Evidence for different pyoverdine-mediated iron uptake systems among Pseudomonas aeruginosa strains

Spread of Pseudomonas aeruginosa ST274 Clone in Different Niches: Resistome, Virulome, and Phylogenetic Relationship

Pseudomonas aeruginosa ST274 is an international epidemic high-risk clone, mostly associated with hospital settings and appears to colonize cystic fibrosis (CF) patients worldwide. To understand the relevant mechanisms for its success, the biological and genomic characteristics of 11 ST274-P. aeruginosa strains from clinical and non-clinical origins were analyzed. The extensively drug-resistant (XDR/DTR), the non-susceptible to at least one agent (modR), and the lasR-truncated (by ISPsp7) strains showed a chronic infection phenotype characterized by loss of serotype-specific antigenicity and low motility. Furthermore, the XDR/DTR and modR strains presented low pigment production and biofilm formation, which were very high in the lasR-truncated strain. Their whole genome sequences were compared with other 14 ST274-P. aeruginosa genomes available in the NCBI database, and certain associations have been primarily detected: blaOXA-486 and blaPDC-24 genes, serotype O:3, exoS+/exoU- genotype, group V of type IV pili, and pyoverdine locus class II. Other general molecular markers highlight the absence of vqsM and pldA/tleS genes and the presence of the same mutational pattern in genes involving two-component sensor-regulator systems PmrAB and CreBD, exotoxin A, quorum-sensing RhlI, beta-lactamase expression regulator AmpD, PBP1A, or FusA2 elongation factor G. The proportionated ST274-P. aeruginosa results could serve as the basis for more specific studies focused on better antibiotic stewardship and new therapeutic developments.

Prevalence of antibiotic resistance and virulent factors in nosocomial clinical isolates of Pseudomonas aeruginosa from Panamá

Background

Pseudomonas aeruginosa is an important causative agent of nosocomial infections. As pathogen, P. aeruginosa is of increasing clinical importance due to its ability to develop high-level multidrug resistance (MDR).

Methods

The aim of the present study was to better understand the intrinsic virulence of circulating strains of Pseudomonas aeruginosa, by surveying and characterizing the antibiotic resistance profiles and prevalence of virulence factors in 51 clinical isolates of P. aeruginosa obtained from children admitted to Hospital del Niño-Panamá during the period of October 2016 until March 2017. Antimicrobial susceptibilities were assessed by determining the minimum inhibitory concentration for 12 antibiotics against P. aeruginosa clinical isolates using the VITEK system (https://www.biomerieux.com). Additionally, all isolates were examined by Polymerase Chain Reaction (PCR) for the presence of components of the MexAB-OprM efflux pump system (mexABR) and pyoverdine receptor genes and betalactamases resistance genes (ESBL) using gene-specific primers.

Results

A total of 51 pyoverdine producing clinical isolates were analyzed, all of which expressed resistance genes such as genes of the MexAB-OprM efflux pump system (mexABR) and pyoverdine receptor genes (fpvA). Out of 51 MDR isolates, 22 were ESBL producers. The most common ESBL gene was blaTEM expressed by 43% of the isolates. The isolates tested in this study showed increased resistance to antibiotics in the following categories: (i) penicillins (ampicillin (69%), piperacillin (22%); (ii) pyrimethamines (trimethoprim, 65%); (iii) nitrofurans (nitrofurantoin, 63%), and (iv) third-generation cephalosporin cefotaxime (53%). These results underscore a high prevalence of MDR amongst clinical isolates from Panama.

Conclusions

The present study indicates that prevalence of BlaTEM-carrying strains is increasing with subsequent multidrug resistance in Panamá and as well reported worldwide. The virulent factors identified in this study provide valuable information regarding the prevalence of resistance genes and their potential impact on treatments that exploit the unique physiology of the pathogen. To prevent further spread of MDR, the proportions of resistant strains of Pseudomonas aeruginosa should be constantly evaluated on healthcare institutions of Panamá. More importantly, this information can be used to better understand the evolution and dissemination of strains hoping to prevent the development of resistance in Pseudomonas aeruginosa. Future studies quantifying the expression of these virulent genes will emphasize on the acquisition of multidrug resistance.

The biosynthesis of pyoverdines

Pyoverdines are fluorescent siderophores of pseudomonads that play important roles for growth under iron-limiting conditions. The production of pyoverdines by fluorescent pseudomonads permits their colonization of hosts ranging from humans to plants. Prominent examples include pathogenic or non-pathogenic species such as Pseudomonas aeruginosa, P. putida, P. syringae, or P. fluorescens. Many distinct pyoverdines have been identified, all of which have a dihydroxyquinoline fluorophore in common, derived from oxidative cyclizations of non-ribosomal peptides. These serve as precursor of pyoverdines and are commonly known as ferribactins. Ferribactins of distinct species or even strains often differ in their sequence, resulting in a large variety of pyoverdines. However, synthesis of all ferribactins begins with an L-Glu/D-Tyr/L-Dab sequence, and the fluorophore is generated from the D-Tyr/L-Dab residues. In addition, the initial L-Glu residue is modified to various acids and amides that are responsible for the range of distinguishable pyoverdines in individual strains. While ferribactin synthesis is a cytoplasmic process, the maturation to the fluorescent pyoverdine as well as the tailoring of the initial glutamate are exclusively periplasmic processes that have been a mystery until recently. Here we review the current knowledge of pyoverdine biosynthesis with a focus on the recent advancements regarding the periplasmic maturation and tailoring reactions.

The effect of cheats on siderophore diversity in Pseudomonas aeruginosa

Cooperation can be maintained if cooperative behaviours are preferentially directed towards other cooperative individuals. Tag-based cooperation (greenbeards) - where cooperation benefits individuals with the same tag as the actor - is one way to achieve this. Tag-based cooperation can be exploited by individuals who maintain the specific tag but do not cooperate, and selection to escape this exploitation can result in the evolution of tag diversity. We tested key predictions crucial for the evolution of cheat-mediated tag diversity using the production of iron-scavenging pyoverdine by the opportunistic pathogen, Pseduomonas aeruginosa as a model system. Using two strains that produce different pyoverdine types and their respective cheats, we show that cheats outcompete their homologous pyoverdine producer, but are outcompeted by the heterologous producer in well-mixed environments. As a consequence, co-inoculating two types of pyoverdine producer and one type of pyoverdine cheat resulted in the pyoverdine type whose cheat was not present having a large fitness advantage. Theory suggests that in such interactions, cheats can maintain tag diversity in spatially structured environments, but that tag-based cooperation will be lost in well-mixed populations, regardless of tag diversity. We saw that when all pyoverdine producers and cheats were co-inoculated in well-mixed environments, both types of pyoverdine producers were outcompeted, whereas spatial structure (agar plates and compost microcosms), rather than maintaining diversity, resulted in the domination of one pyoverdine producer. These results suggest cheats may play a more limited role in the evolution of pyoverdine diversity than predicted.

Bifunctional antimicrobial conjugates and hybrid antimicrobials

Covering: up to the end of 2016Novel antimicrobial drugs are continuously needed to counteract bacterial resistance development. An innovative molecular design strategy for novel antibiotic drugs is based on the hybridization of an antibiotic with a second functional entity. Such conjugates can be grouped into two major categories. In the first category (antimicrobial hybrids), both functional elements of the hybrid exert antimicrobial activity. Due to the dual targeting, resistance development can be significantly impaired, the pharmacokinetic properties can be superior compared to combination therapies with the single antibiotics, and the antibacterial potency is often enhanced in a synergistic manner. In the second category (antimicrobial conjugates), one functional moiety controls the accumulation of the other part of the conjugate, e.g. by mediating an active transport into the bacterial cell or blocking the efflux. This approach is mostly applied to translocate compounds across the cell envelope of Gram-negative bacteria through membrane-embedded transporters (e.g. siderophore transporters) that provide nutrition and signalling compounds to the cell. Such 'Trojan Horse' approaches can expand the antibacterial activity of compounds against Gram-negative pathogens, or offer new options for natural products that could not be developed as standalone antibiotics, e.g. due to their toxicity.

Iron Uptake Analysis in a Set of Clinical Isolates of Pseudomonas putida

Pseudomonas putida strains are frequent inhabitants of soil and aquatic niches and they are occasionally isolated from hospital environments. As the available iron sources in human tissues, edaphic, and aquatic niches are different, we have analyzed iron-uptake related genes in different P. putida strains that were isolated from all these environments. We found that these isolates can be grouped into different clades according to the genetics of siderophore biosynthesis and recycling. The pyoverdine locus of the six P. putida clinical isolates that have so far been completely sequenced, are not closely related; three strains (P. putida HB13667, HB3267, and NBRC14164T) are grouped in Clade I and the other three in Clade II, suggesting possible different origins and evolution. In one clinical strain, P. putida HB4184, the production of siderophores is induced under high osmolarity conditions. The pyoverdine locus in this strain is closely related to that of strain P. putida HB001 which was isolated from sandy shore soil of the Yellow Sea in Korean marine sand, suggesting their possible origin, and evolution. The acquisition of two unique TonB-dependent transporters for xenosiderophore acquisition, similar to those existing in the opportunistic pathogen P. aeruginosa PAO, is an interesting adaptation trait of the clinical strain P. putida H8234 that may confer adaptive advantages under low iron availability conditions.

An overview of the biological metal uptake pathways in Pseudomonas aeruginosa

Biological metal ions, including Co, Cu, Fe, Mg, Mn, Mo, Ni and Zn ions, are necessary for the survival and the growth of all microorganisms. Their biological functions are linked to their particular chemical properties: they play a role in structuring macromolecules and/or act as co-factors catalyzing diverse biochemical reactions. These metal ions are also essential for microbial pathogens during infection: they are involved in bacterial metabolism and various virulence factor functions. Therefore, during infection, bacteria need to acquire biological metal ions from the host such that there is competition for these ions between the bacterium and the host. Evidence is increasingly emerging of "nutritional immunity" against pathogens in the hosts; this includes strategies making access to metals difficult for infecting bacteria. It is clear that biological metals play key roles during infection and in the battle between the pathogens and the host. Here, we summarize current knowledge about the strategies used by Pseudomonas aeruginosa to access the various biological metals it requires. P. aeruginosa is a medically significant Gram-negative bacterial opportunistic pathogen that can cause severe chronic lung infections in cystic fibrosis patients and that is responsible for nosocomial infections worldwide.

Rapid Identification of Pseudomonas spp. via Raman Spectroscopy Using Pyoverdine as Capture Probe

Pyoverdine is a substance which is excreted by fluorescent pseudomonads in order to scavenge iron from their environment. Due to specific receptors of the bacterial cell wall, the iron loaded pyoverdine molecules are recognized and transported into the cell. This process can be exploited for developing efficient isolation and enrichment strategies for members of the Pseudomonas genus, which are capable of colonizing various environments and also include human pathogens like P. aeruginosa and the less virulent P. fluorescens. A significant advantage over antibody based systems is the fact that siderophores like pyoverdine can be considered as "immutable ligands," since the probability for mutations within the siderophore uptake systems of bacteria is very low. While each species of Pseudomonas usually produces structurally unique pyoverdines, which can be utilized only by the producer strain, cross reactivity does occur. In order to achieve a reliable identification of the captured pathogens, further investigations of the isolated cells are necessary. In this proof of concept study, we combine the advantages of an isolation strategy relying on "immutable ligands" with the high specificity and speed of Raman microspectroscopy. In order to isolate the bacterial cells, pyoverdine was immobilized covalently on planar aluminum chip substrates. After capturing, single cell Raman spectra of the isolated species were acquired. Due to the specific spectroscopic fingerprint of each species, the bacteria can be identified. This approach allows a very rapid detection of potential pathogens, since time-consuming culturing steps are unnecessary. We could prove that pyoverdine based isolation of bacteria is fully Raman compatible and further investigated the capability of this approach by isolating and identifying P. aeruginosa and P. fluorescens from tap water samples, which are both opportunistic pathogens and can pose a threat for immunocompromised patients.

Analysis of the draft genome of Pseudomonas fluorescens ATCC17400 indicates a capacity to take up iron from a wide range of sources, including different exogenous pyoverdines

All fluorescent pseudomonads (Pseudomonas aeruginosa, P. putida, P. fluorescens, P. syringae and others) are known to produce the high-affinity peptidic yellow-green fluorescent siderophore pyoverdine. These siderophores have peptide chains that are quite diverse and more than 50 pyoverdine structures have been elucidated. In the majority of the cases, a Pseudomonas species is also able to produce a second siderophore of lower affinity for iron. Pseudomonas fluorescens ATCC 17400 has been shown to produce a unique second siderophore, (thio)quinolobactin, which has an antimicrobial activity against the phytopathogenic Oomycete Pythium debaryanum. We show that this strain has the capacity to utilize 16 different pyoverdines, suggesting the presence of several ferripyoverdine receptors. Analysis of the draft genome of P. fluorescens ATCC 17400 confirmed the presence of 55 TonB-dependent receptors, the largest so far for Pseudomonas, among which 15 are predicted to be ferripyoverdine receptors (Fpv). Phylogenetic analysis revealed the presence of two different clades containing ferripyoverdine receptors, with sequences similar to the P. aeruginosa type II FpvA forming a separate cluster. Among the other receptors we confirmed the presence of the QbsI (thio)quinolobactin receptor, an ferri-achromobactin and an ornicorrugatin receptor, several catecholate and four putative heme receptors. Twenty five of the receptors genes were found to be associated with genes encoding extracytoplasmic sigma factors (ECF σ) and transmembrane anti-σ sensors.

The deletion of TonB-dependent receptor genes is part of the genome reduction process that occurs during adaptation of Pseudomonas aeruginosa to the cystic fibrosis lung

Chronic Pseudomonas aeruginosa infections are the main cause of morbidity among patients with cystic fibrosis (CF) due to persistent lung inflammation caused by interaction between this bacterium and the immune system. Longitudinal studies of clonally related isolates of a dominant CF clone have indicated that genome reduction frequently occurs during adaptation of P. aeruginosa in the CF lung. In this study, we have evaluated the P. aeruginosa population structure of patients attending the Universitair Ziekenhuis Brussel (UZ Brussel) CF reference center using a combination of genotyping methods. Although the UZ Brussel P. aeruginosa CF population is characterized by the absence of a dominant CF clone, some potential interpatient transmissions could be detected. Interestingly, one of these clones showed deletion of the alternative type I ferripyoverdine receptor gene fpvB. Furthermore, we found that several other TonB-dependent receptors are deleted as well. The genome of one potentially transmissible CF clone was sequenced, revealing large deleted regions including all type III secretion system genes and several virulence genes. Remarkably, a large number of deleted genes are shared between the P. aeruginosa CF clone described in this study and isolates belonging to the dominant Copenhagen CF DK2 clone, suggesting parallel evolution.

Siderophore-dependent iron uptake systems as gates for antibiotic Trojan horse strategies against Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen responsible for nosocomial infections. The prevalence of antibiotic-resistant P. aeruginosa strains is increasing, necessitating the urgent development of new strategies to improve the control of this pathogen. Its bacterial envelope constitutes of an outer and an inner membrane enclosing the periplasm. This structure plays a key role in the resistance of the pathogen, by decreasing the penetration and the biological impact of many antibiotics. However, this barrier may also be seen as the "Achilles heel" of the bacterium as some of its functions provide opportunities for breaching bacterial defenses. Siderophore-dependent iron uptake systems act as gates in the bacterial envelope and could be used in a "Trojan horse" strategy, in which the conjugation of an antibiotic to a siderophore could significantly increase the biological activity of the antibiotic, by enhancing its transport into the bacterium. In this review, we provide an overview of the various siderophore-antibiotic conjugates that have been developed for use against P. aeruginosa and show that an accurate knowledge of the structural and functional features of the proteins involved in this transmembrane transport is required for the design and synthesis of effective siderophore-antibiotic Trojan horse conjugates.

Pseudomonas aeruginosa adapts its iron uptake strategies in function of the type of infections

Pseudomonas aeruginosa is a Gram-negative γ-Proteobacterium which is known for its capacity to colonize various niches, including some invertebrate and vertebrate hosts, making it one of the most frequent bacteria causing opportunistic infections. P. aeruginosa is able to cause acute as well as chronic infections and it uses different colonization and virulence factors to do so. Infections range from septicemia, urinary infections, burn wound colonization, and chronic colonization of the lungs of cystic fibrosis patients. Like the vast majority of organisms, P. aeruginosa needs iron to sustain growth. P. aeruginosa utilizes different strategies to take up iron, depending on the type of infection it causes. Two siderophores are produced by this bacterium, pyoverdine and pyochelin, characterized by high and low affinities for iron respectively. P. aeruginosa is also able to utilize different siderophores from other microorganisms (siderophore piracy). It can also take up heme from hemoproteins via two different systems. Under microaerobic or anaerobic conditions, P. aeruginosa is also able to take up ferrous iron via its Feo system using redox-cycling phenazines. Depending on the type of infection, P. aeruginosa can therefore adapt by switching from one iron uptake system to another as we will describe in this short review.

A combinatorial approach to the structure elucidation of a pyoverdine siderophore produced by a Pseudomonas putida isolate and the use of pyoverdine as a taxonomic marker for typing P. putida subspecies

The structure of a pyoverdine produced by Pseudomonas putida, W15Oct28, was elucidated by combining mass spectrometric methods and bioinformatics by the analysis of non-ribosomal peptide synthetase genes present in the newly sequenced genome. The only form of pyoverdine produced by P. putida W15Oct28 is characterized to contain α-ketoglutaric acid as acyl side chain, a dihydropyoverdine chromophore, and a 12 amino acid peptide chain. The peptide chain is unique among all pyoverdines produced by Pseudomonas subspecies strains. It was characterized as -L-Asp-L-Ala-D-AOHOrn-L-Thr-Gly-c[L-Thr(O-)-L-Hse-D-Hya-L-Ser-L-Orn-L-Hse-L-Ser-O-]. The chemical formula and the detected and calculated molecular weight of this pyoverdine are: C65H93N17O32, detected mass 1624.6404 Da, calculated mass 1624.6245. Additionally, pyoverdine structures from both literature reports and bioinformatics prediction of the genome sequenced P. putida strains are summarized allowing us to propose a scheme based on pyoverdines structures as tool for the phylogeny of P. putida. This study shows the strength of the combination of in silico analysis together with analytical data and literature mining in determining the structure of secondary metabolites such as peptidic siderophores.

Pseudomonas aeruginosa population structure revisited

At present there are strong indications that Pseudomonas aeruginosa exhibits an epidemic population structure; clinical isolates are indistinguishable from environmental isolates, and they do not exhibit a specific (disease) habitat selection. However, some important issues, such as the worldwide emergence of highly transmissible P. aeruginosa clones among cystic fibrosis (CF) patients and the spread and persistence of multidrug resistant (MDR) strains in hospital wards with high antibiotic pressure, remain contentious. To further investigate the population structure of P. aeruginosa, eight parameters were analyzed and combined for 328 unrelated isolates, collected over the last 125 years from 69 localities in 30 countries on five continents, from diverse clinical (human and animal) and environmental habitats. The analysed parameters were: i) O serotype, ii) Fluorescent Amplified-Fragment Length Polymorphism (FALFP) pattern, nucleotide sequences of outer membrane protein genes, iii) oprI, iv) oprL, v) oprD, vi) pyoverdine receptor gene profile (fpvA type and fpvB prevalence), and prevalence of vii) exoenzyme genes exoS and exoU and viii) group I pilin glycosyltransferase gene tfpO. These traits were combined and analysed using biological data analysis software and visualized in the form of a minimum spanning tree (MST). We revealed a network of relationships between all analyzed parameters and non-congruence between experiments. At the same time we observed several conserved clones, characterized by an almost identical data set. These observations confirm the nonclonal epidemic population structure of P. aeruginosa, a superficially clonal structure with frequent recombinations, in which occasionally highly successful epidemic clones arise. One of these clones is the renown and widespread MDR serotype O12 clone. On the other hand, we found no evidence for a widespread CF transmissible clone. All but one of the 43 analysed CF strains belonged to a ubiquitous P. aeruginosa "core lineage" and typically exhibited the exoS(+)/exoU(-) genotype and group B oprL and oprD alleles. This is to our knowledge the first report of an MST analysis conducted on a polyphasic data set.

Distribution and evolution of ferripyoverdine receptors in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a ubiquitous gram-negative bacterium, which is also able to cause severe opportunistic infections in humans. The colonization of the host is importantly affected by the production of the high-affinity iron (III) scavenging peptidic siderophore pyoverdine. The species P. aeruginosa can be divided into three subgroups ('siderovars'), each characterized by the production of a specific pyoverdine and receptor (FpvA). We used a multiplex PCR to determine the FpvA siderovar on 345 P. aeruginosa strains from environmental or clinical origin. We found about the same proportion of each type in clinical strains, while FpvA type I was slightly over-represented (49%) in environmental strains. Our multiplex PCR also detected the presence or absence of an additional receptor for type I pyoverdine (FpvB). The fpvB gene was in fact present in the vast majority of P. aeruginosa strains (93%), regardless of their siderovar or their origin. Finally, molecular analyses of fpvA and fpvB genes highlighted a complex evolutionary history, probably linked to the central role of iron acquisition in the ecology and virulence of P. aeruginosa.

New insights into the metal specificity of the Pseudomonas aeruginosa pyoverdine-iron uptake pathway

Pyoverdine (PvdI) is the major siderophore secreted by Pseudomonas aeruginosa PAOI in order to get access to iron. After being loaded with iron in the extracellular medium, PvdI is transported across the bacterial outer membrane by the transporter, FpvAI. We used the spectral properties of PvdI to show that in addition to Fe(3+), this siderophore also chelates, but with lower efficiencies, all the 16 metals used in our screening. Afterwards, FpvAI at the cell surface binds Ag(+), Al(3+), Cd(2+), Co(2+), Cu(2+), Fe(3+), Ga(3+), Hg(2+), Mn(2+), Ni(2+) or Zn(2+) in complex with PvdI. We used Inductively Coupled Plasma-Atomic Emission Spectrometry to monitor metal uptake in P. aeruginosa: TonB-dependent uptake, in the presence of PvdI, was only efficient for Fe(3+). Cu(2+), Ga(3+), Mn(2+) and Ni(2+) were also transported into the cell but with lower uptake rates. The presence of Al(3+), Cu(2+), Ga(3+), Mn(2+), Ni(2+) and Zn(2+) in the extracellular medium induced PvdI production in P. aeruginosa. All these data allow a better understanding of the behaviour of the PvdI uptake pathway in the presence of metals other than iron: FpvAI at the cell surface has broad metal specificity at the binding stage and it is highly selective for Fe(3+) only during the uptake process.

Loss of the oxidative stress regulator OxyR in Pseudomonas aeruginosa PAO1 impairs growth under iron-limited conditions

Pyoverdine is the main siderophore secreted by fluorescent pseudomonads to scavenge iron in the extracellular environment. Iron uptake, however, needs to be tightly regulated, because free iron stimulates the formation of highly toxic oxygen derivatives. In the opportunistic pathogen Pseudomonas aeruginosa, the transcriptional regulator OxyR plays a key role in the upregulation of defense mechanisms against oxidative stress as it stimulates the expression of the antioxidant genes katB, ahpB and ahpCF after contact with oxidative stress-generating agents. Inactivation of the oxyR gene in Pseudomonas fluorescens ATCC 17400 and in P. aeruginosa PAO1 impairs pyoverdine-mediated iron uptake. The pyoverdine utilization defect can be restored by complementation with the oxyR gene of P. aeruginosa, as well as by adding catalase. Growth of the oxyR mutant in low- or high-iron media is also impaired at a low, but not at a high inoculum density. Uptake of radioactive (59)Fe pyoverdine is, however, not affected by the oxyR mutation, nor is the transcription of the fpvA gene encoding the ferripyoverdine receptor, suggesting that the defect lies in the inability to remove iron from the ferrisiderophore.

Pyocin S2 (Sa) kills Pseudomonas aeruginosa strains via the FpvA type I ferripyoverdine receptor

Soluble (S-type) pyocins are Pseudomonas aeruginosa bacteriocins that kill nonimmune P. aeruginosa strains via a specific receptor. The genes coding for pyocin Sa (consisting of a killing protein and an immunity protein) were cloned and expressed in Escherichia coli. Sequence analysis revealed that Sa is identical to pyocin S2. Seventy-nine strains of P. aeruginosa were tested for their sensitivity to pyocins S1, S2, and S3, and their ferripyoverdine receptors were typed by multiplex PCR. No strain was found to be sensitive to both S2 and S3, suggesting that the receptors for these two pyocins cannot coexist in one strain. As expected, all S3-sensitive strains had the type II ferripyoverdine receptor fpvA gene, confirming our previous reports. S1 killed strains irrespective of the type of ferripyoverdine receptor they produced. All S2-sensitive strains had the type I fpvA gene, and the inactivation of type I fpvA in an S2-sensitive strain conferred resistance to the S2 pyocin. Accordingly, complementation with type I fpvA in trans restored sensitivity to S2. Some S2-resistant type I fpvA-positive strains were detected, the majority (all but five) of which had the S1-S2 immunity gene. Comparison of type I fpvA sequences from immunity gene-negative S2-sensitive and S2-resistant strains revealed only a valine-to-isoleucine substitution at position 46 of type I FpvA. However, both type I fpvA genes conferred the capacity for type I pyoverdine utilization and sensitivity to S2. When these two type I fpvA genes were introduced into strain 7NSK2 carrying mutations in type II fpvA (encoding the type II pyoverdine receptor) and fpvB (encoding the alternative type I receptor), growth in the presence of type I pyoverdine was observed and the strain became sensitive to S2. We also found that type I pyoverdine could signal type II pyoverdine production via the type I FpvA receptor in 7NSK2.

Global Pseudomonas aeruginosa biodiversity as reflected in a Belgian river

The biodiversity of the bacterium Pseudomonas aeruginosa in an aquatic environment (the Woluwe River, Brussels, Belgium) was analysed. Surface water was sampled bimonthly over a 1-year period (2000-2001) at seven sites evenly dispersed over the river. Total bacterial counts were performed and P. aeruginosa strains were isolated on a selective medium. A weighed out sample of 100 randomly chosen presumptive P. aeruginosa isolates was further analysed. A set of data consisting of the nucleotide sequence of the oprL gene, a DNA-based fingerprint (amplified fragment length polymorphism, AFLP), serotype, pyoverdine type and antibiogram (MICs of 21 clinically relevant antibiotics) was assembled. These data were integrated with those previously obtained for 73 P. aeruginosa clinical and environmental isolates collected across the world. The combined results were analysed and compared using biological data analysis software. Our findings indicate a positive relationship between the extent of pollution and the prevalence of P. aeruginosa. Surprisingly, the Woluwe River P. aeruginosa community was almost as diverse as the global P. aeruginosa population. Indeed, the Woluwe River harboured members of nearly all successful clonal complexes. With the exception of one multidrug-resistant (MDR) strain, belonging to a ubiquitous and clinically relevant serotype O11 clone, antibiotic resistance levels were relatively low. These findings illustrate the significance of river water as a reservoir and source of distribution of potentially pathogenic P. aeruginosa strains and could have repercussions on antinosocomial infection strategies.

Evidence for diversifying selection at the pyoverdine locus of Pseudomonas aeruginosa

Pyoverdine is the primary siderophore of the gram-negative bacterium Pseudomonas aeruginosa. The pyoverdine region was recently identified as the most divergent locus alignable between strains in the P. aeruginosa genome. Here we report the nucleotide sequence and analysis of more than 50 kb in the pyoverdine region from nine strains of P. aeruginosa. There are three divergent sequence types in the pyoverdine region, which correspond to the three structural types of pyoverdine. The pyoverdine outer membrane receptor fpvA may be driving diversity at the locus: it is the most divergent alignable gene in the region, is the only gene that showed substantial intratype variation that did not appear to be generated by recombination, and shows evidence of positive selection. The hypothetical membrane protein PA2403 also shows evidence of positive selection; residues on one side of the membrane after protein folding are under positive selection. R', previously identified as a type IV strain, is clearly derived from a type III strain via a 3.4-kb deletion which removes one amino acid from the pyoverdine side chain peptide. This deletion represents a natural modification of the product of a nonribosomal peptide synthetase enzyme, whose consequences are predictive from the DNA sequence. There is also linkage disequilibrium between the pyoverdine region and pvdY, a pyoverdine gene separated by 30 kb from the pyoverdine region. The pyoverdine region shows evidence of horizontal transfer; we propose that some alleles in the region were introduced from other soil bacteria and have been subsequently maintained by diversifying selection.

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.

The Pseudomonas siderophore quinolobactin is synthesized from xanthurenic acid, an intermediate of the kynurenine pathway

To cope with iron deficiency fluorescent pseudomonads produce pyoverdines which are complex peptidic siderophores that very efficiently scavenge iron. In addition to pyoverdine some species also produce other siderophores. Recently, it was shown that Pseudomonas fluorescens ATCC 17400 produces the siderophore quinolobactin, an 8-hydroxy-4-methoxy-2-quinoline carboxylic acid (Mossialos, D., Meyer, J.M., Budzikiewicz, H., Wolff, U., Koedam, N., Baysse, C., Anjaiah, V., and Cornelis, P. (2000) Appl Environ Microbiol 66: 487-492). The entire quinolobactin biosynthetic, transport and uptake gene cluster, consisting out of two operons comprising 12 open reading frames, was cloned and sequenced. Based on the genes present and physiological complementation assays a biosynthetic pathway for quinolobactin is proposed. Surprisingly, this pathway turned out to combine genes derived from the eukaryotic tryptophan-xanthurenic acid branch of the kynurenine pathway and from the pathway for the biosynthesis of pyridine-2,6-bis(thiocarboxylic acid) from P. stutzeri, PDTC. These results clearly show the involvement of the tryptophan-kynurenine-xanthurenic acid pathway in the synthesis of an authentic quinoline siderophore.

Identification of type II and type III pyoverdine receptors from Pseudomonas aeruginosa

Pseudomonas aeruginosa produces, under conditions of iron limitation, a high-affinity siderophore, pyoverdine (PVD), which is recognized at the level of the outer membrane by a specific TonB-dependent receptor, FpvA. So far, for P. aeruginosa, three different PVDs, differing in their peptide chain, have been described (types I-III), but only the FpvA receptor for type I is known. Two PVD-producing P. aeruginosa strains, one type II and one type III, were mutagenized by a mini-TnphoA3 transposon. In each case, one mutant unable to grow in the presence of the strong iron chelator ethylenediaminedihydroxyphenylacetic acid (EDDHA) and the cognate PVD was selected. The first mutant, which had an insertion in the pvdE gene, upstream of fpvA, was unable to take up type II PVD and showed resistance to pyocin S3, which is known to use type II FpvA as receptor. The second mutant was unable to take up type III PVD and had the transposon insertion in fpvA. Cosmid libraries of the respective type II and type III PVD wild-type strains were constructed and screened for clones restoring the capacity to grow in the presence of PVD. From the respective complementing genomic fragments, type II and type III fpvA sequences were determined. When in trans, type II and type III fpvA restored PVD production, uptake, growth in the presence of EDDHA and, in the case of type II fpvA, pyocin S3 sensitivity. Complementation of fpvA mutants obtained by allelic exchange was achieved by the presence of cognate fpvA in trans. All three receptors posses an N-terminal extension of about 70 amino acids, similar to FecA of Escherichia coli, but only FpvAI has a TAT export sequence at its N-terminal end.

Diversity of siderophore-mediated iron uptake systems in fluorescent pseudomonads: not only pyoverdines

Fluorescent pseudomonads are gamma-proteobacteria known for their capacity to colonize various ecological niches. This adaptability is reflected by their sophisticated and diverse iron uptake systems. The majority of fluorescent pseudomonads produce complex peptidic siderophores called pyoverdines or pseudobactins, which are very efficient iron scavengers. A tremendous variety of pyoverdines has been observed, each species producing a different pyoverdine. This variety can be used as an interesting tool to study the diversity and taxonomy of fluorescent pseudomonads. Other siderophores, including newly described ones, are also produced by pseudomonads, sometimes endowed with interesting properties in addition to iron scavenging, such as formation of complexes with other metals or antimicrobial activity. Factors other than iron limitation, and different regulatory proteins also seem to influence the production of siderophores in pseudomonads and are reviewed here as well. Another peculiarity of pseudomonads is their ability to use a large number of heterologous siderophores via different TonB-dependent receptors. A first genomic analysis of receptors in four different fluorescent pseudomonads suggests that their siderophore ligand repertoire is likely to overlap, and that not all receptors recognize siderophores as ligands.

Pseudomonas aeruginosa displays an epidemic population structure

Bacteria can have population structures ranging from the fully sexual to the highly clonal. Despite numerous studies, the population structure of Pseudomonas aeruginosa is still somewhat contentious. We used a polyphasic approach in order to shed new light on this issue. A data set consisting of three outer membrane (lipo)protein gene sequences (oprI, oprL and oprD), a DNA-based fingerprint (amplified fragment length polymorphism), serotype and pyoverdine type of 73 P. aeruginosa clinical and environmental isolates, collected across the world, was analysed using biological data analysis software. We observed a clear mosaicism in the results, non-congruence between results of different typing methods and a microscale mosaic structure in the oprD gene. Hence, in this network, we also observed some clonal complexes characterized by an almost identical data set. The most recent clones exhibited serotypes O1, 6, 11 and 12. No obvious correlation was observed between these dominant clones and habitat or, with the exception of some recent clones, geographical origin. Our results are consistent with, and even clarify, some seemingly contradictory results in earlier epidemiological studies. Therefore, we suggest an epidemic population structure for P. aeruginosa, comparable with that of Neisseria meningitidis, a superficially clonal structure with frequent recombinations, in which occasionally highly successful epidemic clones arise.

Siderophore-mediated iron uptake in fluorescent Pseudomonas: characterization of the pyoverdine-receptor binding site of three cross-reacting pyoverdines

Two Pseudomonas fluorescens and one Pseudomonas aeruginosa strains, although producing structurally different pyoverdines, demonstrated highly efficient cross-reactions when tested for pyoverdine-mediated iron uptake. A ferripyoverdine receptor-deficient mutant of the P. aeruginosa strain was unable to use any of the three pyoverdines. Moreover, the three strains presented each a specific outer membrane siderophore-receptor pattern. Thus, the capacity of using heterologous pyoverdines was related not to the presence of supplementary specific ferripyoverdine receptors but to the existence within the respective pyoverdine-peptide chains of a common dipeptide motif which should act as the receptor-binding site for the three pyoverdines. Other pyoverdines sharing the same motif but at another position within the peptide chain were not efficient in iron transport, demonstrating the importance of the spatial position of the binding site.

Synthesis and activities of pyoverdin-quinolone adducts: a prospective approach to a specific Therapy against Pseudomonas aeruginosa

Pseudomonas aeruginosa is particularly resistant to most all the antibiotics presently available, essentially because of the very low permeability of its outer membrane. To overcome this, we synthesized four siderophore-based antibiotics formed by two quinolones - norfloxacin and benzonaphthyridone - bound to the pyoverdin of P. aeruginosa ATCC 15692 via two types of spacer arms: one stable and the other readily hydrolyzable. From the comparison of their antibacterial properties with those of the two unbound quinolones, we reached the following conclusions: (a) The adducts inhibit Escherichia coli's gyrase showing that the dissociation of the compounds is not necessary for their activity. However, the presence of the pyoverdin moiety on the molecule decreases the inhibition activity compared to the antibiotic alone. (b) They facilitate the uptake of (55)Fe using the specific pyoverdin-mediated iron-transport system of the bacterium. No uptake was observed either with P. aeruginosa ATCC 27853, which produces a structurally different pyoverdin, or with P. aeruginosa K690, which is a mutant of P. aeruginosa ATCC 15692 lacking FpvA, the outer-membrane pyoverdin receptor. (c) MIC determinations have shown that only strains P. aeruginosa ATCC 15692 and the derived outer-membrane receptor-producing but pyoverdin-deficient P. aeruginosa IA1 mutant present higher susceptibility to the pyoverdin-quinolone adducts, whereas P. aeruginosa ATCC 27853 and K690 are much more resistant. (d) Growth inhibition by these adducts confirmed these results and showed that the adducts with the hydrolyzable spacer arm have better activity than those with the stable one and that the labile spacer arm adducts present much higher activity than the quinolones alone. These results show clearly that the penetration of the antibiotic into the cells is favored when this latter is coupled with pyoverdin: Only the strains possessing the appropriate outer-membrane receptor present higher susceptibility to the adduct. In this case the antibiotic uses the pyoverdin-mediated iron-transport system. Furthermore, better efficiency is obtained when the spacer arm is labile and favors the antibiotic release inside the cell, allowing better inhibition of gyrase.

Pseudobactin biogenesis in the plant growth-promoting rhizobacterium Pseudomonas strain B10: identification and functional analysis of the L-ornithine N(5)-oxygenase (psbA) gene

Pseudobactin(B10), the fluorescent siderophore produced by the rhizobacterium Pseudomonas strain B10, contains the hydroxamate ligand D-N(5)-hydroxyornithine (D-N(5)-OH-Orn). We cloned the L-Orn N(5)-oxygenase (psbA) gene from a genomic library of Pseudomonas strain B10 and demonstrated that PsbA is involved in the conversion of L-Orn to its N(5)-OH derivative. PsbA shows significant similarity to microbial omega-amino acid hydroxylases containing flavin adenine dinucleotide and NADP cofactor-binding sites and the FATGY signature of the putative substrate recognition pocket. The psbA gene is monocistronic, and its transcription is negatively controlled by iron. A site-specific psbA mutant of Pseudomonas strain B10 was biochemically complemented with the precursor L-N(5)-OH-Orn, suggesting that L-Orn is hydroxylated before conversion to the D isomer. The L-Orn N(5)-hydroxylase-defective mutants of Pseudomonas strain B10 and Pseudomonas aeruginosa PAO1 were much less effective than the parental strains in suppressing the growth of the phytopathogen Erwinia carotovora in iron-poor medium. The extent of in vitro inhibition of E. carotovora was strictly iron dependent and directly correlated with the amount of released siderophores. These data strengthen the role of fluorescent siderophores in biocontrol of deleterious rhizomicroorganisms.

Fitness in soil and rhizosphere of Pseudomonas fluorescens C7R12 compared with a C7R12 mutant affected in pyoverdine synthesis and uptake

Fluorescent pseudomonads have evolved an efficient strategy of iron uptake based on the synthesis of the siderophore pyoverdine and its relevant outer membrane receptor. The possible implication of pyoverdine synthesis and uptake on the ecological competence of a model strain (Pseudomonas fluorescens C7R12) in soil habitats was evaluated using a pyoverdine minus mutant (PL1) obtained by random insertion of the transposon Tn5. The Tn5 flanking DNA was amplified by inverse PCR and sequenced. The nucleotide sequence was found to show a high level of identity with pvsB, a pyoverdine synthetase. As expected, the mutant PL1 was significantly more susceptible to iron starvation than the wild-type strain despite its ability to produce another unknown siderophore. As with the wild-type strain, the mutant PL1 was able to incorporate the wild-type pyoverdine and five pyoverdines of foreign origin, but at a significantly lower rate despite the similarity of the outer membrane protein patterns of the two strains. The survival kinetics of the wild-type and of the pyoverdine minus mutant, in bulk and rhizosphere soil, were compared under gnotobiotic and non-gnotobiotic conditions. In gnotobiotic model systems, both strains, when inoculated separately, showed a similar survival in soil and rhizosphere, suggesting that iron was not a limiting factor. In contrast, when inoculated together, the bacterial competition was favorable to the pyoverdine producer C7R12. The efficient fitness of PL1 in the presence of the indigenous microflora, even when coinoculated with C7R12, is assumed to be related to its ability to uptake heterologous pyoverdines. Altogether, these results suggest that pyoverdine-mediated iron uptake is involved in the ecological competence of the strain P. fluorescens C7R12.

The pyoverdin receptor FpvA, a TonB-dependent receptor involved in iron uptake by Pseudomonas aeruginosa (review)

Iron is an important element, essential for the growth of almost all living cells. Because of the high insolubility of iron(III) in aerobic conditions, many gram-negative bacteria produce, under iron limitation, small iron-chelating compounds called siderophores, together with new outer-membrane proteins, which function as receptors for the ferrisiderophores. Pseudomonas aeruginosa, an important human opportunistic pathogen, produces at least three known siderophores when grown in iron-deficient conditions: pyochelin, salicylate and pyoverdin. This review focuses on pyoverdin and on the ability of FpvA to bind iron-free and ferric-PaA pyoverdin, in the light of recent information gained from biochemical and biophysical studies and of the recently solved 3D-structures of the related ferrichrome FhuA and enterobactin FepA receptors in Escherichia coli.

Quinolobactin, a new siderophore of Pseudomonas fluorescens ATCC 17400, the production of which is repressed by the cognate pyoverdine

Transposon mutant strain 3G6 of Pseudomonas fluorescens ATCC 17400 which was deficient in pyoverdine production, was found to produce another iron-chelating molecule; this molecule was identified as 8-hydroxy-4-methoxy-quinaldic acid (designated quinolobactin). The pyoverdine-deficient mutant produced a supplementary 75-kDa iron-repressed outer membrane protein (IROMP) in addition to the 85-kDa IROMP present in the wild type. The mutant was also characterized by substantially increased uptake of (59)Fe-quinolobactin. The 75-kDa IROMP was produced by the wild type after induction by quinolobactin-containing culture supernatants obtained from the pyoverdine-negative mutant or by purified quinolobactin. Conversely, adding purified wild-type pyoverdine to the growth medium resulted in suppression of the 75-kDa IROMP in the pyoverdine-deficient mutant; however, suppression was not observed when Pseudomonas aeruginosa PAO1 pyoverdine, a siderophore utilized by strain 3G6, was added to the culture. Therefore, we assume that the quinolobactin receptor is the 75-kDa IROMP and that the quinolobactin-mediated iron uptake system is repressed by the cognate pyoverdine.

The response of Pseudomonas aeruginosa to iron: genetics, biochemistry and virulence

During the past decade significant progress has been made towards identifying some of the schemes that Pseudomonas aeruginosa uses to obtain iron and towards cataloguing and characterizing many of the genes and gene products that are likely to play a role in these processes. This review will largely recount what we have learned in the past few years about how P. aeruginosa regulates its acquisition, intake and, to some extent, trafficking of iron, and the role of iron acquisition systems in the virulence of this remarkable opportunistic pathogen. More specifically, the genetics, biochemistry and biology of an essential regulator (Ferric uptake regulator - Fur) and a Fur-regulated alternative sigma factor (PvdS), which are central to these processes, will be discussed. These regulatory proteins directly or indirectly regulate a substantial number of other genes encoding proteins with remarkably diverse functions. These genes include: (i) other regulatory genes, (ii) genes involved in basic metabolic processes (e.g. Krebs cycle), (iii) genes required to survive oxidative stress (e.g. superoxide dismutase), (iv) genes necessary for scavenging iron (e.g. siderophores and their cognate receptors) or genes that contribute to the virulence (e.g. exotoxin A) of this opportunistic pathogen. Despite this recent expansion of knowledge about the response of P. aeruginosa to iron, many significant biological issues surrounding iron acquisition still need to be addressed. Virtually nothing is known about which of the distinct iron acquisition mechanisms P. aeruginosa brings to bear on these questions outside the laboratory, whether it be in soil, in a pipeline, on plants or in the lungs of cystic fibrosis patients.

Synthesis and biological evaluation of a pyoverdin-beta-lactam conjugate: a new type of arginine-specific cross-linking in aqueous solution

Arginine specific reagents such as phenylglyoxal and other alpha-dioxo compounds react with arginine side chains by forming adducts with a stoichiometry of 2:1 or a mixture of 2:1 and 1:1. These adducts are labile in neutral and slightly alkaline aqueous solution. We developed a new type of cross-linking reaction with aliphatic beta-dioxo compounds. They can be used for the well-defined, irreversible covalent attachment of molecules carrying a primary amino group to arginyl residues of water soluble peptides. The reaction proceeds under mild conditions in aqueous solution, essentially without the formation of side products. A pyoverdin-cephalexin conjugate was synthesized in order to promote its cellular uptake by Pseudomonas aeruginosa. Preliminary biological investigations of the conjugate indicated that it enters the bacterial cell via the pyoverdin-mediated iron uptake pathway.

Uptake of pyocin S3 occurs through the outer membrane ferripyoverdine type II receptor of Pseudomonas aeruginosa

Pyocin S3 was found to kill exclusively Pseudomonas aeruginosa isolates producing type II pyoverdine (exemplified by strain ATCC 27853). Killing was specifically inhibited by addition of type II ferripyoverdine. All Tn5 mutants resistant to pyocin S3 were defective for pyoverdine-mediated iron uptake and failed to produce an 85-kDa iron-repressed outer membrane protein. We conclude that this protein is probably the type II ferripyoverdine receptor that is used by pyocin S3 to gain entry into the cell.

The ferripyoverdine receptor FpvA of Pseudomonas aeruginosa PAO1 recognizes the ferripyoverdines of P. aeruginosa PAO1 and P. fluorescens ATCC 13525

FpvA, the ferripyoverdine outer membrane receptor of Pseudomonas aeruginosa ATCC 15692 (PAO1 strain), is not specific to the pyoverdine produced by PAO1, but is also able to recognize the structurally different (ferri)pyoverdine of P. fluorescens ATCC 13525. The specificity of FpvA was assessed by iron uptake competitions using the wild-type strains P. aeruginosa ATCC 15692 and P. fluorescens ATCC 13525 and their respective ferripyoverdines, and by fpvA gene complementation of a FpvA-deficient mutant of P. aeruginosa ATCC 15692. The receptor mutant was able to utilize none of the two pyoverdines, while the same but fpvA-complemented mutant recovered simultaneously the ability to incorporate iron thanks to each of the two siderophores. The broad specificity of recognition of FpvA is viewed as an advantage for the strain in iron competition. Moreover, it allows an interesting approach for the understanding of the recognition mechanism between a (ferri)pyoverdine and its cognate outer membrane receptor.

Bacterial iron transport: 1H NMR determination of the three-dimensional structure of the gallium complex of pyoverdin G4R, the peptidic siderophore of Pseudomonas putida G4R

Among the fluorescent Pseudomonas species, Pseudomonas putida is a rare case of a nitrogen-fixing bacterium that transforms nitrogen into ammonia. When grown under iron-deficient conditions, it produces two major pyoverdins: pyoverdin G4R and pyoverdin G4RA. Their primary structures have been established using FAB-MS and one- and two-dimensional 15N, 13C, and 1H NMR on both the unlabeled and 15N-labeled compounds [Salah El Din, A. L. M., et al. (1997) Tetrahedron 53, 12539-12552]. The two pyoverdins have a common chromophore derived from 2,3-diamino-6,7-dihydroxyquinoline. The chromophore is bound to the linear heptapeptide L-Asp-L-Orn-D-beta-threo-OHAsp-L-Dab-Gly-L-Ser-L-cyclo-OHOrn . Circular dichroism spectra suggest that the absolute configuration of the metal complex is Delta. The three-dimensional structure in solution of pyoverdin G4R-Ga(III) was determined after interpretation of two-dimensional 1H NMR spectra recorded at 283 and 303 K. The complex is tightly defined with a compact structure with a Delta absolute configuration. The site of complexation of the metal ion is found to be located on the surface of the molecule, showing that the ion can be released without large conformational changes, while the polar groups of the peptide chain, which may be responsible for the recognition of the receptor, are placed on the opposite side of the overall shape. The three-dimensional structure of pyoverdin G4R-Ga(III) is compared with those of other pyoverdins, and the role of the structure in iron uptake is discussed.

Siderotyping of fluorescent pseudomonads: characterization of pyoverdines of Pseudomonas fluorescens and Pseudomonas putida strains from Antarctica

Five independent fluorescent pseudomonad isolates originating from Antarctica were analysed for their pyoverdine systems. A pyoverdine-related siderotyping, which involved pyoverdine-induced growth stimulation, pyoverdine-mediated iron uptake, pyoverdine analysis by electrophoresis and isoelectric focusing, revealed three different pyoverdine-related siderotypes among the five isolates. One siderotype, including Pseudomonas fluorescens 1W and P. fluorescens 10CW, was identical to that of P. fluorescens ATCC 13525. Two other strains, P. fluorescens 9AW and Pseudomonas putida 9BW, showed identical pyoverdine-related behaviour to each other, whereas the fifth strain, P. fluorescens 51W, had unique features compared to the other strains or to a set of 12 fluorescent Pseudomonas strains used as comparison material. Elucidation of the structure of the pyoverdines produced by the Antarctic strains supported the accuracy of the siderotyping methodology by confirming that pyoverdines from strains 1W and 10CW had the same structures as the P. fluorescens ATCC 13525 pyoverdine, whereas the 9AW and 9BW pyoverdines are probably identical with the pyoverdine of P. fluorescens strain 244. Pyoverdine from strain 51W appeared to be a novel pyoverdine since its structure was different from all previously established pyoverdine structures. Together with the conclusion that the Antarctic Pseudomonas strains have no special features at the level of their pyoverdines and pyoverdine-mediated iron metabolism compared to worldwide strains, the present work demonstrates that siderotyping provides a rapid means of screening for novel pyoverdines.

Analysis of epidemic Pseudomonas aeruginosa isolates by isoelectric focusing of pyoverdine and RAPD-PCR: modern tools for an integrated anti-nosocomial infection strategy in burn wound centres

The Gram-negative bacterium Pseudomonas aeruginosa is an important life-threatening nosocomial pathogen in burn units. In this study we analysed epidemic P. aeruginosa isolates from patients and their hospital environment using two new molecular techniques in order to establish strain relatedness for epidemiological purposes. One technique was pyoverdine typing by isoelectric focusing (PVD-IEF) and the other was a genomic PCR-based fingerprinting technique called random amplification of polymorphic DNA actually referred to as RAPD-PCR. The described short epidemic (6 weeks) included 37 consecutive isolates from 9 different patients as well as two environmental isolates recovered, at the same time, from one of the hydrotherapy facilities. Only two of the three known pyoverdine types of P. aeruginosa could be found. Type I was absent while type II represented 49 per cent and type III, 51 per cent of the isolates. The two consecutive isolates from the environment were both of type III. The RAPD-PCR fingerprinting discriminated four patterns. Profile 1 represented 60 per cent; profile 2, 34 per cent; and profiles 3 and 4 only 3 per cent of the isolates respectively. The environmental isolates also had a RAPD-PCR 1 profile, arguing for the hydrotherapy facility as a possible contamination source. Prompt measures could prevent an outbreak. The study demonstrates the applicability of the techniques in a routine microbiology lab as well as their usefulness, in combination with other techniques, in the fight against nosocomial infections, which are so critical in burn units. Both techniques showed undoubtable evidence of the occurrence of polymicrobial infection of individual patients by P. aeruginosa species. Meanwhile pyoverdine typing by IEF seems suited to studying more profoundly the role of pyoverdines in burns.

Iron acquisition from transferrin and lactoferrin by Pseudomonas aeruginosa pyoverdin

Growth of Pseudomonas aeruginosa ATCC 15692 was promoted when the strain was cultured in an iron-depleted succinate medium, supplemented with transferrin at 30%, 60% and 100% and lactoferrin at 60% and 100% iron-saturation. No significant differences between cell growth and pyoverdin production were observed when transferrin iron saturation was increased from 30% to 100%; however, cell growth and pyoverdin production were strongly dependent on lactoferrin iron saturation. Lower lactoferrin iron saturation (< 30%) resulted in more pyoverdin production and reduced cell growth. Incubation of pyoverdin (1.0 microM) with 10.0 microM transferrin (30%, 60% and 100% iron-saturated) or lactoferrin (60% and 100% iron-saturated) led to quenching of pyoverdin fluorescence. Also, 24 h incubation of pyoverdin (20.0 microM) with these two proteins (20.0 microM, 30%, 60% and 100% iron-saturated transferrin and 60% and 100% iron-saturated lactoferrin) at 25 degrees C resulted in increased absorbance at 460 nm. Both the fluorescence quenching and absorbance increases were iron-saturation-dependent. Taken together, these results support the conclusion that at physiological pH, P. aeruginosa pyoverdin can acquire from partially iron-saturated transferrin or lactoferrin.