Differentially Evolved Genes of Salmonella Pathogenicity Islands: Insights into the Mechanism of Host Specificity in Salmonella

BACKGROUND: The species Salmonella enterica (S. enterica) includes many serovars that cause disease in avian and mammalian hosts. These serovars differ greatly in their host range and their degree of host adaptation. The host specificity of S. enterica serovars appears to be a complex phenomenon governed by multiple factors acting at different stages of the infection process, which makes identification of the cause/s of host specificity solely by experimental methods difficult. METHODOLOGY/PRINCIPAL FINDINGS: In this study, we have employed a molecular evolution and phylogenetics based approach to identify genes that might play important roles in conferring host specificity to different serovars of S. enterica. These genes are 'differentially evolved' in different S. enterica serovars. This list of 'differentially evolved' genes includes genes that encode translocon proteins (SipD, SseC and SseD) of both Salmonella pathogenicity islands 1 and 2 encoded type three secretion systems, sptP, which encodes an effector protein that inhibits the mitogen-activated protein kinase pathway of the host cell, and genes which encode effector proteins (SseF and SifA) that are important in placing the Salmonella-containing vacuole in a juxtanuclear position. CONCLUSIONS/SIGNIFICANCE: Analysis of known functions of these 'differentially evolved genes' indicates that the products of these genes directly interact with the host cell and manipulate its functions and thereby confer host specificity, at least in part, to different serovars of S. enterica that are considered in this study

Helical packing of needles from functionally altered Shigella type III secretion systems.

Gram-negative bacteria commonly interact with eukaryotic host cells using type III secretion systems (TTSSs or secretons), which comprise cytoplasmic, transmembrane and extracellular domains. The extracellular domain is a hollow needle-like structure protruding 60 nm beyond the bacterial surface. The TTSS is activated to transfer bacterial proteins directly into a host cell only upon physical contact with the target cell. We showed previously that the monomer of the Shigella flexneri needle, MxiH, assembles into a helical structure with parameters similar to those defining the architecture of the extracellular components of bacterial flagella. By analogy with flagella, which are known to exist in different helical states, we proposed that changes in the helical packing of the needle might be used to sense host cell contact. Here, we show that, on the contrary, mutations within MxiH that lock the TTSS into altered secretion states do not detectably alter the helical packing of needles. This implies that either: (1) host cell contact is signalled through the TTSS via helical changes in the needle that are significantly smaller than those linked to structural changes in the flagellar filament and therefore too small to be detected by our analysis methods or (2) that signal transduction in this system occurs via a novel molecular mechanism

Expression, purification, crystallization and preliminary crystallographic analysis of MxiH, a subunit of the Shigella flexneri type III secretion system needle

Caption reads: "Victory Parkway at Rockdale Ave., Cincinnati, Ohio.

Self-chaperoning of the type III secretion system needle tip proteins IpaD and BipD.

Bacteria expressing type III secretion systems (T3SS) have been responsible for the deaths of millions worldwide, acting as key virulence elements in diseases ranging from plague to typhoid fever. The T3SS is composed of a basal body, which traverses both bacterial membranes, and an external needle through which effector proteins are secreted. We report multiple crystal structures of two proteins that sit at the tip of the needle and are essential for virulence: IpaD from Shigella flexneri and BipD from Burkholderia pseudomallei. The structures reveal that the N-terminal domains of the molecules are intramolecular chaperones that prevent premature oligomerization, as well as sharing structural homology with proteins involved in eukaryotic actin rearrangement. Crystal packing has allowed us to construct a model for the tip complex that is supported by mutations designed using the structure

Evaluating Burkholderia pseudomallei Bip proteins as vaccines and Bip antibodies as detection agents.

Burkholderia pseudomallei is a biothreat agent and an important natural pathogen, causing melioidosis in humans and animals. A type III secretion system (TTSS-3) has been shown to be critical for virulence. Because TTSS components from other pathogens have been used successfully as diagnostic agents and as experimental vaccines, it was investigated whether this was the case for BipB, BipC and BipD, components of B. pseudomallei's TTSS-3. The sequences of BipB, BipC and BipD were found to be highly conserved among B. pseudomallei and B. mallei isolates. A collection of monoclonal antibodies (mAbs) specific for each Bip protein was obtained. Most recognized both native and denatured Bip protein. Burkholderia pseudomallei or B. mallei did not express detectable BipB or BipD under the growth conditions used. However, anti-BipD mAbs did recognize the TTSS needle structures of a Shigella strain engineered to express BipD. The authors did not find that BipB, BipC or BipD are protective antigens because vaccination of mice with any single protein did not result in protection against experimental melioidosis. Enzyme-linked immunosorbent assay (ELISA) studies showed that human melioidosis patients had antibodies to BipB and BipD. However, these ELISAs had low diagnostic accuracy in endemic regions, possibly due to previous patient exposure to B. pseudomallei