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Background   Salmonella enterica  serovar Typhi and Typhimurium are closely related serovars as indicated by >96% DNA sequence identity between shared genes. Nevertheless,  S . Typhi is a strictly human-specific pathogen causing a systemic disease, typhoid fever. In contrast,  S . Typhimurium is a broad host range pathogen causing only a self-limited gastroenteritis in immunocompetent humans. We hypothesize that these differences have arisen because some genes are unique to each serovar either gained by horizontal gene transfer or by the loss of gene activity due to mutation, such as pseudogenes.  S . Typhi has 5% of genes as pseudogenes, much more than  S . Typhimurium which contains 1%. As a consequence,  S . Typhi lacks several protein effectors implicated in invasion, proliferation and/or translocation by the type III secretion system that are fully functional proteins in  S . Typhimurium. SseJ, one of these effectors, corresponds to an acyltransferase/lipase that participates in SCV biogenesis in human epithelial cell lines and is needed for full virulence of  S . Typhimurium. In  S . Typhi,  sseJ  is a pseudogene. Therefore, we suggest that  sseJ  inactivation in  S . Typhi has an important role in the development of the systemic infection.    Results  We investigated whether the  S . Typhi  trans -complemented with the functional  sseJ  gene from  S . Typhimurium (STM) affects the cytotoxicity toward cultured cell lines. It was found that  S . Typhi harbouring  sseJ STM   presents a similar cytotoxicity level and intracellular retention/proliferation of cultured epithelial cells (HT-29 or HEp-2) as wild type  S . Typhimurium. These phenotypes are significantly different from wild type  S . Typhi    Conclusions  Based on our results we conclude that the mutation that inactivate the  sseJ  gene in  S . Typhi resulted in evident changes in the behaviour of bacteria in contact with eukaryotic cells, plausibly contributing to the  S . Typhi adaptation to the systemic infection in humans.

S. Typhimurium sseJ gene decreases the S. Typhi cytotoxicity toward cultured epithelial cells