Tight control of mycotoxin biosynthesis gene expression in Aspergillus flavus by temperature as revealed by RNA-Seq

Study on the metabolic changes and regulatory mechanism of Aspergillus flavus conidia germination

Aspergillus flavus conidia are widespread in air; they attach to food and feed crops and secrete aflatoxins, which results in serious contamination. Germination of A. flavus conidia is the most critical step in contamination of food by A. flavus. This study aims to gain an insight into A. flavus conidia through dormancy to germination to provide a theoretical basis for inhibition of A. flavus conidia germination. The morphological changes and regulation mechanism of A. flavus conidia germination at 0, 4, 8, and 12 hours were observed. Transcriptomic and metabolomic analyses showed that conidia became active from dormancy (0 hour) to the initial stage of germination (4 hours), cellular respiration and energy metabolism increased, and amino acids and lipids were synthesized rapidly. The number of differentially expressed genes and differential metabolites was highest at this stage. Besides, we found that conidia germination had selectivity for different carbon and nitrogen sources. Compared with monosaccharides, disaccharides, as the only carbon source, significantly promoted the germination of conidia. Moreover, MepA, one of genes in the ammonium transporter family was studied. The gene deletion mutant ΔMepA had a significant growth defect, and the expression of MeaA was significantly upregulated in ΔMepA compared with the wild-type, indicating that both MepA and MeaA played an important role in transporting ammonium ions.IMPORTANCEThis is the first study to use combined transcriptomic and metabolomics analyses to explore the biological changes during germination of Aspergillus flavus conidia. The biological process with the highest changes occurred in 0-4 hours at the initial stage of germination. Compared with polysaccharides, monosaccharides significantly increased the size of conidia, while significantly decreasing the germination rate of conidia. Both MeaA and MepA were involved in ammonia transport and metabolism during conidia germination.

Pathogenicity is associated with population structure in a fungal pathogen of humans

Aspergillus flavus is a clinically and agriculturally important saprotrophic fungus responsible for severe human infections and extensive crop losses. We analyzed genomic data from 250 (95 clinical and 155 environmental) A. flavus isolates from 9 countries, including 70 newly sequenced clinical isolates, to examine population and pan-genome structure and their relationship to pathogenicity. We identified five A. flavus populations, including a new population, D, corresponding to distinct clades in the genome-wide phylogeny. Strikingly, > 75% of clinical isolates were from population D. Accessory genes, including genes within biosynthetic gene clusters, were significantly more common in some populations but rare in others. Population D was enriched for genes associated with zinc ion binding, lipid metabolism, and certain types of hydrolase activity. In contrast to the major human pathogen Aspergillus fumigatus, A. flavus pathogenicity in humans is strongly associated with population structure, making it a great system for investigating how population-specific genes contribute to pathogenicity.

Role of RNA Modifications, Especially m6A, in Aflatoxin Biosynthesis of Aspergillus flavus

RNA modifications play key roles in eukaryotes, but the functions in Aspergillus flavus are still unknown. Temperature has been reported previously to be a critical environmental factor that regulates the aflatoxin production of A. flavus, but much remains to be learned about the molecular networks. Here, we demonstrated that 12 kinds of RNA modifications in A. flavus were significantly changed under 29 °C compared to 37 °C incubation; among them, m6A was further verified by a colorimetric method. Then, the transcriptome-wide m6A methylome and m6A-altered genes were comprehensively illuminated through methylated RNA immunoprecipitation sequencing and RNA sequencing, from which 22 differentially methylated and expressed transcripts under 29 °C were screened out. It is especially notable that AFCA_009549, an aflatoxin biosynthetic pathway gene (aflQ), and the m6A methylation of its 332nd adenine in the mRNA significantly affect aflatoxin biosynthesis in A. flavus both on media and crop kernels. The content of sterigmatocystin in both ΔaflQ and aflQA332C strains was significantly higher than that in the WT strain. Together, these findings reveal that RNA modifications are associated with secondary metabolite biosynthesis of A. flavus.

Aflatoxigenic Aspergillus Modulates Aflatoxin-B1 Levels through an Antioxidative Mechanism

Aflatoxins (AFs) are considered to play important functions in species of Aspergillus section Flavi including an antioxidative role, as a deterrent against fungivorous insects, and in antibiosis. Atoxigenic Flavi are known to degrade AF-B1 (B1). To better understand the purpose of AF degradation, we investigated the degradation of B1 and AF-G1 (G1) in an antioxidative role in Flavi. Atoxigenic and toxigenic Flavi were treated with artificial B1 and G1 with or without the antioxidant selenium (Se), which is expected to affect levels of AF. After incubations, AF levels were measured by HPLC. To estimate which population would likely be favoured between toxigenic and atoxigenic Flavi under Se, we investigated the fitness, by spore count, of the Flavi as a result of exposure to 0, 0.40, and 0.86 µg/g Se in 3%-sucrose cornmeal agar (3gCMA). Results showed that levels B1 in medium without Se were reduced in all isolates, while G1 did not significantly change. When the medium was treated with Se, toxigenic Flavi significantly digested less B1, while levels of G1 significantly increased. Se did not affect the digestion of B1 in atoxigenic Flavi, and also did not alter levels of G1. Furthermore, atoxigenic strains were significantly fitter than toxigenic strains at Se 0.86 µg/g 3gCMA. Findings show that while atoxigenic Flavi degraded B1, toxigenic Flavi modulated its levels through an antioxidative mechanism to levels less than they produced. Furthermore, B1 was preferred in the antioxidative role compared to G1 in the toxigenic isolates. The higher fitness of atoxigenic over toxigenic counterparts at a plant non-lethal dose of 0.86 µg/g would be a useful attribute for integration in the broader biocontrol prospects of toxigenic Flavi.

Impacts of Temperature and Water Activity Interactions on Growth, Aflatoxin B1 Production and Expression of Major Biosynthetic Genes of AFB1 in Aspergillus flavus Isolates

The contamination of peanuts, with Aspergillus flavus and subsequent aflatoxins (AFs) is considered to be one of the most serious, safety problems in the world. Water activity (a_w) and temperature are limiting, factors for fungal growth and aflatoxin production during storage. The objectives of this study were to integrate data on the effects of temperature (34, 37, and 42 °C) and water activity (a_w; 0.85, 0.90, and 0.95) on growth rate aflatoxin B1 (AFB1) production and up- or-downregulation of the molecular expression of biosynthetic AFB1 genes divided into three types based on their A. flavus isolate composition and AFB1 capacity in vitro: A. flavus KSU114 (high producer), A. flavus KSU114 (low producer), and A. flavus KSU121 (non-producer). The A. flavus isolates were shown to be resilient in terms of growth on yeast extract sucrose agar media when exposed to temperature and water activity as pivotal environmental factors. The optimal conditions for the fungal growth of three isolates were a temperature of 34 °C and water activity of 0.95 a_w; there was very slow fungal growth at the highest temperature of 42 °C, with different a_w values causing inhibited fungal growth. The AFB1 production for the three isolates followed the same pattern with one exception: A. flavus KSU114 failed to produce any AFB1 at 42 °C with different a_w values. All tested genes of A. flavus were significantly up- or downregulated under three levels of interaction between temperature and a_w. The late structural genes of the pathway were significantly upregulated at 34 °C under a_w 0.95, although aflR, aflS and most of the early structural genes were upregulated. Compared to 34 °C with an a_w value of 0.95, most of the expressed genes were significantly downregulated at 37 and 42 °C with a_w values of 0.85 and 0.90. Additionally, two regulatory genes were downregulated under the same conditions. The expression level of laeA was also completely associated with AFB1 production, while the expression level of brlA was linked to A. flavus colonization. This information is required to forecast the actual effects of climate change on A. flavus. The findings can be applied to improve specific food technology processes and create prevention strategies to limit the concentrations of potential carcinogenic substances in peanuts and their derivatives.

Integrated metabolomics and lipidomics analyses suggest the temperature-dependent lipid desaturation promotes aflatoxin biosynthesis in Aspergillus flavus

Temperature is one of the main factors affecting aflatoxin (AF) biosynthesis in Aspergillus flavus. Previous studies showed that AF biosynthesis is elevated in A. flavus at temperatures between 28°C-30°C, while it is inhibited at temperatures above 30°C. However, little is known about the metabolic mechanism underlying temperature-regulated AF biosynthesis. In this study, we integrated metabolomic and lipidomic analyses to investigate the endogenous metabolism of A. flavus across 6 days of mycelia growth at 28°C (optimal AF production) and 37°C (no AF production). Results showed that both metabolite and lipid profiles were significantly altered at different temperatures. In particular, metabolites involved in carbohydrate and amino acid metabolism were up-regulated at 37°C on the second day but down-regulated from days three to six. Moreover, lipidomics and targeted fatty acids analyses of mycelia samples revealed a distinct pattern of lipid species and free fatty acids desaturation. High degrees of polyunsaturation of most lipid species at 28°C were positively correlated with AF production. These results provide new insights into the underlying metabolic changes in A. flavus under temperature stress.

Differential Expression of Genes Related to Growth and Aflatoxin Synthesis in Aspergillus flavus When Inhibited by Bacillus velezensis Strain B2

Aspergillus flavus is a saprophytic soil fungus that infects and contaminates seed crops with the highly carcinogenic aflatoxin, which brings health hazards to animals and humans. In this study, bacterial strains B1 and B2 isolated from the rhizosphere soil of camellia sinensis had significant antagonistic activities against A. flavus. Based on the phylogenetic analysis of 16SrDNA gene sequence, bacterial strains B1 and B2 were identified as Bacillus tequilensis and Bacillus velezensis, respectively. In addition, the transcriptome analysis showed that some genes related to A. flavus growth and aflatoxin synthesis were differential expressed and 16 genes in the aflatoxin synthesis gene cluster showed down-regulation trends when inhibited by Bacillus velezensis strain B2. We guessed that the Bacillus velezensis strain B2 may secrete some secondary metabolites, which regulate the related gene transcription of A. flavus to inhibit growth and aflatoxin production. In summary, this work provided the foundation for the more effective biocontrol of A. flavus infection and aflatoxin contamination by the determination of differential expression of genes related to growth and aflatoxin synthesis in A. flavus when inhibited by B. velezensis strain B2.

Climate Change and Effects on Molds and Mycotoxins

Earth’s climate is undergoing adverse global changes as an unequivocal result of anthropogenic activity. The occurring environmental changes are slowly shaping the balance between plant growth and related fungal diseases. Climate (temperature, available water, and light quality/quantity; as well as extreme drought, desertification, and fluctuations of humid/dry cycles) represents the most important agroecosystem factor influencing the life cycle stages of fungi and their ability to colonize crops, survive, and produce toxins. The ability of mycotoxigenic fungi to respond to Climate Change (CC) may induce a shift in their geographical distribution and in the pattern of mycotoxin occurrence. The present review examines the available evidence on the impact of CC factors on growth and mycotoxin production by the key mycotoxigenic fungi belonging to the genera Aspergillus, Penicillium, and Fusarium, which include several species producing mycotoxins of the greatest concern worldwide: aflatoxins (AFs), ochratoxins, and fumonisins (FUMs).

SfgA Renders Aspergillus flavus More Stable to the External Environment

sfgA is known as a key negative transcriptional regulator gene of asexual sporulation and sterigmatocystin production in Aspergillus nidulans. However, here, we found that the homolog sfgA gene shows a broad and complex regulatory role in governing growth, conidiation, sclerotia formation, secondary metabolism, and environmental stress responses in Aspergillus flavus. When sfgA was deleted in A. flavus, the fungal growth was slowed, but the conidiation was significantly increased, and the sclerotia formation displayed different behavior at different temperatures, which increased at 30 °C but decreased at 36 °C. In addition, sfgA regulated aflatoxin biosynthesis in a complex way that was associated with the changes in cultured conditions, and the increased production of aflatoxin in the ∆sfgA mutant was associated with a decrease in sclerotia size. Furthermore, the ∆sfgA mutant exhibited sensitivity to osmotic, oxidative, and cell wall stresses but still produced dense conidia. Transcriptome data indicated that numerous development- and secondary-metabolism-related genes were expressed differently when sfgA was deleted. Additionally, we also found that sfgA functions downstream of fluG in A. flavus, which is consistent with the genetic position in FluG-mediated conidiation in A. nidulans. Collectively, sfgA plays a critical role in the development, secondary metabolism, and stress responses of A. flavus, and sfgA renders A. flavus more stable to the external environment.

Screening of Bacillus velezensis E2 and the Inhibitory Effect of Its Antifungal Substances on Aspergillus flavus

Aspergilus flavus is the main pathogenic fungus that causes food mold. Effective control of A. flavus contamination is essential to ensure food safety. The lipopeptides (LPs) produced by Bacillus strains have been shown to have an obvious antifungal effect on molds. In this study, an antagonist strain of Bacillus velezensis with obvious antifungal activity against A. flavus was isolated from the surface of healthy rice. Using HPLC-MS analysis, the main components of LPs produced by strain E2 were identified as fengycin and iturins. Further investigations showed that LPs could inhibit the spore germination, and even cause abnormal expansion of hyphae and cell rupture. Transcriptomic analyses showed that some genes, involved in ribosome biogenesis in eukaryotes (NOG1, KRE33) and aflatoxin biosynthesis (aflK, aflR, veA, omtA) pathways in A. flavus were significantly down-regulated by LPs. In conclusion, this study provides novel insights into the cellular and molecular antifungal mechanisms of LPs against grain A. flavus contamination.

The Solvent Dimethyl Sulfoxide Affects Physiology, Transcriptome and Secondary Metabolism of Aspergillus flavus

Dimethyl sulfoxide (DSMO) is a simple molecule widely used because of its great solvating ability, but this solvent also has little-known biological effects, especially on fungi. Aspergillus flavus is a notorious pathogenic fungus which may contaminate a large variety of crops worldwide by producing aflatoxins, endangering at the same time food safety and international trade. The aim of this study was to characterize the effect of DMSO on A. flavus including developmental parameters such as germination and sporulation, as well as its transcriptome profile using high-throughput RNA-sequencing assay and its impact on secondary metabolism (SM). After DMSO exposure, A. flavus displayed depigmented conidia in a dose-dependent manner. The four-day exposition of cultures to two doses of DMSO, chosen on the basis of depigmentation intensity (35 mM “low” and 282 mM “high”), led to no significant impact on fungal growth, germination or sporulation. However, transcriptomic data analysis showed that 4891 genes were differentially regulated in response to DMSO (46% of studied transcripts). A total of 4650 genes were specifically regulated in response to the highest dose of DMSO, while only 19 genes were modulated upon exposure to the lowest dose. Secondary metabolites clusters genes were widely affected by the DMSO, with 91% of clusters impacted at the highest dose. Among these, aflatoxins, cyclopiazonic acid and ustiloxin B clusters were totally under-expressed. The genes belonging to the AFB1 cluster were the most negatively modulated ones, the two doses leading to 63% and 100% inhibition of the AFB1 production, respectively. The SM analysis also showed the disappearance of ustiloxin B and a 10-fold reduction of cyclopiazonic acid level when A. flavus was treated by the higher DMSO dose. In conclusion, the present study showed that DMSO impacted widely A. flavus’ transcriptome, including secondary metabolism gene clusters with the aflatoxins at the head of down-regulated ones. The solvent also inhibits conidial pigmentation, which could illustrate common regulatory mechanisms between aflatoxins and fungal pigment pathways. Because of its effect on major metabolites synthesis, DMSO should not be used as solvent especially in studies testing anti-aflatoxinogenic compounds.

Inhibition of Aspergillus flavus growth and aflatoxin B1 production by natamycin

Aspergillus flavus causes huge crop losses, reduces crop quality and has adverse effects on human and animal health. A large amount of food contaminated with aflatoxin can greatly increase the risk of liver cancer. Therefore, prevention and control of aflatoxin production have aroused attention of research in various countries. Natamycin extracted from Streptomyces spp. has been widely used in production practice due to its good specificity and safety. Here, we found that natamycin could significantly inhibit fungal growth, conidia germination, ergosterol and AFB1 production by A. flavus in a dose-dependent manner. Scanning electron microscope analysis indicated that the number of conidia was decreased, the outer wall of conidia was destroyed, and the mycelia were shrivelled and tangled by natamycin. RNA-Seq data indicated that natamycin inhibited fungal growth and conidia development of A. flavus by significantly down-regulating some genes involved in ergosterol biosynthesis, such as Erg13, HMG1 and HMG2. It inhibited conidia germination by significantly down-regulating some genes related to conidia development, such as FluG and VosA. After natamycin exposure, the decreased ratio of aflS/aflR caused by the down-regulation of all the structural genes, which subsequently resulted in the suppression of AFB1 production. In conclusion, this study served to reveal the inhibitory mechanisms of natamycin on fungal growth and AFB1 biosynthesis in A. flavus and to provide solid evidence for its application in controlling AFB1 contamination.

Fusarium verticillioides and Aspergillus flavus Co-Occurrence Influences Plant and Fungal Transcriptional Profiles in Maize Kernels and In Vitro

Climate change will increase the co-occurrence of Fusarium verticillioides and Aspergillus flavus, along with their mycotoxins, in European maize. In this study, the expression profiles of two pathogenesis-related (PR) genes and four mycotoxin biosynthetic genes, FUM1 and FUM13, fumonisin pathway, and aflR and aflD, aflatoxin pathway, as well as mycotoxin production, were examined in kernels and in artificial medium after a single inoculation with F. verticillioides or A. flavus or with the two fungi in combination. Different temperature regimes (20, 25 and 30 °C) over a time-course of 21 days were also considered. In maize kernels, PR genes showed the strongest induction at 25 °C in the earlier days post inoculation (dpi)with both fungi inoculated singularly. A similar behaviour was maintained with fungi co-occurrence, but with enhanced defence response at 9 dpi under 20 °C. Regarding FUM genes, in the kernels inoculated with F. verticillioides the maximal transcript levels occurred at 6 dpi at 25 °C. At this temperature regime, expression values decreased with the co-occurrence of A. flavus, where the highest gene induction was detected at 20 °C. Similar results were observed in fungi grown in vitro, whilst A. flavus presence determined lower levels of expression along the entire time-course. As concerns afl genes, considering both A. flavus alone and in combination, the most elevated transcript accumulation occurred at 30 °C during all time-course both in infected kernels and in fungi grown in vitro. Regarding mycotoxin production, no significant differences were found among temperatures for kernel contamination, whereas in vitro the highest production was registered at 25 °C for aflatoxin B1 and at 20 °C for fumonisins in the case of single inoculation. In fungal co-occurrence, both mycotoxins resulted reduced at all the temperatures considered compared to the amount produced with single inoculation.

Updates on the Functions and Molecular Mechanisms of the Genes Involved in Aspergillus flavus Development and Biosynthesis of Aflatoxins

Aspergillus flavus (A. flavus) is a ubiquitous and opportunistic fungal pathogen that causes invasive and non-invasive aspergillosis in humans and animals. This fungus is also capable of infecting a large number of agriculture crops (e.g., peanuts, maze, cotton seeds, rice, etc.), causing economic losses and posing serious food-safety concerns when these crops are contaminated with aflatoxins, the most potent naturally occurring carcinogens. In particular, A. flavus and aflatoxins are intensely studied, and they continue to receive considerable attention due to their detrimental effects on humans, animals, and crops. Although several studies have been published focusing on the biosynthesis of the aforementioned secondary metabolites, some of the molecular mechanisms (e.g., posttranslational modifications, transcription factors, transcriptome, proteomics, metabolomics and transcriptome, etc.) involved in the fungal development and aflatoxin biosynthesis in A. flavus are still not fully understood. In this study, a review of the recently published studies on the function of the genes and the molecular mechanisms involved in development of A. flavus and the production of its secondary metabolites is presented. It is hoped that the information provided in this review will help readers to develop effective strategies to reduce A. flavus infection and aflatoxin production.

Effect of Photosensitization Mediated by Curcumin on Carotenoid and Aflatoxin Content in Different Maize Varieties

Mycotoxins are naturally occurring toxins produced by certain types of fungi that contaminate food and feed, posing serious health risks to human and livestock. This study evaluated the combination of blue light with curcumin to inactivate Aspergillus flavus spores, its effect on aflatoxin B1 (AFB1) production and maintaining carotenoid content in three maize varieties. The study was first conducted in vitro, and the spore suspensions (104 CFU·mL−1) were treated with four curcumin concentrations (25 and 50 µM in ethanol, 1000 and 1250 µM in propylene glycol) and illuminated at different light doses from 0 to 130.3 J·cm−2. The photoinactivation efficiency was light-dose dependent with the highest photoinactivation of 2.3 log CFU·mL−1 achieved using 1000 µM curcumin at 104.2 J·cm−2. Scanning electron microscopy revealed cell wall deformations as well as less density in photosensitized cells. Photosensitization of maize kernels gave rise to a complete reduction in the viability of A. flavus and therefore inhibition of AFB1 production, while no significant (p > 0.05) effect was observed using either light or curcumin. Moreover, photosensitization did not affect the carotenoids in all the studied maize varieties. The results suggest that photosensitization is a green alternative preservation technique to decontaminate maize kernels and reduce consumer exposure to AFB1 without any effect on carotenoid content.

The Regulatory Mechanism of Water Activities on Aflatoxins Biosynthesis and Conidia Development, and Transcription Factor AtfB Is Involved in This Regulation

Peanuts are frequently infected by Aspergillus strains and then contaminated by aflatoxins (AF), which brings out economic losses and health risks. AF production is affected by diverse environmental factors, especially water activity (a_w). In this study, A. flavus was inoculated into peanuts with different a_w (0.90, 0.95, and 0.99). Both AFB₁ yield and conidia production showed the highest level in a_w 0.90 treatment. Transcriptional level analyses indicated that AF biosynthesis genes, especially the middle- and later-stage genes, were significantly up-regulated in a_w 0.90 than a_w 0.95 and 0.99. AtfB could be the pivotal regulator response to a_w variations, and could further regulate downstream genes, especially AF biosynthesis genes. The expressions of conidia genes and relevant regulators were also more up-regulated at a_w 0.90 than a_w 0.95 and 0.99, suggesting that the relative lower a_w could increase A. flavus conidia development. Furthermore, transcription factors involved in sexual development and nitrogen metabolism were also modulated by different a_w. This research partly clarified the regulatory mechanism of a_w on AF biosynthesis and A. flavus development and it would supply some advice for AF prevention in food storage.

Brazilian Coffee Production and the Future Microbiome and Mycotoxin Profile Considering the Climate Change Scenario

Brazil holds a series of favorable climatic conditions for agricultural production including the hours and intensity of sunlight, the availability of agricultural land and water resources, as well as diverse climates, soils and biomes. Amidst such diversity, Brazilian coffee producers have obtained various standards of qualities and aromas, between the arabica and robusta species, which each present a wide variety of lineages. However, temperatures in coffee producing municipalities in Brazil have increased by about 0.25 °C per decade and annual precipitation has decreased. Therefore, the agricultural sector may face serious challenges in the upcoming decades due to crop sensitivity to water shortages and thermal stress. Furthermore, higher temperatures may reduce the quality of the culture and increase pressure from pests and diseases, reducing worldwide agricultural production. The impacts of climate change directly affect the coffee microbiota. Within the climate change scenario, aflatoxins, which are more toxic than OTA, may become dominant, promoting greater food insecurity surrounding coffee production. Thus, closer attention on the part of authorities is fundamental to stimulate replacement of areas that are apt for coffee production, in line with changes in climate zoning, in order to avoid scarcity of coffee in the world market.

Mycobiome analysis for distinguishing the geographical origins of sesame seeds

Sesame (Sesamum indicum) is one of the most widely cultivated crops in Asia and Africa. The identification of the geographical origins of sesame seeds is important for the detection of fraudulent samples. This study was conducted to build a prediction model and suggest potential biomarkers for distinguishing the geographical origins of sesame seeds using mycobiome (fungal microbiome) analysis coupled with multivariate statistical analysis. Sesame seeds were collected from 25 cities in Korea, six cities in China, and five sites in other countries (Ethiopia, India, Nigeria, and Pakistan). According to the expression of fungal internal transcribed spacer (ITS) sequences in sesame seeds, 21 fungal genera were identified in sesame seeds from various countries. The optimal partial least squares-discriminant analysis model was established by applying two components with unit variance scaling. Based on seven-fold cross validation, the predictive model had 94.4% (Korea vs. China/other countries), 91.7% (China vs. Korea/other countries), and 88.9% (other countries vs. Korea/China) accuracy in determining the geographical origins of sesame seeds. Alternaria, Aspergillus, and Macrophomina were suggested as the potential fungal genera to differentiate the geographical origins of sesame seeds. This study demonstrated that mycobiome analysis could be used as a complementary method for distinguishing the geographical origins of raw sesame seeds.

Fungal G-Protein-Coupled Receptors: A Promising Mediator of the Impact of Extracellular Signals on Biosynthesis of Ochratoxin A

G-protein-coupled receptors (GPCRs) are transmembrane receptors involved in transducing signals from the external environment inside the cell, which enables fungi to coordinate cell transport, metabolism, and growth to promote their survival, reproduction, and virulence. There are 14 classes of GPCRs in fungi involved in sensing various ligands. In this paper, the synthesis of mycotoxins that are GPCR-mediated is discussed with respect to ligands, environmental stimuli, and intra-/interspecific communication. Despite their apparent importance in fungal biology, very little is known about the role of ochratoxin A (OTA) biosynthesis by Aspergillus ochraceus and the ligands that are involved. Fortunately, increasing evidence shows that the GPCR that involves the AF/ST (sterigmatocystin) pathway in fungi belongs to the same genus. Therefore, we speculate that GPCRs play an important role in a variety of environmental signals and downstream pathways in OTA biosynthesis. The verification of this inference will result in a more controllable GPCR target for control of fungal contamination in the future.

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.

Toxigenic Fungi and Mycotoxins in a Climate Change Scenario: Ecology, Genomics, Distribution, Prediction and Prevention of the Risk

Toxigenic fungi and mycotoxins are very common in food crops, with noticeable differences in their host specificity in terms of pathogenicity and toxin contamination. In addition, such crops may be infected with mixtures of mycotoxigenic fungi, resulting in multi-mycotoxin contamination. Climate represents the key factor in driving the fungal community structure and mycotoxin contamination levels pre- and post-harvest. Thus, there is significant interest in understanding the impact of interacting climate change-related abiotic factors (especially increased temperature, elevated CO2 and extremes in water availability) on the relative risks of mycotoxin contamination and impacts on food safety and security. We have thus examined the available information from the last decade on relative risks of mycotoxin contamination under future climate change scenarios and identified the gaps in knowledge. This has included the available scientific information on the ecology, genomics, distribution of toxigenic fungi and intervention strategies for mycotoxin control worldwide. In addition, some suggestions for prediction and prevention of mycotoxin risks are summarized together with future perspectives and research needs for a better understanding of the impacts of climate change scenarios.

Inhibitory effect ofEnterobacter cloacae 3J1EC onAspergillus flavus 3.4408 growth and aflatoxin production

Aspergillus flavus can easily infect major agricultural products and produce aflatoxin. In this study, we investigated the effect of the biocontrol bacteriumEnterobacter cloacae 3J1EC on the growth ofA. flavus strain 3.4408. The biocontrol bacterium played a key role in preventing infection byA. flavus. E. cloacae 3J1EC was found to inhibit the growth ofA. flavus 3.4408 mycelial pellets and reduce the production of aflatoxin by 96.9%. We found differential expression between the control and the treatment groups in the transcriptome ofA. flavus 3.4408. Gene ontology (GO) analysis indicated thatE. cloacae 3J1EC induced the down-regulated expression of cellular component and molecular function, while its effects on the up-regulated expression indicated the relationship of biological process and molecular function. Thus, these results suggest thatE. cloacae 3J1EC decreased aflatoxin production via down-regulated gene expression in terms of aflatoxin biosynthesis. In summary,E. cloacae 3J1EC can be employed as an alternative for the biological control ofA. flavus 3.4408.

Recent progress of the effect of environmental factors on Aspergillus flavus growth and aflatoxins production on foods

The contamination of Aspergillus flavus and subsequent aflatoxins (AFs) has been considered as one of the most serious food safety problems due to their acute and chronic adverse effects on humans and animals. This review collects the available information from recent years on the effect of the major environmental factors such as water activity (aw), temperature, CO2, and pH on the fungal growth, the expression of AFs-related genes, and AFs production by A. flavus on foods. In particular, the relationship between the relative expression of key regulatory (aflR and aflS) and structural genes (aflD, aflO, aflQ, etc.) and AFs production under different environmental conditions are collected and discussed. The information collected in this review can be used to design control strategies of A. flavus and AFs contamination in practical applications, primarily during storage and processing. These data suggest that integrating various post-harvest methods with synergistic functions may be more efficient for the control of A. flavus growth and AFs production, although the individual environmental factors alone have an impact.

Ethanol Inhibits Aflatoxin B1 Biosynthesis in Aspergillus flavus by Up-Regulating Oxidative Stress-Related Genes

As the most carcinogenic, toxic, and economically costly mycotoxins, aflatoxin B₁ (AFB₁) is primarily biosynthesized by Aspergillus flavus and Aspergillus parasiticus. Aflatoxin biosynthesis is related to oxidative stress and functions as a second line of defense from excessive reactive oxygen species. Here, we find that ethanol can inhibit fungal growth and AFB₁ production by A. flavus in a dose-dependent manner. Then, the ethanol’s molecular mechanism of action on AFB₁ biosynthesis was revealed using a comparative transcriptomic analysis. RNA-Seq data indicated that all the genes except for aflC in the aflatoxin gene cluster were down-regulated by 3.5% ethanol. The drastic repression of aflatoxin structural genes including the complete inhibition of aflK and aflLa may be correlated with the down-regulation of the transcription regulator genes aflR and aflS in the cluster. This may be due to the repression of several global regulator genes and the subsequent overexpression of some oxidative stress-related genes. The suppression of several key aflatoxin genes including aflR, aflD, aflM, and aflP may also be associated with the decreased expression of the global regulator gene veA. In particular, ethanol exposure caused the decreased expression of stress response transcription factor srrA and the overexpression of bZIP transcription factor ap-1, C₂H₂ transcription factors msnA and mtfA, together with the enhanced levels of anti-oxidant enzymatic genes including Cat, Cat1, Cat2, CatA, and Cu, Zn superoxide dismutase gene sod1. Taken together, these RNA-Seq data strongly suggest that ethanol inhibits AFB₁ biosynthesis by A. flavus via enhancing fungal oxidative stress response. In conclusion, this study served to reveal the anti-aflatoxigenic mechanisms of ethanol in A. flavus and to provide solid evidence for its use in controlling AFB₁ contamination.

Aflatoxins in Uganda: An Encyclopedic Review of the Etiology, Epidemiology, Detection, Quantification, Exposure Assessment, Reduction, and Control

Uganda is an agrarian country where farming employs more than 60% of the population. Aflatoxins remain a scourge in the country, unprecedentedly reducing the nutritional and economic value of agricultural foods. This review was sought to synthetize the country's major findings in relation to the mycotoxins' etiology, epidemiology, detection, quantification, exposure assessment, control, and reduction in different matrices. Electronic results indicate that aflatoxins in Uganda are produced by Aspergillus flavus and A. parasiticus and have been reported in maize, sorghum, sesame, beans, sunflower, millet, peanuts, and cassava. The causes and proliferation of aflatoxigenic contamination of Ugandan foods have been largely due to poor pre-, peri-, and postharvest activities, poor government legislation, lack of awareness, and low levels of education among farmers, entrepreneurs, and consumers on this plague. Little diet diversity has exacerbated the risk of exposure to aflatoxins in Uganda because most of the staple foods are aflatoxin-prone. On the detection and control, these are still marginal, though some devoted scholars have devised and validated a sensitive portable device for on-site aflatoxin detection in maize and shown that starter cultures used for making some cereal-based beverages have the potential to bind aflatoxins. More efforts should be geared towards awareness creation and vaccination against hepatitis B and hepatitis A to reduce the risk of development of liver cancer among the populace.

Effect of temperature on growth, gene expression, and aflatoxin production by Aspergillus nomius isolated from Brazil nuts

Aspergillus nomius is a potent producer of aflatoxins B and G and is one of the most common species of fungi found in Brazil nuts. Temperature is considered a major abiotic factor that influences fungal colonization and aflatoxin production in nuts during pre- and post-harvest. Therefore, assessment of the response of aflatoxigenic species to different temperatures is important to add information about the understanding of aflatoxin production by Aspergillus nomius and may help in the development of new strategies to prevent aflatoxin contamination. The aim of this study was to evaluate the effect of temperature (25, 30, and 35 °C) on the radial growth, aflatoxin production (B and G), and aflatoxin gene expression of seven A. nomius strains isolated from Brazil nuts. The optimal temperature for growth was 30 °C and was also the best condition for the expression of the aflR, aflD, and aflQ genes. However, maximum production of aflatoxins B and G occurred at 25 °C. Interestingly, high expression of the structural gene aflQ was observed in the maximum aflatoxin production condition (25 °C). The present study demonstrates that temperature may influence aflatoxin production by A. nomius. The combination of molecular and physiological data aids the understanding of the aflatoxigenic species response to different temperatures and can assist in predicting the driving environmental factors that influence aflatoxin contamination of Brazil nuts.

Dual Transcriptional Profile of Aspergillus flavus during Co-Culture with Listeria monocytogenes and Aflatoxin B1 Production: A Pathogen-Pathogen Interaction

The objective of this study was to investigate the effect of growth temperature and co-culture of Aspergillus flavus with Listeria monocytogenes on the production of Aflatoxin B1 (AFB1) and the transcriptional profile of associated regulatory and biosynthetic genes. The transcription of virulence- and homeostasis-associated genes of L. monocytogenes was also assessed. For this purpose, mono- and co-cultures of L. monocytogenes strain LQC 15257 and A. flavus strain 18.4 were inoculated into Malt Extract broth and allowed to grow for seven days at 25 °C and 30 °C. AFB1 quantification was performed by HPLC analysis and gene expression assessment by RT-qPCR. AFB1 production was lower at 30 °C compared to 25 °C during monoculture and also lower during co-cultures at both temperatures. This was accompanied by downregulation of aflM, aflR, aflP, and aflS during monoculture and aflM and aflS during co-culture at 30 °C. On the other hand, transcription of prfA, plcA, plcB, inlA, inlB, inlJ, murE, accA, acpP, as well as fapR, was not affected. sigB gene was downregulated after co-culture with the fungus at 25 °C and hly was downregulated after monoculture at 30 °C compared to 25 °C. In this work, the molecular interactions between A. flavus and L. monocytogenes were studied for the first time, offering a novel insight into their co-occurrence. Monitoring of their toxigenic and virulence potential at the molecular level revealed a complex dynamic in natural ecosystems.

The birth, evolution and death of metabolic gene clusters in fungi

Fungi contain a remarkable diversity of both primary and secondary metabolic pathways involved in ecologically specialized or accessory functions. Genes in these pathways are frequently physically linked on fungal chromosomes, forming metabolic gene clusters (MGCs). In this Review, we describe the diversity in the structure and content of fungal MGCs, their population-level and species-level variation, the evolutionary mechanisms that underlie their formation, maintenance and decay, and their ecological and evolutionary impact on fungal populations. We also discuss MGCs from other eukaryotes and the reasons for their preponderance in fungi. Improved knowledge of the evolutionary life cycle of MGCs will advance our understanding of the ecology of specialized metabolism and of the interplay between the lifestyle of an organism and genome architecture.

Differential regulation of mycelial growth and aflatoxin biosynthesis by Aspergillus flavus under different temperatures as revealed by strand-specific RNA-Seq

Although several regulatory pathways have been reported for Aspergillus flavus, the regulation of aflatoxin production and mycelial growth under different temperatures remains unclear. In this study, A. flavus differentially expressed genes (DEGs) and regulatory pathways were analyzed under three temperatures, by strand‐specific RNA‐Seq. Results show that a total of 2,428 and 1,474 DEGs were identified in fungal mycelia cultured at 20°C and 37°C, respectively, as compared with the control (28°C). Approximately ~ 79% of DEGs in the 37°C samples were up‐regulated genes, while ~ 63% of DEGs in the 20°C samples were down‐regulated genes. Most of the DEG pathways enriched by lower temperatures differed from those enriched by higher temperatures, while only a small portion of the pathways were shared by A. flavus grown under different temperatures. Aflatoxin biosynthesis, Butanoate metabolism, oxidation–reduction process, and benzene‐containing compound metabolic process were the shared down‐regulated pathways, while steroid biosynthesis, oxidoreductase activity, cellular protein modification process, DNA binding, protein complex were the shared up‐regulated pathways between lower and higher temperatures. The shared genes and pathways are the key regulatory candidates for aflatoxin biosynthesis with changes of temperature. In addition, the identification of both up‐regulated and down‐regulated genes provides a useful gene set for further investigation of the aflatoxin biosynthesis among Aspergillus.

Gene expression of Aspergillus flavus strains on a cheese model system to control aflatoxin production

The expression of genes associated with aflatoxin biosynthesis by different Aspergillus flavus strains growing on a cheese model system has not been studied. To control aflatoxin biosynthesis, it would be useful to understand the changes in gene expression during cheesemaking and relate those changes to toxin production. The objective of this study was to evaluate the effects of pH, water activity, and temperature on the expression of 2 regulatory genes (aflR and aflS) and 1 structural gene (aflP) involved in aflatoxin biosynthesis, using 3 aflatoxigenic A. flavus strains growing on a cheese-based medium and reverse-transcription real-time PCR. The gene expression patterns were influenced by A. flavus strain and environmental conditions. The structural gene aflP and the regulatory genes aflR and aflS showed similar expression patterns in each A. flavus strain, but we also observed inter-strain differences. We observed the highest expression levels at 6 and 9 d of incubation by A. flavus strains CQ8 and CQ103, and saw a decrease in the days following. Strain CQ7 showed the lowest expression of these genes. We observed the highest expression levels of these genes at pH 5.5, water activity 0.95, and 20 to 25°C; strain CQ103 showed a different pattern for the aflS gene, with maximum expression at pH 6.0 on d 6 of incubation. For the 3 strains, we found a strong correlation between the relative expression of the aflR and aflS genes and the concentration of aflatoxins under conditions that simulated cheese ripening. Control strategies to avoid aflatoxin contamination during cheesemaking could use the detection of regulatory gene expression.

Interaction of water activity and temperature on the growth, gene expression and aflatoxin production by Aspergillus flavus on paddy and polished rice

Water activity (a_w) and temperature are two pivotal environmental factors affecting Aspergillus flavus growth and aflatoxin production. Here, we found that AFB₁ production on polished rice can occur over a wider range of temperature × a_w levels than that on paddies. For fungal growth on polished rice, the optimum conditions were a_w 0.92-0.96 and 28-37 °C. The maximum amounts of AFB₁ on polished rice was observed at 33 °C and a_w 0.96. Compared to 33 °C, all tested genes of A. flavus on polished rice were significantly up-regulated at 25 °C under a_w 0.96. The late structural genes of pathway were significantly down-regulated at 37 °C under a_w 0.96, although aflR and aflS and most of early structural genes were up-regulated. Compared to a_w 0.96, most of pathway genes were significantly down-regulated at a_w 0.90 and 0.99 under 33 °C, although two regulatory genes were up-regulated at a_w 0.90.

Docking analysis of hexanoic acid and quercetin with seven domains of polyketide synthase A provided insight into quercetin-mediated aflatoxin biosynthesis inhibition in Aspergillus flavus

Studies on phytochemicals as anti-aflatoxigenic agents have gained importance including quercetin. Thus, to understand the molecular mechanism behind inhibition of aflatoxin biosynthesis by quercetin, interaction study with polyketide synthase A (PksA) of Aspergillus flavus was undertaken. The 3D structure of seven domains of PksA was modeled using SWISS-MODEL server and docking studies were performed by Autodock tools-1.5.6. Docking energies of both the ligands (quercetin and hexanoic acid) were compared with each of the domains of PksA enzyme. Binding energy for quercetin was lesser that ranged from - 7.1 to - 5.25 kcal/mol in comparison to hexanoic acid (- 4.74 to - 3.54 kcal/mol). LigPlot analysis showed the formation of 12 H bonds in case of quercetin and 8 H bonds in hexanoic acid. During an interaction with acyltransferase domain, both ligands showed H bond formation at Arg63 position. Also, in product template domain, quercetin creates four H bonds in comparison to one in hexanoic acid. Our quantitative RT-PCR analysis of genes from aflatoxin biosynthesis showed downregulation of pksA, aflD, aflR, aflP and aflS at 24 h time point in comparison to 7 h in quercetin-treated A. flavus. Overall results revealed that quercetin exhibited the highest level of binding potential (more number of H bonds) with PksA domain in comparison to hexanoic acid; thus, quercetin possibly inhibits via competitively binding to the domains of polyketide synthase, a key enzyme of aflatoxin biosynthetic pathway. Further, we propose that key enzymes from aflatoxin biosynthetic pathway in aflatoxin-producing Aspergilli could be explored further using other phytochemicals as inhibitors.

Ethanol Inhibits Aflatoxin B1 Biosynthesis in Aspergillus flavus by Up-Regulating Oxidative Stress-Related Genes

As the most carcinogenic, toxic, and economically costly mycotoxins, aflatoxin B₁ (AFB₁) is primarily biosynthesized by Aspergillus flavus and Aspergillus parasiticus. Aflatoxin biosynthesis is related to oxidative stress and functions as a second line of defense from excessive reactive oxygen species. Here, we find that ethanol can inhibit fungal growth and AFB₁ production by A. flavus in a dose-dependent manner. Then, the ethanol's molecular mechanism of action on AFB₁ biosynthesis was revealed using a comparative transcriptomic analysis. RNA-Seq data indicated that all the genes except for aflC in the aflatoxin gene cluster were down-regulated by 3.5% ethanol. The drastic repression of aflatoxin structural genes including the complete inhibition of aflK and aflLa may be correlated with the down-regulation of the transcription regulator genes aflR and aflS in the cluster. This may be due to the repression of several global regulator genes and the subsequent overexpression of some oxidative stress-related genes. The suppression of several key aflatoxin genes including aflR, aflD, aflM, and aflP may also be associated with the decreased expression of the global regulator gene veA. In particular, ethanol exposure caused the decreased expression of stress response transcription factor srrA and the overexpression of bZIP transcription factor ap-1, C₂H₂ transcription factors msnA and mtfA, together with the enhanced levels of anti-oxidant enzymatic genes including Cat, Cat1, Cat2, CatA, and Cu, Zn superoxide dismutase gene sod1. Taken together, these RNA-Seq data strongly suggest that ethanol inhibits AFB₁ biosynthesis by A. flavus via enhancing fungal oxidative stress response. In conclusion, this study served to reveal the anti-aflatoxigenic mechanisms of ethanol in A. flavus and to provide solid evidence for its use in controlling AFB₁ contamination.

MGL_3741 gene contributes to pathogenicity of Malassezia globosa in pityriasis versicolor

Dihydroxyacid dehydratase (DHAD) is a key enzyme in biosynthetic pathway of isoleucine and valine. This pathway is absent in human but exists in various organisms such as fungi. Using RNA-seq analysis in this study, we identified MGL_3741gene which encodes DHAD protein in Malassezia globosa (M. globosa). Furthermore, we found that mentioned gene is homologous to the Ustilago maydis, Saccharomyces cerevisiae, Aspergillus flavus, and Aspergillus fumigatus ILV3P. For understanding the probable role of this gene in pathogenicity of M. globosa, we applied Real-time PCR to investigate the differentially expressed of the MGL_3741 gene in healthy and pathogenic states. Our results indicate a significant difference between two mentioned stats. These results revealed that ILV3-like gene in M. globosa can be related to the pathogenicity of this yeast.

Effect of Temperature and Relative Humidity on Growth of Aspergillus and Penicillium spp. and Biocontrol Activity of Pseudomonas protegens AS15 against Aflatoxigenic Aspergillus flavus in Stored Rice Grains

In this study, we evaluated the effect of different temperatures (10, 20, 30, and 40 °C) and relative humidities (RHs; 12, 44, 76, and 98%) on populations of predominant grain fungi (Aspergillus candidus, Aspergillus flavus, Aspergillus fumigatus, Penicillium fellutanum, and Penicillium islandicum) and the biocontrol activity of Pseudomonas protegens AS15 against aflatoxigenic A. flavus KCCM 60330 in stored rice. Populations of all the tested fungi in inoculated rice grains were significantly enhanced by both increased temperature and RH. Multiple linear regression analysis revealed that one unit increase of temperature resulted in greater effects than that of RH on fungal populations. When rice grains were treated with P. protegens AS15 prior to inoculation with A. flavus KCCM 60330, fungal populations and aflatoxin production in the inoculated grains were significantly reduced compared with the grains untreated with strain AS15 regardless of temperature and RH (except 12% RH for fungal population). In addition, bacterial populations in grains were significantly enhanced with increasing temperature and RH, regardless of bacterial treatment. Higher bacterial populations were detected in biocontrol strain-treated grains than in untreated control grains. To our knowledge, this is the first report showing consistent biocontrol activity of P. protegens against A. flavus population and aflatoxin production in stored rice grains under various environmental conditions of temperature and RH.

RNA-Seq Profile Reveals Th-1 and Th-17-Type of Immune Responses in Mice Infected Systemically with Aspergillus fumigatus

With the increasing numbers of immunocompromised hosts, Aspergillus fumigatus emerges as a lethal opportunistic fungal pathogen. Understanding innate and acquired immunity responses of the host is important for a better therapeutic strategy to deal with aspergillosis patients. To determine the transcriptome in the kidneys in aspergillosis, we employed RNA-Seq to obtain single 76-base reads of whole-genome transcripts of murine kidneys on a temporal basis (days 0; uninfected, 1, 2, 3 and 8) during invasive aspergillosis. A total of 6284 transcripts were downregulated, and 5602 were upregulated compared to baseline expression. Gene ontology enrichment analysis identified genes involved in innate and adaptive immune response, as well as iron binding and homeostasis, among others. Our results showed activation of pathogen recognition receptors, e.g., β-defensins, C-type lectins (e.g., dectin-1), Toll-like receptors (TLR-2, TLR-3, TLR-8, TLR-9 and TLR-13), as well as Ptx-3 and C-reactive protein among the soluble receptors. Upregulated transcripts encoding various differentiating cytokines and effector proinflammatory cytokines, as well as those encoding for chemokines and chemokine receptors, revealed Th-1 and Th-17-type immune responses. These studies form a basic dataset for experimental prioritization, including other target organs, to determine the global response of the host against Aspergillus infection.

Large-Scale Comparative Analysis of Eugenol-Induced/Repressed Genes Expression in Aspergillus flavus Using RNA-seq

Aflatoxin B₁ (AFB₁), which is mainly produced by Aspergillus flavus and Aspergillus parasiticus, is the most toxic and hepatocarcinogenic polyketide known. Chemical fungicides are currently utilized to reduce this fungal contaminant, but they are potentially harmful to human health and the environment. Therefore, natural anti-aflatoxigenic products are used as sustainable alternatives to control food and feed contamination. For example, eugenol, presents in many essential oils, has been identified as an aflatoxin inhibitor. However, its exact mechanism of inhibition is yet to be clarified. In this study, the anti-aflatoxigenic mechanism of eugenol in A. flavus was determined using a comparative transcriptomic approach. Twenty of twenty-nine genes in the aflatoxin biosynthetic pathway were down-regulated by eugenol. The most strongly down-regulated gene was aflMa, followed by aflI, aflJ, aflCa, aflH, aflNa, aflE, aflG, aflM, aflD, and aflP. However, the expression of the regulator gene aflR did not change significantly and the expression of aflS was slightly up-regulated. The down-regulation of the global regulator gene veA resulted in the up-regulation of srrA, and the down-regulation of ap-1 and mtfA. The early developmental regulator brlA was profoundly up-regulated in A. flavus after eugenol treatment. These results suggested a model in which eugenol improves fungal development by up-regulating the expression of brlA by the suppression of veA expression and inhibits aflatoxin production through the suppression of veA expression. Exposure to eugenol also caused dysregulated transcript levels of the G protein-coupled receptors (GPCRs) and oxylipins genes. A Gene Ontology analysis indicated that the genes that were highly responsive to eugenol were mainly enriched in RNA-binding functions, suggesting that post-transcriptional modification plays a pivotal role in aflatoxin biosynthesis. KEGG analysis showed that ribosome biogenesis was the most dysregulated pathway, suggesting that eugenol dysregulates ribosome biogenesis, which then interrupts the biosynthesis of Nor-1, Ver-1, and OmtA, and prevents aflatoxisomes performing their normal function in aflatoxin production. In conclusion, our results indicated that eugenol inhibited AFB₁ production by modulating the expression of structural genes in aflatoxin pathway, fungal antioxidant status, post-transcriptional modifications and biosynthesis of backbone enzymes in A. flavus.

Growth medium and incubation temperature alter the Pseudogymnoascus destructans transcriptome: implications in identifying virulence factors

Pseudogymnoascus destructans is the causal agent of bat white-nose syndrome (WNS), which is devastating some North American bat populations. Previous transcriptome studies provided insight regarding the molecular mechanisms involved in WNS; however, it is unclear how different environmental parameters could influence pathogenicity. This information could be useful in developing management strategies to mitigate the negative impacts of P. destructans on bats. We cultured three P. destructans isolates from Atlantic Canada on two growth media (potato dextrose agar and Sabouraud dextrose agar) that differ in their nitrogen source, and at two separate incubation temperatures (4 C and 15 C) that approximate the temperature range of bat hibernacula during the winter and a temperature within its optimal mycelial growth range. We conducted RNA sequencing to determine transcript levels in each sample and performed differential gene expression (DGE) analyses to test the influence of growth medium and incubation temperature on gene expression. We also compared our in vitro results with previous RNA-sequencing data sets generated from P. destructans growing on the wings of a susceptible host, Myotis lucifugus. Our findings point to a critical role for substrate and incubation temperature in influencing the P. destructans transcriptome. DGE analyses suggested that growth medium plays a larger role than temperature in determining P. destructans gene expression and that although the psychrophilic fungus responds to different nitrogen sources, it may have evolved for continued growth at a broad range of low temperatures. Further, our data suggest that down-regulation of the RNA-interference pathway and increased fatty acid metabolism are involved in the P. destructans-bat interaction. Finally, we speculate that to reduce the activation of host defense responses, P. destructans minimizes changes in the expression of genes encoding secreted proteins during bat colonization.

Current Status of Mycotoxin Contamination of Food Commodities in Zimbabwe

Agricultural products, especially cereal grains, serve as staple foods in sub-Saharan Africa. However, climatic conditions in this region can lead to contamination of these commodities by moulds, with subsequent production of mycotoxins posing health risks to both humans and animals. There is limited documentation on the occurrence of mycotoxins in sub-Saharan African countries, leading to the exposure of their populations to a wide variety of mycotoxins through consumption of contaminated foods. This review aims at highlighting the current status of mycotoxin contamination of food products in Zimbabwe and recommended strategies of reducing this problem. Zimbabwe is one of the African countries with very little information with regards to mycotoxin contamination of its food commodities, both on the market and at household levels. Even though evidence of multitoxin occurrence in some food commodities such as maize and other staple foods exist, available published research focuses only on Aspergillus and Fusarium mycotoxins, namely aflatoxins, deoxynivalenol (DON), trichothecenes, fumonisins, and zearalenone (ZEA). Occurrence of mycotoxins in the food chain has been mainly associated with poor agricultural practices. Analysis of mycotoxins has been done mainly using chromatographic and immunological methods. Zimbabwe has adopted European standards, but the legislation is quite flexible, with testing for mycotoxin contamination in food commodities being done voluntarily or upon request. Therefore, the country needs to tighten its legislation as well as adopt stricter standards that will improve the food safety and security of the masses.

Aspergillus flavus Secondary Metabolites: More than Just Aflatoxins

Aspergillus flavus is best known for producing the family of potent carcinogenic secondary metabolites known as aflatoxins. However, this opportunistic plant and animal pathogen also produces numerous other secondary metabolites, many of which have also been shown to be toxic. While about forty of these secondary metabolites have been identified from A. flavus cultures, analysis of the genome has predicted the existence of at least 56 secondary metabolite gene clusters. Many of these gene clusters are not expressed during growth of the fungus on standard laboratory media. This presents researchers with a major challenge of devising novel strategies to manipulate the fungus and its genome so as to activate secondary metabolite gene expression and allow identification of associated cluster metabolites. In this review, we discuss the genetic, biochemical and bioinformatic methods that are being used to identify previously uncharacterized secondary metabolite gene clusters and their associated metabolites. It is important to identify as many of these compounds as possible to determine their bioactivity with respect to fungal development, survival, virulence and especially with respect to any potential synergistic toxic effects with aflatoxin.

Advances in molecular and genomic research to safeguard food and feed supply from aflatoxin contamination

Worldwide recognition that aflatoxin contamination of agricultural commodities by the fungus Aspergillus flavus is a global problem has significantly benefitted from global collaboration for understanding the contaminating fungus, as well as for developing and implementing solutions against the contamination. The effort to address this serious food and feed safety issue has led to a detailed understanding of the taxonomy, ecology, physiology, genomics and evolution of A. flavus, as well as strategies to reduce or control pre-harvest aflatoxin contamination, including (1) biological control, using atoxigenic aspergilli, (2) proteomic and genomic analyses for identifying resistance factors in maize as potential breeding markers to enable development of resistant maize lines, and (3) enhancing host-resistance by bioengineering of susceptible crops, such as cotton, maize, peanut and tree nuts. A post-harvest measure to prevent the occurrence of aflatoxin contamination in storage is also an important component for reducing exposure of populations worldwide to aflatoxins in food and feed supplies. The effect of environmental changes on aflatoxin contamination levels has recently become an important aspect for study to anticipate future contamination levels. The ability of A. flavus to produce dozens of secondary metabolites, in addition to aflatoxins, has created a new avenue of research for understanding the role these metabolites play in the survival and biodiversity of this fungus. The understanding of A. flavus, the aflatoxin contamination problem, and control measures to prevent the contamination has become a unique example for an integrated approach to safeguard global food and feed safety.

Proteome analysis of Aspergillus flavus isolate-specific responses to oxidative stress in relationship to aflatoxin production capability

Aspergillus flavus is an opportunistic pathogen of plants such as maize and peanut under conducive conditions such as drought stress resulting in significant aflatoxin production. Drought-associated oxidative stress also exacerbates aflatoxin production by A. flavus. The objectives of this study were to use proteomics to provide insights into the pathogen responses to H₂O₂-derived oxidative stress, and to identify potential biomarkers and targets for host resistance breeding. Three isolates, AF13, NRRL3357, and K54A with high, moderate, and no aflatoxin production, were cultured in medium supplemented with varying levels of H₂O₂, and examined using an iTRAQ (Isobaric Tags for Relative and Absolute Quantification) approach. Overall, 1,173 proteins were identified and 220 were differentially expressed (DEPs). Observed DEPs encompassed metabolic pathways including antioxidants, carbohydrates, pathogenicity, and secondary metabolism. Increased lytic enzyme, secondary metabolite, and developmental pathway expression in AF13 was correlated with oxidative stress tolerance, likely assisting in plant infection and microbial competition. Elevated expression of energy and cellular component production in NRRL3357 and K54A implies a focus on oxidative damage remediation. These trends explain isolate-to-isolate variation in oxidative stress tolerance and provide insights into mechanisms relevant to host plant interactions under drought stress allowing for more targeted efforts in host resistance research.

Isolation and characterization of Aspergillus flavus strains in China

Important staple foods (peanuts, maize and rice) are susceptible to contamination by aflatoxin (AF)-producing fungi such as Aspergillus flavus. The objective of this study was to explore non-aflatoxin-producing (atoxigenic) A. flavus strains as biocontrol agents for the control of AFs. In the current study, a total of 724 A. flavus strains were isolated from different regions of China. Polyphasic approaches were utilized for species identification. Non-aflatoxin and non-cyclopiazonic acid (CPA)-producing strains were further screened for aflatoxin B₁ (AFB₁) biosynthesis pathway gene clusters using a PCR assay. Strains lacking an amplicon for the regulatory gene aflR were then analyzed for the presence of the other 28 biosynthetic genes. Only 229 (32%) of the A. flavus strains were found to be atoxigenic. Smaller (S) sclerotial phenotypes were dominant (51%) compared to large (L, 34%) and non-sclerotial (NS, 15%) phenotypes. Among the atoxigenic strains, 24 strains were PCR-negative for the fas-1 and aflJ genes. Sixteen (67%) atoxigenic A. flavus strains were PCRnegative for 10 or more of the biosynthetic genes. Altogether, 18 new PCR product patterns were observed, indicating great diversity in the AFB₁ biosynthesis pathway. The current study demonstrates that many atoxigenic A. flavus strains can be isolated from different regions of China. In the future laboratory as well as field based studies are recommended to test these atoxigenic strains as biocontrol agents for aflatoxin contamination.