The aliphatic amidase AmiE is involved in regulation of Pseudomonas aeruginosa virulence.

We have previously shown that the eukaryotic C-type natriuretic peptide hormone (CNP) regulates Pseudomonas aeruginosa virulence and biofilm formation after binding on the AmiC sensor, triggering the amiE transcription. Herein, the involvement of the aliphatic amidase AmiE in P. aeruginosa virulence regulation has been investigated. The proteome analysis of an AmiE over-producing strain (AmiE+) revealed an expression change for 138 proteins, including some that are involved in motility, synthesis of quorum sensing compounds and virulence regulation. We observed that the AmiE+ strain produced less biofilm compared to the wild type, and over-produced rhamnolipids. In the same line, AmiE is involved in P. aeruginosa motilities (swarming and twitching) and production of the quorum sensing molecules N-acyl homoserine lactones and Pseudomonas Quinolone Signal (PQS). We observed that AmiE overproduction reduced levels of HCN and pyocyanin causing a decreased virulence in different hosts (i.e. Dictyostelium discoideum and Caenorhabditis elegans). This phenotype was further confirmed in a mouse model of acute lung infection, in which AmiE overproduction resulted in an almost fully virulence decrease. Taken together, our data suggest that, in addition to its role in bacterial secondary metabolism, AmiE is involved in P. aeruginosa virulence regulation by modulating pilus synthesis and cell-to-cell communication.

Characterisation of non-autoinducing tropodithietic Acid (TDA) production from marine sponge Pseudovibrio species.

The search for new antimicrobial compounds has gained added momentum in recent years, paralleled by the exponential rise in resistance to most known classes of current antibiotics. While modifications of existing drugs have brought some limited clinical success, there remains a critical need for new classes of antimicrobial compound to which key clinical pathogens will be naive. This has provided the context and impetus to marine biodiscovery programmes that seek to isolate and characterize new activities from the aquatic ecosystem. One new antibiotic to emerge from these initiatives is the antibacterial compound tropodithietic acid (TDA). The aim of this study was to provide insight into the bioactivity of and the factors governing the production of TDA in marine Pseudovibrio isolates from a collection of marine sponges. The TDA produced by these Pseudovibrio isolates exhibited potent antimicrobial activity against a broad spectrum of clinical pathogens, while TDA tolerance was frequent in non-TDA producing marine isolates. Comparative genomics analysis suggested a high degree of conservation among the tda biosynthetic clusters while expression studies revealed coordinated regulation of TDA synthesis upon transition from log to stationary phase growth, which was not induced by TDA itself or by the presence of the C10-acyl homoserine lactone quorum sensing signal molecule.

The impact of simvastatin on pulmonary effectors of Pseudomonas aeruginosa infection.

The statin family of cholesterol-lowering drugs is known to have pleiotropic properties which include anti-inflammatory and immunomodulatory effects. Statins exert their pleiotropic effects by altering expression of human immune regulators including pro-inflammatory cytokines. Previously we found that statins modulate virulence phenotypes of the human pathogen Pseudomonas aeruginosa, and sought to investigate if simvastatin could alter the host response to this organism in lung epithelial cells. Simvastatin increased the expression of the P. aeruginosa target genes KLF2, KLF6, IL-8 and CCL20. Furthermore, both simvastatin and P. aeruginosa induced alternative splicing of KLF6. The novel effect of simvastatin on wtKLF6 expression was found to be responsible for induction of the KLF6 regulated genes CCL20 and iNOS. Simvastatin also increased the adhesion of P. aeruginosa to host cells, without altering invasion or cytotoxicity. This study demonstrated that simvastatin had several novel effects on the pulmonary cellular immune response.

Bile acids repress hypoxia-inducible factor 1 signaling and modulate the airway immune response.

Gastroesophageal reflux (GER) frequently occurs in patients with respiratory disease and is particularly prevalent in patients with cystic fibrosis. GER is a condition in which the duodenogastric contents of the stomach leak into the esophagus, in many cases resulting in aspiration into the respiratory tract. As such, the presence of GER-derived bile acids (BAs) has been confirmed in the bronchoalveolar lavage fluid and sputum of affected patients. We have recently shown that bile causes cystic fibrosis-associated bacterial pathogens to adopt a chronic lifestyle and may constitute a major host trigger underlying respiratory infection. The current study shows that BAs elicit a specific response in humans in which they repress hypoxia-inducible factor 1α (HIF-1α) protein, an emerging master regulator in response to infection and inflammation. HIF-1α repression was shown to occur through the 26S proteasome machinery via the prolyl hydroxylase domain (PHD) pathway. Further analysis of the downstream inflammatory response showed that HIF-1α repression by BAs can significantly modulate the immune response of airway epithelial cells, correlating with a decrease in interleukin-8 (IL-8) production, while IL-6 production was strongly increased. Importantly, the effects of BAs on cytokine production can also be more dominant than the bacterium-mediated effects. However, the effect of BAs on cytokine levels cannot be fully explained by their ability to repress HIF-1α, which is not surprising, given the complexity of the immune regulatory network. The suppression of HIF-1 signaling by bile acids may have a significant influence on the progression and outcome of respiratory disease, and the molecular mechanism underpinning this response warrants further investigation.

A new regulator of pathogenicity (bvlR) is required for full virulence and tight microcolony formation in Pseudomonas aeruginosa.

LysR-type transcriptional regulators (LTTRs) are the most common family of transcriptional regulators found in the opportunistic pathogen Pseudomonas aeruginosa. They are known to regulate a wide variety of virulence determinants and have emerged recently as positive global regulators of pathogenicity in a broad spectrum of important bacterial pathogens. However, in spite of their key role in modulating expression of key virulence determinants underpinning pathogenic traits associated with the process of infection, surprisingly few are found to be transcriptionally altered by contact with host cells. BvlR (PA14_26880) an LTTR of previously unknown function, has been shown to be induced in response to host cell contact, and was therefore investigated for its potential role in virulence. BvlR expression was found to play a pivotal role in the regulation of acute virulence determinants such as type III secretion system and exotoxin A production. BvlR also played a key role in P. aeruginosa pathogenicity within the Caenorhabditis elegans acute model of infection. Loss of BvlR led to an inability to form tight microcolonies, a key step in biofilm formation in the cystic fibrosis lung, although surface attachment was increased. Unusually for LTTRs, BvlR was shown to exert its influence through the transcriptional repression of many genes, including the virulence-associated cupA and alg genes. This highlights the importance of BvlR as a new virulence regulator in P. aeruginosa with a central role in modulating key events in the pathogen-host interactome.

Identification of novel phytase genes from an agricultural soil-derived metagenome.

Environmental microorganisms are emerging as an important source of new enzymes for wide-scale industrial application. In this study, novel phytase genes were identified from a soil microbial community. For this, a function-based screening approach was utilized for the identification of phytase activity in a metagenomic library derived from an agricultural soil. Two novel phytases were identified. Interestingly, one of these phytases is an unusual histidine acid phosphatase family phytase, as the conserved motif of the active site of PhyX possesses an additional amino acid residue. The second phytase belongs to a new type, which is encoded by multiple open reading frames (ORFs) and is different to all phytases known to date, which are encoded by a single ORF.

A non-classical LysR-type transcriptional regulator PA2206 is required for an effective oxidative stress response in Pseudomonas aeruginosa.

LysR-type transcriptional regulators (LTTRs) are emerging as key circuit components in regulating microbial stress responses and are implicated in modulating oxidative stress in the human opportunistic pathogen Pseudomonas aeruginosa. The oxidative stress response encapsulates several strategies to overcome the deleterious effects of reactive oxygen species. However, many of the regulatory components and associated molecular mechanisms underpinning this key adaptive response remain to be characterised. Comparative analysis of publically available transcriptomic datasets led to the identification of a novel LTTR, PA2206, whose expression was altered in response to a range of host signals in addition to oxidative stress. PA2206 was found to be required for tolerance to H(2)O(2)in vitro and lethality in vivo in the Zebrafish embryo model of infection. Transcriptomic analysis in the presence of H(2)O(2) showed that PA2206 altered the expression of 58 genes, including a large repertoire of oxidative stress and iron responsive genes, independent of the master regulator of oxidative stress, OxyR. Contrary to the classic mechanism of LysR regulation, PA2206 did not autoregulate its own expression and did not influence expression of adjacent or divergently transcribed genes. The PA2214-15 operon was identified as a direct target of PA2206 with truncated promoter fragments revealing binding to the 5'-ATTGCCTGGGGTTAT-3' LysR box adjacent to the predicted -35 region. PA2206 also interacted with the pvdS promoter suggesting a global dimension to the PA2206 regulon, and suggests PA2206 is an important regulatory component of P. aeruginosa adaptation during oxidative stress.

Respiratory pathogens adopt a chronic lifestyle in response to bile.

Chronic respiratory infections are a major cause of morbidity and mortality, most particularly in Cystic Fibrosis (CF) patients. The recent finding that gastro-esophageal reflux (GER) frequently occurs in CF patients led us to investigate the impact of bile on the behaviour of Pseudomonas aeruginosa and other CF-associated respiratory pathogens. Bile increased biofilm formation, Type Six Secretion, and quorum sensing in P. aeruginosa, all of which are associated with the switch from acute to persistent infection. Furthermore, bile negatively influenced Type Three Secretion and swarming motility in P. aeruginosa, phenotypes associated with acute infection. Bile also modulated biofilm formation in a range of other CF-associated respiratory pathogens, including Burkholderia cepacia and Staphylococcus aureus. Therefore, our results suggest that GER-derived bile may be a host determinant contributing to chronic respiratory infection.

Pseudomonas aeruginosa Alkyl quinolones repress hypoxia-inducible factor 1 (HIF-1) signaling through HIF-1α degradation.

The transcription factor hypoxia-inducible factor 1 (HIF-1) has recently emerged to be a crucial regulator of the immune response following pathogen perception, including the response to the important human pathogen Pseudomonas aeruginosa. However, as mechanisms involved in HIF-1 activation by bacterial pathogens are not fully characterized, understanding how bacteria and bacterial compounds impact on HIF-1α stabilization remains a major challenge. In this context, we have focused on the effect of secreted factors of P. aeruginosa on HIF-1 regulation. Surprisingly, we found that P. aeruginosa cell-free supernatant significantly repressed HIF-1α protein levels. Further characterization revealed that HIF-1α downregulation was dependent on a subset of key secreted factors involved in P. aeruginosa pathogenesis, the 2-alkyl-4-quinolone (AQ) quorum sensing (QS) signaling molecules, and in particular the pseudomonas quinolone signal (PQS). Under hypoxic conditions, the AQ-dependent downregulation of HIF-1α was linked to the suppressed induction of the important HIF-1 target gene hexokinase II. Furthermore, we demonstrated that AQ molecules directly target HIF-1α protein degradation through the 26S-proteasome proteolytic pathway but independently of the prolyl hydroxylase domain (PHD). In conclusion, this is the first report showing that bacterial molecules can repress HIF-1α protein levels. Manipulation of HIF-1 signaling by P. aeruginosa AQs could have major consequences for the host response to infection and may facilitate the infective properties of this pathogen.

Evidence of bacteriophage-mediated horizontal transfer of bacterial 16S rRNA genes in the viral metagenome of the marine sponge Hymeniacidon perlevis.

Marine sponges have never been directly examined with respect to the presence of viruses or their potential involvement in horizontal gene transfer. Here we demonstrate for the first time, to our knowledge, the presence of viruses in the marine sponge Hymeniacidon perlevis. Moreover, bacterial 16S rDNA was detected in DNA isolated from these viruses, indicating that phage-derived transduction appears to occur in H. perlevis. Phylogenetic analysis revealed that bacterial 16S rDNA isolated from sponge-derived viral and total DNA differed significantly, indicating that not all species are equally involved in transduction.

MexT functions as a redox-responsive regulator modulating disulfide stress resistance in Pseudomonas aeruginosa.

MexT is a global LysR transcriptional regulator known to modulate antibiotic resistance and virulence in Pseudomonas aeruginosa. In this study, a novel role for MexT in mediating intrinsic disulfide stress resistance was demonstrated, representing the first identified phenotype associated with inactivation of this regulator in wild-type cells. Disruption of mexT resulted in increased susceptibility to the disulfide stress elicitor diamide [diazenedicarboxylic acid bis(N,N,-di-methylamide)]. This compound is known to elicit a specific stress response via depletion of reduced glutathione and alteration of the cellular redox environment, implicating MexT in redox control. In support of this, MexT-regulated targets, including the MexEF-OprN multidrug efflux system, were induced by subinhibitory concentrations of diamide. A mexF insertion mutant also exhibited increased diamide susceptibility, implicating the MexEF-OprN efflux system in MexT-associated disulfide stress resistance. Purified MexT protein was observed to form an oligomeric complex in the presence of oxidized glutathione, with a calculated redox potential of -189 mV. This value far exceeds the thiol-disulfide redox potential of the bacterial cytoplasm, ensuring that MexT remains reduced under normal physiological conditions. MexT is activated by mutational disruption of the predicted quinone oxidoreductase encoded by mexS. Alterations in the cellular redox state were observed in a mexS mutant (PA14nfxC), supporting a model whereby the perception of MexS-associated redox signals by MexT leads to the induction of the MexEF-OprN efflux system, which, in turn, may mediate disulfide stress resistance via efflux of electrophilic compounds.

The Pseudomonas quinolone signal (PQS), and its precursor HHQ, modulate interspecies and interkingdom behaviour.

The Pseudomonas quinolone signal (PQS), and its precursor 2-heptyl-4-quinolone (HHQ), play a key role in coordinating virulence in the important cystic fibrosis pathogen Pseudomonas aeruginosa. The discovery of HHQ analogues in Burkholderia and other microorganisms led us to investigate the possibility that these compounds can influence interspecies behaviour. We found that surface-associated phenotypes were repressed in Gram-positive and Gram-negative bacteria as well as in pathogenic yeast in response to PQS and HHQ. Motility was repressed in a broad range of bacteria, while biofilm formation in Bacillus subtilis and Candida albicans was repressed in the presence of HHQ, though initial adhesion was unaffected. Furthermore, HHQ exhibited potent bacteriostatic activity against several Gram-negative bacteria, including pathogenic Vibrio vulnificus. Structure-function analysis using synthetic analogues provided an insight into the molecular properties that underpin the ability of these compounds to influence microbial behaviour, revealing the alkyl chain to be fundamental. Defining the influence of these molecules on microbial-eukaryotic-host interactions will facilitate future therapeutic strategies which seek to combat microorganisms that are recalcitrant to conventional antimicrobial agents.

Impaired expression of hypoxia-inducible factor-1α in cystic fibrosis airway epithelial cells - a role for HIF-1 in the pathophysiology of CF?

The continuous infection-inflammation cycle plays a crucial role in the progression of cystic fibrosis (CF) disease. This noxious loop can be aggravated by a reduced partial pressure of oxygen in the blood, hypoxemia, present in CF patients. These interconnected factors, hypoxia, inflammation and infection, by stabilizing the hypoxia-inducible factor-1α (HIF-1α) protein subunit, are able to activate the transcription factor HIF-1. To date, data investigating the potential role of HIF-1 in CF are scarce. Our results demonstrated that HIF-1α protein expression was altered in CF-affected compared to CFTR-corrected airway epithelial cells in unsimulated and simulated hypoxic conditions. In contrast, when CF-affected cells were infected with Pseudomonas aeruginosa, HIF-1α was more stabilized compared to CFTR-corrected cells. As HIF-1 is linked with an efficient immune response and pulmonary complications in cystic fibrosis, this difference in HIF-1α protein levels could have an impact in the CF pathology and the persistence of P. aeruginosa infection.

Characterisation of carbapenem-resistant Acinetobacter baumannii outbreak strains producing OXA-58 in Turkey.

The aim of this study was to characterise the molecular epidemiology and mechanisms of carbapenem resistance of nosocomial Acinetobacter baumannii isolates in a new university hospital in Turkey. A total of 145 carbapenem-resistant A. baumannii (CRAB) isolates were collected during the period 2003-2006. All isolates were typed by amplified fragment length polymorphism (AFLP) analysis. AFLP analysis showed three predominant clusters consisting of 72, 20 and 12 clinical strains as well as some smaller clusters and 23 unique strains. The three main clonal AFLP types corresponded to three major antibiotic susceptibility patterns. One environmental isolate was found related to the major outbreak clone. The reference type strains of European clones I, II and III were also typed by AFLP and analysed for clonal similarity. Polymerase chain reaction (PCR) analysis of different carbapenem resistance genes showed that strains from each of the three main clusters as well as 79% of the remaining strains harboured the bla(OXA-58) gene. No genes encoding the metallo-beta-lactamases GIM-1, SIM-1, SPM-1, IMP-like and VIM-like or the oxacillinases OXA-24-like and OXA-23-like were detected. In conclusion, multiple clones of CRAB strains producing OXA-58-type oxacillinase were responsible for a sustained CRAB outbreak occurring in a hospital in Turkey. These isolates were not associated with A. baumannii strains of the major European clones I, II or III.

Emergence of extended-spectrum beta-lactamase and fluoroquinolone resistance genes among Irish multidrug-resistant isolates.

This study sought to identify mechanisms behind resistance to third-generation cephalosporins and ciprofloxacin in Irish multidrug-resistant Enterobacteriaceae isolates. The most prevalent extended-spectrum beta-lactamase genes identified were bla SHV-12 and bla CTX-M-15. These were associated with the fluoroquinolone resistance genes aac(6')-IB-cr, qnrA and qnrB, not previously reported in Irish isolates.

Antibiotic selection leads to inadvertent selection of nfxC-type phenotypic mutants in Pseudomonas aeruginosa.

Spontaneous nfxC-type phenotypic mutants of Pseudomonas aeruginosa are characterized by chloramphenicol resistance and increased expression of the multidrug efflux pump, MexEF-OprN. These mutants frequently arise during the propagation of bacteria in laboratory conditions. This study shows that some of the discrepancies in transcriptomic/phenotypic profiles of two rsmA mutants, PAZH13 (rsmA mutant in strain PAO1) and PAKΔrsmA, are due to the nfxC-type nature of PAZH13. This indicates that awareness of the possible occurrence of these spontaneous nfxC-type phenotypes is necessary. Screening of mexE levels could provide clarity when an nfxC-type phenotype is suspected.

Integron class 1 reservoir among highly resistant gram-negative microorganisms recovered at a Dutch teaching hospital.

Integrons play an important role in the dissemination of resistance genes among bacteria. Nearly 70% of highly resistant gram-negative bacteria isolated at a tertiary care hospital harbored an integron. Epidemiologic analysis suggests that horizontal gene transfer is an important mechanism of resistance spread and has a greater contribution than cross-transmission to levels of resistance in settings where highly resistant gram-negative bacteria are endemic.

MexT modulates virulence determinants in Pseudomonas aeruginosa independent of the MexEF-OprN efflux pump.

In the human pathogen Pseudomonas aeruginosa, the LysR-family regulator MexT modulates the induction of the tripartite MexEF-OprN resistance nodulation-division multi-drug efflux system resulting in increased resistance to diverse antibiotics. The MexEF-OprN system is normally quiescent in wild-type cells, but is highly induced in nfxC-type phenotypic mutants in a MexT dependent manner. In addition to antibiotic resistance, induction of mexEF-oprN in nfxC-type mutants has been linked to reduced levels of homoserine lactone-dependent virulence traits, including pyocyanin, elastase, rhamnolipids and PQS and to reduced expression of type three secretion effector proteins. In this study, MexT is overexpressed in wild-type PAO1 and an isogenic mexEF deletion mutant to determine if MexT regulates diverse virulence phenotypes dependent or independent of MexEF-OprN. It is shown that MexT regulates type three secretion, pyocyanin production and early surface attachment independent of MexEF-OprN. In contrast, MexT modulation of the expression of the virulence genes rhlA, lasB and hcnB is dependent on MexEF-OprN, which apparently mediates these effects via efflux of cell-signaling intermediates. The data presented demonstrates that MexT may play a more global role in modulating P. aeruginosa virulence than previously reported and suggests that MexT regulates diverse targets that mediate phenotypic alterations independent of MexEF-OprN.

Subinhibitory concentrations of the cationic antimicrobial peptide colistin induce the pseudomonas quinolone signal in Pseudomonas aeruginosa.

Colistin is an important cationic antimicrobial peptide (CAMP) in the fight against Pseudomonas aeruginosa infection in cystic fibrosis (CF) lungs. The effects of subinhibitory concentrations of colistin on gene expression in P. aeruginosa were investigated by transcriptome and functional genomic approaches. Analysis revealed altered expression of 30 genes representing a variety of pathways associated with virulence and bacterial colonization in chronic infection. These included response to osmotic stress, motility, and biofilm formation, as well as genes associated with LPS modification and quorum sensing (QS). Most striking was the upregulation of Pseudomonas quinolone signal (PQS) biosynthesis genes, including pqsH, pqsB and pqsE, and the phenazine biosynthesis operon. Induction of this central component of the QS network following exposure to subinhibitory concentrations of colistin may represent a switch to a more robust population, with increased fitness in the competitive environment of the CF lung.