Salmonella Typhimurium invasion of HEp-2 epithelial cells in vitro is increased by N-acylhomoserine lactone quorum sensing signals

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.