Two-component regulatory systems in Pseudomonas aeruginosa: an intricate network mediating fimbrial and efflux pump gene expression

Pseudomonas aeruginosa is responsible for chronic and acute infections in humans. Chronic infections are associated with production of fimbriae and the formation of a biofilm. The two-component system Roc1 is named after its role in the regulation of cup genes, which encode components of a machinery allowing assembly of fimbriae. A non-characterized gene cluster, roc2, encodes components homologous to the Roc1 system. We show that cross-regulation occurs between the Roc1 and Roc2 signalling pathways. We demonstrate that the sensors RocS2 and RocS1 converge on the response regulator RocA1 to control cupC gene expression. This control is independent of the response regulator RocA2. Instead, we show that these sensors act via the RocA2 response regulator to repress the mexAB-oprM genes. These genes encode a multidrug efflux pump and are upregulated in the rocA2 mutant, which is less susceptible to antibiotics. It has been reported that in cystic fibrosis lungs, in which P. aeruginosa adopts the biofilm lifestyle, most isolates have an inactive MexAB-OprM pump. The concomitant RocS2-dependent upregulation of cupC genes (biofilm formation) and downregulation of mexAB-oprM genes (antibiotic resistance) is in agreement with this observation. It suggests that the Roc systems may sense the environment in the cystic fibrosis lung.     

Integrated regulatory network in Pseudomonas syringae reveals dynamics of virulence

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Regulation of virulence and antibiotic resistance by two-component regulatory systems in Pseudomonas aeruginosa

The Gram-negative opportunistic pathogen Pseudomonas aeruginosa ubiquitously inhabits soil and water habitats and also causes serious, often antibiotic resistant, infections in immunocompromised patients (e.g. cystic fibrosis). This versatility is mediated in part by a large repertoire of two-component regulatory systems that appear instrumental in the regulation of both virulence processes and resistance to antimicrobials. Major two-component regulatory system proteins demonstrated to regulate these diverse processes include PhoP–PhoQ, GacA–GacS, RetS, LadS, and AlgR, among others. Here, we summarize the current body of knowledge of these and other two-component systems that provides insight into the complex regulation of virulence and resistance in P. aeruginosa .     

Pseudomonas aeruginosa proteases and corneal virulence

Pseudomonas aeruginosa is a common cause of corneal infections, particularly among users of soft contact lenses. Previous studies with chemically induced mutants deficient in alkaline protease (AP) or elastase (LasB) suggested that these proteases contributed to the rapid liquifactive stromal necrosis characteristic of P. aeruginosa corneal infections. Because these mutants might harbor other chromosomal changes that could affect virulence, the role of these proteases in the pathogenesis of corneal disease (as well as a second elastase, LasA protease) was reexamined by constructing isogenic mutants deficient only in these enzymes. Allelic exchange was used to construct mutants of P. aeruginosa PAO1-V deficient in AP (PAO1-V AP[ - ]), LasB and LasA protease (PDO801 LasB[ - ]), or all three proteases (PDO801 TM). These mutants were then evaluated for virulence using mouse scratch and rabbit intrastromal injection models of corneal disease. Loss of AP significantly increased disease scores in the rabbit (P < 0.030) but not the mouse (P > 0.060) model of infection. Loss of both elastases had no effect on ocular virulence in either animal model of corneal disease (P > 0.100). The loss of all three proteases significantly decreased disease scores in the rabbit (P < 0.035), but not in the mouse (P > 0.110). Taken together, these data suggest that AP, LasB, and LasA protease are not essential for initiating or maintaining a corneal infection. Furthermore, AP appears to be an important mediator of pathology depending on the location of the organism within the cornea and whether or not concomitant elastolytic activity is present.     

Modeling the quorum sensing regulatory network of human-pathogenic Pseudomonas aeruginosa

The biochemical network underlying quorum sensing in human-pathogenic Pseudomonas aeruginosa is one of the best characterized. Mathematical modeling is required to untangle the complexity of its architecture and dynamics. We present a qualitative model of the P. aeruginosa quorum-sensing network including interactions between the las and rhl modules, the signaling molecule PQS and the regulatory proteins Mvfr and VfR. Simulations exemplify the model to reproduce natural network behavior and suggest quorum-sensing responses to pharmacological interference.     

Siderophore-mediated cooperation and virulence in Pseudomonas aeruginosa

Why should organisms cooperate with each other? Helping close relatives that are likely to share the same genes (kin selection) is one important explanation that is likely to apply across taxa. The production of metabolically costly extracellular iron-scavenging molecules (siderophores) by microorganisms is a cooperative behaviour because it benefits nearby conspecifics. We review experiments focusing on the production of the primary siderophore (pyoverdin) of the opportunistic bacterial pathogen, Pseudomonas aeruginosa, which test kin selection theories that seek to explain the evolution of cooperation. First, cooperation is indeed favoured when individuals interact with their close relatives and when there is competition between groups of cooperators and noncooperators, such that the benefit of cooperation can be realized. Second, the relative success of cheats and cooperators is a function of their frequencies within populations. Third, elevated mutation rates can confer a selective disadvantage under conditions when cooperation is beneficial, because high mutation rates reduce how closely bacteria are related to each other. Fourth, cooperative pyoverdin production is also shown to be favoured by kin selection in vivo (caterpillars), and results in more virulent infections. Finally, we briefly outline ongoing and future work using this experimental system.     

Cooperation and virulence in acute Pseudomonas aeruginosa infections

Background  

Efficient host exploitation by parasites is frequently likely to depend on cooperative behaviour. Under these conditions, mixed-strain infections are predicted to show lower virulence (host mortality) than are single-clone infections, due to competition favouring non-contributing social 'cheats' whose presence will reduce within-host growth. We tested this hypothesis using the cooperative production of iron-scavenging siderophores by the pathogenic bacterium Pseudomonas aeruginosa in an insect host.     

Pseudomonas aeruginosa exoproducts as pulmonary virulence factors

An experimental animal model of chronic pseudomonas pneumonia was used to document the production of potential virulence factors by Pseudomonas aeruginosa during the infection. The production of exotoxin A, proteolytic enzymes, and the serotype-specific lipopolysaccharide and slime-layer antigens during the infection was examined by solid-phase radioimmunoassay of serum from infected rats and by indirect immunofluorescence tests of their lung tissue. Rats inoculated intratracheally with purified bacterial exoproducts, delivered alone or in combination, developed pulmonary histopathology similar to that induced by the experimental infection. The results indicate that these exoproducts are produced during the course of the pulmonary infection and suggest that they are involved in the observed lung pathology.     

Small regulatory RNAs in Pseudomonas aeruginosa

The opportunistic human pathogen Pseudomonas aeruginosa is frequently associated with nosocomial infections, and can be life threatening in immunosuppressed, cancer and cystic fibrosis patients. Virulence in P. aeruginosa is combinatorial, and results from the activation of several genetic programs that regulate motility, attachment to the host epithelium as well as the synthesis of exotoxins. The pathogen has a high survival capacity in the host owing to its metabolic versatility, nutrient scavenging and resistance against both, antibiotics and immune defenses. Adaptive responses to various environmental stresses and stimuli are often regulated by small regulatory RNAs (sRNA). In this review, we summarize the current knowledge on the regulation and function of P. aeruginosa sRNAs that titrate regulatory proteins, base-pair with target mRNAs, and which are derived from CRISPR elements.     

Flagella, motility and invasive virulence of Pseudomonas aeruginosa

The role of motility as a virulence factor in Pseudomonas aeruginosa burn wound sepsis was examined using mutants deficient in the Fla or Mot phenotype. Physiological profiles of parental strains and Fla- and Mot- mutants were similar with respect to antibiograms, O antigen types, growth rates, and proteolytic, exotoxin A and phospholipase activities, providing evidence for isogenicity. Lethality studies using a subcutaneous mouse burn model showed that three Fla- mutants and one Mot- mutant were much less virulent (10(2) to 10(5) times) than the parent wild-type. Topical challenges in the flame burn model showed that a Fla- mutant of strain M-2 was approximately tenfold less virulent. A reduction in virulence, although somewhat less than tenfold, was also observed in the scald burn model for M-2 Fla-, and Mot- strains. Tissue colonization experiments revealed a characteristic, rapidly systemic infection in burned mice challenged with wild-type organisms. Nonmotile mutants similarly proliferated in the burn wound, but the characteristic bacteraemia and systemic invasion were markedly absent. The infection remained localized in the skin wound and the mice survived. The pattern of infection by nonmotile mutants in the colonization studies was very similar to that obtained with Fla+ cells in burned animals passively treated with antiflagellar antibody. These results add substantial support to the concept of motility as a P. aeruginosa virulence factor in invasive infections.     

铜绿假单胞菌的双组分系统

双组分系统是存在于原核和少部分真核生物细胞中的信号转导系统,主要由组氨酸蛋白激酶和反应调节蛋白组成,通过感应外界环境信号、信号输入、磷酸基团传递、信号输出等环节调节基因表达,使细胞能更加适应环境变化。铜绿假单胞菌为条件致病菌,其双组分系统构成多样、功能复杂且参与介导耐药性产生,因此铜绿假单胞菌的双组分系统日益引起人们关注。本文对铜绿假单胞菌双组分系统的组成、信号转导机制、种类、研究方法及其临床意义进行了综述。     

A new quorum-sensing inhibitor attenuates virulence and decreases antibiotic resistance in Pseudomonas aeruginosa

Quorum sensing (QS) has been a novel target for the treatment of infectious diseases. Here structural analogs of Pseudomonas aeruginosa autoinducer N-acyl homoserine lactone (AHL) were investigated for QS inhibitor (QSI) activity and a novel QSI was discovered, N-decanoyl-L-homoserine benzyl ester (C2). Virulence assays showed that C2 down-regulated total protease and elastase activities, as well as the production of rhamnolipid, that are controlled by QS in P. aeruginosa wild-type strain PAO1 without affecting growth. C2 was also shown to inhibit swarming motility of PAO1. Using a microdilution checkerboard method, we identified synergistic interactions between C2 and several antibiotics, tobramycin, gentamycin, cefepime, and meropenem. Data from real-time RT-PCR suggested that C2 inhibited the expression of lasR (29.67%), lasI (21.57%), rhlR (28.20%), and rhlI (29.03%).     

Genomic analysis reveals that Pseudomonas aeruginosa virulence is combinatorial

Background

Pseudomonas aeruginosa is a ubiquitous environmental bacterium and an important opportunistic human pathogen. Generally, the acquisition of genes in the form of pathogenicity islands distinguishes pathogenic isolates from nonpathogens. We therefore sequenced a highly virulent strain of P. aeruginosa, PA14, and compared it with a previously sequenced (and less pathogenic) strain, PAO1, to identify novel virulence genes.

Results

The PA14 and PAO1 genomes are remarkably similar, although PA14 has a slightly larger genome (6.5 megabses [Mb]) than does PAO1 (6.3 Mb). We identified 58 PA14 gene clusters that are absent in PAO1 to determine which of these genes, if any, contribute to its enhanced virulence in a Caenorhabditis elegans pathogenicity model. First, we tested 18 additional diverse strains in the C. elegans model and observed a wide range of pathogenic potential; however, genotyping these strains using a custom microarray showed that the presence of PA14 genes that are absent in PAO1 did not correlate with the virulence of these strains. Second, we utilized a full-genome nonredundant mutant library of PA14 to identify five genes (absent in PAO1) required for C. elegans killing. Surprisingly, although these five genes are present in many other P. aeruginosa strains, they do not correlate with virulence in C. elegans.

Conclusion

Genes required for pathogenicity in one strain of P. aeruginosa are neither required for nor predictive of virulence in other strains. We therefore propose that virulence in this organism is both multifactorial and combinatorial, the result of a pool of pathogenicity-related genes that interact in various combinations in different genetic backgrounds.     

The contribution of exoproducts to virulence of Pseudomonas aeruginosa

Pseudomonas aeruginosa produces a large number of extracellular products which may contribute to its virulence. We have employed a genetic approach to determine the contribution of toxin A, exoenzyme S, elastase and alkaline protease to the pathogenesis of P. aeruginosa. Mutations have been introduced with chemicals or transposons. Mutants have been identified using immunological, chemical, or toxicity assays. Mutants were extensively characterized in vitro to ascertain that they were identical to their parent strain except for the production of the desired product. Appropriate mutants were compared with their parent strains in several animal models: the burned mouse model, the mouse corneal infection model, and a rat model of chronic lung infection. The data indicate that virulence of P. aeruginosa is multifactorial. Further, the relative contribution of a given P. aeruginosa product may vary with the type of infection.     

Adenylate kinase as a virulence factor of Pseudomonas aeruginosa

Adenylate kinase (AK; ATP:AMP phosphotransferase, EC 2.7.4.3) is a ubiquitous enzyme that contributes to the homeostasis of adenine nucleotides in eukaryotic and prokaryotic cells. AK catalyzes the reversible reaction Mg. ATP + AMP <--> Mg. ADP + ADP. In this study we show that AK secreted by the pathogenic strains of Pseudomonas aeruginosa appears to play an important role in macrophage cell death. We purified and characterized AK from the growth medium of a cystic fibrosis isolate strain of P. aeruginosa 8821 and hyperproduced it as a fusion protein with glutathione S-transferase. We demonstrated enhanced macrophage cell death in the presence of both the secreted and recombinant purified AK and its substrates AMP plus ATP or ADP. These data suggested that AK converts its substrates to a mixture of AMP, ADP, and ATP, which are potentially more cytotoxic than ATP alone. In addition, we observed increased macrophage killing in the presence of AK and ATP alone. Since the presence of ATPase activity on the macrophages was confirmed in the present work, external macrophage-effluxed ATP is converted to ADP, which in turn can be transformed by AK into a cytotoxic mixture of three adenine nucleotides. Evidence is presented in this study that secreted AK was detected in macrophages during infection with P. aeruginosa. Thus, the possible role of secreted AK as a virulence factor is in producing and keeping an intact pool of toxic mixtures of AMP, ADP, and ATP, which allows P. aeruginosa to exert its full virulence.