Aspergillus

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Aspergillus

Aspergillus is a genus of around 200 fungi (moulds) found worldwide. It is a filamentous fungus (made of chains of cells, called hyphae, as opposed to yeast which consists of single round cells). Its natural habitat is in hay and compost.

Aspergillus was first catalogued in 1729 by the Italian priest and biologist Pietro Antonio Micheli as Nova plantarum genera juxta Tournafortii methodum disposita. In studying the shape of the fungus, he was reminded of an aspergillum, an instrument used for the dispersion of holy water, thus the name.

Growth and distribution

Aspergillus species are highly aerobic and are found in almost all oxygen-rich environments, where they commonly grow as molds on the surface of a substrate, as a result of the high oxygen tension. Commonly, fungi are grown on carbon-rich substrates, mainly monosaccharides such as glucose. Aspergillus, however, can also secrete amylase enzymes, which allow it to use polysaccharides as a carbon source, for example starch. As a result of this, Aspergillus species are common contaminants of starchy foods, for example bread and potato, and grow in or on many plants and trees.

In addition to growth on carbon sources such as glucose, fructose, maltose, and starch, many species of Aspergillus demonstrate oligotrophy: they are capable of growing in nutrient-depleted environments, or environments in which there is a complete lack of key nutrients. A. niger is a prime example of this; it can be found growing on damp walls, or as a major component of mildew. It will also grow in environments lacking a carbon source, instead using salts and nitrogen sources such as ammonia and nitrate for both growth and protein production, although in such environments the fastidious nature of the organism is truly revealed.

Commercial importance

Species of Aspergillus are without doubt the most commercially important fungi in the world, being exploited for the production of both primary and secondary metabolites. Perhaps the most well-known application of A. niger is as the major source of citric acid; indeed, this organism accounts for over 99% of the citric acid production globally, or more than 4.5 million tonnes per annum. A. niger is also commonly used for the production of native and foreign enzymes, examples of which include glucose oxidase and hen egg white lysozyme. In these instances, the culture is rarely grown on a solid substrate, although this is still common practice in Japan, but is more often grown as a submerged culture in a bioreactor. In this way, the most important parameters can be strictly controlled, and maximal productivity can be achieved. It also makes it far easier to separate the chemical or enzyme of importance from the medium, and is therefore far more cost-effective.

Research

A. nidulans has been used as a research organism for many years and was used by Guido Pontecorvo to demonstrate parasexuality in fungi. Recently, A. nidulans was one of the pioneering organisms to have its genome sequenced by researchers at the Broad Institute. Now, many Aspergillus species have had their genomes sequenced.

Pathogens

Some Aspergillus species cause serious disease in humans and animals, and can be pathogenic. The most common causing invasive disease are Aspergillus fumigatus and Aspergillus flavus. The most common causing allergic disease are Aspergillus fumigatus and Aspergillus clavatus. Other species are important as agricultural pathogens. Aspergillus spp. cause disease on many grain crops, especially maize and synthesise mycotoxins including aflatoxin.

Aspergillosis

Aspergillosis is the group of diseases caused by Aspergillus. The symptoms include fever, cough, chest pain or breathlessness, which also occur in many other illnesses so diagnosis can be hard. Usually, only patients with already weakened immune systems or who suffer other lung conditions are susceptible.

In man, the major forms of disease are:

Allergic aspergillosis (affects asthma, cystic fibrosis and sinusitis patients).
Acute invasive aspergillosis (risk increases if patient has weakened immunity such as some cancer patients and those in chemotherapy).
Disseminated invasive aspergillosis (widespread through body).

External links

Aspergillus | Parasitic fungi

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