Peptide siderophores

Discovery of Siderophore and Metallophore Production in the Aerobic Anoxygenic Phototrophs

Aerobic anoxygenic phototrophs have been isolated from a rich variety of environments including marine ecosystems, freshwater and meromictic lakes, hypersaline springs, and biological soil crusts, all in the hopes of understanding their ecological niche. Over 100 isolates were chosen for this study, representing 44 species from 27 genera. Interactions with Fe³⁺ and other metal(loid) cations such as Mg²⁺, V³⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Se⁴⁺ and Te²⁺ were tested using a chromeazurol S assay to detect siderophore or metallophore production, respectively. Representatives from 20 species in 14 genera of α-Proteobacteria, or 30% of strains, produced highly diffusible siderophores that could bind one or more metal(loid)s, with activity strength as follows: Fe > Zn > V > Te > Cu > Mn > Mg > Se > Ni > Co. In addition, γ-proteobacterial Chromocurvus halotolerans, strain EG19 excreted a brown compound into growth medium, which was purified and confirmed to act as a siderophore. It had an approximate size of ~341 Da and drew similarities to the siderophore rhodotorulic acid, a member of the hydroxamate group, previously found only among yeasts. This study is the first to discover siderophore production to be widespread among the aerobic anoxygenic phototrophs, which may be another key method of metal(loid) chelation and potential detoxification within their environments.

68Ga-labelled desferrioxamine-B for bacterial infection imaging

Purpose

With the increase of especially hospital-acquired infections, timely and accurate diagnosis of bacterial infections is crucial for effective patient care. Molecular imaging has the potential for specific and sensitive detection of infections. Siderophores are iron-specific chelators recognized by specific bacterial transporters, representing one of few fundamental differences between bacterial and mammalian cells. Replacing iron by gallium-68 without loss of bioactivity is possible allowing molecular imaging by positron emission tomography (PET). Here, we report on the preclinical evaluation of the clinically used siderophore, desferrioxamine-B (Desferal®, DFO-B), radiolabelled with ⁶⁸Ga for imaging of bacterial infections.

Methods

In vitro characterization of [⁶⁸Ga]Ga-DFO-B included partition coefficient, protein binding and stability determination. Specific uptake of [⁶⁸Ga]Ga-DFO-B was tested in vitro in different microbial cultures. In vivo biodistribution was studied in healthy mice and dosimetric estimation for human setting performed. PET/CT imaging was carried out in animal infection models, representing the most common pathogens.

Results

DFO-B was labelled with ⁶⁸Ga with high radiochemical purity and displayed hydrophilic properties, low protein binding and high stability in human serum and PBS. The high in vitro uptake of [⁶⁸Ga]Ga-DFO-B in selected strains of Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus agalactiae could be blocked with an excess of iron-DFO-B. [⁶⁸Ga]Ga-DFO-B showed rapid renal excretion and minimal retention in blood and other organs in healthy mice. Estimated human absorbed dose was 0.02 mSv/MBq. PET/CT images of animal infection models displayed high and specific accumulation of [⁶⁸Ga]Ga-DFO-B in both P. aeruginosa and S. aureus infections with excellent image contrast. No uptake was found in sterile inflammation, heat-inactivated P. aeruginosa or S. aureus and Escherichia coli lacking DFO-B transporters.

Conclusion

DFO-B can be easily radiolabelled with ⁶⁸Ga and displayed suitable in vitro characteristics and excellent pharmacokinetics in mice. The high and specific uptake of [⁶⁸Ga]Ga-DFO-B by P. aeruginosa and S. aureus was confirmed both in vitro and in vivo, proving the potential of [⁶⁸Ga]Ga-DFO-B for specific imaging of bacterial infections. As DFO-B is used in clinic for many years and the estimated radiation dose is lower than for other ⁶⁸Ga-labelled radiopharmaceuticals, we believe that [⁶⁸Ga]Ga-DFO-B has a great potential for clinical translation.

Electronic supplementary material

The online version of this article (10.1007/s00259-020-04948-y) contains supplementary material, which is available to authorized users.

Imaging of Pseudomonas aeruginosa infection with Ga-68 labelled pyoverdine for positron emission tomography

Pseudomonas aeruginosa is an increasingly prevalent opportunistic pathogen that causes a variety of life-threatening nosocomial infections. Novel strategies for the development of new antibacterial treatments as well as diagnostic tools are needed. One of the novel diagnostic strategies for the detection of infection could be the utilization of siderophores. Siderophores are low-molecular-weight chelators produced by microbes to scavenge essential iron. Replacing iron in siderophores by suitable radiometals, such as Ga-68 for positron emission tomography (PET) imaging, opens approaches for targeted imaging of infection. Here we report on pyoverdine PAO1 (PVD-PAO1), a siderophore produced by P. aeruginosa, labelled with Ga-68 for specific imaging of Pseudomonas infections. PVD-PAO1 was labelled with Ga-68 with high radiochemical purity. The resulting complex showed hydrophilic properties, low protein binding and high stability in human serum. In vitro uptake of ⁶⁸Ga-PVD-PAO1 was highly dependent on the type of microbial culture. In normal mice ⁶⁸Ga-PVD-PAO1 showed rapid pharmacokinetics with urinary excretion. PET imaging in infected animals displayed specific accumulation of ⁶⁸Ga-PVD-PAO1 in infected tissues and better distribution than clinically used ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) and ⁶⁸Ga-citrate. Ga-68 labelled pyoverdine PAO1 seems to be a promising agent for imaging of P. aeruginosa infections by means of PET.

Nonribosomal peptides for iron acquisition: pyochelin biosynthesis as a case study

Microbes synthesize small, iron-chelating molecules known as siderophores to acquire iron from the environment. One way siderophores are generated is by nonribosomal peptide synthetases (NRPSs). The bioactive peptides generated by NRPS enzymes have unique chemical features, which are incorporated by accessory and tailoring domains or proteins. The first part of this review summarizes recent progress in NRPS structural biology. The second part uses the biosynthesis of pyochelin, a siderophore from Pseudomonas aeruginosa, as a case study to examine enzymatic methods for generating the observed diversity in NRPS-derived natural products.

Evaluation of Stress-Induced Microbial Siderophore from Pseudomonas aeruginosa Strain S1 as a Potential Matrix Metalloproteinase Inhibitor in Wound Healing Applications

Matrix metalloproteinases (MMPs) are zinc-dependent proteolytic enzymes capable of causing various inflammatory and various degenerative diseases if over-expressed. The active site of these enzymes is a zinc binding motif which binds to the specific site on the substrate and induce degradation. Hence an inhibitor is required to form a complex with zinc motif which hampers the binding ability of MMPs. To obtain novel MMPs inhibitor for wound healing, the chelating activity of siderophore from the microbial source was focused. During screening for siderophore production, strain S1 produced the highest amount of siderophore in the minimal salts medium. The isolate was confirmed as Pseudomonas aeruginosa strain S1 based on 16S rRNA gene sequencing and phylogenetic analysis. The activity of the siderophore was assayed using chrome azurol sulphonate and purified by the chromatographic techniques. The structural evidence through Fourier transform infrared and nuclear magnetic resonance spectra revealed that the isolated siderophore is a catecholate type with the distinctive characters. The positive results of calcein and fluozin-3 assays indicate that siderophore could bind to divalent metal ions, namely Fe(2+) and Zn(2+). As the siderophore compound focused on wound healing property, the in vitro studies revealed the viability of NH3T3 fibroblast cells and its efficiency in matrix modulating was confirmed through gelatin zymogram.

Whole Genome Sequencing of the Symbiont Pseudovibrio sp. from the Intertidal Marine Sponge Polymastia penicillus Revealed a Gene Repertoire for Host-Switching Permissive Lifestyle

Sponges harbor a complex consortium of microbial communities living in symbiotic relationship benefiting each other through the integration of metabolites. The mechanisms influencing a successful microbial association with a sponge partner are yet to be fully understood. Here, we sequenced the genome of Pseudovibrio sp. POLY-S9 strain isolated from the intertidal marine sponge Polymastia penicillus sampled from the Atlantic coast of Portugal to identify the genomic features favoring the symbiotic relationship. The draft genome revealed an exceptionally large genome size of 6.6 Mbp compared with the previously reported genomes of the genus Pseudovibrio isolated from a coral and a sponge larva. Our genomic study detected the presence of several biosynthetic gene clusters—polyketide synthase, nonribosomal peptide synthetase and siderophore—affirming the potential ability of the genus Pseudovibrio to produce a wide variety of metabolic compounds. Moreover, we identified a repertoire of genes encoding adaptive symbioses factors (eukaryotic-like proteins), such as the ankyrin repeats, tetratrico peptide repeats, and Sel1 repeats that improve the attachment to the eukaryotic hosts and the avoidance of the host’s immune response. The genome also harbored a large number of mobile elements (∼5%) and gene transfer agents, which explains the massive genome expansion and suggests a possible mechanism of horizontal gene transfer. In conclusion, the genome of POLY-S9 exhibited an increase in size, number of mobile DNA, multiple metabolite gene clusters, and secretion systems, likely to influence the genome diversification and the evolvability.

Racemization in reverse: evidence that D-amino acid toxicity on Earth is controlled by bacteria with racemases

D-amino acids are toxic for life on Earth. Yet, they form constantly due to geochemical racemization and bacterial growth (the cell walls of which contain D-amino acids), raising the fundamental question of how they ultimately are recycled. This study provides evidence that bacteria use D-amino acids as a source of nitrogen by running enzymatic racemization in reverse. Consequently, when soils are inundated with racemic amino acids, resident bacteria consume D- as well as L-enantiomers, either simultaneously or sequentially depending on the level of their racemase activity. Bacteria thus protect life on Earth by keeping environments D-amino acid free.

Extracellular electron transport-mediated Fe(III) reduction by a community of alkaliphilic bacteria that use flavins as electron shuttles

The biochemical and molecular mechanisms used by alkaliphilic bacterial communities to reduce metals in the environment are currently unknown. We demonstrate that an alkaliphilic (pH > 9) consortium dominated by Tissierella, Clostridium, and Alkaliphilus spp. is capable of using iron (Fe(3+)) as a final electron acceptor under anaerobic conditions. Iron reduction is associated with the production of a freely diffusible species that, upon rudimentary purification and subsequent spectroscopic, high-performance liquid chromatography, and electrochemical analysis, has been identified as a flavin species displaying properties indistinguishable from those of riboflavin. Due to the link between iron reduction and the onset of flavin production, it is likely that riboflavin has an import role in extracellular metal reduction by this alkaliphilic community.

Burkholderia cepacia XXVI siderophore with biocontrol capacity against Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is the causal agent of anthracnose in mango. Burkholderia cepacia XXVI, isolated from mango rhizosphere and identified by 16S rDNA sequencing as a member of B. cepacia complex, was more effective than 6 other mango rhizosphere bacteria in inhibiting the model mango pathogen, C. gloeosporioides ATCC MYA 456. Biocontrol of this pathogen was demonstrated on Petri-dishes containing PDA by > 90 % reduction of surface colonization. The nature of the biocontrol metabolite(s) was characterized via a variety of tests. The inhibition was almost exclusively due to production of agar-diffusible, not volatile, metabolite(s). The diffusible metabolite(s) underwent thermal degradation at 70 and 121 °C (1 atm). Tests for indole acetic acid production and lytic enzyme activities (cellulase, glucanase and chitinase) by B. cepacia XXVI were negative, indicating that these metabolites were not involved in the biocontrol effect. Based on halo formation and growth inhibition of the pathogen on the diagnostic medium, CAS-agar, as well as colorimetric tests we surmised that strain XXVI produced a hydroxamate siderophore involved in the biocontrol effect observed. The minimal inhibitory concentration test showed that 0.64 μg ml(-1) of siderophore (Deferoxamine mesylate salt-equivalent) was sufficient to achieve 91.1 % inhibition of the pathogen growth on Petri-dishes containing PDA. The biocontrol capacity against C. gloeosporioides ATCC MYA 456 correlated directly with the siderophore production by B. cepacia XXVI: the highest concentration of siderophore production in PDB on day 7, 1.7 μg ml(-1) (Deferoxamine mesylate salt-equivalent), promoted a pathogen growth inhibition of 94.9 %. The growth of 5 additional strains of C. gloeosporioides (isolated from mango "Ataulfo" orchards located in the municipality of Chahuites, State of Oaxaca in Mexico) was also inhibited when confronted with B. cepacia XXVI. Results indicate that B. cepacia XXVI or its siderophore have the potential to be used as a biological control agent against C. gloeosporioides; thus diminishing environmental problems caused by the current practices to control this disease.

In vitro and in vivo evaluation of selected 68Ga-siderophores for infection imaging

Introduction

Siderophores are low-molecular-mass iron chelators serving as iron transporters for almost all bacteria, fungi and some plants. Iron is an essential element for majority of organisms and plays an important role in virulence of pathogenic organisms. ⁶⁸Ga is a positron emitter with complexing properties comparable to those of Fe(III) and readily available from a generator. Initial studies with ⁶⁸Ga-triacetylfusarinine C (TAFC) showed excellent targeting properties in a rat infection model. We report here on the in vitro and in vivo evaluation of other siderophores radiolabelled with ⁶⁸Ga as potential radiopharmaceuticals for infection imaging.

Methods

⁶⁸Ga labelling was performed using acetate buffer. Stability, log P and protein binding values were determined. In vitro uptake was tested using iron-deficient and iron-sufficient Aspergillus fumigatus (A.f.) cultures. Biodistribution of ⁶⁸Ga-siderophores was studied in Balb/c mice.

Results

Significant differences among studied siderophores were observed in labelling efficiency, stability and protein binding. Uptake in A.f. cultures was highly dependent on iron load and type of the siderophore. In mice, ⁶⁸Ga-TAFC and ⁶⁸Ga-ferrioxamine E (FOXE) showed rapid renal excretion and low blood values even at a short period after injection; in contrast, ⁶⁸Ga-ferricrocin and ⁶⁸Ga-ferrichrome revealed high retention in blood and ⁶⁸Ga-fusarinine C showed very high kidney retention.

Conclusions

Some of the studied siderophores bind ⁶⁸Ga with high affinity and stability, especially ⁶⁸Ga-TAFC and ⁶⁸Ga-FOXE. Low values of protein binding, high and specific uptake in A.f., and excellent in vivo biodistribution make them favourable agents for Aspergillus infection imaging.

Genome of alkaliphilic Bacillus pseudofirmus OF4 reveals adaptations that support the ability to grow in an external pH range from 7.5 to 11.4

Bacillus pseudofirmus OF4 is an extreme but facultative alkaliphile that grows non-fermentatively in a pH range from 7.5 to above 11.4 and can withstand large sudden increases in external pH. It is a model organism for studies of bioenergetics at high pH, at which energy demands are higher than at neutral pH because both cytoplasmic pH homeostasis and ATP synthesis require more energy. The alkaliphile also tolerates a cytoplasmic pH > 9.0 at external pH values at which the pH homeostasis capacity is exceeded, and manages other stresses that are exacerbated at alkaline pH, e.g. sodium, oxidative and cell wall stresses. The genome of B. pseudofirmus OF4 includes two plasmids that are lost from some mutants without viability loss. The plasmids may provide a reservoir of mobile elements that promote adaptive chromosomal rearrangements under particular environmental conditions. The genome also reveals a more acidic pI profile for proteins exposed on the outer surface than found in neutralophiles. A large array of transporters and regulatory genes are predicted to protect the alkaliphile from its overlapping stresses. In addition, unanticipated metabolic versatility was observed, which could ensure requisite energy for alkaliphily under diverse conditions.

Dual pathways for copper uptake by methanotrophic bacteria

Methanobactin (Mb), a 1217-Da copper chelator produced by the methanotroph Methylosinus trichosporium OB3b, is hypothesized to mediate copper acquisition from the environment, particularly from insoluble copper mineral sources. Although indirect evidence suggests that Mb provides copper for the regulation and activity of methane monooxygenase enzymes, experimental data for direct uptake of copper loaded Mb (Cu-Mb) are lacking. Uptake of intact Cu-Mb by M. trichosporium OB3b was demonstrated by isotopic and fluorescent labeling experiments. Confocal microscopy data indicate that Cu-Mb is localized in the cytoplasm. Both Cu-Mb and unchelated Cu are taken up by M. trichosporium OB3b, but by different mechanisms. Uptake of unchelated Cu is inhibited by spermine, suggesting a porin-dependent passive transport process. By contrast, uptake of Cu-Mb is inhibited by the uncoupling agents carbonyl cyanide m-chlorophenylhydrazone and methylamine, but not by spermine, consistent with an active transport process. Cu-Mb from M. trichosporium OB3b can also be internalized by other strains of methanotroph, but not by Escherichia coli, suggesting that Cu-Mb uptake is specific to methanotrophic bacteria. These findings are consistent with a key role for Cu-Mb in copper acquisition by methanotrophs and have important implications for further investigation of the copper uptake machinery.

Draft genome sequence of the thermoalkaliphilic Caldalkalibacillus thermarum strain TA2.A1

The genes and molecular machines that allow for a thermoalkaliphilic lifestyle have not been defined. To address this goal, we report on the improved high-quality draft genome sequence of Caldalkalibacillus thermarum strain TA2.A1, an obligately aerobic bacterium that grows optimally at pH 9.5 and 65 to 70°C on a wide variety of carbon and energy sources.

68Ga-siderophores for PET imaging of invasive pulmonary aspergillosis: proof of principle

The diagnosis of invasive pulmonary aspergillosis (IPA) is difficult and lacks specificity and sensitivity. In the pathophysiology of Aspergillus fumigatus, iron plays an essential role as a nutrient during infection. A. fumigatus uses a specific and highly efficient iron uptake mechanism based on iron-complexing ferric ion Fe(III) siderophores, which are a requirement for A. fumigatus virulence. We aimed to evaluate the potential of siderophores radiolabeled with (68)Ga, a positron emitter with complexing properties comparable to those of Fe(III), as a radiopharmaceutical for imaging IPA.

METHODS

(68)Ga radiolabeling of the A. fumigatus siderophores desferri-triacetylfusarinine C (TAFC) and desferri-ferricrocin (FC) was performed at high specific activity. Stability, protein binding, and log P values were determined. In vitro uptake in A. fumigatus cultures was tested under varying conditions. Biodistribution was studied in healthy noninfected BALB/c mice, and uptake was studied in a model of A. fumigatus infection using immunosuppressed Lewis rats.

RESULTS

High-specific-activity (68)Ga labeling could be achieved, and resulting complexes were stable in serum, toward diethylenetriaminepentaacetic acid and Fe(III) challenge. Both siderophores showed hydrophilic properties ((68)Ga-TAFC, log P = -2.59; (68)Ga-FC, log P = -3.17) with low values of protein binding for (68)Ga-TAFC (<2%). Uptake of both siderophores was highly dependent on the mycelial iron load and could be blocked with an excess (10 microM) of siderophore or NaN(3), indicating specific, energy-dependent uptake. In noninfected mice, (68)Ga-TAFC showed rapid renal excretion and low blood values (1.6 +/- 0.37 percentage injected dose per gram [%ID/g] at 30 min); in urine only intact (68)Ga-TAFC was detected. In contrast, (68)Ga-FC revealed high retention in blood (16.1 +/- 1.07 %ID/g at 90 min) and rapid metabolism. In the rat IPA model, lung uptake of (68)Ga-TAFC was dependent on the severity of infection, with less than 0.04 %ID/g in control rats (n = 5) and 0.29 +/- 0.11 %ID/g in mildly infected (n = 3) and 0.95 +/- 0.37 %ID/g in severely infected (n = 4) rats. PET showed focal accumulation in infected lung tissue.

CONCLUSION

Both siderophores bound (68)Ga with high affinity, and (68)Ga-TAFC, especially, showed high stability. (68)Ga-TAFC displayed highly selective accumulation by A. fumigatus subspecies in vitro and in vivo. The high and specific uptake by A. fumigatus proves the potential of (68)Ga-labeled siderophores for the specific detection of A. fumigatus during infection. They hold promise as new PET agents for IPA.

Enzymatic tailoring of ornithine in the biosynthesis of the Rhizobium cyclic trihydroxamate siderophore vicibactin

To acquire iron, the N(2)-fixing, symbiotic bacterium Rhizobium sp. produce the cyclic trihydroxamate siderophore vicibactin, containing a 30-membered trilactone scaffold. Herein we report the overproduction and purification of the six proteins VbsACGOLS in the bacterial host Escherichia coli and the reconstitution of the biosynthesis of vicibactin from primary metabolites. The flavoprotein VbsO acts as a pathway-initiating l-ornithine N(5)-hydroxylase, followed by VbsA, which transfers (R)-3-hydroxybutyryl- from the CoA thioester to N(5)-hydroxyornithine to yield N(5)-((R)-3-hydroxybutyryl)-N(5)-hydroxy-l-ornithine. VbsL is a PLP-dependent epimerase acting at C(2) of the 10 atom monomer unit. VbsS, a nonribosomal peptide synthetase free-standing module, then activates N(5)-((R)-3-hydroxybutyryl)-N(5)-hydroxy-d-ornithine as the AMP anhydride on the way to cyclotrimerization to the vicibactin scaffold. The last step, tris-acetylation of the C(2) amino group of desacetyl-d-vicibactin to the mature siderophore vicibactin, is catalyzed distributively by VbsC, using three molecules of acetyl-CoA.

Analysis of achromobactin biosynthesis by Pseudomonas syringae pv. syringae B728a

Pseudomonas syringae pv. syringae B728a is known to produce the siderophore pyoverdine under iron-limited conditions. It has also been proposed that this pathovar has the ability to produce a second siderophore, achromobactin. Here we present genetic and biochemical evidence supporting the hypothesis that P. syringae pv. syringae B728a produces both of these siderophores. We show that strains unable to synthesize either pyoverdine or achromobactin are unable to grow under iron-limiting conditions, which is consistent with these two molecules being the only siderophores synthesized by P. syringae pv. syringae B728a. Enzymes associated with achromobactin biosynthesis were purified and analyzed for substrate recognition. We showed that AcsD, AcsA, and AcsC together are able to condense citrate, ethanolamine, 2,4-diaminobutyrate, and alpha-ketoglutarate into achromobactin. Replacement of ethanolamine with ethylene diamine or 1,3-diaminopropane in these reactions resulted in the formation of achromobactin analogs that were biologically active. This work provides insights into the biosynthetic steps in the formation of achromobactin and is the first in vitro reconstitution of achromobactin biosynthesis.

Structure elucidation and biosynthesis of fuscachelins, peptide siderophores from the moderate thermophile Thermobifida fusca

Bacteria belonging to the order Actinomycetales have proven to be an important source of biologically active and often therapeutically useful natural products. The characterization of orphan biosynthetic gene clusters is an emerging and valuable approach to the discovery of novel small molecules. Analysis of the recently sequenced genome of the thermophilic actinomycete Thermobifida fusca revealed an orphan nonribosomal peptide biosynthetic gene cluster coding for an unknown siderophore natural product. T. fusca is a model organism for the study of thermostable cellulases and is a major degrader of plant cell walls. Here, we report the isolation and structure elucidation of the fuscachelins, siderophore natural products produced by T. fusca. In addition, we report the purification and biochemical characterization of the termination module of the nonribosomal peptide synthetase. Biochemical analysis of adenylation domain specificity supports the assignment of this gene cluster as the producer of the fuscachelin siderophores. The proposed nonribosomal peptide biosynthetic pathway exhibits several atypical features, including a macrocyclizing thioesterase that produces a 10-membered cyclic depsipeptide and a nonlinear assembly line, resulting in the unique heterodimeric architecture of the siderophore natural product.

Copper methanobactin: a molecule whose time has come

Copper plays a key role in the physiology of methanotrophs. One way that these bacteria meet their high copper requirement is by the biosynthesis and release of high affinity copper binding compounds called methanobactins. Recent advances in methanobactin characterization include the first crystal structure, detailed spectroscopic analyses, and studies of metal ion specificity. Methanobactin may function in copper uptake, regulation of methane monooxygenase expression, protection against copper toxicity, and particulate methane monooxygenase activity. Methanobactin can extract copper from insoluble minerals and could be important for mineral weathering. Many methanobactin properties are reminiscent of iron siderophores, suggesting a similar mechanism of handling. Methanobactin-like compounds have also been identified in yeast mitochondria, suggesting that these molecules are a more universal phenomenon.

Siderophore production by Pseudomonas stutzeri under aerobic and anaerobic conditions

The siderophore production of the facultative anaerobe Pseudomonas stutzeri, strain CCUG 36651, grown under both aerobic and anaerobic conditions, was investigated by liquid chromatography and mass spectrometry. The bacterial strain has been isolated at a 626-m depth at the Aspö Hard Rock Laboratory, where experiments concerning the geological disposal of nuclear waste are performed. In bacterial culture extracts, the iron in the siderophore complexes was replaced by gallium to facilitate siderophore identification by mass spectrometry. P. stutzeri was shown to produce ferrioxamine E (nocardamine) as the main siderophore together with ferrioxamine G and two cyclic ferrioxamines having molecular masses 14 and 28 atomic mass units lower than that of ferrioxamine E, suggested to be ferrioxamine D(2) and ferrioxamine X(1), respectively. In contrast, no siderophores were observed from anaerobically grown P. stutzeri. None of the siderophores produced by aerobically grown P. stutzeri were found in anaerobic natural water samples from the Aspö Hard Rock Laboratory.

The Legionella pneumophila iraAB locus is required for iron assimilation, intracellular infection, and virulence

Legionella pneumophila, a facultative intracellular parasite of human alveolar macrophages and protozoa, causes Legionnaires' disease. Using mini-Tn10 mutagenesis, we previously isolated a L. pneumophila mutant that was hypersensitive to iron chelators. This mutant, NU216, and its allelic equivalent, NU216R, were also defective for intracellular infection, particularly in iron-deficient host cells. To determine whether NU216R was attenuated for virulence, we assessed its ability to cause disease in guinea pigs following intratracheal inoculation. NU216R-infected animals yielded 1,000-fold fewer bacteria from their lungs and spleen compared to wild-type-130b-infected animals that had received a 50-fold-lower dose. Moreover, NU216R-infected animals subsequently cleared the bacteria from these sites. While infection with 130b resulted in high fever, weight loss, and ruffled fur, inoculation with NU216R did not elicit any signs of disease. DNA sequence analysis revealed that the transposon insertion in NU216R lies in the first open reading frame of a two-gene operon. This open reading frame (iraA) encodes a 272-amino-acid protein that shows sequence similarity to methyltransferases. The second open reading frame (iraB) encodes a 501-amino-acid protein that is highly similar to di- and tripeptide transporters from both prokaryotes and eukaryotes. Southern hybridization analyses determined that the iraAB locus was largely limited to strains of L. pneumophila, the most pathogenic of the Legionella species. A newly derived mutant containing a targeted disruption of iraB showed reduced ability to grow under iron-depleted extracellular conditions, but it did not have an infectivity defect in the macrophage-like U937 cells. These data suggest that iraA is critical for virulence of L. pneumophila while iraB is involved in a novel method of iron acquisition which may utilize iron-loaded peptides.