Surface iron polymers and hydroxy acids. A model of iron supply in sideramine-free fungi

Factors Impacting Outcome and Prognosis of Invasive Fungal Sinusitis: How Vital is Iron Metabolism?

Iron is an important micronutrient involved in cell biology through vital reactions. We examined the correlations between iron metabolism parameters and the course of invasive fungal sinusitis. Patients with invasive fungal sinusitis were enrolled. Serum iron and ferritin levels, total iron-binding capacity, and transferrin saturation were measured at the initiation of treatment. Patients were followed for 6 months, and the clinical course was categorised as improvement or worsening/death. A total of 35 patients were enrolled. The average ferritin levels in mucormycosis patients was 944.9 ng/ml, versus 110.7 ng/ml for aspergillosis patients. Iron levels were significantly lower in mucormycosis than in aspergillosis (29.14 µg/dl vs. 68.55 µg/dl). Total iron-binding capacity was significantly different between the two groups (16.76 µg/dl vs. 330.36 µg/dl). After 6 months, improvement, worsening, and death were noted for 18, 8, and 9 patients, respectively. Higher iron levels and lower ferritin levels were linked with improvement. Total iron-binding capacity was significantly higher in improved patients (2314 vs. 151). Iron metabolism parameters play significant roles in the preemptive judgment of the course of fungal sinusitis. Based on these findings, studies on drugs affecting iron metabolism should be conducted.

Regulatory and pathogenic mechanisms in response to iron deficiency and excess in fungi

Iron is an essential element for all eukaryote organisms because of its redox properties, which are important for many biological processes such as DNA synthesis, mitochondrial respiration, oxygen transport, lipid, and carbon metabolism. For this reason, living organisms have developed different strategies and mechanisms to optimally regulate iron acquisition, transport, storage, and uptake in different environmental responses. Moreover, iron plays an essential role during microbial infections. Saccharomyces cerevisiae has been of key importance for decrypting iron homeostasis and regulation mechanisms in eukaryotes. Specifically, the transcription factors Aft1/Aft2 and Yap5 regulate the expression of genes to control iron metabolism in response to its deficiency or excess, adapting to the cell's iron requirements and its availability in the environment. We also review which iron-related virulence factors have the most common fungal human pathogens (Aspergillus fumigatus, Cryptococcus neoformans, and Candida albicans). These factors are essential for adaptation in different host niches during pathogenesis, including different fungal-specific iron-uptake mechanisms. While being necessary for virulence, they provide hope for developing novel antifungal treatments, which are currently scarce and usually toxic for patients. In this review, we provide a compilation of the current knowledge about the metabolic response to iron deficiency and excess in fungi.

A Penicillium expansum glucose oxidase-encoding gene, GOX2, is essential for gluconic acid production and acidification during colonization of deciduous fruit

Penicillium expansum, the causal agent of blue mold rot, causes severe postharvest maceration of fruit through secretion of total, d-gluconic acid (GLA). Two P. expansum glucose oxidase (GOX)-encoding genes, GOX1 and GOX2, were analyzed. GOX activity and GLA accumulation were strongly related to GOX2 expression, which increased with pH to a maximum at pH 7.0, whereas GOX1 was expressed at pH 4.0, where no GOX activity or extracellular GLA were detected. This differential expression was also observed at the leading edge of the decaying tissue, where GOX2 expression was dominant. The roles of the GOX genes in pathogenicity were further studied through i) development of P. expansum goxRNAi mutants exhibiting differential downregulation of GOX2, ii) heterologous expression of the P. expansum GOX2 gene in the nondeciduous fruit-pathogen P. chrysogenum, and iii) modulation of GLA production by FeSO(4) chelation. Interestingly, in P. expansum, pH and GLA production elicited opposite effects on germination and biomass accumulation: 26% of spores germinated at pH 7.0 when GOX activity and GLA were highest whereas, in P. chrysogenum at the same pH, when GLA did not accumulate, 72% of spores germinated. Moreover, heterologous expression of P. expansum GOX2 in P. chrysogenum resulted in enhanced GLA production and reduced germination, suggesting negative regulation of spore germination and GLA production. These results demonstrate that pH modulation, mediated by GLA accumulation, is an important factor in generating the initial signal or signals for fungal development leading to host-tissue colonization by P. expansum.

Ecology of siderophores with special reference to the fungi

Ecology of siderophores, as described in the present review, analyzes the factors that allow the production and function of siderophores under various environmental conditions. Microorganisms that excrete siderophores are able to grow in natural low-iron environments by extracting residual iron from insoluble iron hydroxides, protein-bound iron or from other iron chelates. Compared to the predominantly mobile bacteria, the fungi represent mostly immobile microorganisms that rely on local nutrient concentrations. Feeding the immobile is a general strategy of fungi and plants, which depend on the local nutrient resources. This also applies to iron nutrition, which can be improved by excretion of siderophores. Most fungi produce a variety of different siderophores, which cover a wide range of physico-chemical properties in order to overcome adverse local conditions of iron solubility. Resource zones will be temporally and spatially dynamic which eventually results in conidiospore production, transport to new places and outgrow of mycelia from conidiospores. Typically, extracellular and intracellular siderophores exist in fungi which function either in transport or storage of ferric iron. Consequently, extracellular and intracellular reduction of siderophores may occur depending on the fungal strain, although in most fungi transport of the intact siderophore iron complex has been observed. Regulation of siderophore biosynthesis is essential in fungi and allows an economic use of siderophores and metabolic resources. Finally, the chemical stability of fungal siderophores is an important aspect of microbial life in soil and in the rhizosphere. Thus, insolubility of iron in the environment is counteracted by dissolution and chelation through organic acids and siderophores by various fungi.

Simultaneous P-solubilizing and biocontrol activity of microorganisms: potentials and future trends

Phosphate (P)-solubilizing microorganisms as a group form an important part of the microorganisms, which benefit plant growth and development. Growth promotion and increased uptake of phosphate are not the only mechanisms by which these microorganisms exert a positive effect on plants. Microbially mediated solubilization of insoluble phosphates through release of organic acids is often combined with production of other metabolites, which take part in biological control against soilborne phytopathogens. In vitro studies show the potential of P-solubilizing microorganisms for the simultaneous synthesis and release of pathogen-suppressing metabolites, mainly siderophores, phytohormones, and lytic enzymes. Further trends in this field are discussed, suggesting a number of biotechnological approaches through physiological and biochemical studies using various microorganisms.

Iron gathering of opportunistic pathogenic fungi. A mini review

Iron is an essential nutrient for most organisms because it serves as a catalytic cofactor in oxidation-reduction reactions. Iron is rather unavailable because it occurs in its insoluble ferric form in oxides and hydroxides, while in serum of mammalian hosts is highly bound to carrier proteins such as transferrin, so the free iron concentration is extremely low insufficient for microbial growth. Therefore, many organisms have developed different iron-scavenging systems for solubilizing ferric iron and transporting it into cells across the fungal membrane. There are three major mechanisms by which fungi can obtain iron from the host: (a) utilization of a high affinity iron permease to transport iron intracellularly, (b) production and secretion of low molecular weight iron-specific chelators (siderophores), (c) utilization of a hem oxygenase to acquire iron from hemin. Patients with elevated levels of available serum iron treated with iron chelator, deferoxamine to remedy iron overload conditions have an increased susceptibility of invasive zygomycosis. Presumably deferoxamine predisposes patients to Zygomycetes infections by acting as a siderophore]. The frequency of zygomycosis is increasing in recent years and these infections respond very poorly to currently available antifungal agents, so new approaches to develop strategies to prevent and treat zygomycosis are urgently needed. Siderophores and iron-transport proteins have been suggested to function as virulence factors because the acquisition of iron is a crucial pathogenetic event. Biosynthesis and uptake of siderophores represent possible targets for antifungal therapy.

Development and application of an assay for uranyl complexation by fungal metabolites, including siderophores

An assay to detect UO(2)(2+) complexation was developed based on the chrome azurol S (CAS) assay for siderophores (B. Schwyn and J. B. Neilands, Anal. Biochem. 160:47-56, 1987) and was used to investigate the ability of fungal metabolites to complex actinides. In this assay the discoloration of two dyed agars (one containing a CAS-Fe(3+) dye and the other containing a CAS-UO(2)(2+) dye) caused by ligands was quantified. The assay was tested by using the siderophore desferrioxamine B (DFO), and the results showed that there was a regular, reproducible relationship between discoloration and the amount of siderophore added. The ratio of the discoloration on the CAS-UO(2)(2+) agar to the discoloration on the CAS-Fe(3+) agar was independent of the amount of siderophore added. A total of 113 fungi and yeasts were isolated from three soil samples taken from the Peak District National Park. The fungi were screened for the production of UO(2)(2+) chelators by using the CAS-based assay and were also tested specifically for hydroxamate siderophore production by using the hydroxamate siderophore auxotroph Aureobacterium flavescens JG-9. This organism is highly sensitive to the presence of hydroxamate siderophores. However, the CAS-based assay was found to be less sensitive than the A. flavescens JG-9 assay. No significant difference between the results for each site for the two tests was found. Three isolates were selected for further study and were identified as two Pencillium species and a Mucor species. Our results show that the new assay can be effectively used to screen fungi for the production of UO(2)(2+) chelating ligands. We suggest that hydroxamate siderophores can be produced by mucoraceous fungi.

Acquisition, transport, and storage of iron by pathogenic fungi

Iron is required by most living systems. A great variety of means of acquisition, avenues of uptake, and methods of storage are used by pathogenic fungi to ensure a supply of the essential metal. Solubilization of insoluble iron polymers is the first step in iron assimilation. The two methods most commonly used by microorganisms for solubilization of iron are reduction and chelation. Reduction of ferric iron to ferrous iron by enzymatic or nonenzymatic means is a common mechanism among pathogenic yeasts. Under conditions of iron starvation, many fungi synthesize iron chelators known as siderophores. Two classes of compounds that function in iron gathering are commonly observed: hydroxamates and polycarboxylates. Two major responses to iron stress in fungi are a high-affinity ferric iron reductase and siderophore synthesis. Regulation of these two mechanisms at the molecular level has received attention. Uptake of siderophores is a diverse process, which varies among the different classes of compounds. Since free iron is toxic, it must be stored for further metabolic use. Polyphosphates, ferritins, and siderophores themselves have been described as storage molecules. The iron-gathering mechanisms used by a pathogen in an infected host are largely unknown and can only be posited on the basis of in vitro studies at present.

The Bradyrhizobium japonicum fegA gene encodes an iron-regulated outer membrane protein with similarity to hydroxamate-type siderophore receptors

Iron is important in the symbiosis between soybean and its nitrogen-fixing endosymbiont Bradyrhizobium japonicum, yet little is known about rhizobial iron acquisition strategies. Analysis of outer membrane proteins (OMPs) from B. japonicum 61A152 identified three iron-regulated OMPs in the size range of several known receptors for Fe(III)-scavenging siderophores. One of the iron-regulated proteins, FegA, was purified and microsequenced, and a reverse genetics approach was used to clone a fegA-containing DNA fragment. Sequencing of this fragment revealed a single open reading frame of 750 amino acids. A putative N-terminal signal sequence of 14 amino acids which would result in a mature protein of 736 amino acids with a molecular mass of 80,851 Da was predicted. FegA shares significant amino acid similarity with several Fe(III)-siderophore receptors from gram-negative bacteria and has greater than 50% amino acid similarity and 33% amino acid identity with two [corrected] bacterial receptors for hydroxamate-type Fe(III)-siderophores. A dendrogram describing total inferred sequence similarity among 36 TonB-dependent OMPs was constructed; FegA grouped with Fe(III)-hydroxamate receptors. The transcriptional start site of fegA was mapped by primer extension analysis, and a putative Fur-binding site was found in the promoter. Primer extension and RNA slot blot analysis demonstrated that fegA was expressed only in cells grown under iron-limiting conditions. This is the first report of the cloning of a gene encoding a putative Fe(III)-siderophore receptor from nitrogen-fixing rhizobia.

Coordination Chemistry of the Carboxylate Type Siderophore Rhizoferrin: The Iron(III) Complex and Its Metal Analogs

Rhizoferrin is a member of a new class of siderophores (microbial iron transport compounds) based on carboxylate and hydroxy donor groups rather than the commonly encountered hydroxamates and catecholates. We have studied the coordination chemistry of rhizoferrin (Rf), as a representative of this group, with Fe(3+), Rh(3+), Cr(3+), Al(3+), Ga(3+), VO(2+), and Cu(2+). The metal complexes have been studied by UV-vis, CD, NMR, and EPR spectroscopies and mass spectrometry. The formation constants for the iron complex have also been measured and yield a log K(LFe) of 25.3. The Rh and Cr rhizoferrin complexes are unusual in that they appear to adopt a chirality about the metal center that is the opposite of the native iron analog. Several of the alternative metal ion complexes are found to have biological activity toward Morganella morganii in a plate type assay.

Citrate as a siderophore in Bradyrhizobium japonicum

Under iron-limiting conditions, many bacteria secrete ferric iron-specific ligands, generically termed siderophores, to aid in the sequestering and transport of iron. One strain of the nitrogen-fixing soybean symbiont Bradyrhizobium japonicum, 61A152, was shown to produce a siderophore when 20 B. japonicum strains were screened with all six chemical assays commonly used to detect such production. Production by strain 61A152 was detected via the chrome azurol S assay, a general test for siderophores which is independent of siderophore structure. The iron-chelating compound was neither a catechol nor a hydroxamate and was ninhydrin negative. It was determined to be citric acid via a combination of thin-layer chromatography and high-voltage paper electrophoresis; this identification was verified by a specific enzymatic assay for citric acid. The inverse correlation which was observed between citric acid release and the iron content of the medium suggested that ferric citrate could serve as an iron source. This was confirmed via growth and transport assays. Exogenously added ferric citrate could be used to overcome iron starvation, and iron-deficient cells actively transported radiolabeled ferric citrate. These results, taken together, indicate a role for ferric citrate in the iron nutrition of this strain, which has been shown to be an efficient nitrogen-fixing strain on a variety of soybean cultivars.

Iron limitation and its effect on membrane proteins and siderophore transport in Neurospora crassa

Cells of the fungus Neurospora crassa were grown under iron-deficient and iron-sufficient conditions and their plasma membrane proteins were compared. Three strains were studied: N. crassa 74A (wild type), a siderophore-free mutant N. crassa (arg-5 ota aga) as well as a 'slime' variant of N. crassa which lacks a cell wall. Plasma membranes were purified, solubilized and analyzed by one-dimensional SDS/polyacrylamide gel electrophoresis yielding approximately 50 distinct protein bands with molecular masses in the range 14-160 kDa. Iron-sufficient and iron-deficient growth resulted in nearly identical plasma membrane protein profiles in all strains. Although minor alterations in the proportion of certain proteins could be detected, significant overproduction of certain membrane proteins during iron limitation could not be observed. Transport of 55Fe-labeled siderophores seems to be correlated to the degree of iron limitation. For example, transport rates were enhanced fivefold after 16 h of growth in iron-deficient medium compared to growth in iron-sufficient medium. Extraction and HPLC measurement of siderophores from conidiospores yielded approximately 10(-15) mol/spore, indicating that germination tubes and young cells used for transport measurements are not iron-deficient. It is suggested that the putative transport systems for siderophores in fungal plasma membranes are constitutively expressed and enhanced uptake of siderophores during iron limitation is rather the result of cellular transport regulation mechanisms.

Effect of hypertonic solutes upon the polysaccharide capsule in Cryptococcus neoformans

The polysaccharide capsule is a characteristic virulence factor in the yeast-pathogen, Cryptococcus neoformans. Growth in hypertonic growth media results in yeast cells with visibly smaller capsules. We investigated this suppression quantitatively, using a chemical assay for cell-bound and dissolved capsular polysaccharide. Molar NaCl suppressed production of cell-bound polysaccharide by a factor of 2.5- to 5-fold. The possibility of salt-induced physico-chemical contraction of capsular gel was tested by dialysis of fixed cells from hypotonic medium against medium containing 1 M NaCl and against the original medium again, while capsular thickness, packed cell volume and cell-bound polysaccharide were followed. We detected a physical contraction of gel following dialysis against medium containing 1 M NaCl. Mutants which gave mucoid colonies on hypertonic agar were isolated. One of these gave twice as much polysaccharide as the wild type when cultivated in medium containing 1 M NaCl. The hypercapsular trait was passed through serial outcrosses to the wild type and segregated as a chromosomal gene. This mutant may represent a gene which regulates production of capsular polysaccharide.

Ferrous iron transport in Streptococcus mutans

Radioiron uptake from 59FeCl3 by Streptococcus mutans OMZ176 was increased by anaerobiosis, sodium ascorbate, and phenazine methosulfate (PMS), although there was a 10-min lag before PMS stimulation was evident. The reductant ascorbate may have provided ferrous iron. The PMS was reduced by the cells, and the reduced PMS then may have generated ferrous iron for transport; reduced PMS also may have depleted dissolved oxygen. We conclude that S. mutans transports only ferrous iron, utilizing reductants furnished by glucose metabolism to reduce iron prior to its uptake.

Derepression of the Azotobacter vinelandii siderophore system, using iron-containing minerals to limit iron repletion

Azotobacter vinelandii solubilized iron from certain minerals using only dihydroxybenzoic acid, which appeared to be produced constitutively. Solubilization of iron from other minerals required dihydroxybenzoic acid and the siderophore N,N'-bis-(2,3- dihydroxybenzoyl )-L-lysine ( azotochelin ) or these chelators plus the yellow-green fluorescent siderophore azotobactin . In addition to this sequential production of siderophores, cells also demonstrated partial to hyperproduction relative to the iron-limited control. The iron sources which caused partial derepression of the siderophores caused derepression of all the high-molecular-weight iron-repressible outer membrane proteins except a 77,000-molecular-weight protein, which appeared to be coordinated with azotobactin production. Increased siderophore production correlated with increased production of outer membrane proteins with molecular weights of 93,000, 85,000, and 77,000, but an 81,000-molecular-weight iron-repressible protein appeared at a constant level despite the degree of derepression. When iron was readily available, it appeared to complex with a 60,000-molecular-weight protein believed to form a surface layer on the A. vinelandii cell.

Iron uptake from ferrichrome A and iron citrate in Ustilago sphaerogena

Double radioactive label transport assays with iron, chromium, and gallium chelates were used to investigate the mechanism of iron uptake by Ustilago sphaerogena. In iron-deficient cells, ferrichrome A iron was taken up without appreciable uptake of the ligand. Iron-sufficient cells partially accumulated the ligand with the metal. The chromium- and gallium-containing analogs of ferrichrome A were transported as intact chelates. Ferrichrome A iron uptake was inhibited by dipyridyl. The data suggest that the intact ferrichrome A chelate binds to a specific receptor, the iron is then separated from the ligand at the membrane by reduction, and the metal is released to the inside of the cell while the ligand is released to the exterior. The reduction step is not transport rate limiting. Iron chelated to citrate was taken up by an energy-dependent process. The citrate ligand was not taken up with the metal. Uptake was sensitive to dipyridyl and ferrozine. Chromic ion chelated to citrate was not transported, suggesting that the iron, rather than the chelate, is recognized by the receptor or that reduction of the metal is required for transport.

Iron acquisition by Neisseria meningitidis in vitro

Assays employing iron-limited solid and liquid, defined and complex media were devised to test the iron requirements of Neisseria meningitidis. A variety of tests yielded no evidence for the secretion of a soluble iron-binding substance (siderophore) by the meningococci. The meningococci were unable to use iron bound to some common hydroxamate- and catechol-type siderophores or even compete with them for iron in the growth medium. A total of 20 strains of meningococci, differing widely in their virulence for mice, were similar in ability to acquire iron from a variety of iron-containing substances; the iron in such compounds as hog gastric mucin, citrate, hemoglobin, and myoglobin was easily acquired, whereas the iron in compounds such as ferrioxamine B, ferrichrome,ferritin, Imferon, cytochrome c, FePO4, and [Fe(OH)3]n was not readily available. No correlation was noted between the ability of particular strains to obtain iron from compounds and virulence in mice. Iron complexed or chelated with a number of metabolic organic acids, polyphosphates, and several synthetic polycarboxylic acids was readily available to all strains, even though some of the compounds used had high effective binding constants for iron and all were in 3- or 10-fold molar excess over the iron present. The addition of some of these iron-complexing substances (e.g., citrate and pyrophosphate) in iron-free form made many biologically important iron compounds that are normally inaccessible to the meningococci readily available.