The DnaK/DnaJ Chaperone Machinery of Salmonella enterica Serovar Typhimurium Is Essential for Invasion of Epithelial Cells and Survival within Macrophages, Leading to Systemic Infection

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Abstract
Salmonella enterica serovar Typhimurium, similar to various facultative intracellular pathogens, has been shown to respond to the hostile conditions inside macrophages of the host organism by inducing stress proteins, such as DnaK. DnaK forms a chaperone machinery with the cochaperones DnaJ and GrpE. To elucidate the role of the DnaK chaperone machinery in the pathogenesis of S. enterica serovar Typhimurium, we first constructed an insertional mutation in the dnaK-dnaJ operon of pathogenic strain χ3306. The DnaK/DnaJ-depleted mutant was temperature sensitive for growth, that is, nonviable above 39°C. We then isolated a spontaneously occurring revertant of the dnaK-dnaJ-disrupted mutant at 39°C and used it for infection of mice. The mutant lost the ability to cause a lethal systemic disease in mice. The impaired ability for virulence was restored when a functional copy of the dnaK-dnaJ operon was provided, suggesting that the DnaK/DnaJ chaperone machinery is required by Salmonella for the systemic infection of mice. This result also indicates that with respect to the DnaK/DnaJ chaperone machinery, the cellular requirements for growth at a high temperature are not identical to the cellular requirements for the pathogenesis of Salmonella. Macrophage survival assays revealed that the DnaK/DnaJ-depleted mutant could not survive or proliferate at all within macrophages. Of further interest are the findings that the mutant could neither invade cultured epithelial cells nor secrete any of the invasion proteins encoded within Salmonella pathogenicity island 1. This is the first time that the DnaK/DnaJ chaperone machinery has been shown to be involved in bacterial invasion of epithelial cells.