The SopEPhi phage integrates into the ssrA gene of Salmonella enterica serovar Typhimurium A36 and is closely related to the Fels-2 prophage.

Salmonella spp. are enteropathogenic gram-negative bacteria that use a large array of virulence factors to colonize the host, manipulate host cells, and resist the host's defense mechanisms. Even closely related Salmonella strains have different repertoires of virulence factors. Bacteriophages contribute substantially to this diversity. There is increasing evidence that the reassortment of virulence factor repertoires by converting phages like the GIFSY phages and SopEPhi may represent an important mechanism in the adaptation of Salmonella spp. to specific hosts and to the emergence of new epidemic strains. Here, we have analyzed in more detail SopEPhi, a P2-like phage from Salmonella enterica serovar Typhimurium DT204 that encodes the virulence factor SopE. We have cloned and characterized the attachment site (att) of SopEPhi and found that its 47-bp core sequence overlaps the 3' terminus of the ssrA gene of serovar Typhimurium. Furthermore, we have demonstrated integration of SopEPhi into the cloned attB site of serovar Typhimurium A36. Sequence analysis of the plasmid-borne prophage revealed that SopEPhi is closely related to (60 to 100% identity over 80% of the genome) but clearly distinct from the Fels-2 prophage of serovar Typhimurium LT2 and from P2-like phages in the serovar Typhi CT18 genome. Our results demonstrate that there is considerable variation among the P2-like phages present in closely related Salmonella spp.

Phage mediated horizontal transfer of the sopE1 gene increases enteropathogenicity of Salmonella enterica serotype Typhimurium for calves.

Epidemiological evidence shows that the sopE1 gene is associated with Salmonella Typhimurium phage types causing epidemics in cattle. In this study we demonstrate that horizontal transfer of the sopE1 gene by lysogenic conversion with the SopEphi increased enteropathogenicity of S. Typhimurium in the bovine ligated ileal loop model. These data support the hypothesis that phage mediated horizontal transfer of the sopE1 gene contributes to the emergence of epidemic cattle-associated S. Typhimurium clones.

The dual role of wild phages for horizontal gene transfer among Salmonella strains.

Salmonella bacteriophages seem to mediate horizontal transfer of virulence functions among Salmonella strains in two different ways: by general transduction and also by lysogenic conversion. The majority of wild phages isolated from Salmonella strains belong to the P22 like phages and were able to transduce. Our data show that the lysogenic conversion is generally accompanied by changes in the susceptibility to the typing phages used for epidemiological purposes. Similar phage type conversions to S. Typhimurium DT104 could be detected upon lysogenization with two other S. Typhimurium strains. For some S. Typhimurium strains the typical phage pattern is actually associated with alterations of virulence characteristics. For example, all tested wild type isolates of phage types DT49 and DT204 were found to be SopE phi-lysogens. The Anderson typing phages interfere with the prophages and/or cryptic phages and so the complex genetic short-term evolution can be demonstrated in the lab. This is one reason for the successful application of phage typing in Salmonella epidemiology since the 50s.

Characterization of effector proteins translocated via the SPI1 type III secretion system of Salmonella typhimurium.

Salmonella spp. employ a conserved type III secretion system encoded within the pathogenicity island 1 (SPI1; centisome 63) to translocate effector proteins into the host cytosol. The translocated effector proteins trigger diverse responses including bacterial internalization. In a mutation analysis we have defined the set of effector proteins mediating tissue culture cell invasion. This set includes sopE2 (centisome 40-42), sopB (SPI5, centisome 20) and in the case of S. typhimurium SL1344 also the phage-encoded effector sopE (SopEphi, centisome 59-60). A triple mutant SL1344 derivative deficient of SopE, SopE2 and SopB was more than 100-fold attenuated in tissue culture cell invasion. Phylogenetic analyses indicate that the last common ancestor of all contemporary Salmonella lineages already harbored all genes necessary for host cell invasion, namely the SPI1 type III secretion system, sopE2 and sopB. SopE, which is 70% identical to sopE2 is only present in some Salmonella strains and emerged later well after the divergence of the contemporary Salmonella lineages. Interestingly, S. typhimurium strains that harbor sopE are associated with epidemics, arguing that sopE is one of the factors determining the "fitness" of a strain. We found that SopE can specifically activate a different set of host cellular RhoGTPases than SopE2. This allows the bacteria to fine tune host cellular responses very precisely and may offer an explanation for the improved epidemic fitness of sopE-positive S. typhimurium strains.