Aspergillus

Aspergillus is a genus of around 200 molds found throughout much of nature worldwide. Aspergillus was first catalogued in 1729 by the Italian priest and biologist Pietro Antonio Micheli. Viewing the fungi under a microscope, Micheli was reminded of the shape of an aspergillum (holy water sprinkler), and named the genus accordingly.

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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 grow on carbon-rich substrates such as monosaccharides (such as glucose) and polysaccharides (such as amylose). Aspergillus species are common contaminants of starchy foods (such as bread and potatoes), and grow in or on many plants and trees.

In addition to growth on carbon sources, many species of Aspergillus demonstrate oligotrophy where 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, as a major component of mildew.

Commercial importance

Species of Aspergillus are without a doubt an important microorganism, both medically and commercially. Some species can cause infection in humans and other animals. Others are important in commercial microbial fermentations.

In Asian countries, alcoholic beverages such as Japanese sake are made from rice, rather than from grapes, barley or corn. This fermentation is carried out entirely by microorganisms, instead of by plant amylases, or animal amylases (saliva) and yeast. Firstly, koji mold such as Aspergillus oryzae is used to convert the starch in the rice to sugars (saccharification), which are subsequently fermented by other microorganisms, such as yeast (Saccharomyces) and lactic acid bacteria.

Perhaps the most well-known application of A. niger is as the major source of citric acid; this organism accounts for over 99% of global citric acid production, or more than 4.5 million tonnes per annum. A. niger is also commonly used for the production of native and foreign enzymes, including 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, including A. fumigatus, A. oryzae and A. niger.

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. Aspergillus flavus produces aflatoxin which is both a toxin and a carcinogen, and which can potentially contaminate foods such as nuts. 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 synthesize 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 difficult. Usually, only patients with already weakened immune systems or who suffer other lung conditions are susceptible.

In humans, the major forms of disease are:

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

See also

• Mycotoxin
• Sick building syndrome (SBS)
• Mold health issues
• aspergilloma