Microbial siderophore-mediated transport

Microbial iron chelates, called siderophores, are synthesized by bacteria and fungi in response to low iron availability in the environment. The present review summarizes structural details of siderophore ligands with respect to their transport properties. This presentation is largely centred on the occurrence and function of siderophores in the various bacterial and fungal genera.     

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.     

Iron and siderophores in fungal–host interactions

Most fungi and bacteria express specific mechanisms for the acquisition of iron from the hosts they infect for their own survival. This is primarily because iron plays a key catalytic role in various vital cellular reactions in conjunction with the fact that iron is not freely available in these environments due to host sequestration. High-affinity iron uptake systems, such as siderophore-mediated iron uptake and reductive iron assimilation, enable fungi to acquire limited iron from animal or plant hosts. Regulating iron uptake is crucial to maintain iron homeostasis, a state necessary to avoid iron-induced toxicity from iron abundance, while simultaneously supplying iron required for biochemical demand. Siderophores play diverse roles in fungal–host interactions, many of which have been principally delineated from gene deletions in non-ribosomal peptide synthetases, enzymes required for siderophore biosynthesis. These analyses have demonstrated that siderophores are required for virulence, resistance to oxidative stress, asexual/sexual development, iron storage, and protection against iron-induced toxicity in some fungal organisms. In this review, the strategies fungi employ to obtain iron, siderophore biosynthesis, and the regulatory mechanisms governing iron homeostasis will be discussed with an emphasis on siderophore function and relevance for fungal organisms in their interactions with their hosts.     

Rhizosphere interactions and siderophores

Certain plant growth-promoting pseudomonads inhibit deleterious and pathogenic rhizosphere bacteria and fungi by producing siderophores. Properties of a siderophore transport system which might provide a competitive advantage under iron stress conditions include ability to utilize other organisms' siderophores, higher Fe(III) stability constant, faster kinetics of dissolution of Fe(III) minerals, more efficient transport system, and resistance to degradation. In order to determine the concentration and localization of siderophores in the rhizosphere monoclonal antibodies (Mabs) to ferric pseudobactin, the siderophore of Pseudomonas putida B10, have been developed. Several Mabs cross reacted differently with various pseudobactins. A growth medium has been developed for the study for siderophore-mediated rhizosphere interactions in the laboratory.     

Structural and stereochemical aspects of iron transport in fungi

A variety of fungi are known to overproduce and excrete desferri-siderophores under iron limitation. After complexing with ferric iron, octahedral complexes are formed and taken up by siderophore-specific transport systems. These systems represent energy consuming systems as inferred from their sensitivity to respiratory inhibitors, uncouplers and changes of the membrane potential and are able to recognize structure and stereochemical configuration of the various siderophore molecules. Ferrichromes, the most common siderophores in fungi, are generally recognized as Lambda-cis coordination complexes. Triacetylfusarinins, although prevailing as Delta-cis optical isomers in aqueous solution, are assumed to be taken up after isomerization to the corresponding Lambda-cis complexes. However, coprogens which also show a predominant Delta-absolute configuration in solution seem to be transported without prior isomerization. When both, ferrichromes as well as triacetylfusarines or coprogens are taken up, competition during uptake is observed, suggesting the presence of a common transport system during translocation of siderophores across the fungal plasma membrane.     

The crucial role of the Aspergillus fumigatus siderophore system in interaction with alveolar macrophages

Iron plays a central role in manifestation of infections for a variety of pathogens. To ensure an adequate supply with iron, Aspergillus fumigatus employs extra- and intracellular siderophores (low-molecular mass iron chelators), which are of importance for fungal growth in particular during iron starvation. Here we show that the lack of extracellular siderophores, and especially, the lack of the entire siderophore system cause in immunosuppressed mice in vivo (i) a reduced extracellular growth rate, (ii) a reduced intracellular growth rate in alveolar macrophages, and (iii) an increased susceptibility to conidial growth inhibition by alveolar macrophages. These data underline the crucial role of the fungal siderophore system not only for extracellular growth but also in the interaction with the host immune cells. Moreover, the hyphal growth rate within alveolar macrophages compared to extracellular lavage fluid was significantly decreased indicating that, besides elimination of fungal conidia, inhibition of pathogenic growth is a function of macrophages.     

Hydroxamate Production as a High Affinity Iron Acquisition Mechanism in Paracoccidioides Spp

Iron is a micronutrient required by almost all living organisms, including fungi. Although this metal is abundant, its bioavailability is low either in aerobic environments or within mammalian hosts. As a consequence, pathogenic microorganisms evolved high affinity iron acquisition mechanisms which include the production and uptake of siderophores. Here we investigated the utilization of these molecules by species of the Paracoccidioides genus, the causative agents of a systemic mycosis. It was demonstrated that iron starvation induces the expression of Paracoccidioides ortholog genes for siderophore biosynthesis and transport. Reversed-phase HPLC analysis revealed that the fungus produces and secretes coprogen B, which generates dimerumic acid as a breakdown product. Ferricrocin and ferrichrome C were detected in Paracoccidioides as the intracellular produced siderophores. Moreover, the fungus is also able to grow in presence of siderophores as the only iron sources, demonstrating that beyond producing, Paracoccidioides is also able to utilize siderophores for growth, including the xenosiderophore ferrioxamine. Exposure to exogenous ferrioxamine and dimerumic acid increased fungus survival during co-cultivation with macrophages indicating that these molecules play a role during host-pathogen interaction. Furthermore, cross-feeding experiments revealed that Paracoccidioides siderophores promotes growth of Aspergillus nidulans strain unable to produce these iron chelators. Together, these data denote that synthesis and utilization of siderophores is a mechanism used by Paracoccidioides to surpass iron limitation. As iron paucity is found within the host, siderophore production may be related to fungus pathogenicity.     

Combat of iron-deprivation through a plant growth promoting fluorescent Pseudomonas strain GRP3A in mung bean (Vigna radiata L. Wilzeck)

Microorganisms and plants sustain themselves under iron-deprived conditions by releasing siderophores. Among others, fluorescent pseudomonads are known to exert extensive biocontrol action against soil and root borne phytopathogens through release of antimicrobials and siderophores. In this study, production and regulation of siderophores by fluorescent Pseudomonas strain GRP3A was studied. Among various media tested, standard succinate medium (SSM) promoted maximum siderophore production of 56.59 mg l(-1). There were low levels of siderophore in complex media like King's B medium, trypticase soya medium and nutrient medium (41.27, 29.86 and 27.63 mg l(-1)), respectively. In defferrated SSM, siderophore level was quantified to be 68.74 mg l(-1). Supplementation with iron (FeCl3) resulted in decreased siderophore levels depending on concentration. Siderophore production was promoted by Zn2+ (78.94 mg l(-1)), Cu2+ (68.80 mg l(-1)) whereas Co2+ (57.33 mg l(-1)) and Fe3+ reduced siderophore production (37.44 mg l(-1) as compared to control (55.97 mg l(-1)). Strain GRP3A showed plant growth promotion under iron limited conditions.     

Fe(III)-complexes of the tripodal trishydroxamate siderophore basidiochrome: potential biological implications

One method of mobilization of iron by mycorrhizal organisms is through the secretion of small organic chelators called siderophores. Hydroxamate donor chelators are a common type of siderophore that is frequently used by fungal organisms. The primary siderophore that is produced by fungi from the genera Ceratobasidium and Rhizoctonia is the tripodal trishydroxamate siderophore basidiochrome. To gain some insight into the iron uptake mechanisms of these symbiotic fungi, the iron binding characteristics of basidiochrome were determined. It was found that basidiochrome exhibits a log β₁₁₀ of 27.8 ± 0.1 and a pFe value of 25.0. These values are similar to those of another fungal trishydroxamate siderophore, ferrichrome. The similarity in iron affinity between the two siderophores suggests that the structure of the backbone has little influence in complex formation due to the length of the pendant arms, although the identity of the terminating groups of the pendant arms is likely related to complex stability. The role of basidiochrome in the biogeochemical cycling of iron is also discussed.     

Siderophore-based detection of Fe(iii) and microbial pathogens

Siderophores are low-molecular-weight iron chelators that are produced and exported by bacteria, fungi and plants during periods of nutrient deprivation. The structures, biosynthetic logic, and coordination chemistry of these molecules have fascinated chemists for decades. Studies of such fundamental phenomena guide the use of siderophores and siderophore conjugates in a variety of medicinal applications that include iron-chelation therapies and drug delivery. Sensing applications constitute another important facet of siderophore-based technologies. The high affinities of siderophores for both ferric ions and siderophore receptors, proteins expressed on the cell surface that are required for ferric siderophore import, indicate that these small molecules may be employed for the selective capture of metal ions, proteins, and live bacteria. This minireview summaries progress in methods that utilize native bacterial and fungal siderophore scaffolds for the detection of Fe(iii) or microbial pathogens.     

Iron transport-mediated antagonism between plant growth-promoting and plant-deleterious Pseudomonas strains

Both plant growth-promoting Pseudomonas B10 and its yellow-green, fluorescent iron transport agent (siderophore) pseudobactin enhance potato growth and biologically control certain soil-borne fungal diseases in part by depriving specific root-colonizing endemic microorganisms including phytopathogens of iron(III), thus inhibiting their growth. The present study examines this mode of iron deprivation. The growth inhibition of certain bean-deleterious fluorescent pseudomonads by specific bean-beneficial fluorescent pseudomonads is due in part to the inability of susceptible strains to utilize siderophores from beneficial strains to transport iron(III). Conversely, deleterious strains which were able to utilize siderophores from beneficial strains were not inhibited. The ability of a given pseudomonad to utilize another pseudomonad's siderophore may depend upon its possessing a specific outer membrane receptor protein for that pseudomonad's ferric siderophore. Siderophore-mediated competition for iron in microbial systems appears to be a widespread phenomenon.     

An overview of siderophores for iron acquisition in microorganisms living in the extreme

Siderophores are iron-chelating molecules produced by microbes when intracellular iron concentrations are low. Low iron triggers a cascade of gene activation, allowing the cell to survive due to the synthesis of important proteins involved in siderophore synthesis and transport. Generally, siderophores are classified by their functional groups as catecholates, hydroxamates and hydroxycarboxylates. Although other chemical structural modifications and functional groups can be found. The functional groups participate in the iron-chelating process when the ferri-siderophore complex is formed. Classified as acidophiles, alkaliphiles, halophiles, thermophiles, psychrophiles, piezophiles, extremophiles have particular iron requirements depending on the environmental conditions in where they grow. Most of the work done in siderophore production by extremophiles is based in siderophore concentration and/or genomic studies determining the presence of siderophore synthesis and transport genes. Siderophores produced by extremophiles are not well known and more work needs to be done to elucidate chemical structures and their role in microorganism survival and metal cycling in extreme environments.     

Fate of ferrisiderophores after import across bacterial outer membranes: different iron release strategies are observed in the cytoplasm or periplasm depending on the siderophore pathways

Siderophore production and utilization is one of the major strategies deployed by bacteria to get access to iron, a key nutrient for bacterial growth. The biological function of siderophores is to solubilize iron in the bacterial environment and to shuttle it back to the cytoplasm of the microorganisms. This uptake process for Gram-negative species involves TonB-dependent transporters for translocation across the outer membranes. In Escherichia coli and many other Gram-negative bacteria, ABC transporters associated with periplasmic binding proteins import ferrisiderophores across cytoplasmic membranes. Recent data reveal that in some siderophore pathways, this step can also be carried out by proton-motive force-dependent permeases, for example the ferrichrome and ferripyochelin pathways in Pseudomonas aeruginosa. Iron is then released from the siderophores in the bacterial cytoplasm by different enzymatic mechanisms depending on the nature of the siderophore. Another strategy has been reported for the pyoverdine pathway in P. aeruginosa: iron is released from the siderophore in the periplasm and only siderophore-free iron is transported into the cytoplasm by an ABC transporter having two atypical periplasmic binding proteins. This review presents recent findings concerning both ferrisiderophore and siderophore-free iron transport across bacterial cytoplasmic membranes and considers current knowledge about the mechanisms involved in iron release from siderophores.     

Utilization of Fe<Superscript>3+</Superscript>-acinetoferrin analogs as an iron source by <Emphasis Type="Italic">Mycobacterium tuberculosis</Emphasis>

Mycobacterium tuberculosis, the causative agent of human tuberculosis, synthesizes and secretes siderophores in order to compete for iron (an essential micronutrient). Successful iron acquisition allows M. tuberculosis to survive and proliferate under the iron-deficient conditions encountered in the host. To examine structural determinants important for iron siderophore transport in this pathogen, the citrate-based siderophores petrobactin, acinetoferrin and various acinetoferrin homologs were synthesized and used as iron transport probes. Mutant strains of M. tuberculosis deficient in native siderophore synthesis or transport were utilized to better understand the mechanisms involved in iron delivery via the synthetic siderophores. Acinetoferrin and its derivatives, especially those containing a cyclic imide group, were able to deliver iron or gallium into M. tuberculosis which promoted or inhibited, respectively, the growth of this pathogen. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Iron gathering by zoopathogenic fungi

Iron is a metal required by most microorganisms and is prominently used in the transfer of electrons during metabolism. The gathering of iron is, then, an essential process and its fulfillment becomes a crucial pathogenetic event for zoopathogenic fungi. Iron is rather unavailable because it occurs on the earth's surface in its insoluble ferric form in oxides and hydroxides. In the infected host iron is bound to proteins such as transferrin and ferritin. Solubilization of ferric iron is the major problem confronting microorganisms. This process is achieved by two major mechanisms: ferric reduction and siderophore utilization. Ferric reductase is frequently accompanied by a copper oxidase transport system. There is one example of direct ferric iron transport apparently without prior reduction. Ferric reduction may also be accomplished by low molecular mass compounds. Some fungi have evolved a process of iron acquisition involving the synthesis of iron-gathering compounds called siderophores. Even those fungi that do not synthesize siderophores have developed permeases for transport of such compounds formed by other organisms. Fungi can also reductively release iron from siderophores and transport the ferrous iron often by the copper oxidase transport system. There is a great diversity of iron-gathering mechanisms expressed by pathogenic fungi and such diversity may be found even in a single species.