ExoU expression by Pseudomonas aeruginosa correlates with acute cytotoxicity and epithelial injury

Prevalence of type III secretion genes in clinical and environmental isolates of Pseudomonas aeruginosa

The type III secretion system of Pseudomonas aeruginosa transports four known effector proteins: ExoS, ExoT, ExoU and ExoY. However, the prevalence of the type III secretion system genes or the effector-encoding genes in clinical and environmental isolates of P. aeruginosa has not been well studied. Southern hybridization analyses and PCR were performed on over 100 P. aeruginosa isolates to determine the distribution of these genes. Clinical isolates were obtained from urine, endotracheal, blood and wound specimens, from the sputum of cystic fibrosis (CF) patients, and from non-hospital environmental sites. The popB gene was used as a marker for the presence of the large chromosomal locus encoding the type III secretion machinery proteins. Each isolate contained the popB gene, indicating that at least a portion of this large chromosomal locus was present in all isolates. Likewise, each isolate contained exoT-like sequences. In contrast, the exoS, exoU and exoY genes were variable traits. Overall, 72% of examined isolates contained the exoS gene, 28% contained the exoU gene, and 89% contained the exoY gene. Interestingly, an inverse correlation was noted between the presence of the exoS and exoU genes in that all isolates except two contained either exoS or exoU but not both. No significant difference in exoS, exoU or exoY prevalence was observed between clinical and environmental isolates or between isolates cultured from different disease sites except for CF respiratory isolates. CF isolates harboured the exoU gene less frequently and the exoS gene more frequently than did isolates from some of the other sites of infection, including the respiratory tract of patients without CF. These results suggest that the P. aeruginosa type III secretion system is present in nearly all clinical and environmental isolates but that individual isolates and populations of isolates from distinct disease sites differ in their effector genotypes. The ubiquity of type III secretion genes in clinical isolates is consistent with an important role for this system in human disease.

Epidemic population structure of Pseudomonas aeruginosa: evidence for a clone that is pathogenic to the eye and that has a distinct combination of virulence factors

The genetic structure of a population of Pseudomonas aeruginosa, isolated from patients with keratitis, endophthalmitis, and contact lens-associated red eye, contact lens storage cases, urine, ear, blood, lungs, wounds, feces, and the environment was determined by multilocus enzyme electrophoresis. The presence and characteristics of virulence factors were determined by restriction fragment length polymorphism analysis with DNA probes for lasA, lasB, aprA, exoS, exoT, exoU, and ctx and by zymography of staphylolysin, elastase, and alkaline protease. These analyses revealed an epidemic population structure of P. aeruginosa, characterized by frequent recombination in which a particular successful clone may increase, predominate for a time, and then disappear as a result of recombination. Epidemic clones were found among isolates from patients with keratitis. They were characterized by high activity of a hitherto-unrecognized size variant of elastase, high alkaline protease activity, and possession of the exoU gene encoding the cytotoxic exoenzyme U. These virulence determinants are not exclusive traits in strains causing keratitis, as strains with other properties may cause keratitis in the presence of predisposing conditions. There were no uniform patterns of characteristics of isolates from other types of infection; however, all strains from urinary tract infections possessed the exoS gene, all strains from environment and feces and the major part of keratitis and wound isolates exhibited high elastase and alkaline protease activity, and all strains from feces showed high staphylolysin activity, indicating that these virulence factors may be important in the pathogenesis of these infectious diseases.

Growth phase-dependent invasion of Pseudomonas aeruginosa and its survival within HeLa cells

Clinical isolates of Pseudomonas aeruginosa are classified into invasive and noninvasive (cytolytic) strains. In a noninvasive PA103 background, ExoS and ExoT have recently been shown to function as anti-internalization factors. However, these two factors seemed not to have such a function in an invasive strain PAK background. In this study, using HeLa tissue culture cells, we observed that the internalization of invasive strain PAK is dependent on its growth phases, with the stationary-phase cells internalized about 100-fold more efficiently than the exponential-phase cells. This growth phase-dependent internalization was not observed in the noninvasive PA103 strain. Further analysis of various mutant derivatives of the invasive PAK and the noninvasive PA103 strains demonstrated that ExoS or ExoT that is injected into host cells by a type III secretion machinery functions as an anti-internalization factor in both types of strains. In correlation with the growth phase-dependent internalization, the invasive strain PAK translocates much higher amount of ExoS and ExoT into HeLa cells when it is in an exponential-growth phase than when it is in a stationary-growth phase, whereas the translocation of ExoT by the noninvasive strain PA103 is consistently high regardless of the growth phases, suggesting a difference in the regulatory mechanism of type III secretion between the two types of strains. Consistent with the invasive phenotype of the parent strain, an internalized PAK derivative survived well within the HeLa cells, whereas the viability of internalized PA103 derivative was dramatically decreased and completely cleared within 48 h. These results indicate that the invasive strains of P. aeruginosa have evolved the mechanism of intracellular survival, whereas the noninvasive P. aeruginosa strains have lost or not acquired the ability to survive within the epithelial cells.

Multiple domains are required for the toxic activity of Pseudomonas aeruginosa ExoU

Expression of ExoU by Pseudomonas aeruginosa is correlated with acute cytotoxicity in a number of epithelial and macrophage cell lines. In vivo, ExoU is responsible for epithelial injury. The absence of a known motif or significant homology with other proteins suggests that ExoU may possess a new mechanism of toxicity. To study the intracellular effects of ExoU, we developed a transient-transfection system in Chinese hamster ovary cells. Transfection with full-length but not truncated forms of ExoU inhibited reporter gene expression. Inhibition of reporter activity after cotransfection with ExoU-encoding constructs was correlated with cellular permeability and death. The toxicity of truncated versions of ExoU could be restored by coexpression of the remainder of the molecule from separate plasmids in trans. This strategy was used to map N- and C-terminal regions of ExoU that are necessary but not sufficient for toxicity. Disruption of a middle region of the protein reduces toxicity. This portion of the molecule is postulated to allow the N- and C-terminal regions to functionally complement one another. In contrast to ExoS and ExoT, native and recombinant ExoU molecules do not oligomerize or form aggregates. The complex domain structure of ExoU suggests that, like other P. aeruginosa-encoded type III effectors (ExoS and ExoT), ExoU toxicity may result from a molecule that possesses more than one activity.

Comparison of the exoS gene and protein expression in soil and clinical isolates of Pseudomonas aeruginosa

Exoenzyme S (ExoS) is translocated into eukaryotic cells by the type III secretory process and has been hypothesized to function in conjunction with other virulence factors in the pathogenesis of Pseudomonas aeruginosa. To gain further understanding of how ExoS might contribute to P. aeruginosa survival and virulence, ExoS expression and the structural gene sequence were determined in P. aeruginosa soil isolates and compared with ExoS of clinical isolates. Significantly higher levels of ExoS ADP-ribosyltransferase (ADPRT) activity were detected in culture supernatants of soil isolates compared to those of clinical isolates. The higher levels of ADPRT activity of soil isolates reflected both the increased production of ExoS and the production of ExoS having a higher specific activity. ExoS structural gene sequence comparisons found the gene to be highly conserved among soil and clinical isolates, with the greatest number of nonsynonymous substitutions occurring within the region of ExoS encoding GAP function. The lack of amino acid changes in the ADPRT region in association with a higher specific activity implies that other factors produced by P. aeruginosa or residues outside the ADPRT region are affecting ExoS ADPRT activity. The data are consistent with ExoS being integral to P. aeruginosa survival in the soil and suggest that, in the transition of P. aeruginosa from the soil to certain clinical settings, the loss of ExoS expression is favored.

Resistance of Pseudomonas aeruginosa isolates to hydrogel contact lens disinfection correlates with cytotoxic activity

One of the most common pathogens in infection of hydrogel contact lens wearers is Pseudomonas aeruginosa, which can gain access to the eye via contamination of the lens, lens case, and lens care solutions. Only one strain per species is used in current regulatory testing for the marketing of chemical contact lens disinfectants. The aim of this study was to determine whether P. aeruginosa strains vary in their susceptibility to hydrogel contact lens disinfectants. A method for rapidly screening bacterial susceptibility to contact lens disinfectants was developed, based on measurement of the MIC. The susceptibility of 35 P. aeruginosa isolates to two chemical disinfectants was found to vary among strains. MICs ranged from 6.25 to 100% for both disinfectants at 37 degrees C, and a number of strains were not inhibited by a 100% disinfectant concentration in the lens case environment at room temperature (22 degrees C). Resistance to disinfection appeared to be an inherent rather than acquired trait, since some resistant strains had been isolated prior to the introduction of the disinfectants and some susceptible P. aeruginosa strains could not be made more resistant by repeated disinfectant exposure. A number of P. aeruginosa strains which were comparatively more resistant to short-term disinfectant exposure also demonstrated the ability to grow to levels above the initial inoculum in one chemical disinfectant after long-term (24 to 48 h) disinfectant exposure. Resistance was correlated with acute cytotoxic activity toward corneal epithelial cells and with exsA, which encodes a protein that regulates cytotoxicity via a complex type III secretion system. These results suggest that chemical disinfection solutions may select for contamination with cytotoxic strains. Further investigation of the mechanisms and factors responsible for resistance may also lead to strategies for reducing adverse responses to contact lens wear.

Pseudomonas aeruginosa ExoT inhibits in vitro lung epithelial wound repair

The nosocomial pathogen Pseudomonas aeruginosa causes clinical infection in the setting of pre-existing epithelial tissue damage, an association that is mirrored by the increased ability of P. aeruginosa to bind, invade and damage injured epithelial cells in vitro. In this study, we report that P. aeruginosa inhibits the process of epithelial wound repair in vitro through the type III-secreted bacterial protein ExoT, a GTPase-activating protein (GAP) for Rho family GTPases. This inhibition primarily targets cells at the edge of the wound, and causes actin cytoskeleton collapse, cell rounding and cell detachment. ExoT-dependent inhibition of wound repair is mediated through the GAP activity of this bacterial protein, as mutations in ExoT that alter the conserved arginine (R149) within the GAP domain abolish the ability of P. aeruginosa to inhibit wound closure. Because ExoT can also inhibit P. aeruginosa internalization by phagocytes and epithelial cells, this protein may contribute to the in vivo virulence of P. aeruginosa by allowing organisms both to overcome local host defences, such as an intact epithelial barrier, and to evade phagocytosis by immune effector cells.

Exoenzyme T of Pseudomonas aeruginosa elicits cytotoxicity without interfering with Ras signal transduction

One virulence strategy used by the opportunistic pathogen Pseudomonas aeruginosa is to target toxic proteins into eukaryotic cells by a type III secretion mechanism. Two of these proteins, ExoS and ExoT, show 75% homology on amino acid level. However, compared with ExoS, ExoT exhibits highly reduced ADP-ribosylating activity and the role of ExoT in pathogenesis is poorly understood. To study the biological effect of ExoT, we used a strategy by which ExoT was delivered into host cells by the heterologous type III secretion system of Yersinia pseudotuberculosis. ExoT was found to induce a rounded cell morphology and to mediate disruption of actin microfilaments, similar to that induced by an ADP-ribosylation defective ExoS (E381A) and the related cytotoxin YopE of Y. pseudotuberculosis. In contrast to ExoS, ExoT had no major effect on cell viability and did not modify or inactivate Ras by ADP-ribosylation in vivo. However, similar to ExoS and YopE, ExoT exhibited GAP (GTPase activating protein) activity on RhoA GTPase in vitro. Interestingly, ExoT(R149K), deficient for GAP activity, still caused a morphological change of HeLa cells. Based on our findings, we suggest that the ADP-ribosylating activity of ExoT target another, as yet unidentified, host protein that is distinct from Ras.

Pseudomonas aeruginosa-induced apoptosis involves mitochondria and stress-activated protein kinases

Pseudomonas aeruginosa, a gram-negative facultative pathogen, causes severe infections in immunocompromised and cystic fibrosis patients. However, the molecular details of the interaction between P. aeruginosa and mammalian cells are still largely unknown. Here we demonstrate that infection of human conjunctiva epithelial Chang cells with the well-characterized P. aeruginosa strain PAO-I results in rapid induction of apoptosis. Apoptosis was mediated by mitochondrial alterations, in particular mitochondrial depolarization, synthesis of reactive oxygen intermediates, and release of cytochrome c, as well as an activation of Jun N-terminal kinases (JNK). Stimulation of these events was dependent on upregulation of CD95 on infected cells, and a deficiency of CD95 or the CD95 ligand prevented mitochondrial changes, JNK activation, and apoptosis upon infection. Further, efficient apoptosis of Chang epithelial cells required infection with live P. aeruginosa, adhesion but not invasion of the bacteria, and expression of the type III secretion system in PAO-I. The data indicate a type III secretion system-dependent, sequential activation of several signaling pathways by P. aeruginosa PAO-I, resulting in apoptosis of the infected cell.

Pore-forming activity of type III system-secreted proteins leads to oncosis of Pseudomonas aeruginosa-infected macrophages

The Pseudomonas aeruginosa cystic fibrosis isolate CHA induces type III secretion system-dependent but ExoU-independent oncosis of neutrophils and macrophages. Time-lapse microscopy of the infection process revealed the rapid accumulation of motile bacteria around infected cells undergoing the process of oncosis, a phenomenon we termed pack swarming. Characterization of the non-chemotactic CHAcheZ mutant showed that pack swarming is a bacterial chemotactic response to infected macrophages. A non-cytotoxic mutant, lacking the type III-secreted proteins PcrV, PopB and PopD, was able to pack swarm only in the presence of the parental strain CHA or when macrophages were pretreated with the pore-forming toxin streptolysin O. Interaction of P. aeruginosa with red blood cells (RBCs) showed that the contact-dependent haemolysis provoked by CHA requires secretion via the type III system and the PcrV, PopB/PopD proteins. The pore inserted into RBC membrane was estimated from osmoprotection experiments to be between 2.8 and 3.5 nm. CHA-infected macrophages could be protected from cell lysis with PEG3350, indicating that the pore introduced into RBC and macrophage membranes is of similar size. The time course uptake of the vital fluorescent dye, Yo-Pro-1, into infected macrophages confirmed that the formation of transmembrane pores by CHA precedes cellular oncosis. Therefore, CHA-induced macrophage death results from a pore-forming activity that is dependent on the intact pcrGVHpopBD operon.

Assembly and function of type III secretory systems

Type III secretion systems allow Yersinia spp., Salmonella spp., Shigella spp., Bordetella spp., and Pseudomonas aeruginosa and enteropathogenic Escherichia coli adhering at the surface of a eukaryotic cell to inject bacterial proteins across the two bacterial membranes and the eukaryotic cell membrane to destroy or subvert the target cell. These systems consist of a secretion apparatus, made of approximately 25 proteins, and an array of proteins released by this apparatus. Some of these released proteins are "effectors," which are delivered into the cytosol of the target cell, whereas the others are "translocators," which help the effectors to cross the membrane of the eukaryotic cell. Most of the effectors act on the cytoskeleton or on intracellular-signaling cascades. A protein injected by the enteropathogenic E. coli serves as a membrane receptor for the docking of the bacterium itself at the surface of the cell. Type III secretion systems also occur in plant pathogens where they are involved both in causing disease in susceptible hosts and in eliciting the so-called hypersensitive response in resistant or nonhost plants. They consist of 15-20 Hrp proteins building a secretion apparatus and two groups of effectors: harpins and avirulence proteins. Harpins are presumably secreted in the extracellular compartment, whereas avirulence proteins are thought to be targeted into plant cells. Although a coherent picture is clearly emerging, basic questions remain to be answered. In particular, little is known about how the type III apparatus fits together to deliver proteins in animal cells. It is even more mysterious for plant cells where a thick wall has to be crossed. In spite of these haunting questions, type III secretion appears as a fascinating trans-kingdom communication device.

Sera from adult patients with cystic fibrosis contain antibodies to Pseudomonas aeruginosa type III apparatus

Expression of type III proteins of Pseudomonas aeruginosa in patients with cystic fibrosis (CF) was investigated by measuring the immune response against components of the type III pathway. Twenty-three of the 33 sera contained antibodies against PcrV, a protein involved in translocation of type III cytotoxins into eukaryotic cells, and 11 of 33 had antibodies against ExoS, while most CF sera contained antibodies against PopB and PopD, components of the type III apparatus. These data indicate that P. aeruginosa commonly expresses components of the type III translocation apparatus in adult CF patients.

Lack of adherence of clinical isolates of Pseudomonas aeruginosa to asialo-GM(1) on epithelial cells

Numerous studies have reported that asialo-GM(1), gangliotetraosylceramide, or moieties serve as epithelial cell receptors for Pseudomonas aeruginosa. Usually this interaction is confirmed with antibodies to asialo-GM(1). However, few, if any, of these reports have evaluated the binding of fresh clinical isolates of P. aeruginosa to asialo-GM(1) or the specificity of the antibodies for the asialo-GM(1) antigen. We confirmed that asialo-GM(1) dissolved in dimethyl sulfoxide could be added to the apical membrane of Madin-Darby canine kidney cells growing as a polarized epithelium on Transwell membranes (J. C. Comolli, L. L. Waite, K. E. Mostov, and J. N. Engel, Infect. Immun. 67:3207-3214, 1999) and that such treatment enhanced the binding of P. aeruginosa strain PA103. However, no other P. aeruginosa strain, including eight different clinical isolates, exhibited enhanced binding to asialo-GM(1)-treated cells. Studies with commercially available antibodies to asialo-GM(1) showed that these preparations had high titers of antibody to P. aeruginosa antigens, including whole cells, purified lipopolysaccharide (LPS), and pili. Inhibition studies showed that adsorption of an antiserum to asialo-GM(1) with P. aeruginosa cells could remove the reactivity of antibodies to asialo-GM(1), and adsorption of this serum with asialo-GM(1) removed antibody binding to P. aeruginosa LPS. Antibodies in sera raised to asialo-GM(1) were observed to bind to P. aeruginosa cells by immunoelectron microscopy. Antibodies to asialo-GM(1) inhibited formation of a biofilm by P. aeruginosa in the absence of mammalian cells, indicating a direct inhibition of bacterial cell-cell interactions. These findings demonstrate that asialo-GM(1) is not a major cellular receptor for clinical isolates of P. aeruginosa and that commercially available antibodies raised to this antigen contain high titers of antibody to multiple P. aeruginosa antigens, which do not interfere with the binding of P. aeruginosa to mammalian cells but possibly interfere with the binding of P. aeruginosa cells to each other.

Expression of ExsA in trans confers type III secretion system-dependent cytotoxicity on noncytotoxic Pseudomonas aeruginosa cystic fibrosis isolates

Twelve Pseudomonas aeruginosa cystic fibrosis isolates that are not able to exert a type III secretion system (TTSS)-dependent cytotoxicity towards phagocytes have been further studied. The strains, although possessing TTSS genes and exsA, which encodes a positive regulator of the TTSS regulon, showed no transcriptional activation of the exsCBA regulatory operon. The expression of exsA in trans restored the in vitro secretion of TTSS proteins and ex vivo cytotoxicity.

Triggering the ExoS regulon of Pseudomonas aeruginosa: A GFP-reporter analysis of exoenzyme (Exo) S, ExoT and ExoU synthesis

The ExoS regulon of Pseudomonas aeruginosa encodes diverse type III secreted effector proteins which have been shown to exert cytotoxic effects in cell culture experiments. However, little information exists about the environmental conditions and stimuli for upregulation of the ExoS regulon. Translational reporter fusion proteins of exoenzyme (Exo) S, ExoT and ExoU, as well as the type II secreted exotoxin A (ETA) to the green fluorescent protein (GFP), were constructed in order to compare exoprotein production under diverse growth conditions. Reporter protein activity was recorded by FACS-analysis and by conventional and confocal laser scanning microscopy. Low ion concentration induced co-ordinated upregulation of ExoS, ExoT and ExoU with a maximum effect at 37 degrees C. A dose-dependent upregulation was seen with human serum or increasing NaCl concentrations. A type III secretion-negative pcrD mutant of P. aeruginosa showed a weak ExoS response to environmental stimuli, compared with the parental strain, suggesting a negative regulatory mechanism. Co-culture with the mammalian cell lines J774A.1 or HeLa led to rapid upregulation of ExoS, ExoT and ExoU synthesis. These data suggest that the ExoS regulon of P. aeruginosa can be triggered by a variety of environmental signals as well as by cell contact with eukaryotic cells.

The role of quorum sensing in the in vivo virulence of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that causes a wide variety of infections. The cell-density-dependent signaling mechanisms known as quorum sensing play a role in several of these infections including corneal, lung and burn wound infections. In addition, the quorum-sensing systems contribute to the ability of P. aeruginosa to form biofilms on medically important devices. The quorum-sensing systems accomplish their effect by controlling the production of different virulence factors and by manipulating the host immune response.

Pseudomonas aeruginosa exoenzyme S, a bifunctional type-III secreted cytotoxin

Our recent studies have shown ExoS to be a bifunctional type-III secreted cytotoxin. Intracellular expression of the amino terminus of ExoS (C234) in eukaryotic cells stimulates actin reorganization without cytotoxicity, which involves small-molecular-weight GTPases of the Rho subfamily. Expression of the carboxyl terminus of ExoS comprises an ADP-ribosyltransferase domain, which is cytotoxic when expressed in cultured cells (Pederson and Barbieri, 1998). Rho and Ras are molecular switches, which control numerous cellular processes. Recent signaling studies suggest that there is crosstalk between Rho and Ras (Keely et al, 1997). Ras and Rho also contribute to wound healing processes and tissue regeneration. Recent studies have shown that microinjection of endothelial cells with activated Ras stimulated their motility, while microinjection of Ras-blocking antibodies inhibited cellular motility that is a component of the wound healing process (Fox et al., 1994). In addition, hepatocyte growth factor/scatter factor (HGF/ SF) and epidermal growth factor stimulate cellular motility through the Ras signal transduction pathway (Ridley et al., 1995). Rac and Rho are also involved in motility and tissue regeneration, since dominant negative Rac inhibits the cellular motility stimulated by HGF/SF (Santos et al., 1997) and inhibition of Rho by either C. difficile ToxA and ToxB or the C. botulinum C3 transferase inhibits wound healing (Santos et al., 1997). Inhibition of tissue regeneration and wound healing appear to play a role in the pathogenesis of C. difficile, since treatment of gastrointestinal mucosa with C. difficile ToxA and ToxB alone inhibits regeneration of the gastric mucosa. Thus, ExoS may contribute to the establishment of P. aeruginosa infections by inhibiting wound healing and tissue regeneration by two mechanisms. The amino terminus of ExoS could inhibit Rho function and wound healing in a manner similar to C. difficile. Alternatively, ExoS could inhibit the cellular motility and angiogenesis required for wound healing by ADP-ribosylating Ras. Through the inhibition of tissue regeneration and wound healing, ExoS may play a pivotal role in chronic disease by maintaining sites of colonization. Inhibition of Ras or Rho signaling may also interfere with both innate and acquired immunity. Small-molecular-weight GTP-binding proteins of the Ras superfamily are required for cellular processes, such as phagocytosis, as Rho proteins contribute to phagocytosis (Caron and Hall, 1998). Since Ras functions upstream of Rho in cellular signaling processes (Ridley et al., 1995), ADP-ribosylation of Ras by ExoS or the inhibition of Rho function by C234 may inhibit phagocytosis of P. aeruginosa by macrophages. Other studies indicate that Ras plays a role in T cell activation (Cantrell, 1994). Thus, ExoS may inhibit acquired immunity by inhibiting T-cell activation.

Pseudomonas aeruginosa mediated apoptosis requires the ADP-ribosylating activity of exoS

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that primarily infects immunocompromised individuals and patients with cystic fibrosis. Using a tissue culture system, invasive strains of P. aeruginosa were discovered to induce apoptosis at high frequency in HeLa and other epithelial and fibroblast cell lines. This apoptotic phenotype in the infected cells was determined by several criteria including (i) visual changes in cell morphology, (ii) induction of chromatin condensation and nuclear marginalization, (iii) the presence of a high percentage of cells with subG1 DNA content, and (iv) activation of caspase-3 activity. Induction of the type III secretion machinery, but not invasion of P. aeruginosa is required for induction of apoptosis. The apoptosis phenotype is independent of the cytoskeletal rearrangements that occur in the host cell early after infection. Mutants in P. aeruginosa exoS fail to induce apoptosis and complementation with wild-type exoS restored the apoptosis-inducing capacity, demonstrating that ExoS is the effector molecule. Analysis of exoS activity mutants shows that the ADP-ribosylating capacity of ExoS is essential for inducing the apoptotic pathway.

Acquisition of expression of the Pseudomonas aeruginosa ExoU cytotoxin leads to increased bacterial virulence in a murine model of acute pneumonia and systemic spread

Pseudomonas aeruginosa is the nosocomial bacterial pathogen most commonly isolated from the respiratory tract. Animal models of this infection are extremely valuable for studies of virulence and immunity. We thus evaluated the utility of a simple model of acute pneumonia for analyzing P. aeruginosa virulence by characterizing the course of bacterial infection in BALB/c mice following application of bacteria to the nares of anesthetized animals. Bacterial aspiration into the lungs was rapid, and 67 to 100% of the inoculum could be recovered within minutes from the lungs, with 0.1 to 1% of the inoculum found intracellularly shortly after infection. At later time points up to 10% of the bacteria were intracellular, as revealed by gentamicin exclusion assays on single-cell suspensions of infected lungs. Expression of exoenzyme U (ExoU) by P. aeruginosa is associated with a cytotoxic effect on epithelial cells in vitro and virulence in animal models. Insertional mutations in the exoU gene confer a noncytotoxic phenotype on mutant strains and decrease virulence for animals. We used the model of acute pneumonia to determine whether introduction of the exoU gene into noncytotoxic strains of P. aeruginosa lacking this gene affected virulence. Seven phenotypically noncytotoxic P. aeruginosa strains were transformed with pUCP19exoUspcU which carries the exoU gene and its associated chaperone. Three of these strains became cytotoxic to cultured epithelial cells in vitro. These strains all secreted ExoU, as confirmed by detection of the ExoU protein with specific antisera. The 50% lethal dose of exoU-expressing strains was significantly lower for all three P. aeruginosa isolates carrying plasmid pUCP19exoUspcU than for the isogenic exoU-negative strains. mRNA specific for ExoU was readily detected in the lungs of animals infected with the transformed P. aeruginosa strains. Introduction of the exoU gene confers a cytotoxic phenotype on some, but not all, otherwise-noncytotoxic P. aeruginosa strains and, for recombinant strains that could express ExoU, there was markedly increased virulence in a murine model of acute pneumonia and systemic spread.

Intracellular localization and processing of Pseudomonas aeruginosa ExoS in eukaryotic cells

ExoS is a type III cytotoxin of Pseudomonas aeruginosa, which modulates two eukaryotic signalling pathways. The N-terminus (residues 1-234) is a GTPase activating protein (GAP) for RhoGTPases, while the C-terminus (residues 232-453) encodes an ADP-ribosyltransferase. Utilizing a series of N-terminal deletion peptides of ExoS and an epitope-tagged full-length ExoS, two independent domains have been identified within the N-terminus of ExoS that are involved in intracellular localization and expression of GAP activity. N-terminal peptides of ExoS localized to the perinuclear region of CHO cells, and a membrane localization domain was localized between residues 36 and 78 of ExoS. The capacity to elicit CHO cell rounding and express GAP activity resided within residues 90-234 of ExoS, which showed that membrane localization was not required to elicit actin reorganization. ExoS was present in CHO cells as a full-length form, which fractionated with membranes, and as an N-terminally processed fragment, which localized to the cytosol. Thus, ExoS localizes in eukaryotic cells to the perinuclear region and is processed to a soluble fragment, which possesses both the GAP and ADP-ribosyltransferase activities.

Type III secretion: a bacterial device for close combat with cells of their eukaryotic host

Salmonella, Shigella, Yersinia, Pseudomonas aeruginosa, enteropathogenic Escherichia coli and several plant-pathogenic Gram-negative bacteria use a new type of systems called 'type III secretion' to attack their host. These systems are activated by contact with a eukaryotic cell membrane and they allow bacteria to inject bacterial proteins across the two bacterial membranes and the eukaryotic cell membrane to reach a given compartment and destroy or subvert the target cell. These systems consist of a secretion apparatus made up of about 25 individual proteins and a set of proteins released by this apparatus. Some of these released proteins are 'effectors' that are delivered by extracellular bacteria into the cytosol of the target cell while the others are 'translocators' that help the 'effectors' to cross the membrane of the eukaryotic cell. Most of the 'effectors' act on the cytoskeleton or on intracellular signalling cascades. One of the proteins injected by the enteropathogenic E. coli serves as a membrane receptor for the docking of the bacterium itself at the surface of the cell.

Pseudomonas aeruginosa cystic fibrosis isolates induce rapid, type III secretion-dependent, but ExoU-independent, oncosis of macrophages and polymorphonuclear neutrophils

Pseudomonas aeruginosa, an opportunistic pathogen responsible most notably for severe infections in cystic fibrosis (CF) patients, utilizes the type III secretion system for eukaryotic cell intoxication. The CF clinical isolate CHA shows toxicity towards human polymorphonuclear neutrophils (PMNs) which is dependent on the type III secretion system but independent of the cytotoxin ExoU. In the present study, the cytotoxicity of this strain toward human and murine macrophages was demonstrated. In low-multiplicity infections (multiplicity of infection, 10), approximately 40% of the cells die within 60 min. Analysis of CHA-infected cells by transmission electron microscopy, DNA fragmentation assay, and Hoechst staining revealed the hallmarks of oncosis: cellular and nuclear swelling, disintegration of the plasma membrane, and absence of DNA fragmentation. A panel of 29 P. aeruginosa CF isolates was screened for type III system genotype, protein secretion profile, and cytotoxicity toward PMNs and macrophages. This study showed that six CF isolates were able to induce rapid ExoU-independent oncosis on phagocyte cells.

Identification of a chitin-binding protein secreted by Pseudomonas aeruginosa

One of the major proteins secreted by Pseudomonas aeruginosa is a 43-kDa protein, which is cleaved by elastase into smaller fragments, including a 30-kDa and a 23-kDa fragment. The N-terminal 23-kDa fragment was previously suggested as corresponding to a staphylolytic protease and was designated LasD (S. Park and D. R. Galloway, Mol. Microbiol. 16:263-270, 1995). However, the sequence of the gene encoding this 43-kDa protein revealed that the N-terminal half of the protein is homologous to the chitin-binding proteins CHB1 of Streptomyces olivaceoviridis and CBP21 of Serratia marcescens and to the cellulose-binding protein p40 of Streptomyces halstedii. Furthermore, a short C-terminal fragment shows homology to a part of chitinase A of Vibrio harveyi. The full-length 43-kDa protein could bind chitin and was thereby protected against the proteolytic activity of elastase, whereas the degradation products did not bind chitin. The purified 43-kDa chitin-binding protein had no staphylolytic activity, and comparison of the enzymatic activities in the extracellular medium of a wild-type strain and a chitin-binding protein-deficient mutant indicated that the 43-kDa protein supports neither chitinolytic nor staphylolytic activity. We conclude that the 43-kDa protein, which was found to be produced by many clinical isolates of P. aeruginosa, is a chitin-binding protein, and we propose to name it CbpD (chitin-binding protein D).

ExoT of cytotoxic Pseudomonas aeruginosa prevents uptake by corneal epithelial cells

The presence of invasion-inhibitory activity that is regulated by the transcriptional activator ExsA of cytotoxic Pseudomonas aeruginosa has previously been proposed. The results of this study show that both ExoT and ExoS, known type III secreted effector proteins of P. aeruginosa that are regulated by ExsA, possess this activity. Invasion was reduced 94.4% by ExoT and 96.0% by ExoS. Invasion-inhibitory activity is not linked to ADP-ribosylation activity, at least for ExoS, since a noncatalytic mutant also inhibits uptake by an epithelial cell line (invasion was reduced 96. 0% by ExoSE381A).

Cell death of human polymorphonuclear neutrophils induced by a Pseudomonas aeruginosa cystic fibrosis isolate requires a functional type III secretion system

With a coincubation model incorporating Pseudomonas aeruginosa and human polymorphonuclear neutrophils (PMNs), a cystic fibrosis (CF) P. aeruginosa isolate has been shown to resist the bactericidal action of PMNs and to induce their cellular death. An isogenic mutant of this CF isolate in which the type III secretion system was rendered nonfunctional was unable to induce cellular death of PMNs.

Pseudomonas aeruginosa induces type-III-secretion-mediated apoptosis of macrophages and epithelial cells

Pseudomonas aeruginosa is a gram-negative opportunistic pathogen that is cytotoxic towards a variety of eukaryotic cells. To investigate the effect of this bacterium on macrophages, we infected J774A.1 cells and primary bone-marrow-derived murine macrophages with the P. aeruginosa strain PA103 in vitro. PA103 caused type-III-secretion-dependent killing of macrophages within 2 h of infection. Only a portion of the killing required the putative cytotoxin ExoU. By three criteria, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling assays, cytoplasmic nucleosome assays, and Hoechst staining, the ExoU-independent but type-III-secretion-dependent killing exhibited features of apoptosis. Extracellular bacteria were capable of inducing apoptosis, and some laboratory and clinical isolates of P. aeruginosa induced significantly higher levels of this form of cell death than others. Interestingly, HeLa cells but not Madin-Darby canine kidney cells were susceptible to type-III-secretion-mediated apoptosis under the conditions of these assays. These findings are consistent with a model in which the P. aeruginosa type III secretion system transports at least two factors that kill macrophages: ExoU, which causes necrosis, and a second, as yet unidentified, effector protein, which induces apoptosis. Such killing may contribute to the ability of this organism to persist and disseminate within infected patients.

Pathogenesis of septic shock in Pseudomonas aeruginosa pneumonia

The pathogenesis of septic shock occurring after Pseudomonas aeruginosa pneumonia was studied in a rabbit model. The airspace instillation of the cytotoxic P. aeruginosa strain PA103 into the rabbit caused a consistent alveolar epithelial injury, progressive bacteremia, and septic shock. The lung instillation of a noncytotoxic, isogenic mutant strain (PA103DeltaUT), which is defective for production of type III secreted toxins, did not cause either systemic inflammatory response or septic shock, despite a potent inflammatory response in the lung. The intravenous injection of PA103 did not cause shock or an increase in TNF-alpha, despite the fact that the animals were bacteremic. The systemic administration of either anti-TNF-alpha serum or recombinant human IL-10 improved both septic shock and bacteremia in the animals that were instilled with PA103. Radiolabeled TNF-alpha instilled in the lung significantly leaked into the circulation only in the presence of alveolar epithelial injury. We conclude that injury to the alveolar epithelium allows the release of proinflammatory mediators into the circulation that are primarily responsible for septic shock. Our results demonstrate the importance of compartmentalization of inflammatory mediators in the lung, and the crucial role of bacterial cytotoxins in causing alveolar epithelial damage in the pathogenesis of acute septic shock in P. aeruginosa pneumonia.

Airway epithelial tight junctions and binding and cytotoxicity of Pseudomonas aeruginosa

The role of tight junctions in the binding and cytoxicity of Pseudomonas aeruginosa to apical or basolateral membranes of lung airway epithelial cells was tested with fluorescence microscopy on living cells. Binding of noncytotoxic P. aeruginosa strain O1 was assessed with P. aeruginosa that expressed green fluorescent protein. Binding of cytotoxic P. aeruginosa strain 6206 was assessed with FITC-labeled P. aeruginosa; cytotoxicity was determined from nuclear uptake of the impermeant dye propidium iodide. The role of direct contact of P. aeruginosa to epithelial cells was tested with filters with small (0.45-micrometer) or large (2.0-micrometer) pores. High transepithelial resistance (R(t)) Calu-3 and cultured bovine tracheal monolayers (R(t) > 1,000 Omega. cm(2)) bound P. aeruginosa very infrequently (<1 P. aeruginosa/100 cells) at the apical membrane, but P. aeruginosa bound frequently to cells near "free edges" at holes, wounds, islands, and perimeters; cytotoxicity required direct interaction with basolateral membranes. Wounded high R(t) epithelia showed increased P. aeruginosa binding and cytotoxicity at the free edges because basolateral membranes were accessible to P. aeruginosa, and dead and living cells near the wound bound P. aeruginosa similarly. Compared with high R(t) epithelia, low R(t) CFT1 (R(t) = 100-200 Omega. cm(2)) and EGTA-treated Calu-3 monolayers were 25 times more susceptible to P. aeruginosa binding throughout the monolayer. Cytotoxicity to CFT1 cells (throughout the confluent monolayer, not only at the free edge) occurred after a shorter delay (0.25 vs. 2.0 h) and then five times faster than to Calu-3 cells, indicating that the time course of P. aeruginosa cytotoxicity may be limited by the rate of gaining access through tight junctions and that this occurred faster in low R(t) than in high R(t) airway epithelia. Cytotoxicity appeared to occur in a sequential process that led first to a loss of fura 2 and a later uptake of propidium iodide. P. aeruginosa bound three times more frequently to regions between cells (tight junctions?) than to cell membranes of low R(t) CFT1 cells.

Pili binding to asialo-GM1 on epithelial cells can mediate cytotoxicity or bacterial internalization by Pseudomonas aeruginosa

The interaction of Pseudomonas aeruginosa type IV pili and the glycosphingolipid asialo-GM1 (aGM1) can mediate bacterial adherence to epithelial cells, but the steps subsequent to this adherence have not been elucidated. To investigate the result of the interaction of pili and aGM1, we used polarized epithelial monolayers of Madin-Darby canine kidney (MDCK) cells in culture, which contained little detectable aGM1 on their apical surface but were able to incorporate exogenous aGM1. Compared to an untreated monolayer, P. aeruginosa PA103 displayed an eightfold increase in association with and fivefold more cytotoxicity toward MDCK cells pretreated with aGM1. Cytotoxicity of either carrier-treated or aGM1-treated monolayers required the type III secreted protein ExoU. Asialo-GM1 pretreatment of MDCK monolayers likewise augmented bacterial internalization of an isogenic invasive strain approximately fourfold. These increases were not seen in monolayers treated with GM1, the sialyated form of the glycolipid, and were inhibited by treatment with an antibody to aGM1. Also, the aGM1-mediated adhesion, cytotoxicity, and internalization required intact type IV pili since nonpiliated PA103 mutants were unaffected by aGM1 pretreatment of MDCK cells. These results demonstrate that epithelial cell injury and bacterial internalization can proceed from the same adhesin-receptor interaction, and they indicate that P. aeruginosa exoproducts solely determine the steps subsequent to adhesion.

Macrophages and epithelial cells respond differently to the Pseudomonas aeruginosa type III secretion system

The multiple effects of Pseudomonas aeruginosa type III secretion have largely been attributed to variations in cytotoxin expression between strains. Here we show that the target cell type is also important. While lung epithelial cells showed significant changes in morphology but not viability when infected with P. aeruginosa, macrophages were efficiently killed by P. aeruginosa. Both responses were dependent on the type III secretion system.

Expression of interleukin-6 in the cornea in response to infection with different strains of Pseudomonas aeruginosa

Strains of Pseudomonas aeruginosa causing keratitis can be either cytotoxic (6206) or invasive (6294), while a strain (Paer1) causing contact lens-induced acute red eye has been shown to be neither. In situ hybridization was used to examine the location and identity of cells expressing interleukin-6 (IL-6) mRNA in the murine cornea and changes in expression in response to infection with different strains of P. aeruginosa. The number of IL-6-positive cells was determined by image analysis. IL-6 protein levels were measured by an enzyme-linked immunosorbent assay. BALB/c mice were challenged by use of the wounded-cornea model with P. aeruginosa 6294, 6206, or Paer1 (2 x 10(6) CFU). At time intervals up to 24 h, postchallenge corneal tissue was probed for IL-6 mRNA. IL-6 mRNA expression was rapidly elevated in the epithelium in response to strains 6294 and 6206. At the conclusion of the experiments, infiltrating inflammatory cells also stained positively for IL-6 mRNA. In contrast, corneas challenged with strain Paer1 showed significant upregulation of IL-6 mRNA only at 4 h postchallenge. Three distinct patterns of IL-6 mRNA expression in the mouse cornea occur in response to these three ocular isolates of P. aeruginosa. The data obtained for mRNA expression in the cornea for all three strains of P. aeruginosa correlated well with IL-6 protein analysis of whole-eye homogenates. Differences in the cytokine responses to these strains correlate with differences in the pathology associated with each strain and may offer an opportunity to develop strategies for the improved management of ocular inflammation.

Biological effects of Pseudomonas aeruginosa type III-secreted proteins on CHO cells

A strain of Pseudomonas aeruginosa that fails to express known type III-secreted effector proteins was constructed as an expression host. Individual effectors were expressed in trans, and their biological effects on CHO cells were assessed in an acute cellular infection model. Intoxication with ExoS, ExoT, or ExoY resulted in alterations in cell morphology. As shown in previous genetic studies, ExoU expression was linked to acute cytotoxicity.

Active and passive immunization with the Pseudomonas V antigen protects against type III intoxication and lung injury

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that can cause fatal acute lung infections in critically ill individuals. Damage to the lung epithelium is associated with the expression of toxins that are directly injected into eukaryotic cells through a type Ill-mediated secretion and translocation mechanism. Here we show that the P. aeruginosa homolog of the Yersinia V antigen, PcrV, is involved in the translocation of type III toxins. Vaccination against PcrV ensured the survival of challenged mice and decreased lung inflammation and injury. Antibodies to PcrV inhibited the translocation of type III toxins.

Use of an Animal Model in Studies of Bacterial Corneal Infection

Despite medical advancements in available therapies, bacterial corneal infection frequently results in vision loss. Contact lens wear is a common predisposing factor for corneal infection; other reported risk factors are dry eye syndrome, blepharitis, trauma, and surgery. Both the immune defense against infection and the pathogenic mechanisms bacteria employ have been studied in vitro. However, there are complex interactions between the pathogen, the immune system, and the corneal tissue in vivo. Animal models allow the researcher to take the results of in vitro assays and validate their role in corneal infection in a living organism. A murine model is frequently used for studies of the pathogenesis of corneal infection caused by Pseudomonas aeruginosa. In this study, a modified scoring system is introduced that was designed to increase the information derived from this infection model. The new system includes evaluation of area, density, and surface characteristics of the ulceration. Results of in vitro experiments had previously indicated that ExsA, a transcriptional regulator of virulence-associated proteins, was important in pathogenesis of corneal infection caused by P. aeruginosa. Here we use the new scoring system to demonstrate in vivo that ExsA is involved.

Cystic fibrosis transmembrane conductance regulator-mediated corneal epithelial cell ingestion of Pseudomonas aeruginosa is a key component in the pathogenesis of experimental murine keratitis

Previous findings indicate that the cystic fibrosis transmembrane conductance regulator (CFTR) is a ligand for Pseudomonas aeruginosa ingestion into respiratory epithelial cells. In experimental murine keratitis, P. aeruginosa enters corneal epithelial cells. We determined the importance of CFTR-mediated uptake of P. aeruginosa by corneal cells in experimental eye infections. Entry of noncytotoxic (exoU) P. aeruginosa into human and rabbit corneal cell cultures was inhibited with monoclonal antibodies and peptides specific to CFTR amino acids 108 to 117. Immunofluorescence microscopy and flow cytometry demonstrated CFTR in the intact murine corneal epithelium, and electron microscopy showed that CFTR binds to P. aeruginosa following corneal cell ingestion. In experimental murine eye infections, multiple additions of 5 nM CFTR peptide 103-117 to inocula of either cytotoxic (exoU+) or noncytotoxic P. aeruginosa resulted in large reductions in bacteria in the eye and markedly lessened eye pathology. Compared with wild-type C57BL/6 mice, heterozygous DeltaF508 Cftr mice infected with P. aeruginosa had an approximately 10-fold reduction in bacterial levels in the eye and consequent reductions in eye pathology. Homozygous DeltaF508 Cftr mice were nearly completely resistant to P. aeruginosa corneal infection. CFTR-mediated internalization of P. aeruginosa by buried corneal epithelial cells is critical to the pathogenesis of experimental eye infection, while in the lung, P. aeruginosa uptake by surface epithelial cells enhances P. aeruginosa clearance from this tissue.

Identification and characterization of SpcU, a chaperone required for efficient secretion of the ExoU cytotoxin

In recent studies, we have shown that Pseudomonas aeruginosa strains that are acutely cytotoxic in vitro damage the lung epithelium in vivo. Genetic analysis indicated that the factor responsible for acute cytotoxicity was controlled by ExsA and therefore was part of the exoenzyme S regulon. The specific virulence determinant responsible for epithelial damage in vivo and cytotoxicity in vitro was subsequently mapped to the exoU locus. The present studies are focused on a genetic characterization of the exoU locus. Northern blot analyses and complementation experiments indicated that a region downstream of exoU was expressed and that the expression of this region corresponded to increased ExoU secretion. DNA sequence analysis of a region downstream of exoU identified several potential coding regions. One of these open reading frames, SpcU (specific Pseudomonas chaperone for ExoU), encoded a small 15-kDa acidic protein (137 amino acids [pI 4.4]) that possessed a leucine-rich motif associated with the Syc family of cytosolic chaperones for the Yersinia Yops. T7 expression analysis and nickel chromatography of histidine-tagged proteins indicated that ExoU and SpcU associated as a noncovalent complex when coexpressed in Escherichia coli. The association of ExoU and SpcU required amino acids 3 to 123 of ExoU. In P. aeruginosa, ExoU and SpcU are coordinately expressed as an operon that is controlled at the transcriptional level by ExsA.

ExoY, an adenylate cyclase secreted by the Pseudomonas aeruginosa type III system

The exoenzyme S regulon is a set of coordinately regulated virulence genes of Pseudomonas aeruginosa. Proteins encoded by the regulon include a type III secretion and translocation apparatus, regulators of gene expression, and effector proteins. The effector proteins include two enzymes with ADP-ribosyltransferase activity (ExoS and ExoT) and an acute cytotoxin (ExoU). In this study, we identified ExoY as a fourth effector protein of the regulon. ExoY is homologous to the extracellular adenylate cyclases of Bordetella pertussis (CyaA) and Bacillus anthracis (EF). The homology among the three adenylate cyclases is limited to two short regions, one of which possesses an ATP-binding motif. In assays for adenylate cyclase activity, recombinant ExoY (rExoY) catalyzed the formation of cAMP with a specific activity similar to the basal activity of CyaA. In contrast to CyaA and EF, rExoY activity was not stimulated or activated by calmodulin. A 500-fold stimulation of activity was detected following the addition of a cytosolic extract from Chinese hamster ovary (CHO) cells. These results indicate that a eukaryotic factor, distinct from calmodulin, enhances rExoY catalysis. Site-directed mutagenesis of residues within the putative active site of ExoY abolished adenylate cyclase activity. Infection of CHO cells with ExoY-producing strains of P. aeruginosa resulted in the intracellular accumulation of cAMP. cAMP accumulation within CHO cells depended on an intact type III translocation apparatus, demonstrating that ExoY is directly translocated into the eukaryotic cytosol.

Pseudomonas aeruginosa induces changes in fluid transport across airway surface epithelia

Fluid transport across cultures of bovine tracheal epithelium was measured with a capacitance probe technique. Baseline fluid absorption (Jv) across bovine cells of 3.2 microliter. cm-2. h-1 was inhibited by approximately 78% after 1 h of exposure to suspensions of Pseudomonas aeruginosa, with a concomitant decrease in transepithelial potential (TEP) and increase in transepithelial resistance (Rt). Effects of P. aeruginosa were blocked by amiloride, which decreased Jv by 112% from baseline of 2.35 +/- 1.25 microliter. cm-2. h-1, increased Rt by 101% from baseline of 610 +/- 257 Omega. cm2, and decreased TEP by 91% from baseline of -55 +/- 18.5 mV. Microelectrode studies suggested that effects of P. aeruginosa on amiloride-sensitive Na absorption were due in part to a block of basolateral membrane K channels. In the presence of Cl transport inhibitors [5-nitro-2-(3-phenylpropylamino)-benzoic acid, H2-DIDS, and bumetanide], P. aeruginosa induced a fluid secretion of approximately 2.5 +/- 0.4 microliter. cm-2. h-1 and decreased Rt without changing TEP. However, these changes were abolished when the transport inhibitors were used in a medium in which Cl was replaced by an impermeant organic anion. Filtrates of P. aeruginosa suspensions had no effect on Jv, TEP, or Rt. Mutants lacking exotoxin A or rhamnolipids or with defective lipopolysaccharide still inhibited fluid absorption and altered bioelectrical properties. By contrast, mutations in the rpoN gene encoding a sigma factor of RNA polymerase abolished actions of P. aeruginosa. In vivo, changes in transepithelial salt and water transport induced by P. aeruginosa may alter viscosity and ionic composition of airway secretions so as to foster further bacterial colonization.

Intracellular expression of the ADP-ribosyltransferase domain of Pseudomonas exoenzyme S is cytotoxic to eukaryotic cells

Exoenzyme S of Pseudomonas aeruginosa is an ADP-ribosyltransferase, which is secreted via a type III-dependent secretion mechanism and has been demonstrated to exert cytotoxic effects on eukaryotic cells. Alignment studies predict that the amino-terminus of exoenzyme S has limited primary amino acid homology with the YopE cytotoxin of Yersinia, while biochemical studies have localized the FAS-dependent ADP-ribosyltransferase activity to the carboxyl-terminus. Thus, exoenzyme S could interfere with host cell physiology via several independent mechanisms. The goal of this study was to define the role of the ADP-ribosyltransferase domain in the modulation of eukaryotic cell physiology. The carboxyl-terminal 222 amino acids of exoenzyme S, which represent the FAS-dependent ADP-ribosyltransferase domain (termed deltaN222), and a point mutant, deltaN222-E381A, which possesses a 2000-fold reduction in the capacity to ADP-ribosylate, were transiently expressed in eukaryotic cells under the control of the immediate early CMV promoter. Lysates from cells transfected with deltaN222 expressed ADP-ribosyltransferase activity. Co-transfection of deltaN222, but not deltaN222-E381A, resulted in a decrease in the steady-state levels of two reporter proteins, green fluorescent protein and luciferase, in both CHO and Vero cells. In addition, transfection with deltaN222 resulted in a greater percentage of cells staining with trypan blue than when cells were transfected with either deltaN222-E381A or control plasmid. Together, these data indicate that expression of the ADP-ribosyltransferase domain of exoenzyme S is cytotoxic to eukaryotic cells.

Functional conservation of the effector protein translocators PopB/YopB and PopD/YopD of Pseudomonas aeruginosa and Yersinia pseudotuberculosis

Virulent Yersinia species cause systemic infections in rodents, and Y. pestis is highly pathogenic for humans. Pseudomonas aeruginosa, on the other hand, is an opportunistic pathogen, which normally infects only compromised individuals. Surprisingly, these pathogens both encode highly related contact-dependent secretion systems for the targeting of toxins into eukaryotic cells. In Yersinia, YopB and YopD direct the translocation of the secreted Yop effectors across the target cell membrane. In this study, we have analysed the function of the YopB and YopD homologues, PopB and PopD, encoded by P. aeruginosa. Expression of the pcrGVHpopBD operon in defined translocation-deficient mutants (yopB/yopD) of Yersinia resulted in complete complementation of the cell contact-dependent, YopE-induced cytotoxicity of Y. pseudotuberculosis on HeLa cells. We demonstrated that the complementation fully restored the ability of Y. pseudotuberculosis to translocate the effector molecules YopE and YopH into the HeLa cells. Similar to YopB, PopB induced a lytic effect on infected erythrocytes. The lytic activity induced by PopB could be prevented if the erythrocytes were infected in the presence of sugars larger than 3 nm in diameter, indicating that PopB induced a pore of similar size compared with that induced by YopB. Our findings show that the contact-dependent toxin-targeting mechanisms of Y. pseudotuberculosis and P. aeruginosa are conserved at the molecular level and that the translocator proteins are functionally interchangeable. Based on these similarities, we suggest that the translocation of toxins such as ExoS, ExoT and ExoU by P. aeruginosa across the eukaryotic cell membrane occurs via a pore induced by PopB.

YopT, a new Yersinia Yop effector protein, affects the cytoskeleton of host cells

Extracellular Yersinia disarm the immune system of their host by injecting effector Yop proteins into the cytosol of target cells. Five effectors have been described: YopE, YopH, YpkA/YopO, YopP and YopM. Delivery of these effectors by Yersinia adhering at the cell surface requires other Yops (translocators) including YopB. Effector and translocator Yops are secreted by the type III Ysc secretion apparatus, and some Yops also need a specific cytosolic chaperone, called Syc. In this paper, we describe a new Yop, which we have called YopT (35.5kDa). Its secretion required an intact Ysc apparatus and SycT (15.0kDa, pl4.4), a new chaperone resembling SycE. Infection of macrophages with a Yersinia, producing a hybrid YopT-adenylate cyclase, led to the accumulation of intracellular cAMP, indicating that YopT is delivered into the cytosol of eukaryotic cells. Infection of HeLa cells with a mutant strain devoid of the five known Yop effectors (deltaHOPEM strain) but producing YopT resulted in the alteration of the cell cytoskeleton and the disruption of the actin filament structure. This cytotoxic effect was caused by YopT and dependent on YopB. YopT is thus a new effector Yop and a new bacterial toxin affecting the cytoskeleton of eukaryotic cells.

In vitro cellular toxicity predicts Pseudomonas aeruginosa virulence in lung infections

The role of quorum sensing by Pseudomonas aeruginosa in producing cytotoxicity has not been fully investigated. Strains of P. aeruginosa have been characterized as having an invasive or a cytotoxic phenotype (S. M. J. Fleiszig et al., Infect. Immun. 65:579-586, 1997). We noted that the application of a large inoculum of the invasive strain 6294 caused cytotoxicity of cultured epithelial cells. To investigate this dose-related cytotoxicity, we compared the behavior of 6294 to that of another invasive strain, PAO1, and determined whether the cytotoxicity could be related to quorum sensing. Both invasive strains, 6294 and PAO1, appear to have quorum-sensing systems that were operative when large doses of bacteria were applied to cultured lung epithelial cells or instilled into the lungs of animals. Nonetheless, only 6294 was cytotoxic. Cytotoxicity induced by 6294 correlated with increased elastase production. These experiments suggest that there are multiple mechanisms for the induction of cytotoxicity, pathology, and mortality in vivo. However, in vivo cytotoxicity and mortality, but not pathology, could be predicted by quantitative in vitro cellular damage experiments utilizing a range of bacteria-to-cell ratios. It appears that quorum sensing may inversely correlate with virulence in that strains that produced PAI [N-(3-oxododecanoyl) homoserine lactone] also appeared to attract more polymorphonuclear leukocytes in vivo and were possibly eliminated more quickly. In addition, exoproduct production in bacteriological medium in vitro may differ significantly from exoproduct expression from infections in vivo or during cocultivation of bacteria with tissue culture cells.

Modification of Ras in eukaryotic cells by Pseudomonas aeruginosa exoenzyme S

Genetic and functional data suggest that Pseudomonas aeruginosa exoenzyme S (ExoS), an ADP-ribosyltransferase, is translocated into eukaryotic cells by a bacterial type III secretory mechanism activated by contact between bacteria and host cells. Although purified ExoS is not toxic to eukaryotic cells, ExoS-producing bacteria cause reduced proliferation and viability, possibly mediated by bacterially translocated ExoS. To investigate the activity of translocated ExoS, we examined in vivo modification of Ras, a preferred in vitro substrate. The ExoS-producing strain P. aeruginosa 388 and an isogenic mutant strain, 388DeltaexoS, which fails to produce ExoS, were cocultured with HT29 colon carcinoma cells. Ras was found to be ADP-ribosylated during coculture with 388 but not with 388DeltaexoS, and Ras modification by 388 corresponded with reduction in HT29 cell DNA synthesis. Active translocation by bacteria was found to be required, since exogenous ExoS, alone or in the presence of 388DeltaexoS, was unable to modify intracellular Ras. Other ExoS-producing strains caused modification of Ras, indicating that this is not a strain-specific event. ADP-ribosylation of Rap1, an additional Ras family substrate for ExoS in vitro, was not detectable in vivo under conditions sufficient for Ras modification, suggesting possible ExoS substrate preference among Ras-related proteins. These results confirm that intracellular Ras is modified by bacterially translocated ExoS and that the inhibition of target cell proliferation correlates with the efficiency of Ras modification.

Type III protein secretion systems in bacterial pathogens of animals and plants

Various gram-negative animal and plant pathogens use a novel, sec-independent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion systems inject (translocate) proteins into the cytosol of eukaryotic cells, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and other cellular processes. Accordingly, some type III secretion systems are activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pathogenesis phenotypes. In contrast to the secreted virulence factors, most of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion apparatus show additional homologies to flagellar biosynthetic proteins, while a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones which specifically bind to individual secreted proteins play an important role in type III protein secretion, apparently by preventing premature interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizontal genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental stimuli by networks of transcription factors. This review comprises a comparison of the structure, function, regulation, and impact on host cells of the type III secretion systems in the animal pathogens Yersinia spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimurium, enteropathogenic Escherichia coli, and Chlamydia spp. and the plant pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearum, Xanthomonas campestris, and Rhizobium spp.

Defects in type III secretion correlate with internalization of Pseudomonas aeruginosa by epithelial cells

Previous characterization of Pseudomonas aeruginosa clinical isolates has demonstrated an inverse correlation between cytotoxicity and internalization by epithelial cells. To further investigate this relationship, we tested PA103, a cytotoxic P. aeruginosa strain, and 33 isogenic noncytotoxic transposon mutants for internalization by Madin-Darby canine kidney cells. The majority of the mutants were not internalized, demonstrating that an inverse correlation between cytotoxicity and bacterial uptake by epithelial cells is not absolute. Six of the noncytotoxic mutants, however, demonstrated measurable levels of internalization by standard aminoglycoside exclusion assays even though internalization of wild-type strain PA103 was not detectable. All six had evidence of protein secretion defects involving two proteins, a 40-kDa protein and a 32-kDa protein. These proteins, designated PepB (for Pseudomonas exoprotein B) and PepD, respectively, each had characteristics of type III transported proteins. In addition, nucleotide sequencing studies demonstrated that PepB and PepD are homologs of YopB and YopD, respectively, type III secreted proteins of Yersinia spp. necessary for the translocation of effector molecules into the cytoplasmic compartment of eukaryotic cells. Thus, while many mutations in PA103 result in loss of cytotoxicity without an appreciable increase in internalization, defects in transport of type III secretion proteins PepB and PepD correlate with both loss of cytotoxicity and gain of internalization. These results are consistent with type III secretion of an inhibitor of internalization that requires PepB and PepD for translocation into the host cell.

Pseudomonas aeruginosa invasion and cytotoxicity are independent events, both of which involve protein tyrosine kinase activity

Pseudomonas aeruginosa clinical isolates exhibit invasive or cytotoxic phenotypes. Cytotoxic strains acquire some of the characteristics of invasive strains when a regulatory gene, exsA, that controls the expression of several extracellular proteins, is inactivated. exsA mutants are not cytotoxic and can be detected within epithelial cells by gentamicin survival assays. The purpose of this study was to determine whether epithelial cell invasion precedes and/or is essential for cytotoxicity. This was tested by measuring invasion (gentamicin survival) and cytotoxicity (trypan blue staining) of PA103 mutants deficient in specific exsA-regulated proteins and by testing the effect of drugs that inhibit invasion for their effect on cytotoxicity. A transposon mutant in the exsA-regulated extracellular factor exoU was neither cytotoxic nor invasive. Furthermore, several of the drugs that inhibited invasion did not prevent cytotoxicity. These results show that invasion and cytotoxicity are mutually exclusive events, inversely regulated by an exsA-encoded invasion inhibitor(s). Both involve host cell protein tyrosine kinase (PTK) activity, but they differ in that invasion requires Src family tyrosine kinases and calcium-calmodulin activity. PTK inhibitor drugs such as genistein may have therapeutic potential through their ability to block both invasive and cytotoxicity pathways via an action on the host cell.