Nramp1 is a transporter that pumps divalent cations from the vacuoles of phagocytic cells and is associated with the innate resistance of mice to diverse intracellular pathogens. We demonstrate that sitA and mntH, genes encoding high-affinity metal ion uptake systems in Salmonella enterica serovar Typhimurium, are upregulated when Salmonella is internalized by Nramp1-expressing macrophages and play an essential role in systemic infection of congenic Nramp1-expressing mice.

Salmonella enterica is a gram-negative, facultative intracellular pathogen that causes disease symptoms ranging from gastroenteritis to typhoid fever. A key virulence strategy is the translocation of bacterial effector proteins into the host cell, mediated by the type III secretion systems (TTSSs) encoded in Salmonella pathogenicity island 1 (SPI-1) and SPI-2. In S. enterica serovar Typhimurium LT2, we identified the protein products of STM4157 and STM2137 as novel candidate secreted proteins by comparison to known secreted proteins from enterohemorrhagic Escherichia coli and Citrobacter rodentium. The STM4157 and STM2137 proteins, which we have designated SseK1 and SseK2, respectively, are 61% identical at the amino acid level and differ mainly in their N termini. Western analysis showed that in vitro accumulation and secretion of these proteins in serovar Typhimurium were affected by mutations in the two-component systems SsrA/B and PhoP/Q, which are key mediators of intracellular growth and survival. SPI-2 TTSS-dependent translocation of recombinant SseK1:: Cya was evident at 9 h postinfection of epithelial cells, while translocation of SseK2::Cya was not detected until 21 h. Remarkably, the translocation signal for SseK1 was contained within the N-terminal 32 amino acids. Fractionation of infected epithelial cells revealed that following translocation SseK1 localizes to the host cytosol, which is unusual among the currently known Salmonella effectors. Phenotypic analysis of ⌬sseK1, ⌬sseK2, and ⌬sseK1/⌬sseK2 mutants provided evidence for a role that was not critical during systemic infection. In summary, this work demonstrates that SseK1 and SseK2 are novel translocated proteins of serovar Typhimurium.

Azotobacter vinelandii strain UA22 was produced by pTn5luxAB mutagenesis, such that the promoterless luxAB genes were transcribed in an iron-repressible manner. Tn5luxAB was localized to a fragment of chromosomal DNA encoding the thrS, infC, rpmI, rplT, pheS and pheT genes, with Tn5 inserted in the 3'-end of pheS. The isolation of this mutation in an essential gene was possible because of polyploidy in Azotobacter, such that strain UA22 carried both wild-type and mutant alleles of pheS. Phenylalanyl-tRNA synthetase activity and PHES::luxAB reporter activity was partially repressed under iron-sufficient conditions and fully derepressed under iron-limited conditions. The ferric uptake regulator (Fur) bound to a DNA sequence immediately upstream of luxAB, within the pheS gene, but PHES::luxAB reporter activity was not affected by phenylalanine availability. This suggests there is novel regulation of pheST in A. vinelandii by iron availability.

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The host-pathogen interaction involves a myriad of initiations and responses from both sides. Bacterial pathogens such as enteropathogenic Escherichia coli (EPEC) and Salmonella enterica have numerous virulence factors that interact with and alter signaling components of the host cell to initiate responses that are beneficial to pathogen survival and persistence. The study of Salmonella and EPEC infection reveals intricate connections between host signal transduction, cytoskeletal architecture, membrane trafficking, and cytokine gene expression. The emerging picture includes elements of molecular mimicry by bacterial effectors and bacterial subversion of typical host events, with the result that EPEC is able to survive and persist in an extracellular milieu, while Salmonella establishes an intracellular niche and is able to spread systemically throughout the host. This review focuses on recent advances in our understanding of the signaling events stemming from the host-pathogen interactions specific to Salmonella and EPEC.