Tom Volk's Fungus of the Month for February 1997

This month's fungus is Aspergillus.

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AspergillusThere is probably no other genus of fungi so useful to humans that is also so harmful to humans. Members of this genus produce many industrially useful enzymes, chemicals, and foods. Yet others produce deadly carcinogenic toxins, and some may even grow through a person's lungs as if it were a loaf of bread.

Eurotium and Aspergillus Aspergillus is a member of the "deuteromycetes," or Fungi Imperfecti, which is a group (technically) reserved for fungi for which there is no known sexual state. However, the sexual state (the teleomorph) for many of the species of Aspergillus is known, and most of these are in the Ascomycota genus Emericella, which forms cleistothecia (closed ascocarps) under certain conditions. With DNA evidence forthcoming, it is likely that all members of the genus Aspergillus are closely related and should be considered members of the Ascomycota. The picture to the right links to a larger 56kb image.

Aspergillus species are very difficult to distinguish from one another without a great deal of practice. (See Raper and Fennel 1965. The Genus Aspergillus. The Williams & Wilkins Co. Baltimore.) . There is now a computerized key to the genus Aspergillus by Keith Seifert, Biosystematics Centre, Agriculture Canada, Ottawa, Ontario K1A OCG, Canada. It's called "ASP45. A Computer key for the identification of Aspergillus species." I haven't seen this key, Keith tells me it's an even better key than Fuskey, Keith's incredible online Fusarium interactive key.

I should also mention that some significant progress is being made with regard to distinguishing the Aspergillus species using molecular biological techniques. According to Toby Feibelman, who's working with Peter Cotty on this problem at the USDA's Southern Regional Research Lab in New Orleans, molecular biology seems to be confirming the species delimitations set up by the conidiophore morphologists, for the most part. However there are some unexplained anomalies with the results so far using different strains. Work is still in progress!

Penicillium vs. Aspergillus--- a paintbrush vs. a toilet bowl brush.

PenicilliumSome people have difficulty distinguishing between the genera Aspergillus and Penicillium, or remembering which is which. It's very easy to remember if you know what the genus names mean. Both of them are named for the shapes and arrangements of the conidia on their conidiophores. Aspergillus is named for a device called an aspergillum, which is used by a priest at a Catholic mass to sprinkle holy water on the faithful. It is basically a circle (called the vesicle) with a handle and spikes around the outside. Some people claim it reminds them more of a toilet bowl brush. Penicillium (pictured here) comes from the Latin word for (paint)brush, penicillus, where there are many "bristles" that emanate from the top of a flat or even somewhat rounded surface. The two genera are pretty easily distinguished with some practice.

Industrial uses of Aspergillus

If we believe all the commercials on television, we could not imagine having a day without a soft drink. One of the main ingredients of many soft drinks is citric acid. But it is far too expensive to isolate the citric acid from citrus fruits. For that reason, almost all of the citric acid in cola drinks is produced by large-scale vat fermentation of Aspergillus niger.

Maybe this should be the new logo...?

the joy of fungi

Or perhaps the catch phrase should be:        

Things go better with Aspergillus TM !

Another Aspergillus species is used in food production. Authentic soy sauce is fermented with the fungus Aspergillus oryzae, again in large vats. The fungus gives soy sauce its distinctive flavor. Unfermented soy sauce is just not as tasty.

Some Aspergillus species can be harmful to humans

Some species of Aspergillus, notably A. flavus and A. parasiticus produce a secondary metabolite known as aflatoxin. Aflatoxin was first discovered by observing a "plague" that occurred on many turkey farms, particularly in England in the early 1960's, where the turkeys died rapidly and in great numbers. Since the causal agent for the disease was not known, it was simply called "Turkey X disease." Mycologists later discovered that the turkeys had been fed peanut meal contaminated with Aspergillus flavus, Aspergillus parasiticus, and related species. Although aflatoxin is widely known for its carcinogenic properties, under the conditions present at that time these fungi produced aflatoxin in such great quantities as to rapidly kill the turkeys. This is no longer a problem since the peanut meal, if used to feed the birds, is monitored very closely for the fungus. There is no chance of turkey containing aflatoxin today. There is, however, still a chance for aflatoxin to be in peanut butter.

Some Aspergillus species can even act directly as human pathogens, causing a lung disease called Aspergillosis. It can be a very severe disease in immunocompromised patients. There's lots more information on Aspergillosis and other fungal diseases of humans at the Medical Mycology Page at the Medical Mycology Research Center at the University of Texas Medical Branch at Galveston. I highly recommend reading about Aspergillosis and looking at the great pictures at this site.

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