Salmonella enterica serovar Typhimurium requires the type III secretion system encoded by Salmonella pathogenicity island 1 (SPI1) and controlled by the master regulator, HilA, to penetrate the intestinal epithelium. Numerous regulators affect virulence through influence on this system, including the proteolytic component ClpP, the stationary phase regulator RpoS and the carbon-storage regulator CsrA. However, the mechanism behind the ClpP regulation is not fully understood. To elucidate this we examined differentially expressed genes in a ΔclpP mutant compared with WT using global transcriptomic analysis. SPI1 and SPI4 virulence genes were significantly downregulated in the ΔclpP mutant, whereas several RpoS-dependent genes and the fliC gene encoding flagellin were upregulated. While the ΔclpP mutant was attenuated in cell invasion, this attenuation was not present in a ΔclpP/rpoS : : amp double mutant, suggesting the repression of invasion was directed through RpoS. The expression of the csrA virulence regulator was increased in the ΔclpP mutant and decreased in the rpoS : : amp and ΔclpP/rpoS : : amp mutants, indicating that ClpP affects the csrA expression level as well. Thus, this study suggests that ClpP affects SPI1 expression and thereby virulence indirectly through its regulation of both RpoS and CsrA.

Department of Systems Biology Soeltoft Plads Building 221 2800 Kongens Lyngby Denmark

Department of Veterinary Disease Biology Faculty of Health and Medical Sciences University of Copenhagen Stigboejlen 4 1870 Frederiksberg C Denmark

School of Veterinary Medicine and Science University of Nottingham Loughborough LE12 5RD UK

The National Food Institute Technical University of Denmark Moerkhoej Bygade 19 2860 Soeborg Denmark

Veterinary Research Institute Hudcova 70 621 00 Brno Czech Republic

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