The effects of selected cotton-leaf volatiles on growth, development and aflatoxin production of Aspergillus parasiticus

Natural inhibitors: A sustainable way to combat aflatoxins

Among a few hundred mycotoxins, aflatoxins had always posed a major threat to the world. Apart from A. flavus, A. parasiticus, and A. nomius of Aspergillus genus, which are most toxin-producing strains, several fungal bodies including Fusarium, Penicillium, and Alternaria that can biosynthesis aflatoxins. Basically, there are four different types of aflatoxins (Aflatoxin B1 (AFB1), Aflatoxin B2 (AFB2), Aflatoxin G1 (AFG1), Aflatoxin G2 (AFG2)) are produced as secondary metabolites. There are certainly other types of aflatoxins found but they are the by-products of these toxins. The fungal agents generally infect the food crops during harvesting, storing, and/or transporting; making a heavy post-harvest as well as economic loss in both developed and developing countries. And while ingesting the crop products, these toxins get into the dietary system causing aflatoxicosis, liver cirrhosis, etc. Therefore, it is imperative to search for certain ways to control the spread of infections and/or production of these toxins which may also not harm the crop harvest. In this review, we are going to discuss some sustainable methods that can effectively control the spread of infection and inhibit the biosynthesis of aflatoxins.

Control of toxigenic Aspergillus spp. in dried figs by volatile organic compounds (VOCs) from antagonistic yeasts

Aspergillus flavus and Aspergillus niger are fungi which can contaminate dried figs before and after harvest and consequently produce aflatoxins (AFs) and ochratoxin A (OTA). Many approaches have been applied to minimise the growth of these filamentous fungi, mainly involving the use of synthetic fungicides which are limited due to their negative impact on human health and the environment. In this context, biocontrol is a recent approach that needs to be explored. This study evaluated the potential of three volatile organic compounds (VOCs), octanoic acid (OA), 2-phenylethyl acetate (2PEA) and furfuryl acetate (FA), produced by Hanseniaspora uvarum and Hanseniaspora opuntiae yeasts on the growth, germination, gene expression and production of AFs and OTA by A. flavus M144 and A. niger M185 on dried fig-based agar and the incidence rates in dried figs. Two of the three VOCs evaluated (2PEA and FA) effectively controlled A. flavus M144 and A. niger M185 by using at least amounts of 50 μL (715 μL/L in the headspace) for FA and 100 μL (1430 μL/L in the headspace) for 2PEA in dried figs. One of the mode of actions of both compounds consists in early repressing the expression of genes involved in the biosynthesis of AFs (aflR) and OTA (pks) of A. flavus and A. niger, respectively. The results of this study support the application of 2PEA and FA at the early post-harvest stages of dried figs to control mycotoxin accumulation.

Sisters in structure but different in character, some benzaldehyde and cinnamaldehyde derivatives differentially tune Aspergillus flavus secondary metabolism

Great are the expectations for a new generation of antimicrobials, and strenuous are the research efforts towards the exploration of diverse molecular scaffolds-possibly of natural origin - aimed at the synthesis of new compounds against the spread of hazardous fungi. Also high but winding are the paths leading to the definition of biological targets specifically fitting the drug's structural characteristics. The present study is addressed to inspect differential biological behaviours of cinnamaldehyde and benzaldehyde thiosemicarbazone scaffolds, exploiting the secondary metabolism of the mycotoxigenic phytopathogen Aspergillus flavus. Interestingly, owing to modifications on the parent chemical scaffold, some thiosemicarbazones displayed an increased specificity against one or more developmental processes (conidia germination, aflatoxin biosynthesis, sclerotia production) of A. flavus biology. Through the comparative analysis of results, the ligand-based screening strategy here described has allowed us to delineate which modifications are more promising for distinct purposes: from the control of mycotoxins contamination in food and feed commodities, to the environmental management of microbial pathogens, to the investigation of specific structure-activity features for new generation drug discovery.

The molecular mechanisms that underpin the biological benefits of full-spectrum cannabis extract in the treatment of neuropathic pain and inflammation

Cannabis has been shown to be beneficial in the treatment of pain and inflammatory diseases. The biological effect of cannabis is mainly attributed to two major cannabinoids, tetrahydrocannabinol and cannabidiol. In the majority of studies to-date, a purified tetrahydrocannabinol and cannabidiol alone or in combination have been extensively examined in many studies for the treatment of numerous disorders including pain and inflammation. However, few studies have investigated the biological benefits of full-spectrum cannabis plant extract. Given that cannabis is known to generate a large number of cannabinoids along with numerous other biologically relevant products including terpenes, studies involving purified tetrahydrocannabinol and/or cannabidiol do not consider the potential biological benefits of the full-spectrum cannabis extracts. This may be especially true in the case of cannabis as a potential treatment of pain and inflammation. Herein, we review the pre-clinical physiological and molecular mechanisms in biological systems that are affected by cannabis.

Navel Orange Peel Essential Oil To Control Food Spoilage Molds in Potato Slices

The aim of this study was to investigate the efficacy of navel orange, Citrus sinensis (L.) Osbeck, peel essential oil (NOPEO) for inhibiting spoilage fungi in potato slices. Sixteen different components accounting for 99.79% of the headspace components of NOPEO were identified by gas chromatography-mass spectrometry. d-Limonene was the major component of NOPEO. Antifungal activity of NOPEO was tested in vitro and in vivo against four foodborne fungi. A MIC of NOPEO against the four fungal species was 9.40 μL/mL_air. NOPEO provided about 74, 74, 73, and 69% protection against Aspergillus niger, Mucor wutungkiao, Penicillium funiculosum, and Rhizopus oryzae at 2.00 μL/mL_air concentration, respectively. NOPEO has been demonstrated to significantly improve the microbiological quality of potato slices.

Comparison of antimicrobial activities of natural essential oils and synthetic fragrances against selected environmental pathogens

Plant essential oils (EOs) are known to inhibit the growth of bacteria and fungi. Whether these antimicrobial effects are comparable to synthetic household products is less clear. Furthermore, limited research is available on the potential additive effect of blending EOs. In this investigation, a new EO blend containing orange, patchouli, peppermint, and clary sage was compared to its individual single oils and to three household products–air freshener, liquid soap, and body spray–for their ability to inhibit the growth of Staphylococcus aureus, Streptococcus pneumoniae, Pseudonomas aeruginosa, and Aspergillus brasiliensis in the disc-diffusion assay. The new EO blend significantly inhibited the growth of the four microorganisms. The zones of inhibition of new EO blend were greater than the air freshener and similar to the liquid soap and body spray, with the exception of Str. pneumoniae in which the body spray provided greater inhibitory zone. The new EO blend and the single oils, with the exception of peppermint, equally inhibited the growth of S. aureus and Str. pneumoniae suggesting no additive effect. P. aeruginosa and A. brasiliensis showed variable susceptibility to all EOs except for no susceptibility to orange and limonene. No difference was found between (−) and (+)-limonene; whereas, (+)-menthol showed greater effect than (−)-menthol. In conclusion, blending the EO of orange, patchouli, peppermint, and clary sage was beneficial in inhibiting the growth of S. aureus, Str. pneumoniae, P. aeruginosa, and A. brasiliensis providing a natural antimicrobial fragrance option over synthetics fragrances used in soaps, body sprays, and air fresheners.

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A new essential oil blend inhibited the growth of four common environmental pathogens.

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The antimicrobial activity of a natural essential oil blend was similar or better to three synthetic products.

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Peppermint essential oil was the main ingredient in the essential oil blend that provided the antimicrobial activity.

Aflatoxin-Exposure of Vibrio gazogenes as a Novel System for the Generation of Aflatoxin Synthesis Inhibitors

Aflatoxin is a mycotoxin and a secondary metabolite, and the most potent known liver carcinogen that contaminates several important crops, and represents a significant threat to public health and the economy. Available approaches reported thus far have been insufficient to eliminate this threat, and therefore provide the rational to explore novel methods for preventing aflatoxin accumulation in the environment. Many terrestrial plants and microbes that share ecological niches and encounter the aflatoxin producers have the ability to synthesize compounds that inhibit aflatoxin synthesis. However, reports of natural aflatoxin inhibitors from marine ecosystem components that do not share ecological niches with the aflatoxin producers are rare. Here, we show that a non-pathogenic marine bacterium, Vibrio gazogenes, when exposed to low non-toxic doses of aflatoxin B₁, demonstrates a shift in its metabolic output and synthesizes a metabolite fraction that inhibits aflatoxin synthesis without affecting hyphal growth in the model aflatoxin producer, Aspergillus parasiticus. The molecular mass of the predominant metabolite in this fraction was also different from the known prodigiosins, which are the known antifungal secondary metabolites synthesized by this Vibrio. Gene expression analyses using RT-PCR demonstrate that this metabolite fraction inhibits aflatoxin synthesis by down-regulating the expression of early-, middle-, and late- growth stage aflatoxin genes, the aflatoxin pathway regulator, aflR and one global regulator of secondary metabolism, laeA. Our study establishes a novel system for generation of aflatoxin synthesis inhibitors, and emphasizes the potential of the under-explored Vibrio’s silent genome for generating new modulators of fungal secondary metabolism.

Suppression of Aflatoxin Biosynthesis in Aspergillus flavus by 2-Phenylethanol Is Associated with Stimulated Growth and Decreased Degradation of Branched-Chain Amino Acids

The saprophytic soil fungus Aspergillus flavus infects crops and produces aflatoxin. Pichia anomala, which is a biocontrol yeast and produces the major volatile 2-phenylethanol (2-PE), is able to reduce growth of A. flavus and aflatoxin production when applied onto pistachio trees. High levels of 2-PE are lethal to A. flavus and other fungi. However, at low levels, the underlying mechanism of 2-PE to inhibit aflatoxin production remains unclear. In this study, we characterized the temporal transcriptome response of A. flavus to 2-PE at a subinhibitory level (1 µL/mL) using RNA-Seq technology and bioinformatics tools. The treatment during the entire 72 h experimental period resulted in 131 of the total A. flavus 13,485 genes to be significantly impacted, of which 82 genes exhibited decreased expression. They included those encoding conidiation proteins and involved in cyclopiazonic acid biosynthesis. All genes in the aflatoxin gene cluster were also significantly decreased during the first 48 h treatment. Gene Ontology (GO) analyses showed that biological processes with GO terms related to catabolism of propionate and branched-chain amino acids (valine, leucine and isoleucine) were significantly enriched in the down-regulated gene group, while those associated with ribosome biogenesis, translation, and biosynthesis of α-amino acids were over-represented among the up-regulated genes. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that metabolic pathways negatively impacted among the down-regulated genes parallel to those active at 30 °C, a condition conducive to aflatoxin biosynthesis. In contrast, metabolic pathways positively related to the up-regulated gene group resembled those at 37 °C, which favors rapid fungal growth and is inhibitory to aflatoxin biosynthesis. The results showed that 2-PE at a low level stimulated active growth of A. flavus but concomitantly rendered decreased activities in branched-chain amino acid degradation. Since secondary metabolism occurs after active growth has ceased, this growth stimulation resulted in suppression of expression of aflatoxin biosynthesis genes. On the other hand, increased activities in degradation pathways for branched-chain amino acids probably are required for the activation of the aflatoxin pathway by providing building blocks and energy regeneration. Metabolic flux in primary metabolism apparently has an important role in the expression of genes of secondary metabolism.

Deep sequencing analysis of transcriptomes in Aspergillus flavus in response to resveratrol

Background

Resveratrol has been reported as a natural phytoalexin that inhibits infection or the growth of certain fungi including Aspergillus flavus. Our previous research revealed that aflatoxin production in A. flavus was reduced in medium with resveratrol. To understand the molecular mechanism of the A. flavus response to resveratrol treatment, the high-throughput paired-end RNA-Seq was applied to analyze the transcriptomic profiles of A. flavus.

Results

In total, 366 and 87 genes of A. flavus were significantly up- and down- regulated, respectively, when the fungus was treated with resveratrol. Gene Ontology (GO) functional enrichment analysis revealed that 48 significantly differentially expressed genes were involved in 6 different terms. Most genes in the aflatoxin biosynthetic pathway genes cluster (#54) did not show a significant change when A. flavus was treated with resveratrol, but 23 of the 30 genes in the #54 cluster were down-regulated. The transcription of aflA and aflB was significantly suppressed under resveratrol treatment, resulting in an insufficient amount of the starter unit hexanoate for aflatoxin biosynthesis. In addition, resveratrol significantly increased the activity of antioxidative enzymes that destroy radicals, leading to decreased aflatoxin production. Moreover, stuA, fluG, flbC, and others genes involved in mycelial and conidial development were down-regulated, which disrupted the cell’s orderly differentiation and blocked conidia formation and mycelia development. The transcripts of laeA and veA were slightly inhibited by resveratrol, which may partly decrease aflatoxin production and depress conidia formation.

Conclusions

Resveratrol can affect the expression of A. flavus genes that are related to developmental and secondary metabolic processes, resulting in decreased aflatoxin production and conidia formation and could also cause abnormal mycelia development. These results provide insight into the transcriptome of A. flavus in response to resveratrol and a new clew for further study in regulation of aflatoxin biosynthesis in A. flavus.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0513-6) contains supplementary material, which is available to authorized users.

Effects of thiamine on growth, aflatoxin production, and aflr gene expression in A.parasiticus

Background and Purpose:

Mycotoxins are secondary fungal metabolites with a very high diversity that are produced by some species of Aspergillus which frequently leads to contaminate food and agricultural products. Recently, elimination of aflatoxin contamination in food and feed has been considered by scientists worldwide. Although, the antibacterial and antifungal effects of vitamins as natural compounds have been proven, the mechanism of vitamins effect on Aspergillus parasiticus growth and aflatoxin production is not yet clear. In this study, the effect of thiamine (vitamin B₁) was studied on Aspergillus parasiticus growth, aflatoxins production and the afIR gene expression.

Materials and Methods:

A standard strain of Aspergillus parasiticus was applied for performing antifungal susceptibility test in different concentrations of thiamine. Antifungal susceptibility test was performed according to CLSI M38-A2 document. The concentration of aflatoxin was determined by HPLC. Moreover, the quantitative changes in the aflR gene expression were analyzed by Real Time PCR method.

Results:

The minimum inhibitory concentration was yielded as > 500 mg/ml. However, HPLC analysis results showed that aflatoxin production reduced in samples treated with 500 mg/ml of thiamine. In addition, the level of afIR gene expression was significantly reduced after treating with 500 and 250 mg/ml of vitamin B₁.

Conclusion:

Based on the obtained results, thiamine could not inhibit the fungal growth completely. However, the rate of afIR gene expression and aflatoxin production was significantly reduced after fungal treating with thiamine. Consequently, using natural compounds such as vitamins may be regarded as potential antitoxic agent in food industry and the industries related to agriculture.

Current understanding on aflatoxin biosynthesis and future perspective in reducing aflatoxin contamination

Traditional molecular techniques have been used in research in discovering the genes and enzymes that are involved in aflatoxin formation and genetic regulation. We cloned most, if not all, of the aflatoxin pathway genes. A consensus gene cluster for aflatoxin biosynthesis was discovered in 2005. The factors that affect aflatoxin formation have been studied. In this report, the author summarized the current status of research progress and future possibilities that may be used for solving aflatoxin contamination.

Phytochemicals to suppress Fusarium head blight in wheat-chickpea rotation

Fusarium diseases cause major economic losses in wheat-based crop rotations. Volatile organic compounds (VOC) in wheat and rotation crops, such as chickpea, may negatively impact pathogenic Fusarium. Using the headspace GC-MS method, 16 VOC were found in greenhouse-grown wheat leaves: dimethylamine, 2-methyl-1-propanol, octanoic acid-ethyl ester, acetic acid, 2-ethyl-1-hexanol, nonanoic acid-ethyl ester, nonanol, N-ethyl-benzenamine, naphthalene, butylated hydroxytoluene, dimethoxy methane, phenol, 3-methyl-phenol, 3,4-dimethoxy-phenol, 2,4-bis (1,1-dimethyethyl)-phenol, and 1,4,7,10,13,16-hexaoxacyclooctadecane; and 10 VOC in field-grown chickpea leaves: ethanol, 1-penten-3-ol, 1-hexanol, cis-3-hexen-1-ol, trans-2-hexen-1-ol, trans-2-hexenal, 3-methyl-1-butanol, 3-hydroxy-2-butanone, 3-methyl-benzaldehyde and naphthalene. Also found was 1-penten-3-ol in chickpea roots and in the root nodules of two of the three cultivars tested. Chickpea VOC production pattern was related (P=0.023) to Ascochyta blight severity, suggesting that 1-penten-3-ol and cis-3-hexen-1-ol were induced by Ascochyta rabiei. Bioassays conducted in Petri plates established that chickpea-produced VOC used in isolation were generally more potent against Fusarium graminearum and Fusarium avenaceum than wheat-produced VOC, except for 2-ethyl-1-hexanol, which was rare in wheat and toxic to both Fusarium and tetraploid wheat. Whereas exposure to 1-penten-3-ol and 2-methyl-1-propanol could suppress radial growth by over 50% and octanoic acid-ethyl ester, nonanol, and nonanoic acid-ethyl ester had only weak effects, F. graminearum and F. avenaceum growth was completely inhibited by exposure to trans-2-hexenal, trans-2-hexen-1-ol, cis-3-hexen-1-ol, and 1-hexanol. Among these VOC, trans-2-hexenal and 1-hexanol protected wheat seedlings against F. avenaceum and F. graminearum, respectively, in a controlled condition experiment. Genetic variation in the production of 2-ethyl-1-hexanol, a potent VOC produced in low amount by wheat, suggests the possibility of selecting Fusarium resistance in wheat on the basis of leaf VOC concentration. Results also suggests that the level of Fusarium inoculum in chickpea-wheat rotation systems may be reduced by growing chickpea genotypes with high root and shoot levels of trans-2-hexen-1-ol and 1-hexanol.

Willow volatiles influence growth, development, and secondary metabolism in Aspergillus parasiticus

Aflatoxin is a mycotoxin and the most potent naturally occurring carcinogen in many animals. Aflatoxin contamination of food and feed crops causes a significant global burden on human and animal health. However, available methods to eliminate aflatoxin from food and feed are not fully effective. Our goal is to discover novel, efficient, and practical methods to control aflatoxin contamination in crops during storage. In the present study, we tested the effect of volatiles produced by willow (Salix acutifolia and Salix babylonica) and maple (Acer saccharinum) bark on fungal growth, development, and aflatoxin production by the fungus Aspergillus parasiticus, one economically important aflatoxin producer. S. acutifolia bark volatiles nearly eliminated aflatoxin accumulation (>90% reduction) by A. parasiticus grown on a minimal agar medium. The decrease in aflatoxin accumulation correlated with a twofold reduction in ver-1 (encodes a middle aflatoxin pathway enzyme) transcript level. Expression data also indicate that one histone H4 acetyltransferase, MYST3, may play a role in epigenetic control of aflatoxin gene transcription in response to volatile exposure. Volatiles derived from wood bark samples also increased fungal growth up to 20% and/or enhanced conidiospore development. Solid-phase microextraction-gas chromatographic-mass spectrometric analysis of bark samples identified sets of shared and unique volatile compounds that may mediate the observed regulatory effects on growth, development, and aflatoxin synthesis. This work provides an experimental basis for the use of willow industry by-products to control aflatoxin contamination in food and feed crops.

The lipid language of plant-fungal interactions

Lipid mediated cross-kingdom communication between hosts and pathogens is a rapidly emerging field in molecular plant-fungal interactions. Amidst our growing understanding of fungal and plant chemical cross-talk lies the distinct, yet little studied, role for a group of oxygenated lipids derived from polyunsaturated fatty acids, termed oxylipins. Endogenous fungal oxylipins are known for their roles in carrying out pathogenic strategies to successfully colonize their host, reproduce, and synthesize toxins. While plant oxylipins also have functions in reproduction and development, they are largely recognized as agents that facilitate resistance to pathogen attack. Here we review the composition and endogenous functions of oxylipins produced by both plants and fungi and introduce evidence which suggests that fungal pathogens exploit host oxylipins to facilitate their own virulence and pathogenic development. Specifically, we describe how fungi induce plant lipid metabolism to utilize plant oxylipins in order to promote G-protein-mediated regulation of sporulation and mycotoxin production in the fungus. The use of host-ligand mimicry (i.e. coronatine) to manipulate plant defense responses that benefit the fungus are also implicated.

Study of the Effect of Methyl Jasmonate Concentration on Aflatoxin B(1) Biosynthesis by Aspergillus parasiticus in Yeast Extract Sucrose Medium

Aflatoxin B(1) (AFB(1)) is a carcinogenic metabolite produced by certain Aspergillus species on agricultural commodities. AFB(1) biosynthesis is affected by jasmonic acid and also by its methylester (MeJA), a plant growth regulator derived from linoleic acid. This study reports the effect of MeJA on the growth of A. parasiticus and AFB(1) output in yeast extract sucrose (YES) medium when added at three different concentrations; namely, 10(-2) M, 10(-4) M, and 10(-6) M. AFB(1) determination was performed by immunoaffinity and HPLC. MeJA at 10(-4) and 10(-6) M concentrations had no significant effect on mycelial growth but did affect AFB(1) production after the 7th day of incubation; on the 12th day, AFB(1) production was increased by 212.7% and 141.6% compared to the control samples (addition of 10(-6) M and 10(-4) M MeJA, resp.). Treatment of A. parasiticus cultures with 10(-2) M MeJA inhibited mycelial growth and AFB(1) production as well. These results suggest that the effect of MeJA on AFB(1) biosynthesis by A. parasiticus depends on the MeJA concentration used.

Aspergillus volatiles regulate aflatoxin synthesis and asexual sporulation in Aspergillus parasiticus

Aspergillus parasiticus is one primary source of aflatoxin contamination in economically important crops. To prevent the potential health and economic impacts of aflatoxin contamination, our goal is to develop practical strategies to reduce aflatoxin synthesis on susceptible crops. One focus is to identify biological and environmental factors that regulate aflatoxin synthesis and to manipulate these factors to control aflatoxin biosynthesis in the field or during crop storage. In the current study, we analyzed the effects of aspergillus volatiles on growth, development, aflatoxin biosynthesis, and promoter activity in the filamentous fungus A. parasiticus. When colonies of Aspergillus nidulans and A. parasiticus were incubated in the same growth chamber, we observed a significant reduction in aflatoxin synthesis and asexual sporulation by A. parasiticus. Analysis of the headspace gases demonstrated that A. nidulans produced much larger quantities of 2-buten-1-ol (CA) and 2-ethyl-1-hexanol (EH) than A. parasiticus. In its pure form, EH inhibited growth and increased aflatoxin accumulation in A. parasiticus at all doses tested; EH also stimulated aflatoxin transcript accumulation. In contrast, CA exerted dose-dependent up-regulatory or down-regulatory effects on aflatoxin accumulation, conidiation, and aflatoxin transcript accumulation. Experiments with reporter strains carrying nor-1 promoter deletions and mutations suggested that the differential effects of CA were mediated through separate regulatory regions in the nor-1 promoter. The potential efficacy of CA as a tool for analysis of transcriptional regulation of aflatoxin biosynthesis is discussed. We also identify a novel, rapid, and reliable method to assess norsolorinic acid accumulation in solid culture using a Chroma Meter CR-300 apparatus.

Understanding the genetics of regulation of aflatoxin production and Aspergillus flavus development

Aflatoxins are polyketide-derived, toxic, and carcinogenic secondary metabolites produced primarily by two fungal species, Aspergillus flavus and A. parasiticus, on crops such as corn, peanuts, cottonseed, and treenuts. Regulatory guidelines issued by the U.S. Food and Drug Administration (FDA) prevent sale of commodities if contamination by these toxins exceeds certain levels. The biosynthesis of these toxins has been extensively studied. About 15 stable precursors have been identified. The genes involved in encoding the proteins required for the oxidative and regulatory steps in the biosynthesis are clustered in a 70 kb portion of chromosome 3 in the A. flavus genome. With the characterization of the gene cluster, new insights into the cellular processes that govern the genes involved in aflatoxin biosynthesis have been revealed, but the signaling processes that turn on aflatoxin biosynthesis during fungal contamination of crops are still not well understood. New molecular technologies, such as gene microarray analyses, quantitative polymerase chain reaction (PCR), and chromatin immunoprecipitation are being used to understand how physiological stress, environmental and soil conditions, receptivity of the plant, and fungal virulence lead to episodic outbreaks of aflatoxin contamination in certain commercially important crops. With this fundamental understanding, we will be better able to design improved non-aflatoxigenic biocompetitive Aspergillus strains and develop inhibitors of aflatoxin production (native to affected crops or otherwise) amenable to agricultural application for enhancing host-resistance against fungal invasion or toxin production. Comparisons of aflatoxin-producing species with other fungal species that retain some of the genes required for aflatoxin formation is expected to provide insight into the evolution of the aflatoxin gene cluster, and its role in fungal physiology. Therefore, information on how and why the fungus makes the toxin will be valuable for developing an effective and lasting strategy for control of aflatoxin contamination.

Ultrastructural evidences of growth inhibitory effects of a novel biocide, Akacid(plus), on an aflatoxigenic Aspergillus parasiticus

The effects of Akacid(plus), a novel member of guanidine-based polymeric compounds recently introduced as a potent inhibitor of fungal growth and aflatoxin biosynthesis were studied on Aspergillus parasiticus by transmission electron microscopy (TEM). The toxigenic fungus was cultured on yeast extract-sucrose broth in presence of serial two-fold concentrations of Akacid(plus) (1.5-96 microL/50 mL medium) for 96 h at 28 degrees C with shaking. Mycelial samples exposed to fungistatic concentrations of compound (1.5-48 microL) were processed for TEM. Corresponding to the growth inhibition, TEM observations revealed morphological anomalies in fungal compartments. The results demonstrated that Akacid(plus) targets the plasma membrane of the hyphae by its breaking down at variable intervals with the formation of small membrane-bound vesicles inside the cytoplasm, while no obvious damage was observed on the cell wall. A marked depletion of cytoplasmic contents of hyphae accompanied with lysis and disruption of membranes of major organelles such as nuclei, mitochondria and endoplasmic reticulum indicates that in high fungistatic concentrations, Akacid(plus) passes not only through the cell wall but also through the plasma membrane and then interact with membranous structures of the cytoplasmic organelles. Ultrastructural changes of fungal compartments exposed to Akacid(plus) in relation to the fungal growth and aflatoxin biosynthesis are discussed.

Real-time detection of common microbial volatile organic compounds from medically important fungi by Selected Ion Flow Tube-Mass Spectrometry (SIFT-MS)

We describe a new method, Selected Ion Flow Tube-Mass Spectrometry (SIFT-MS) for the rapid and sensitive real-time detection and quantification of volatile organic compounds from medically important fungi, grown on a range of laboratory media. SIFT-MS utilises the chemical ionisation reactions of mass-selected ions to characterise volatile organic compounds (VOCs) that are produced as metabolites from fungi. This technique has the distinct advantage over others in that it readily detects low molecular weight, reactive volatiles, and allows for real-time, quantitative monitoring. The fungi examined in this study were Aspergillus flavus, Aspergillus fumigatus, Candida albicans, Mucor racemosus, Fusarium solani, and Cryptococcus neoformans grown on or in malt extract agar, Columbia agar, Sabouraud's dextrose agar, blood agar, and brain-heart infusion broth. Common metabolites (ethanol, methanol, acetone, acetaldehyde, methanethiol, and crotonaldehyde) were detected and quantified. We found the fingerprint of volatiles, in terms of presence and quantity of volatiles to be strongly dependent on the culture medium, both in terms of variety and quantity of volatiles produced, but may form the basis for species specific identification of medically important fungi.

Ethylene modulates development and toxin biosynthesis in aspergillus possibly via an ethylene sensor-mediated signaling pathway

Ethylene, a biologically active natural compound, inhibited aflatoxin accumulation by Aspergillus parasiticus on a solid growth medium in a dose-dependent manner at concentrations of 0.1 to 150 ppm. The activity of the nor-1 promoter (an early aflatoxin gene) was reduced to nondetectable levels by similar quantities of ethylene, suggesting that the inhibitory effect on toxin synthesis occurred, at least in part, at the level of transcription. The inhibitory effect of ethylene on aflatoxin accumulation was also observed when A. parasiticus was grown on raw peanuts. Under similar growth conditions and doses, ethylene strongly inhibited development of asci and ascospores in Aspergillus nidulans, with no detectable effect on Hülle cell formation, conidiation, or sterigmatocystin accumulation. During early growth, A. parasiticus and A. nidulans produced ethylene with approximately twofold higher quantities measured in continuous light than in the dark. 1-Methylcyclopropene (an inhibitor of ethylene receptors in plants), light, CO2, temperature, and growth medium composition altered the effect of ethylene on A. nidulans and A. parasiticus. These observations are consistent with the existence of an ethylene sensor molecule that mediates the function of an ethylene-responsive signaling pathway(s) in Aspergillus.

Effects of volatile aldehydes from Aspergillus-resistant varieties of corn on Aspergillus parasiticus growth and aflatoxin biosynthesis

The fungi Aspergillus flavus and Aspergillus parasiticus produce a potent class of hepatocarcinogens known as aflatoxins. Corn-derived volatile compounds have been previously found to affect growth and aflatoxin production in A. flavus. In this study, the effects on A. parasiticus of three corn-derived volatile compounds, n-decyl aldehyde, hexanal and octanal, were measured. These three compounds were previously found to be variably expressed in five Aspergillus-resistant maize strains and three susceptible strains. In this study, A. parasiticus radial growth was restricted least by n-decyl aldehyde and most by octanal. Treatments of 100 microl of both hexanal and octanal were found to completely inhibit radial growth of the fungus using an agar plate assay method. While the volatile compound n-decyl aldehyde had less of an effect on radial growth than the other volatiles, the n-decyl aldehyde treated colonies had a predominance of uniquely aerial hyphae. These colony structures were found to have more complex hyphae and significantly fewer conidiophores than the control and other aldehyde treatments. Furthermore, aflatoxin production by the fungus was reduced by n-decyl aldehyde and hexanal, but was stimulated by octanal. The results presented here indicate that all three volatile compounds reduce radial growth but only n-decyl aldehyde significantly inhibits aflatoxin biosynthesis in A. parasiticus.