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

The type III pseudomonal exotoxin U activates the c-Jun NH2-terminal kinase pathway and increases human epithelial interleukin-8 production

Microbial interactions with host cell signaling pathways are key determinants of the host cell response to infection. Many toxins secreted by bacterial type III secretion systems either stimulate or inhibit the host inflammatory response. We investigated the role of type III secreted toxins of the lung pathogen Pseudomonas aeruginosa in the inflammatory response of human respiratory epithelial cells to infection. Using bacteria with specific gene deletions, we found that interleukin-8 production by these cells was almost entirely dependent on bacterial type III secretion of exotoxin U (ExoU), a phospholipase, although other bacterial factors are involved. ExoU activated the c-Jun NH(2)-terminal kinase pathway, stimulating the phosphorylation and activation of mitogen-activated kinase kinase 4, c-Jun NH(2)-terminal kinase, and c-Jun. This in turn increased levels of transcriptionally competent activator protein-1. Although this pathway was dependent on the lipase activity of ExoU, it was independent of cell death. Activation of mitogen-activated kinase signaling by ExoU in this fashion is a novel mechanism by which a bacterial product can initiate a host inflammatory response, and it may result in increased epithelial permeability and bacterial spread.

Type III secretion-dependent modulation of innate immunity as one of multiple factors regulated by Pseudomonas aeruginosa RetS

Mutation of retS (rtsM) of Pseudomonas aeruginosa strain PA103 reduces its virulence in both ocular and respiratory murine models of infection. In vitro, retS mutants exhibit loss of the ExsA-regulated type III secretion system (TTSS), reduced twitching motility, and a decrease in association with, invasion of, and survival within corneal epithelial cells. In addition, transcription of multiple other virulence genes is positively and negatively affected by retS mutation. Since our published data show that ExoU and ExoT, the two TTSS effectors encoded by strain PA103, each confer virulence in this corneal model, we hypothesized that loss of virulence of retS mutants follows loss of type III secretion. Corneal pathology, bacterial colonization, and phagocyte infiltration were compared for wild-type PA103, retS mutants, and various TTSS mutants after infection with approximately 10(6) CFU bacteria. Results showed that either a retS or an exsA (TTSS) mutation delayed disease progression, as illustrated by reduced severity scores and colonization levels during the first 48 h postinfection. Surprisingly, retS mutant infections then became more severe than those involving exsA mutants. By day 7, colonization levels of retS mutants even surpassed those of wild-type bacteria (more than twofold, P = 0.028). Although retS mutants caused more severe opacification of central corneas than both the wild type and the exsA mutants, neither mutant caused the peripheral ring opacity commonly associated with wild-type infection, suggesting that the TTSS was involved. Histological experiments with retS and various TTSS mutants showed that ring opacification required ExoU but not ExoT and that it consisted of dense polymorphonuclear phagocyte infiltration at the corneal periphery and the absence of any cell type in the central cornea. These data suggest that these P. aeruginosa TTSS effectors have different effects on innate immunity and that RetS influences virulence beyond its effects on the TTSS.

Analysis of FimX, a phosphodiesterase that governs twitching motility in Pseudomonas aeruginosa

Type IV pili (Tfp) are polar surface structures of Pseudomonas aeruginosa required for twitching motility, biofilm formation and adherence. One protein required for the assembly of tfp is FimX, which possesses both GGDEF and EAL domains characteristic of diguanylate cyclases and phosphodiesterases respectively. In this work we demonstrate that FimX has phosphodiesterase activity towards bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP), but does not show diguanylate cyclase activity. Instead, the imperfect GGDEF domain of FimX likely serves to activate phosphodiesterase activity when bound to GTP, as has recently been described for the Caulobacter crescentus composite GGDEF-EAL protein, CC3396. Bacteria expressing FimX in which either the GGDEF or EAL domain is deleted or mutated have phenotypes indistinguishable from a DeltafimX strain, demonstrating the importance of both domains to function. Previous work has shown that FimX localizes to the bacterial pole. In this work we show that restriction of FimX to a single pole requires intact GGDEF and EAL domains. Deletion of the amino-terminal REC domain of FimX, which contains a putative polar localization signal, results in a protein that still supports intermediate levels of pilus assembly and function. RFP-FimXDeltaREC, unlike RFP-FimX, is no longer localized to the bacterial pole, while transmission electron microscopy shows that surface pili can originate from non-polar sites in this mutant. Although DeltafimX mutants show limited in vitro cytotoxicity, they are as virulent as the wild-type strain in a murine model of acute pneumonia.

Eukaryotic localization, activation and ubiquitinylation of a bacterial type III secreted toxin

Type III secretion is a widespread method whereby Gram-negative bacteria introduce toxins into eukaryotic cells. These toxins mimic or subvert a normal cellular process by interacting with a specific target, although how toxins reach their site of action is unclear. We set out to investigate the intracellular localization of a type III toxin of Pseudomonas aeruginosa called ExoU, which has phospholipase activity and requires a eukaryotic factor for activity. We found that ExoU is localized to the plasma membrane and undergoes modification within the cell by addition of two ubiquitin molecules at lysine-178. A region of five amino acids at position 679-683 near the C-terminus of the ExoU protein controls both membrane localization and ubiquitinylation. Site-directed mutagenesis identified a tryptophan at position 681 as crucial for these effects. We found that the same region at position 679-683 was also required for cell toxicity produced by ExoU as well as in vitro phospholipase activity. Localization of the phospholipase ExoU to the plasma membrane is thus required for activation and allows efficient utilization of adjacent substrate phospholipids.

Acquisition and evolution of the exoU locus in Pseudomonas aeruginosa

ExoU is a potent Pseudomonas aeruginosa cytotoxin translocated into host cells by the type III secretion system. A comparison of genomes of various P. aeruginosa strains showed that that the ExoU determinant is found in the same polymorphic region of the chromosome near a tRNA(Lys) gene, suggesting that exoU is a horizontally acquired virulence determinant. We used yeast recombinational cloning to characterize four distinct ExoU-encoding DNA segments. We then sequenced and annotated three of these four genomic regions. The sequence of the largest DNA segment, named ExoU island A, revealed many plasmid- and genomic island-associated genes, most of which have been conserved across a broad set of beta- and gamma-Proteobacteria. Comparison of the sequenced ExoU-encoding genomic islands to the corresponding PAO1 tRNA(Lys)-linked genomic island, the pathogenicity islands of strain PA14, and pKLC102 of clone C strains allowed us to propose a mechanism for the origin and transmission of the ExoU determinant. The evolutionary history very likely involved transposition of the ExoU determinant onto a transmissible plasmid, followed by transfer of the plasmid into different P. aeruginosa strains. The plasmid subsequently integrated into a tRNA(Lys) gene in the chromosome of each recipient, where it acquired insertion sequences and underwent deletions and rearrangements. We have also applied yeast recombinational cloning to facilitate a targeted mutagenesis of ExoU island A, further demonstrating the utility of the specific features of the yeast capture vector for functional analyses of genes on large horizontally acquired genetic elements.

Type III secretion system-associated toxins, proteases, serotypes, and antibiotic resistance of Pseudomonas aeruginosa isolates associated with keratitis

The association between possession of toxin gene-related type III secretory system, protease profiles, O serotypes, and antibiotic resistance patterns was characterized genetically and phenotypically in 46 keratitis isolates of Pseudomonas aeruginosa. There was no significant difference in exoU or exoS prevalence among the keratitis strains. Distinct protease profiles were seen in isolates harboring either exoU or exoS genes. One hundred percent (13/13) of serotype E (O:11) strains contained type III secretion system-associated cytotoxin gene exoU. Multidrug resistance was identified in 4% of Australian and 29% of Indian isolates. None of the Australian isolates was resistant to ciprofloxacin. In general, the rate of multidrug resistance in the exoU positive cytotoxic and serotype E (O:11) strains was significantly higher than in exoS positive invasive strains (p < 0.01). The results suggest that multidrug resistance may be more commonly associated with the corneal isolates of P. aeruginosa having type III secretion system-associated cytotoxin gene exoU and belonging to serotype E (O:11) group.

Regulation of pulmonary Pseudomonas aeruginosa infection by the transcriptional repressor Gfi1

Pulmonary Pseudomonas aeruginosa infections are characterized by the release of proinflammatory mediators, focal induction of apoptosis in respiratory epithelial cells and internalization of the bacteria. Here, we demonstrate that the transcriptional regulator Gfi1 is critically involved in the regulation of proinflammatory cytokine release and the induction of apoptosis in respiratory epithelial cells and macrophages upon P. aeruginosa infection. Gfi1-deficient mice responded to a pulmonary P. aeruginosa infection with uncontrolled pulmonary release of interleukin (IL)-1 and tumour necrosis factor (TNF)-alpha, sepsis and death, which were delayed by injection of IL-1- and TNF-alpha-neutralizing antibodies. The uncontrolled release of cytokines seems to be caused by a failure of Gfi1-deficient respiratory epithelial cells in large to small bronchi and macrophages to respond to P. aeruginosa infection with an induction of apoptosis. Pharmacological inhibition of apoptosis in wild-type mice by intravenous injection of the broad-spectrum caspase inhibitor zVAD-cmk mimicked the phenotype of Gfi1-deficient mice and resulted in a profound sensitization of mice to P. aeruginosa, an increased release of cytokines, sepsis and death of the animals. Thus, Gfi1 controls apoptosis of respiratory epithelial cells and macrophages upon infection with P. aeruginosa. Inhibition of apoptosis by Gfi1 deficiency or caspase blockers sensitizes mice to P. aeruginosa infections, suggesting that apoptosis functions as a novel defence mechanisms in the regulation of the local innate immune response.

Members of a Legionella pneumophila family of proteins with ExoU (phospholipase A) active sites are translocated to target cells

Legionella pneumophila replicates within alveolar macrophages, causing a severe pneumonia termed Legionnaires' disease. The bacterium resides within a vacuole that escapes immediate transport to the host lysosome. Instead, the vacuole interacts with the early secretory pathway to establish an environment suitable for rapid multiplication. A type IV secretion system is central to the pathogenicity of the bacterium, and many protein substrates that are translocated by this system to the host cell have been identified. One of these, VipD, was found to interrupt the late secretory pathway when overproduced in Saccharomyces cerevisiae. We independently identified VipD in a previous study and have further characterized this protein as well as its three paralogs. The vipD gene belongs to a family of L. pneumophila open reading frames that are predicted to contain a phospholipase A domain with sequence similarity to the type III-secreted toxin ExoU from Pseudomonas aeruginosa. Similarly to other known translocated proteins of L. pneumophila, VipD is strongly induced in early stationary phase, a time when the bacterium is most virulent. Detergent extraction studies of infected macrophages confirm that VipD is translocated into host cells via the type IV secretion system. A second assay for translocation revealed that two paralogs of VipD, VpdA and VpdB, also have translocation signals recognized by the type IV system. A strain lacking VipD and its three paralogs grew at wild-type rates in murine macrophages, although secondary mutations that cause growth defects in strains lacking VipD accumulate. The quadruple mutant displayed a growth advantage in the amoebal host Dictyostelium discoideum, indicating that the protein family may modulate intracellular growth in a complex fashion. VipD is mildly toxic when overproduced in eukaryotic cells, and the toxicity is partially dependent on the putative phospholipase active site. VipD and its paralogs therefore define a family of translocated proteins that may assist in the establishment of a vacuole suitable for bacterial replication through functioning as a phospholipase.

Identification of multi-drug resistant Pseudomonas aeruginosa clinical isolates that are highly disruptive to the intestinal epithelial barrier

Background

Multi-drug resistant Pseudomonas aeruginosa nosocomial infections are increasingly recognized worldwide. In this study, we focused on the virulence of multi-drug resistant clinical strains P. aeruginosa against the intestinal epithelial barrier, since P. aeruginosa can cause lethal sepsis from within the intestinal tract of critically ill and immuno-compromised patients via mechanisms involving disruption of epithelial barrier function.

Methods

We screened consecutively isolated multi-drug resistant P. aeruginosa clinical strains for their ability to disrupt the integrity of human cultured intestinal epithelial cells (Caco-2) and correlated these finding to related virulence phenotypes such as adhesiveness, motility, biofilm formation, and cytotoxicity.

Results

Results demonstrated that the majority of the multi-drug resistant P. aeruginosa clinical strains were attenuated in their ability to disrupt the barrier function of cultured intestinal epithelial cells. Three distinct genotypes were found that displayed an extreme epithelial barrier-disrupting phenotype. These strains were characterized and found to harbor the exoU gene and to display high swimming motility and adhesiveness.

Conclusion

These data suggest that detailed phenotypic analysis of the behavior of multi-drug resistant P. aeruginosa against the intestinal epithelium has the potential to identify strains most likely to place patients at risk for lethal gut-derived sepsis. Surveillance of colonizing strains of P. aeruginosa in critically ill patients beyond antibiotic sensitivity is warranted.

A C-terminal domain targets the Pseudomonas aeruginosa cytotoxin ExoU to the plasma membrane of host cells

ExoU, a phospholipase injected into host cells by the type III secretion system of Pseudomonas aeruginosa, leads to rapid cytolytic cell death. Although the importance of ExoU in infection is well established, the mechanism by which this toxin kills host cells is less clear. To gain insight into how ExoU causes cell death, we examined its subcellular localization following transfection or type III secretion/translocation into HeLa cells. Although rapid cell lysis precluded visualization of wild-type ExoU by fluorescence microscopy, catalytically inactive toxin was readily detected at the periphery of HeLa cells. Biochemical analysis confirmed that ExoU was targeted to the membrane fraction of transfected cells. Visualization of ExoU peptides fused with green fluorescent protein indicated that the domain responsible for this targeting was in the C terminus of ExoU, between residues 550 and 687. Localization to the plasma membrane occurred within 1 h of expression, which is consistent with the kinetics of cytotoxicity. Together, these results indicate that a domain between residues 550 and 687 of ExoU targets this toxin to the plasma membrane, a process that may be important in cytotoxicity.

Interactions between effector proteins of the Pseudomonas aeruginosa type III secretion system do not significantly affect several measures of disease severity in mammals

The effector proteins of the type III secretion systems of many bacterial pathogens act in a coordinated manner to subvert host cells and facilitate the development and progression of disease. It is unclear whether interactions between the type-III-secreted proteins of Pseudomonas aeruginosa result in similar effects on the disease process. We have previously characterized the contributions to pathogenesis of the type-III-secreted proteins ExoS, ExoT and ExoU when secreted individually. In this study, we extend our prior work to determine whether these proteins have greater than expected effects on virulence when secreted in combination. In vitro cytotoxicity and anti-internalization activities were not enhanced when effector proteins were secreted in combinations rather than alone. Likewise in a mouse model of pneumonia, bacterial burden in the lungs, dissemination and mortality attributable to ExoS, ExoT and ExoU were not synergistically increased when combinations of these effector proteins were secreted. Because of the absence of an appreciable synergistic increase in virulence when multiple effector proteins were secreted in combination, we conclude that any cooperation between ExoS, ExoT and ExoU does not translate into a synergistically significant enhancement of disease severity as measured by these assays.

The global regulatory proteins LetA and RpoS control phospholipase A, lysophospholipase A, acyltransferase, and other hydrolytic activities of Legionella pneumophila JR32

Legionella pneumophila possesses a variety of secreted and cell-associated hydrolytic activities that could be involved in pathogenesis. The activities include phospholipase A, lysophospholipase A, glycerophospholipid:cholesterol acyltransferase, lipase, protease, phosphatase, RNase, and p-nitrophenylphosphorylcholine (p-NPPC) hydrolase. Up to now, there have been no data available on the regulation of the enzymes in L. pneumophila and no data at all concerning the regulation of bacterial phospholipases A. Therefore, we used L. pneumophila mutants in the genes coding for the global regulatory proteins RpoS and LetA to investigate the dependency of hydrolytic activities on a global regulatory network proposed to control important virulence traits in L. pneumophila. Our results show that both L. pneumophila rpoS and letA mutants exhibit on the one hand a dramatic reduction of secreted phospholipase A and glycerophospholipid:cholesterol acyltransferase activities, while on the other hand secreted lysophospholipase A and lipase activities were significantly increased during late logarithmic growth phase. The cell-associated phospholipase A, lysophospholipase A, and p-NPPC hydrolase activities, as well as the secreted protease, phosphatase, and p-NPPC hydrolase activities were significantly decreased in both of the mutant strains. Only cell-associated phosphatase activity was slightly increased. In contrast, RNase activity was not affected. The expression of plaC, coding for a secreted acyltransferase, phospholipase A, and lysophospholipase A, was found to be regulated by LetA and RpoS. In conclusion, our results show that RpoS and LetA affect phospholipase A, lysophospholipase A, acyltransferase, and other hydrolytic activities of L. pneumophila in a similar way, thereby corroborating the existence of the LetA/RpoS regulation cascade.

Adenoviral gene transfer of a mutant surfactant enzyme ameliorates pseudomonas-induced lung injury

Surfactant deficiency is an important contributor to the acute respiratory distress syndrome, a disorder that commonly occurs after bacterial sepsis. CTP:phosphocholine cytidylyltransferase (CCTalpha) is the rate-limiting enzyme required for the biosynthesis of dipalmitoylphosphatidylcholine (DPPC), the major phospholipid of surfactant. In this study, a cDNA encoding a novel, calpain-resistant mutant CCTalpha enzyme was delivered intratracheally in mice using a replication-deficient adenovirus 5 CTP:phosphocholine cytidylyltransferase construct (Ad5-CCT(Penta)) in models of bacterial sepsis. Ad5-CCT(Penta) gene transfer produced high-level CCTalpha gene expression, increased alveolar surfactant (DPPC) levels and improved lung surface tension and pressure-volume relationships relative to control mice. Pseudomonas aeruginosa (PA103) decreased DPPC synthesis, in part, via calpain-mediated degradation of CCTalpha. Deleterious effects of Pseudomonas on surfactant were lessened after infection with a mutant strain lacking the type III exotoxin, Exo U. Replication-deficient adenovirus 5 CTP:phosphocholine cytidylyltransferase gene delivery improved lung biophysical properties by optimizing surface activity in this Pseudomonas model of proteinase-mediated lung injury. The studies are the first demonstration of in vivo gene transfer of a lipogenic enzyme resulting in improved lung mechanics. The studies suggest that augmentation of DPPC synthesis via gene delivery of CCTalpha can attenuate impaired lung function in surfactant-deficient states such as bacterial sepsis.

Impact of quorum sensing on fitness of Pseudomonas aeruginosa

In Pseudomonas aeruginosa, cell-cell communication based on N-acyl-homoserine lactone (AHL) signal molecules (termed quorum sensing) is known to control the production of extracellular virulence factors. Hence, in pathogenic interactions with host organisms, the quorum-sensing (QS) machinery can confer a selective advantage on P. aeruginosa. However, as shown by transcriptomic and proteomic studies, many intracellular metabolic functions are also regulated by quorum sensing. Some of these serve to regenerate the AHL precursors methionine and S-adenosyl-methionine and to degrade adenosine via inosine and hypoxanthine. The fact that a significant percentage of clinical and environmental isolates of P. aeruginosa is defective for QS because of mutation in the major QS regulatory gene lasR, raises the question of whether the QS machinery can have a negative impact on the organism's fitness. In vitro, lasR mutants have a higher probability to escape lytic death in stationary phase under alkaline conditions than has the QS-proficient wild type. Similar selective forces might also operate in natural environments.

PsrA is a positive transcriptional regulator of the type III secretion system in Pseudomonas aeruginosa

The type III secretion system (TTSS) of Pseudomonas aeruginosa is induced in vivo upon contact with eukaryotic cells and in vitro by calcium depletion in culture medium. We have observed a previously identified protein, PsrA, necessary for full activation of TTSS gene expression in P. aeruginosa. Electrophoretic mobility shift assays showed that recombinant PsrA could bind to the exsCEBA promoter region. A mutant with a deletion in the psrA gene was constructed. Using transcriptional fusions, we demonstrated that PsrA is required for the full activation of transcription of the TTSS regulatory operon exsCEBA and effector exoS, although the deletion mutant still responded to calcium depletion, to serum, and to host cell contact. The psrA mutant showed a marked decrease in the secretion of the type III effectors and weak resistance to phagocyte-like PLB-985 cells. The defect in TTSS transcription and secretion in the psrA mutant could be complemented by expression in trans of psrA. PsrA was previously identified as a transcriptional activator of RpoS, a central regulator during stationary phase. We confirmed with our strain that RpoS has a negative effect on TTSS gene expression. Taken altogether, these results suggest that PsrA is a newly identified activator that is involved in the expression of the TTSS by enhancing the exsCEBA transcriptional level.

Targeting mechanisms of Pseudomonas aeruginosa pathogenesis

Pseudomonas aeruginosa is an opportunistic pathogen responsible for ventilator-acquired pneumonia, acute lower respiratory tract infections in immunocompromised patients and chronic respiratory infections in cystic fibrosis patients. High incidence, infection severity and increasing resistance characterize P. aeruginosa infections, highlighting the need for new therapeutic options. One such option is to target the many pathogenic mechanisms conferred to P. aeruginosa by its large genome encoding many different virulence factors. This article reviews the pathogenic mechanisms and potential therapies targeting these mechanisms in P. aeruginosa respiratory infections.

Eicosanoid-mediated proinflammatory activity of Pseudomonas aeruginosa ExoU

As Pseudomonas aeruginosa ExoU possesses two functional blocks of homology to calcium-independent (iPLA(2)) and cytosolic phospholipase A(2) (cPLA(2)), we addressed the question whether it would exhibit a proinflammatory activity by enhancing the synthesis of eicosanoids by host organisms. Endothelial cells from the HMEC-1 line infected with the ExoU-producing PA103 strain exhibited a potent release of arachidonic acid (AA) that could be significantly inhibited by methyl arachidonyl fluorophosphonate (MAFP), a specific PLA(2) inhibitor, as well as significant amounts of the cyclooxygenase (COX)-derived prostaglandins PGE(2) and PGI(2). Cells infected with an isogenic mutant defective in ExoU synthesis did not differ from non-infected cells in the AA release and produced prostanoids in significantly lower concentrations. Infection by PA103 induced a marked inflammatory response in two different in vivo experimental models. Inoculation of the parental bacteria into mice footpads led to an early increase in the infected limb volume that could be significantly reduced by inhibitors of both COX and lipoxygenase (ibuprofen and NDGA respectively). In an experimental respiratory infection model, bronchoalveolar lavage (BAL) from mice instilled with 10(4) cfu of PA103 exhibited a marked influx of inflammatory cells and PGE(2) release that could be significantly reduced by indomethacin, a non-selective COX inhibitor. Our results suggest that ExoU may contribute to P. aeruginosa pathogenesis by inducing an eicosanoid-mediated inflammatory response of host organisms.

An indirect enzyme-linked immunosorbent assay for rapid and quantitative assessment of Type III virulence phenotypes of Pseudomonas aeruginosa isolates

Background

The presence of a Type III secretion system in clinical isolates of Pseudomonas aeruginosa is associated with severe disease and poor outcomes in infections caused by this pathogen. We describe an indirect enzyme-linked immunosorbent assay that rapidly and quantitatively detects two exotoxins, ExoU and ExoT, and two structural components, PopD and PcrV, of the P. aeruginosa Type III secretion system after in-vitro growth in a calcium-free minimal medium.

Methods

We used this assay to characterize the Type III secretion phenotype of 74 clinical isolates of P. aeruginosa. Findings were compared with results of standard immunoblotting and correlated with Type III secretion-dependent virulence of isolates toward cultured epithelial cells.

Results

Results of the ELISA assay were concordant with immunoblot detection of the secreted antigens for 73 of 74 isolates. The Type III secretion phenotype assessed by this immunoassay predicted bacterial virulence toward epithelial cells in vitro for all but five of the clinical isolates.

Conclusion

The availability of an ELISA assay for rapid detection of Type III secreted virulence factors will facilitate large clinical studies to examine whether the Type III secretion phenotype of a P. aeruginosa isolate predicts the course of clinical disease in a patient and should be taken into account in determining optimal treatment strategies for infected patients.

Factors triggering type III secretion in Pseudomonas aeruginosa

The type III secretion system of Pseudomonas aeruginosa is tightly regulated by various environmental signals, such as low calcium and contact with the host cell. However, the exact signals triggering type III secretion are unknown. The present study describes the finding that secretion of P. aeruginosa type III effector molecules requires protein factors from serum and L broth, designated type III secretion factors (TSFs), in addition to the low-calcium environment. In the absence of TSF or calcium chelator EGTA, basal levels of type III effector molecules are accumulated intracellularly. Addition of TSF and EGTA together effectively triggers the secretion of pre-existing effector molecules in a short time, even before the active expression of type III genes; thus, active type III gene expression does not seem to be a prerequisite for type III secretion. A search for TSF molecules in serum and L broth resulted in the identification of albumin and casein as the functional TSF molecules. Although there is no clear sequence similarity between albumin and casein, both proteins are known to have a low-affinity, high-capacity calcium-binding property. Tests of well-studied calcium-binding proteins seemed to indicate that low-affinity calcium-binding proteins have TSF activity, although the requirement of low-affinity calcium-binding ability for the TSF activity is not clear. P. aeruginosa seems to have evolved a sensing mechanism to detect target cells for type III injection through host-derived proteins in combination with a low-calcium signal. Disruption of the bacterial ability to sense low calcium or TSF might be a valid avenue to the effective control of this bacterial pathogen.

TLRs 2 and 4 are not involved in hypersusceptibility to acute Pseudomonas aeruginosa lung infections

TLRs are implicated in defense against microorganisms. Animal models have demonstrated that the susceptibility to a number of Gram-negative pathogens is linked to TLR4, and thus LPS of many Gram-negative bacteria have been implicated as virulence factors. To assess the role of this pathogen-associated molecular pattern as it is exposed on intact Pseudomonas aeruginosa, the susceptibility of mice lacking TLR4 or both TLR2 and TLR4 was examined in a model of acute Pseudomonas pneumonia. These mutant mice were not hypersusceptible to the Pseudomonas challenge and mounted an effective innate response that cleared the organism despite low levels of TNF-alpha and KC in the airways. Bacterial and neutrophil counts in the lung were similar in control and TLR-deficient mice at 6 and 24 h after infection. MyD88(-/-) mice were, however, hypersusceptible, with 100% of mice dying within 48 h with a lower dose of P. aeruginosa. Of note there were normal levels of IL-6 and G-CSF in the airways of TLR mutant mice that were absent from the MyD88(-/-) mice. Thus, the susceptibility of mice to P. aeruginosa acute lung infection does not go through TLR2 or TLR4, implying that Pseudomonas LPS is not the most important virulence factor in acute pneumonia caused by this organism. Furthermore, G-CSF treatment of infected MyD88(-/-) mice results in improved clearance and survival. Thus, the resistance to infection in TLR2/TLR4(-/-) mice may be linked to G-CSF and possibly IL-6 production.

Pseudomonas aeruginosa: the potential to immunise against infection

Pseudomonas aeruginosa remains a serious pathogen for specific cohorts of patients where chronic infection is a poor prognostic indicator, such as those with cystic fibrosis, burn wounds or those who are immunocompromised. Significant disease burden is associated with a diverse spectrum of both nosocomial and community-acquired infections. To date, vaccines against P. aeruginosa have shown limited and often conflicting efficacy data, especially against heterologous strains, which are increasingly identified as co-colonisers of biofilms. While few studies have gone beyond Phase II clinical trials, a particular concern is the ability of P. aeruginosa to evade the immune system while provoking an immune response that contributes to the destructive nature of infection. Therefore, vaccine development needs to focus on preventing attachment and colonisation, as well as preventing conversion to a mucoid phenotype that is characteristic of the chronic condition that promotes pathology.

Type III secretion: more systems than you think

The type III secretion (T3S) pathway allows bacteria to inject effector proteins into the cytosol of target animal or plant cells. T3S systems evolved into seven families that were distributed among Gram-negative bacteria by horizontal gene transfer. There are probably a few hundred effectors interfering with control and signaling in eukaryotic cells and offering a wealth of new tools to cell biologists.

Actin cytoskeleton disruption by ExoY and its effects on Pseudomonas aeruginosa invasion

Three of the Type III-secreted effectors of Pseudomonas aeruginosa (ExoS, ExoT, and ExoY) each alter mammalian cell morphology in culture without causing a loss of cell viability. For ExoS and ExoT this property involves RhoGAP activity, and leads to actin cytoskeleton disruption and a reduced capacity for internalizing bacteria. ExoY does not possess RhoGAP activity. Instead, cell rounding depends upon its adenylate cyclase catalytic region. Since anti-phagocytic activities of ExoS and ExoT are associated with cell rounding and cytoskeleton disruption, we hypothesized that ExoY would also inhibit P. aeruginosa invasion of epithelial cells coinciding with adenylate cyclase-mediated cytoskeleton disruption. The results showed actin disruption of epithelial cells at 2 h post-infection associated with both adenylate cyclase-active ExoY and its catalytic mutant form ExoYK81M, and which coincided with inhibition of bacterial invasion (76% inhibition by ExoY, and 37% by ExoYK81M). Surprisingly, at 4h post-infection, neither form of ExoY inhibited invasion despite extensive actin disruption. These data suggest that ExoY, like ExoS and ExoT, contains more than one active domain affecting mammalian cell function. The data also suggest that cytoskeleton disruption does not necessarily predict invasion inhibitory activity, supporting the recently proposed model that P. aeruginosa internalization can proceed through more than one pathway.

The type III cytotoxins of Yersinia and Pseudomonas aeruginosa that modulate the actin cytoskeleton

Initial studies of how bacterial toxins modulate the actin cytoskeleton have focused primarily on the mode of action of these toxins. More recently, studies have addressed the molecular interactions of these toxins with host cell signaling pathways and how toxins modulate cellular physiology. Although each individual toxin has a unique mode of action, general themes have started to emerge between bacterial pathogens. During the course of an infection, many pathogenic bacteria produce toxins that target the actin cytoskeleton and its regulatory proteins. Toxins can either act as positive regulators promoting the assembly of filamentous actin structures or, alternatively, as negative regulators promoting actin filament disassembly. Modulation of the actin cytoskeleton facilitates various infectious processes critical for the success of the pathogen. Intracellular bacteria such as Salmonella typhimurium utilize toxins to promote both assembly and disassembly of the actin cytoskeleton during the infection process. Temporal regulation of toxin activities results in internalization of the bacterium by epithelial cells into specialized vacuoles permissive for growth. In contrast, Yersinia utilizes actin modulating toxins to block internalization by professional antigen-presenting cells such as macrophages and dendritic cells. Modulation of the immune response through the production of actin-regulating toxins appears to be a common approach adopted by several extracellular pathogens. Thus the repertoire of actin-modifying toxins produced by various species is specifically tailored to facilitate the lifestyle of the pathogen. The presence of multiple toxins that modulate the activation state of actin shows the importance of interfering with the cytoskeleton to neutralize the host's innate immune system for the survival and growth of Yersinia and P. aeruginosa.

Implications of oxidative stress in the cytotoxicity of Pseudomonas aeruginosa ExoU

ExoU PLA2-like activity has been shown to account for membrane lysis and acute death of infected cells. Translocation of effector proteins by the type III secretion systems depends on close contact between microbial and host cells. Our finding that both the ExoU-producing PA103 Pseudomonas aeruginosa and its mutant obtained by deletion of exoU adhered poorly to endothelial cells (EC) led to the hypothesis that, in some cells, the amount of injected toxin may not be enough to induce cell lysis but cells would suffer from a long-term effect of ExoU intoxication. To address this question, cells were exposed to both bacteria for 1 h and then treated with gentamicin-containing medium, to eliminate infecting microorganisms. After 24 h, the percentage of viable EC in PA103-infected cultures was significantly lower than in cultures exposed to the mutant, as determined by the MTT assay. Cell death was not likely to depend on the ExoU lytic activity since cell labeling with propidium iodide was similar in cultures infected with both bacterial strains. Bacterial cytotoxicity was significantly reduced by MAFP, a specific inhibitor of cPLA2 and iPLA2. Since the PLA2 activity on membrane phospholipids generates free fatty acid, including arachidonic acid (AA), we next compared the bacterial ability to release AA from infected EC. PA103 was shown to induce a potent AA release that was inhibited by MAFP. AA oxidation by oxygenases generates eicosanoids, known to induce both cell death and proliferation. However neither inhibitors of cyclooxygenases (ibuprofen) nor lipoxygenases (NDGA) reduced the ExoU toxicity. Since non-enzymatic oxidation of AA generates reactive radicals, we next investigated the PA103 ability to induce oxidative stress in infected cells. FACS analysis of cell labeling with the C-11 fluor probe and with anti-4-hydroxynonel antibody revealed a significant peroxidation of cell membrane lipids. These results, together with our finding that PA103-infected EC death was significantly attenuated by alpha-tocopherol, led to the conclusion that AA-induced oxidative stress may be another mechanism of cell damage in the course of infection by ExoU-producing P. aeruginosa.

A novel sensor kinase-response regulator hybrid regulates type III secretion and is required for virulence in Pseudomonas aeruginosa

The type III secretion system (TTSS) of Pseudomonas aeruginosa is induced by contact with eukaryotic cells and by growth in low-calcium media. We have identified a protein, RtsM, that is necessary for expression of the TTSS genes in P. aeruginosa. RtsM possesses both histidine kinase and response regulator domains common to two-component signalling proteins, as well as a large predicted periplasmic domain and seven transmembrane domains. Deletion of rtsM resulted in a defect in production and secretion of the type III effectors. Northern blot analysis revealed that mRNAs encoding the effectors ExoT and ExoU are absent in the DeltartsM strain under TTSS-inducing conditions. Using transcriptional fusions, we demonstrated that RtsM is required for transcription of the operons encoding the TTSS effectors and apparatus in response to calcium limitation or to host cell contact. The operon encoding the TTSS regulator ExsA does not respond to calcium limitation, but the basal transcription rate of this operon was lower in deltartsM than in the wild-type parent, PA103. The defect in TTSS effector production and secretion of deltartsM could be complemented by overexpressing ExsA or Vfr, two transcriptional activators involved in TTSS regulation. DeltartsM was markedly less virulent than PA103 in a murine model of acute pneumonia, demonstrating that RtsM is required in vivo. We propose that RtsM is a sensor protein at the start of a signalling cascade that induces expression of the TTSS in response to environmental signals.

Genotypic and phenotypic characteristics of Pseudomonas aeruginosa isolates associated with ulcerative keratitis

A collection of 63 isolates of Pseudomonas aeruginosa associated with ulcerative keratitis, collected from six centres in England, were typed using serotyping and random amplified polymorphic DNA-PCR, and screened for several variable virulence-related genotypes and phenotypes. Sixty-one percent of the isolates were of either serotype O1 or serotype O11, but there was no evidence for a common clone. The majority of isolates (59%) were PCR-positive for exoU rather than for exoS (38%), and carried a-type fliC genes (76%) rather than b-type (24%). Isolates were PCR-positive for pyoverdine-receptor types at a prevalence of 38% for type I, 46 % for type II and 8 % for type III. All but one of the isolates exhibited twitching activity. There was a correlation between the presence of exoS and twitching activity (P = 0.04), suggesting that a combination of exoS genotype and good twitching activity may have a role to play in ExoU-independent corneal virulence.

Alveolar response to Pseudomonas aeruginosa: role of the type III secretion system

The type III secretion system (TTSS) is a specialized cytotoxin-translocating apparatus of gram-negative bacteria which is involved in lung injury, septic shock, and a poor patient outcome. Recent studies have attributed these effects mainly to the ExoU effector protein. However, few studies have focused on the ExoU-independent pathogenicity of the TTSS. For the present study, we compared the pathogenicities of two strains of Pseudomonas aeruginosa in a murine model of acute lung injury. We compared the CHA strain, which has a functional TTSS producing ExoS and ExoT but not ExoU, to an isogenic mutant with an inactivated exsA gene, CHA-D1, which does not express the TTSS at all. Rats challenged with CHA had significantly increased lung injury, as assessed by the wet/dry weight ratio for the lungs and the protein level in bronchoalveolar lavage fluid (BALF) at 12 h, compared to those challenged with CHA-D1. Consistent with these findings, the CHA strain was associated with increased in vitro cytotoxicity on A549 cells, as assessed by the release of lactate dehydrogenase. CHA was also associated at 12 h with a major decrease in polymorphonuclear neutrophils in BALF, with a proinflammatory response, as assessed by the amounts of tumor necrosis factor alpha and interleukin-1beta, and with decreased bacterial clearance from the lungs, ultimately leading to an increased mortality rate. These results demonstrate that the TTSS has a major role in P. aeruginosa pathogenicity independent of the role of ExoU. This report underscores the crucial roles of ExoS and ExoT or other TTSS-related virulence factors in addition to ExoU.

A secreted regulatory protein couples transcription to the secretory activity of the Pseudomonas aeruginosa type III secretion system

The type III secretion system (T3SS) of Pseudomonas aeruginosa is an important virulence determinant. Transcription of the T3SS is highly regulated and intimately coupled to the activity of the type III secretion channel. The secretion channel is generally closed, and transcription is repressed. Inducing signals such as calcium depletion, however, open the secretion channel and derepress transcription of the T3SS. The coupling of transcription with secretion requires three previously identified cytoplasmic regulatory proteins. ExsA is a DNA-binding protein required for transcriptional activation of the entire T3SS. The second regulatory protein, ExsD, functions as anti-activator by directly binding to ExsA. Finally, ExsC functions as an anti-anti-activator by directly binding to and inhibiting ExsD. Although the regulatory roles of ExsC, ExsD, and ExsA were defined through these previous studies, the mechanism of coupling transcription to secretion was unclear. We now report the identification of ExsE as a secreted regulator of the T3SS and provide evidence that ExsE functions as a direct inhibitor of ExsC. When the secretion channel is closed, ExsE is complexed with ExsC in the cytoplasm, and transcription of the T3SS is repressed by sequestration of ExsA by ExsD. We propose that the secretion of ExsE provides an initiating signal that results in an equilibrium shift whereby ExsC becomes preferentially bound to ExsD, thus allowing liberated ExsA to activate transcription of the T3SS. The presence of ExsE homologs in the T3SSs of other bacterial species suggests that this mechanism of coupling transcription to secretion may be commonly used.

Mutation of retS, encoding a putative hybrid two-component regulatory protein in Pseudomonas aeruginosa, attenuates multiple virulence mechanisms

Two-component regulatory systems play an important role in bacterial virulence. We report that mutation of a Pseudomonas aeruginosa gene designated retS (previously designated fimK; accession number PA4856) encoding a putative hybrid two-component regulator, attenuates multiple virulence mechanisms. The retS mutant was selected from a Tn5 transposon library of the cytotoxic P. aeruginosa strain PA103 based upon expression of a small-colony phenotype suggestive of reduced surface-associated "twitching" motility, a property dependent upon type IV pili. Subsequent analysis revealed that the mutant expressed pilin, albeit at lower levels than wild-type PA103. In a murine model of corneal infection, retS mutation was associated with delayed disease development and altered pathology. In vitro, retS mutants demonstrated loss of acute cytotoxic activity towards corneal epithelia as determined by trypan blue exclusion and by LDH release assays (P<0.0001). This coincided with loss of ExsA-regulated type III secretion. Mutation of retS also impaired ExsA-independent pathogenic mechanisms. When compared to the exsA mutant of PA103, retS mutants exhibited reduced epithelial adherence and invasion and reduced intracellular survival within the cells after invasion. Time-lapse video microscopy revealed that retS mutants, compared to exsA mutants, had a reduced capacity to access, and move along, the basal cell surfaces of corneal epithelial cell monolayers. Taken together, these data suggest that the protein encoded by retS regulates various properties of P. aeruginosa including both ExsA-dependent and ExsA-independent virulence mechanisms.

ExsE, a secreted regulator of type III secretion genes in Pseudomonas aeruginosa

Type III secretion systems are toxin delivery systems that are present in a large number of pathogens. A hallmark of all type III secretion systems studied to date is that expression of one or more of their components is induced upon cell contact. It has been proposed that this induction is controlled by a negative regulator that is itself secreted by means of the type III secretion machinery. Although candidate proteins for this negative regulator have been proposed in a number of systems, for the most part, a direct demonstration of their role in regulation is lacking. Here, we report the discovery of ExsE, a negative regulator of type III secretion gene expression in Pseudomonas aeruginosa. Deletion of exsE deregulates expression of the type III secretion genes. We provide evidence that ExsE is itself secreted by means of the type III secretion machinery and physically interacts with ExsC, a positive regulator of the type III secretion regulon. Taken together, these data demonstrate that ExsE is the secreted negative regulator that couples triggering of the type III secretion machinery to induction of the type III secretion genes.

Passive anti-PcrV treatment protects burned mice against Pseudomonas aeruginosa challenge

The type III secretion system consists of secreted exoproducts and structural components, such as PcrV, and this system plays an important role in the virulence of Pseudomonas aeruginosa in burned hosts. The purpose of this study was to determine if passive anti-PcrV treatment would protect burned mice from fatal P. aeruginosa challenge, and to determine the type III exoproduct phenotype of the P. aeruginosa used as challenge strains. Antiserum was raised in rabbits. Mice were given a third degree burn, challenged with a lethal dose of P. aeruginosa, and treated with either anti-PcrV or control immunoglobulin intraperitoneally. Protection against three different pseudomonads was tested. Genotyping by PCR and phenotyping by immunoblots showed the P. aeruginosa strains to all be of the invasive type III phenotype: ExoS+ and/or ExoT+, ExoU-, ExoY+. Against all strains, the anti-PcrV treatment yielded significantly better survival (p<0.05) than the control immunoglobulin treatment. Duration of significant protection was improved by giving a second injection of PcrV antisera at 24h postburn. Hence, passive anti-PcrV immunization could protect burned mice against fatal challenge with P. aeruginosa of an invasive type III phenotype. This immunotherapy might be explored further as possible treatment for highly antibiotic resistant P. aeruginosa infections in burned hosts.

Role of the type III secreted exoenzymes S, T, and Y in systemic spread of Pseudomonas aeruginosa PAO1 in vivo

Pseudomonas aeruginosa uses a dedicated type III secretion system to deliver toxins directly into the cytoplasm of host cells. While progress has been made in elucidating the function of type III-secreted toxins in vitro, the in vivo functions of the type III-secreted exoenzymes are less well understood, particularly for the sequenced strain PAO1. Therefore, we have systematically deleted the genes for the three known type III effector molecules (exoS, exoT, and exoY) in P. aeruginosa PAO1 and assayed the effect of the deletions, both singly and in combination, on cytotoxicity in vitro and in vivo. We found that the type III secretion system acts differently on different cell types, causing an exoST-dependent rounding of a lung epithelial-like cell line in contrast to causing an exoSTY-independent but translocase (popB)-dependent lysis of a macrophage cell line. We utilized an in vivo competitive infection model to test each of our mutants, examining replication in the lung and spread to secondary sites such as the blood and spleen. Type III mutants inoculated intranasally exhibited only a minor defect in replication and survival in the lung, but popB and exoSTY triple mutants were profoundly defective in their ability to spread systemically. Intravenous injection of the mutants indicated that the type III secretion machinery is required for survival in the blood. Furthermore, our findings suggest that the effector-independent popB-dependent cytotoxicity that we and others have observed in vitro in macrophage cell lines may not be of great importance in vivo.

Activities of Pseudomonas aeruginosa effectors secreted by the Type III secretion system in vitro and during infection

Pseudomonas aeruginosa utilizes a number of distinct pathways to secrete proteins that play various roles during infection. These include the type II secretion system, which is responsible for the secretion of the majority of exoproducts into the surrounding environment, including toxins and degradative enzymes. In contrast, the type III secretion system mediates the delivery of protein effectors directly into the cytoplasm of the host cell. Using tissue culture assays and a mouse acute-pneumonia model, we have determined the contribution of each of the type III effectors during infection. In strain PAK, ExoS is the major cytotoxin required for colonization and dissemination during infection. ExoT confers protection of tissue culture cells from type III-dependent lysis, while ExoY seemed to have little effect on cytotoxicity. ExoU is over 100-fold more cytotoxic than ExoS. The cytotoxicity of type II secretion was determined following deletion of the genes for the more toxic type III secretion system. The participation of these secretion systems during lifelong colonization of cystic fibrosis (CF) patients is unclear. By comparing clonal strains from the same patient isolated at the initial onset of P. aeruginosa infection and more than a decade later, after chronic colonization has been established, we show that initial strains are more cytotoxic than chronic strains that have evolved to reduce type III secretion. Constitutive expression of genes for the type III secretion system restored ExoS secretion but did not always reestablish cytotoxicity, suggesting that CF strains accumulate a number of mutations to reduce bacterial toxicity to the host.

An in vivo inducible gene of Pseudomonas aeruginosa encodes an anti-ExsA to suppress the type III secretion system

We have previously reported on the isolation of in vivo inducible genes of Pseudomonas aeruginosa using IVET system. One of such genes isolated from burn mouse infection model encodes a short open reading frame with unknown function. In this study, we demonstrate that this gene product specifically suppresses the expression of type III secretion genes in P. aeruginosa, thus named PtrA (Pseudomonas type III repressor A). A direct interaction between the PtrA and type III transcriptional activator ExsA was demonstrated, suggesting that its repressor function is probably realized through inhibition of the ExsA protein function. Indeed, an elevated expression of the exsA compensates the repressor effect of the PtrA. Interestingly, expression of the ptrA is highly and specifically induced by copper cation. A copper- responsive two-component regulatory system, copR-copS, has also been identified and shown to be essential for the copper resistance in P. aeruginosa as well as the activation of ptrA in response to the copper signal. Elevated expression of the ptrA during the infection of mouse burn wound suggests that P. aeruginosa has evolved tight regulatory systems to shut down energy-expensive type III secretion apparatus in response to specific environmental signals, such as copper stress.

Characterization of phospholipase activity of the Pseudomonas aeruginosa type III cytotoxin, ExoU

Recombinant ExoU (rExoU) and yeast extract were used to optimize an in vitro phospholipase assay as a basis for identifying the mechanism for enzyme activation and substrate specificity. Our results support a model in which a eukaryotic protein cofactor or complex facilitates the interaction of rExoU with phospholipid substrates.

Pseudomonas aeruginosa causes acute lung injury via the catalytic activity of the patatin-like phospholipase domain of ExoU

OBJECTIVE

Acute lung injury in Pseudomonas aeruginosa pneumonia depends primarily on ExoU toxin being delivered directly into the eukaryotic cell cytosol through the type III secretion system. The amino-acid sequence of ExoU has a potato patatin-like phospholipase domain, similar to the sequence of mammalian Ca-independent phospholipase A2. We examined whether the acute lung injury caused by cytotoxic P. aeruginosa was dependent on the patatin-like phospholipase domain of ExoU.

DESIGN

Laboratory investigation using an established mouse model for P. aeruginosa pneumonia with quantitative measurements of acute lung injury and mortality.

SETTING

University experimental research laboratory.

SUBJECTS

Balb/c mice.

INTERVENTIONS

First, a site-directional mutation was introduced in the predicted catalytically active site of the patatin-like phospholipase domain of recombinant ExoU protein. The effect of the mutation on the catalytic activity of ExoU was tested by the in vitro lysophospholipase A assay. Second, the same site-directional mutation was introduced into the exoU gene of P. aeruginosa PA103. Mice were intratracheally infected with either a wild-type P. aeruginosa strain PA103 or an isogenic mutant containing the mutation in exoU. Acute epithelial lung injury, lung edema, bacteremia, and mortality were evaluated quantitatively.

MEASUREMENTS AND MAIN RESULTS

Recombinant ExoU had lysophospholipase A activity. Site-directional mutations in the predicted catalytic site of ExoU caused a loss of the lysophospholipase A activity. Whereas the airspace instillation of PA103 caused acute lung injury and death of the infected mice, the airspace instillation of isogenic mutants secreting catalytically inactive ExoU were noncytotoxic and did not cause acute lung injury or death of the infected mice.

CONCLUSION

Virulent P. aeruginosa causes acute lung injury and death by the cytotoxic activity derived from the patatin-like phospholipase domain of ExoU.

Pathogen-host interactions in Pseudomonas aeruginosa pneumonia

Pseudomonas aeruginosa is an important pathogen causing a wide range of acute and chronic infections. P. aeruginosa rarely causes infection in the normal host, but is an efficient opportunistic pathogen causing serious infections in patients who are mechanically ventilated, individuals who are immunocompromised, and patients with malignancies or HIV infection. Among these risk groups, the most vulnerable hosts are neutropenic and patients who are mechanically ventilated. In addition, P. aeruginosa is the most prevalent chronic infection contributing to the pathogenesis of cystic fibrosis. Because of the ubiquitous nature of P. aeruginosa and its ability to develop resistance to antibiotics, it continues to be problematic from a treatment perspective. The pathogenicity of P. aeruginosa is largely caused by multiple bacterial virulence factors and genetic flexibility enabling it to survive in varied environments. Lung injury associated with P. aeruginosa infection results from both the direct destructive effects of the organism on the lung parenchyma and exuberant host immune responses. This article focuses on the major bacterial virulence factors and important aspects of the host immunity that are involved in the pathogenesis of serious P. aeruginosa infection. In addition to antibiotic therapy, strategies directed toward enhancing host defense and/or limiting excessive inflammation could be important to improve outcome in P. aeruginosa lung infections.

Type III secretion phenotypes of Pseudomonas aeruginosa strains change during infection of individuals with cystic fibrosis

Pseudomonas aeruginosa is a frequent cause of respiratory exacerbations in individuals with cystic fibrosis. An important virulence determinant of this pathogen is its type III protein secretion system. In this study, the type III secretion properties of 435 P. aeruginosa respiratory isolates from 56 chronically infected individuals with cystic fibrosis were investigated. Although it had been previously reported that 75 to 90% of P. aeruginosa isolates from patients with hospital-acquired pneumonia secreted type III proteins, only 12% of isolates from cystic fibrosis patients did so, with nearly all of these isolates secreting ExoS and ExoT but not ExoU. Despite the low overall prevalence of type III protein-secreting isolates, at least one secreting isolate was cultured from one-third of cystic fibrosis patients. Interestingly, the fraction of cystic fibrosis patient isolates capable of secreting type III proteins decreased with duration of infection. Although 90% of isolates from the environment, the presumed reservoir for the majority of P. aeruginosa strains that infect patients with cystic fibrosis, secreted type III proteins, only 49% of isolates from newly infected children, 18% of isolates from chronically infected children, and 4% of isolates from chronically infected adults with cystic fibrosis secreted these proteins. Within individual patients, isolates of clonal origin differed in their secretion phenotypes, indicating that as strains persisted in cystic fibrosis patient airways, their type III protein secretion properties changed. Together, these findings indicate that following infection of cystic fibrosis patient airways, P. aeruginosa strains gradually change from a type III protein secretion-positive phenotype to a secretion-negative phenotype.

ExoU is a potent intracellular phospholipase

The combination of a large genome encoding metabolic versatility and conserved secreted virulence determinants makes Pseudomonas aeruginosa a model pathogen that can be used to study host-parasite interactions in many eukaryotic hosts. One of the virulence regulons that likely plays a role in the ability of P. aeruginosa to avoid innate immune clearance in mammals is a type III secretion system (TTSS). Upon cellular contact, the P. aeruginosa TTSS is capable of delivering a combination of at least four different effector proteins, exoenzyme S (ExoS), ExoT, ExoU, and ExoY. Two of the four translocated proteins, ExoS and ExoU, are cytotoxic to cells during infection and transfection. The mechanism of cytotoxicity of ExoS is unclear. ExoU, however, has recently been characterized as a member of the phospholipase A family of enzymes, possessing at least phospholipase A2 activity. Similar to ExoS, ExoT and ExoY, ExoU requires either a eukaryotic-specific modification or cofactor for its activity in vitro. The biologic effects of minimal expression of ExoU in yeast can be visualized by membrane damage to different organelles and fragmentation of the vacuole. In mammalian cells, the direct injection of ExoU causes irreversible damage to cellular membranes and rapid necrotic death. ExoU likely represents a unique enzyme and is the first identified phopholipase virulence factor that is translocated into the cytosol by TTSS.

Functional regions of the Pseudomonas aeruginosa cytotoxin ExoU

ExoU, a potent patatin-like phospholipase, causes rapid cell death following its injection into host cells by the Pseudomonas aeruginosa type III secretion system. To better define regions of ExoU required for cytotoxicity, transposon-based linker insertion mutagenesis followed by site-directed mutagenesis of individual residues was employed by using a Saccharomyces cerevisiae model system. Random insertion of five amino acids identified multiple regions within ExoU that are required for cell killing. Five regions were chosen for further characterization: three corresponded to the oxyanion hole, hydrolase motif, and catalytic aspartate motif of the patatin-like domain within the N-terminal half of ExoU; one corresponded to an uncharacterized part of the patatin-like domain; and one corresponded to a region near the C terminus. Specific individual amino acid substitutions in each of the four N-terminal regions prevented killing of yeast and significantly reduced phospholipase activity. Whereas five amino acid insertions in the fifth region near the C terminus markedly reduced cytotoxicity and phospholipase activity, substitution of individual amino acids did not abolish either activity. To determine whether each of the five identified regions of ExoU was also essential for cytotoxicity in human cells, representative mutant forms of ExoU fused to green fluorescent protein were expressed in HeLa cells. These variants of ExoU were readily visualized and caused minimal cytotoxicity to HeLa cells, while wild-type ExoU fused to green fluorescent protein induced significant cell lysis and no detectable fluorescence. Thus, a minimum of five regions, including one which is well removed from the patatin-like domain, are required for the cytotoxicity and phospholipase activity of ExoU.

Relative contributions of Pseudomonas aeruginosa ExoU, ExoS, and ExoT to virulence in the lung

Pseudomonas aeruginosa uses a type III secretion system to promote development of severe disease, particularly in patients with impaired immune defenses. While the biochemical and enzymatic functions of ExoU, ExoS, and ExoT, three effector proteins secreted by this system, are well defined, the relative roles of each protein in the pathogenesis of acute infections is not clearly understood. Since ExoU and ExoS are usually not secreted by the same strain, it has been difficult to directly compare the effects of these proteins during infection. In the work described here, several isogenic mutants of a bacterial strain that naturally secretes ExoU, ExoS, and ExoT were generated to carefully evaluate the relative contribution of each effector protein to pathogenesis in a mouse model of acute pneumonia. Measurements of mortality, bacterial persistence in the lung, and dissemination indicated that secretion of ExoU had the greatest impact on virulence while secretion of ExoS had an intermediate effect and ExoT had a minor effect. It is of note that these results conclusively show for the first time that ExoS is a virulence factor. Infection with isogenic mutants secreting wild-type ExoS, ExoS defective in GTPase-activating protein (GAP) activity, or ExoS defective in ADP-ribosyltransferase activity demonstrated that the virulence of ExoS was largely dependent on its ADP-ribosyltransferase activity. The GAP activity of this protein had only a minor effect in vivo. The relative virulence associated with each of these type III effector proteins may have important prognostic implications for patients infected with P. aeruginosa.

A novel anti-anti-activator mechanism regulates expression of the Pseudomonas aeruginosa type III secretion system

Expression of the Pseudomonas aeruginosa type III secretion system (TTSS) is coupled to the secretion status of the cells. Environmental signals such as calcium depletion activate the type III secretion channel and, as a consequence, type III gene transcription is derepressed. Two proteins, ExsA and ExsD, were shown previously to play a role in coupling transcription to secretion. ExsA is an activator of TTSS gene transcription, and ExsD is an anti-activator of ExsA. In the absence of environmental secretion cues, ExsD binds ExsA and inhibits transcription. Here, we describe the characterization of ExsC as an anti-anti-activator of TTSS expression. Transcription of the TTSS is repressed in an exsC mutant and is derepressed upon ExsC overexpression. The dependence on exsC for transcription is relieved in the absence of exsD, suggesting that ExsC and ExsD function together to regulate transcription. Consistent with this idea, ExsC interacts with ExsD in bacterial two-hybrid and co-purification assays. We propose a model in which the anti-anti-activator (ExsC) binds to and sequesters the anti-activator (ExsD) under low Ca(2+) conditions, freeing ExsA and allowing for transcription of the TTSS. The P. aeruginosa system represents the first example of an anti-activator/anti-anti-activator pair controlling transcription of a TTSS.

The adenylate cyclase toxins

Cyclic AMP is a ubiquitous messenger that integrates many processes of the cell. Diverse families of adenylate cyclases and phosphodiesterases stringently regulate the intracellular concentration of cAMP. Any alteration in the cytosolic concentration of cAMP has a profound effect on the various processes of the cell. Disruption of these cellular processes in vivo is often the most critical event in the pathogenesis of infectious diseases for animals and humans. Many pathogenic bacteria secrete toxins to alter the intracellular concentration of cAMP. These toxins either disrupt the normal regulation of the host cell's adenylate cyclases/phosphodiesterases or they themselves catalyze the synthesis of cAMP in the host cell. The latter are known as the adenylate cyclase toxins. Four such toxins have been identified: the invasive adenylate cyclase of Bordetella pertussis, the edema factor of Bacillus anthracis, ExoY of Pseudomonas aeruginosa, and the adenylate cyclase of Yersinia pestis. These adenylate cyclase toxins enter the eukaryotic host cells and get activated by eukaryotic cofactors, like calmodulin, to trigger the synthesis of cAMP in these cells. By accumulating cAMP in the target cells, these toxins either modulate the cellular function or completely deactivate the cell for further function. The immune effector cells appear to be the primary target of these adenylate cyclase toxins. By accumulating cAMP in the immune effector cells, these adenylate cyclase toxins poison the immune system and thus facilitate the survival of the bacteria in the host.

A therapeutic strategy against the shared virulence mechanism utilized by both Yersinia pestis and Pseudomonas aeruginosa

Yersinia pestis, which causes pneumonic plague in healthy individuals, has the potential to be used for biologic warfare. Pseudomonas aeruginosis is an opportunistic pathogen that causes severe pneumonia in immunocompromised patients. There is evidence that these two pathogens use a highly homologous virulence mechanism, the type III secretion system. The type III secretion systems of both Yersinia and P. aeruginosa possess a protein named V-antigen which can be used as a target for immunotherapy against bioterrorism and opportunistic infections. This article summarizes the recent progress of V-antigen studies in Yersinia and P. aeruginosa.

Analysis of quorum sensing-deficient clinical isolates of Pseudomonas aeruginosa

Pseudomonas aeruginosa produces multiple virulence factors and causes different types of infections. Previous clinical studies identified P. aeruginosa isolates that lack individual virulence factors. However, the impact of losing several virulence factors simultaneously on the in vivo virulence of P. aeruginosa is not completely understood. The P. aeruginosa cell-to-cell communication system, or quorum sensing (QS), controls the production of several virulence factors. Animal studies using constructed QS mutants indicated that loss of the QS system severely impacts the virulence of P. aeruginosa. In this study, we tried to determine if deficiency within the QS system compromises the ability of P. aeruginosa to establish infections in humans. We have identified five QS-deficient strains through screening 200 isolates from patients with urinary tract, lower respiratory tract and wound infections. These strains lacked LasB and LasA activities and produced either no or very low levels of the autoinducers N-(3-oxododecanoyl) homoserine lactone and N-butyryl homoserine lactone. PCR analysis revealed that three isolates contained all four QS genes (lasI, lasR, rhlI and rhlR) while two isolates lacked both the lasR and rhlR genes. We also examined the five isolates for other virulence factors. The isolates produced variable levels of exotoxin A and, with one exception, were deficient in pyocyanin production. One isolate produced the type III secretion system (TTSS) effector proteins ExoS and ExoT, two isolates produced ExoT only and two isolates produced no TTSS proteins. The isolates produced weak to moderate biofilms on abiotic surfaces. Analysis of the patients' data revealed that two of the isolates represented a single strain that was isolated twice from the same patient within a 1 month interval. One QS-deficient clinical isolate (CI-1) lacked all tested virulence factors and produced a weak biofilm. These results suggest that naturally occurring QS-deficient strains of P. aeruginosa do occur and are capable of causing infections; and, that besides the known virulence factors, additional factors may contribute to the ability of certain strains such as CI-1 to establish an infection.

Pseudomonas aeruginosa ExoS and ExoT

ExoS and ExoT are bi-functional type-III cytotoxins of Pseudomonas aeruginosa that share 76% primary amino acid homology and contain N-terminal RhoGAP domains and C-terminal ADP-ribosylation domains. The Rho GAP activities of ExoS and ExoT appear to be biochemically and biologically identical, targeting Rho, Rac, and Cdc42. Expression of the RhoGAP domain in mammalian cells results in the disruption of the actin cytoskeleton and interference of phagocytosis. Expression of the ADP-ribosyltransferase domain of ExoS elicits a cytotoxic phenotype in cultured cells, while expression of ExoT appears to interfere with host cell phagocytic activity. Recent studies showed that ExoS and ExoT ADP-ribosylate different substrates. While ExoS has poly-substrate specificity and can ADP-ribosylate numerous host proteins, ExoT ADP-ribosylates a more restricted subset of host proteins including the Crk proteins. Protein modeling predicts that electrostatic interactions contribute to the substrate specificity of the ADP-ribosyltransferase domains of ExoS and ExoT.

Phages and the evolution of bacterial pathogens: from genomic rearrangements to lysogenic conversion

Comparative genomics demonstrated that the chromosomes from bacteria and their viruses (bacteriophages) are coevolving. This process is most evident for bacterial pathogens where the majority contain prophages or phage remnants integrated into the bacterial DNA. Many prophages from bacterial pathogens encode virulence factors. Two situations can be distinguished: Vibrio cholerae, Shiga toxin-producing Escherichia coli, Corynebacterium diphtheriae, and Clostridium botulinum depend on a specific prophage-encoded toxin for causing a specific disease, whereas Staphylococcus aureus, Streptococcus pyogenes, and Salmonella enterica serovar Typhimurium harbor a multitude of prophages and each phage-encoded virulence or fitness factor makes an incremental contribution to the fitness of the lysogen. These prophages behave like "swarms" of related prophages. Prophage diversification seems to be fueled by the frequent transfer of phage material by recombination with superinfecting phages, resident prophages, or occasional acquisition of other mobile DNA elements or bacterial chromosomal genes. Prophages also contribute to the diversification of the bacterial genome architecture. In many cases, they actually represent a large fraction of the strain-specific DNA sequences. In addition, they can serve as anchoring points for genome inversions. The current review presents the available genomics and biological data on prophages from bacterial pathogens in an evolutionary framework.

The galU Gene of Pseudomonas aeruginosa is required for corneal infection and efficient systemic spread following pneumonia but not for infection confined to the lung

Acute pneumonias and corneal infections due to Pseudomonas aeruginosa are typically caused by lipopolysaccharide (LPS)-smooth strains. In cystic fibrosis patients, however, LPS-rough strains of P. aeruginosa, which lack O antigen, can survive in the lung and cause chronic infection. It is not clear whether an LPS-rough phenotype affects cytotoxicity related to the type III secretion system (TTSS). We previously reported that interruption of the galU gene in P. aeruginosa results in production of a rough LPS and truncated LPS core. Here we evaluated the role of the galU gene in the pathogenesis of murine lung and eye infections and in cytotoxicity due to the TTSS effector ExoU. We studied galU mutants of strain PAO1, of its cytotoxic variant expressing ExoU from a plasmid, and of the inherently cytotoxic strain PA103. The galU mutants were more serum sensitive than the parental strains but remained cytotoxic in vitro. In a corneal infection model, the galU mutants were significantly attenuated. In an acute pneumonia model, the 50% lethal doses of the galU mutants were higher than those of the corresponding wild-type strains, yet these mutants could cause mortality and severe pneumonia, as judged by histology, even with minimal systemic spread. These findings suggest that the galU gene is required for corneal infection and for efficient systemic spread following lung infection but is not required for infection confined to the lung. Host defenses in the lung appear to be insufficient to control infection with LPS-rough P. aeruginosa when local bacterial levels are high.