Siderophore-mediated signaling regulates virulence factor production in Pseudomonasaeruginosa

Transcriptional profiling of Mycoplasma hyopneumoniae during iron depletion using microarrays

Mycoplasma hyopneumoniae, the causative agent of swine enzootic pneumonia and a major component of the porcine respiratory disease complex, continues to confound swine producers despite control programmes worldwide. The disease is chronic and self-limiting, but the host is subject to immunopathological changes that potentiate respiratory disease associated with other pathogens. The response of M. hyopneumoniae to environmental stress is of interest because of its relevance to virulence mechanisms in other bacterial pathogens. One of these stressors, iron deprivation, is a prominent feature of the host innate immune response, and most certainly impacts growth of mycoplasmas in vivo. To study this, microarray technology was applied to the transcriptome analysis of M. hyopneumoniae during iron deprivation. An array consisting of 632 of the 698 ORFs in the genome was used to compare the mRNA isolated from organisms grown under normal laboratory conditions with that from organisms subjected to iron deprivation with the chelator 2,2'-dipyridyl. This analysis identified 27 genes that were either up- or down-regulated in response to low-iron growth conditions (P<0.01), with an estimated false discovery rate below 10 %. These included genes encoding transport proteins, enzymes involved in energy metabolism, and components of the translation process. Ten of the 27 identified genes had no assigned function. These studies indicate that M. hyopneumoniae can respond to changes in environmental conditions, but the mechanism employed remains unknown.

Enhancement of in vitro growth of pathogenic bacteria by norepinephrine: importance of inoculum density and role of transferrin

Norepinephrine is a stress hormone that enhances bacterial growth. We examined the effects of a small inoculum on the norepinephrine-induced growth of species previously reported to be unaffected by norepinephrine. The results indicated that a reduced inoculum density is essential for observing norepinephrine-induced effects. Additional studies using serum-free media suggested that transferrin plays a role in norepinephrine-induced growth.

Quorum quenching by an N-acyl-homoserine lactone acylase from Pseudomonas aeruginosa PAO1

The virulence of the opportunistic human pathogen Pseudomonas aeruginosa PAO1 is controlled by an N-acyl-homoserine lactone (AHL)-dependent quorum-sensing system. During functional analysis of putative acylase genes in the P. aeruginosa PAO1 genome, the PA2385 gene was found to encode an acylase that removes the fatty acid side chain from the homoserine lactone (HSL) nucleus of AHL-dependent quorum-sensing signal molecules. Analysis showed that the posttranslational processing of the acylase and the hydrolysis reaction type are similar to those of the beta-lactam acylases, strongly suggesting that the PA2385 protein is a member of the N-terminal nucleophile hydrolase superfamily. In a bioassay, the purified acylase was shown to degrade AHLs with side chains ranging in length from 11 to 14 carbons at physiologically relevant low concentrations. The substituent at the 3' position of the side chain did not affect activity, indicating broad-range AHL quorum-quenching activity. Of the two main AHL signal molecules of P. aeruginosa PAO1, N-butanoyl-l-homoserine lactone (C4-HSL) and N-(3-oxododecanoyl)-l-homoserine lactone (3-oxo-C12-HSL), only 3-oxo-C12-HSL is degraded by the enzyme. Addition of the purified protein to P. aeruginosa PAO1 cultures completely inhibited accumulation of 3-oxo-C12-HSL and production of the signal molecule 2-heptyl-3-hydroxy-4(1H)-quinolone and reduced production of the virulence factors elastase and pyocyanin. Similar results were obtained when the PA2385 gene was overexpressed in P. aeruginosa. These results demonstrate that the protein has in situ quorum-quenching activity. The quorum-quenching AHL acylase may enable P. aeruginosa PAO1 to modulate its own quorum-sensing-dependent pathogenic potential and, moreover, offers possibilities for novel antipseudomonal therapies.

Characterization of a gene encoding an acetylase required for pyoverdine synthesis in Pseudomonas aeruginosa

Strains of Pseudomonas aeruginosa secrete one of three pyoverdine siderophores (types I to III). We have characterized a gene, pvdY(II) (for the pvdY gene present in type II P. aeruginosa strains), that is only present in strains that make type II pyoverdine. A mutation in pvdY(II) prevented pyoverdine synthesis. Bioinformatic, genetic, and biochemical approaches indicate that the PvdYII enzyme catalyzes acetylation of hydroxyornithine. Expression of pvdY(II) is repressed by the presence of iron and upregulated by the presence of type II pyoverdine. Characterization of pvdY(II) provides insights into the molecular basis for production of different pyoverdines by different strains of P. aeruginosa.

Heme and a five-amino-acid hemophore region form the bipartite stimulus triggering the has signaling cascade

Bacterial cells sense the extracellular environment and adapt to that environment by activating gene regulation circuits, often by means of signaling molecules. The Serratia marcescens hemophore is a signaling molecule that acts as an extracellular heme-scavenging protein. The heme-loaded hemophore interacts with its cognate receptor (HasR), triggering transmembrane signaling and turning on transcription of hemophore-dependent heme uptake genes. We investigated the features of the holo-hemophore, the only HasR ligand known to act as an inducer. We used a hemophore mutant that does not deliver its heme and a HasR mutant that does not bind heme, and we showed that heme transfer from the hemophore to the receptor is necessary for induction. Using a hemophore mutant that does not bind heme and that blocks heme transport, we demonstrated that two molecules that do not interact (heme and the mutant hemophore) may nonetheless induce this system. These findings suggest that hemophore-mediated induction and heme transport involve different mechanisms. The hemophore region important for induction was precisely localized to amino acids 50 to 55, which lie in one of the two HasR-binding hemophore regions. This bipartite stimulus probably corresponds to a physiological process because heme is transferred to the receptor before apo-hemophore release. This bipartite regulation mechanism may allow the bacterium to adjust its heme transport mechanism to the perceived environmental heme concentration.

The heterologous siderophores ferrioxamine B and ferrichrome activate signaling pathways in Pseudomonas aeruginosa

Pseudomonas aeruginosa secretes two siderophores, pyoverdine and pyochelin, under iron-limiting conditions. These siderophores are recognized at the cell surface by specific outer membrane receptors, also known as TonB-dependent receptors. In addition, this bacterium is also able to incorporate many heterologous siderophores of bacterial or fungal origin, which is reflected by the presence of 32 additional genes encoding putative TonB-dependent receptors. In this work, we have used a proteomic approach to identify the inducing conditions for P. aeruginosa TonB-dependent receptors. In total, 11 of these receptors could be discerned under various conditions. Two of them are only produced in the presence of the hydroxamate siderophores ferrioxamine B and ferrichrome. Regulation of their synthesis is affected by both iron and the presence of a cognate siderophore. Analysis of the P. aeruginosa genome showed that both receptor genes are located next to a regulatory locus encoding an extracytoplasmic function sigma factor and a transmembrane sensor. The involvement of this putative regulatory locus in the specific induction of the ferrioxamine B and ferrichrome receptors has been demonstrated. These results show that P. aeruginosa has evolved multiple specific regulatory systems to allow the regulation of TonB-dependent receptors.

Genetic and transcriptional analysis of the siderophore malleobactin biosynthesis and transport genes in the human pathogen Burkholderia pseudomallei K96243

Burkholderia pseudomallei is a gram-negative facultative intracellular pathogen that causes melioidosis, an invasive disease of humans and animals. To address the response of this bacterium to iron-limiting conditions, we first performed a global transcriptional analysis of RNA extracted from bacteria grown under iron-limiting and iron-rich conditions by microarrays. We focused our study on those open reading frames (ORFs) induced under iron limitation, which encoded predicted proteins that could be involved in the biosynthesis and uptake of the siderophore malleobactin. We purified this siderophore and determined that it consisted of at least three compounds with different molecular weights. We demonstrated that ORFs BPSL1776 and BPSL1774, designated mbaA and mbaF, respectively, are involved in the biosynthesis of malleobactin, while BPSL1775, named fmtA, is involved in its transport. These genes are in an operon with two other ORFs (mbaJ and mbaI) whose transcription is under the control of MbaS, a protein that belongs to the extracytoplasmic function sigma factors. Interestingly, the transcription of the mbaA, fmtA, and mbaS genes is not controlled by the availability of the siderophore malleobactin.

Mutational analysis of an extracytoplasmic-function sigma factor to investigate its interactions with RNA polymerase and DNA

The extracytoplasmic-function (ECF) family of sigma factors comprises a large group of proteins required for synthesis of a wide variety of extracytoplasmic products by bacteria. Residues important for core RNA polymerase (RNAP) binding, DNA melting, and promoter recognition have been identified in conserved regions 2 and 4.2 of primary sigma factors. Seventeen residues in region 2 and eight residues in region 4.2 of an ECF sigma factor, PvdS from Pseudomonas aeruginosa, were selected for alanine-scanning mutagenesis on the basis of sequence alignments with other sigma factors. Fourteen of the mutations in region 2 had a significant effect on protein function in an in vivo assay. Four proteins with alterations in regions 2.1 and 2.2 were purified as His-tagged fusions, and all showed a reduced affinity for core RNAP in vitro, consistent with a role in core binding. Region 2.3 and 2.4 mutant proteins retained the ability to bind core RNAP, but four mutants had reduced or no ability to cause core RNA polymerase to bind promoter DNA in a band-shift assay, identifying residues important for DNA binding. All mutations in region 4.2 reduced the activity of PvdS in vivo. Two of the region 4.2 mutant proteins were purified, and each showed a reduced ability to cause core RNA polymerase to bind to promoter DNA. The results show that some residues in PvdS have functions equivalent to those of corresponding residues in primary sigma factors; however, they also show that several residues not shared with primary sigma factors contribute to protein function.

Influence of the regulatory protein RsmA on cellular functions in Pseudomonas aeruginosa PAO1, as revealed by transcriptome analysis

RsmA is a posttranscriptional regulatory protein in Pseudomonas aeruginosa that works in tandem with a small non-coding regulatory RNA molecule, RsmB (RsmZ), to regulate the expression of several virulence-related genes, including the N-acyl-homoserine lactone synthase genes lasI and rhlI, and the hydrogen cyanide and rhamnolipid biosynthetic operons. Although these targets of direct RsmA regulation have been identified, the full impact of RsmA on cellular activities is not as yet understood. To address this issue the transcriptome profiles of P. aeruginosa PAO1 and an isogenic rsmA mutant were compared. Loss of RsmA altered the expression of genes involved in a variety of pathways and systems important for virulence, including iron acquisition, biosynthesis of the Pseudomonas quinolone signal (PQS), the formation of multidrug efflux pumps, and motility. Not all of these effects can be explained through the established regulatory roles of RsmA. This study thus provides both a first step towards the identification of further genes under RsmA posttranscriptional control in P. aeruginosa and a fuller understanding of the broader impact of RsmA on cellular functions.

Targeting mechanisms of Pseudomonas aeruginosa pathogenesis

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

Variations on a theme: diverse N-acyl homoserine lactone-mediated quorum sensing mechanisms in gram-negative bacteria

Many Gram-negative bacteria employ a mechanism of cell-cell communication known as quorum sensing (QS). The role of QS is to enable the cells in a culture to coordinate their gene expression profile with changes in the population cell density. The best characterized mechanisms of QS employ N-acylated homoserine lactones (AHLs) as signalling molecules. These AHLs are made by enzymes known as LuxI homologs, and accumulate in the culture supernatant at a rate proportional to the increase in cell density. Once the AHL concentration exceeds a certain threshold value, these ligands bind to intracellular receptors known as LuxR homologs. The latter are transcriptional regulators, whose activity alters upon binding the AHL ligand, thereby eliciting a change in gene transcription. Over the last five years, it has become increasingly obvious that this is a rather simplistic view of AHL-dependent QS, and that in fact, there is considerable diversity in the way in which LuxI-R homologs operate. The aim of the current review is to describe these variations on the basic theme, and to show how functional genomics is revolutionizing our understanding of QS-controlled regulons.

Complex regulation of AprA metalloprotease in Pseudomonas fluorescens M114: evidence for the involvement of iron, the ECF sigma factor, PbrA and pseudobactin M114 siderophore

In the soil bacterium Pseudomonas fluorescens M114, extracellular proteolytic activity and fluorescent siderophore (pseudobactin M114) production were previously shown to be co-ordinately negatively regulated in response to environmental iron levels. An iron-starvation extracytoplasmic function sigma factor, PbrA, required for the transcription of siderophore biosynthetic genes, was also implicated in M114 protease regulation. The current study centred on the characterization and genetic regulation of the gene(s) responsible for protease production in M114. A serralysin-type metalloprotease gene, aprA, was identified and found to encode the major, if not only, extracellular protease produced by this strain. The expression of aprA and its protein product were found to be subject to complex regulation. Transcription analysis confirmed that PbrA was required for full aprA transcription under low iron conditions, while the ferric uptake regulator, Fur, was implicated in aprA repression under high iron conditions. Interestingly, the iron regulation of AprA was dependent on culture conditions, with PbrA-independent AprA-mediated proteolytic activity observed on skim milk agar supplemented with yeast extract, when supplied with iron or purified pseudobactin M114. These effects were not observed on skim milk agar without yeast extract. PbrA-independent aprA expression was also observed from a truncated transcriptional fusion when grown in sucrose asparagine tryptone broth supplied with iron or purified pseudobactin M114. Thus, experimental evidence suggested that iron mediated its effects via transcriptional activation by PbrA under low iron conditions, while an as-yet-unidentified sigma factor(s) may be required for the PbrA-independent aprA expression and AprA proteolytic activity induced by siderophore and iron.

Reciprocal regulation of pyoluteorin production with membrane transporter gene expression in Pseudomonas fluorescens Pf-5

Pyoluteorin is a chlorinated polyketide antibiotic secreted by the rhizosphere bacterium Pseudomonas fluorescens Pf-5. Genes encoding enzymes and transcriptional regulators involved in pyoluteorin production are clustered in the genome of Pf-5. Sequence analysis of genes adjacent to the known pyoluteorin biosynthetic gene cluster revealed the presence of an ABC transporter system. We disrupted two putative ABC transporter genes by inserting transcriptional fusions to an ice nucleation reporter gene. Mutations in pltI and pltJ, which are predicted to encode a membrane fusion protein and an ATP-binding cassette of the ABC transporter, respectively, greatly reduced pyoluteorin production by Pf-5. During the transition from exponential growth to stationary phase, populations of a pltI mutant were lower than those of a pltI+ strain in a culture medium containing pyoluteorin, suggesting a role for the transport system in efflux and the resistance of Pf-5 to the antibiotic. Although pltI or pltJ mutant strains displayed low pyoluteorin production, they did not accumulate proportionately more of the antibiotic intracellularly, indicating that pltI and pltJ do not encode an exclusive exporter for pyoluteorin. Transcription of the putative pyoluteorin efflux genes pltI and pltJ was enhanced by exogenous pyoluteorin. These new observations parallel an earlier finding that pyoluteorin enhances the transcription of pyoluteorin biosynthesis genes and pyoluteorin production in Pf-5. This report provides evidence of a coordination of pyoluteorin production and the transcription of genes encoding a linked transport apparatus, wherein each requires the other for optimal expression.

PchR-box recognition by the AraC-type regulator PchR of Pseudomonas aeruginosa requires the siderophore pyochelin as an effector

Under iron limitation, the opportunistic human pathogen Pseudomonas aeruginosa produces the siderophore pyochelin. When secreted into the extracellular environment, pyochelin complexes ferric ions and delivers them, via the outer membrane receptor FptA, to the bacterial cytoplasm. Extracellular pyochelin also acts as a signalling molecule, inducing the expression of pyochelin biosynthesis and uptake genes by a mechanism involving the AraC-type regulator PchR. We have identified a 32 bp conserved sequence element (PchR-box) in promoter regions of pyochelin-controlled genes and we show that the PchR-box in the pchR-pchDCBA intergenic region is essential for the induction of the pyochelin biosynthetic operon pchDCBA and the repression of the divergently transcribed pchR gene. PchR was purified as a fusion with maltose-binding protein (MBP). Mobility shift assays demonstrated specific binding of MBP-PchR to the PchR-box in the presence, but not in the absence of pyochelin and iron. PchR-box mutations that interfered with pyochelin-dependent regulation in vivo, also affected pyochelin-dependent PchR-box recognition in vitro. We conclude that pyochelin, probably in its iron-loaded state, is the intracellular effector required for PchR-mediated regulation. The fact that extracellular pyochelin triggers this regulation suggests that the siderophore can enter the cytoplasm.

Identification of a twin-arginine translocation system in Pseudomonas syringae pv. tomato DC3000 and its contribution to pathogenicity and fitness

The bacterial plant pathogen Pseudomonas syringae pv. tomato DC3000 (DC3000) causes disease in Arabidopsis thaliana and tomato plants, and it elicits the hypersensitive response in nonhost plants such as Nicotiana tabacum and Nicotiana benthamiana. While these events chiefly depend upon the type III protein secretion system and the effector proteins that this system translocates into plant cells, additional factors have been shown to contribute to DC3000 virulence and still many others are likely to exist. Therefore, we explored the contribution of the twin-arginine translocation (Tat) system to the physiology of DC3000. We found that a tatC mutant strain of DC3000 displayed a number of phenotypes, including loss of motility on soft agar plates, deficiency in siderophore synthesis and iron acquisition, sensitivity to copper, loss of extracellular phospholipase activity, and attenuated virulence in host plant leaves. In the latter case, we provide evidence that decreased virulence of tatC mutants likely arises from a synergistic combination of (i) compromised fitness of bacteria in planta; (ii) decreased efficiency of type III translocation; and (iii) cytoplasmically retained virulence factors. Finally, we demonstrate a novel broad-host-range genetic reporter based on the green fluorescent protein for the identification of Tat-targeted secreted virulence factors that should be generally applicable to any gram-negative bacterium. Collectively, our evidence supports the notion that virulence of DC3000 is a multifactorial process and that the Tat system is an important virulence determinant of this phytopathogenic bacterium.

Hypermutability Impedes Cooperation in Pathogenic Bacteria

When the supply of beneficial mutations limits adaptation, bacterial mutator alleles can reach high frequencies by hitchhiking with advantageous mutations. However, when populations are well adapted to their environments, the increased rate of deleterious mutations makes hypermutability selectively disadvantageous. Here, we consider a further cost of hypermutability: its potential to break down cooperation (group-beneficial behavior that is costly to the individual). This probably occurs for three reasons. First, an increased rate at which 'cheating' genotypes are generated; second, an increased probability of producing efficient cheats; and third, a decrease in relatedness (not addressed in the present study). We used Pseudomonas aeruginosa's production of extracellular iron-scavenging molecules, siderophores, to determine if cheating evolved more readily in mutator populations. Siderophore production is costly to individual bacteria but benefits all nearby cells. Siderophore-deficient cheats therefore have a selective advantage within populations. We observed the de novo evolution and subsequent increase in frequency of siderophore cheats within both wild-type and mutator populations for 200 generations. Cheats appeared and increased in frequency more rapidly in mutator populations. The presence of cheats was costly to the group, as shown by a negative correlation between cheat frequency and population density.

Iron salts perturb biofilm formation and disrupt existing biofilms of Pseudomonas aeruginosa

Bacterial biofilms are thought to aid in the survivability of a variety of intractable infections in humans. Specifically, biofilm production in Pseudomonas aeruginosa has been shown to play a significant role in chronic infection of cystic fibrosis (CF) patients. Unfortunately, no clinically effective inhibitors of biofilm formation are available. A rapid screen of 4509 compounds for nonantibiotic biofilm inhibitors in Pseudomonas aeruginosa PA14 was executed in 384-well plates. Among those compounds, ferric ammonium citrate inhibited biofilm formation in a dose-dependent manner; other iron salts functioned similarly. In addition to biofilm inhibition in static culture, pregrown biofilms could be disrupted and cleared by switching to iron-rich media in flow-chamber experiments. Furthermore, P. aeruginosa strains taken from the sputum of 20 CF patients showed a similar response to elevated iron levels. Previous expression-profiling analyses demonstrated that high levels of iron repress the expression of genes whose products are essential for scavenging iron and that expression of these genes is critical for virulence. Our results, combined with existing transcriptional-profiling data, now indicate that elevated iron concentrations repress the expression of certain genes essential for biofilm production in P. aeruginosa.

Modulation of lung epithelial functions by Pseudomonas aeruginosa

Microorganisms gain access to the airways and respiratory epithelial surface during normal breathing. Most inhaled microbes are trapped on the mucous layer coating the nasal epithelium and upper respiratory tract, and are cleared by ciliary motion. Microorganisms reaching the alveolar spaces are deposited on the pulmonary epithelium. This contact initiates complex offensive and defensive strategies by both parties. Here, we briefly outline how the pulmonary pathogen Pseudomonas aeruginosa uses multi-pronged strategies that include cell surface appendages, and secreted and injected virulence determinants to switch from an unobtrusive soil bacterium to a pathogen for lung epithelium colonization. Understanding the complex interactions between the lung epithelium and P. aeruginosa might enable more effective therapeutic strategies against infection in cystic fibrosis and immuno-compromised individuals.

FpvIR control of fpvA ferric pyoverdine receptor gene expression in Pseudomonas aeruginosa: demonstration of an interaction between FpvI and FpvR and identification of mutations in each compromising this interaction

FpvR is a presumed cytoplasmic membrane-associated anti-sigma factor that controls the activities of extracytoplasmic function sigma factors PvdS and FpvI responsible for transcription of pyoverdine biosynthetic genes and the ferric pyoverdine receptor gene, fpvA, respectively. Using deletion analysis and an in vivo bacterial two-hybrid system, FpvR interaction with these sigma factors was confirmed and shown to involve the cytoplasmic N-terminal 67 amino acid resides of FpvR. FpvR bound specifically to a C-terminal region of FpvI corresponding to region 4 of the sigma(70) family of sigma factors. FpvR and FpvI mutant proteins compromised for this interaction were generated by random and site-directed PCR mutagenesis and invariably contained secondary structure-altering proline substitution in predicted alpha-helices within the FpvR N terminus or FpvI region 4. PvdS was shown to bind to the same N-terminal region of FpvR, and FpvR mutations compromising FpvI binding also compromised PvdS binding, although some mutations had a markedly greater impact on PvdS binding. Apparently, these two sigma factors bind to FpvR in a substantially similar but not identical fashion. Intriguingly, defects in FpvR binding correlated with a substantial drop in yields of the FpvI and to a lesser extent PvdS sigma factors, suggesting that FpvR-bound FpvI and PvdS are stable while free and active sigma factor is prone to turnover.

Effects of azithromycin on clinical isolates of Pseudomonas aeruginosa from cystic fibrosis patients

There is considerable interest in the use of azithromycin for the treatment of lung disease in patients with cystic fibrosis (CF). Although its mechanism of action as an inhibitor of bacterial protein synthesis has been well-established, it is less clear how azithromycin ameliorates the lung disease associated with Pseudomonas aeruginosa, which is considered to be resistant to the drug. We tested the effects of azithromycin on clinical isolates (CIs) from CF patients and compared them with laboratory reference strains to establish how this drug might interfere with the production of bacterial virulence factors that are relevant to the pathogenesis of airway disease in CF patients. Azithromycin inhibited P aeruginosa PAO1 protein synthesis by 80%, inhibiting bacterial growth and the expression of immunostimulatory exoproducts such as pyocyanin, as well as the gene products necessary for biofilm formation. In contrast, the effects of azithromycin on CIs of P aeruginosa were much more variable, due in large part to their slow growth and limited exoproduct expression. Culture supernatants for two of three clinical strains induced appreciable CXCL8 expression from cultured epithelial cells. Azithromycin treatment of the organisms inhibited 65 to 70% of this induction; azithromycin had no direct effect on the ability of either normal cells or CF epithelial cells to produce CXCL8. Azithromycin does decrease the P aeruginosa synthesis of immunostimulatory exoproducts and is likely to be most effective against planktonic, actively growing bacteria. This effect is less predictable against CIs than the prototypic strain PAO1.

An iron uptake operon required for proper nodule development in the Bradyrhizobium japonicum-soybean symbiosis

Rhizobia live in the soil or enter into a nitrogen-fixing symbiosis with a suitable host plant. Each environment presents different challenges with respect to iron acquisition. The soybean symbiont Bradyrhizobium japonicum 61A152 can utilize a variety of siderophores (Fe[III]-specific ligands). Purification of iron-regulated outer membrane proteins had previously allowed the cloning of a gene, fegA, from B. japonicum 61A152, whose predicted protein shares significant amino acid similarity with known TonB-dependent siderophore receptors. Here, we show that fegA is in an operon with a gene, fegB, that is predicted to encode an inner membrane protein. Characterization of fegAB and fegB mutants shows that bothfegA and fegB are required for utilization of the siderophore ferrichrome. Whereas thefegB mutant forms a normal symbiosis, the fegAB mutant has a dramatic phenotype in planta. Six weeks after inoculation with a fegAB strain, soybean nodules do not contain leghemoglobin and do not fix nitrogen. Infected cells contain few symbiosomes and are filled with vesicles. As ferrichrome is a fungal siderophore not likely to be available in nodules, the symbiotic defect suggests that the fegAB operon is serving a different function in planta, possibly one involved in signaling between the two partners.

Involvement of AlgQ in transcriptional regulation of pyoverdine genes in Pseudomonas aeruginosa PAO1

In response to iron limitation, Pseudomonas aeruginosa produces the fluorescent siderophore pyoverdine. Transcription of pyoverdine biosynthetic (pvd) genes is driven by the iron starvation sigma factor PvdS, which is negatively regulated by the Fur-Fe(II) holorepressor. We studied the effect of AlgQ, the Escherichia coli Rsd orthologue, on pyoverdine production by P. aeruginosa PAO1. AlgQ is a global regulatory protein which activates alginate, ppGpp, and inorganic polyphosphate synthesis through a cascade involving nucleoside diphosphate kinase (Ndk). AlgQ is also capable of interacting with region 4 of RpoD. In a reconstituted E. coli system, PvdS-dependent transcription from the pvdA promoter was doubled by the multicopy algQ gene. The P. aeruginosa DeltaalgQ mutant exhibited a moderate but reproducible reduction in pyoverdine production compared with wild-type PAO1, as a result of a decline in transcription of pvd genes. PvdS expression was not affected by the algQ mutation. Single-copy algQ fully restored pyoverdine production and expression of pvd genes in the DeltaalgQ mutant, while ndk did not. An increased intracellular concentration of RpoD mimicked the DeltaalgQ phenotype, whereas PvdS overexpression suppressed the algQ mutation. E. coli rsd could partially substitute for algQ in transcriptional modulation of pvd genes. We propose that AlgQ acts as an anti-sigma factor for RpoD, eliciting core RNA polymerase recruitment by PvdS and transcription initiation at pvd promoters. AlgQ provides a link between the pyoverdine and alginate regulatory networks. These systems have similarities in responsiveness and physiological function: both depend on alternative sigma factors, respond to nutrient starvation, and act as virulence determinants for P. aeruginosa.

Mutational analysis of a bifunctional ferrisiderophore receptor and signal-transducing protein from Pseudomonas aeruginosa

The FpvA protein of Pseudomonas aeruginosa strain PAO1 mediates uptake of a siderophore, ferripyoverdine. It is also a component of a signal transduction pathway that controls production of an exotoxin, a protease, pyoverdine, and FpvA itself. The purpose of the research described here was to dissect these different functions of FpvA. Signaling involves an N-terminal domain of FpvA, and it was shown that this domain is probably located in the periplasm, as expected. Short peptides were inserted at 36 sites within FpvA by linker insertion mutagenesis. The effects of these mutations on the presence of FpvA in the outer membrane, on FpvA-mediated uptake of ferripyoverdine, and on pyoverdine synthesis and gene expression were determined. Five of the mutations resulted in the absence of FpvA from the outer membrane of the bacteria. All of the remaining mutations eliminated either the transport or signaling function of FpvA and most affected both functions. Three mutations prevented transport of ferripyoverdine but had no effect on the signal transduction pathway showing that transport of ferripyoverdine is not required for the trans-membrane signaling process. Conversely, eight mutations affected pyoverdine-mediated signaling but had no effect on transport of ferripyoverdine. These data show that insertions throughout FpvA resulted in loss of function and that signaling and transport are separate and discrete functions of FpvA.

Iron and Pseudomonas aeruginosa biofilm formation

Iron serves as a signal in Pseudomonas aeruginosa biofilm development. We examined the influence of mutations in known and putative iron acquisition-signaling genes on biofilm morphology. In iron-sufficient medium, mutants that cannot obtain iron through the high-affinity pyoverdine iron acquisition system form thin biofilms similar to those formed by the parent under low iron conditions. If an iron source for a different iron acquisition system is provided to a pyoverdine mutant, normal biofilm development occurs. This enabled us to identify iron uptake gene clusters that likely serve in transport of ferric citrate and ferrioxamine. We suggest that the functional iron signal for P. aeruginosa biofilm development is active transport of chelated iron or the level of internal iron. If the signal is internal iron levels, then a factor likely to be involved in iron signaling is the cytoplasmic ferric uptake regulator protein, Fur, which controls expression of iron-responsive genes. In support of a Fur involvement, we found that with low iron a Fur mutant was able to organize into more mature biofilms than was the parent. The two known Fur-controlled small regulatory RNAs (PrrF1 and F2) do not appear to mediate iron control of biofilm development. This information establishes a mechanistic basis for iron control of P. aeruginosa biofilm formation.

Expression of the quorum-sensing regulatory protein LasR is strongly affected by iron and oxygen concentrations in cultures of Pseudomonas aeruginosa irrespective of cell density

The expression of the transcriptional regulatory protein LasR, a main component of the quorum-sensing (QS) system in Pseudomonas aeruginosa, was recently found to be sensitive to several environmental factors in addition to its dependency on cell density. However, the inherent effects of the different factors have seldom been separately demonstrated due to concurrent changes of culture conditions in typical experimental settings. Furthermore, the interplays of the different factors are unknown. In this work, the effects and interplay of iron concentration and dissolved oxygen tension (pO(2)) on the expression of lasR in P. aeruginosa were studied in defined growth media with varied iron concentration and pO(2) values in computer-controlled batch and continuous cultures. beta-Galactosidase activity in a recombinant P. aeruginosa PAO1 (NCCB 2452) strain with a lasRp-lacZ fusion was used as a reporter for lasR expression. In batch culture with a constant pO(2) approximately 10 % air saturation, a strong correlation between the exhaustion of iron and the increase of lasR expression was observed. In continuous culture with nearly constant cell density but varied pO(2) values, lasR expression generally increased with increasing oxidative stress with the exception of growth under O(2)-limited conditions (pO(2) approximately equal to 0 %). Under O(2) limitation, the expression of lasR strongly depended on the concentration of iron. It showed a nearly twofold increase in cells grown under iron deprivation in comparison with cells grown in iron-replete conditions and reached the expression level seen at high oxidative stress. A preliminary proteomic analysis was carried out for extracellular proteins in samples from batch cultures grown under different iron concentrations. Several of the extracellular proteins (e.g. AprA, LasB, PrpL) which were up-regulated under iron-limited conditions were found to be QS regulated proteins. Thus, this study clearly shows the links between QS and genes involved in iron and oxygen regulation in P. aeruginosa.

Making 'sense' of metabolism: autoinducer-2, LUXS and pathogenic bacteria

Bacteria exploit many mechanisms to communicate with each other and their surroundings. Mechanisms using small diffusible signals to coordinate behaviour with cell density (quorum sensing) frequently contribute to pathogenicity. However, pathogens must also be able to acquire nutrients and replicate to successfully invade their host. One quorum-sensing system, based on the possession of LuxS, bears the unique feature of contributing directly to metabolism, and therefore has the potential to influence both gene regulation and bacterial fitness. Here, we discuss the influence that LuxS and its product, autoinducer-2, have on virulence, relating the current evidence to the preferred niche of the pathogen and the underlying mechanisms involved.

Biological control of soil-borne pathogens by fluorescent pseudomonads

Particular bacterial strains in certain natural environments prevent infectious diseases of plant roots. How these bacteria achieve this protection from pathogenic fungi has been analysed in detail in biocontrol strains of fluorescent pseudomonads. During root colonization, these bacteria produce antifungal antibiotics, elicit induced systemic resistance in the host plant or interfere specifically with fungal pathogenicity factors. Before engaging in these activities, biocontrol bacteria go through several regulatory processes at the transcriptional and post-transcriptional levels.

Characterization and genetic manipulation of peptide synthetases in Pseudomonas aeruginosa PAO1 in order to generate novel pyoverdines

PvdD, a nonribosomal peptide synthetase (NRPS) of Pseudomonas aeruginosa PAO1, incorporates two L-threonines into the siderophore pyoverdine. A pvdD mutant did not synthesize pyoverdine and lacked a high Mr iron-regulated cytoplasmic protein (IRCP). Analysis of other IRCPs and the P. aeruginosa genome enabled the remaining pyoverdine NRPSs to be identified. The pvdD mutation could be complemented in trans, enabling design of plasmid-based systems for the generation of novel pyoverdines. Introduction of a truncated pvdD gene resulted in attenuated forms of pyoverdine, and introduction of L-threonine-incorporating NRPSs from other organisms restored pyoverdine production to mutant cells. This is the first successful rational in vivo modification of NRPS modules outside of Bacillus subtilis. The systems employed did not allow incorporation of other residues into pyoverdine, indicating that there are multiple elements contributing toward substrate specificity in NRPSs.

Effect of pH on production of virulence factors by biofilm cells ofPseudomonas aeruginosa

Effect of pH on production of extracellular virulence factors of Pseudomonas aeruginosa grown on catheter in biofilm was determined. Alginate and proteinase production was higher at pH 8; in contrast, siderophores (pyochelin and pyoverdin) were synthesized more intensively at pH 5.

Pathogen-host interactions in Pseudomonas aeruginosa pneumonia

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

Bacterial iron sources: from siderophores to hemophores

Iron is an essential element for most organisms, including bacteria. The oxidized form is insoluble, and the reduced form is highly toxic for most macromolecules and, in biological systems, is generally sequestrated by iron- and heme-carrier proteins. Thus, despite its abundance on earth, there is practically no free iron available for bacteria whatever biotope they colonize. To fulfill their iron needs, bacteria have multiple iron acquisition systems, reflecting the diversity of their potential biotopes. The iron/heme acquisition systems in bacteria have one of two general mechanisms. The first involves direct contact between the bacterium and the exogenous iron/heme sources. The second mechanism relies on molecules (siderophores and hemophores) synthesized and released by bacteria into the extracellular medium; these molecules scavenge iron or heme from various sources. Recent genetic, biochemical, and crystallographic studies have allowed substantial progress in describing molecular mechanisms of siderophore and hemophore interactions with the outer membrane receptors, transport through the inner membrane, iron storage, and regulation of genes encoding biosynthesis and uptake proteins.

Transcriptome analysis of Pseudomonas aeruginosa after interaction with human airway epithelial cells

The transcriptional profile of Pseudomonas aeruginosa after interactions with primary normal human airway epithelial cells was determined using Affymetrix GeneChip technology. Gene expression profiles indicated that various genes involved in phosphate acquisition and iron scavenging were differentially regulated.

Functional characterization of an aminotransferase required for pyoverdine siderophore biosynthesis in Pseudomonas aeruginosa PAO1

The fluorescent dihydroxyquinoline chromophore of the pyoverdine siderophore in Pseudomonas is a condensation product of D-tyrosine and l-2,4-diaminobutyrate. Both pvdH and asd (encoding aspartate beta-semialdehyde dehydrogenase) knockout mutants of Pseudomonas aeruginosa PAO1 were unable to synthesize pyoverdine under iron-limiting conditions in the absence of l-2,4-diaminobutyrate in the culture media. The pvdH gene was subcloned, and the gene product was hyperexpressed and purified from P. aeruginosa PAO1. PvdH was found to catalyze an aminotransferase reaction, interconverting aspartate beta-semialdehyde and l-2,4-diaminobutyrate. Steady-state kinetic analysis with a novel coupled assay established that the enzyme adopts a ping-pong kinetic mechanism and has the highest specificity for alpha-ketoglutarate. The specificity of the enzyme toward the smaller keto acid pyruvate is 41-fold lower. The enzyme has negligible activity toward other keto acids tested. Homologues of PvdH were present in the genomes of other Pseudomonas spp. These homologues were found in the DNA loci of the corresponding genomes that contain other pyoverdine synthesis genes. This suggests that there is a general mechanism of l-2,4-diaminobutyrate synthesis in Pseudomonas strains that produce the pyoverdine siderophore.

FpvB, an alternative type I ferripyoverdine receptor of Pseudomonas aeruginosa

Under conditions of iron limitation, Pseudomonas aeruginosa secretes a high-affinity siderophore pyoverdine to scavenge Fe(III) in the extracellular environment and shuttle it into the cell. Uptake of the pyoverdine-Fe(III) complex is mediated by a specific outer-membrane receptor protein, FpvA (ferripyoverdine receptor). Three P. aeruginosa siderovars can be distinguished, each producing a different pyoverdine (type I-III) and a cognate FpvA receptor. Growth of an fpvA mutant of P. aeruginosa PAO1 (type I) under iron-limiting conditions can still be stimulated by its cognate pyoverdine, suggesting the presence of an alternative uptake route for type I ferripyoverdine. In silico analysis of the PAO1 genome revealed that the product of gene PA4168 has a high similarity with FpvA. Inactivation of PA4168 (termed fpvB) in an fpvA mutant totally abolished the capacity to utilize type I pyoverdine. The expression of fpvB is induced by iron limitation in Casamino acids (CAA) and in M9-glucose medium, but, unlike fpvA, not in a complex deferrated medium containing glycerol as carbon source. The fpvB gene was also detected in other P. aeruginosa isolates, including strains producing type II and type III pyoverdines. Inactivation of the fpvB homologues in these strains impaired their capacity to utilize type I ferripyoverdine as a source of iron. Accordingly, introduction of fpvB in trans restored the capacity to utilize type I ferripyoverdine.

Quorum-sensing antagonist (5Z)-4-bromo-5-(bromomethylene)-3-butyl-2(5H)-furanone influences siderophore biosynthesis in Pseudomonas putida and Pseudomonas aeruginosa

Siderophore synthesis of Pseudomonas putida F1 was found to be regulated by quorum sensing since normalized siderophore production (per cell) increased 4.2-fold with cell density after the cells entered middle exponential phase; similarly, normalized siderophore concentrations in Pseudomonas aeruginosa JB2 increased 28-fold, and a 5.5-fold increase was seen for P. aeruginosa PAO1. Further evidence of the link between quorum sensing and siderophore synthesis of P. putida F1 was that the quorum-sensing-disrupter (5Z)-4-bromo-5-(bromomethylene)-3-butyl-2(5H)-furanone (furanone) from the marine red alga Delisea pulchra was found to inhibit the formation of the siderophore produced by P. putida F1 in a concentration-dependent manner, with 57% siderophore synthesis repressed by 100 microg/ml furanone. In contrast, this furanone did not affect the siderophore synthesis of Burkholderia cepacia G4 at 20-40 microg/ml, and stimulated siderophore synthesis of P. aeruginosa JB2 2.5- to 3.7-fold at 20-100 microg/ml. Similarly, 100 microg/ml furanone stimulated siderophore synthesis in P. aeruginosa PAO1 about 3.5-fold. The furanone appears to interact with the quorum-sensing machinery of P. aeruginosa PAO1 since it stimulates less siderophore synthesis in the P. aeruginosa qscR quorum-sensing mutant (QscR is a negative regulator of LasI, an acylated homoserine lactone synthase).

Genetic characterization of pcpS, encoding the multifunctional phosphopantetheinyl transferase of Pseudomonas aeruginosa

Fatty acid synthases (primary metabolism), non-ribosomal peptide synthases and polyketide synthases (secondary metabolism) contain phosphopantetheinyl (Ppant)-dependent carrier proteins that must be made functionally active by transfer of the 4'-Ppant moiety from coenzyme A. These reactions are usually catalysed by dedicated Ppant transferases. Although rich in Ppant-dependent carrier proteins, it was previously shown that Pseudomonas aeruginosa possesses only one Ppant transferase, encoded by pcpS, which functions in both primary and secondary metabolism. Consistent with this notion are our findings that pcpS can genetically complement mutations in the Escherichia coli acpS and entD genes, encoding the apo-acyl carrier protein (ACP) synthase of fatty acid synthesis and a Ppant transferase of enterobactin synthesis, respectively. It also complements a Bacillus subtilis sfp mutation affecting a gene encoding a Ppant transferase essential for surfactin synthesis. A pcpS insertion mutant could only be constructed in a strain carrying the E. coli acpS gene on a chromosomally integrated element in trans, implying that the in vitro essentiality of pcpS is due to its requirement for activation of apo-ACP of fatty acid synthesis. The conditional pcpS mutant is non-fluorescent, does not produce pyoverdine and pyochelin, and does not grow in the presence of iron chelators. The data presented here for the first time confirm that PcpS plays an essential role in both fatty acid and siderophore metabolism.

Haemophore-mediated signalling in Serratia marcescens: a new mode of regulation for an extra cytoplasmic function (ECF) sigma factor involved in haem acquisition

Bacterial extra cytoplasmic function (ECF) sigma factors control a wide range of cell envelope activities including iron and haem uptake systems. Sigma activity is usually inhibited by membrane-bound antisigma. An extra cytoplasmic signal modulates sigma-antisigma interactions and thereby leads to the transcription of the target operon. Sigma and antisigma genes generally belong to one autoregulated operon. However, ECF sigma and antisigma genes involved in iron acquisition, also called iron starvation ECF, are non-autoregulated exceptions to this rule. We fully reconstituted the has signalling cascade of Serratia marcescens in Escherichia coli. Binding of the haem-loaded haemophore to the outer membrane receptor, HasR, inactivates the antisigma HasS, turning on HasI and thereby allowing has operon transcription. Deletion of the HasR N-terminal extension, present in all characterized outer membrane receptors endowed with signal transduction capacity, abolished the inducing activity but not the transport activity. Induction required the TonB-ExbB-ExbD complex. HasI, like the other iron starvation sigma, is iron repressed but not autoregulated. We found an entirely new regulation for the antisigma hasS gene, the transcription of which is HasI dependent. We suggest that the has system is both activated and repressed by the availability of external haem. When there is enough haem, the HasS antisigma activity is turned off and HasI induces the transcription of hasS. This leads to the storage of inactive HasS molecules which become active when HasR is not occupied by holo-haemophore ligand molecules: as soon as there is a haem shortage transcription is turned off. Positive autoregulation of ECF sigma and antisigma genes is usually considered as a mechanism for amplifying a perceived signal. However, our findings suggest, on the contrary, that antisigma regulation allows fine-tuning to the external signal. The biological significance of ECF sigma and antisigma autoregulation may need to be reconsidered.

Induction of pseudo-periodic oscillation in voltage-gated sodium channel properties is dependent on the duration of prolonged depolarization

The neuronal voltage-gated sodium channels play a vital role in the action potential waveform shaping and propagation. Here, we report the effects of prolonged depolarization (1-160 s) on the detailed kinetics of activation, fast inactivation and recovery from slow inactivation in the rNa(v)1.2a voltage-gated sodium channel alpha-subunit expressed in Chinese hamster ovary (CHO) cells. Wavelet analysis revealed that the duration and amplitude of a prolonged sustained depolarization altered all the steady state and kinetic parameters of the channel in a pseudo-oscillatory fashion with time-variable period and amplitude, often superimposed on a linear trend. The half steady state activation potential showed a reversible depolarizing shift of 5-10 mV with duration of prolonged depolarization, while half steady state inactivation potential showed a hyperpolarizing shift of 43-55 mV. The time periods for most of the parameters relating to activation and fast and slow inactivation, lie close to 28-30 s, suggesting coupling of these kinetic processes through an oscillatory mechanism. Co-expression of the beta1-subunit affected the time periods of oscillation (close to 22 s for alpha + beta1) in steady state activation parameters. Application of a pulse protocol that mimicked paroxysmal depolarizing shift (PDS), a kind of depolarization seen in epileptic discharges, instead of a sustained depolarization, also caused oscillatory behaviour in the rNav1.2a alpha-subunit. This inherent pseudo-oscillatory mechanism may regulate excitability of the neurons, account for the epileptic discharges and subthreshold membrane potential oscillation and offer a molecular memory mechanism intrinsic to the neurons, independent of synaptic plasticity.

Positive autoregulation and signaling properties of pyoluteorin, an antibiotic produced by the biological control organism Pseudomonas fluorescens Pf-5

Pseudomonas fluorescens Pf-5, a rhizosphere bacterium, produces a suite of secondary metabolites that are toxic to seed- and root-rotting plant pathogens. Among these are the polyketide compounds pyoluteorin and 2,4-diacetylphloroglucinol. We provide evidence that pyoluteorin production is influenced by positive autoregulation. Addition of pyoluteorin to liquid cultures of Pf-5 enhanced pyoluteorin production. In addition, pyoluteorin and 2,4-diacetylphloroglucinol mutually inhibit one another's production in Pf-5. For pyoluteorin, both positive autoregulation and negative influences on production by 2,4-diacetylphloroglucinol were demonstrated at the transcriptional level by measuring activity from transcriptional fusions of an ice nucleation reporter gene (inaZ) to three separate pyoluteorin biosynthetic genes. The occurrence of pyoluteorin autoregulation in the rhizosphere was assessed on cucumber seedlings in pasteurized soil with cross-feeding experiments. In the rhizosphere, expression of a pyoluteorin biosynthesis gene by a pyoluteorin-deficient mutant of Pf-5 was enhanced by pyoluteorin produced by coinoculated cells of Pf-5. These data establish that the polyketide pyoluteorin is an autoregulatory compound and functions as a signal molecule influencing the spectrum of secondary metabolites produced by the bacterial cell.

Physiological and molecular genetic evaluation of the dechlorination agent, pyridine-2,6-bis(monothiocarboxylic acid) (PDTC) as a secondary siderophore of Pseudomonas

The bacterial metabolite and transition metal chelator pyridine-2,6-dithiocarboxylic acid (PDTC), promotes a novel and effective means of dechlorination of the toxic and carcinogenic pollutant, carbon tetrachloride. Pyridine-2,6-dithiocarboxylic acid has been presumed to act as a siderophore in the Pseudomonas strains known to produce it. To explore further the physiological function of PDTC production, we have examined its regulation, the phenotype of PDTC-negative (pdt) mutants, and envelope proteins associated with PDTC in P. putida strain DSM 3601. Aspects of the regulation of PDTC production and outer membrane protein composition were consistent with siderophore function. Pyridine-2,6-dithiocarboxylic acid production was coordinated with production of the well-characterized siderophore pyoverdine; exogenously added pyoverdine led to decreased PDTC production, and added PDTC led to decreased pyoverdine production. Positive regulation of a chromosomal pdtI-xylE transcriptional fusion, and of a 66 kDa outer membrane protein (IROMP), was seen in response to exogenous PDTC. Tests with transition metal chelators indicated that PDTC could provide a benefit under conditions of metal limitation; the loss of PDTC biosynthetic capacity caused by a pdtI transposon insertion resulted in increased sensitivity to 1,10-phenanthroline, a chelator that has high affinity for a range of divalent transition metals (e.g. Fe(2+), Cu(2+), Zn(2+)). Exogenously added PDTC could also suppress a phenotype of pyoverdine-negative (Pvd-) mutants, that of sensitivity to EDDHA, a chelator with higher affinity and specificity for Fe(3+). Measurement of 59Fe incorporation showed uptake from 59Fe:PDTC by DSM 3601 grown in low-iron medium, but not by cells grown in high iron medium, or by the pdtI mutant, which did not show expression of the 66 kDa envelope protein. These data verified a siderophore function for PDTC, and have implicated it in the uptake of transition metals in addition to iron.

Bacillus anthracis requires siderophore biosynthesis for growth in macrophages and mouse virulence

Systemic anthrax infections can be characterized as proceeding in stages, beginning with an early intracellular establishment stage within phagocytes that is followed by extracelluar stages involving massive bacteraemia, sepsis and death. Because most bacteria require iron, and the host limits iron availability through homeostatic mechanisms, we hypothesized that B. anthracis requires a high-affinity mechanism of iron acquisition during its growth stages. Two putative types of siderophore synthesis operons, named Bacillus anthracis catechol, bac (anthrabactin), and anthrax siderophore biosynthesis, asb (anthrachelin), were identified. Directed gene deletions in both anthrabactin and anthrachelin pathways were generated in a B. anthracis (Sterne) 34F2 background resulting in mutations in asbA and bacCEBF. A decrease in siderophore production was observed during iron-depleted growth in both the DeltaasbA and DeltabacCEBF strains, but only the DeltaasbA strain was attenuated for growth under these conditions. In addition, the DeltaasbA strain was severely attenuated both for growth in macrophages (MPhi) and for virulence in mice. In contrast, the DeltabacCEBF strain did not differ phenotypically from the parental strain. These findings support a requirement for anthrachelin but not anthrabactin in iron assimilation during the intracellular stage of anthrax.

RhuR, an extracytoplasmic function sigma factor activator, is essential for heme-dependent expression of the outer membrane heme and hemoprotein receptor of Bordetella avium

Genes involved in iron (Fe) acquisition often are regulated in response to the local availability of Fe. In many bacteria, Fe-dependent responsiveness is mediated by Fur, a global Fe-dependent transcriptional repressor. Tighter regulatory control of Fur-responsive genes is afforded by incorporating additional regulators into Fur-dependent regulatory cascades. RhuI, a Fur-dependent extracytoplasmic function sigma factor of Bordetella avium, in response to the dual stimulation of Fe starvation and the presence of heme (or hemoproteins), regulates P(bhuR), a heme-responsive promoter which directs expression of the bhuRSTUV heme utilization operon. While BhuR, the outer membrane heme receptor, and RhuI have been shown to be indispensable for heme-dependent activation of P(bhuR), collateral components of the regulatory cascade have not been described. In this investigation, RhuR, an integral cytoplasmic membrane protein with homology to anti-sigma factors, is shown to be an essential activator of P(bhuR) expression. The functional domain of RhuR required for heme-dependent activation of P(bhuR) expression was mapped to the N-terminal 97 amino acids of the protein by use of a chimeric RhuR-BlaM fusion. Expression of the chimera in a rhuR mutant rendered P(bhuR) constitutive, thereby decoupling the promoter from heme dependency. Growth studies confirmed that B. avium requires RhuR for optimal utilization of hemoglobin, but not hemin, as a sole source of nutrient Fe. These data imply that B. avium expresses, in addition to the BhuR heme/hemoprotein utilization system, an alternative RhuR-independent heme utilization mechanism. A model is proposed in which RhuR is the functional bridge between BhuR and RhuI in a heme-dependent regulatory cascade.

Expression of l-ornithine Nδ-oxygenase (PvdA) in fluorescent Pseudomonas species: an immunochemical and in silico study

Omega-amino acid monooxygenases (EC 1.14.13.-), catalysing the formation of hydroxamate precursors of microbial siderophores (e.g., pyoverdine), have so far eluded structural and biochemical characterisation. Here, the expression of recombinant L-ornithine-Ndelta-oxygenase (PvdA) from Pseudomonas aeruginosa PAO1 is reported. A library of eight monoclonal antibodies (MAbs) directed against PvdA has been generated. Two MAb families recognising the N- and C-terminal regions of PvdA were identified. The MAbs made it possible to demonstrate that 45-48 kDa PvdA homologues are expressed in response to iron limitation by different species and strains of fluorescent pseudomonads. Despite the different degrees in sequence similarity between P. aeruginosa PvdA and putative homologues from Pseudomonas fluorescens, Pseudomonas putida, Pseudomonas syringae, Burkholderia cepacia, and Ralstonia solanacearum, in silico domain scanning predicts an impressive conservation of putative cofactor and substrate binding domains. The MAb library was also used to monitor PvdA expression during the transition of P. aeruginosa from iron-sufficient to iron-deficient growth.

In vivo functional genomics of Pseudomonas aeruginosa for high-throughput screening of new virulence factors and antibacterial targets

Pseudomonas aeruginosa is a model for studying opportunistic pathogens that are highly resistant to most classes of antibiotics and cause chronic pulmonary infections. We have developed and adapted a multiplex polymerase chain reaction-based signature-tagged mutagenesis (STM) for high-throughput screening of a collection of 7968 P. aeruginosa mutants in a rat model of chronic respiratory infection. After three rounds of screening, a total of 214 mutants, representing transposition events into 148 open reading frames, were shown to be attenuated in lung infection and were retained for further analysis. As proof of concept supporting this technology, we identified 11 insertions in typical virulence genes such as those coding for pili implicated in motility, attachment and swarming, alginate synthesis and its expression, a mucus transcription regulator, extracellular enzymes such as alkaline protease, esterase and amino peptidase, a rhamnosyl surfactant transferase and a lipopolysaccharide glycosyl transferase. Detailed analysis of the 148 STM mutants, including seven auxotrophs, revealed insertions in 21 of the 26 known gene classes used to characterize sequenced bacterial genomes. We noted that at least 46% of STM mutants identified had insertions in hypothetical proteins or proteins of unknown function and that approximately 40% of all STM mutants had insertions in surface proteins including the outer membrane, the periplasm and the inner membrane. Interestingly, 11 STM mutants attenuated for lung infection were also identified in microarray and transcriptome for quorum sensing and mucoidy production. The remaining 130 mutants were systematically analysed for their capability to express fully known virulence factors. In addition, testing the ability of these mutants to infect alternative model host Drosophila melanogaster revealed 36 STM mutants defective in protease, twitching motility, swimming and swarming. Finally, we identified many genes, the activity of which in respiratory infection was not fully appreciated.

Calcium and magnesium enhance the production of Pseudomonas aeruginosa protease IV, a corneal virulence factor

The effect of calcium and magnesium on protease IV production during the growth of Pseudomonas aeruginosa was investigated. Strain PA103 was grown to stationary phase in medium containing various concentrations of either calcium or magnesium. Culture supernatants were concentrated, standardized relative to cell density, and the pyoverdine concentrations were measured. Overall extracellular protease activity and specific protease IV (lysine endoproteinase) activity were measured with or without TLCK, a serine protease inhibitor effective against protease IV activity. Protease IV activity was also observed by casein zymography. Calcium and magnesium were quantified in the corneas and aqueous humor of rabbits that were inoculated intrastromally with strain PA103. Pyoverdine production was not significantly different in cultures grown in medium with added calcium or magnesium, but extracellular caseinase activity increased in these cultures. Susceptibility of caseinase activity to TLCK inhibition and a specific assay for protease IV indicated that protease IV activity increased in cultures grown in calcium or magnesium. Casein zymography supported the observation that protease IV activity increased in the cultures with added calcium and magnesium. Addition of calcium or magnesium to the protease IV-specific assay had no effect on the catalytic activity of pure protease IV. Infection of rabbit corneas with PA103 did not change the magnesium concentration in either corneas or aqueous humor, but significantly increased the concentration of calcium in corneas. These results indicate that calcium and magnesium enhance the production of protease IV, but not pyoverdine production. Calcium increases in the cornea following infection with P. aeruginosa could favor production of protease IV.

Identification of quorum-sensing regulated proteins in the opportunistic pathogenPseudomonas aeruginosaby proteomics

The Gram-negative bacterium Pseudomonas aeruginosa is an opportunistic human pathogen which is responsible for severe nosocomial infections in immunocompromised patients and is the major pathogen in cystic fibrosis. The bacterium utilizes two interrelated quorum-sensing (QS) systems, which rely on N-acyl-homoserine lactone (AHL) signal molecules, to control the expression of virulence factors and biofilm development. In this study, we compared the protein patterns of the intracellular, extracellular and surface protein fractions of the PAO1 parent strain with those of an isogenic lasI rhlI double mutant by means of two-dimensional gel electrophoresis (2-DE). This analysis showed that the intensities of 23.7% of all detected protein spots differed more than 2.5-fold between the two strains. We only considered those protein spots truly QS regulated that were changed in the mutant in the absence of signal molecules but were rescued to the wild-type situation when the medium was supplemented with AHLs. These protein spots were characterized by MALDI-TOF peptide mapping. Twenty-seven proteins were identified that were previously reported to be AHL controlled, among them several well-characterized virulence factors. For one of the identified proteins, the serine protease PrpL, a biochemical assay was established to verify that expression of this factor is indeed QS regulated. Furthermore, it is shown that the quorum-sensing blocker C-30 specifically interferes with the expression of 67% of the AHL-controlled protein spots of the surface fraction, confirming the high specificity of the compound. Importantly, 20 novel QS-regulated proteins were identified, many of which are involved in iron utilization, suggesting a link between quorum sensing and the iron regulatory system. Two of these proteins, PhuR and HasAp, are components of the two distinct haem-uptake systems present in P. aeruginosa. In agreement with the finding that both proteins are positively regulated by the QS cascade, we show that the lasI rhlI double mutant grows poorly with haemoglobin as the only iron source when compared with the wild type. These results add haemoglobin utilization to the list of phenotypes controlled through QS in P. aeruginosa. The surprisingly high number of AHL-regulated proteins relative to the number of regulated genes suggests that quorum-sensing control also operates via post-transcriptional mechanisms. To strengthen this hypothesis we investigated the role of quorum sensing in the post-translational modification of HasAp, an extracellular protein required for the uptake of free and haemoglobin-bound haem.