Expression of Genes by Aflatoxigenic and Nonaflatoxigenic Strains of Aspergillus flavus Isolated from Brazil Nuts

Aflatoxin profiles of Aspergillus flavus isolates in Sudanese fungal rhinosinusitis

Aspergillus flavus is a commonly encountered pathogen responsible for fungal rhinosinusitis (FRS) in arid regions. The species is known to produce aflatoxins, posing a significant risk to human health. This study aimed to investigate the aflatoxin profiles of A. flavus isolates causing FRS in Sudan. A total of 93 clinical and 34 environmental A. flavus isolates were studied. Aflatoxin profiles were evaluated by phenotypic (thin-layer and high-performance chromatography) and genotypic methods at various temperatures and substrates. Gene expression of aflD and aflR was also analyzed. A total of 42/93 (45%) isolates were positive for aflatoxin B1 and AFB2 by HPLC. When the incubation temperature changed from 28°C to 36°C, the number of positive isolates decreased to 41% (38/93). Genetic analysis revealed that 85% (79/93) of clinical isolates possessed all seven aflatoxin biosynthesis-associated genes, while 27% (14/51) of non-producing isolates lacked specific genes (aflD/aflR/aflS). Mutations were observed in aflS and aflR genes across both aflatoxin-producers and non-producers. Gene expression of aflD and aflR showed the highest expression between the 4th and 6th days of incubation on the Sabouraud medium and on the 9th day of incubation on the RPMI (Roswell Park Memorial Institute) medium. Aspergillus flavus clinical isolates demonstrated aflatoxigenic capabilities, influenced by incubation temperature and substrate. Dynamic aflD and aflR gene expression patterns over time enriched our understanding of aflatoxin production regulation. The overall findings underscored the health risks of Sudanese patients infected by this species, emphasizing the importance of monitoring aflatoxin exposure.

Current Practices for Reference Gene Selection in RT-qPCR of Aspergillus: Outlook and Recommendations for the Future

Aspergillus is a genus of filamentous fungi with vast geographic and ecological distributions. Species within this genus are clinically, agriculturally and biotechnologically relevant, leading to increasing interest in elucidating gene expression dynamics of key metabolic and physiological processes. Reverse-transcription quantitative Polymerase Chain Reaction (RT-qPCR) is a sensitive and specific method of quantifying gene expression. A crucial step for comparing RT-qPCR results between strains and experimental conditions is normalisation to experimentally validated reference gene(s). In this review, we provide a critical analysis of current reference gene selection and validation practices for RT-qPCR gene expression analyses of Aspergillus. Of 90 primary research articles obtained through our PubMed query, 17 experimentally validated the reference gene(s) used. Twenty reference genes were used across the 90 studies, with beta-tubulin being the most used reference gene, followed by actin, 18S rRNA and glyceraldehyde 3-phosphate dehydrogenase. Sixteen of the 90 studies used multiple reference genes for normalisation. Failing to experimentally validate the stability of reference genes can lead to conflicting results, as was the case for four studies. Overall, our review highlights the need to experimentally validate reference genes in RT-qPCR studies of Aspergillus.

Molecular profile of aflatoxigenic and non-aflatoxigenic isolates of Aspergillus flavus isolated from stored maize

Maize is a significant staple crop and utilized in Saudi Arabia as food and feed, but maize is often infected with Aspergillus flavus in tropical and subtropical climates, especially during storage. This study intended at a polyphasic approach, consisting of microscopic morphological, biochemical, and molecular characterizations that were applied to 29 of A. flavus isolates of stored maize, with the goal of characterization and identification of aflatoxigenic and non-aflatoxigenic A. flavus isolates. The technique of real-time PCR (RTi-PCR) was used to detection of A. flavus in stored maize samples, the findings have been very accurate. Centered on macroscopic morphological (primarily colony color and morphology of conidia) and microscopic (morphology of conidia and size) characteristics. Results have shown 23 A. flavus isolates (80%) were categorized as the dark green of colonies also all isolates were rough conidia. The isolates have been two different groups, 16 isolates (62%) had sclerotium-forming and the remaining 13 isolates (38%) had no sclerotium-forming isolates. To the identification of aflatoxigenic isolates of A. flavus in stored maize, we utilized the qualitative methods (easy and inexpensive) like UV test, yellow pigmentation, and ammonia vapor and quantitative method as HPLC (accurate and expensive). the accuracy methods to the identification aflatoxigenicity isolates, vary, and classified in the following descending order: HPLC (100%) > UV method (81%) > yellow pigmentation (YP) and ammonia vapor (AV) (63%). The profile of Aflatoxigenicity of A. flavus isolates by HPLC has been involved in two types first of 11 isolates (38%) have been aflatoxigenic isolates while 18 isolates (62%) were non-aflatoxigenic isolates. The expression of six aflatoxins (AFs) genes (aflD, aflM, aflO, aflP, aflR, and aflQ) was estimated using PCR and RT-PCR. PCR of all genes did not correspond to the aflatoxigenic isolates. The transcriptional analysis of aflO and aflQ was a beneficial marker for discriminating aflatoxigenic from non-aflatoxigenic A. flavus isolates. Also, qRT-PCR indicated that non-aflatoxigenic isolates had a high incidence of defect or downregulation in late AF-genes contrast with early AF-genes. therefore, these non-aflatoxigenic isolates could be critical factors for an efficient and competent strategy for the control of aflatoxin contamination pre-harvest can be considered.

Influence of Light and Water Activity on Growth and Mycotoxin Formation of Selected Isolates of Aspergillus flavus and Aspergillus parasiticus

Aspergillus flavus and A. parasiticus are the main causes of aflatoxin contamination in various foods, particularly grains, as they can thrive in environments with lower water activity and higher temperatures. The growth of Aspergillus and the formation of the mycotoxins aflatoxin and cyclopiazonic acid are strongly influenced by environmental stimuli and can be reduced by modulating parameters such as water activity, pH, temperature and light during the storage. This study has two objectives—on the one hand, to assess how global warming and an increase in exposure to sunlight affect growth and mycotoxin formation, and on the other hand, how the findings from these experiments can be used to reduce fungal growth and mycotoxin formation in stored foods. Using growth substrates with two different water activities (a_w 0.95, a_w 0.98), together with a light incubation device consisting of different chambers equipped with diodes emitting visible light of five different wavelengths (455 nm, 470 nm, 530 nm, 590 nm, 627 nm) plus white light, we analyzed the growth and mycotoxin formation of selected Aspergillus flavus and A. parasiticus isolates. It was shown that light with a wavelength of 455/470 nm alone, but especially in combination with a lower water activity of a_w 0.95, leads to a significant reduction in growth and mycotoxin formation, which was accompanied by reduced transcriptional activity of the responsible mycotoxin biosynthetic genes. Therefore, these results can be used to significantly reduce the growth and the mycotoxin formation of the analyzed fungi during storage and to estimate the trend of fungal infestation by Aspergillus flavus and A. parasiticus in water activity- and light exposure-equivalent climate change scenarios. Mycotoxin-producing aspergilli can be effective and sustainably inhibited using a combination of short-wave light and lowered water activity in the substrate. A higher annual mean temperature accompanying climate change may lead to an increased spread of aflatoxin-producing fungi in areas that were previously too cold for them. On the other hand, there will be regions in the world where contamination with aflatoxin-producing fungi will be reduced due to increased drought and sun exposure.

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.

Fitness Cost of Aflatoxin Production in Aspergillus flavus When Competing with Soil Microbes Could Maintain Balancing Selection

Selective forces that maintain the polymorphism for aflatoxigenic and nonaflatoxigenic individuals of Aspergillus flavus are largely unknown. As soils are widely considered the natural habitat of A. flavus, we hypothesized that aflatoxin production would confer a fitness advantage in the soil environment. To test this hypothesis, we used A. flavus DNA quantified by quantitative PCR (qPCR) as a proxy for fitness of aflatoxigenic and nonaflatoxigenic field isolates grown in soil microcosms. Contrary to predictions, aflatoxigenic isolates had significantly lower fitness than did nonaflatoxigenic isolates in natural soils across three temperatures (25, 37, and 42°C). The addition of aflatoxin to soils (500 ng/g) had no effect on the growth of A. flavus Amplicon sequencing showed that neither the aflatoxin-producing ability of the fungus nor the addition of aflatoxin had a significant effect on the composition of fungal or bacterial communities in soil. We argue that the fitness disadvantage of aflatoxigenic isolates is most likely explained by the metabolic cost of producing aflatoxin. Coupled with a previous report of a selective advantage of aflatoxin production in the presence of some insects, our findings give an ecological explanation for balancing selection resulting in persistent polymorphisms in aflatoxin production.IMPORTANCE Aflatoxin, produced by the fungus Aspergillus flavus, is an extremely potent hepatotoxin that causes acute toxicosis and cancer, and it incurs hundreds of millions of dollars annually in agricultural losses. Despite the importance of this toxin to humans, it has remained unclear what the fungus gains by producing aflatoxin. In fact, not all strains of A. flavus produce aflatoxin. Previous work has shown an advantage to producing aflatoxin in the presence of some insects. Our current work demonstrates the first evidence of a disadvantage to A. flavus in producing aflatoxin when competing with soil microbes. Together, these opposing evolutionary forces could explain the persistence of both aflatoxigenic and nonaflatoxigenic strains through evolutionary time.

Molecular identification of aflatoxigenic Aspergillus species in feedstuff samples

Background and Purpose:

Aflatoxins are naturally produced by some species of Aspergillus, such as A.flavus and A.parasiticus. Aflatoxins reportedly have carcinogenic effects on human, poultry, and livestock, and therefore could be linked to severe human illnesses. Aflatoxin biosynthesis pathway involves different clustered genes, including structural, regular, and unassigned genes. The present study was conducted to detect aflR,aflP, and aflD as three important genes contributing to aflatoxin B1 production cycle in Aspergillus species isolated from the feedstuffs of animal husbandry.

Materials and Methods:

This study was conducted on 25 isolates of A. flavus, A. parasiticus, A. nomius, and A. nidulans, isolated from animal feedstuff as a test group. The test group was compared with two standard strains (i.e., A. flavus and A. parasiticus) as aflatoxigenic reference organisms and negative controls (i.e., A. fumigatus, A.fusarium, and A.penicillium) in terms of the presence of aflR, aflP, and aflD genes using polymerase chain reaction (PCR). The determination of the toxigenicity and aflatoxin production of isolated Aspergillus species was accomplished using thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC).

Results:

The results obtained by the amplification of the selected genes by PCR method for the detection of aflatoxigenic Asprgillus species were significantly correlated with TLC and HPLC results. Accordingly, all samples, having positive results for aflatoxin B1 production in TLC and HPLC, were able to show the amplification of three target genes. However, 4 cases out of 6 (66%) non-aflatoxigenic isolates were positive for three or two genes.

Conclusion:

Based on the findings, the molecular detection of aflatoxin biosynthesis genes (i.e., aflP, aflD, and aflR) could be considered as a quick and reliable method for the detection of aflatoxigenic Aspergillus. Furthermore, this method could be useful in planning and implementing strategies targeted toward improving the safety of human or animal food.

Genetic and Toxigenic Variability within Aspergillus flavus Population Isolated from Maize in Two Diverse Environments in Kenya

Aspergillus flavus is the main producer of carcinogenic aflatoxins in agricultural commodities such as maize. This fungus occurs naturally on crops, and produces aflatoxins when environmental conditions are favorable. The aim of this study is to analyse the genetic variability among 109 A. flavus isolates previously recovered from maize sampled from a known aflatoxin-hotspot (Eastern region, Kenya) and the major maize-growing area in the Rift Valley (Kenya), and to determine their toxigenic potential. DNA analyses of internal transcribed spacer (ITS) regions of ribosomal DNA, partial β-tubulin gene (benA) and calmodulin gene (CaM) sequences were used. The strains were further analyzed for the presence of four aflatoxin-biosynthesis genes in relation to their capability to produce aflatoxins and other metabolites, targeting the regulatory gene aflR and the structural genes aflP, aflD, and aflQ. In addition, the metabolic profile of the fungal strains was unraveled using state-of-the-art LC-MS/MS instrumentation. The three gene-sequence data grouped the isolates into two major clades, A. minisclerotigenes and A. flavus. A. minisclerotigenes was most prevalent in Eastern Kenya, while A. flavus was common in both regions. A. parasiticus was represented by a single isolate collected from Rift Valley. Diversity existed within the A. flavus population, which formed several subclades. An inconsistency in identification of some isolates using the three markers was observed. The calmodulin gene sequences showed wider variation of polymorphisms. The aflatoxin production pattern was not consistent with the presence of aflatoxigenic genes, suggesting an inability of the primers to always detect the genes or presence of genetic mutations. Significant variation was observed in toxin profiles of the isolates. This is the first time that a profound metabolic profiling of A. flavus isolates was done in Kenya. Positive associations were evident for some metabolites, while for others no associations were found and for a few metabolite-pairs negative associations were seen. Additionally, the growth medium influenced the mycotoxin metabolite production. These results confirm the wide variation that exists among the group A. flavus and the need for more insight in clustering the group.