Invasion by Salmonella typhimurium is affected by the direction of flagellar rotation

Characterization of Bartonella bacilliformis flagella and effect of antiflagellin antibodies on invasion of human erythrocytes

Bartonella bacilliformis is the etiologic agent of Oroya fever in humans. Flagellum-mediated motility has been postulated as a major virulence factor for invasion of host cells. To address this hypothesis, we purified and characterized flagella from strain KC584 and then assessed their role in human erythrocyte association and invasion. Electron microscopy of the flagellar preparation showed a high concentration of filaments with a mean wavelength of 800 nm. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblot analysis, and KBr density gradient centrifugation indicated that the flagellar filament is composed of a polypeptide of 42 kDa. The flagellin is partially (ca. 50%) resistant to treatment with trypsin. The first 17 amino acid residues of the N terminus of the mature flagellin protein are GAAILTNDNAMDALQDL and show approximately 46% sequence identity to the residues of the N termini of two Caulobacter crescentus flagellin proteins. A monospecific polyclonal antibodies to the flagellin protein was generated, and its specificity was verified by both immunoblot and immunogold analyses. Human erythrocyte invasion assays performed with bartonellae exposed to the antiflagellin antiserum showed a significant decrease in bacterial association with and invasion of human erythrocytes in comparison with that in bartonellae exposed to preimmune rabbit serum or phosphate-buffered saline (PBS) controls. These results suggest that flagella are an important component in the invasiveness of B. bacilliformis.

Salmonella typhimurium induces membrane ruffling by a growth factor-receptor-independent mechanism

Invasive Salmonella typhimurium induces dramatic actin rearrangements on the membrane surface of mammalian cells as part of its entry mechanism. These changes, which are best characterized as membranous ruffles, closely resemble the membrane changes that occur when a growth factor binds to its receptor. Recently, inhibition of the function of the small GTPases rac and rho in quiescent serum-starved fibroblasts was demonstrated to abolish growth factor-mediated ruffling and stress-fiber formation, respectively. In addition, actin changes induced by the oncogene ras were also shown to be regulated by rac and rho. Because Salmonella-induced actin rearrangements resemble those caused by growth factors, we investigated whether ras, rho, or rac regulates the membrane ruffling elicited by S. typhimurium. Surprisingly, inhibition of the functions of these GTPases had no effect on the ability of invasive S. typhimurium to induce membrane ruffles on a variety of tissue culture cells including Madin-Darby canine kidney cells, Swiss 3T3 fibroblasts, and Hep-2 cells. These results led us to examine the interactions of S. typhimurium with Henle-407 intestinal cells, which lack epidermal growth factor receptor on their membrane surface. We found no difference in the ability of invasive S. typhimurium to induce membrane ruffling and to enter Henle-407 cells with or without the epidermal growth factor receptor on the membrane surface. We, therefore, conclude that invasive S. typhimurium induces membrane ruffling and its own internalization by a rac-independent, growth factor-receptor-independent signaling pathway.

Use of Salmonella for heterologous gene expression and vaccine delivery systems

In the past year, the importance of secretory IgA has been emphasized as fundamental to protection against oral Salmonella infection. In several human trials, aro mutants of Salmonella typhi were highly immunogenic, but still retained the capacity to proceed beyond the gut wall after ingestion. Epitopes of Shiga toxin and influenza hemagglutinin have been expressed in Salmonella surface proteins in work aimed at the construction of hybrid vaccines. Eukaryotic cell involvement in the process of Salmonella attachment/invasion appears to be triggered by host cell phospholipase activation. Our understanding of the number and functions of Salmonella genes involved in the attachment/invasion process has increased considerably--different gene sets are required for invasion of different cell types.

Cellular immunity to intracellular bacteria

Great progress has been made in understanding the mechanisms bacteria use to invade, survive and move within eukaryotic cells. It is clear that bacteria have found ways to manipulate host cell signal transduction pathways and the cytoskeleton to their advantage. To defend against prokaryotic invaders, the immune system has evolved mechanisms for the specific recognition of bacterial antigens.

Role of flagella in adherence, internalization, and translocation of Campylobacter jejuni in nonpolarized and polarized epithelial cell cultures

Previous studies of Campylobacter jejuni have suggested that flagellin is an adhesin for epithelial cells and that motility is a virulence factor of this bacterium. The role of flagella in the interactions of C. jejuni with nonpolarized and polarized epithelial cells was examined with flagellar mutants. Flagellated, nonmotile (flaA flaB+ Mot-) and nonflagellated, nonmotile (flaA flaB Mot-) mutants of C. jejuni were constructed by in vivo homologous recombination and gene replacement techniques. Both classes of mutants were found to adhere to cells of human epithelial origin (INT 407) equally well; however, on the basis of the percentage of the inoculum internalized, internalization of the flaA flaB Mot- mutants was decreased by factors ranging from approximately 30 to 40 compared with the parent. The flaA flaB+ Mot- mutant was internalized by the INT 407 cells at levels six- to sevenfold higher than the flaA flaB Mot- mutants. Both classes of mutants, unlike the parent, were unable to translocate across polarized Caco-2 monolayers. These results indicate that flagella are not involved in C. jejuni adherence to epithelial cells but that they do play a role in internalization. Furthermore, the results suggest that either the motility of C. jejuni or the product of flaA is essential for the bacterium to cross polarized epithelial cell monolayers.

Prolonged inhibition of bacterial protein synthesis abolishes Salmonella invasion

We have found that prolonged inhibition of bacterial protein synthesis abolishes the ability of Salmonella typhimurium to enter HEp-2 cells. Our results suggest that an essential invasion factor has a functional half-life that is seen as a gradual loss of invasiveness in the absence of protein synthesis. Therefore, Salmonella invasiveness appears to be a transient phenotype that is lost unless protein synthesis is maintained. This finding may explain why salmonellae grown to stationary phase lose their ability to enter cultured cells. In addition, a short-lived capacity to enter cells may be important during infection so that bacterial invasiveness is limited to certain times and host sites during pathogenesis.

Immobilization of plasminogen on Escherichia coli flagella

The interaction of plasminogen with flagella of Escherichia coli was investigated. Plasminogen bound to flagella purified from E. coli LE392, a commonly used cloning host, and E. coli IH3069, an O25H1 strain isolated from a case of newborn bacteremia. The binding was inhibited by the lysine analog epsilon-aminocaproic acid, suggesting involvement of the lysine-binding Kringle domains of plasminogen in the binding. Purified flagella enhanced the formation of plasmin activity in the presence of tissue-type plasminogen activator; a similar enhancement was observed with flagella-expressing LE392 cells.

Ability of Proteus mirabilis to invade human urothelial cells is coupled to motility and swarming differentiation

Proteus mirabilis causes serious kidney infections which can involve invasion of host urothelial cells. We present data showing that the ability to invade host urothelial cells is closely coupled to swarming, a form of cyclical multicellular behavior in which vegetative bacteria differentiate into hyperflagellated, filamentous swarm cells capable of coordinated and rapid population migration. Entry into the human urothelial cell line EJ/28 by P. mirabilis U6450 isolated at different stages throughout the swarming cycle was measured by the antibiotic protection assay method and confirmed by electron microscopy. Differentiated filaments entered urothelial cells within 30 min and were 15-fold more invasive (ca. 0.18% entry in 2 h) than an equivalent dry weight of vegetative cells isolated before differentiation, which attained only ca. 0.012% entry in the 2-h assay. The invasive ability of P. mirabilis was modulated in parallel with flagellin levels throughout two cycles of swarming. Septation and division of intracellular swarm cells produced between 50 and 300 vegetative bacteria per human cell, compared with 4 to 12 intracellular bacteria after incubation with vegetative cells. Transposon (Tn5) mutants of P. mirabilis with specific defects in motility and multicellular behavior were compared with the wild-type for the ability to invade. Mutants which lacked flagella (nonmotile nonswarming) were entirely noninvasive, and those which were motile but defective in swarm cell formation (motile nonswarming) were 25-fold less invasive than wild-type vegetative cells. Mutants with defects in the coordination of multicellular migration and the temporal control of consolidation (cyclical reversion of swarm cells to vegetative cells) were reduced ca. 3- to 12-fold in the ability to enter urothelial cells. In contrast, a nonhemolytic transposon mutant which swarmed normally retained over 80% of wild-type invasive ability. Swarm cells and early consolidation cells were at least 10-fold more cytolytic than vegetative cells as a result of their high-level production of hemolysin.