SdiA is therefore believed to be a sensor of AHLs produced by other bacterial species [3], possibly in the mammalian gastrointestinal tract [5]. An interesting question is whether AHLs produced by the gastrointestinal flora © 2011 Nesse et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.