Role of novel adhesins in interaction of Salmonella with the epithelium

Salmonella enterica is an important gastrointestinal pathogen of humans and animals with the ability to invade non-phagocytic cells and to persist and proliferate within mammalian cells. We recently performed the functional characterization of various new pathogenicity islands in Salmonella enterica. While analyzing the virulence functions of such invasion of non-polarized cells and intracellular replication, no specific contributions of SPI4 to SPI9 were observed. Invasion by Salmonella is commonly analyzed using non-polarized epithelial cells. Since these cells do not represent the properties of cells encountered by Salmonella in the intestinal epithelium, we hypothesized that cell culture models with polarized cells should give a natural setting for invasion experiments. Using infection models with polarized epithelial cells such as MDCK, T84 or CaCo-2, we observed that the function of SPI1 and, in addition, of SPI4 is required for efficient invasion. We found that Salmonella is unable to invade polarized epithelial cells without the function of proteins encoded by Salmonella Pathogenicity Island 4 (SPI4) (Gerlach et al., 2008) in cooperation with the SPI1-T3SS.
Image "MDCK-EM.jpg"
The polarized epithelial cell line MDCK was infected with Salmonella wild type. After 1 h of infection, the cells were fixed and subjected to scanning EM. One infected cell is pseudo-colored blue and adhered bacteria are pseudo-colored red. For the effacement of the crush border and reorganization of the apical cells surface of infected cells.

SPI4 encodes a type I secretion system (T1SS). The SPI4-T1SS has only one known substrate protein SiiE, a highly repetitive protein of 595 kDa, a non-fimbrial adhesin that mediates the binding of Salmonella specifically to the apical side of polarized cells (Gerlach et al., 2007). In contrast to T3SS, the architecture of T1SS is rather simple and only 3 subunits are required to mediate the transport of substrate proteins across the envelope of Gram-negative cells. Secretion is energized by an ATPase at the cytoplasmic face of the inner membrane (here SiiF), a membrane fusion protein (SiiD) that traverses the periplasm and a secretin (SiiC) that permits the passage of substrate proteins across the outer membrane. The adhesion of bacteria to host cells requires the temporal connection of the adhesin SiiE to the bacterial as well as to the host cell surface. Thus, the secretion into the exterior medium does not appear compatible with the function of SiiE as adhesin. We observed that SiiE is retained on the bacterial surface of Salmonella during adhesion to polarized cells (Gerlach et al., 2007).
Two further large non-fimbrial adhesins, i.e. SadA as a member of the family of trimeric autotransporter adhesins and BapA as another T1SS-secreted adhesin are currently under investigation in the group.


Future work is directed to

  • investigate the formation of a microcompartment of two protein secretion systems at the zone of contract between Salmonella and its target cell
  • dissect the temporal and spatial organization of a T1SS for the large adhesin SiiE, T3SS for translocated effectors and their ligands in the membrane of eukaryotic target cells
  • analyse the involvement of host cell receptors in microcompartment formation
  • reveal the mechanisms of control of the retention and release of SiiE by regulators SiiA and SiiB during the interaction of Salmonella with host cells.


Funding

The project is currently funded by the DFG, project HE1964/13-2


Collaborations

  • Prof. Dr. Yves Muller, FAU Erlangen-Nürnberg
  • Dr. Roman Gerlach, Robert Koch-Institut, Wernigerode
  • Dr. Dirk Linke, MPI für Entwicklungsbiologie, Tübingen


Original papers

Gerlach, R.G., Claudio, N., Rohde, M., Jäckel, D., Wagner, C., and Hensel, M. (2008). Cooperation of Salmonella pathogenicity islands 1 and 4 is required to breach epithelial barriers. Cell Microbiol 10, 2364-2376.
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Gerlach, R.G., Jäckel, D., Geymeier, N., and Hensel, M. (2007). Salmonella pathogenicity island 4-mediated adhesion is coregulated with invasion genes in Salmonella enterica. Infect Immun 75, 4697-4709.
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Gerlach, R.G., Jäckel, D., Stecher, B., Wagner, C., Lupas, L., Hardt, W.D., and Hensel, M. (2007). Salmonella Pathogenicity Island 4 encodes a giant non-fimbrial adhesin and the cognate type 1 secretion system. Cell Microbiol 9, 1834-1850.
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Reviews

Gerlach, R.G., and Hensel, M. (2007). Protein secretion systems and adhesins: the molecular armory of Gram-negative pathogens. Int J Med Microbiol 297, 401-415.
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Gerlach, R.G., and Hensel, M. (2007). Salmonella pathogenicity islands in host specificity, host pathogen-interactions and antibiotics resistance of Salmonella enterica. Berl Munch Tierarztl Wochenschr 120, 317-327.
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