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Infection and Immunity, June 2003, p. 3196-3205, Vol. 71, No. 6
0019-9567/03/$08.00+0     DOI: 10.1128/IAI.71.6.3196-3205.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Modulation of Virulence by Two Acidified Nitrite-Responsive Loci of Salmonella enterica Serovar Typhimurium

Charles C. Kim,* Denise Monack, and Stanley Falkow

Microbiology and Immunology, Stanford University Medical Center, Stanford, California 94305

Received 28 October 2002/ Returned for modification 13 January 2003/ Accepted 18 March 2003

Two acidified nitrite-inducible genes of Salmonella enterica serovar Typhimurium were identified with a green fluorescent protein-based promoter-trap screen. The nitrite-inducible promoters were located upstream of loci that we designated nipAB and nipC, which correspond to hcp-hcr (hybrid cluster protein) of Escherichia coli and norA of Alcaligenes eutrophus, respectively. Maximal induction of the promoters by nitrite was dependent on pH. The nipAB promoter was regulated by oxygen in an Fnr-dependent manner. The nipC promoter was also regulated by oxygen but in an Fnr-independent manner. The promoters were upregulated in activated RAW264.7 macrophage-like cells, which produce NO via the inducible nitric oxide synthase (iNOS), and the induction was inhibited by aminoguanidine, an inhibitor of iNOS. Although the nipAB and nipC mutants displayed no defects under a variety of in vitro conditions or in tissue culture infections, they exhibited lower oral 50% lethal doses (LD50s) than did the wild type in C57BL/6J mouse infections. The lower LD50s reflected an unexpected increased ability of small inoculating doses of the mutant bacteria to cause lethal infection 2 to 3 weeks after challenge, compared to a similar challenge dose of wild-type bacteria. We conclude that these genes are regulated by physiological nitrogen oxides and that the absence of these bacterial genes in some way diminishes the ability of mice to clear a low dose infection.

* Corresponding author. Mailing address: Microbiology and Immunology, Stanford University Medical Center, 299 Campus Dr., Stanford, CA 94305-5124. Phone: (650) 723-2671. Fax: (650) 723-1837. E-mail: cckim{at}stanford.edu.

Editor: V. J. DiRita

Infection and Immunity, June 2003, p. 3196-3205, Vol. 71, No. 6
0019-9567/03/$08.00+0     DOI: 10.1128/IAI.71.6.3196-3205.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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