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Infection and Immunity, November 1999, p. 5690-5698, Vol. 67, No. 11
0019-9567/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Salmonella typhimurium Virulence Genes Are Induced upon Bacterial Invasion into Phagocytic and Nonphagocytic Cells

Cheryl G. Pfeifer, Sandra L. Marcus, Olivia Steele-Mortimer, Leigh A. Knodler, and B. Brett Finlay*

Departments of Microbiology and Immunology and the Biotechnology Laboratory, Vancouver, British Columbia, Canada V6T 1Z3

Received 12 April 1999/Returned for modification 23 June 1999/Accepted 25 August 1999

Survival and growth of salmonellae within host cells are important aspects of bacterial virulence. We have developed an assay to identify Salmonella typhimurium genes that are induced inside Salmonella-containing vacuoles within macrophage and epithelial cells. A promoterless luciferase gene cassette was inserted randomly into the Salmonella chromosome, and the resulting mutants were screened for genes upregulated in intracellular bacteria compared to extracellular bacteria. We identified four genes in S. typhimurium that were upregulated upon bacterial invasion of both phagocytic and nonphagocytic cells. Expression of these genes was not induced by factors secreted by host cells or media alone. All four genes were induced at early time points (2 to 4 h) postinvasion and continued to be upregulated within host cells at later times (5 to 7 h). One mutant contained an insertion in the ssaR gene, within Salmonella pathogenicity island 2 (SPI-2), which abolished bacterial virulence in a murine typhoid model. Two other mutants contained insertions within SPI-5, one in the sopB/sigD gene and the other in a downstream gene, pipB. The insertions within SPI-5 resulted in the attenuation of S. typhimurium in the mouse model. The fourth mutant contained an insertion within a previously undescribed region of the S. typhimurium chromosome, iicA (induced intracellularly A). We detected no effect on virulence as a result of this insertion. In conclusion, all but one of the genes identified in this study were virulence factors within pathogenicity islands, illustrating the requirement for specific gene expression inside mammalian cells and indicating the key role that virulence factor regulation plays in Salmonella pathogenesis.

* Corresponding author. Mailing address: Biotechnology Laboratory, Room 237, 6174 University Blvd., Vancouver, BC, Canada V6T 1Z3. Phone: (604) 822-2210. Fax: (604) 822-9830. E-mail: bfinlay{at}unixg.ubc.ca.

Infection and Immunity, November 1999, p. 5690-5698, Vol. 67, No. 11
0019-9567/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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