Salmonella host cell invasion emerged by acquisition of a mosaic of separate genetic elements, including Salmonella pathogenicity island 1 (SPI1), SPI5, and sopE2.

Salmonella spp. possess a conserved type III secretion system encoded within the pathogenicity island 1 (SPI1; centisome 63), which mediates translocation of effector proteins into the host cell cytosol to trigger responses such as bacterial internalization. Several translocated effector proteins are encoded in other regions of the Salmonella chromosome. It remains unclear how this complex chromosomal arrangement of genes for the type III apparatus and the effector proteins emerged and how the different effector proteins cooperate to mediate virulence. By Southern blotting, PCR, and phylogenetic analyses of highly diverse Salmonella spp., we show here that effector protein genes located in the core of SPI1 are present in all Salmonella lineages. Surprisingly, the same holds true for several effector protein genes located in distant regions of the Salmonella chromosome, namely, sopB (SPI5, centisome 20), sopD (centisome 64), and sopE2 (centisomes 40 to 42). Our data demonstrate that sopB, sopD, and sopE2, along with SPI1, were already present in the last common ancestor of all contemporary Salmonella spp. Analysis of Salmonella mutants revealed that host cell invasion is mediated by SopB, SopE2, and, in the case of Salmonella enterica serovar Typhimurium SL1344, by SopE: a sopB sopE sopE2-deficient triple mutant was incapable of inducing membrane ruffling and was >100-fold attenuated in host cell invasion. We conclude that host cell invasion emerged early during evolution by acquisition of a mosaic of genetic elements (SPI1 itself, SPI5 [sopB], and sopE2) and that the last common ancestor of all contemporary Salmonella spp. was probably already invasive.

Yersinia enterocolitica-mediated translocation of defined fusion proteins to the cytosol of mammalian cells results in peptide-specific MHC class I-restricted antigen presentation.

Yersinia enterocolitica delivers a set of effector proteins [Yersinia outer proteins (Yop)] into the cytosol of target cells to modulate host cell signal transduction pathways required for the extracellular survival of the bacterium. Secretion and subsequent translocation of Yop across the eukaryotic cell membrane are achieved via a type III secretion system. About 50 - 100 amino acids of the N terminus of Yop are required for chaperone-directed secretion and translocation. In this study, it is demonstrated by immunoblot analysis of Yersinia-infected cultured epithelial cells that one ot these proteins, YopE, can serve as a molecular carrier to deliver protein fragments of the heterologous p60 antigen of Listeria monocytogenes into the cytosol of target cells. T cell activation assays revealed that the observed type III-mediated antigen translocation led to a p60 peptide-specific MHC class I-restricted antigen presentation. Efficient translocation and antigen presentation were strictly dependent on the co-localized expression of hybrid YopE-p60 proteins and the YopE-specific chaperone SycE. These results suggest that the Yersinia type III secretion system may serve as an attractive tool for antigen delivery in Yersinia-based live vaccines to induce cellular immune responses.