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

Study of genotypes and virB4 secretion gene of Bartonella henselae strains from patients with clinically defined cat scratch disease

Bartonella henselae is the causative agent of cat scratch disease (CSD), which usually presents as a self-limiting lymphadenopathy. Occasionally, the bacteria will spread and be responsible for tissue and visceral involvement. Two B. henselae genotypes (genotypes I and II) have been described to be responsible for uncomplicated CSD on the basis of 16S rRNA sequence analysis. A type IV secretion system (T4SS) similar to the virulence-associated VirB system of Agrobacterium tumefaciens was recently identified in the B. henselae Houston-1 genotype I strain. We studied the correlations of the B. henselae genotypes with the clinical presentations and with the presence of T4SS. Isolates originated from CSD patients whose lymph nodes were prospectively analyzed. B. henselae genotype I was identified in 13 of 42 patients (30%). Among these, two teenage twins presented with hepatosplenic CSD and one immunocompetent adult presented with osteomyelitis. Genotype II was detected in 28 of 42 patients (67%), all of whom presented with uncomplicated CSD. The last patient was infected with both genotypes. T4SS was studied by PCR amplification of the virB4 gene. Amplification of virB4 codons 146 to 256, 273 to 357, and 480 to 537 enabled us to detect 66, 90, and 100% of the B. henselae isolates, respectively. Sequence analysis revealed sequence variations that correlated with genotype distribution. Our studies suggest that B. henselae genotype I strains harbor virB4 genes that are different from those harbored by genotype II strains and that genotype I strains might be more pathogenic.

Role of the cystic fibrosis transmembrane conductance regulator in internalization of Pseudomonas aeruginosa by polarized respiratory epithelial cells

Pseudomonas aeruginosa is an important human pathogen, producing lung infection in individuals with cystic fibrosis (CF), patients who are ventilated and those who are neutropenic. The respiratory epithelium provides the initial barrier to infection. Pseudomonas aeruginosa can enter epithelial cells, although the mechanism of entry and the role of intracellular organisms in its life cycle are unclear. We devised a model of infection of polarized human respiratory epithelial cells with P. aeruginosa and investigated the role of the cystic fibrosis transmembrane conductance regulator (CFTR) in adherence, uptake and IL-8 production by human respiratory epithelial cells. We found that a number of P. aeruginosa strains could invade and replicate within cells derived from a patient with CF. Intracellular bacteria did not produce host cell cytotoxicity over a period of 24 h. When these cells were transfected with wild-type CFTR, uptake of bacteria was significantly reduced and release of IL-8 following infection enhanced. We propose that internalized P. aeruginosa may play an important role in the pathogenesis of infection and that, by allowing greater internalization into epithelial cells, mutant CFTR results in an increased susceptibility of bronchial infection with this microbe.

Genetic features of Pseudomonas aeruginosa isolates from cystic fibrosis patients compared with those of isolates from other origins

In order to improve our understanding of the colonization of the pulmonary tract of cystic fibrosis (CF) patients by Pseudomonas aeruginosa, 162 isolates from five different ecological origins were studied. The genetic features of each isolate were determined by random amplification of polymorphic DNA (RAPD) and by searching for eight virulence genes (six known virulence genes, algD, lasB, toxA, plcH, plcN and exoS, and two genes encoding putative neuraminidases, nan1 and nan2). Five RAPD groups were identified. Most of the CF isolates were distributed equally in three of these groups (RA, RB and RC). The CF isolates in RB were related to isolates from a wide variety of origins. The CF isolates in RA were related to a population composed of 65 % of the non-CF isolates from pulmonary tract infections. RC was mainly composed of CF isolates that were related to 30 % of isolates from plants. All genes except exoS and nan1 were present in all isolates. The exoS and nan1 virulence factor genes were most prevalent in CF isolates. exoS, which encodes exoenzyme S, was present in 94 % of CF isolates but also in 80 % of non-CF isolates from pulmonary tract infections. nan1, which encodes a putative neuraminidase, was found in 82.5 % of the isolates from group RC, which was composed largely of CF isolates. In conclusion, three major genogroups of P. aeruginosa isolates, each of which exhibits peculiar genetic features, are able to colonize CF patients. This may have different consequences on the outcome of pulmonary disease.

The ADP ribosyltransferase domain of Pseudomonas aeruginosa ExoT contributes to its biological activities

ExoT is a type III secreted effector protein found in almost all strains of Pseudomonas aeruginosa and is required for full virulence in an animal model of acute pneumonia. It is comprised of an N-terminal domain with GTPase activating protein (GAP) activity towards Rho family GTPases and a C-terminal ADP ribosyltransferase (ADPRT) domain with minimal activity towards a synthetic substrate in vitro. Consistent with its activity as a Rho family GTPase, ExoT has been shown to inhibit P. aeruginosa internalization into epithelial cells and macrophages, disrupt the actin cytoskeleton through a Rho-dependent pathway, and inhibit wound repair in a scrape model of injured epithelium. We have previously shown that mutation of the invariant arginine of the GAP domain to lysine (R149K) results in complete loss of GAP activity in vitro but only partially inhibits ExoT anti-internalization and cell rounding activity. We have constructed in-frame deletions and point mutations within the ADPRT domain in order to test whether this domain might account for the residual activity observed in ExoT GAP mutants. Deletion of a majority of the ADPRT domain (residues 234 to 438) or point mutations of the ADPRT catalytic site (residues 383 to 385) led to distinct changes in host cell morphology and substantially reduced the ability of ExoT to inhibit in vitro epithelial wound healing over a 24-h period. In contrast, only subtle effects on the efficiency of ExoT-induced bacterial internalization were observed in the ADPRT mutant forms. Expression of each domain individually in Saccharomyces cerevisiae was toxic, whereas expression of each of the catalytically inactive mutant domains was not. Collectively, these data demonstrate that the ADPRT domain of ExoT is active in vivo and contributes to the pathogenesis of P. aeruginosa infections.

In vivo phospholipase activity of the Pseudomonas aeruginosa cytotoxin ExoU and protection of mammalian cells with phospholipase A2 inhibitors

A number of clinical isolates of Pseudomonas aeruginosa are cytotoxic to mammalian cells due to the action of the 74-kDa protein ExoU, which is secreted into host cells by the type III secretion system and whose function is unknown. Here we report that the swift and profound cytotoxicity induced by purified ExoU or by an ExoU-expressing strain of P. aeruginosa is blocked by various inhibitors of cytosolic (cPLA2) and Ca2+ -independent (iPLA2) phospholipase A2 enzymes. In contrast, no cytoprotection is offered by inhibitors of secreted phospholipase A2 enzymes or by a number of inhibitors of signal transduction pathways. This suggests that phospholipase A2 inhibitors may represent a novel mode of treatment for acute P. aeruginosa infections. We find that 300-600 molecules of ExoU/cell are required to achieve half-maximal cell killing and that ExoU localizes to the host cell plasma membrane in punctate fashion. We also show that ExoU interacts in vitro with an inhibitor of cPLA2 and iPLA2 enzymes and contains a putative serine-aspartate catalytic dyad homologous to those found in cPLA2 and iPLA2 enzymes. Mutation of either the serine or the aspartate renders ExoU non-cytotoxic. Although no phospholipase or esterase activity is detected in vitro, significant phospholipase activity is detected in vivo, suggesting that ExoU requires one or more host cell factors for activation as a membrane-lytic and cytotoxic phospholipase.

Bacterial biofilms: an emerging link to disease pathogenesis

The role of biofilms in the pathogenesis of some chronic human infections is now widely accepted. However, the criteria used to determine whether a given infection is caused by biofilms remain unclear. In this chapter we discuss three infections that are caused by biofilms--infectious kidney stones, bacterial endocarditis, and cystic fibrosis lung infections--and focus on the role of the biofilm in disease pathogenesis. Biofilms are also important as environmental reservoirs for pathogens, and the biofilm growth mode may provide organisms with survival advantages in natural environments and increase their virulence. The consequences of pathogens living in environmental biofilms and an analysis of some specific environmental biofilm systems are presented.

Single-nucleotide-polymorphism mapping of the Pseudomonas aeruginosa type III secretion toxins for development of a diagnostic multiplex PCR system

We mapped the coding single nucleotide polymorphisms in four toxin genes-exoS, exoT, exoU, and exoY-of the Pseudomonas aeruginosa type III secretion system among several clinical isolates. We then used this information to design a multiplex PCR assay based on the simultaneous amplification of fragments of these genes. Eight strains of known genotype were used to test our multiplex PCR method, which showed 100% sensitivity and specificity in this small sample size. This assay appears to be promising for the rapid and accurate genotyping of the presence of these genes in clinical strains of P. aeruginosa.

Conservation of genome content and virulence determinants among clinical and environmental isolates of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a ubiquitous environmental bacterium capable of causing a variety of life-threatening human infections. The genetic basis for preferential infection of certain immunocompromised patients or individuals with cystic fibrosis by P. aeruginosa is not understood. To establish whether variation in the genomic repertoire of P. aeruginosa strains can be associated with a particular type of infection, we used a whole-genome DNA microarray to determine the genome content of 18 strains isolated from the most common human infections and environmental sources. A remarkable conservation of genes including those encoding nearly all known virulence factors was observed. Phylogenetic analysis of strain-specific genes revealed no correlation between genome content and infection type. Clusters of strain-specific genes in the P. aeruginosa genome, termed variable segments, appear to be preferential sites for the integration of novel genetic material. A specialized cloning vector was developed for capture and analysis of these genomic segments. With this capture system a site associated with the strain-specific ExoU cytotoxin-encoding gene was interrogated and an 80-kb genomic island carrying exoU was identified. These studies demonstrate that P. aeruginosa strains possess a highly conserved genome that encodes genes important for survival in numerous environments and allows it to cause a variety of human infections. The acquisition of novel genetic material, such as the exoU genomic island, through horizontal gene transfer may enhance colonization and survival in different host environments.

Pseudomonas aeruginosa ExoU, a toxin transported by the type III secretion system, kills Saccharomyces cerevisiae

ExoU, a protein transported by the type III secretion system of Pseudomonas aeruginosa, is an important cytotoxin, though its mechanism of action is unclear. Here we show that the intracellular expression of ExoU is cytotoxic to Saccharomyces cerevisiae. Furthermore, internal amino- and carboxyl-terminal deletions confirmed that regions of ExoU previously shown to be essential for killing mammalian cells were also required for killing yeast cells. These findings indicate that S. cerevisiae is a useful model organism for the study of ExoU.

Use of the Galleria mellonella caterpillar as a model host to study the role of the type III secretion system in Pseudomonas aeruginosa pathogenesis

Nonvertebrate model hosts represent valuable tools for the study of host-pathogen interactions because they facilitate the identification of bacterial virulence factors and allow the discovery of novel components involved in host innate immune responses. In this report, we determined that the greater wax moth caterpillar Galleria mellonella is a convenient nonmammalian model host for study of the role of the type III secretion system (TTSS) in Pseudomonas aeruginosa pathogenesis. Based on the observation that a mutation in the TTSS pscD gene of P. aeruginosa strain PA14 resulted in a highly attenuated virulence phenotype in G. mellonella, we examined the roles of the four known effector proteins of P. aeruginosa (ExoS, ExoT, ExoU, and ExoY) in wax moth killing. We determined that in P. aeruginosa strain PA14, only ExoT and ExoU play a significant role in G. mellonella killing. Strain PA14 lacks the coding sequence for the ExoS effector protein and does not seem to express ExoY. Moreover, using Delta exoU Delta exoY, Delta exoT Delta exoY, and Delta exoT Delta exoU double mutants, we determined that individual translocation of either ExoT or ExoU is sufficient to obtain nearly wild-type levels of G. mellonella killing. On the other hand, data obtained with a Delta exoT Delta exoU Delta exoY triple mutant and a Delta pscD mutant suggested that additional, as-yet-unidentified P. aeruginosa components of type III secretion are involved in virulence in G. mellonella. A high level of correlation between the results obtained in the G. mellonella model and the results of cytopathology assays performed with a mammalian tissue culture system validated the use of G. mellonella for the study of the P. aeruginosa TTSS.

O-antigen serotypes and type III secretory toxins in clinical isolates of Pseudomonas aeruginosa

The association of O-antigen serotypes with type III secretory toxins was analyzed in 99 clinical isolates of Pseudomonas aeruginosa. Isolates secreting ExoU were frequently serotyped as O11, but none were serotype O1. Most of the isolates that were nontypeable for O antigen did not secrete type III secretory toxins.

Cell line differences in bacterially translocated ExoS ADP-ribosyltransferase substrate specificity

Exoenzyme S (ExoS) is an ADP-ribosyltransferase (ADPRT) directly translocated into eukaryotic cells by the type III secretory (TTS) process of Pseudomonas aeruginosa. Comparisons of the functional effects of ExoS on human epithelial and murine fibroblastic cells showed that human epithelial cells exhibited an overall increased sensitivity to the effects of bacterially translocated ExoS on cell proliferation, morphology and re-adherence. ExoS was also found to ADP-ribosylate a greater number of low-molecular-mass G (LMMG) proteins in human epithelial cells, as compared to murine fibroblasts. Examination of the cellular mechanism for differences in ExoS ADPRT substrate modification found that the more restricted pattern of substrate modification in murine fibroblasts was not linked to the efficiency of bacterial adherence nor to the efficiency of ExoS internalization by the TTS process. In exploring the cellular nature of patterns of substrate modification, more extensive substrate modification was detected in human and simian cell lines, while rodent cell lines, including rat, mouse and hamster lines, consistently exhibited the more limited pattern of LMMG protein ADP-ribosylation. Patterns of substrate modification were not altered by cellular transformation and occurred independently of cell type. These studies suggest that eukaryotic cell properties, as recognized through studies of cells of different animal origins, affect the substrate targeting of ExoS ADPRT activity, and that this in turn can influence the severity of effects of ExoS on host-cell function.

Modulation of cytosolic Ca(2+) concentration in airway epithelial cells by Pseudomonas aeruginosa

Modulation of cytosolic (intracellular) Ca(2+) concentration (Ca(i)) may be an important host response when airway epithelial cells are exposed to Pseudomonas aeruginosa. We measured Ca(i) in Calu-3 cells exposed from the apical or basolateral surface to cytotoxic and noncytotoxic strains of P. aeruginosa. Apical addition of either noncytotoxic strains or cytotoxic strains failed to affect Ca(i) over a 3-h time period, nor were changes observed after basolateral addition of noncytotoxic strains. In contrast, basolateral addition of cytotoxic strains caused a slow increase in Ca(i) from 100 nM to 200 to 400 nM. This increase began after 20 to 50 min and persisted for an additional 30 to 75 min, at which time the cells became nonviable. P. aeruginosa-induced increases in Ca(i) were blocked by the addition of the Ca channel blocker La(3+) to the basolateral but not to the apical chamber. Likewise, replacing the basolateral but not the apical medium with Ca-free solution prevented P. aeruginosa-mediated changes in Ca(i). With isogenic mutants of PA103, we demonstrated that the type III secretion apparatus, the type III-secreted effector ExoU, and type IV pili were necessary for increased Ca(i). We propose that translocation of ExoU through the basolateral surface of polarized airway epithelial cells via the type III secretion apparatus leads to release of Ca stored in the endoplasmic reticulum and activation of Ca channels in the basolateral membranes of epithelial cells.

Virulence as a target for antimicrobial chemotherapy

Bacterial resistance to present day antibiotics has become a dangerous threat to public health. Consequently, the pharmaceutical industry must provide new agents and novel classes to combat bacterial disease and to stay a step ahead of the rapid evolution of bacterial resistance mechanisms. The need for novel antibacterials has resulted in a search for previously unexplored targets for chemotherapy, utilising the new techniques of genomics to identify them. Several targets currently under investigation are involved in the process of bacterial virulence. These targets are unique in that their inhibition, by definition, should interfere with the process of infection rather than with bacterial viability. If successful, virulence inhibition may represent a 'kinder, gentler' approach to chemotherapy in which the pathogen is disarmed rather than killed outright.

The human pathogen Pseudomonas aeruginosa utilizes conserved virulence pathways to infect the social amoeba Dictyostelium discoideum

Genetically accessible host models are useful for studying microbial pathogenesis because they offer the means to identify novel strategies that pathogens use to evade immune mechanisms, cause cellular injury, and induce disease. We have developed conditions under which the human pathogen Pseudomonas aeruginosa infects Dictyostelium discoideum, a genetically tractable eukaryotic organism. When D. discoideum is plated on nutrient agar plates with different P. aeruginosa strains, the bacteria form lawns on these plates with amoebae embedded in them. Virulent P. aeruginosa strains kill these amoebae and leave an intact bacterial lawn. A number of P. aeruginosa mutants have been identified that are avirulent in this assay. Amoebae feed on these bacteria and form plaques in their bacterial lawns. One avirulent mutant strain carries an insertional mutation in the lasR gene. LasR is a transcription factor that controls a number of virulence genes in a density-dependent fashion. Another class of avirulent P. aeruginosa mutants is defective in type III secretion. One mutant lacks the PscJ protein, a structural component of the secretion apparatus, suggesting that cytotoxins are injected into the D. discoideum cell. One of these cytotoxins is ExoU, and exoU mutants are avirulent toward D. discoideum. Complementation of the lasR and exoU mutations restores virulence. Therefore, P. aeruginosa uses conserved virulence pathways to kill D. discoideum.

Type III protein secretion is associated with poor clinical outcomes in patients with ventilator-associated pneumonia caused by Pseudomonas aeruginosa

OBJECTIVE

Pseudomonas aeruginosa is a frequent cause of ventilator-associated pneumonia. Recent evidence suggests that production of type III secretion proteins is correlated with increased pathogenicity in both cellular and animal models of infection. The objective of this study was to determine whether this system contributes to disease severity in humans with ventilator-associated pneumonia.

DESIGN

Retrospective pilot cohort study.

SETTING

University hospital.

PATIENTS

Thirty-five mechanically ventilated patients with bronchoscopically confirmed ventilator-associated pneumonia caused by P. aeruginosa.

MEASUREMENTS AND MAIN RESULTS

Ventilator-associated pneumonia was categorized as severe (patients died or had a recurrence of their pneumonia despite appropriate antibiotic therapy) or mild (patients uneventfully recovered from their pneumonia). The type III secretion genotypes and phenotypes of isolates cultured from the patients with ventilator-associated pneumonia were determined. Whereas every examined isolate harbored type III secretion genes, only 27 (77%) were capable of secreting detectable amounts of type III proteins in vitro. Twenty-two (81%) of the patients infected with these 27 isolates had severe disease. Of the eight isolates that did not secrete type III proteins, only three (38%) were cultured from patients with severe disease. Thus, infection with a type-III-secreting isolate correlated with severe disease (p < .05). In vitro assays indicated that ExoU, the type III effector protein most closely linked to mortality in animal models, was secreted in detectable amounts in vitro by 10 (29%) of the 35 examined isolates. Nine (90%) of these 10 isolates were cultured from patients with severe disease (p < .05 when compared with the nonsecreting isolates). In contrast, ExoS was secreted by 16 (46%) of the 35 examined isolates. Twelve (75%) of these 16 isolates were cultured from patients with severe disease (p = .14 when compared with the nonsecreting isolates).

CONCLUSIONS

In patients with ventilator-associated pneumonia, type-III-secreting isolates were associated with worse clinical outcomes, suggesting that this secretion system plays an important role in human disease. Our findings support the hypothesis that antibodies targeted against these proteins may be useful as adjunctive therapy in intubated patients with P. aeruginosa colonization or infection.