Salmonella typhi Ty2 lacks the chromosomal region encoding the adjacent ompD and smvA genes present in the Salmonella typhimurium genome

Santiviago, C.A., and Mora, G.C. Departamento de Genética Molecular y Microbiología, Facultad de Ciencias Biológicas, P. Universidad Católica de Chile.

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Salmonella typhi causes a systemic disease only in its human host. The closely related Salmonella typhimurium causes a typhoid-like systemic disease in the mouse, but causes only a self-limited gastroenteritis in the human host. Although these two species share about 98-99% DNA sequence similarity, they display a remarkably different host specifity. This contrasting ability to infect their respective hosts may lay in profound genome differences In this study we have shown that the majority of Salmonella species most frequently associated with the systemic infection of vertebrate hosts produce a major outer membrane porin OmpD. However, this protein is absent from the outer membrane protein profiles of S. typhi strain Ty2 and 14 clinical isolates examined by SDS-PAGE. PCR primers flanking the ompD coding sequence amplify this gene from the genomes of all tested Salmonella species, with the exception of S. typhi strains. Consistent with this result, a PCR-amplified ompD fragment do not hybridize with DNA isolated from the S. typhi strains. We have extended this study to smvA gene, which is mapping next to ompD in the S. typhimurium chromosome and is responsible for the methyl viologen resistance phenotype. In contrast to all Salmonella species tested, S. typhi strains presented a methyl viologen sensitive phenotype. As suggested by hybridization and PCR amplification analyses, S. typhi Ty2 chromosome is also missing the smvA gene, however smvA is present in the clinical isolates tested. We have introduced S. typhimurium ompD and smvA genes into S. typhi Ty2 by generalized transduction with phage P22. The recombinant S. typhi Ty2 strain harboring ompD and smvA genes expresses OmpD in the outer membrane, however remains methyl viologen sensitive. Taken together ompD deletion and methyl viologen sensitive phenotype may contribute to the highly restricted host-specificity of S. typhi.