Characterization of siderophores produced by different species of the dermatophytic fungiMicrosporum andTrichophyton

Biosynthesis Pathways, Transport Mechanisms and Biotechnological Applications of Fungal Siderophores

Iron (Fe) is the fourth most abundant element on earth and represents an essential nutrient for life. As a fundamental mineral element for cell growth and development, iron is available for uptake as ferric ions, which are usually oxidized into complex oxyhydroxide polymers, insoluble under aerobic conditions. In these conditions, the bioavailability of iron is dramatically reduced. As a result, microorganisms face problems of iron acquisition, especially under low concentrations of this element. However, some microbes have evolved mechanisms for obtaining ferric irons from the extracellular medium or environment by forming small molecules often regarded as siderophores. Siderophores are high affinity iron-binding molecules produced by a repertoire of proteins found in the cytoplasm of cyanobacteria, bacteria, fungi, and plants. Common groups of siderophores include hydroxamates, catecholates, carboxylates, and hydroximates. The hydroxamate siderophores are commonly synthesized by fungi. L-ornithine is a biosynthetic precursor of siderophores, which is synthesized from multimodular large enzyme complexes through non-ribosomal peptide synthetases (NRPSs), while siderophore-Fe chelators cell wall mannoproteins (FIT1, FIT2, and FIT3) help the retention of siderophores. S. cerevisiae, for example, can express these proteins in two genetically separate systems (reductive and nonreductive) in the plasma membrane. These proteins can convert Fe (III) into Fe (II) by a ferrous-specific metalloreductase enzyme complex and flavin reductases (FREs). However, regulation of the siderophore through Fur Box protein on the DNA promoter region and its activation or repression depend primarily on the Fe availability in the external medium. Siderophores are essential due to their wide range of applications in biotechnology, medicine, bioremediation of heavy metal polluted environments, biocontrol of plant pathogens, and plant growth enhancement.

HapX Mediates Iron Homeostasis in the Pathogenic Dermatophyte Arthroderma benhamiae but Is Dispensable for Virulence

For many pathogenic fungi, siderophore-mediated iron acquisition is essential for virulence. The process of siderophore production and further mechanisms to adapt to iron limitation are strictly controlled in fungi to maintain iron homeostasis. Here we demonstrate that the human pathogenic dermatophyte Arthroderma benhamiae produces the hydroxamate siderophores ferricrocin and ferrichrome C. Additionally, we show that the iron regulator HapX is crucial for the adaptation to iron starvation and iron excess, but is dispensable for virulence of A. benhamiae. Deletion of hapX caused downregulation of siderophore biosynthesis genes leading to a decreased production of siderophores during iron starvation. Furthermore, HapX was required for transcriptional repression of genes involved in iron-dependent pathways during iron-depleted conditions. Additionally, the ΔhapX mutant of A. benhamiae was sensitive to high-iron concentrations indicating that HapX also contributes to iron detoxification. In contrast to other pathogenic fungi, HapX of A. benhamiae was redundant for virulence and a ΔhapX mutant was still able to infect keratinized host tissues in vitro. Our findings underline the highly conserved role of the transcription factor HapX for maintaining iron homeostasis in ascomycetous fungi but, unlike in many other human and plant pathogenic fungi, HapX of A. benhamiae is not a virulence determinant.

Direct detection of fungal siderophores on bats with white-nose syndrome via fluorescence microscopy-guided ambient ionization mass spectrometry

White-nose syndrome (WNS) caused by the pathogenic fungus Pseudogymnoascus destructans is decimating the populations of several hibernating North American bat species. Little is known about the molecular interplay between pathogen and host in this disease. Fluorescence microscopy ambient ionization mass spectrometry was used to generate metabolic profiles from the wings of both healthy and diseased bats of the genus Myotis. Fungal siderophores, molecules that scavenge iron from the environment, were detected on the wings of bats with WNS, but not on healthy bats. This work is among the first examples in which microbial molecules are directly detected from an infected host and highlights the ability of atmospheric ionization methodologies to provide direct molecular insight into infection.

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.

Over-expression of genes coding for proline oxidase, riboflavin kinase, cytochrome c oxidase and an MFS transporter induced by acriflavin in Trichophyton rubrum

Acriflavin (3,6-acridinediamine) and other acridine derivatives act in both prokaryotic and eukaryotic cells at the level of DNA-coiling enzymes (topoisomerases) causing the stabilization of the enzyme-DNA cleavable complex. In order to better understand the mode of action of acriflavin, Differential Display RT-PCR was used to isolate transcripts specifically over-expressed during exposure of Trichophyton rubrum mycelia to this drug. Five transcripts, whose differential expressions were confirmed by Northern blotting, revealed genes not previously described in this dermatophyte. Functional grouping identified putative enzymes possibly involved in the mitochondrial respiratory electron-transport chain and in iron transport. These results may be relevant to our understanding of the molecular events involved in the stress response of T. rubrum to acriflavin.

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.

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 impact of biotechnology on hyphomycetous fungal insect biocontrol agents

The potential for the control of insect pests by entomopathogenic fungi has been touted for decades, if not centuries. Only recently have advances in biotechnology provided the tools for indepth analysis of the mechanisms involved in pathogenesis and host death at the molecular level. This review outlines the current state of knowledge regarding the mode of infection and targets several key components that are amenable to improvement via biotechnology. Realization of the considerable economic potential of fungal bioinsecticides can occur only through a combined and coordinated effort involving fundamental science, formulation technology and field applications.