Iron - A Key Nexus in the Virulence of Aspergillus fumigatus

Iron acquisition strategies in pathogenic fungi

Iron plays a crucial role in various biological processes, including enzyme function, DNA replication, energy production, oxygen transport, lipid, and carbon metabolism. Although it is abundant in the Earth's crust, its bioavailability is restricted by the insolubility of ferric iron (Fe³+) and the auto-oxidation of ferrous iron (Fe²+) in oxygen-rich environments. This limitation poses significant challenges for all organisms, including fungi, which have developed intricate mechanisms for iron acquisition and utilization. These mechanisms include reductive iron uptake, siderophore production/transport, and heme utilization. Fungi employ a variety of enzymes-such as ferric reductases, ferroxidases, permeases, and transporters-to regulate intracellular iron levels effectively. The challenge is heightened for pathogenic fungi during infection, as they must compete with the host's iron-binding proteins like transferrin and lactoferrin, which sequester iron to restrict pathogen growth. This review delves into the iron acquisition strategies of medically important fungi, emphasizing the roles of reductive iron uptake and siderophore pathways. Understanding these mechanisms is vital for enhancing our knowledge of fungal pathogenesis and developing effective treatments. By targeting these iron acquisition processes, new antifungal therapies can be formulated more effectively to combat fungal infections.

Antimicrobial Agent Trimethoprim Influences Chemical Interactions in Cystic Fibrosis Pathogens via the ham Gene Cluster

The fungus Aspergillus fumigatus and the bacterium Burkholderia cenocepacia cause fatal respiratory infections in immunocompromised humans and patients with lung disease, such as cystic fibrosis (CF). In dual infections, antagonistic interactions contribute to increased mortality. These interactions are further altered by the presence of antimicrobial and antifungal agents. However, studies performed to date on chemical interactions between clinical B. cenocepacia and A. fumigatus have focused on pathogens in isolation and do not include the most abundant chemical signal, i.e., clinically administered therapeutics, present in the lung. Here, we characterize small molecule-mediated interactions between B. cenocepacia and A. fumigatus and their shift in response to trimethoprim exposure by using metabolomics and mass spectrometry imaging. Using these methods, we report that the production of several small-molecule natural products of both the bacteria and the fungus is affected by cocultivation and exposure to trimethoprim. By systematic analysis of metabolomics data, we hypothesize that the B. cenocepacia-encoded ham gene cluster plays a role in the trimethoprim-mediated alteration of bacterial-fungal interactions. We support our findings by generating a genetically modified strain lacking the ham gene cluster and querying its interaction with A. fumigatus. Using comparative analyses of the extracts of wild-type and knockout strains, we report the inactivation of a bacterially produced antifungal compound, fragin, by A. fumigatus, which was verified by the addition of purified fragin to the A. fumigatus culture. Furthermore, we report that trimethoprim does not inhibit fungal growth, but affects the biochemical pathway for DHN-melanin biosynthesis, an important antifungal drug target, altering the pigmentation of the fungal conidia and is associated with modification of ergosterol to ergosteryl-3β-O-l-valine in coculture. This study demonstrates the impact of therapeutics on shaping microbial and fungal metabolomes, which influence interkingdom interactions and the expression of virulence factors. Our findings enhance the understanding of the complexity of chemical interactions between therapeutic compounds, bacteria, and fungi and may contribute to the development of selective treatments.

The Role of FpfetC from Fusarium proliferatum in Iron Acquisition, Fumonisin B1 Production, and Virulence

Iron is an essential micronutrient required for the fungal growth and propagation. Fusarium proliferatum is the causal agent of rice spikelet rot disease. In this study, we characterized the role of F. proliferatum multicopper ferroxidase (FpfetC), which mediated the oxidization of ferrous to ferric iron in the reductive system of iron assimilation. Deletion of FpfetC led to impaired growth under iron-deprived conditions, and the growth defect could be restored by exogenous iron. Compared to wild-type Fp9 strain, ΔFpfetC showed increased conidiation, resistance to copper stress, and sensitivity to zinc stress. FpfetC deficiency rendered a transcription remodeling of genes involved in high-affinity iron assimilation, iron homeostasis and iron storage. Moreover, production of fumonisin B1 (FB1) and transcript levels of fumonisin biosynthesis (Fpfums) genes were elevated in ΔFpfetC. ΔFpfetC exhibited hypervirulence to rice, accompanied with aggravation of invasive hyphae and activation of siderophore synthesis at the sites of inoculation. Additionally, disruption of FpfetC attenuated penetration ability to cellophane membrane under iron starvation. Taken together, these results demonstrated that FpfetC played important roles in iron uptake, conidiation, response to metal stress, fumonisin biosynthesis, and virulence in F. proliferatum.

An environmental isolate of Pseudomonas, 20EI1, reduces Aspergillus flavus growth in an iron-dependent manner and alters secondary metabolism

Introduction

Aspergillus flavus is an opportunistic pathogenic fungus that infects oilseed crops worldwide. When colonizing plants, it produces mycotoxins, including carcinogenic compounds such as aflatoxins. Mycotoxin contamination results in an important economic and health impact. The design of new strategies to control A. flavus colonization and mycotoxin contamination is paramount.

Methods

The biocontrol potential of a promising new isolate of Pseudomonas spp., 20EI1 against A. flavus was assessed using bioassays and microscopy. To further elucidate the nature of this bacterial-fungal interaction, we also performed chemical and transcriptomics analyses.

Results

In the present study, Pseudomonas spp., 20EI1 was able to reduce the growth of A. flavus. Furthermore, we determined that this growth inhibition is iron-dependent. In addition, Pseudomonas 20EI1 reduced or blocked the production of aflatoxin, as well as cyclopiazonic acid and kojic acid. Expression of iron-related genes was altered in the presence of the bacteria and genes involved in the production of aflatoxin were down-regulated. Iron supplementation partially reestablished their expression. Expression of other secondary metabolite (SM) genes was also reduced by the bacteria, including genes of clusters involved in cyclopiazonic acid, kojic acid and imizoquin biosynthesis, while genes of the cluster corresponding to aspergillicin, a siderophore, were upregulated. Interestingly, the global SM regulatory gene mtfA was significantly upregulated by 20EI1, which could have contributed to the observed alterations in SM.

Discussion

Our results suggest that Pseudomonas 20EI1 is a promising biocontrol against A. flavus, and provide further insight into this iron-dependent bacterial-fungal interaction affecting the expression of numerous genes, among them those involved in SM.

The Significance of Mono- and Dual-Effective Agents in the Development of New Antifungal Strategies

Invasive fungal infections (IFIs) pose significant challenges in clinical settings, particularly due to their high morbidity and mortality rates. The rising incidence of these infections, coupled with increasing antifungal resistance, underscores the urgent need for novel therapeutic strategies. Current antifungal drugs target the fungal cell membrane, cell wall, or intracellular components, but resistance mechanisms such as altered drug-target interactions, enhanced efflux, and adaptive cellular responses have diminished their efficacy. Recent research has highlighted the potential of dual inhibitors that simultaneously target multiple pathways or enzymes involved in fungal growth and survival. Combining pharmacophores, such as lanosterol 14α-demethylase (CYP51), heat shock protein 90 (HSP90), histone deacetylase (HDAC), and squalene epoxidase (SE) inhibitors, has led to the development of compounds with enhanced antifungal activity and reduced resistance. This dual-target approach, along with novel chemical scaffolds, not only represents a promising strategy for combating antifungal resistance but is also being utilized in the development of anticancer agents. This review explores the development of new antifungal agents that employ mono-, dual-, or multi-target strategies to combat IFIs. We discuss emerging antifungal targets, resistance mechanisms, and innovative therapeutic approaches that offer hope in managing these challenging infections.

Quantitative proteomic analysis reveals Ga(III) polypyridyl catecholate complexes disrupt Aspergillus fumigatus mitochondrial function

Infections caused by the airborne fungal pathogen, Aspergillus fumigatus, are increasing in severity due to growing numbers of immunocompromised individuals and the increasing incidence of antifungal drug resistance, exacerbating treatment challenges. Gallium has proven to be a strong candidate in the fight against microbial pathogens due to its iron-mimicking capability and substitution of Ga(III) in place of Fe(III), disrupting iron-dependent pathways. Since the antimicrobial properties of 2,2'-bipyridine and derivatives have been previously reported, we assessed the in vitro activity and proteomic effects of a recently reported heteroleptic Ga(III) polypyridyl catecholate compound against A. fumigatus. This compound has demonstrated promising growth-inhibition and impact on the A. fumigatus proteome compared to untreated controls. Proteins associated with DNA replication and repair mechanisms along with lipid metabolism and the oxidative stress responses were elevated in abundance compared to control. Crucially, a large number of mitochondrial proteins were reduced in abundance. Respiration is an important source of energy to fuel metabolic processes required for growth, survival and virulence, the disruption of which may be a viable strategy for the treatment of microbial infections.

Resynthesis of Damaged Fe-S Cluster Proteins Protects Aspergillus fumigatus Against Oxidative Stress in the Absence of Mn-Superoxide Dismutase

The importance of manganese superoxide dismutase (Mn-SOD), an evolutionarily ancient metalloenzyme that maintains the integrity and function of mitochondria, was studied in oxidative stress-treated Aspergillus fumigatus cultures. Deletion of the Mn-SOD gene (sodB) increased both the menadione sodium bisulfite (MSB)-elicited oxidative stress and the deferiprone (DFP)-induced iron limitation stress sensitivity of the strain. Moreover, DFP treatment enhanced the MSB sensitivity of both the gene deletion mutant and the reference strain. The lack of SodB also increased the susceptibility of conidia to killing by human macrophages. Concurring with the stress sensitivity data, RNS sequencing data also demonstrated that the deletion of sodB largely altered the MSB-induced oxidative stress response. The difference between the oxidative stress responses of the two strains manifested mainly in the intensity of the response. Importantly, upregulation of "Ribosome protein", "Iron uptake", and "Fe-S cluster assembly" genes, alterations in the transcription of "Fe-S cluster protein" genes, and downregulation of "Heme binding protein" genes under MSB stress were characteristic only for the ΔsodB gene deletion mutant. We assume that the elevated superoxide level generated by MSB treatment may have destroyed Fe-S cluster proteins of mitochondria in the absence of SodB. This intensified the resynthesis of Fe-S cluster proteins, which was accompanied with enhanced translation and iron acquisition, leading to increased DFP sensitivity.

Nanoparticle-Mediated Delivery of Deferasirox: A Promising Strategy Against Invasive Aspergillosis

BACKGROUND

Invasive aspergillosis (IA) is a deadly fungal lung infection. Antifungal resistance and treatment side effects are major concerns. Iron chelators are vital for IA management, but systemic use can cause side effects. We developed nanoparticles (NPs) to selectively deliver the iron chelator deferasirox (DFX) for IA treatment.

METHODS

DFX was encapsulated in poly(lactic-co-glycolic acid) (PLGA) NPs using a single emulsion solvent evaporation method. The NPs were characterized by light scattering and electron microscopy. DFX loading efficiency and release were assessed spectrophotometrically. Toxicity was evaluated using SRB, luciferase, and XTT assays. Therapeutic efficacy was tested in an IA mouse model, assessing fungal burden by qPCR and biodistribution via imaging.

RESULTS

DFX-NPs had a size of ~50 nm and a charge of ~-30 mV, with a loading efficiency of ~80%. Release kinetics showed DFX release via diffusion and bioerosion. The EC50 of DFX-NPs was significantly lower (p < 0.001) than the free drug, and they were significantly less toxic (p < 0.0001) in mammalian cell cultures. In vivo, NP treatment significantly reduced Af burden (p < 0.05).

CONCLUSION

The designed DFX-NPs effectively target and kill Af with minimal toxicity to mammalian cells. The significant in vivo therapeutic efficacy suggests these NPs could be a safe and effective treatment for IA.

Exploring the Siderophore Portfolio for Mass Spectrometry-Based Diagnosis of Scedosporiosis and Lomentosporiosis

Scedosporium apiospermum and Lomentospora prolificans secrete siderophores (iron scavengers) during hyphal proliferation. Siderophores are virulence factors and potential clinical biomarkers of invasive scedosporiosis and lomentosporiosis. Both strains secreted a uniform spectrum of siderophores, including coprogen B (CopB), N α-methyl-coprogen B, dimethyl-coprogen, and ferricrocin, with N α-methyl-coprogen B being the fastest secreted and most abundant coprogen. Under iron and zinc restriction, reflecting a nutrient-limited host environment, L. prolificans secreted 45 times more CopB than did S. apiospermum, presumably contributing to its higher virulence. This robust mobilization of CopB was further enhanced by zinc surplus. Additionally, two novel cyclic peptides, Scedocyclin A and B, were characterized inScedosporium boydii using the de novo sequencing tool CycloBranch. Utilizing matrix-assisted laser desorption/ionization, the portfolio of coprogens detected had limits of detection and quantitation of 4.9 and 14.6 fmol/spot in complex matrices, respectively, making them strong candidates for the next-generation, routine diagnosis of invasive scedosporiosis and lomentosporiosis through the Biotyper siderotyping.

Biosynthesis of iron-chelating terramides A-C and their role in Aspergillus terreus infection

Fungal natural products from various species often feature hydroxamic acid motifs that have the ability to chelate iron. These compounds have an array of medicinally and ecologically relevant activities. Through genome mining, gene deletion in the host Aspergillus terreus, and heterologous expression experiments, this study has revealed that a nonribosomal peptide synthetase (NRPS) TamA and a specialized cytochrome P450 monooxygenase TamB catalyze the sequential biosynthetic reactions in the formation of terramides A-C, a series of diketopiperazines (DKPs) with hydroxamic acid motifs. Feeding experiments showed that TamB catalyzes an unprecedented di-hydroxylation of the amide nitrogens in the diketopiperazine core. This tailoring reaction led to the formation of two bidentate iron-binding sites per molecule with an unusual iron-binding stoichiometry. The structure of the terramide A-Fe complex was characterized by liquid chromatography-mass spectrometry (LC-MS), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and electron paramagnetic resonance spectroscopy (EPR). Antimicrobial assays showed that the iron-binding motifs are crucial for the activity against bacteria and fungi. Murine infection experiments indicated that terramide production is crucial for the virulence of A. terreus and could be a potential antifungal drug target.

AcuM and AcuK: The global regulators controlling multiple cellular metabolisms in a dimorphic fungus Talaromyces marneffei

Talaromycosis is a fungal infection caused by an opportunistic dimorphic fungus Talaromyces marneffei. During infection, T. marneffei resides inside phagosomes of human host macrophages where the fungus encounters nutrient scarcities and host-derived oxidative stressors. Previously, we showed that the deletion of acuK, a gene encoding Zn(2)Cys(6) transcription factor, caused a decreased ability for T. marneffei to defend against macrophages, as well as a growth impairment in T. marneffei on both low iron-containing medium and gluconeogenic substrate-containing medium. In this study, a paralogous gene acuM was deleted and characterized. The ΔacuM mutant showed similar defects with the ΔacuK mutant, suggesting their common role in gluconeogenesis and iron homeostasis. Unlike the pathogenic mold Aspergillus fumigatus, the ΔacuK and ΔacuM mutants unexpectedly exhibited normal siderophore production and did not show lower expression levels of genes involved in iron uptake and siderophore synthesis. To identify additional target genes of AcuK and AcuM, RNA-sequencing analysis was performed in the ΔacuK and ΔacuM strains growing in a synthetic dextrose medium with 1% glucose at 25 °C for 36 hours. Downregulated genes in both mutants participated in iron-consuming processes, especially in mitochondrial metabolism and anti-oxidative stress. Importantly, the ΔacuM mutant was sensitive to the oxidative stressors menadione and hydrogen peroxide while the ΔacuK mutant was sensitive to only hydrogen peroxide. The yeast form of both mutants demonstrated a more severe defect in antioxidant properties than the mold form. Moreover, ribosomal and ribosomal biogenesis genes were expressed at significantly lower levels in both mutants, suggesting that AcuK and AcuM could affect the protein translation process in T. marneffei. Our study highlighted the role of AcuK and AcuM as global regulators that control multiple cellular adaptations under various harsh environmental conditions during host infection. These transcription factors could be potentially exploited as therapeutic targets for the treatment of this neglected infectious disease.

Morphological responses of two Aspergillus oryzae strains to various metal ions at different concentrations

Aspergillus species take up various metal ions from environment. The morphology of Aspergillus oryzae strains can vary under the influence of various metal ions. Here, the effects of Ti4+, V3+, Sr2+, Ba2+, Al3+, Fe2+, Zn2+, Mn2+, Ca2+, and Cu2+ on morphological parameters of A. oryzae strains RIB40 and RIB143 were estimated. Colony diameter, conidiation, vesicle head size, and stipe width in both strains varied with concentration. Ti4+, Sr2+, Ba2+, Al3+, Fe2+, and Ca2+ affected conidiation in similar tendency between two strains. The effects of Ti4+, V3+, Sr2+, and Ba2+ on the morphology of A. oryzae are reported here for the first time. Induction of growth of both strains by 0.0001% Ti4+ may help the fermentation industry. Induction of conidiation in RIB40 by 0.001% Cu2+ confirmed previous results that low concentrations of Cu2+ promote the growth of Aspergillus. The most novel finding is that 0.001% Zn2+ increased the vesicle head size in RIB40; possible reasons are discussed.

Two ferrous iron transporter-like proteins independently participate in asexual development under iron limitation and virulence in Beauveria bassiana

Reductive assimilation pathway involves ferric reductase and ferrous iron transporter, which is integral for fungal iron acquisition. A family of ferric reductase-like proteins has been functionally characterized in the filamentous entomopathogenic fungus Beauveria bassiana. In this investigation, two ferrous iron transporter-like proteins (Ftr) were functionally annotated in B. bassiana. BbFtr1 and BbFtr2 displayed high similarity in structure and were associated with the plasma and nuclear membrane. Their losses had no negatively influence on fungal growth on various nutrients and development under the iron-replete condition. Single mutants of BbFTR1 and BbFTR2 displayed the iron-availability dependent developmental defects, and double mutant exhibited the significantly impaired developmental potential under the iron-limited conditions. In insect bioassay, the double mutant also showed the weaker virulence than either of two single disruption mutants. These results suggested that two ferrous iron transporter-like proteins function independently in fungal physiologies under the iron-deficient condition. Intriguingly, a bZIP transcription factor BbHapX was required for expression of BbFTR1 and BbFTR2 under iron-depleted conditions. This study enhances our understanding of the iron uptake system in the filamentous entomopathogenic fungi.

Biomimetic Analogues of the Desferrioxamine E Siderophore for PET Imaging of Invasive Aspergillosis: Targeting Properties and Species Specificity

The pathogenic fungus Aspergillus fumigatus utilizes a cyclic ferrioxamine E (FOXE) siderophore to acquire iron from the host. Biomimetic FOXE analogues were labeled with gallium-68 for molecular imaging with PET. [68Ga]Ga(III)-FOXE analogues were internalized in A. fumigatus cells via Sit1. Uptake of [68Ga]Ga(III)-FOX 2-5, the most structurally alike analogue to FOXE, was high by both A. fumigatus and bacterial Staphylococcus aureus. However, altering the ring size provoked species-specific uptake between these two microbes: ring size shortening by one methylene unit (FOX 2-4) increased uptake by A. fumigatus compared to that by S. aureus, whereas lengthening the ring (FOX 2-6 and 3-5) had the opposite effect. These results were consistent both in vitro and in vivo, including PET imaging in infection models. Overall, this study provided valuable structural insights into the specificity of siderophore uptake and, for the first time, opened up ways for selective targeting and imaging of microbial pathogens by siderophore derivatization.

Manganese homeostasis modulates fungal virulence and stress tolerance in Candida albicans

Due to the scarcity of transition metals within the human host, fungal pathogens have evolved sophisticated mechanisms to uptake and utilize these micronutrients at the infection interface. While considerable attention was turned to iron and copper acquisition mechanisms and their importance in fungal fitness, less was done regarding either the role of manganese (Mn) in infectious processes or the cellular mechanism by which fungal cells achieve their Mn-homeostasis. Here, we undertook transcriptional profiling in the pathogenic fungus Candida albicans experiencing both Mn starvation and excess to capture biological processes that are modulated by this metal. We uncovered that Mn scarcity influences diverse processes associated with fungal fitness including invasion of host cells and antifungal sensitivity. We show that Mn levels influence the abundance of iron and zinc emphasizing the complex crosstalk between metals. The deletion of SMF12, a member of Mn Nramp transporters, confirmed its contribution to Mn uptake. smf12 was unable to form hyphae and damage host cells and exhibited sensitivity to azoles. We found that the unfolded protein response (UPR), likely activated by decreased glycosylation under Mn limitation, was required to recover growth when cells were shifted from an Mn-starved to an Mn-repleted medium. RNA-seq profiling of cells exposed to Mn excess revealed that UPR was also activated. Furthermore, the UPR signaling axis Ire1-Hac1 was required to bypass Mn toxicity. Collectively, this study underscores the importance of Mn homeostasis in fungal virulence and comprehensively provides a portrait of biological functions that are modulated by Mn in a fungal pathogen.

IMPORTANCE

Transition metals such as manganese provide considerable functionality across biological systems as they are used as cofactors for many catalytic enzymes. The availability of manganese is very limited inside the human body. Consequently, pathogenic microbes have evolved sophisticated mechanisms to uptake this micronutrient inside the human host to sustain their growth and cause infections. Here, we undertook a comprehensive approach to understand how manganese availability impacts the biology of the prevalent fungal pathogen, Candida albicans. We uncovered that manganese homeostasis in this pathogen modulates different biological processes that are essential for host infection which underscores the value of targeting fungal manganese homeostasis for potential antifungal therapeutics development.

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.

Radiotracer Development for Fungal-Specific Imaging: Past, Present, and Future

Invasive fungal infections have become a major challenge for public health, mainly due to the growing numbers of immunocompromised patients, with high morbidity and mortality. Currently, conventional imaging modalities such as computed tomography and magnetic resonance imaging contribute largely to the noninvasive diagnosis and treatment evaluation of those infections. These techniques, however, often fall short when a fast, noninvasive and specific diagnosis of fungal infection is necessary. Molecular imaging, especially using nuclear medicine-based techniques, aims to develop fungal-specific radiotracers that can be tested in preclinical models and eventually translated to human applications. In the last few decades, multiple radioligands have been developed and tested as potential fungal-specific tracers. These include radiolabeled peptides, antifungal drugs, siderophores, fungal-specific antibodies, and sugars. In this review, we provide an overview of the pros and cons of the available radiotracers. We also address the future prospects of fungal-specific imaging.

How metals fuel fungal virulence, yet promote anti-fungal immunity

Invasive fungal infections represent a significant global health problem, and present several clinical challenges, including limited treatment options, increasing rates of antifungal drug resistance and compounding comorbidities in affected patients. Metals, such as copper, iron and zinc, are critical for various biological and cellular processes across phyla. In mammals, these metals are important determinants of immune responses, but pathogenic microbes, including fungi, also require access to these metals to fuel their own growth and drive expression of major virulence traits. Therefore, host immune cells have developed strategies to either restrict access to metals to induce starvation of invading pathogens or deploy toxic concentrations within phagosomes to cause metal poisoning. In this Review, we describe the mechanisms regulating fungal scavenging and detoxification of copper, iron and zinc and the importance of these mechanisms for virulence and infection. We also outline how these metals are involved in host immune responses and the consequences of metal deficiencies or overloads on how the host controls invasive fungal infections.

Microbiological Non-Culture-Based Methods for Diagnosing Invasive Pulmonary Aspergillosis in ICU Patients

The diagnosis of invasive pulmonary aspergillosis (IPA) in intensive care unit (ICU) patients is crucial since most clinical signs are not specific to invasive fungal infections. To detect an IPA, different criteria should be considered. Next to host factors and radiological signs, microbiological criteria should be fulfilled. For microbiological diagnostics, different methods are available. Next to the conventional culture-based approaches like staining and culture, non-culture-based methods can increase sensitivity and improve time-to-result. Besides fungal biomarkers, like galactomannan and (1→3)-β-D-glucan as nonspecific tools, molecular-based methods can also offer detection of resistance determinants. The detection of novel biomarkers or targets is promising. In this review, we evaluate and discuss the value of non-culture-based microbiological methods (galactomannan, (1→3)-β-D-glucan, Aspergillus PCR, new biomarker/targets) for diagnosing IPA in ICU patients.

The Transcriptome Response to Azole Compounds in Aspergillus fumigatus Shows Differential Gene Expression across Pathways Essential for Azole Resistance and Cell Survival

The opportunistic pathogen Aspergillus fumigatus is found on all continents and thrives in soil and agricultural environments. Its ability to readily adapt to novel environments and to produce billions of spores led to the spread of azole-resistant A. fumigatus across the globe, posing a threat to many immunocompromised patients, including critically ill patients with severe influenza or COVID-19. In our study, we sought to compare the adaptational response to azoles from A. fumigatus isolates that differ in azole susceptibility and genetic background. To gain more insight into how short-term adaptation to stressful azole compounds is managed through gene expression, we conducted an RNA-sequencing study on the response of A. fumigatus to itraconazole and the newest clinically approved azole, isavuconazole. We observed many similarities in ergosterol biosynthesis up-regulation across isolates, with the exception of the pan-azole-resistant isolate, which showed very little differential regulation in comparison to other isolates. Additionally, we found differential regulation of membrane efflux transporters, secondary metabolites, iron metabolism, and various stress response and cell signaling mechanisms.

Soil Aspergillus Species, Pathogenicity and Control Perspectives

Five Aspergillus sections have members that are established agricultural pests and producers of different metabolites, threatening global food safety. Most of these pathogenic Aspergillus species have been isolated from almost all major biomes. The soil remains the primary habitat for most of these cryptic fungi. This review explored some of the ecological attributes that have contributed immensely to the success of the pathogenicity of some members of the genus Aspergillus over time. Hence, the virulence factors of the genus Aspergillus, their ecology and others were reviewed. Furthermore, some biological control techniques were recommended. Pathogenic effects of Aspergillus species are entirely accidental; therefore, the virulence evolution prediction model in such species becomes a challenge, unlike their obligate parasite counterparts. In all, differences in virulence among organisms involved both conserved and species-specific genetic factors. If the impacts of climate change continue, new cryptic Aspergillus species will emerge and mycotoxin contamination risks will increase in all ecosystems, as these species can metabolically adjust to nutritional and biophysical challenges. As most of their gene clusters are silent, fungi continue to be a source of underexplored bioactive compounds. The World Soil Charter recognizes the relevance of soil biodiversity in supporting healthy soil functions. The question of how a balance may be struck between supporting healthy soil biodiversity and the control of toxic fungi species in the field to ensure food security is therefore pertinent. Numerous advanced strategies and biocontrol methods so far remain the most environmentally sustainable solution to the control of toxigenic fungi in the field.

Regulation of High-Affinity Iron Acquisition, Including Acquisition Mediated by the Iron Permease FtrA, Is Coordinated by AtrR, SrbA, and SreA in Aspergillus fumigatus

Iron acquisition is crucial for virulence of the human pathogen Aspergillus fumigatus. Previous studies indicated that this mold regulates iron uptake via both siderophores and reductive iron assimilation by the GATA factor SreA and the SREBP regulator SrbA. Here, characterization of loss of function as well as hyperactive alleles revealed that transcriptional activation of iron uptake depends additionally on the Zn2Cys6 regulator AtrR, most likely via cooperation with SrbA. Mutational analysis of the promoter of the iron permease-encoding ftrA gene identified a 210-bp sequence, which is both essential and sufficient to impart iron regulation. Further studies located functional sequences, densely packed within 75 bp, that largely resemble binding motifs for SrbA, SreA, and AtrR. The latter, confirmed by chromatin immunoprecipitation (ChIP) analysis, is the first one not fully matching the 5'-CGGN12CCG-3' consensus sequence. The results presented here emphasize for the first time the direct involvement of SrbA, AtrR, and SreA in iron regulation. The essential role of both AtrR and SrbA in activation of iron acquisition underlines the coordination of iron homeostasis with biosynthesis of ergosterol and heme as well as adaptation to hypoxia. The rationale is most likely the iron dependence of these pathways along with the enzymatic link of biosynthesis of ergosterol and siderophores. IMPORTANCE Aspergillus fumigatus is the most common filamentous fungal pathogen infecting humans. Iron acquisition via siderophores has previously been shown to be essential for virulence of this mold species. Here, we demonstrate that AtrR, a transcription factor previously shown to control ergosterol biosynthesis, azole resistance, and adaptation to hypoxia, is essential for activation of iron acquisition, including siderophore biosynthesis and uptake. Dissection of an iron-regulated promoter identified binding motifs for AtrR and the two previously identified regulators of iron acquisition, SrbA and SreA. Altogether, this study identified a new regulator required for maintenance of iron homeostasis, revealed insights into promoter architecture for iron regulation, and emphasized the coordinated regulation of iron homeostasis ergosterol biosynthesis and adaptation to hypoxia.

Whole-genome and comparative genome analysis of Mucor racemosus C isolated from Yongchuan Douchi

Mucor racemosus is the predominant fungal in the zhiqu stage of the fermentation of Yongchuan Douchi (Mucor-type), which plays an important role in the fermentation process of Yongchuan Douchi. However, there is a lack of information on the genetic analysis of M. racemosus. In this study, we isolated and identified M. racemosus C (accession no JAPEHQ000000000) from Yongchuan Douchi and analyzed the physiological indicators, then genomic information of the strain to perform a comprehensive analysis of its fermentation capacity and safety. M. racemosus C had neutral protease activity up to 68.051 U/mL at 30 °C and alkaline protease activity up to 57.367 U/mL at 25 °C. In addition, comparing the genomic data with the COGs database (NCBI), it was predicted that M. racemosus C undergoes extensive amino acid metabolism, making C suitable for the production of fermented foods (e.g., Douchi, Syoyu, and sufu). Finally, we performed virulence genes and resistance genes analysis, hemolysis experiment, aflatoxins assay, antibiotic resistance assay to evaluate the safety of M. racemosus C, and the results showed that M. racemosus C was safe, non-toxin-producing and non-hemolytic.

The Regulatory Hub of Siderophore Biosynthesis in the Phytopathogenic Fungus Alternaria alternata

A GATA zinc finger-containing repressor (AaSreA) suppresses siderophore biosynthesis in the phytopathogenic fungus Alternaria alternata under iron-replete conditions. In this study, targeted gene deletion revealed two bZIP-containing transcription factors (AaHapX and AaAtf1) and three CCAAT-binding proteins (AaHapB, AaHapC, and AaHapE) that positively regulate gene expression in siderophore production. This is a novel phenotype regarding Atf1 and siderophore biosynthesis. Quantitative RT-PCR analyses revealed that only AaHapX and AaSreA were regulated by iron. AaSreA and AaHapX form a transcriptional feedback negative loop to regulate iron acquisition in response to the availability of environmental iron. Under iron-limited conditions, AaAtf1 enhanced the expression of AaNps6, thus playing a positive role in siderophore production. However, under nutrient-rich conditions, AaAtf1 plays a negative role in resistance to sugar-induced osmotic stress, and AaHapX plays a negative role in resistance to salt-induced osmotic stress. Virulence assays performed on detached citrus leaves revealed that AaHapX and AaAtf1 play no role in fungal pathogenicity. However, fungal strains carrying the AaHapB, AaHapC, or AaHapE deletion failed to incite necrotic lesions, likely due to severe growth deficiency. Our results revealed that siderophore biosynthesis and iron homeostasis are regulated by a well-organized network in A. alternata.

Invasive aspergillosis in liver transplant recipients

BACKGROUND

Liver transplantation is increasing worldwide with underlying pathologies dominated by metabolic and alcoholic diseases in developed countries.

METHODS

We provide a narrative review of invasive aspergillosis (IA) in liver transplant (LT) recipients. We searched PubMed and Google Scholar for references without language and time restrictions.

RESULTS

The incidence of IA in LT recipients is low (1.8%), while mortality is high (∼50%). It occurs mainly early (<3 months) after LT. Some risk factors have been identified before (corticosteroid, renal, and liver failure), during (massive transfusion and duration of surgical procedure), and after transplantation (intensive care unit stay, re-transplantation, re-operation). Diagnosis can be difficult and therefore requires full radiological and clinicobiological collaboration. Accurate identification of Aspergillus species is recommended due to the cryptic species, and susceptibility testing is crucial given the increasing resistance of Aspergillus fumigatus to azoles. It is recommended to reduce the dose of tacrolimus (50%) and to closely monitor the trough level when introducing voriconazole, isavuconazole, and posaconazole. Surgery should be discussed on a case-by-case basis. Antifungal prophylaxis is recommended in high-risk patients. Environmental preventative measures should be implemented to prevent outbreaks of nosocomial aspergillosis in LT recipient units.

CONCLUSION

IA remains a very serious disease in LT patients and should be promptly sought and, if possible, prevented by clinicians when risk factors are identified.

Molecular Regulatory Mechanism of the Iron-Ion-Promoted Asexual Sporulation of Antrodia cinnamomea in Submerged Fermentation Revealed by Comparative Transcriptomics

Antrodia cinnamomea is a precious edible and medicinal fungus with activities of antitumor, antivirus, and immunoregulation. Fe²⁺ was found to promote the asexual sporulation of A. cinnamomea markedly, but the molecular regulatory mechanism of the effect is unclear. In the present study, comparative transcriptomics analysis using RNA sequencing (RNA-seq) and real time quantitative PCR (RT-qPCR) were conducted on A. cinnamomea mycelia cultured in the presence or absence of Fe²⁺ to reveal the molecular regulatory mechanisms underlying iron-ion-promoted asexual sporulation. The obtained mechanism is as follows: A. cinnamomea acquires iron ions through reductive iron assimilation (RIA) and siderophore-mediated iron assimilation (SIA). In RIA, ferrous iron ions are directly transported into cells by the high-affinity protein complex formed by a ferroxidase (FetC) and an Fe transporter permease (FtrA). In SIA, siderophores are secreted externally to chelate the iron in the extracellular environment. Then, the chelates are transported into cells through the siderophore channels (Sit1/MirB) on the cell membrane and hydrolyzed by a hydrolase (EstB) in the cell to release iron ions. The O-methyltransferase TpcA and the regulatory protein URBS1 promote the synthesis of siderophores. HapX and SreA respond to and maintain the balance of the intercellular concentration of iron ions. Furthermore, HapX and SreA promote the expression of flbD and abaA, respectively. In addition, iron ions promote the expression of relevant genes in the cell wall integrity signaling pathway, thereby accelerating the cell wall synthesis and maturation of spores. This study contributes to the rational adjustment and control of the sporulation of A. cinnamomea and thereby improves the efficiency of the preparation of inoculum for submerged fermentation.

Siderophore-Based Noninvasive Differentiation of Aspergillus fumigatus Colonization and Invasion in Pulmonary Aspergillosis

Germination from conidia to hyphae and hyphal propagation of Aspergillus fumigatus are the key pathogenic steps in the development of invasive pulmonary aspergillosis (IPA). By applying in vitro observations in a clinical study of 13 patients diagnosed with probable IPA, here, we show that the transition from colonization to the A. fumigatus invasive stage is accompanied by the secretion of triacetylfusarinine C (TafC), triacetylfusarinine B (TafB), and ferricrocin (Fc) siderophores into urine, with strikingly better sensitivity performance than serum sampling. The best-performing index, the TafC/creatinine index, with a median value of 17.2, provided 92.3% detection sensitivity (95% confidence interval [CI], 64.0 to 99.8%) and 100% specificity (95% CI, 84.6 to 100%), i.e., substantially better than the corresponding indications provided by galactomannan (GM) and β-d-glucan (BDG) serology. For the same patient cohort, the serum GM and BDG sensitivities were 46.2 and 76.9%, respectively, and their specificities were 86.4 and 63.6%, respectively. The time-dependent specific appearance of siderophores in the host's urine represents an impactful clinical diagnostic advantage in the early discrimination of invasive aspergillosis from colonization. A favorable concentration of TafC in a clinical specimen distant from a deep infection site enables the noninvasive sampling of patients suffering from IPA. IMPORTANCE The importance of this research lies in the demonstration that siderophore analysis can distinguish between asymptomatic colonization and invasive pulmonary aspergillosis. We found clear associations between phases of fungal development, from conidial germination to the proliferative stage of invasive aspergillosis, and changes in secondary metabolite secretion. The critical extracellular fungal metabolites triacetylfusarinines C and B are produced during the polarized germination or postpolarized growth phase and reflect the morphological status of the proliferating pathogen. False positivity in Aspergillus diagnostics is minimized as mammalian cells do not synthesize Aspergillus siderophore or mycotoxin molecules.

Interconnected Set of Enzymes Provide Lysine Biosynthetic Intermediates and Ornithine Derivatives as Key Precursors for the Biosynthesis of Bioactive Secondary Metabolites

Bacteria, filamentous fungi, and plants synthesize thousands of secondary metabolites with important biological and pharmacological activities. The biosynthesis of these metabolites is performed by networks of complex enzymes such as non-ribosomal peptide synthetases, polyketide synthases, and terpenoid biosynthetic enzymes. The efficient production of these metabolites is dependent upon the supply of precursors that arise from primary metabolism. In the last decades, an impressive array of biosynthetic enzymes that provide specific precursors and intermediates leading to secondary metabolites biosynthesis has been reported. Suitable knowledge of the elaborated pathways that synthesize these precursors or intermediates is essential for advancing chemical biology and the production of natural or semisynthetic biological products. Two of the more prolific routes that provide key precursors in the biosynthesis of antitumor, immunosuppressant, antifungal, or antibacterial compounds are the lysine and ornithine pathways, which are involved in the biosynthesis of β-lactams and other non-ribosomal peptides, and bacterial and fungal siderophores. Detailed analysis of the molecular genetics and biochemistry of the enzyme system shows that they are formed by closely related components. Particularly the focus of this study is on molecular genetics and the enzymatic steps that lead to the formation of intermediates of the lysine pathway, such as α-aminoadipic acid, saccharopine, pipecolic acid, and related compounds, and of ornithine-derived molecules, such as N⁵-Acetyl-N⁵-Hydroxyornithine and N⁵-anhydromevalonyl-N⁵-hydroxyornithine, which are precursors of siderophores. We provide evidence that shows interesting functional relationships between the genes encoding the enzymes that synthesize these products. This information will contribute to a better understanding of the possibilities of advancing the industrial applications of synthetic biology.

Genetic Engineering of Talaromyces marneffei to Enhance Siderophore Production and Preliminary Testing for Medical Application Potential

Siderophores are compounds with low molecular weight with a high affinity and specificity for ferric iron, which is produced by bacteria and fungi. Fungal siderophores have been characterized and their feasibility for clinical applications has been investigated. Fungi may be limited in slow growth and low siderophore production; however, they have advantages of high diversity and affinity. Hence, the purpose of this study was to generate a genetically modified strain in Talaromyces marneffei that enhanced siderophore production and to identify the characteristics of siderophore to guide its medical application. SreA is a transcription factor that negatively controls iron acquisition mechanisms. Therefore, we deleted the sreA gene to enhance the siderophore production and found that the null mutant of sreA (ΔsreA) produced a high amount of extracellular siderophores. The produced siderophore was characterized using HPLC-MS, HPLC-DAD, FTIR, and 1H- and 13C-NMR techniques and identified as a coprogen B. The compound showed a powerful iron-binding activity and could reduce labile iron pool levels in iron-loaded hepatocellular carcinoma (Huh7) cells. In addition, the coprogen B showed no toxicity to the Huh7 cells, demonstrating its potential to serve as an ideal iron chelator. Moreover, it inhibits the growth of Candida albicans and Escherichia coli in a dose-dependent manner. Thus, we have generated the siderophore-enhancing strain of T. marneffei, and the coprogen B isolated from this strain could be useful in the development of a new iron-chelating agent or other medical applications.

A GATA-type transcription factor SreA affects manganese susceptibility by regulating the expression of iron uptake-related genes

SreA has been identified as a GATA-type transcription factor that represses iron uptake to avoid iron excess during iron sufficiency. However, knowledge about whether SreA also affects the homeostasis of other divalent metal ions is limited. In this study, by screening Aspergillus fumigatus transcription factor deletion mutant libraries, we demonstrate that the sreA deletion mutant shows the greatest tolerance to MnCl₂ among the tested divalent metal ions. Fe and Mn stimuli are able to enhance the expression of SreA with the different time-dependent manner, while the expression of SreA contributes to Mn²⁺ tolerance. Lack of SreA results in abnormally increased expression of a series of siderophore biosynthesis genes and iron transport-related genes, especially under MnCl₂ treatment. Further mechanistic exploration indicated that lack of SreA exacerbates abnormal iron uptake, and iron excess inhibits cellular Mn content; thus, deletion of sreA results in Mn tolerance. Thus, findings in this study have demonstrated a new unexplored function for the transcription factor SreA in regulation of the Mn²⁺ tolerance.

Uptake of the Siderophore Triacetylfusarinine C, but Not Fusarinine C, Is Crucial for Virulence of Aspergillus fumigatus

Siderophores play an important role in fungal virulence, serving as trackers for in vivo imaging and as biomarkers of fungal infections. However, siderophore uptake is only partially characterized. As the major cause of aspergillosis, Aspergillus fumigatus is one of the most common airborne fungal pathogens of humans. Here, we demonstrate that this mold species mediates the uptake of iron chelated by the secreted siderophores triacetylfusarinine C (TAFC) and fusarinine C by the major facilitator-type transporters MirB and MirD, respectively. In a murine aspergillosis model, MirB but not MirD was found to be crucial for virulence, indicating that TAFC-mediated uptake plays a dominant role during infection. In the absence of MirB, TAFC becomes inhibitory by decreasing iron availability because the mutant is not able to recognize iron that is chelated by TAFC. MirB-mediated transport was found to tolerate the conjugation of fluorescein isothiocyanate to triacetylfusarinine C, which might aid in the development of siderophore-based antifungals in a Trojan horse approach, particularly as the role of MirB in pathogenicity restrains its mutational inactivation. Taken together, this study identified the first eukaryotic siderophore transporter that is crucial for virulence and elucidated its translational potential as well as its evolutionary conservation. IMPORTANCE Aspergillus fumigatus is responsible for thousands of cases of invasive fungal disease annually. For iron uptake, A. fumigatus secretes so-called siderophores, which are taken up after the binding of environmental iron. Moreover, A. fumigatus can utilize siderophore types that are produced by other fungi or bacteria. Fungal siderophores raised considerable interest due to their role in virulence and their potential for the diagnosis and treatment of fungal infections. Here, we demonstrate that the siderophore transporter MirB is crucial for the virulence of A. fumigatus, which reveals that its substrate, triacetylfusarinine C, is the most important siderophore during infection. We found that in the absence of MirB, TAFC becomes inhibitory by decreasing the availability of environmental iron and that MirB-mediated transport tolerates the derivatization of its substrate, which might aid in the development of siderophore-based antifungals. This study significantly improved the understanding of fungal iron homeostasis and the role of siderophores in interactions with the host.

Induction of Iron Stress in Hepatocellular Carcinoma Cell Lines by Siderophore of Aspergillus nidulans Towards Promising Anticancer Effect

Hepatocellular carcinoma is among the leading causes of cancer-related deaths worldwide and needs efficient and feasible approach of treatment. Present study focuses on exploring the anticancer activity of a secondary metabolite called siderophore of Aspergillus nidulans against hepatocellular carcinoma cell line HepG2. These small peptides are produced by microorganisms including fungi for scavenging iron from its surroundings. Fungi including Aspergillus spp. are known to produce siderophores under iron-limited conditions. Siderophores have high affinity towards iron and are classified into various types. In the present study, siderophore isolated and purified from fungal cultures was confirmed to be of hydroxamate type by chrome azurol sulfonate and Atkin's assay. HPLC analysis confirmed purity while LC-ESI-MS revealed that the siderophore is triacetyl fusigen. Cancerous cells, HepG2, grown under siderophore treatment showed inhibition in growth and proliferation in a dose- and time-dependent manner. Reduction in viability and metabolic activity was evident upon treatment as seen in trypan blue, MTT and WST assay. Fluorescent staining using PI and DAPI confirmed the same while DCFDA staining revealed increased reactive oxygen species production which might have led to cell death and deterioration. Such increase in ROS has been correlated with iron accumulation by assessing intracellular iron level through ICP-MS. To assess the effect of siderophore treatment on normal cells, WRL-68, same assays were carried out but the effect was mostly non-significant up to 48 h. Thus, present work suggests that an optimum dose of siderophore purified from A. nidulans culture might prove a useful anticancer agent.

Synergistic Antifungal Effect of a Combination of Iron Deficiency and Calcium Supplementation

Fungal diseases have become a major public health issue worldwide. Increasing drug resistance and the limited number of available antifungals result in high morbidity and mortality. Metal-based drugs have been reported to be therapeutic agents against major protozoan diseases, but knowledge of their ability to function as antifungals is limited. In this study, we found that calcium supplementation combined with iron deficiency causes dramatic growth inhibition of the human fungal pathogens Aspergillus fumigatus, Candida albicans, and Cryptococcus neoformans. Calcium induces the downregulation of iron uptake-related genes and, in particular, causes a decrease in the expression of the transcription factor HapX, which tends to transcriptionally activate siderophore-mediated iron acquisition under iron-deficient conditions. Iron deficiency causes calcium overload and the overproduction of intracellular reactive oxygen species (ROS), and perturbed ion homeostasis suppresses fungal growth. These phenomena are consistently identified in azole-resistant A. fumigatus isolates. The findings here imply that low iron availability lets cells mistakenly absorb calcium as a substitute, causing calcium abnormalities. Thus, there is a mutual effect between iron and calcium in fungal pathogens, and the combination of calcium with an iron chelator could serve to improve antifungal therapy. IMPORTANCE Millions of immunocompromised people are at a higher risk of developing different types of severe fungal diseases. The limited number of antifungals and the emergence of antimicrobial resistance highlight an urgent need for new strategies against invasive fungal infections. Here, we report that calcium can interfere with iron absorption of fungal pathogens, especially in iron-limited environments. Thus, a combination of calcium supplementation with an iron chelator inhibits the growth of human fungal pathogens, including Aspergillus fumigatus, Candida albicans, and Cryptococcus neoformans. Moreover, we demonstrate that iron deficiency induces a nonspecific calcium uptake response, which results in toxic levels of metal. Findings in this study suggest that a microenvironment with excess calcium and limited iron is an efficient strategy to curb the growth of fungal pathogens, especially for drug-resistant isolates.

Aspergillus Utilizes Extracellular Heme as an Iron Source During Invasive Pneumonia, Driving Infection Severity

BACKGROUND

Depriving microbes of iron is critical to host defense. Hemeproteins, the largest source of iron within vertebrates, are abundant in infected tissues in aspergillosis due to hemorrhage, but Aspergillus species have been thought to lack heme import mechanisms. We hypothesized that heme provides iron to Aspergillus during invasive pneumonia, thereby worsening the outcomes of the infection.

METHODS

We assessed the effect of heme on fungal phenotype in various in vitro conditions and in a neutropenic mouse model of invasive pulmonary aspergillosis.

RESULTS

In mice with neutropenic invasive aspergillosis, we found a progressive and compartmentalized increase in lung heme iron. Fungal cells cultured under low iron conditions took up heme, resulting in increased fungal iron content, resolution of iron starvation, increased conidiation, and enhanced resistance to oxidative stress. Intrapulmonary administration of heme to mice with neutropenic invasive aspergillosis resulted in markedly increased lung fungal burden, lung injury, and mortality, whereas administration of heme analogs or heme with killed Aspergillus did not. Finally, infection caused by fungal germlings cultured in the presence of heme resulted in a more severe infection.

CONCLUSIONS

Invasive aspergillosis induces local hemolysis in infected tissues, thereby supplying heme iron to the fungus, leading to lethal infection.

The Siderophore Transporters Sit1 and Sit2 Are Essential for Utilization of Ferrichrome-, Ferrioxamine- and Coprogen-Type Siderophores in Aspergillus fumigatus

Siderophore-mediated acquisition of iron has been shown to be indispensable for the virulence of several fungal pathogens, the siderophore transporter Sit1 was found to mediate uptake of the novel antifungal drug VL-2397, and siderophores were shown to be useful as biomarkers as well as for imaging of fungal infections. However, siderophore uptake in filamentous fungi is poorly characterized. The opportunistic human pathogen Aspergillus fumigatus possesses five putative siderophore transporters. Here, we demonstrate that the siderophore transporters Sit1 and Sit2 have overlapping, as well as unique, substrate specificities. With respect to ferrichrome-type siderophores, the utilization of ferrirhodin and ferrirubin depended exclusively on Sit2, use of ferrichrome A depended mainly on Sit1, and utilization of ferrichrome, ferricrocin, and ferrichrysin was mediated by both transporters. Moreover, both Sit1 and Sit2 mediated use of the coprogen-type siderophores coprogen and coprogen B, while only Sit1 transported the bacterial ferrioxamine-type xenosiderophores ferrioxamines B, G, and E. Neither Sit1 nor Sit2 were important for the utilization of the endogenous siderophores fusarinine C and triacetylfusarinine C. Furthermore, A. fumigatus was found to lack utilization of the xenosiderophores schizokinen, basidiochrome, rhizoferrin, ornibactin, rhodotorulic acid, and enterobactin. Taken together, this study characterized siderophore use by A. fumigatus and substrate characteristics of Sit1 and Sit2.

Sporulation environment drives phenotypic variation in the pathogen Aspergillus fumigatus

Aspergillus fumigatus causes more than 300,000 life-threatening infections annually and is widespread across varied environments with a single colony producing thousands of conidia, genetically identical dormant spores. Conidia are easily wind-dispersed to new environments where they can germinate and, if inhaled by susceptible hosts, cause disease. Using high-throughput single-cell analysis via flow cytometry we analyzed conidia produced and germinated in nine environmentally and medically relevant conditions (complete medium, minimal medium, high temperature, excess copper, excess iron, limited iron, excess salt, excess reactive oxygen species, and limited zinc). We found that germination phenotypes vary among genetically identical individuals, that the environment of spore production determines the size of spores and the degree of germination heterogeneity, and that the environment of spore production impacts virulence in a Galleria mellonella host.

Antifungal Siderophore Conjugates for Theranostic Applications in Invasive Pulmonary Aspergillosis Using Low-Molecular TAFC Scaffolds

Invasive pulmonary aspergillosis (IPA) is a life-threatening form of fungal infection, primarily in immunocompromised patients and associated with significant mortality. Diagnostic procedures are often invasive and/or time consuming and existing antifungals can be constrained by dose-limiting toxicity and drug interaction. In this study, we modified triacetylfusarinine C (TAFC), the main siderophore produced by the opportunistic pathogen Aspergillus fumigatus (A. fumigatus), with antifungal molecules to perform antifungal susceptibility tests and molecular imaging. A variation of small organic molecules (eflornithine, fludioxonil, thiomersal, fluoroorotic acid (FOA), cyanine 5 (Cy5) with antifungal activity were coupled to diacetylfusarinine C (DAFC), resulting in a "Trojan horse" to deliver antifungal compounds specifically into A. fumigatus hyphae by the major facilitator transporter MirB. Radioactive labeling with gallium-68 allowed us to perform in vitro characterization (distribution coefficient, stability, uptake assay) as well as biodistribution experiments and PET/CT imaging in an IPA rat infection model. Compounds chelated with stable gallium were used for antifungal susceptibility tests. [Ga]DAFC-fludioxonil, -FOA, and -Cy5 revealed a MirB-dependent active uptake with fungal growth inhibition at 16 µg/mL after 24 h. Visualization of an A. fumigatus infection in lungs of a rat was possible with gallium-68-labeled compounds using PET/CT. Heterogeneous biodistribution patterns revealed the immense influence of the antifungal moiety conjugated to DAFC. Overall, novel antifungal siderophore conjugates with promising fungal growth inhibition and the possibility to perform PET imaging combine both therapeutic and diagnostic potential in a theranostic compound for IPA caused by A. fumigatus.

Transcription factor control of virulence in phytopathogenic fungi

Plant-pathogenic fungi are a significant threat to economic and food security worldwide. Novel protection strategies are required and therefore it is critical we understand the mechanisms by which these pathogens cause disease. Virulence factors and pathogenicity genes have been identified, but in many cases their roles remain elusive. It is becoming increasingly clear that gene regulation is vital to enable plant infection and transcription factors play an essential role. Efforts to determine their regulatory functions in plant-pathogenic fungi have expanded since the annotation of fungal genomes revealed the ubiquity of transcription factors from a broad range of families. This review establishes the significance of transcription factors as regulatory elements in plant-pathogenic fungi and provides a systematic overview of those that have been functionally characterized. Detailed analysis is provided on regulators from well-characterized families controlling various aspects of fungal metabolism, development, stress tolerance, and the production of virulence factors such as effectors and secondary metabolites. This covers conserved transcription factors with either specialized or nonspecialized roles, as well as recently identified regulators targeting key virulence pathways. Fundamental knowledge of transcription factor regulation in plant-pathogenic fungi provides avenues to identify novel virulence factors and improve our understanding of the regulatory networks linked to pathogen evolution, while transcription factors can themselves be specifically targeted for disease control. Areas requiring further insight regarding the molecular mechanisms and/or specific classes of transcription factors are identified, and direction for future investigation is presented.

Nitrogen, Iron and Zinc Acquisition: Key Nutrients to Aspergillus fumigatus Virulence

Aspergillus fumigatus is a ubiquitous soil decomposer and an opportunistic pathogen that is characterized by its large metabolic machinery for acquiring nutrients from media. Lately, an ever-increasing number of genes involved in fungal nutrition has been associated with its virulence. Of these, nitrogen, iron, and zinc metabolism-related genes are particularly noteworthy, since 78% of them have a direct implication in virulence. In this review, we describe the sensing, uptake and regulation process of the acquisition of these nutrients, the connections between pathways and the virulence-implicated genes. Nevertheless, only 40% of the genes mentioned in this review have been assayed for roles in virulence, leaving a wide field of knowledge that remains uncertain and might offer new therapeutic and diagnostic targets.

Expression Patterns in Reductive Iron Assimilation and Functional Consequences during Phagocytosis of Lichtheimia corymbifera, an Emerging Cause of Mucormycosis

Iron is an essential micronutrient for most organisms and fungi are no exception. Iron uptake by fungi is facilitated by receptor-mediated internalization of siderophores, heme and reductive iron assimilation (RIA). The RIA employs three protein groups: (i) the ferric reductases (Fre5 proteins), (ii) the multicopper ferroxidases (Fet3) and (iii) the high-affinity iron permeases (Ftr1). Phenotyping under different iron concentrations revealed detrimental effects on spore swelling and hyphal formation under iron depletion, but yeast-like morphology under iron excess. Since access to iron is limited during pathogenesis, pathogens are placed under stress due to nutrient limitations. To combat this, gene duplication and differential gene expression of key iron uptake genes are utilized to acquire iron against the deleterious effects of iron depletion. In the genome of the human pathogenic fungus L. corymbifera, three, four and three copies were identified for FRE5, FTR1 and FET3 genes, respectively. As in other fungi, FET3 and FTR1 are syntenic and co-expressed in L. corymbifera. Expression of FRE5, FTR1 and FET3 genes is highly up-regulated during iron limitation (Fe-), but lower during iron excess (Fe+). Fe- dependent upregulation of gene expression takes place in LcFRE5 II and III, LcFTR1 I and II, as well as LcFET3 I and II suggesting a functional role in pathogenesis. The syntenic LcFTR1 I–LcFET3 I gene pair is co-expressed during germination, whereas LcFTR1 II- LcFET3 II is co-expressed during hyphal proliferation. LcFTR1 I, II and IV were overexpressed in Saccharomyces cerevisiae to represent high and moderate expression of intracellular transport of Fe3+, respectively. Challenge of macrophages with the yeast mutants revealed no obvious role for LcFTR1 I, but possible functions of LcFTR1 II and IVs in recognition by macrophages. RIA expression pattern was used for a new model of interaction between L. corymbifera and macrophages.

Galleria mellonella as a screening tool to study virulence factors of Aspergillus fumigatus

The invertebrate Galleria mellonella has increasingly and widely been used in the last few years to study complex host–microbe interactions. Aspergillus fumigatus is one of the most pathogenic fungi causing life-threatening diseases in humans and animals. Galleria mellonella larvae has been proven as a reliable model for the analysis of pathogenesis and virulence factors, enable to screen a large number of A. fumigatus strains. This review describes the different uses of G. mellonella to study A. fumigatus and provides a comparison of the different protocols to trace fungal pathogenicity. The review also includes a summary of the diverse mutants tested in G. mellonella, and their respective contribution to A. fumigatus virulence. Previous investigations indicated that G. mellonella should be considered as an interesting tool even though a mammalian model may be required to complete and verify initial data.

Transcriptomics Reveal the Survival Strategies of Enterococcus mundtii in the Gut of Spodoptera littoralis

The complex interaction between a higher organism and its resident gut flora is a subject of immense interest in the field of symbiosis. Many insects harbor a complex community of microorganisms in their gut. Larvae of Spodoptera littoralis, a lepidopteran pest, house a bacterial community that varies both spatially (along the length of the gut) and temporally (during the insect's life cycle). To monitor the rapid adaptation of microbes to conditions in the gut, a GFP-tagged reporter strain of E. mundtii, a major player in the gut community, was constructed. After early-instar S. littoralis larvae were fed with the tagged microbes, these were recovered from the larval fore- and hindgut by flow cytometry. The fluorescent reporter confirmed the persistence of E. mundtii in the gut. RNA-sequencing of the sorted bacteria highlighted various strategies of the symbiont's survival, including upregulated pathways for tolerating alkaline stress, forming biofilms and two-component signaling systems for quorum sensing, and resisting oxidative stress. Although these symbionts depend on the host for amino acid and fatty acids, differential regulation among various metabolic pathways points to an enriched lysine synthesis pathway of E. mundtii in the hindgut of the larvae.

Conservation and Loss of a Putative Iron Utilization Gene Cluster among Genotypes of Aspergillus flavus

Iron is an essential component for growth and development. Despite relative abundance in the environment, bioavailability of iron is limited due to oxidation by atmospheric oxygen into insoluble ferric iron. Filamentous fungi have developed diverse pathways to uptake and use iron. In the current study, a putative iron utilization gene cluster (IUC) in Aspergillus flavus was identified and characterized. Gene analyses indicate A. flavus may use reductive as well as siderophore-mediated iron uptake and utilization pathways. The ferroxidation and iron permeation process, in which iron transport depends on the coupling of these two activities, mediates the reductive pathway. The IUC identified in this work includes six genes and is located in a highly polymorphic region of the genome. Diversity among A. flavus genotypes is manifested in the structure of the IUC, which ranged from complete deletion to a region disabled by multiple indels. Molecular profiling of A. flavus populations suggests lineage-specific loss of IUC. The observed variation among A. flavus genotypes in iron utilization and the lineage-specific loss of the iron utilization genes in several A. flavus clonal lineages provide insight on evolution of iron acquisition and utilization within Aspergillus section Flavi. The potential divergence in capacity to acquire iron should be taken into account when selecting A. flavus active ingredients for biocontrol in niches where climate change may alter iron availability.

Siderophore Scaffold as Carrier for Antifungal Peptides in Therapy of Aspergillus fumigatus Infections

Antifungal resistance of human fungal pathogens represents an increasing challenge in modern medicine. Short antimicrobial peptides (AMP) display a promising class of antifungals with a different mode of action, but lack target specificity and metabolic stability. In this study the hexapeptide PAF26 (Ac-dArg-dLys-dLys-dTrp-dPhe-dTrp-NH2) and the three amino acid long peptide NLF (H2N-Asn-Leu-dPhe-COOH) were coupled to diacetylfusarinine C (DAFC), a derivative of the siderophore triacetylfusarinine C (TAFC) of Aspergillus fumigatus, to achieve targeted delivery for treatment of invasive aspergillosis. Conjugated compounds in various modifications were labelled with radioactive gallium-68 to perform in vitro and in vivo characterizations. LogD, serum stability, uptake- growth promotion- and minimal inhibitory concentration assays were performed, as well as in vivo stability tests and biodistribution in BALB/c mice. Uptake and growth assays revealed specific internalization of the siderophore conjugates by A. fumigatus. They showed a high stability in human serum and also in the blood of BALB/c mice but metabolites in urine, probably due to degradation in the kidneys. Only PAF26 showed growth inhibition at 8 µg/ml which was lost after conjugation to DAFC. Despite their lacking antifungal activity conjugates based on a siderophore scaffold have a potential to provide the basis for a new class of antifungals, which allow the combination of imaging by using PET/CT with targeted treatment, thereby opening a theranostic approach for personalized therapy.

Iron Assimilation during Emerging Infections Caused by Opportunistic Fungi with emphasis on Mucorales and the Development of Antifungal Resistance

Iron is a key transition metal required by most microorganisms and is prominently utilised in the transfer of electrons during metabolic reactions. The acquisition of iron is essential and becomes a crucial pathogenic event for opportunistic fungi. Iron is not readily available in the natural environment as it exists in its insoluble ferric form, i.e., in oxides and hydroxides. During infection, the host iron is bound to proteins such as transferrin, ferritin, and haemoglobin. As such, access to iron is one of the major hurdles that fungal pathogens must overcome in an immunocompromised host. Thus, these opportunistic fungi utilise three major iron acquisition systems to overcome this limiting factor for growth and proliferation. To date, numerous iron acquisition pathways have been fully characterised, with key components of these systems having major roles in virulence. Most recently, proteins involved in these pathways have been linked to the development of antifungal resistance. Here, we provide a detailed review of our current knowledge of iron acquisition in opportunistic fungi, and the role iron may have on the development of resistance to antifungals with emphasis on species of the fungal basal lineage order Mucorales, the causative agents of mucormycosis.

Iron in infection and immunity

Iron is an essential micronutrient for virtually all living cells. In infectious diseases, both invading pathogens and mammalian cells including those of the immune system require iron to sustain their function, metabolism and proliferation. On the one hand, microbial iron uptake is linked to the virulence of most human pathogens. On the other hand, the sequestration of iron from bacteria and other microorganisms is an efficient strategy of host defense in line with the principles of 'nutritional immunity'. In an acute infection, host-driven iron withdrawal inhibits the growth of pathogens. Chronic immune activation due to persistent infection, autoimmune disease or malignancy however, sequesters iron not only from infectious agents, autoreactive lymphocytes and neoplastic cells but also from erythroid progenitors. This is one of the key mechanisms which collectively result in the anemia of chronic inflammation. In this review, we highlight the most important interconnections between iron metabolism and immunity, focusing on host defense against relevant infections and on the clinical consequences of anemia of inflammation.

Absent regulation of iron acquisition by the copper regulator Mac1 in A. fumigatus

Aspergillus fumigatus is the most common cause of invasive aspergillosis, a life-threatening infection mainly affecting immunocompromised patients. The essential metals copper and iron play crucial roles in virulence of this mold. Recently, the copper-regulatory transcription factor Mac1 was reported to additionally be involved in the control of iron acquisition. However, in the current study, neither growth assays on solid and in liquid media, analysis of siderophore production nor expression analysis of genes involved in iron acquisition indicated the involvement of Mac1 in the regulation of iron uptake in A. fumigatus.

Adaptation to iron deficiency in human pathogenic fungi

Iron is an essential micronutrient for virtually all eukaryotic organisms and plays a central role during microbial infections. Invasive fungal diseases are associated with strikingly high rates of mortality, but their impact on human health is usually underestimated. Upon a fungal infection, hosts restrict iron availability in order to limit the growth and virulence of the pathogen. Here, we use two model yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe, to delve into the response to iron deficiency of human fungal pathogens, such as Candida glabrata, Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans. Fungi possess common and species-specific mechanisms to acquire iron and to control the response to iron limitation. Upon iron scarcity, fungi activate a wide range of elegant strategies to capture and import exogenous iron, mobilize iron from intracellular stores, and modulate their metabolism to economize and prioritize iron utilization. Hence, iron homeostasis genes represent remarkable virulence factors that can be used as targets for the development of novel antifungal treatments.

Expression of Talaromyces marneffei acuM and acuK Genes in Gluconeogenic Substrates and Various Iron Concentrations

Talaromyces marneffei is an opportunistic, dimorphic fungal pathogen that causes a disseminated infection in people with a weakened immunological status. The ability of this fungus to acquire nutrients inside the harsh environment of the macrophage phagosome is presumed to contribute to its pathogenicity. The transcription factors AcuM and AcuK are known to regulate gluconeogenesis and iron acquisition in Aspergillus fumigatus. This study demonstrated that they are also involved in both of these processes in the dimorphic fungus T. marneffei. Expression of acuM and acuK genes was determined by real time-polymerase chain reaction (RT-PCR) on the cells grown in media containing gluconeogenic substrates and various iron concentrations. We found that the acuM and acuK transcript levels were sequentially reduced when growing the fungus in increasing amounts of iron. The acuM transcript was upregulated in the gluconeogenic condition, while the acuK transcript showed upregulation only in the acetate medium in the yeast phase. These results suggest the involvement of acuM and acuK in gluconeogenesis and iron homeostasis in T. marneffei.