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      Actin assessment in addition to specific immuno-fluorescence staining to demonstrate rickettsial growth in cell culture

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          Rickettsiae are able to spread within infected cell mono-layers by modifying intra-cellular actin formations. The study analyzes whether a visualization of actin modifications in addition to specific immuno-fluorescence staining of rickettsiae might facilitate the proof of rickettsial growth in cell culture.

          Cell mono-layers of Vero E6 und BGM cells were infected with Rickettsia honei. Intra-cellular actin was fluorescence stained with TRITC-(tetra-methyl-5,6-isothiocyanate)-labeled phalloidin in addition to specific immuno-fluorescence staining of rickettsiae with FITC-(fluorescein-isothiocyanate)-labeled antibodies. DNA of bacteria and cells was counter-stained with DAPI (4′,6- diamino-2-phenyl-indole). Cell cultures infected with Vaccinia virus were used as positive controls, cell cultures infected with Coxiella burnetii as negative controls.

          High concentrations of R. honei are necessary to demonstrate characteristic modifications of the intra-cellular actin. This effect is more pronounced in Vero E6 cells than in BGM cells.

          Actin staining with phalloidin is not suited for an early proof of rickettsial growth in cell culture but may confirm unclear findings in specific immuno-fluorescence staining in case of sufficient bacterial density.

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          Most cited references 9

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          Actin-based motility of intracellular microbial pathogens.

          A diverse group of intracellular microorganisms, including Listeria monocytogenes, Shigella spp., Rickettsia spp., and vaccinia virus, utilize actin-based motility to move within and spread between mammalian host cells. These organisms have in common a pathogenic life cycle that involves a stage within the cytoplasm of mammalian host cells. Within the cytoplasm of host cells, these organisms activate components of the cellular actin assembly machinery to induce the formation of actin tails on the microbial surface. The assembly of these actin tails provides force that propels the organisms through the cell cytoplasm to the cell periphery or into adjacent cells. Each of these organisms utilizes preexisting mammalian pathways of actin rearrangement to induce its own actin-based motility. Particularly remarkable is that while all of these microbes use the same or overlapping pathways, each intercepts the pathway at a different step. In addition, the microbial molecules involved are each distinctly different from the others. Taken together, these observations suggest that each of these microbes separately and convergently evolved a mechanism to utilize the cellular actin assembly machinery. The current understanding of the molecular mechanisms of microbial actin-based motility is the subject of this review.
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            A comparative study of the actin-based motilities of the pathogenic bacteria Listeria monocytogenes, Shigella flexneri and Rickettsia conorii.

             I Lasa,  H Ohayon,  H Gantelet (1999)
            Listeria monocytogenes, Shigella flexneri, and Rickettsia conorii are three bacterial pathogens that are able to polymerize actin into 'comet tail' structures and move within the cytosol of infected cells. The actin-based motilities of L. monocytogenes and S. flexneri are known to require the bacterial proteins ActA and IcsA, respectively, and several mammalian cytoskeleton proteins including the Arp2/3 complex and VASP (vasodilator-stimulated phosphoprotein) for L. monocytogenes and vinculin and N-WASP (the neural Wiskott-Aldrich syndrome protein) for S. flexneri. In contrast, little is known about the motility of R. conorii. In the present study, we have analysed the actin-based motility of this bacterium in comparison to that of L. monocytogenes and S. flexneri. Rickettsia moved at least three times more slowly than Listeria and Shigella in both infected cells and Xenopus laevis egg extracts. Decoration of actin with the S1 subfragment of myosin in infected cells showed that the comet tails of Rickettsia have a structure strikingly different from those of L. monocytogenes or S. flexneri. In Listeria and Shigella tails, actin filaments form a branching network while Rickettsia tails display longer and not cross-linked actin filaments. Immunofluorescence studies revealed that the two host proteins, VASP and (&agr;)-actinin colocalized with actin in the tails of Rickettsia but neither the Arp2/3 complex which we detected in the Shigella actin tails, nor N-WASP, were detected in Rickettsia actin tails. Taken together, these results suggest that R. conorii may use a different mechanism of actin polymerization.
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              Directional actin polymerization associated with spotted fever group Rickettsia infection of Vero cells.

              Members of the spotted fever group (SFG) of rickettsiae spread rapidly from cell to cell by an unknown mechanism(s). Staining of Rickettsia rickettsii-infected Vero cells with rhodamine phalloidin demonstrated unique actin filaments associated with one pole of intracellular rickettsiae. F-actin tails greater than 70 microns in length were seen extending from rickettsiae. Treatment of infected cells with chloramphenicol eliminated rickettsia-associated F-actin tails, suggesting that de novo protein synthesis of one or more rickettsial proteins is required for tail formation. Rickettsiae were coated with F-actin as early as 15 min postinfection, and tail formation was detected by 30 min. A survey of virulent and avirulent species within the SFG rickettsiae demonstrated that all formed actin tails. Typhus group rickettsiae, which do not spread directly from cell to cell, lacked F-actin tails entirely or exhibited only very short tails. Transmission electron microscopy demonstrated fibrillar material in close association with R. rickettsii but not Rickettsia prowazekii. Biochemical evidence that actin polymerization plays a role in movement was provided by showing that transit of R. rickettsii from infected cells into the cell culture medium was inhibited by treatment of host cells with cytochalasin D. These data suggest that the cell-to-cell transmission of SFG rickettsiae may be aided by induction of actin polymerization in a fashion similar to that described for Shigella flexneri and Listeria monocytogenes.

                Author and article information

                European Journal of Microbiology and Immunology
                Akadémiai Kiadó, co-published with Springer Science+Business Media B.V., Formerly Kluwer Academic Publishers B.V.
                1 September 2013
                : 3
                : 3
                : 198-203
                [ 1 ] Department of Tropical Medicine at the Bernhard Nocht Institute, German Armed Forces Hospital of Hamburg, Bernhard Nocht street 74, D-20359, Hamburg, Germany
                [ 2 ] Institute for Microbiology, Virology and Hygiene, University Hospital Rostock, Rostock, Germany
                [ 3 ] Institute for Microbiology of the German Armed Forces, Munich, Germany
                Author notes


                Hagen Frickmann and Elmar Schröpfer contributed equally to this work.

                [* ] 0049-40-6947-28743, 0049-40-6947-28709, Frickmann@
                Original Article


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