Aspergillus flavus

Description and significance
Aspergillus flavus is a filamentous fungus. It produces a mycotoxin called aflatoxin B that is carcinogenic. It causes liver damage and is one of the mycotoxins used as a biologcal weapon. When crops are infected with A. Flavus their value decreases because of the potential human or animal exposure to the aflatoxin. A. flavus is mainly a saprophyte, meaning it obtains its nutrients from dead or decaying material, but it can also be a pathogen to plants and animals including humans.

Pathology
A. flavus is an opportunistic pathogen, which allows it to infect animals, humans and plants. It is not virulent in healthy viable tissues. It infects seeds by entering wounds and holes from which insect larval have exited, and by entering from the vascular tissue. A. flavus targets seeds with poor viability. A. flavus can be carried to the surface of corn by insects. Also the corn can be colonized but not infected until it is matured. It can also infect cotton by entering from natural openings and traveling up into the boll. In the boll the fungus can be there for 25 days but not infect until the boll matures. In peanuts the fungus can infect seeds and the plant as it grows, it also can penetrate the pod.5

In humans A. flavus is associated with aspergillosis, a secondary respiratory infection in immuno-compromised patients. In certain areas it is also the cause of infections. 2 Infections A. flavus can cause are corneal, otomycotic, and nasoorbital infections.3 A. flavus is also an allergen. It causes allergic bronchopulmonary aspergillosis. 5

Ecology
Aspergillus flavus can be found all over the world; however it is found to be abundant in places of warm temperature. Field contamination is mostly present in areas with high temperatures and drought. It is also abundant in areas with temperate climates during warm drought years. In the US it affects the corn crop in the south eastern area. 5

A. flavus grows in temperatures of 25-42˚C, but the optimal temperature for growth is 37˚C. It survives through winter as a mycelium or sclerotia. Sclerotia is a resistant structure that is developed into hyphae or conidia. The conidia are then scattered into air and soil by insects or wind.

Cell structure and metabolism
A. flavus is also known as a mold. Like other molds it also grows by producing hyphae. The network of hyphae, or mycelium,are responsible for secreting enzymes. The enzymes secreted are used to break down complex food sources. The complex food sources are broken down into small molecules which are then absorbed by the myceilium, which uses the small molecules to grow. Mycelium contain conidia or asexual spores, both the mycelium and conidia can be seen with the naked eye while the hyphae cannot. Young conidia in A. flavus are yellow green and as the spores age they turn a darker green. A. flavus can use many nutrient sources but being a saphrophyte it uses primarily dead or decaying materials.

Genome structure
The genome of A. flavus strain NRRL 3357 has been sequenced. It has 2761 scaffolds and each scaffold has anywhere between 4.5 Mbp to 200 bp. It has approximately 13,487 genes and the average length of each gene is 1485 bp.

Application to Biotechnology
The genome was sequenced at The Institute for Genomic Research in Maryland. It has also been entered into the BLAST database.

Current Research
With the sequencing of the genome done, current research is focused on using the sequence to understand the fungus, to yield crops that are resistant to the fungus and to determine tools to control the production of the aflatoxin. One of the ways that the control of the production of aflatoxin is being researched is to see how the use of chemicals can weaken the oxidative stress response. ( oxidative stress has been shown to increase the production of aflatoxin.) There have been many unsuccessful experiments conducted on yielding A. flavus resistant crops of peanuts and corn. Currently research is being done to cultivate genes that are resistant to A. flavus, afflatoxin production and plant stress. These genes will then be used to create resistant crops. 5