Role of exoenzyme S in chronic Pseudomonas aeruginosa lung infections

Revisiting the virulence hallmarks of Pseudomonas aeruginosa: a chronicle through the perspective of quorum sensing

Pseudomonas aeruginosa is an opportunistic pathogen and the leading cause of mortality among immunocompromised patients in clinical setups. The hallmarks of virulence in P. aeruginosa encompass six biologically competent attributes that cumulatively drive disease progression in a multistep manner. These multifaceted hallmarks lay the principal foundation for rationalizing the complexities of pseudomonal infections. They include factors for host colonization and bacterial motility, biofilm formation, production of destructive enzymes, toxic secondary metabolites, iron-chelating siderophores and toxins. This arsenal of virulence hallmarks is fostered and stringently regulated by the bacterial signalling system called quorum sensing (QS). The central regulatory functions of QS in controlling the timely expression of these virulence hallmarks for adaptation and survival drive the disease outcome. This review describes the intricate mechanisms of QS in P. aeruginosa and its role in shaping bacterial responses, boosting bacterial fitness. We summarize the virulence hallmarks of P. aeruginosa, relating them with the QS circuitry in clinical infections. We also examine the role of QS in the development of drug resistance and propose a novel antivirulence therapy to combat P. aeruginosa infections. This can prove to be a next-generation therapy that may eventually become refractory to the use of conventional antimicrobial treatments.

Bacterial Biofilm Control by Perturbation of Bacterial Signaling Processes

The development of effective strategies to combat biofilm infections by means of either mechanical or chemical approaches could dramatically change today’s treatment procedures for the benefit of thousands of patients. Remarkably, considering the increased focus on biofilms in general, there has still not been invented and/or developed any simple, efficient and reliable methods with which to “chemically” eradicate biofilm infections. This underlines the resilience of infective agents present as biofilms and it further emphasizes the insufficiency of today’s approaches used to combat chronic infections. A potential method for biofilm dismantling is chemical interception of regulatory processes that are specifically involved in the biofilm mode of life. In particular, bacterial cell to cell signaling called “Quorum Sensing” together with intracellular signaling by bis-(3′-5′)-cyclic-dimeric guanosine monophosphate (cyclic-di-GMP) have gained a lot of attention over the last two decades. More recently, regulatory processes governed by two component regulatory systems and small non-coding RNAs have been increasingly investigated. Here, we review novel findings and potentials of using small molecules to target and modulate these regulatory processes in the bacterium Pseudomonas aeruginosa to decrease its pathogenic potential.

The molecular mechanism of acute lung injury caused by Pseudomonas aeruginosa: from bacterial pathogenesis to host response

Pseudomonas aeruginosa is the most common gram-negative pathogen causing pneumonia in immunocompromised patients. Acute lung injury induced by bacterial exoproducts is associated with a poor outcome in P. aeruginosa pneumonia. The major pathogenic toxins among the exoproducts of P. aeruginosa and the mechanism by which they cause acute lung injury have been investigated: exoenzyme S and co-regulated toxins were found to contribute to acute lung injury. P. aeruginosa secretes these toxins through the recently defined type III secretion system (TTSS), by which gram-negative bacteria directly translocate toxins into the cytosol of target eukaryotic cells. TTSS comprises the secretion apparatus (termed the injectisome), translocators, secreted toxins, and regulatory components. In the P. aeruginosa genome, a pathogenic gene cluster, the exoenzyme S regulon, encodes genes underlying the regulation, secretion, and translocation of TTSS. Four type III secretory toxins, namely ExoS, ExoT, ExoU, and ExoY, have been identified in P. aeruginosa. ExoS is a 49-kDa form of exoenzyme S, a bifunctional toxin that exerts ADP-ribosyltransferase and GTPase-activating protein (GAP) activity to disrupt endocytosis, the actin cytoskeleton, and cell proliferation. ExoT, a 53-kDa form of exoenzyme S with 75% sequence homology to ExoS, also exerts GAP activity to interfere with cell morphology and motility. ExoY is a nucleotidal cyclase that increases the intracellular levels of cyclic adenosine and guanosine monophosphates, resulting in edema formation. ExoU, which exhibits phospholipase A2 activity activated by host cell ubiquitination after translocation, is a major pathogenic cytotoxin that causes alveolar epithelial injury and macrophage necrosis. Approximately 20% of clinical isolates also secrete ExoU, a gene encoded within an insertional pathogenic gene cluster named P. aeruginosa pathogenicity island-2. The ExoU secretory phenotype is associated with a poor clinical outcome in P. aeruginosa pneumonia. Blockade of translocation by TTSS or inhibition of the enzymatic activity of translocated toxins has the potential to decrease acute lung injury and improve clinical outcome.

Targeting quorum sensing in Pseudomonas aeruginosa biofilms: current and emerging inhibitors

Bacterial resistance to conventional antibiotics combined with an increasing acknowledgement of the role of biofilms in chronic infections has led to a growing interest in new antimicrobial strategies that target the biofilm mode of growth. In the aggregated biofilm mode, cell-to-cell communication systems involved in the process known as quorum sensing regulate coordinated expression of virulence with immune shielding mechanisms and antibiotic resistance. For two decades, the potential of interference with quorum sensing by small chemical compounds has been investigated with the aim of developing alternative antibacterial strategies. Here, we review state of the art research of quorum sensing inhibitors against the opportunistic human pathogen Pseudomonas aeruginosa, which is found in a number of biofilm-associated infections and identified as the predominant organism infecting the lungs of cystic fibrosis patients.

Clinical significance of microbial infection and adaptation in cystic fibrosis

A select group of microorganisms inhabit the airways of individuals with cystic fibrosis. Once established within the pulmonary environment in these patients, many of these microbes adapt by altering aspects of their structure and physiology. Some of these microbes and adaptations are associated with more rapid deterioration in lung function and overall clinical status, whereas others appear to have little effect. Here we review current evidence supporting or refuting a role for the different microbes and their adaptations in contributing to poor clinical outcomes in cystic fibrosis.

Interference of Pseudomonas aeruginosa signalling and biofilm formation for infection control

Pseudomonas aeruginosa is the best described bacterium with regards to quorum sensing (QS), in vitro biofilm formation and the development of antibiotic tolerance. Biofilms composed of P. aeruginosa are thought to be the underlying cause of many chronic infections, including those in wounds and in the lungs of patients with cystic fibrosis. In this review, we provide an overview of the molecular mechanisms involved in QS, QS-enabled virulence, biofilm formation and biofilm-enabled antibiotic tolerance. We now have substantial knowledge of the multicellular behaviour of P. aeruginosa in vitro. A major task for the future is to investigate how such in vitro data correlate with the in vivo behaviour of P. aeruginosa, and how to treat chronic infections of this bacterium in patients.

Quorum-sensing blockade as a strategy for enhancing host defences against bacterial pathogens

Conventional antibiotics target the growth and the basal life processes of bacteria leading to growth arrest and cell death. The selective force that is inherently linked to this mode of action eventually selects out antibiotic-resistant variants. The most obvious alternative to antibiotic-mediated killing or growth inhibition would be to attenuate the bacteria with respect to pathogenicity. The realization that Pseudomonas aeruginosa, and a number of other pathogens, controls much of their virulence arsenal by means of extracellular signal molecules in a process denoted quorum sensing (QS) gave rise to a new 'drug target rush'. Recently, QS has been shown to be involved in the development of tolerance to various antimicrobial treatments and immune modulation. The regulation of virulence via QS confers a strategic advantage over host defences. Consequently, a drug capable of blocking QS is likely to increase the susceptibility of the infecting organism to host defences and its clearance from the host. The use of QS signal blockers to attenuate bacterial pathogenicity, rather than bacterial growth, is therefore highly attractive, particularly with respect to the emergence of multi-antibiotic resistant bacteria.

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.

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.

Different domains of Pseudomonas aeruginosa exoenzyme S activate distinct TLRs

Some bacterial products possess multiple immunomodulatory effects and thereby complex mechanisms of action. Exogenous administration of an important Pseudomonas aeruginosa virulence factor, exoenzyme S (ExoS) induces potent monocyte activation leading to the production of numerous proinflammatory cytokines and chemokines. However, ExoS is also injected directly into target cells, inducing cell death through its multiple effects on signaling pathways. This study addresses the mechanisms used by ExoS to induce monocyte activation. Exogenous administration resulted in specific internalization of ExoS via an actin-dependent mechanism. However, ExoS-mediated cellular activation was not inhibited if internalization was blocked, suggesting an alternate mechanism of activation. ExoS bound a saturable and specific receptor on the surface of monocytic cells. ExoS, LPS, and peptidoglycan were all able to induce tolerance and cross-tolerance to each other suggesting the involvement of a TLR in ExoS-recognition. ExoS activated monocytic cells via a myeloid differentiation Ag-88 pathway, using both TLR2 and the TLR4/MD-2/CD14 complex for cellular activation. Interestingly, the TLR2 activity was localized to the C-terminal domain of ExoS while the TLR4 activity was localized to the N-terminal domain. This study provides the first example of how different domains of the same molecule activate two TLRs, and also highlights the possible overlapping pathophysiological processes possessed by microbial toxins.

Effect of metabolic imbalance on expression of type III secretion genes in Pseudomonas aeruginosa

The type III secretion system is a dedicated machinery used by many pathogens to deliver toxins directly into the cytoplasm of a target cell. Expression and secretion of the type III effectors are triggered by cell contact. In Pseudomonas aeruginosa and Yersinia spp., expression can be triggered in vitro by removing calcium from the medium. The mechanism underlying either mode of regulation is unclear. Here we characterize a transposon insertion mutant of P. aeruginosa PAO1 that displays a marked defect in cytotoxicity. The insertion is located upstream of several genes involved in histidine utilization and impedes the ability of PAO1 to intoxicate eukaryotic cells effectively in a type III-dependent fashion. This inhibition depends on the presence of histidine in the medium and appears to depend on the excessive uptake and catabolism of histidine. The defect in cytotoxicity is mirrored by a decrease in exoS expression. Other parameters such as growth or piliation are unaffected. The cytotoxicity defect is partially complemented by an insertion mutation in cbrA that also causes overexpression of cbrB. The cbrAB two-component system has been implicated in sensing and responding to a carbon-nitrogen imbalance. Taken together, these results suggest that the metabolic state of the cell influences expression of the type III regulon.

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.

Exoenzyme S shows selective ADP-ribosylation and GTPase-activating protein (GAP) activities towards small GTPases in vivo

Intracellular targeting of the Pseudomonas aeruginosa toxins exoenzyme S (ExoS) and exoenzyme T (ExoT) initially results in disruption of the actin microfilament structure of eukaryotic cells. ExoS and ExoT are bifunctional cytotoxins, with N-terminal GTPase-activating protein (GAP) and C-terminal ADP-ribosyltransferase activities. We show that ExoS can modify multiple GTPases of the Ras superfamily in vivo. In contrast, ExoT shows no ADP-ribosylation activity towards any of the GTPases tested in vivo. We further examined ExoS targets in vivo and observed that ExoS modulates the activity of several of these small GTP-binding proteins, such as Ras, Rap1, Rap2, Ral, Rac1, RhoA and Cdc42. We suggest that ExoS is the major ADP-ribosyltransferase protein modulating small GTPase function encoded by P. aeruginosa. Furthermore, we show that the GAP activity of ExoS abrogates the activation of RhoA, Cdc42 and Rap1.

An attractive surface: gram-negative bacterial biofilms

In nature, most bacteria live in close association with surfaces as complex communities referred to as biofilms. Community members within these compact microbial consortia show extraordinary resistance to conventional antibiotics, biocides, and hydrodynamic shear forces when compared to their planktonic counterparts. The buildup of these surface-associated bacterial communities is a highly organized and complex process that requires many signal transduction mechanisms to orchestrate the different stages of development. In this review, we describe several types of signal transduction that Gram-negative bacteria employ during the adhesion and expansion stages of biofilm formation, as well as discuss quorum-sensing in relation to the production of virulence factors.

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

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

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

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

Pseudomonas aeruginosa exoenzyme S induces transcriptional expression of proinflammatory cytokines and chemokines

Pseudomonas aeruginosa infection of cystic fibrosis patients causes lung damage that is substantially orchestrated by cytokines. In this study, multi-gene probe analysis was used to characterize the ability of the P. aeruginosa mitogen, exoenzyme S, to induce proinflammatory and immunoregulatory cytokines and chemokines. Exoenzyme S strongly induced transcription of proinflammatory cytokines and chemokines (tumor necrosis factor alpha, interleukin-1alpha [IL-1alpha], IL-1beta, IL-6, IL-8, MIP-1alpha, MIP-1beta, MCP-1, RANTES, and I-309), modest transcription of immunoregulatory cytokines (IL-10 and IL-12p40), and weak transcription of Th1 cytokines (IL-2 and gamma interferon). The response occurred early and subsided without evolving over time. These data suggest that cells responding to exoenzyme S would rapidly express proinflammatory cytokines and chemokines that may contribute to pulmonary inflammation in cystic fibrosis.

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

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

Pseudomonas aeruginosa exoenzyme S stimulates murine lymphocyte proliferation in vitro

The exuberant immunoinflammatory response that is associated with Pseudomonas aeruginosa infection is the major source of the morbidity and mortality in cystic fibrosis (CF) patients. Previous studies have established that an exoproduct of P. aeruginosa (exoenzyme S) is a mitogen for human T lymphocytes and activates a larger percentage of T cells than most superantigens, which may contribute to the immunoinflammatory response. An animal model would facilitate studies of the pathophysiologic consequences of this activation. As a first step toward developing an animal model, the murine lymphocyte response to exoenzyme S was examined. When stimulated with exoenzyme S, splenocytes isolated from naive mice entered S phase and proliferated. The optimum response occurred after 2 to 3 days in culture, at 4 x 10(5) cells per well and 5.0 micrograms of exoenzyme S per ml. The response was not due to lipopolysaccharide, since Rhodobacter sphaeroides lipid A antagonist did not block the response. Other preparations of exoenzyme S stimulated lymphocyte proliferation, since the response to recombinant exoenzyme S (rHisExo S) cloned from strain 388 was similar to the response to exoenzyme S from strain DG1. There was evidence that genetic variability influenced the response, since A/J, CBA/J, and C57BL/6 mice were high responders and BALB/cJ mice were low responders following stimulation with exoenzyme S. Both splenic T and B lymphocytes entered the cell cycle in response to exoenzyme S. Thus, murine lymphocytes, like human lymphocytes, respond to P. aeruginosa exoenzyme S, which supports the development of a murine model that may facilitate our understanding of the role that exoenzyme S plays in the pathogenesis of P. aeruginosa infections in CF patients.

RecombinantPseudomonasexoenzyme S and exoenzyme S fromPseudomonas aeruginosaDG1 share the ability to stimulate T lymphocyte proliferation

Exoenzyme S from P. aeruginosa DG1 and recombinant exoenzyme S derived from strain 388 have distinct characteristics, which has led to a controversy about their homology and their pathophysiologic consequences. We have been investigating the ability of exoenzyme S to activate T lymphocytes, and therefore performed studies to determine whether exoenzyme S from P. aeruginosa DG1 and recombinant exoenzyme S derived from strain 388 and expressed in Pseudomonas aeruginosa PA103 or in E. coli BL21(DE3), could induce T lymphocyte activation and proliferation. Both preparations were able to activate T cells and induce lymphocyte proliferation at similar levels as measured by flow cytometry of surface-activation markers and DNA synthesis, respectively. Further, a monoclonal antibody raised against exoenzyme S from strain DG1 partially neutralized T cell activation induced by recombinant exoenzyme S and bound to it in an immunoblot suggesting that the epitope responsible for T cell activation is shared by exoenzyme S from strain DG1 and recombinant exoenzyme S. These data suggest that the two different preparations of exoenzyme S, despite biochemical differences, share the characteristic that is responsible for T lymphocyte activation.Key words: exoenzyme S, Pseudomonas aeruginosa, T lymphocyte, cystic fibrosis.

Differential sensitivity of human epithelial cells to Pseudomonas aeruginosa exoenzyme S

Exoenzyme S (ExoS) is an ADP-ribosyltransferase produced and directly translocated into eukaryotic cells by the opportunistic pathogen Pseudomonas aeruginosa. Model systems that allow bacterial translocation of ExoS have found ExoS to have multiple effects on eukaryotic cell function, affecting DNA synthesis, actin cytoskeletal structure, and cell matrix adherence. To understand mechanisms underlying differences observed in cell sensitivities to ExoS, we examined the effects of bacterially translocated ExoS on multiple human epithelial cell lines. Of the cell lines examined, confluent normal kidney (NK) epithelial cells were most resistant to ExoS, while tumor-derived cell lines were highly sensitive to ExoS. Analysis of the mechanisms of resistance indicated that cell association as well as an intrinsic resistance to morphological alterations were associated with increased resistance to ExoS. Conversely, increased sensitivity to ExoS appeared to be linked to epithelial cell growth, with tumor cells capable of undergoing non-contact-inhibited, anchorage-independent growth all being sensitive to ExoS, and NK cells becoming sensitive to ExoS when subconfluent and growing. Consistent with the possibility that growth-related, actin-based structures are involved in sensitivity to ExoS, scanning electron microscopy revealed cellular extensions from sensitive, growing cells to bacteria, which were not readily evident in resistant cells. In all studies, the severity of effects of ExoS on cell function directly correlated with the degree of Ras modification, indicating that sensitivity to ExoS in some manner related to the efficiency of ExoS translocation and its ADP-ribosylation of Ras. Our results suggest that factors expressed by growing epithelial cells are required for the bacterial contact-dependent translocation of ExoS; as normal epithelial cells differentiate into polarized confluent monolayers, expression of these factors is altered, and cells in turn become more resistant to the effects of ExoS.

ADP-ribosylation of oncogenic Ras proteins by pseudomonas aeruginosa exoenzyme S in vivo

The exoenzyme S (ExoS)-producing Pseudomonas aeruginosa strain, 388, and corresponding ExoS knock-out strain, 388deltaexoS, were used in a bacterial and mammalian co-culture system as a model for the contact-dependent delivery of ExoS into host cells. Examination of DNA synthesis and Ras ADP-ribosylation in tumour cell lines expressing normal and mutant Ras revealed a decrease in DNA synthesis concomitant with ADP-ribosylation of Ras proteins after exposure to ExoS-producing bacteria, but not after exposure to non-ExoS-producing bacteria. Examination of normal H-Ras, K-Ras and N-Ras by two-dimensional electrophoresis after exposure to bacteria revealed differences in the degree of ADP-ribosylation by ExoS, with H-Ras being modified most extensively. ADP-ribosylation of oncogenic forms of Ras was examined in vivo using cancer lines expressing mutant forms of H-, N- or K-Ras. The mutant Ras proteins were modified in a manner qualitatively similar to their normal counterparts. Using Ras/Raf-1 co-immunoprecipitation after co-culture, it was found that exposure to ExoS-producing bacteria caused a decrease in the amount of Raf-1 associated with EGF-activated Ras and oncogenic Ras. The results from this study indicate that ExoS ADP-ribosylates both normal and mutant Ras proteins in vivo and inhibits signalling through Ras.

Modification of Ras in eukaryotic cells by Pseudomonas aeruginosa exoenzyme S

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

Pseudomonas aeruginosa exoenzyme S ADP-ribosylates Ras at multiple sites

Pseudomonas aeruginosa exoenzyme S (ExoS) ADP-ribosylated Ras to a stoichiometry of approximately 2 molecules of ADP-ribose incorporated per molecule of Ras, which suggested that ExoS could ADP-ribosylate Ras at more than one arginine residue. SDS-polyacrylamide gel electrophoresis analysis showed that ADP-ribosylated Ras possessed a slower mobility than non-ADP-ribosylated Ras. Analysis of the ADP-ribosylation of in vitro transcribed/translated Ras by ExoS identified two electrophoretically shifted forms of Ras, which was consistent with the ADP-ribosylation of Ras at two distinct arginine residues. Analysis of ADP-ribosylated in vitro transcribed/translated Ras mutants possessing individual Arg-to-Ala substitutions showed that Arg-41 was the preferred site of ADP-ribosylation and that the second ADP-ribosylation event occurred at a slower rate than the ADP-ribosylation at Arg-41, but did not occur at a specific arginine residue. Analysis of bacterially expressed wild-type RasDeltaCAAX and RasDeltaCAAXR41K supported the conclusion that Arg-41 was the preferred site of ADP-ribosylation. Arg-41 is located adjacent to the switch 1 region of Ras, which is involved in effector interactions. Introduction of ExoS into eukaryotic cells inhibited Ras-mediated eukaryotic signal transduction since infection of PC-12 cells with an ExoS-producing strain of P. aeruginosa inhibited nerve growth factor-stimulated neurite formation. This is the first demonstration that ExoS disrupts a Ras-mediated signal transduction pathway.

Ecto-ADP-ribosyltransferase activity of Pseudomonas aeruginosa exoenzyme S

Pseudomonas aeruginosa produces two ADP-ribosyltransferases, exotoxin A and exoenzyme S (ExoS). Although the physiological target protein remains to be defined, ExoS has been shown to ADP-ribosylate several eukaryotic proteins in vitro, including vimentin and members of the family of low-molecular-weight GTP-binding proteins. Recently, ExoS ADP-ribosyltransferase activity has been detected in the pleural fluid of rabbits infected with P. aeruginosa. This observation prompted an examination of the potential for ExoS to function as an ecto-ADP-ribosyltransferase. We have observed that ExoS preferentially ADP-ribosylated two extracellular serum proteins with molecular masses of 150 and 27 kDa. The ADP-ribosylation of these serum proteins by ExoS was stimulated by, but not dependent upon, exogenous FAS (for factor activating exoenzyme S), which indicated that serum contained endogenous FAS activity. Biochemical analysis showed that the 150-kDa ADP-ribosylated protein was immunoglobulin of the immunoglobulin G (IgG) and IgA classes. Subtyping showed that ExoS preferentially ADP-ribosylated human IgG3 and that ADP-ribosylation occurred within its Fc region. The 27-kDa protein ADP-ribosylated by ExoS was determined to be apolipoprotein A1. These data demonstrate ecto-ADP-ribosyltransferase activity by ExoS. This may extend the potential physiological consequences of ExoS during infection by P. aeruginosa beyond the implicated type III secretion-mediated intracellular delivery of ExoS into sensitive eukaryotic cells.

Effects of differential expression of the 49-kilodalton exoenzyme S by Pseudomonas aeruginosa on cultured eukaryotic cells

Production of the ADP-ribosylating enzyme exoenzyme S (ExoS) by Pseudomonas aeruginosa has been associated with increased virulence. Previous studies, however, have been unable to confirm an effect of soluble ExoS in cell culture or animal model systems. To determine if bacteria must come in contact with target cells in order for an effect of ExoS to be observed, coculture systems were developed to compare the effects of ExoS- and non-ExoS-producing bacteria on eukaryotic cell function. The two P. aeruginosa strains used in these studies, 388 and 388delta exoS, maintained genetic identity, with the exception that strain 388delta exoS lacked production of the 49-kDa form of ExoS. When bacteria were cocultured with Detroit 532 fibroblastic cells, ExoS-producing 388 bacteria caused a significant decrease in DNA synthesis and viability compared to the decrease caused by non-ExoS-producing 388delta exoS bacteria. Maximal differences between the two strains were observed when 10(4) to 10(7) CFU of bacteria/ml were cocultured with Detroit cells for 4 or 6 h. Both strains were effective in eliminating Detroit cell DNA synthesis after a 20-h coculture period. Secreted ExoS had no effect on Detroit cell growth and viability, indicating that bacteria must have contact with target cells for the effect of ExoS on cellular function to be observed. Similar effects on cell proliferation and viability were observed when the two strains were cocultured with the KB epithelioid cell line. ExoS-associated decreases in eukaryotic cell viability were not found to be mediated by an inhibition of protein synthesis. These studies confirm that the 49-kDa ExoS contributes to the cellular pathogenesis of P. aeruginosa by interfering with eukaryotic cell growth and viability. In addition, the coculture system developed which recognizes this effect should provide a means for defining the function of ExoS in vivo.

Genetic analysis of exoenzyme S expression by Pseudomonas aeruginosa

The dissemination of Pseudomonas aeruginosa to the bloodstream increases the likelihood of developing fatal sepsis. In experimental models, the ability to disseminate is linked to expression of the exoenzyme S pathway. Genetic and biochemical analysis of the pathway has led to the identification of the two structural genes encoding exoenzyme S, exoS and exoT. A key regulator of several loci of the pathway has been identified as a DNA-binding protein with transcriptional activation properties. Preliminary evidence suggests that exoenzyme S and the Yop virulence determinants of yersiniae share homology among proteins involved in their synthesis and secretion. With the addition of exoS and exoT to the molecular arsenal, questions concerning in vivo toxicity and target specificities of exoenzyme S can be directly addressed.

Pseudomonas aeruginosa exoenzyme S induces proliferation of human T lymphocytes

Pseudomonas aeruginosa is a gram-negative bacterium that is responsible for devastating acute and chronic infections, which include bronchiectasis in cystic fibrosis, nosocomial pneumonia, and infection of burn wounds. Previous studies have demonstrated that these patients have impaired host responses, including cell-mediated immune responses, which are important in anti-Pseudomonas host defense. The P. aeruginosa exoproduct, exoenzyme S, has a number of characteristics which suggest that it might be important in cell-mediated immunity. To determine whether exoenzyme S activates lymphocytes to proliferate, peripheral blood mononuclear cells (PBMC) from normal volunteers were stimulated with purified exoenzyme S, and the lymphocyte response was assessed by measuring [3H]thymidine uptake and by counting the number of cells after various times in culture. Ninety-five percent of healthy adult donors had a lymphocyte response to exoenzyme S. The optimal lymphocyte response occurred on day 7, with 4 x 10(5) PBMC per microtiter well when cells were stimulated with 10 micrograms exoenzyme S per ml. [3H]thymidine uptake correlated with an increase in the number of mononuclear cells, indicating that proliferation occurred. In unseparated PBMC, T cells, and to a lesser extent B cells, proliferated. Purified T cells proliferated, while purified B cells proliferated only after the addition of irradiated T cells. Thus, T lymphocytes are necessary and sufficient for the proliferative response to exoenzyme S. We speculate that exoenzyme S from P. aeruginosa is important in T-lymphocyte-mediated host defense to P. aeruginosa. In strategies to enhance impaired cell-mediated immunity, exoenzyme S should be considered as a potential stimulant.

Expression of recombinant exoenzyme S of Pseudomonas aeruginosa

The structural gene for the 49-kDa form of exoenzyme S (exoS) isolated from Pseudomonas aeruginosa 388 was expressed in both Escherichia coli and P. aeruginosa PA103. Expression of exoS in E. coli under the transcriptional regulation of the T7 promoter yielded a soluble cytosolic protein with an apparent molecular mass of 49 kDa, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Expression of exoS in P. aeruginosa PA103 under the transcriptional regulation of the 0.9 kbp of Pseudomonas chromosomal DNA flanking the 5' end of exoS yielded a nitrilotriacetic acid-inducible extracellular protein with an apparent molecular mass of 49 kDa. Recombinant ExoS (rExoS) reacted with the anti-49-kDa form of exoenzyme S immunoglobulin G, existed as an aggregate as determined by gel filtration chromatography, and ADP-ribosylated soybean trypsin inhibitor at a specific activity that was similar (within twofold) to that of native exoenzyme S. Allelic exchange of exoS with a tetracycline gene cartridge yielded a strain of P. aeruginosa 388 that did not express detectable amounts of either ExoS in an immunoblot analysis using the anti-49-kDa form of exoenzyme S immunoglobulin G or ADP-ribosyltransferase activity under standard enzyme assay conditions. Expression of catalytically active rExoS in E. coli demonstrated that exoS was necessary and sufficient for the factor-activating exoenzyme S-dependent ADP-ribosyltransferase activity of exoenzyme S. Expression of nitrilotriacetic acid-inducible rExoS in P. aeruginosa PA103 demonstrated that the 0.9 kbp of Pseudomonas chromosomal DNA flanking the 5' end of exoS encoded a functional exoenzyme S promoter. Expression analysis and allelic exchange experiments suggest that the 49- and 53-kDa forms of exoenzyme S are encoded by separate genes.

Purification and characterization of exoenzyme S from Pseudomonas aeruginosa 388

Exoenzyme S was purified > 1,500-fold from the culture supernatant fluid of Pseudomonas aeruginosa 388 at high yield without utilization of solvents or detergents. Two proteins, with apparent molecular sizes of 53 and 49 kDa, cofractionated with exoenzyme S activity. Rabbit anti-49-kDa-protein immunoglobulin G was prepared by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis-purified 49-kDa protein as immunogen. Anti-49-kDa-protein IgG inhibited the ADP-ribosyltransferase activity of purified exoenzyme S in a dose-dependent manner, which indicated a role for the 49-kDa protein in the ADP-ribosylation reaction. Analysis by ultrafiltration showed that exoenzyme S activity and the 53- and 49-kDa proteins cofractionated and that exoenzyme S was apparently > 300 kDa in size. Urea (8 M) and 1.0% Triton X-100 reversibly decreased the apparent molecular sizes of exoenzyme S activity and the 53- and 49-kDa proteins to between 30 and 100 kDa.

ADP-ribosylation of p21ras and related proteins by Pseudomonas aeruginosa exoenzyme S

Pseudomonas aeruginosa exoenzyme S ADP-ribosylates p21ras and several related proteins. ADP-ribosylation of p21ras does not alter interactions with guanine nucleotides. The ras-related GTP-binding proteins, including Rab3, Rab4, Ral, Rap1A, and Rap2, are also substrates; given these results, we propose a model for the role of exoenzyme S in pathogenesis.

Glycolipid receptor binding specificity of exoenzyme S from Pseudomonas aeruginosa

By use of the tlc overlay procedure we have shown that exoenzyme S extracted from cultures of Pseudomonas aeruginosa specifically binds to the glycolipids asialoGM1, asialoGM2 and to a lesser extent lactosyl ceramide. More significantly, strong binding was also observed to the glycerolipid receptor we have detected for Helicobacter pylori (Lancet ii, 238-241.1989). Exoenzyme S can be extracted in a toxic and nontoxic form. Toxicity correlated with ability to bind the H. pylori receptor. This species was the only receptor detected in the most sensitive cell lines. The relative binding of exoenzyme S to the ganglio series glycolipids and the glycerolipid receptor was modified in a reciprocal manner in the presence of metal ions, suggesting that exoenzyme S has two interrelated receptor binding sites.

Microbiology of airway disease in patients with cystic fibrosis

Individuals with cystic fibrosis have abbreviated life spans primarily due to chronic airway infection. A limited number of types of organisms are responsible for these infections, with Staphylococcus aureus and Pseudomonas aeruginosa being of primary importance. In the pre-antibiotic era, greater than 90% of deaths due to infection were caused by S. aureus and death usually occurred in the first 2 years of life. With the advent of effective antistaphylococcal therapy, life spans increased and P. aeruginosa became the pathogen of primary importance. P. aeruginosa isolates recovered from patients with cystic fibrosis have a unique phenotypic characteristic referred to as "mucoid." The mucoid phenotype is due to the production of a mucoid exopolysaccharide. A mucoid exopolysaccharide is believed to play a central role in the establishment of chronic pseudomonal lung infection in these patients. A third organism, Pseudomonas cepacia, has recently been detected in the airways of older patients with cystic fibrosis and is associated with increased mortality. The virulence of P. cepacia is not understood, but the organism is extremely refractory to antimicrobial therapy. Other bacteria, including Haemophilus influenzae and members of the family Enterobacteriaceae, appear to play a secondary role in airway infection. Aspergillus fumigatus is the most important fungal agent causing allergic bronchopulmonary disease. The role of viruses has only recently been examined. At least in some patients with cystic fibrosis, respiratory syncytial virus may be important in predisposing to subsequent bacterial infections.

Cloning and expression of Pseudomonas aeruginosa flagellin in Escherichia coli

The flagellin gene was isolated from a Pseudomonas aeruginosa PAO1 genomic bank by conjugation into a PA103 Fla- strain. Flagellin DNA was transferred from motile recipient PA103 Fla+ cells by transformation into Escherichia coli. We show that transformed E. coli expresses flagellin protein. Export of flagellin to the E. coli cell surface was suggested by positive colony blots of unlysed cells and by isolation of flagellin protein from E. coli supernatants.

Exoenzyme S of Pseudomonas aeruginosa ADP-ribosylates the intermediate filament protein vimentin

Exoenzyme S, which had been thought to be unselective, catalyzes the ADP-ribosylation of only a subset of cellular proteins. The intermediate filament protein vimentin is one of the more abundant substrates. Disassembled vimentin, and proteolytic fragments of vimentin that cannot form filaments, is more readily ADP-ribosylated than is filamentous vimentin.

Inhibition of Pseudomonas aeruginosa exoenzyme expression by subinhibitory antibiotic concentrations

We examined the effects of subinhibitory concentrations of ciprofloxacin, tobramycin, and ceftazidime on Pseudomonas aeruginosa exoenzyme expression in vitro and in vivo. Exotoxin A, exoenzyme S, phospholipase C, elastase, and total protease activities were suppressed by antibiotics at concentrations as low as 1/20 of the MIC over a 24-h period in broth. Continuous 10-day exposure of P. aeruginosa DG1 broth cultures to antibiotic levels equal to 1/10 of the MIC reduced exoenzyme S activity in all treatment groups. Elastase activity was reduced only by ciprofloxacin and tobramycin treatment. This suppressive effect of the antibiotics persisted throughout the 10 days and was not influenced by the increase in MIC of ciprofloxacin detected during the course of the experiment. Rats chronically infected with P. aeruginosa were treated with subinhibitory doses of antibiotics and compared with untreated controls. Bacterial numbers in lung homogenates from each of the four study groups were identical. However, the lungs from antibiotic-treated rats had significantly less histological damage than those from control rats (P less than 0.001). The protective effect was greatest for ciprofloxacin and tobramycin. Further, P. aeruginosa isolates from ciprofloxacin- and tobramycin-treated rats demonstrated significantly less exoenzyme S and elastase activity than isolates from untreated rats (P less than 0.001). Isolates from ceftazidime-treated lungs expressed less exoenzyme S activity (P less than 0.001) but an equivalent amount of elastase activity as isolates from controls. The suppression of P. aeruginosa exoenzymes may arrest progressive lung injury during chronic P. aeruginosa lung infections.

Purification of Pseudomonas aeruginosa exoenzyme S

Pseudomonas aeruginosa produces two distinct ADP-ribosyl transferases, exotoxin A and exoenzyme S, which differ in a number of properties including substrate specificity. Exoenzyme S was purified from culture supernatants of P. aeruginosa DG1. The procedure for purification consists of four major steps: ammonium sulfate precipitation, anion-exchange chromatography on DEAE-Sephacel, acetone precipitation in the presence of 1 M NaCl, and G-100 Superfine gel filtration chromatography. Exoenzyme S was monitored during purification by an assay for ADP-ribosyl transferase activity, mouse toxicity, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified material exhibited ADP-ribosyl transferase activity, reacted with monoclonal antibodies to exoenzyme S, and was toxic to mice and a variety of tissue culture cell lines.

Pseudomonas aeruginosa: biology, mechanisms of virulence, epidemiology

Pseudomonas aeruginosa is a gram-negative pathogen, versatile and opportunistic in terms of its genetics, metabolic potential, and mechanisms of virulence. This versatility enables it to respond to variable and frequently adverse environmental conditions. Considered by many to be an aerobic organism, it is capable of growing anaerobically if certain substrates are available, for example, nitrates or arginine. Diversity of mechanisms of genetic exchange, including transformation, transduction, and conjugation, help P. aeruginosa adapt to changing conditions by acquiring new genetic information. Genetic manipulations have been exploited in recent years to study the basic biology of this bacterial species and the roles of its numerous virulence factors. Recently, transposon mutagenesis techniques and recombinant DNA methods (cloning) have been used to study some of the virulence factors of P. aeruginosa. The pathogenesis of P. aeruginosa infections is multifactorial, as manifested by the numerous toxins, or virulence factors, it produces and the variety of diseases it causes. P. aeruginosa is invasive and toxigenic. Infections appear to occur in stages: bacterial adherence, colonization, invasion and dissemination, and systemic or toxemic disease. Virulence factors can contribute to one or several stages of pathogenesis. Surface factors, including pili, lipopolysaccharide, and polysaccharide slime (alginate), probably contribute to the first two stages. Polysaccharide slime and lipopolysaccharide may also contribute to other processes later in the course of infection. Toxins, including exotoxin A and phospholipase C (hemolysin), and proteases of P. aeruginosa may contribute to tissue damage and dissemination. They may also aid in the procurement of nutrients required by the bacteria in the early stages of infection. The significance of the different virulence factors probably depends on the infection. Alginate production and phospholipase C are likely to have special significance in respiratory infections, particularly in cystic fibrosis.