Effect of heterologous expression of Salmonella typhimurium OmpD porin in Salmonella typhi: Entry into and proliferation within culture cells and virulence in mice

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

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S. typhi is the only Salmonella serovar that grows exclusively in humans causing typhoid fever. On the other hand, S. typhimurium  produces a typhoid-like disease in mice, being less pathogenic for humans. The lack of OmpD porin is the major difference between S. typhi  and S. typhimurium outer membrane (OM) proteins. Although OmpD is present only in the S. typhimurium  OM, ompD::Tn10 transduction from S. typhimurium into S. typhi chromosome suggests the presence of the homologous gene in the latter. Therefore, it is posible that ompD is a silent gene in S. typhi. We transduced, via P22 phage, the S. typhimurium ompD gene into S. typhi and studied its expression in the S. typhi genetic background. Under acidic culture conditions (pH 5.0), the porin is repressed. It is induced under anaerobic growth conditions in a Fnr-dependent manner, as seen using fnr::Tn10 mutants. Furthermore, no regulation is detectable by medium osmolarity, accordingly to the observation that an envZ null mutation does not affect its expression. These regulations are also observed for ompD in S. typhimurium. To evaluate whether OmpD has some role in host specificity, we have tested our strains to asses their ability to entry into and proliferate within J774 cells, a murine derived macrophage-like cell line. Our results show that S. typhi OmpD+ penetrates J774 cells as efficiently as the wild type strain, however, its intracellular proliferation is four-fold higher. These results indicate that the heterologous OmpD is somehow enhancing S. typhi  intramacrophage survival. In addition, we have assayed virulence by intraperitoneal injection of bacteria into BALB/c mice. Using a S. typhimurium virulent strain, we observed that the lack of OmpD porin does not modify the median lethal dose (LD50) obtained with the wild type parental strain. When S. typhi strains were assayed, it was necessary to inject bacteria in mucin suspension. Fourteen days after inoculation, S. typhi OmpD+ showed the highest viable count recovered from spleen. Again, this result supports that S. typhi survival in mice has been enhanced by OmpD porin presence. The question whether ompD is present in S. typhi still remains.