In this work it is shown that the majority of Salmonella serovars most frequently associated with the systemic infection of vertebrate hosts produce a major outer-membrane porin, OmpD. However, OmpD is absent from the outer-membrane protein profiles of Salmonella typhi strain Ty2 and 26 clinical isolates of S. typhi examined by SDS-PAGE. To determine whether the ompD gene is present in S. typhi, primers internal to the ompD coding sequence were used to amplify the gene by PCR. With the exception of S. typhi strains, the ompD gene was amplified from the genomes of all Salmonella serovars tested. Consistently, a specific ompD probe did not hybridize with DNA isolated from the S. typhi strains. Taken together, these results demonstrate that S. typhi does not produce OmpD due to the absence of the ompD gene. Furthermore, it was investigated whether the deletion of ompD extended to smvA. This gene is adjacent to ompD in the Salmonella typhimurium chromosome and encodes a protein involved in the resistance to methyl viologen, a superoxide-generating agent. Although PCR failed to amplify the smvA gene from the S. typhi strain Ty2 genome, it was possible to amplify it from the chromosome of the clinical strains. On the other hand, hybridization analyses showed that the smvA gene is present in all the S. typhi strains tested. In contrast to the other Salmonella serovars, S. typhi strains Ty2 and the clinical isolates showed sensitivity to methyl viologen, suggesting that smvA gene is inactive in S. typhi. In conclusion, the ompD-smvA region is variable in structure among Salmonella serovars. It is hypothesized that the absence of ompD may suggest a role in host specificity.
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