Aspergillus flavus
1. Classification
a. Higher order taxa
Eukaryota; Ascomycota; Eurotiomycetes; Eurotiales; Trichocomaceae
b. Species
Aspergillus flavus
2. Description and significance
Aspergillus flavus is a pathogenic fungus in the phylum Ascomycota. This species is known primarily for its ability to produce a potent toxin and carcinogen known as aflatoxin (1). Aflatoxin is known to contaminate many types of crop seeds, but in the field it is predominantly problematic for maize, peanuts, cotton seed, and tree nuts (1). A. flavus also has a great impact on human health, in which immunosuppressed people are most susceptible to infection by this fungus (1). A. flavus may be found in any type of climate, but it is most common in warm temperate zones and environments with low water levels and higher temperatures (1).
3. Genome structure
The genus Aspergillus consists of 250 recognizable species. These species are further divided into different groups (2). A. flavus is part of the Flavi group, which is notable for its aflatoxin-producing fungi. This section also includes A. parasiticus. The A. flavus genome consists of 37 million base pairs arranged into eight different chromosomes (4). It was found that the genome contains 13,485 predicted protein coding regions, including several secondary metabolite biosynthetic gene clusters (3). Further research on the secondary metabolite biosynthetic gene clusters of this species revealed that there were more than 56 biosynthetic gene clusters (4). The secondary metabolite biosynthetic gene clusters responsible for the production of aflatoxin consists of 25 genes, spanning 70 kb DNA sections located near the telomere of the third chromosome (4).
4. Cell structure
The microscopic characteristics of A. flavus align with the other organisms in the Aspergillus genus. The hyphae of A. flavus are partitioned by a septum and are hyaline, giving them a glossy appearance (2). The organism is a circular vesicle, with protruding filamentous extensions (2). In cell cultures, A. flavus are known to grow as yellow-green colonies and are 65-70 mm in diameter on Czapek yeast extract (1). As the spores mature, they transition into a darker green color (1).
5. Metabolic processes
A. flavus grows and produces aflatoxin in the presence of oil-rich agricultural crops including maize, peanuts, and cotton seeds. The biosynthesis of aflatoxin involves a regulatory mechanism mediated by pathway-specific genes aflR and aflS. The aflR gene expresses the DNA binding promoter 'AflR (4). Expression of aflatoxin is upregulated when AflR is bound to the promoter of the aflatoxin gene (4). This binding is not required for aflatoxin production, however. Aflatoxin will still be expressed in low levels without any AflR present in the cell (4). Moreover, AflS, a regulatory protein expressed by aflS, takes an important role in aflatoxin biosynthesis by acting as a transcriptional enhancer or co-activator of AflR (4). The exact role of AflS is still unknown, but recent research has shown that AflS was required to properly transport AflR to and from the nucleus (4).
Additional gene clusters have been shown to regulate the production of aflatoxin, one of which is veA (5). A deletion of this gene causes a down regulation of aflatoxin, in addition to the prevention of the formation of sclerotia, a part of the cell membrane that helps the fungal cell to survive environmental extremes for long periods (5). Since veA regulates both aflatoxin and sclerotia, it is presumed that the protein encoded by this gene is involved in controlling both processes, but the exact mechanism of how veA regulates these is still unknown (5). In addition to veA, laeA has also been shown to be required for aflatoxin and sclerotial formation (6). LaeA is shown to be involved in the conversion of heterochromatin, where gene expression is suppressed, to euchromatin, where gene expression is promoted (5). Both veA and laeA are known to be global regulators since they regulate several gene clusters in the cell (4). VeA, LaeA, and VelB form a heterotrimer, known as the Velvet Complex, in the nucleus to control fungal development and secondary metabolite production (4).
6. Ecology
A. flavus inhabits a broad range of environments. The prevalence of A. flavus has led some researchers to deem it ubiquitous, as scientists have found species of Aspergillus every time they sought to look for them (7). This claim was supported by a meta-analysis that revealed that A. flavus was found at all latitudes studied. Despite the capacity to colonize a wide range of environments, A. flavus grows more readily in warm climates and thrives in tropical latitudes (7). Specifically, A. flavus responds best to temperatures between 36oC and 38oC. Additionally, A. flavus flourishes in drier environments, and is also represented in desert regions (7).
A. flavus is similarly diverse in the ecological roles it fulfills within an environment. It is primarily a saprophyte in soil, and acts to recycle nutrients (7). A. flavus is also an opportunistic parasite capable of infesting a wide range of hosts. Although it has been documented within avian and mammalian hosts, A. flavus is primarily a parasite that affects plants, some of which include a large number of economically important crops (7).
7. Pathology
How does this organism cause disease? Human, animal, plant hosts? Virulence factors, as well as patient symptoms.
7. Key microorganisms
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8. Current Research
Include information about how this microbe (or related microbes) are currently being studied and for what purpose
9. References
It is required that you add at least five primary research articles (in same format as the sample reference below) that corresponds to the info that you added to this page. [Sample reference] Faller, A., and Schleifer, K. "Modified Oxidase and Benzidine Tests for Separation of Staphylococci from Micrococci". Journal of Clinical Microbiology. 1981. Volume 13. p. 1031-1035.
