Studies on Regulatory Mechanism of Salmonella Pathogenesis by Proteome Analysis

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Intracellular pathogens, including Salmonella, which maintain long-term resistance within host phagocytes elicit a variety of genetic programs to help them adapt to the hostile environmental conditions encountered within the phagosome. To elucidate the genetic programs for Salmonella pathogenesis expressed in host after infection, a proteome analysis of proteins newly induced in Salmonella after phagocytosis by macrophages was performed. Since prominent among these programs was the heat shock response, the, role of molecular chaperones and AAA+ proteases which are members of the heat shock proteins as virulence proteins have been studied.(1)The disruption of ClpXP or Lon of serovar Typhimurium results a loss of virulence to mice. The attenuation could be due to the impaired ability to survive and replicate within macrophage cells, suggesting that ClpXP and Lon are critically important for the systemic Salmonella infection of mice. We found that the ClpXP mutant and Lon mutant persist i … More n the BALB/c mice for long periods of time without causing an overwhelming systemic infection, suggesting a possible candidate of Salmonella live vaccine. We therefore examined whether oral immunization with the lon mutant or clpXP mutant protects mice against subsequent oral challenge with virulent serovar Typhimurium. The results suggested that a single oral immunization of the lon and clpXP mutants should be effective to protect against the colonization of wild-type serovar Typhimurium within the intestinal tract.(2)We demonstrated that depletion of ATP-dependent Lon protease in serovar Typhimurium induces rapid and massive apoptosis in macrophages by a mechanism involving both caspases-1 and -3. This excessive induction of apoptosis was abrogated by disruption of invF, which is required for the expression of the Salmonella pathogenicity island 1(SPI1) genes. Expression of hilA, a central regulator of SPI1 transcription, was repressed in the macrophages after phagocytosis, but this gene was continuously expressed when the Lon mutant grew within the macrophages, so the SPI1 proteins accumulated. Thus, the increase in macrophage apoptosis induced by the Lon mutant could be due to continued expression of SPI1 genes under conditions where they are normally repressed. Once Salmonella has established a systemic infection, excess apoptosis of macrophages cells upon which the organism is reliant would be detrimental to the pathogen. Therefore, the Lon protease may be required to suppress apoptosis sufficiently to allow time for the bacterium to replicate, escape, and invade new macrophages.(3)The DnaK/DnaJ-depleted mutant lost the ability to causes a lethal systemic disease in mice. Macrophage-survival assays revealed that the 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 by Salmonella pathogenicity island 1. Less